Vertical Axis Wind Turbine Testing
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Vertical Axis Wind Turbine Testing Final Report
Declaration I hereby certify that this material which I now submit for assessment on the program of study leading to the award of “Bachelor of Mechanical Engineering Honours Degree”, is entirely my own work and has not been submitted for any academic purpose other than in partial fulfilment for that stated above.
Signed:
___________________________ Robert Mc Auley
Date
_____________________________
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Acknowledgements The objectives set out for this project were ambitious and required a large amount of work covering many different areas of the engineering profession. The success of the project was greatly helped by a number of individuals who provided invaluable advice in their respective fields. These individuals helped the progress of this project by dedicating their time to providing prompt responses to any queries regarding certain phases of the project. I would like to thank the following people for helping me to make this project a success. Dr. Fergal Boyle
Project Supervisor
Sean Keane
Rapid Prototyping Technician
Martin Byrne
Lab Technician
I would also like to thank my friends and family for the help they provided throughout the course of the project, in particular Hugh O’Reilly for his help with the wind tunnel testing, Stephen Kirwan for his help with the spin down testing and L.A Mc Auley Ltd. for the use of their metal fabrication facilities.
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Abstract The aim of this project was to carry out performance testing on a vertical axis wind turbine (VAWT) designed by a company called Brí Toinne Teoranta. The first section of the project involved a background study into the history of wind turbines and wind energy at a global level. Once a clear understanding was developed for the importance of sustainable energy sources, an investigation was then carried out into the testing of wind turbines. A manufacturing process had to be specified for the designers’ complex blade geometry. A detailed investigation was carried out into the area of rapid prototyping and 3D printing processes which were available at DIT Bolton Street. Several prototypes of the blades were manufactured before a manufacturing process was decided upon for the final blades. A testing methodology was developed by carrying out research into the different methods of applying torques and measuring the power produced by a VAWT. A decision was made to proceed with the development of a Prony brake apparatus and a magnetic particle brake (MPB) apparatus. A test rig was designed and fabricated to accommodate both of these testing methods. The test rig was designed to work specifically with the wind tunnel situated in DIT Bolton Street. A careful approach was taken to the wind tunnel testing of the VAWT, with preliminary tests being carried out to ensure that the apparatus was working safely and correctly. Phase 1 of the Prony brake testing was carried out providing performance curves for the turbine at different wind speeds. The mechanical losses in the test rig were assessed and reduced before commencing phase 2 of the Prony brake testing. The performance curves for the VAWT were converted into dimensionless form and an experiment was designed to calculate the magnitude of the mechanical losses in the bearings of the VAWT test rig. Several components were designed to allow testing to be carried out using the MPB. The MPB wind tunnel testing was documented along with any issues which arose. All of the data from the wind tunnel testing was carefully analysed and documented in a way that would accommodate future testing using the designed testing methodologies and test rig.
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Contents Chapter 1: Introduction ................................................................................................................. 1 1.1 Project Background ............................................................................................................... 1 1.2 Project Aims/Objectives ....................................................................................................... 2 1.3 Project Management ............................................................................................................. 2 1.3.1 Time Plan ....................................................................................................................... 2 1.3.2 Objective Tree Method .................................................................................................. 3 Chapter 2: Background ................................................................................................................. 5 2.1 Introduction ........................................................................................................................... 5 2.2 Energy ................................................................................................................................... 6 2.3 Wind Energy ......................................................................................................................... 7 2.3.1 Wind Energy in Ireland .................................................................................................. 7 2.3.2 Benefits of Wind Energy ............................................................................................... 8 2.3.3 Measuring wind energy.................................................................................................. 9 2.4 Wind Turbine Fundamentals............................................................................................... 10 2.4.1 Power ........................................................................................................................... 10 2.4.2 Wind Turbine Design Variation................................................................................... 11 2.4.3 Aerodynamics of Wind turbines .................................................................................. 13 2.5 Orientation of axis of rotation ............................................................................................. 16 2.6 VAWT Development .......................................................................................................... 19 2.6.1 Rotor Design ................................................................................................................ 19 2.6.2 The Flettner Rotor ........................................................................................................ 20 2.6.3 Savonius Rotor ............................................................................................................. 21 2.6.4 Helical Rotor ................................................................................................................ 22 2.6.5 VAWT Case study ....................................................................................................... 23 2.7 Summary ............................................................................................................................. 24 Chapter 3: Rotor Fabrication ....................................................................................................... 25 3.1 Introduction ......................................................................................................................... 25 3.2 Rotor Design ....................................................................................................................... 25 3.3 Sizing of rotor (scale).......................................................................................................... 26
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3.4 Blade Modifications ............................................................................................................ 27 3.5 Mounting Rotor ................................................................................................................... 28 3.6 Rotor Manufacture .............................................................................................................. 29 3.6.1 Considerations.............................................................................................................. 29 3.6.2 Selection Process ......................................................................................................... 29 3.6.3 Analysis of Results ...................................................................................................... 31 3.6.4 Final Blade ................................................................................................................... 33 3.7 Summary ............................................................................................................................. 34 Chapter 4: Test Rig Design ......................................................................................................... 36 4.1 Introduction ......................................................................................................................... 36 4.2 Dimensional Analysis of VAWT ........................................................................................ 36 4.3 Methods of applying torque (Brakes) ................................................................................. 40 4.4 Prony Brake ........................................................................................................................ 41 4.4.1 Background .................................................................................................................. 41 4.4.2 Derivation of Prony brake formula .............................................................................. 41 4.4.3 Prony Brake Behaviour ................................................................................................ 45 4.4.4 Investigation ................................................................................................................. 47 4.4.5 Prony Brake Design/Manufacture................................................................................ 48 4.4.6 Proposed Testing Methodology ................................................................................... 52 4.5 Magnetic Particle Brake ...................................................................................................... 53 4.5.1 Principle of operation ................................................................................................... 53 4.5.2 Brake sizing ................................................................................................................. 54 4.5.3 Power Supply ............................................................................................................... 54 4.5.4 MPB Mounting ............................................................................................................ 55 4.5.5 M.P.B Calibration ........................................................................................................ 58 4.6 Measuring Rpm ................................................................................................................... 59 4.7 Frame Design & Fabrication ............................................................................................... 60 4.8 Summary ............................................................................................................................. 63 Chapter 5: Wind Tunnel Testing ................................................................................................. 65 5.1 Introduction ......................................................................................................................... 65 5.2 Wind Tunnel Set-up ............................................................................................................ 65 vi
5.3 Prony Brake Set-up ............................................................................................................. 67 5.4 Prony Brake testing phase 1 ................................................................................................ 68 5.5 Prony Brake testing Phase 2 ............................................................................................... 72 5.6 Mechanical Losses .............................................................................................................. 74 5.6.1 Theory .......................................................................................................................... 74 5.6.2 Procedure ..................................................................................................................... 75 5.6.3 Sample calculation ....................................................................................................... 76 5.6.4 Drag Calculation .......................................................................................................... 78 5.7 Magnetic Particle Brake testing .......................................................................................... 82 5.8 Summary ............................................................................................................................. 83 Chapter 6: Conclusion/ Recommendation .................................................................................. 85 6.1 Conclusion .......................................................................................................................... 85 6.2 Recommendations ............................................................................................................... 87 Bibliography ................................................................................................................................. 88 Appendix A: Rapid Prototyping Screenshots and Code ............................................................... 89 Appendix B: MPB Specifications ................................................................................................. 97 Appendix C: Engineering Drawings ........................................................................................... 103 Appendix D: Bearing Specifications ......................................................................................... 108 Appendix E: Wind Tunnel Testing Log………………………………………………………...110
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List of Figures Figure 1 (Predicted Test Rig Design) ............................................................................................. 1 Figure 2 (Project Time Plan)........................................................................................................... 3 Figure 3 (Test Rig Design Tree) ..................................................................................................... 4 Figure 4 (Graph showing energy consumption pattern (in million tonnes oil equivalent)) ............ 6 Figure 5: (Wind-sourced electricity in Ireland 2000-2009. Source: Eir Grid & SEAI EPSSU ) ... 8 Figure 6 (Various Power Curves for wind turbines) ..................................................................... 11 Figure 7 (Drag-type rotor) ............................................................................................................ 12 Figure 8 (Left: Darreius type lift turbine, Right: Propeller type lift turbine) ............................... 12 Figure 9 (Airfoil Geometry)[8] ..................................................................................................... 13 Figure 10 (Aerodynamic Forces)[8] ............................................................................................. 15 Figure 11 (Power Curve Comparison)[7] ..................................................................................... 18 Figure 12 (VAWTs used in comparison)[7] ................................................................................. 18 Figure 13 (VAWT Rotor Designs) ............................................................................................... 19 Figure 14 (Flettner rotors used on ships) ..................................................................................... 20 Figure 15 (Savonius Rotor)[10] .................................................................................................... 21 Figure 16 (Gorlov helical rotor) ................................................................................................... 22 Figure 17 (Helical VAWT from Quietrevolution)[12] ................................................................. 23 Figure 18 (The Eole 3.8MW VAWT) ............................................................................................. 23 Figure 19 (Rotor Design) .............................................................................................................. 25 Figure 20 (Wind tunnel dimensions) ............................................................................................ 26 Figure 21 (Rotor size) ................................................................................................................... 27 Figure 22 (Blades with modified holes)........................................................................................ 28 Figure 23 (Hub design) ................................................................................................................. 29 Figure 24 (Powder Blade Prototype) ............................................................................................ 32 Figure 25 (ABS Plastic Blade Prototype) ..................................................................................... 33 Figure 26 (Catalyst Screenshot) .................................................................................................... 34 Figure 27 (Final Blades) ............................................................................................................... 35 Figure 28 (Prony Brake fundamentals) ......................................................................................... 42 Figure 29 (Prony Brake Derivation) ............................................................................................. 42 Figure 30 (Prony Brake Behavior 1) ............................................................................................. 45 viii
Figure 31 (Prony Brake Behavior 2) ............................................................................................. 46 Figure 32 (Prony Brake Behavior 3) ............................................................................................. 46 Figure 33 (Prony Brake Design) ................................................................................................... 49 Figure 34 (Tensioning Mechanism) .............................................................................................. 50 Figure 35 (Spring Attachment) ..................................................................................................... 51 Figure 36 (Prony Brake Mounting)............................................................................................... 51 Figure 37 (Prony Brake Assembly) .............................................................................................. 52 Figure 38 (Magnetic Particle Brake)............................................................................................. 53 Figure 39 (Torque Vs Current) ..................................................................................................... 54 Figure 40 (Power Supply) ............................................................................................................. 55 Figure 41 (MPB Setup) ................................................................................................................. 55 Figure 42 (MPB Coupling) ........................................................................................................... 56 Figure 43 (MPB Bracket).............................................................................................................. 56 Figure 44 (MPB Assembly) .......................................................................................................... 57 Figure 45 (Fabricated MPB Assembly) ........................................................................................ 57 Figure 46 (MPB Calibration) ........................................................................................................ 58 Figure 47 (MPB Extension Lead) ................................................................................................. 59 Figure 48 (MPB and Power Supply) ............................................................................................. 59 Figure 49 (Laser Tachometer) ...................................................................................................... 60 Figure 50 (Test Rig Frame)........................................................................................................... 61 Figure 51 (Frame Clamps) ............................................................................................................ 61 Figure 52 (Frame Top Features) ................................................................................................... 62 Figure 53 (Frame and Brake Assembly) ....................................................................................... 63 Figure 54 (Test Rig in Place) ........................................................................................................ 63 Figure 55 (Wind Tunnel DIT Bolton Street) ................................................................................ 66 Figure 56 (Wind Tunnel Controls) ............................................................................................... 66 Figure 57 (Prony Brake Testing) .................................................................................................. 67 Figure 58 (Prony Brake Phase 1 Power Curves) .......................................................................... 69 Figure 59 (Prony Brake Phase 1 Dimensionless Power Curves) .................................................. 70 Figure 60 (Prony Brake Phase 1 Repeatability Test) .................................................................... 71 Figure 61 (Prony Brake Phase 1 Torque Measurements) ............................................................. 71 ix
Figure 62 (Prony Brake Phase 2 Power) ....................................................................................... 72 Figure 63 (Prony Brake Phase 2 Torque) ..................................................................................... 73 Figure 64 (Prony Brake Phase 2 Dimensionless Power Curves) .................................................. 74 Figure 65 (Spin Down Test) ......................................................................................................... 76 Figure 66 (Spin Down Test) ......................................................................................................... 78 Figure 67 (Friction Torque in Bearings) ....................................................................................... 80 Figure 68 (Torque Losses and Useful torque) .............................................................................. 80 Figure 69 (Total power curve 22m/s) ........................................................................................... 81 Figure 70 (MPB Testing) .............................................................................................................. 82 Figure 71 (MPB Minimum current) .............................................................................................. 83
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List of Tables Table 1 (Selecting manufacturing process)................................................................................... 31 Table 2 (List of variables) ............................................................................................................. 37 Table 3 (Brake Selection) ............................................................................................................. 41 Table 4 (Recorded data) ................................................................................................................ 77 Table 5 (Average Rpm Values) .................................................................................................... 79
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Chapter 1: Introduction 1.1 Project Background The fluid mechanics department in DIT Bolton Street were approached by a company called Brí Toinne Teoranta with a design for a vertical axis wind turbine (VAWT). The company had completed the preliminary design of the turbine but had not carried out any testing to determine its performance. It was decided that the mechanical testing of this VAWT design would be undertaken as an undergraduate final year project. On accepting the project, videos of an early version of the turbine operating in a wind tunnel were provided by the company to show the principle of operation of the turbine. These videos were carefully analysed in order to gain an understanding of the challenges that were ahead for the project. The company also provided the relevant CAD files for the unique turbine blades, which would allow the blades to be manufactured in accordance with the company’s design. The following dissertation was compiled to document the approach used in testing the Brí Toinne Teoranta turbine design along with the testing methodologies which were devised for analysing the turbines performance.
Figure 1 (Predicted Test Rig Design)
1.2 Project Aims/Objectives The aim of this project was to design and build a test rig for a vertical axis wind turbine (VAWT) which can test the performance of the turbine. It was decided that by measuring the mechanical power output and plotting dimensionless power curves, the performance of the turbine could be assessed. A series of objectives were devised in order to successfully complete the project. The main objectives of the project were as follows. 1. Carry out research into the area of wind energy and develop an understanding for the fundamentals of wind power generation. 2. Manufacture the turbine blades designed by Brí Toinne Teoranta to a high standard. 3. Design a testing methodology to obtain performance curves for the turbine. 4. Design and fabricate a suitable test rig for the wind tunnel testing. 5. Carry out wind tunnel testing on the VAWT. 6. Analyse the results obtained from the wind tunnel testing. 7. Present the findings from testing in report form.
1.3 Project Management Before any work was started for the project it was decided that the correct project management procedures should be carried out in order to ensure the project was executed in a professional manner. Both the direction and the time management for the project were carefully planned out to avoid any time being wasted on areas which were not relevant to the project. 1.3.1 Time Plan The time plan shown in Figure 2 below was devised in order to create deadlines within the duration of the project itself. The project was divided up into five main sections which were research, test rig design, turbine and rig manufacture, wind tunnel testing and analysis. These sections were set out in a logical fashion, with specific deadlines for each section.
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Figure 2 (Project Time Plan) 1.3.2 Objective Tree Method In order to develop a greater understanding of what was required by the project brief, the chart shown in Figure 3 was developed using the objective tree method for the design of a VAWT test rig. A few basic requirements of the test rig were defined so that smaller objectives could be put in place to meet these requirements. The requirements were that the test rig be safe to operate, have a long lifespan, have the ability to test different rotor designs using different braking mechanisms and have a simple design which can be easily adapted for future requirements. In order for the test rig to be safe to operate it was decided that any controls which the operator must use should be outside the operating area of the turbine. It was also decided that appropriate locking fasteners should be used on the turbine and test rig in order to prevent any part of the apparatus coming loose due to vibrations during operation. In order to ensure that the test rig has a long lifespan, it was decided that the rig should be made from durable materials and should be rigid in its construction. It was also decided that an appropriate surface finish should be applied to surfaces of the test rig which may be prone to rusting. It was decided that in order to make the test rig useful for future projects, it should be multifunctional in the sense that it can be used with different types of rotors, which can be easily removed from the rig. It was also kept in mind that the test rig should be able to accommodate
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different methods of applying a braking torque to the rotor, so that comparison tests could be easily carried out. The final basic requirement of the test rig was that it could be easily adapted. It was decided that to meet this requirement the turbine should be capable of being quickly and easily detached from the wind tunnel, have a means of staying upright while maintenance work was being carried out on it and be reasonably portable. When all of the basic requirements for the test rig design were analysed and understood, a decision was made to carry out research into the areas of wind energy and VAWT technology.
Figure 3 (Test Rig Design Tree)
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Chapter 2: Background 2.1 Introduction The following chapter contains a literature review which was carried out in the areas of energy, wind energy, wind turbine fundamentals and the development of vertical axis wind turbines. Before any work could be done on designing the vertical axis wind turbine test rig, a good understanding of the history of wind energy was obtained. In the following chapter, the history and the development of the wind turbine is discussed, from its early conception in the form of the windmill to the modern day electricity generating devices with which the world is now so familiar. The area of aerodynamics is explored in this chapter, pointing out the various characteristics of aerofoils and the concepts behind aerofoil performance. This is a crucial area as the turbine being tested in this project is a lift type device which has an aerofoil cross section. The development of the VAWT was investigated, along with the various different rotor designs which have been developed over the years. In this chapter the different rotor design are discussed in order to gain an understanding of the performance expected from the unique VAWT rotor design presented by Brí Toinne Teoranta.
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2.2 Energy “The environmental implications of the continued global energy system’s dependence on fossil fuels call for urgent action across the world”[1] Most of the world’s energy currently comes from non-renewable sources as indicated in Figure 4 below. This graph from the BP statistical review of world energy 2011 gives a striking indication as to the worlds dependence on fossil fuels like oil and coal, and taking into account the fact that these resources will one day run out makes it a matter of urgency to pursue the development of renewable energy technologies. It is vital that the world is able to reduce the amount energy being produced from non-renewable sources.
Figure 4 (Graph showing energy consumption pattern (in million tonnes oil equivalent))
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2.3 Wind Energy Wind energy has been used as an effective resource since the ancient times, when it was harnessed to propel sail boats. According to historic research the first people to use the wind to drive actual machinery, appears to have been the Persians, who used very early forms of windmills to grind wheat. Conquests allowed this technology to eventually spread to Holland, where the use of wind energy was embraced and became a very important tool. In the American West the power of the wind was used to drive saw mills, water pumps and cereal grinders. In these early versions of wind energy conversion machinery, the wind machines were connected directly to a mechanical load. The first substantial electricity generating wind turbine was built by Charles Brush in Cleveland Ohio, and ran for twelve years, from 1888 to 1900, supplying electricity to his home. This early wind turbine was a bulky device which had multiple rotors and because of its size rotated quite slowly thus having to be geared up in order to satisfy the rotational speed required of the generator. The production of electricity using wind energy has had to overcome many stumbling blocks in order to become the viable, efficient process it is today. After World War II for example, when the price of oil dropped and almost all interest in alternate energy was lost, wind energy was put on the backburner in favour of the cheaper electricity being produced in power plants. The 1973 oil crisis however, re-ignited interest in the area of wind energy and this led to early forms of wind farms being developed. Cost it seems is the deciding factor when the use of alternate energy is in question. The environmental friendliness of an energy source alone will not motivate investors enough to part with their cash, but if there is money to be made from the production of electricity, then it becomes a very interesting prospect. [2] 2.3.1 Wind Energy in Ireland Every year, wind energy is making a bigger contribution to the electricity supplied throughout Ireland. At the end of June 2010 it was reported that there were 110 wind farms metered in Ireland, bringing the total installed capacity for wind up to 1,379MW. The national target for the year 2020 is to have 40% of our electricity coming from renewable sources, an estimated 5,5006,000 MW of wind generation is required to achieve this target. [3] 7
Looking at the data taken from the 2009 IEA Annual report for Ireland there is a promising growth in wind power in Ireland. In Figure 5 below it is clear to see that there has been a significant rise in the amount of wind sourced electricity being used in Ireland since the year 2000. This dramatic increase makes the national targets for renewable electricity look like they are a realistic goal. [4]
Figure 5: (Wind-sourced electricity in Ireland 2000-2009. Source: Eir Grid & SEAI EPSSU ) 2.3.2 Benefits of Wind Energy Wind energy is classed as a renewable source of energy and has certain benefits associated with it when compared with other non-renewable processes used to produce power. A common theme amongst renewable energies is that they can be described as “clean” energy sources. A “clean” energy source does not produce any emissions like nitrogen oxide, sulphur dioxide, mercury and carbon dioxide, which pollute the air. This means that not only can wind energy provide the world with extra capacity for creating electricity; it can do so without producing any extra emissions. When a country has got a well established system for producing electricity using wind energy in place, it can then start to decrease the demand on electricity produced in power plants, hence decreasing the amount of fossil fuels which will be consumed on a daily basis. The development and progression of wind energy as a source of electricity has benefits on a domestic level also. As the wind energy industry grows, there will be a diversification in the market whereby the majority of the world’s electricity will no longer be coming from power plants burning fossil fuels. This means that when there are dramatic increases in the price of oil 8
and other fossil fuels around the world, the cost of electricity for customers will not be as dramatically affected. In an ideal situation, 100% of the energy supplied to a customer would be from a wind energy source and electricity prices would not be affected at all by the cost of fossil fuels. [5] 2.3.3 Measuring wind energy When choosing a source of energy which will be used to power a machine, it is crucial to be able to measure exactly the amount of power the machine can produce using this energy source. It is easy to calculate the performance and power input of a machine which will be powered by fossil fuels, because of the set calorific value for the fuel. This set value guarantees a certain amount of energy output from the machine, and when the machines efficiency is taken into account, the performance of the machine can be calculated over any given period of time. Wind energy however does not have this certainty of performance attached to it. There are many factors which make harnessing the winds energy in a consistent and efficient way, a very complicated process. Wind speed is one of these factors. The power input for a turbine is calculated from knowing the wind speed. The faster the wind, the more power can be extracted from it. The problem with this is that the wind speed is constantly fluctuating, so the wind turbine does not have a constant power input, making calculations and efficiencies very complicated. Another factor which affects the performance of a wind turbine substantially is its placement. The placement of a wind turbine has to be exact in order to achieve the maximum possible power output.[6]
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2.4 Wind Turbine Fundamentals A wind turbines main objective is to harness the power of the wind and convert it into some useful form of energy. The modern day design of wind turbines did not come about by chance, but was formed from constant upgrading and experimenting carried out over many years. There are a few fundamental equations used to quantify the performance of a wind turbine. 2.4.1 Power It is crucial to know the amount of power that can be gathered by a wind turbine, the equation for this power is given by
(2.1)
Where Cp is the coefficient of power for the turbine,
is the density of the air which is flowing
through the turbine, A is the swept area of the turbine (the area which the blades or rotor sweeps through), and
is the wind speed.
The Cp for a wind turbine is the way in which the aerodynamic efficiency of the turbine is quantified. Cp is a function of the tip speed ratio λ. This is the ratio of speed at the tip of the turbine blade to wind speed and is given by
(2.2)
Where
is the rotational frequency, R is the radius of the turbine and
is the wind speed. The
efficiency and performance of a wind turbine is usually displayed using power curves. Figure 6 below shows plots of power coefficient versus tip speed ratio for various different wind turbine types. The theoretical maximum power coefficient is known as the Betz limit and is 0.59 for an ideal wind turbine. This Betz efficiency is marked as the ideal efficiency of propeller-type turbine in Figure 6 below. [7]
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Figure 6 (Various Power Curves for wind turbines) 2.4.2 Wind Turbine Design Variation Wind turbines fall under the category of either a drag-type turbine or a lift-type turbine. Modern day wind turbines mostly fall into the lift-type category, but it is worth knowing the principles of both categories. 2.4.2.1 Drag turbines In a drag type device a force which acts in the same direction as the wind is blowing is exerted on the blades or paddles of the turbine. This is the same principle by which sail boats operate, as the wind exerts a force on the sails. In a turbine which works solely on the principle of drag, the surface on which the wind is exerting a force cannot move faster than the speed of the wind. This fact limits the tip speed ratio and hence the overall efficiency of these drag type turbines. Many of the earlier vertical axis wind turbine designs used drag rotors, such as the bucket type wind turbine shown in Figure 7. 11
Figure 7 (Drag-type rotor) 2.4.2.2 Lift turbines In a lift type turbine like the ones shown in Figure 8 below the force generated by the wind acts perpendicular to the direction that the wind is blowing. It should also be noted that in a lift type turbine, the maximum speed of the blade is not limited to the speed at which the wind is blowing as in drag type turbines. This means that lift type turbines can have much larger tip speed ratios than their drag type counterparts. In order to fully understand exactly how lift type turbines operate, an investigation must be done into the area of aerodynamics and aerofoil technology.
Figure 8 (Left: Darreius type lift turbine, Right: Propeller type lift turbine)
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2.4.3 Aerodynamics of Wind turbines 2.4.3.1 Geometry of aerodynamic profiles In lift type wind turbines the blades of the rotor have an airfoil cross section. The amount of lift force which is exerted on the blade depends strongly on the shape of the airfoil section used. There are various geometric parameters on which the aerodynamic characteristics of the air foil depend. These parameters shown in include the leading-edge radius, the mean camber line, the maximum thickness and the thickness distribution of the profile, and the trailing edge angle.[8]
Figure 9 (Airfoil Geometry)[8] Leading-Edge Radius The leading-edge radius of an airfoil section is the radius of a circle centered on a line which is tangent to the leading edge camber connecting the points of tangency on the upper and lower surfaces of the airfoil with the leading edge. The centre of the leading edge radius is located such that the cambered section of the airfoil is projected out and overhangs slightly the leading edge point. [8]
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Chord Line The chord line of an airfoil section is the straight line connecting the leading and trailing edge. The angle of attack of an airfoil is the angle which exists between the chord line and the direction of the free stream fluid flow. [8] Mean Camber Line When a locus of points located half way between the top and bottom surfaces of the airfoil section is plotted, the resulting line is called the mean camber line. One of the effects of a change of camber is a change in the zero-lift angle of attack, α0l. Symmetric airfoil sections have zero lift at zero angle of attack, and likewise zero lift occurs for sections with positive camber when their angle of attack is negative. [8] Maximum Thickness and Thickness Distribution The maximum thickness and thickness distribution have a large impact on the aerodynamic characteristics of the airfoil section. An increase in the maximum thickness of an airfoil increases the maximum lift coefficient for the airfoil. An increase in the maximum thickness of an airfoil section also increases the maximum local velocity to which a fluid particle will accelerate as it flows around the airfoil. As a result of this the minimum pressure value is smallest for the thickest airfoil. There is an adverse pressure gradient associated with the deceleration of the flow between the point on the airfoil where the minimum pressure occurs to the trailing edge. This pressure gradient is largest for the thickest airfoil and the larger the pressure gradient the larger the boundary layer will be, therefore boundary layer separation will occur more easily and hence the drag values for the airfoil section will increase. The thickness distribution of an airfoil section affects the pressure distribution and the character of the boundary layer. Moving the location of the maximum thickness of the airfoil towards the leading edge of the airfoil will result in a decrease in the pressure gradient at the central region of the airfoil. This decrease in pressure gradient leads to a more stabilised boundary layer and can
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promote the possibility of a laminar boundary layer which is favourable because of it lower skin friction drag than occurs with turbulent boundary layers. [8] Trailing-Edge angle The trailing-edge of an airfoil affects the location of the aerodynamic centre. The theoretical location of the aerodynamic centre of a thin airfoil in a subsonic stream is located at the quarter chord. [8] Aerodynamic forces The motion of air around an airfoil section produces variations in pressure and velocity which result in aerodynamic forces and moments. Viscous forces are neglected apart from when they occur in a small area near the surface of the airfoil called the boundary layer, a region in which the large velocity gradient results in large viscous forces. If these boundary layer forces are neglected then standard equations of motion can be used to analyse the three main forces which occur in an airfoil section; lift, drag and side force. [8] The primary forces shown in Figure 10 contribute to the main forces which occur in the airfoil. Lift is a component of force which acts upward, perpendicular to the direction of the undisturbed free-stream velocity. The primary cause of the lift force is the pressure forces acting on the airfoil surface. Drag is the net aerodynamic force which acts in the same direction as the freestream velocity. The drag force is due to a combination of pressure forces and skin friction forces which act on the surface of the airfoil. Side force is a force which acts perpendicular to both the lift force and the drag force. [8]
Figure 10 (Aerodynamic Forces)[8] 15
2.5 Orientation of axis of rotation The concept of the wind turbine has taken many different physical forms since it was first introduced. Wind turbines can be divided up into two categories Horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs).As things stand today, the HAWT dominates as the most widely used type of wind turbine across the world. This does not mean however that the concept of the VAWT should be discarded as a novel idea. There are various advantages and disadvantages associated with both the HAWT and the VAWT, some of which will now be discussed. HAWTs and VAWTs have been developed almost in parallel, but there has been less interest and investment in the development of VAWTs. This is one of the key reasons why the HAWT dominates the wind power world today. One of the main differences between the VAWT and the HAWT is the VAWTs ability to accept wind which is blowing in any direction. This ability makes the VAWT very suited to areas where there are changeable and gusting winds, such as the tops of large buildings. In the case of HAWTs a yaw mechanism is required to point the propeller of the turbine into the wind and hold it there. The time it takes the HAWT to point into the wind can be regarded as downtime in which it is not producing as much electricity as it could be. The yawing mechanism also adds extra cost to the production of the wind turbine, as does the control system used to control the yaw. Because of the orientation of the axis of a VAWT, it is possible to place the power generating equipment at ground level. This is a major advantage from the point of view of maintenance and monitoring of the electricity generating equipment. Another advantage of having the power generating equipment at ground level is that when selecting a generator, the criteria for selection does not have to include minimizing the weight and physical size of the generator, and the sole focus of the selection can be choosing the most efficient generator for the task. The blades of a HAWT have to be self supporting because they are only attached to the turbine at one end. It has been claimed that the HAWT has reached its maximum possible size, and that the reversing gravity loads on the blades limits their progression. These reversing gravity loads however, do not occur in VAWTs, which theoretically have no maximum possible size. 16
Guy wires can be used to support VAWTs which means that the main shaft of the turbine can be of a smaller diameter. This is not the case with HAWTs, guy wires cannot be used on HAWTs because they would interfere with the rotation of the propeller. In a direct drive wind turbine the rotor or propeller is connected directly to the generator without the drive going through a gearbox. The power generating equipment associated with direct drive machinery is usually more bulky than the usual equipment used in a system with a gearbox. This means that the nacelle (the area in a HAWT where the generating equipment is housed) will be heavier and hence the turbine support mast will have to be larger. In a VAWT however this will not be an issue as the power generating equipment can be located at ground level. One of the problems Associated with VAWTs is the torque ripple which occurs in the rotors. This torque ripple causes cyclic loading of the blades of the turbine which can lead to failure of the blades. In HAWTs there is constant torque acting on all of the blades, so the issue of fatigue due to cyclic loading does not arise. A study was carried out comparing the power curves of three different turbines, a Darrieus type, an H-type, and a standard HAWT. The power curves are shown in Figure 11 below. The power curves were formed by plotting the coefficient of power (Cp) against the tip speed ratio (λ). The three turbines which are being compared in Figure 11 are a 100kw H-rotor VAWT, a 500kW Darrieus VAWT (shown in Figure 12) and the HAWT data comes from the National Renewable Energy Laboratory in the USA, and is said to represent the data associated with a typical HAWT. The high values of Cp which can be seen for the HAWT show how much more developed the HAWT is compared to the two VAWTs which are also plotted. It should be noted however that the maximum value for coefficient of power for the Darreius type turbine is not very far off the HAWT value, and considering how much more field testing and research has been carried out on the HAWT, the performance gap between the two turbine concepts could easily be closed. [7]
17
Figure 11 (Power Curve Comparison)[7]
Figure 12 (VAWTs used in comparison)[7]
18
2.6 VAWT Development 2.6.1 Rotor Design The development and design of an efficient rotor is perhaps the most important step in making VAWT’s more efficient as devices for gathering wind energy. The rotor of a VAWT must be designed so that it can deal with pulsating torques and cyclic loading over the lifetime of the wind turbine. Unlike the rotors used on HAWTs, VAWT rotors can differ greatly in both appearance and in the fundamentals of aerodynamics which they use. It is necessary to have an understanding of how the various rotor designs for VAWTs originated and the problems associated with each design. In this section the background of the following rotors will be discussed.
Flettner Rotor
Savonius Rotor
Darius Rotor
H-Type Rotor
Helical Rotor
Figure 13 (VAWT Rotor Designs)
19
2.6.2 The Flettner Rotor The history of the VAWT begins with a German engineer named Anton Flettner who came up with the Flettner rotor. Although the Flettner rotor was never actually used as a VAWT rotor, it inspired the design of the Savonius rotor which will be discussed later on. The Flettner rotor uses the Magnus effect to turn wind energy into a lateral thrust in the direction which is perpendicular to that of the wind. The rotor itself is a large cylinder which is revolved in order to create a difference in pressure on both sides of the cylinder. This pressure difference between the two sides of the rotating cylinder results in a lift force in the direction of the lower pressure. The rotor that Flettner had designed was used on ships to propel the ship forward using wind which was approaching the ship from its side; this process is illustrated in Figure 14. On these ships, the rotation of Flettners rotors was powered by diesel engines. The reliability and speed which became associated with the conventional propeller powered ships meant that the Flettner rotor ceased to be used for the purposes of propelling ships. [9]
Figure 14 (Flettner rotors used on ships)
20
2.6.3 Savonius Rotor A Finish man by the name of Sigurd Savonius decided that it would be possible to use the wind energy harnessed from offshore winds to turn the Flettner rotors which were used on ships, removing the need for the diesel engines on these ships. Savonius took the Cylindrical Flettner rotor and cut in into two semi-circles. The semi circles were then offset from the centre of the rotor along the cutting plane, creating two semi circular cups. The gap between the cups at the centre of the rotor meant that the air would flow into one of the cups and pass through the gap, thus having a thrust effect on the rotor on the other side also. When the Savonius rotor was tested against a Flettner rotor of comparable size, the Savonius rotor produced a greater lateral thrust than the Flettner rotor. Despite this increased lateral force, there was no real need for the rotor as a replacement for the Flettner rotor as it had never taken off as a widely used method of propelling ships. [9]
Figure 15 (Savonius Rotor)[10] The Savonius rotor uses drag as its driving force. It has been used as a rotor for water current turbines to good effect. The Savonius rotor has various advantages associated with it, such as the simplicity of its design and the ease with which it can be manufactured. This makes the Savonius rotor an interesting, economical possibility for converting wind energy to electricity in underdeveloped areas of the world.
21
2.6.4 Helical Rotor The helical rotor is a relatively new rotor design when compared with the others mentioned in this report. The helical rotor was developed for use on a hydraulic turbine in the early nineties, harnessing the different types of ocean currents to produce electricity. The Gorlov helical type rotor shown in Figure 16, was developed in order to utilise all of the advantages of the Darreius rotor, without any of the disadvantages. The basic principle behind the helical rotor is that instead of straight blades which are used in Darrieus rotors, the blades follow a spiral around the outer circumference of the rotor. This spiral helps to get rid of the pulsating torque and the vibration problems which were associated with the Darrieus rotor when it was tested in water.[11]
Figure 16 (Gorlov helical rotor) A company form the UK called quietrevolution has developed a helical VAWT shown in Figure 17 called the Q5, which is designed to be used in urban areas where the turbine will be operating close to the general public. The helical design eliminates vibration in the turbine, so it can be attached onto buildings without causing any detectable shaking of the building. The company also says that the helical design eliminates any noise from the turbine and provides it with a robust structure. [12] 22
Figure 17 (Helical VAWT from Quietrevolution)[12] 2.6.5 VAWT Case study There have been a few reasonably successful attempts at creating large scale VAWTs in the past, for example a VAWT called the Eole shown in Figure 18, which was built in 1986 by an American company called FloWind. The Eole was a 96m tall Darrieus turbine which as the largest VAWT ever built had a maximum power output of 3.8MW. During its five year lifetime the Eole produced 12GWh of electricity, reaching power levels of around 2.7MW. Failure of the bottom bearing in the Eole resulted in the turbine being shut down. The existence of this multimegawatt Darrieus type VAWT shows that it is realistic to believe that the VAWT could someday be just as popular as the HAWT. [7]
Figure 18 (The Eole 3.8MW VAWT) 23
2.7 Summary The research carried out in this chapter provided a good insight into the history of wind turbines and some of the fundamental principles on which their operation depends. The areas investigated also gave a good rounded insight into the position of wind energy in the world today. The issue of global energy demands was investigated and it was clear that alternative energy sources such as wind power generation will be crucial in meeting these demands in the future. The sustainable and environmentally friendly nature of wind energy was also investigated and it was clear from the information obtained that wind energy will play a leading role in reducing the amount of fossil fuels being used worldwide. The potential for wind power generation in Ireland was explored briefly in this chapter. Ireland’s large Atlantic coastline gives it great potential to eventually source almost all of its power from renewable sources like the wind. It is likely that in a strong economic climate, there will be huge investment in developing offshore and onshore wind farms along the coast of Ireland. The fundamentals of wind power generation were discussed in this chapter which involved the differences between drag and lift turbines, the area of aerodynamics, the theory behind calculating the power produced by a wind turbine and the differences between horizontal and vertical axis turbines. Particular attention was paid to the development of the vertical axis wind turbine along with the different design concepts which have been devised in recent years. It was clear that the VAWT had not seen the same commercial success as the HAWT but this could have been due to a lack of investment into the development of the VAWT. The different VAWT rotor designs which have been developed proved to be remarkably different in their appearance and operation. After looking at the different rotor designs, it could be argued that the optimum VAWT rotor design has not yet been discovered, thus making the testing of the rotor designed by Brí Toinne Teoranta a relevant and worthwhile project.
24
Chapter 3: Rotor Fabrication 3.1 Introduction In this chapter the fabrication of the VAWT rotor designed by Brí Toinne Teoranta is explained in detail, documenting the various challenges which were involved with the fabrication of the rotor. The unique rotor design is discussed briefly along with the key features of the blades, followed by the size limitations which were faced and any additional design modifications which had to be made to the blades before manufacture. The complex geometry of the turbine blades meant that the manufacturing process used to make the blades needed careful consideration. The following chapter also contains the details of how the rotor was mounted on a central shaft for operation.
3.2 Rotor Design The vertical axis wind turbine (VAWT) rotor which was provided by Brí Toinne Teorannta is shown in the figure below. The rotor is a hybrid design, incorporating elements of various VAWT rotors such as the Darreius and helical rotors. The rotor consists of three helical blades connected to a central shaft. The aerofoil cross section of the blades creates the lift force in the rotor, causing the rotor to rotate. The helical design of the rotor should help to eliminate pulsating torque in the rotor. This style of rotor is a relatively new design, and there has been very little testing carried out on it in the past.
Figure 19 (Rotor Design)
25
3.3 Sizing of rotor (scale) Before any design modifications could be carried out on the rotor it was necessary to select an approximate diameter for the VAWT rotor. Ideally the rotor should be as large as possible in order to increase the accuracy of the testing and decrease the effect of any losses encountered. There was however some constraints which placed a limit on the maximum diameter of the turbine. The first limiting factor was the physical size of the wind tunnel in D.I.T. Bolton Street. The wind tunnel has a cross section of 500mm by 500mm as shown in Figure 20 below. Previous testing was carried out with this wind tunnel on a turbine of diameter 568mm. The turbine was too large for the wind tunnel and the blades were not experiencing the full free stream for a complete revolution of the rotor, resulting in very low power output from the rotor. In order to avoid this, it was decided that the rotor diameter should be kept well inside the cross section of the wind tunnel. It was acknowledged that reducing the size of the rotor would also limit the turbines power output, but because there was no other wind tunnel available it was considered to be the best course of action.
Figure 20 (Wind tunnel dimensions) The other factor which limited the size of the turbine was the capacity of the machines which would be used to manufacture the blades. After looking at the possible manufacturing techniques 26
which could be applied to the blades, it was concluded that the maximum rotor diameter was limited to 200mm. The 200mm rotor would fit comfortably into the wind tunnel and there would also be significant distance between the tip of the blades and the walls of the wind tunnel. This gap was considered to be important as it helps to prevent blockage effects and should also prevent issues with the boundary layers on the walls of the wind tunnel. Figure 21 below shows the rotor placed in the cross section of the wind tunnel.
Figure 21 (Rotor size)
3.4 Blade Modifications The SolidWorks file for the turbine blade was provided by the designer for use as part of the project. The file was provided for the blade only and they did not include any components for connecting the blades together to make the rotor. The blade was scaled down from the original 300mm diameter to 194mm to enable its manufacture as discussed above. The solid model of the scaled down blade had to be carefully analysed, finding its centre of rotation and other key points so that the blades could be connected together and mounted onto a central shaft. Once the centre of rotation was determined, three blades were evenly spaced in a circle around the centre as shown in Figure 22 below. This allowed for the design of a central hub to hold the blades onto the central shaft. The blade provided by the designer had already got two holes in each of the blade ends. During the scaling down of the blade these holes became too small to be 27
used to fit the blades to the central shaft so it was necessary to replace these with 4.2mm holes which would accommodate an M4 machine screw. The holes also had to be slightly re-located as the new larger holes went too close to the edge of the blade which could potentially cause failure of the blades. The re-located larger holes are also shown in Figure 22 below.
Figure 22 (Blades with modified holes)
3.5 Mounting Rotor After the new holes were located in the blades, the blades had to somehow be connected to the central shaft of the turbine. A component was designed to hold the blades in position and fix them to the central shaft. Firstly a central shaft diameter of 12mm was decided upon. The 12mm shaft was chosen in order to prevent deflection and vibration occurring. The component shown in Figure 23 below was designed using the modified holes in the blades as a template. An M5 grub screw was decided upon as the preferred method of tightening the rotor to the shaft. The grub screw also enables the rotor to be easily removed from the central shaft and to be moved up and down the shaft.
28
Figure 23 (Hub design)
3.6 Rotor Manufacture 3.6.1 Considerations The complex geometry of the rotor meant that the manufacture of the blades was a key area in the success of the VAWT testing. Previous testing was carried out on blades which had imperfections in the surface finish, and this affected the performance of the rotor. It was decided that the blades must have the best possible surface finish and the most accurate replication of the designed geometry possible. Some key characteristics of the blade which needed to be focused on were the accuracy of the trailing edge, the aerofoil profile, the overall surface roughness and the strength of the material used. These features were used as the criteria for selection of the manufacturing process. An investigation was carried out into different manufacturing processes which could be used to manufacture the VAWT blades. Conventional machining techniques such as milling and turning were ruled out due to the complex three dimensional curves which even on an automated machine would have been difficult to achieve. It was decided to investigate the areas of rapid prototyping and 3D printing as methods for the manufacture of the turbine blades. 3.6.2 Selection Process Investigations were carried out into the suitability of three different rapid prototyping machines for the manufacture of the VAWT blades. All three machines were located on the DIT Bolton Street premises and were available for use. The machines available were the Rap-man 3D 29
printer, Z-Corp 3D printer and the Dimension Fused Deposition Modeller. Both the Rap-man and the Dimension machine use a plastic as their working material and the Z-Corp machine uses a powder based material. In order to decide which machine would be used, a number of criteria for selection were decided upon. The three machines were assessed by looking at parts previously manufactured by each machine. The following criteria were marked on a scale of 0-5 in below. Surface Finish A smooth, uniform surface finish is vital to the performance of the VAWT blades. If the surface of the blades is rough and uneven, the lift forces created by the aerofoil cross section will be affected. Geometry Replication Accurate replication of the blade geometry is important so that the blade performs as the designer intended. Accurate blade geometry also means that the current design of the blades is being analysed correctly and design modifications can be implemented following the testing. It is also necessary to manufacture three identical blades in order to ensure that the rotor is balanced. Rigidity The blades of a VAWT are put under a considerable amount of mechanical stress during testing at high wind speeds. It is vital that the blades are manufactured to be as rigid as possible in order to prevent any bending and possible failure of the blades. Compatibility (software) In order to correctly produce the VAWT blades, the software for the rapid prototyping machine should run smoothly without errors or complications. Incorrect use of the software could lead to incorrectly produced parts and in turn wasted materials. Capacity The capacity of the rapid prototyping machine is one of the factors which determine the maximum blade diameter which can be manufactured. 30
Cost of Raw Material Each rapid prototyping machine uses different materials to manufacture parts. The cost of this raw material can vary greatly depending on the properties of the material. Technical Support Past experience and availability of technical support are vital to the successful manufacture of the blades. Making decisions about orientating the blades during manufacture to produce maximum strength and carrying out necessary modifications are made easier if there is a technician available with a lot of past experience working with the rapid prototyping machines. Rap-Man 3D
Z-Corp Dimension
Surface Finish
2
3
3
Geometry Replication
3
2
5
Rigidity
3
1
4
Compatibility(software)
1
5
5
Capacity
5
5
5
Cost of Raw Material
3
4
1
Technical Support
1
5
5
18/35
25/35
28/35
Total:
Table 1 (Selecting manufacturing process) 3.6.3 Analysis of Results From the results shown in Table 1 above it was concluded that the Dimension machine would be most suitable for manufacturing the blades. The ABS plastic material used by the Dimension machine is expensive (approximately 50c per cubic centimetre), therefore it was decided to make a prototype of the blades on the Z-Corp powder based machine, which will be half the cost of a plastic blade. Although the powder prototype would not be strong enough to be used in testing, it would give a good indication as to any slight modifications which had to be made to the blade design, and would enable any changes to be carried out before using the more expensive manufacturing process. The prototype powder blade shown in Figure 24 was analysed for
31
imperfections and possible problems involving geometry. Overall the powder blade proved to be an accurate recreation of the desired geometry, and no major modifications had to be made. The next step was to manufacture a small section of the VAWT blade on the Dimension ABS plastic machine. This small and inexpensive model of the blades cross section was used to assess how the Dimension machine would reproduce the aerofoils characteristics such as the trailing edge. The ABS blade section shown in Figure 25 was inspected and a decision was made to proceed with the manufacture of the blades.
Figure 24 (Powder Blade Prototype)
32
Figure 25 (ABS Plastic Blade Prototype) 3.6.4 Final Blade The Standard Tessellation Language (STL) file for the blade was imported into a software package called catalyst in order to prepare it to be sent to the Dimension machine. This file contains the data required by the Dimension machine to manufacture the blades. The catalyst programme calculates the required support material to be added to the model as shown in Figure 26. This support material is required so that the machine can print out parts of the blade which are not sitting on the base of the machines build area. The orientation of the blade is essential to minimizing the amount of support material required. Several orientations were investigated to see which used the smallest amount of support material. The catalyst software then calculated the total build time for one blade to be 4 hours 51 minutes. The blades were manufactured and the support material was carefully removed from the holes in the blade and the blade geometry.
33
Figure 26 (Catalyst Screenshot)
3.7 Summary Before the investigation into the rotor manufacture began, the wind tunnel in DIT Bolton Street was measured in order to determine the maximum size VAWT which could be tested accurately. The wind tunnel had a cross section of 0.5m by 0.5m so the rotor diameter was immediately limited to within these dimensions. The next area which was investigated was the manufacturing methods available for the manufacture of the blades in DIT Bolton Street. The 3D printing machines available were limited to a maximum dimension of 200mm. This 200mm was then considered the maximum diameter for the turbine blades. The CAD files for the turbine blades presented by Brí Toinne Teoranta were carefully analysed and scaled down to the 200mm limit discussed above. A method for attaching the turbines blades to a central shaft had to be devised as there was no attachment method specified with the blade design. Suitable hubs were designed to attach the three turbine blades in an evenly spaced manner around the central shaft of the turbine. The design of the turbine blades had to be modified slightly by adding a series of holes which would allow the blades to be attached to the central hub. 34
Two different 3D printing based manufacturing processes were available for the manufacture of the turbine blades. Prototypes were made using both of the 3D printing machines. The samples made by each machine were inspected and by using specific selection criteria the appropriate manufacturing method was selected. The final blades shown in Figure 27 below were manufactured on a 3D printer using ABS plastic as the material. The blades were of a reasonably high quality and were deemed appropriate for use in the VAWT wind tunnel testing.
Figure 27 (Final Blades)
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Chapter 4: Test Rig Design 4.1 Introduction In this chapter the test rig design and fabrication is documented, showing the challenges which were faced throughout. Dimensional analysis was carried out for a vertical axis wind turbine (VAWT) in order to determine the relationship between the relevant variables. Using the results from the dimensional analysis a testing methodology was devised to get the performance curves for the VAWT. An appropriate method of applying a mechanical load (braking torque) to the VAWT was selected for use on the test rig. The Prony brake and the magnetic particle brake were investigated in detail and any additional components needed for testing were designed. A test rig was designed and fabricated which supported the braking mechanisms specified and allowed the rpm of the rotor to be measured. The test rig was designed to comply with the objective tree design method carried out in the introduction chapter of this dissertation.
4.2 Dimensional Analysis of VAWT Dimensional analysis is a useful tool which can be used when designing experiments such as the VAWT performance test. Using the Buckingham Pi theorem, the relevant variables for a problem can be combined to produce dimensionless groups known as Pi groups. The steps involved in the Buckingham Pi theorem are as follows 1. List and count the relevant variables associated with the problem. Assign the letter n to the number of variables considered. 2. List the dimensions of each of the variables associated with the problem. 3. Count the total number of dimensions involved with the problem and. Call this number j. 4. Select j repeating variables, which do not form a dimensionless group. 5. Add one additional variable to the j repeating variables and form a power product. Solve to find exponents of the repeating variables which will make the product dimensionless. Repeat the process adding a different variable to the repeating variables each time until n-j dimensionless groups have been formed. 36
6. Finally, verify that all the groups formed are dimensionless and write the dimensionless function. It was decided that the variables which are relevant to the performance of the VAWT are the Power
, the angular velocity
of the fluid roughness
, the free stream velocity
, the diameter of the turbine
, the swept area
, the viscosity of the fluid
, the density and the surface
.
The units and dimensions for each variable are shown in Table 2 below Variable Units Dimensions
Table 2 (List of variables) The number of Pi groups for the problem was calculated as follows
The repeating variables were selected to be the density , the free stream velocity diameter . Each dimensionless group was calculated as follows Pi group 1
37
, and the
Pi group 2
Pi group 3
38
Pi group 4
( Pi group 5
39
)
The dimensionless function can be written as follows
For the VAWT performance test, the coefficient of power varies mainly due to a change in tipspeed ratio. This relationship between the power coefficient and the tip-speed ratio can be used to form dimensionless power curves for the VAWT. These dimensionless power curves allow the turbine to be compared to larger turbines in the same operating conditions. From the dimensional analysis of the VAWT it was concluded that in order to construct the power curves for the turbine, the tip speed ratio must be varied and the torque at each tip speed ratio measured. In order to vary the tip speed ratio a brake must be used to apply a known braking torque to the rotor. Applying the braking torque slows down the rotor and changes the tip speed ratio. It was concluded that to carry out the performance tests the test rig must have a brake which can apply the braking torque and a means of recording the rpm and the wind speed. Once this conclusion was made, the next step was to investigate the different methods of applying a braking torque to the VAWT and measuring its rpm.
4.3 Methods of applying torque (Brakes) An investigation was carried out into the different methods of applying and measuring mechanical torque. After extensive research the most commonly used brakes were found to be the magnetic particle brake (MPB), the Prony brake and the Hysteresis brake. It was considered unnecessary to use all three braking methods for the test rig, so the brakes were rated on a scale of 0 to 5 (5 being the best), under the criteria of cost, accuracy of torque measurement and the range of sizes in which the brakes were available. Table 3 below shows the criteria for selecting the appropriate brake for the test rig.
40
Brake
Cost Accuracy Size Range Total
Magnetic Particle Brake
4
4
4
12
Prony Brake
5
3
4
12
Hysteresis Brake
2
4
2
8
Table 3 (Brake Selection) It was concluded from the selection process that both the magnetic particle brake and the Prony brake were suitable for use in the VAWT test rig. A decision was made to proceed with a more detailed investigation into both the Prony brake and the MPB. This would provide a means of getting performance curves using two different methods and the results could then be compared.
4.4 Prony Brake 4.4.1 Background One of the proposed methods of applying a torque to the VAWT rotor was the Prony brake. The Prony brake was initially introduced to measure the brake horse power of internal combustion engines. It uses frictional forces applied by a belt or rope to a pulley, to apply a load to a rotating pulley or shaft. Due to its simplicity the Prony brake is an inexpensive mechanical method of measuring the maximum torque output from any rotating shaft. The mechanics and operation of the Prony brake were investigated and will now be discussed in detail. 4.4.2 Derivation of Prony brake formula The diagram in Figure 28 shows a pulley rotating at a constant angular velocity
. The frictional
force between the pulley and the belt causes a difference in tension in the two sides of the belt. This tensional difference is used to calculate the power output from the pulley.
41
Figure 28 (Prony Brake fundamentals) In order to derive the equation for the power output it is necessary to take a closer look at the forces which occur at the surface of the pulley. Figure 29 below shows a small section of the surface of the pulley wheel with the reaction force
, the frictional force
and the relevant
angles displayed. It should be noted that for this derivation, the tensional force at assumed to be greater than the tensional force at .
Figure 29 (Prony Brake Derivation)
42
is
Equations for
and
are obtained by equating the vertical and horizontal forces which
gives (4.1) (4.2)
Using a small angle approximation which states that
goes to
and
goes to 1 as
tends towards zero, the above equations can be rearranged to give (
)
(4.3) (4.4)
Again using the approximation that
is very small, the above equations simplify down to the
following (4.5) (4.6)
Dividing the frictional force by the reaction force gives an expression for the coefficient of friction
as shown
(4.7)
Both sides of the equation are integrated between the limits of 0 and
because this is the area in
which the belt or rope is in contact with the surface of the pulley. The integration is carried out as follows
∫
∫
(4.8)
(4.9)
43
(4.10)
The frictional force on a segment of the belt or rope over angle
is given by (4.11)
The work done by the wheel as it turns through an arc of length s is given by (4.12)
Integrating between 0 and
gives the sum of the work done by the contact area. The integration
is carried out as follows ∫
(4.13) (4.14) (4.15)
An equation for the power output from the pulley wheel is obtained by introducing the term which represents the rate of change of the arc length with time as the wheel turns. This term can be further simplified by breaking up the wheel and
term into
, where R is the radius of the pulley
is the angle as previously discussed. (4.16) (4.17)
The term
can be rewritten as the angular velocity
, which gives the following equation for
the power output from the pulley in terms of the angular velocity and the difference in tension across the two sides of the belt or pulley
(4.18)
44
4.4.3 Prony Brake Behaviour The behaviour of the Prony brake under different loading conditions was investigated. Initially no tensional force is exerted on the ropes and the pulley rotates freely as there is almost no force due to friction between the pulley and the rope as shown in Figure 30. The spring balance relating to T2 is displaced downwards as shown in Figure 31. This creates a tension of T2+ΔT2 in the left hand side of the rope, and a tension of T1+ΔT1 in the right hand side of the rope. The frictional force between the rope and the pulley increases and hence the . (Note that it is assumed that ΔT2 > ΔT1)
rotational speed of the pulley drops down to
The tension T2 is increased in Figure 32 until the frictional force between the rope and the pulley is large enough to stop the pulley
. At this point T2-T1 is at its max, and the brake is
applying a torque which is equal to the maximum torque output for the turbine.
Figure 30 (Prony Brake Behavior 1)
45
Figure 31 (Prony Brake Behavior 2)
Figure 32 (Prony Brake Behavior 3) 46
4.4.4 Investigation From the derivation for the Prony brake the power output from the wheel is given by: (4.19)
Using the formula
, and hence dividing across by
the torque output from the wheel is
given as: (4.20)
This formula can now be used to calculate the maximum and minimum tension difference which will occur in the two sides of the Prony brake. In order to calculate the differences in tension in the Prony brake, the radius of the pulley must be assumed to start with. The maximum torque output from the turbine must also be approximated. The increments in which the applied torque can be adjusted also must be approximated, because until the spring stiffness and the physical set-up for the Prony brake is determined, there is no way of knowing the torque increment which will be achievable. After investigating the previous testing which was carried out on this particular wind turbine, and other papers in which wind tunnel testing was carried out on small scale vertical axis wind turbines, it was decided that a maximum torque output of 0.1Nm would be taken as the expected maximum torque value. A pulley radius of 40mm is used as the starting point for the calculation. The effect that changing the pulley radius has on the system will also be investigated. Scenario 1: Maximum Torque of 0.1Nm and pulley radius of 40mm (4.21) (4.22)
Scenario 2: Maximum Torque of 0.1Nm and pulley radius of 20mm
47
(4.23) (4.24)
In order to calculate the amount of data points which can be recorded using the Prony brake, it is important to know how a particular increment of Torque will appear on the spring balances. It is assumed firstly that the tensioning mechanism allows for ten Torque increments between zero and 0.1Nm. Thus the brake will be able to apply Torque in increments of 0.01Nm. The difference in tension which will result from this increment will now be calculated. Scenario 3: A torque of 0.01Nm is applied by the brake, with a pulley radius of 40mm (4.25) (4.26)
Scenario 4: A torque of 0.01Nm is applied by the brake, with a pulley radius of 20mm (4.27) (4.28)
It was concluded from Scenario 3 and Scenario 4 that when the radius of the pulley is decreased, it takes a larger difference in tension across the brake to apply the same torque that was achieved with the bigger pulley. 4.4.5 Prony Brake Design/Manufacture The main components in a Prony brake are the spring balances which measure the tension in the two sides of the belt or rope, and the base which supports the springs. The spring balances chosen must provide adequate increments of torque in order to gradually increase or decrease the angular velocity of the rotor. Choosing the correct size springs is crucial to the success of the experimental set up. Springs are characterised by a spring constant, measured in Newtons per metre (N/m). The spring constant denotes how many Newtons of force are required to displace the spring by one metre.
48
Due to the fact that the maximum torque output for the VAWT is unknown, the spring size was approximated in order to find out from initial testing, the approximate value for the maximum power output. It was decided that after initial testing a spring could be selected which would be closer to the required range of applied torque. The springs purchased for the initial Prony brake, were Salter springs with a range of 0-10N in increments of 0.1N.
Figure 33 (Prony Brake Design) Figure 33 above shows the final SolidWorks model of the Prony brake mechanism designed for use on the VAWT rig. The design process began by making SolidWorks models of the Salter springs specified for the brake. Conventional Prony brakes usually measure the torque of a shaft rotating about a horizontal axis. Due to the orientation of the axis of rotation of the VAWT, it was decided that tension had to be applied in the horizontal axis as shown in Figure 33 above. The frictional forces between the spring balances and the metal base plate were not an issue as the same frictional force is applied to both of the spring balances. A mechanism was designed to allow the tension in the rope or belt to be increased at small increments. The mechanism, shown in Figure 34 below, uses threaded bars to increase the tension in each side of the belt or rope. The nut at position 2 was welded to the angle iron, which held the 49
treaded bar rigid and only allowed it to move in an axial direction when turned. Lock nuts were used at positions 3 and 4, to loosely hold the small piece of angle iron in place. Because the lock nuts were not fixed to the angle iron, as the threaded bar rotates the angle iron will slide along the base plate which in turn tensions the springs. Finally two nuts were locked together at position 1 to make it easier for the operator to turn the threaded bar.
Figure 34 (Tensioning Mechanism) The next step in the design was to attach the spring balances to the small angle iron parts. Figure 35 below shows the method used to connect the springs to the angle iron. The small clips shown are conventionally used for securing grease pipes onto machinery however they were adapted for use in the Prony brake design to save on fabrication time. The clips were fastened onto the small angle iron parts using m6 bolts to hold them in place.
50
Figure 35 (Spring Attachment) The Prony brake assembly was designed so that it could be easily attached to the test rig, and removed to accommodate other methods of applying torque. Figure 36 below shows the mounting bracket which was designed to attach the Prony brake to the test rig. The mounting bracket was designed to be easily detachable in order to accommodate any alternative braking mechanisms which may be used with the test rig in future.
Figure 36 (Prony Brake Mounting) 51
Figure 37 below shows the fabricated Prony brake before any surface finishing was applied to the steel components.
Figure 37 (Prony Brake Assembly)
4.4.6 Proposed Testing Methodology The following basic testing methodology was devised for the Prony Brake. 1. Allow the VAWT to spin with no load applied initially, at a selected wind speed. 2. Gradually apply an opposing torque to the rotor by turning one of the threaded bars on the prony brake anti-clockwise. 3. When the rotor begins to stall, take note of the difference in tension between the two sides of the brake. 4.
Use this force difference to calculate the maximum torque output for the rotor.
5. Divide the maximum torque by the number of data points required (Fifteen for example). This gives the increment at which the torque should be increased. 6. As in step 1 allow the VAWT to spin without a load and gradually apply the torque in increments as calculated in step 5, recording the corresponding angular velocity for each torque increment.
52
4.5 Magnetic Particle Brake 4.5.1 Principle of operation The magnetic particle brake (MPB) shown in Figure 38, was proposed as a method of applying torque to the VAWT. The magnetic particle brake uses the principle of magnetism to apply a torque to its output shaft. The brake does this in a unique way, with no direct contact between the housing and the disc in the centre of the brake. The central cavity in the brake is filled with fine, dry, stainless steel powder. Applying a magnetic field to the powder using the coil in the brake causes the particles to form chains which link the disk to the housing of the brake. Increasing the DC current applied to the coil increases the magnetic field thus increasing the torque applied to the shaft. Using a fine adjustment of current, the torque can be adjusted to suitable increments for the desired application.
Figure 38 (Magnetic Particle Brake) The brake shown in the figure above has an output shaft on both sides of the housing but other models are available with a shaft on one side only. For the VAWT it was decided that only one shaft output was needed.
53
4.5.2 Brake sizing An MPB must be correctly sized so that it matches the expected output torque range of the VAWT. After looking at previous tests carried out on the larger 0.568m diameter rotor, it was concluded that the maximum torque output could be as low as 1Nm. In order to get a power curve for the turbine, there would need to be enough torque increments within the range 0-1Nm. The graph in Figure 39 shows a plot of torque (N.cm) versus input current (% of rated input current). Each MPB is supplied with one of these plots so that the output torque for the brake can be related to the input current for the different size brakes available.
Figure 39 (Torque Vs Current) 4.5.3 Power Supply While operating the MPB for a long period of time the temperature of the brake will increase, causing the temperature of the coil and hence its resistance to increase. This increase in the coil resistance will cause the current drawn from the power source to decrease, resulting in unstable torque output values. For this reason Huco offer a specific constant current power supply, shown in Figure 40 to power their range of MPB’s.
54
The current produced by the power supply is adjusted using a potentiometer on the front face of the unit. A digital read-out showing the value of the current makes it easier to apply precise currents to the MPB.
Figure 40 (Power Supply) 4.5.4 MPB Mounting It was decided that the magnetic particle brake would be coupled to the end of the VAWT’s central shaft at the top of the test rig. A bracket would be designed to hold the casing of the MPB allowing only the shaft to spin. As with the Prony brake, the MPB would be mounted in such a way that it could be easily removed from the test rig.
Figure 41 (MPB Setup) The first step in mounting the MPB onto the end of the central shaft was to design a coupling which would connect the output shaft of the MPB to the central shaft of the VAWT. The coupling shown in Figure 42 below was designed to be attached to both shafts using grub screws 55
which would tighten onto flat spots on each of the shafts. The coupling was made from nylon in order to keep the inertia to a minimum.
Figure 42 (MPB Coupling) The bracket shown in Figure 43 below was designed to hold the MPB in place while a braking torque is being applied to the VAWT. The bracket was designed to match up with the mounting holes on the MPB put in place by the manufacturer. In order to make the alignment and position of the brake adjustable, a long slot was cut into the bracket as shown.
Figure 43 (MPB Bracket) Figure 44 below shows how a length of threaded bar was used to hold the bracket in place. Using the threaded bar meant that the height of the brake could be easily adjusted to accommodate 56
different shafts and couplings on the test rig. The figure also shows how the bracket is easily adjusted using the slot to vary the alignment of the MPB with the central shaft of the VAWT.
Figure 44 (MPB Assembly) Figure 45 below shows the fabricated MPB assembly complete with the threaded bar for support. Additional M5 nuts were used to lock the bracket to the MPB housing.
Figure 45 (Fabricated MPB Assembly)
57
4.5.5 MPB Calibration Before any testing could be carried out with the MPB, there was a calibration procedure to be carried out. Each magnetic particle brake has a specified rated current which must be set as the maximum current which can be put out by the constant current power supply. The MPB was connected to the power supply. Figure 46 below shows the power supply being calibrated. With the % Rated Current potentiometer set to 100%, the span max potentiometer was adjusted until the digital readout matched the rated current for the chosen magnetic particle brake which was 190mA. The % rated current potentiometer was then set to 10% and the span minimum potentiometer was adjusted until the digital readout showed 10% of the rated current value (19mA).
Figure 46 (MPB Calibration) After the brake had been calibrated, an extension lead had to be made so that the power supply could be placed a distance away from the test rig during operation for safety reasons. Figure 47 below shows the extension lead being connected to the power supply and the magnetic particle brake. The lead was made so that it could be easily removed and used with other MPB’s.
58
Figure 47 (MPB Extension Lead)
Finally, Figure 48 below shows the magnetic particle brake connected to the power supply using the extension lead.
Figure 48 (MPB and Power Supply)
4.6 Measuring RPM Both of the investigated load application methods require an additional rpm measurement in order to calculate the power output from the VAWT rotor. 59
It was decided that a laser tachometer would provide the simplest and most cost effective method of measuring the rpm of the rotor at each torque increment. The laser tachometer shown in Figure 49, can be used to easily measure the rpm of any rotating shaft by placing a small strip of reflective tape onto the shaft and pointing the laser at the strip as the shaft rotates. The rpm is displayed on the digital readout of the handheld laser tachometer. To increase the accuracy of the rpm measurement, multiple strips of reflective tape should be evenly spaced around the circumference of the shaft, and the rpm value obtained should be divided by the number of strips present. Due to the small diameter of the VAWT’s central shaft, a light pulley or disc may need to be fitted onto the shaft to allow the reflective tape to be applied correctly. The Ideal system would use a control system for measuring rpm of the rotor, and record it into a spread sheet however this requires expensive rpm sensors to be purchased. It was decided that once the basic testing methodology had been refined, it would be a good idea to look into the computerised method of logging the rpm value at various points.
Figure 49 (Laser Tachometer)
4.7 Frame Design & Fabrication A suitable frame was designed to mount the VAWT to the end of the wind tunnel in DIT Bolton Street. The design began with the basic frame structure shown in Figure 50 below. 50mm angle iron was used for the frame to ensure that it was rigid and to reduce any vibrations that could occur during operation. 60
Figure 50 (Test Rig Frame) In order to attach the frame rigidly to the end of the wind tunnel, a suitable clamping mechanism was designed. The clamping mechanism shown in Figure 51 was designed so that the frame could be quickly and easily removed from the wind tunnel to save on valuable testing time and to accommodate other students using the testing facility.
Figure 51 (Frame Clamps) The frame was designed in such a way that the rotor and central shaft can be easily removed from the test rig, making it easy to carry out any necessary modifications to the rotor. A notch was cut out of the top of the frame to allow the rotor and central shaft to be removed easily as 61
shown in Figure 52 below. Appropriate holes were included in the frame for mounting the bearings and to allow the brakes to be attached.
Figure 52 (Frame Top Features) Suitable bearings had to be selected for the VAWT. Due to the small scale of the turbine, and the expected magnitude of the maximum torque, the losses in the bearings had to be kept to a minimum. The seals were removed from the bearings in order to reduce the friction drag. Figure 53 below shows the test rig assembled with the braking mechanisms attached. The brakes were securely attached using threaded fasteners, ensuring that there was no vibration in during operation.
62
Figure 53 (Frame and Brake Assembly) Figure 54 below shows the test rig in place at the end of the wind with the VAWT rotor also attached. The design and fabrication of the VAWT test rig was considered complete at this point.
Figure 54 (Test Rig in Place)
4.8 Summary The dimensional analysis carried out showed that the power coefficient varies with the tip speed ratio of a VAWT. Using this information a suitable methodology was devised in which the tip speed ratio would be varied by applying a known torque to the VAWT rotor. By measuring the 63
rpm of the rotor at different values of applied torque, the performance curves for the VAWT could be obtained. An investigation was carried out into methods of applying a braking torque to the VAWT. The Prony brake and the magnetic particle brake (MPB) were selected as appropriate braking mechanisms. A Prony brake was designed and fabricated to be used on the test rig. A suitable MPB was purchased and the necessary components were designed to allow the brake to be used on the test rig. A VAWT test rig was designed around the braking mechanisms and the wind tunnel in DIT Bolton Street. The test rig was designed to meet several requirements which were specified using the objective tree design method. The test rig was fabricated and the brakes and VAWT rotor were attached making the test rig ready for wind tunnel testing.
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Chapter 5: Wind Tunnel Testing 5.1 Introduction The following chapter documents the approach taken and the challenges faced during the wind tunnel testing of the fabricated VAWT. Firstly the operation of the wind tunnel used for testing will be discussed, outlining the main controls and the capabilities of the tunnel itself. The Prony brake set up will then be briefly discussed, showing how the brake was set up and describing in detail the procedure used for the Prony brake testing. The first phase of Prony brake testing will be discussed in detail with reference to the performance curves obtained and the general operation of the turbine and test rig. The modifications carried out on the test rig after phase 1 will be discussed followed by the performance results obtained in the phase 2 testing. The mechanical losses in the system will be discussed in detail including several calculations which were carried out in order to quantify the magnitude of the losses due to friction in the bearings. These mechanical losses will then be added to the torque values recorded in the prony brake phase 2 testing. The procedure for the magnetic particle brake testing will be discussed along with the findings of the magnetic particle brake testing.
5.2 Wind Tunnel Set-up The wind tunnel in DIT Bolton Street is shown in Figure 55 below. The motor shown powers a fan which provides wind at a maximum velocity of approximately 24 m/s. The test section has a 500mm square perimeter and is fully transparent to allow for observation during testing.
65
Figure 55 (Wind Tunnel DIT Bolton Street) The wind tunnel controls are shown in Figure 56 below. The control box on the left contains the stop and start buttons along with the isolator switch which is padlocked when the wind tunnel is not in use. The mechanism on the right is used to adjust the speed of the wind produced by the fan. By turning the mechanism a baffle is applied to the fan changing the velocity of the air exiting the fan. An anemometer is placed in the test section of the wind tunnel during testing in order to measure the wind speed.
Figure 56 (Wind Tunnel Controls) 66
5.3 Prony Brake Set-up Figure 57 below shows the Prony brake set up at the wind tunnel in DIT Bolton Street. Before the Prony brake testing commenced, the wind tunnel was run several times to get an understanding of how the VAWT would react to various wind speeds. Once the test rig was considered to be set up in a satisfactory manner, the Prony brake testing of the VAWT commenced.
Figure 57 (Prony Brake Testing)
The following testing methodology was used for the Prony brake tests 1. The VAWT test rig was attached to the end of the wind tunnel, using the clamps on each side of the frame. 2. The height was adjusted so that it was in the centre of the wind tunnel. 3. All fasteners on the rig were checked and tightened. 4. The spring balances were adjusted so that there was no load on the central shaft of the turbine. 5. The wind tunnel was switched on, and wind speed was measured using an anemometer. 6. The wind speed was adjusted to a desired value. 7. The unloaded rpm of the central shaft was measured using the tachometer. 67
8. A load was applied to the central shaft by tensioning the spring balances until the rotor had almost stopped. This was noted as the maximum torque for the particular wind speed. 9. The tension in the spring balances was released allowing the rotor to reach its maximum velocity. 10. A suitable torque increment was decided upon by dividing the maximum torque by the number of data points required. 11. The torque was applied to the central shaft in increments by tensioning the spring balances, and the rpm of the rotor was recorded at each increment using the tachometer. 12. When the maximum torque was reached, the rotor was unloaded and the wind speed of was adjusted to a different value. 13. Steps 7 to 12 were repeated for different wind speeds.
5.4 Prony Brake testing phase 1 The first phase of Prony brake testing was carried out successfully, providing multiple performance curves for the VAWT. For the phase 1 testing the seals had been removed from the bearings to reduce the friction drag. Testing was carried out at multiple wind speeds to provide as much data as possible for the rotor. Figure 58 below shows a plot of the Power versus rpm for the VAWT at different wind speeds. As expected, the maximum power output for the turbine decreased as the wind speed decreased. The curves generated from the testing follow the expected from, with the power varying as the rpm of the VAWT changes. From the plot in Figure 58 it can be seen that at a wind speed of 22.6 m/s the turbine has a maximum power output of approximately 0.26 Watts. This maximum power occurred when the turbine was rotating at approximately 300 rpm.
68
Power vs rpm 0.3
0.25 22.6 m/s
Power (w)
0.2
19.5 m/s 15.3 m/s
0.15
13 m/s
0.1
0.05
0 0
100
200
300
400
500
600
700
n (rpm) Figure 58 (Prony Brake Phase 1 Power Curves) The performance curves form the Prony brake tests were converted into dimensionless form and hence the coefficient of power was plotted against the tip speed ratio for each of the wind speeds. Figure 59 below shows the dimensionless power curves for the phase 1 Prony brake tests. At a wind speed of 22.6 m/s a maximum power coefficient of 0.0013 was achieved at a tip speed ratio of 0.15. The dimensionless power curves collapsed onto each other at low values of tip speed ratio when compared with the non-dimensionless power curves above.
69
Cp vs. λ 0.0014 0.0012 22.6 m/s
0.001
19.5 m/s
Cp
0.0008 15.3 m/s 0.0006
13 m/s
0.0004 0.0002 0 0
0.05
0.1
0.15
0.2
0.25
0.3
λ Figure 59 (Prony Brake Phase 1 Dimensionless Power Curves) A repeatability test was carried out to see if the VAWT testing was consistent. The Prony brake test procedure was carried out at wind speeds of 22.7, 22.2 and 22.1 m/s. Figure 61 below shows the curves which were obtained from the repeatability tests. All three curves follow the same trend and have very similar values of maximum power. The repeatability test shows that the experimental procedure is consistent and this means that it is appropriate to compare experimental values taken from different tests.
70
Power vs. RPM
0.3 0.25
22.7 m/s
Power (W)
0.2
22.2 m/s 22.1 m/s
0.15 0.1 0.05 0 0
100
200
300 n (rpm)
400
500
600
Figure 60 (Prony Brake Phase 1 Repeatability Test) Figure 61 below shows a plot of torque versus rpm for the phase 1 Prony brake tests. The Torque values form the repeatability tests discussed above are also plotted. At a wind speed of approximately 22m/s, a maximum torque of 0.011Nm was obtained at around 100rpm. This torque is considerably smaller than the 1Nm of torque which was expected from the VAWT.
Torque vs. Rpm
0.014
22.7 m/s
0.012
22.2 m/s
Torque (Nm)
0.01
22.1 m/s
0.008
19.5 m/s
0.006
15.3 m/s
0.004 0.002 0 0
100
200
300
400
500
n (rpm)
Figure 61 (Prony Brake Phase 1 Torque Measurements) 71
600
5.5 Prony Brake testing Phase 2 After the phase 1 Prony brake testing was completed, an investigation was carried out into reducing the losses in the VAWT test rig. These losses included any frictional losses in the bearings and any losses due to misalignment in the test rig. Before the phase 1 tests were carried out, the seals were removed from the bearings which made a significant difference to the amount of friction drag in the bearings. For the phase 2 testing the bearings were cleaned out using compressed air to remove any of the grease from the bearings. Although removing the seals and the grease from the bearings would shorten their lifespan considerably in industrial applications, the test rig would not be running for long periods of time. When the test rig was inspected after the phase 1 testing, it was found that there was a small buckle in the central shaft. The central shaft was replaced with a new shaft and the test rig was once again set up at the end of the wind tunnel in DIT Bolton Street.
Power vs. Rpm 0.6 0.5 22.1 m/s Power (W)
0.4
22 m/s (Free Bearings)
0.3 0.2 0.1 0 0
200
400
600 800 n (rpm)
1000
Figure 62 (Prony Brake Phase 2 Power)
72
1200
1400
The Prony brake test procedure was carried out for the modified test rig at a wind speed of 22m/s. Figure 62 above shows a comparison of the power curves for phase 1 and phase 2 testing. Both tests were carried out at approximately 22m/s. By cleaning out the bearings and replacing the central shaft, the maximum power produced by the turbine more than doubled from 0.24 Watts to 0.55 Watts. This shows that the losses in the system were having a significant effect on the performance of the turbine. Similarly it can be seen in Figure 63 below that the torque produced by the rotor was significantly increased by modifying the test rig to reduce losses.
Torque vs. RPM
0.02 0.018
22.7 m/s 0.016 22.1 m/s
Torque (Nm)
0.014
22.5 (Free Bearings)
0.012
22 m/s (Free Bearings)
0.01 0.008 0.006 0.004 0.002 0 0
200
400
600 800 n (rpm)
1000
1200
1400
Figure 63 (Prony Brake Phase 2 Torque) The Prony brake test procedure was repeated for a number of wind speeds. During the phase 2 testing, it was discovered that the rpm began to fluctuate more than in phase 1 which made the rpm readings more difficult to record. This increase in fluctuation could be due to the fact that with the friction in the bearings reduced, the turbine accelerates and decelerates more freely within a single revolution of the turbine. There is also a pulsating torque in the VAWT which causes this acceleration and deceleration of the turbine. Figure 64 below shows the dimensionless power curves for the different wind speeds recorded during phase 2 of the Prony brake testing. The plot shows a significant increase in the maximum 73
power coefficient compared to the phase 2 tests. At 22m/s the power coefficient almost reaches 0.003, compared to 0.0013 in the phase 1 tests. In the phase 2 testing the turbine also reached a maximum tip speed ratio of 0.56 compared to 0.277 in the phase 1 testing.
Cp vs. λ
0.0035 0.003 0.0025
22.5 m/s 0.002
Cp
20.3 m/s 22 m/s
0.0015 0.001 0.0005 0 0
0.1
0.2
0.3
0.4
0.5
0.6
λ Figure 64 (Prony Brake Phase 2 Dimensionless Power Curves)
5.6 Mechanical Losses The mechanical losses in the system, such as the friction losses in the bearings and losses caused by misalignment, have a significant effect on the overall performance of the VAWT, because of the small torques which the VAWT can produce. An investigation was carried out into calculating the magnitude of the mechanical losses so that any torque which was lost in the bearings etc. could be estimated and added on to the recorded values for torque, giving a more accurate representation of the performance of the VAWT. 5.6.1 Theory The driving torque couple
accelerating a shaft carrying a rotor with inertia
plus the frictional torque
as shown in the equation below
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is equal to the inertia
(5.1)
If the same shaft and rotor is being brought to rest by a braking torque
the frictional torque is
now assisting the braking toque and the equation can be written as (5.2)
If the braking torque is equal to zero and the frictional torque is the only thing slowing the rotor down the above equation then becomes (5.3)
This means that if the angular deceleration and the inertia of the shaft and rotor are known, then the torque due to friction for the system can be calculated. This theory shows that if the VAWT rotor was replaced by an object with an easily calculated inertia value, and the deceleration of the rotor is measured, the torque due to friction in the system can then be calculated. This formulation assumes that the rotor has a constant deceleration, but in the case of the VAWT rig, the deceleration will not be constant because the friction losses in the bearings change with rotational speed. It was decided that the frictional torque could be measured by approximating the deceleration over small ranges of rpm, giving a reasonably accurate measure of the systems overall deceleration. A simple experiment was designed to measure the frictional torque in the system at different rpm values. The details of the experiment will now be discussed. 5.6.2 Procedure A simple mechanical experiment was devised to approximate the torque due to friction acting on the VAWT. Firstly the rotor was removed from the VAWT and it was replaced by a bar with a known moment of inertia. The test rig was removed from the wind tunnel and the following test procedure was carried out. 1. Using a battery powered drill, the VAWT central shaft was accelerated and the rpm was measured using a laser tachometer. 2. Once the desired rpm value was reached, the drill was removed and a timer was started. 3. The rotor was allowed to decelerate by a set rpm value. 75
4. When the rotor decelerated to the desired rpm value, the timer was stopped and the time was recorded. 5. Steps 1 to 4 were repeated for different rpm ranges and the relevant data was recorded.
Figure 65 (Spin Down Test) 5.6.3 Sample calculation Table 4 below shows the data recorded during the spin down tests to calculate the friction losses in the bearings. The rpm at which the bar could be rotated was limited by the speed of the battery drill. The experiment was repeated five times for each rpm range and an average value for the deceleration time was calculated. The inertia for the bar used in the experiment was calculated to be as follows
(5.4)
Where M is the mass of the bar, R is the radius of the bar and L is the length of the bar. Subbing in the values the equation becomes
(5.5)
(5.6)
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Upper rpm 500 400 300 200
Lower rpm 400 300 200 100
Average Time (s) 4.676 4.97 6.602 9.144
Table 4 (Recorded data) A sample calculation was carried out for the first row of the table above. The change in angular velocity was calculated as follows (5.7) (5.8) (5.9)
The angular deceleration was calculated as follows (5.10) (5.11)
The frictional torque applied by the bearings was calculated as follows (5.12) (5.13) (5.14)
The results from the spin-down tests are shown in Figure 66 below. The graph shows that the torque due to friction in the bearings increases as the rpm increases.
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Torque (Nm)
Torque vs. RPM 0.16 0.15 0.14 0.13 0.12 0.11 0.1 0.09 0.08 0.07
Spin Down Test
150
200
250
300
350
400
450
500
rpm
Figure 66 (Spin Down Test) 5.6.4 Drag Calculation After the spin-down tests were completed it was decided that there could be an opposing torque in the system due to drag force acting on the rotating bar. Using the theory of drag forces in flow past immersed bodies, the drag on the rotating bar was calculated as follows. The total torque due to drag acting on the bar as it rotates mounted about its centre is given as ∫
(5.15)
Where R is the maximum radius of the bar and dF is the force acting on a small element of the bar at a radius r. Subbing in for the force the equation becomes ∫
Where D is the diameter of the bar, is the angular velocity of the bar and
[
]
is the drag coefficient for the bar,
(5.16)
is the density of air,
is a small element of the radius.
Integrating the above equation gives the total drag torque being applied to the bar as it decelerates which is given by 78
(5.17)
At different rpm values there is a different drag force acting on the rotating bar. Average rpm values were taken to coincide with the values in the spin down tests as shown in Table 5 below. Average rpm 450 350 250 150
Radians 47.12 36.65 26.18 15.71
Table 5 (Average Rpm Values) Subbing in the dimensions of the bar, the drag coefficient for a cylindrical bar and the angular velocity from the first row of the above table the equation becomes (5.18) (5.19)
This value for torque can be subtracted from the torque calculated in the spin down tests to give the actual frictional torque in the bearings. At 450 rpm the actual bearing friction torque is given as follows (5.20) (5.21) (5.22) (5.23)
This calculation was repeated for the other values of rpm and the results were plotted on the graph shown in Figure 67 below.
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Bearing Friction Torque vs. RPM 0.14
Friction Torque Nm
0.12 0.1 0.08 0.06 0.04 0.02 0 150
200
250
300 350 n (rpm)
400
450
500
Figure 67 (Friction Torque in Bearings) The torque required to overcome the friction in the bearings proved to be much larger than the maximum torque recorded in the Prony brake tests. Figure 68 below shows the difference between the frictional losses and the useful torque produced by the VAWT.
Torque vs. RPM 0.14
Torque (Nm)
0.12 0.1 22.5 (Free Bearings)
0.08
Bearing Losses
0.06 0.04 0.02 0 0
100
200
300
400
500
n (rpm)
Figure 68 (Torque Losses and Useful Torque)
80
600
An equation was generated to represent the curve in Figure 67 above. The equation was then used to calculate the frictional losses at specific rpm values from the phase 2 Prony brake tests. The vales for the losses due to friction were then added onto the corresponding value for output torque from the VAWT. Figure 69 below shows a plot of the total dimensionless power curve for a wind speed of 22 m/s. This curve is a combination of the data recorded in the phase 2 Prony brake tests and the bearing losses calculated. The power curve is an ideal power curve as the torque which is lost due to friction in the bearings is not useful torque, but it still has to be overcome by the VAWT. With the calculated losses included the VAWT reaches a maximum power coefficient of 0.09 at a tip speed ratio of 0.25. This is a significant increase on the previously calculated maximum power coefficient.
Cp vs λ
0.1 0.09 0.08 0.07
Total Dimensionless Power Curve 22m/s
Cp
0.06 0.05 0.04 0.03 0.02 0.01 0 0
0.1
0.2
0.3
0.4
λ Figure 69 (Total power curve 22m/s)
81
0.5
0.6
5.7 Magnetic Particle Brake testing The next step in the VAWT testing was to carry out the magnetic particle brake (MPB) tests. The magnetic particle brake was attached to the test rig which was set up as shown in Figure 70 below. The power supply was calibrated as discussed previously and was connected to the MPB.
Figure 70 (MPB Testing) The following test procedure was used for the MPB testing 1. Connect the VAWT test rig to the end of the wind tunnel. 2. Connect and calibrate the constant current power supply. 3. Set the % rated current potentiometer to zero to apply the minimum torque to the VAWT central shaft. 4. Start the wind tunnel and adjust the wind speed to the desired value using an anemometer. 5. Increase the % rated current potentiometer until the turbine is just about to stall and record the value of current from the digital readout at this point. 6.
Divide the current at which stall occurred by the number of data points desired for the test to get the increments at which the current will be increased.
7. Reset the % rated current potentiometer to zero, and apply the current in increments, recording the rpm at each increment using a laser tachometer. 82
Wind tunnel testing could not be carried out using the MPB because the losses due to friction in the MPB were greater than the maximum torque produced by the VAWT at the maximum wind speed possible. The manufacturers of the MPB quoted the minimum torque produced by the brake to be 0.02 Nm which is greater than the maximum torque recorded in phase 2 of the Prony brake testing. Figure 71 below shows the minimum value of current being applied to the MPB with no rotation occurring in the VAWT. There are several solutions to the problem including gearing the MPB instead of connecting it directly to the central shaft of the turbine. Solutions will be discussed in detail in the recommendations section of this report.
Figure 71 (MPB Minimum current)
5.8 Summary The wind tunnel test facility in DIT Bolton Street was used for the VAWT wind tunnel testing. Before any testing took place a good understanding of the facility and the equipment available was obtained. The VAWT test rig was attached to the end of the wind tunnel and an anemometer was put in place to measure the wind speed during testing. The Prony brake apparatus was attached to the test rig and preliminary wind tunnel tests were carried out to gain experience with the operation of the test rig and the turbine. Phase 1 of the Prony brake testing began by running performance tests at several wind speeds. The phase 1 testing resulted in a maximum recorded power coefficient of 0.0013 being recorded. 83
After these tests several modifications were made to the test rig to reduce the mechanical losses in the system. Phase 2 of the Prony brake testing was carried out on the modified test rig with reduced friction in the bearings and a new central shaft. There was a significant increase in the maximum torque produced by the VAWT, giving a maximum power coefficient of almost 0.003. As with the phase 1 testing, the VAWT performance was tested at several wind speeds. Due to the significant impact of the test rig modifications on performance, analysis was carried out into quantifying the magnitude of the frictional losses in the bearings. An experiment was designed to approximate the opposing frictional torque generated by the bearings during operation. The data from the experiment was combined with the performance data from the phase 2 Prony brake tests to give the total power curve for the VAWT. This idealised power curve reached a maximum power coefficient of almost 0.1 at a wind speed of 22m/s. An experimental procedure was devised for the MPB testing of the VAWT. Power curves could not be generated from the MPB testing because the maximum torque which the VAWT could produce was lower than the minimum value of braking torque applied by the MPB. The MPB test however could be carried out on the VAWT if the rotor is redesigned for higher torque output.
84
Chapter 6: Conclusion/ Recommendation 6.1 Conclusion The aim of this project was to carry out performance tests on an innovative vertical axis wind turbine (VAWT) designed by Brí Toinne Teoranta. Overall the project was a great success and all of the original objectives set out were achieved. The objectives set out at the start of the project were as follows 1. Carry out research into the area of wind energy and develop an understanding for the fundamentals of wind power generation. 2. Manufacture the turbine blades designed by Brí Toinne Teoranta to a high standard. 3. Design a testing methodology to obtain performance curves for the turbine. 4. Design and fabricate a suitable test rig for the wind tunnel testing. 5. Carry out wind tunnel testing on the VAWT. 6. Analyse the results obtained from the wind tunnel testing. 7. Present the findings from testing in report form. Before work commenced on the project, a good understanding of the importance of sustainable energy was obtained. It was clear from this research that in order to meet the global energy demand, sustainable energy sources such as wind must be utilised. It was also clear that there was a need to conduct more research into the area of vertical axis wind turbine technology. Vertical axis wind turbines are more favourable than horizontal axis turbines in many applications, making any research and development carried out in the area both relevant and beneficial. This project offered a good opportunity to gain experience in the area of turbine blade manufacture. The 3D CAD files from the designer had to be interpreted and understood so that the correct approach to manufacture could be taken. Several small design modifications had to be made to the turbine blades designed by the manufacturer and components had to be designed to mount the blades to the central shaft of the turbine. Experience was also gained in the area of
85
prototyping components using different manufacturing techniques before deciding upon a final solution. The area of VAWT performance testing was investigated which provided several methodologies for assessing a turbine’s performance. Several selection criteria were used to decide upon the appropriate testing methodology for the test facility available. A more detailed investigation was carried out into the selected testing methodology. A VAWT test rig was designed which incorporated the selected testing apparatus. The test rig design was optimised to ensure that it was both safe and easy to operate. The test rig was designed to work specifically with the available wind tunnel test facility. Great experience was gained in the area of fabrication as the test rig had to be completely fabricated and finished to a high standard. Wind tunnel testing of the VAWT was successfully carried out which yielded excellent results using simple test procedures. The performance curves for the VAWT were successfully obtained for several different wind speeds. The wind tunnel tests were carefully documented so that the testing could be assessed correctly. An understanding of the challenges associated with wind tunnel testing was gained along with valuable experience with troubleshooting any issues as quickly as possible. The results obtained from the VAWT wind tunnel testing were analysed in depth, presenting the data in the form of dimensionless performance curves which allow for comparison between different turbines. Excellent experience was gained in dealing with large amounts of data from various wind tunnel tests and filing the recorded data correctly. Overall this was a very successful project from start to finish. It provided the opportunity to gain experience in a broad range of areas of the engineering profession. The project also made a strong contribution to the wind turbine testing facilities present in DIT Bolton Street, providing valuable experience to everyone involved with the project.
86
6.2 Recommendations This project provides a good foundation for testing to be carried out on different VAWT designs using the methodology devised. This would provide a good opportunity to directly compare the performance of different turbine designs and would also make it possible to begin to optimise the aerodynamic design of turbine blades in future projects. The test rig designed for this project can easily accommodate different VAWT designs and the testing apparatus can be used on future testing projects. Although all of the objectives which were set out for this project were completed successfully, there are certain areas of the VAWT testing methodology which could be refined further in order to improve the testing methodology. These areas include
Minimising mechanical losses
Gearing magnetic particle brake
Refining Prony brake design
The friction losses in the bearings could be minimised by using air bearings or other alternatives which have the lowest possible drag. The magnetic particle brake could be mechanically geared in order to allow it to be used to test turbines which produce low values of torque. The Prony brake mechanism could be refined by using an apparatus which uses an arrangement of strain gauges to measure the torque applied to the VAWT. A method of recording instantaneous values of rpm could be used in conjunction with the refined Prony brake to automate the recording of data during the testing. In an ideal situation the VAWT testing would be carried out in a larger wind tunnel. A larger wind tunnel would provide the opportunity to manufacture a larger version of the designed turbine. The larger turbine would produce larger torques which would be easier to measure and would also reduce the effects of the mechanical losses on the turbines performance. The existing wind tunnel however would be adequate for conducting comparisons between different VAWT designs. Overall this project has provided a good insight into the challenges associated with VAWT testing and it has also created opportunity for future work in the area of wind turbine performance testing. 87
Bibliography [1]
W. E. Council, "2010 Survey of Energy Resources Executive Summary," 2010.
[2]
A. V. Da Rosa, Fundamentals of renewable energy processes. Amsterdam ; Burlington,
MA: Academic, 2005. [3]
SEAI.
(31/10/11).
Wind
Energy.
Available:
http://www.seai.ie/Renewables/Wind_Energy/ [4]
IEA, "IEA Annual report 2009," www.seai.ie2009.
[5]
B. Blank, Swezey, "A Certificate-Based Approach to marketing Green Power and
Constructing New Wind Energy Facilities," Wind Engineering, vol. 26, pp. 63-70, 2002. [6]
United Nations., New Sources of Energy
[7]
S. Eriksson, H. Bernhoff, and M. Leijon, "Evaluation of Different turbine concepts for
wind power," Renewable and Sustainable Energy Reviews, vol. 12, pp. 1419-1434, 2008. [8]
J. J. Bertin and M. L. Smith, Aerodynamics for engineers, 2nd ed. Englewood Cliffs,
N.J.: Prentice Hall, 1989. [9]
T. S. B. Shikha, D.P .Kothari, "Early Development Of modern Vertical Axis Wind
Turbines: A Review," Wind Engineering, vol. 29, p. 288, 2005. [10]
W. Gramlich, "Savonius rotor,"
vol. 2011, ed. Internet, 2011, p. Savonius rotor
schematic. [11]
A. Gorlov, "Development of the helical reaction hydraulic turbine," Northeastern
University Boston, MA1998. [12]
q. ltd. (2010, 31/10/11). Quietrevolution Website.
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Appendix A: Rapid Prototyping Screenshots and Code
89
90
Catalyst Code session {12/2 12:51:00pm > sean.keane opened job C:/CatalystV36/jobs/sean3983/Default.sjb. Catalyst 3.6.1 (2160).} msg {12:51:01pm > Read 6300 facets in 0:00 from "C:/Documents Settings/sean.keane/Desktop/robert/Scaled Blade for Rapid Prototype.STL"} msg {12:51:01pm > STL comment: solid Scaled Blade for Rapid Prototype} msg {12:51:01pm > Bounding box of this STL X, Y, Z:} msg {12:51:01pm >
min = (50.1989, 51.5281, 105.0166)}
msg {12:51:01pm >
max = (148.2176, 248.4719, 179.2500)}
msg {12:51:01pm > Done loading STL file after 1 sec.} msg {12:51:13pm > Slice height: 0.2540} msg {12:51:14pm > Curves too small to build will be removed} msg {12:51:14pm > Done slicing.} msg {12:51:14pm > Supports removed} 91
and
msg {12:51:14pm > Support style: Minimal} msg {12:51:14pm > Inspecting part curves} msg {12:51:14pm > Generating supports} msg {12:51:25pm > Done generating supports after 11 sec.} msg {12:51:27pm > Done writing boundary curves after 2 sec.} msg {12:51:27pm > Saved job: C:/Documents and Settings/sean.keane/Desktop/robert/ssys_scaled blade for rapid prototype/scaled blade for rapid prototype.sjb} msg {12:51:27pm > Part fill style: Perimeter / rasters} msg {12:51:27pm > Part interior style:
Solid - normal}
msg {12:51:27pm > Contour width:
0.5080}
msg {12:51:27pm > Part raster width:
0.5080}
msg {12:51:31pm > Done generating toolpaths after 4 sec.} msg {12:51:32pm > Part start (-0.006986, -0.083297) was outside of modeling envelope; CMB moved to origin.} msg {12:51:37pm > Toolpath Information} msg {12:51:37pm > Est. build time:
4 hr 51 min }
msg {12:51:37pm > Model material:
41.00 cm³}
msg {12:51:37pm > Support material:
4.39 cm³}
msg {12:51:37pm > Done writing CMB file after 5 sec.} msg {12:51:37pm > Job Summary} msg {12:51:37pm > Full job path: C:/Documents and Settings/sean.keane/Desktop/robert/ssys_scaled blade for rapid prototype/scaled blade for rapid prototype.sjb} msg {12:51:37pm > Modeler:
Dimension}
msg {12:51:37pm > Est. build time:
4 hr 51 min }
msg {12:51:37pm > Model material:
41.00 cm³,
T12,
ABS P400}
msg {12:51:37pm > Support material:
4.39 cm³,
T12,
ABS P400R}
msg {12:51:37pm > Slice height: 0.2540} 92
msg {12:51:37pm > Part fill style: Perimeter / rasters} msg {12:51:37pm > Part interior style:
Solid - normal}
msg {12:51:37pm > Contour width:
0.5080}
msg {12:51:37pm > Depth of contours:
6.3500}
msg {12:51:37pm > Part interior depth:
1.0160}
msg {12:51:37pm > Part raster width:
0.5080}
msg {12:51:37pm > Application:
Catalyst 3.6.1, Build version: 2160}
msg {12:51:37pm > Saved job: C:/Documents and Settings/sean.keane/Desktop/robert/ssys_scaled blade for rapid prototype/scaled blade for rapid prototype.sjb, approximate build time 4 hr 51 min } msg {12:51:37pm > Saved job: C:/Documents and Settings/sean.keane/Desktop/robert/ssys_scaled blade for rapid prototype/scaled blade for rapid prototype.sjb} session {12/5 9:01:57am > sean.keane closed job.} session {12/6 1:57:54pm > sean.keane closed job.} session {12/9 12:12:41pm > sean.keane opened job C:/Documents and Settings/sean.keane/Desktop/robert/ssys_scaled blade for rapid prototype/scaled blade for rapid prototype.sjb. Catalyst 3.6.1 (2160).} msg {12:12:41pm > C:/Documents and Settings/sean.keane/Desktop/robert/ssys_scaled blade for rapid prototype/scaled blade for rapid prototype.sbs loaded...} msg {12:12:43pm > Done reading boundary curves after 2 sec.} msg {12:13:00pm > Saved job: C:/Documents and Settings/sean.keane/Desktop/robert/ssys_scaled blade for rapid prototype/scaled blade for rapid prototype.sjb} msg {12:13:00pm > Part fill style: Perimeter / rasters} msg {12:13:00pm > Part interior style:
Solid - normal}
msg {12:13:00pm > Contour width:
0.5080}
msg {12:13:00pm > Part raster width:
0.5080}
msg {12:13:03pm > Done generating toolpaths after 2 sec.} msg {12:13:04pm > Part start (-0.006986, -0.083297) was outside of modeling envelope; CMB moved to origin.} 93
msg {12:13:09pm > Toolpath Information} msg {12:13:09pm > Est. build time:
4 hr 51 min }
msg {12:13:09pm > Model material:
41.00 cm³}
msg {12:13:09pm > Support material:
4.39 cm³}
msg {12:13:10pm > Done writing CMB file after 5 sec.} msg {12:13:10pm > Job Summary} msg {12:13:10pm > Full job path: C:/Documents and Settings/sean.keane/Desktop/robert/ssys_scaled blade for rapid prototype/scaled blade for rapid prototype.sjb} msg {12:13:10pm > Modeler:
Dimension}
msg {12:13:10pm > Est. build time:
4 hr 51 min }
msg {12:13:10pm > Model material:
41.00 cm³,
T12,
ABS P400}
msg {12:13:10pm > Support material:
4.39 cm³,
T12,
ABS P400R}
msg {12:13:10pm > Slice height: 0.2540} msg {12:13:10pm > Part fill style: Perimeter / rasters} msg {12:13:10pm > Part interior style:
Solid - normal}
msg {12:13:10pm > Contour width:
0.5080}
msg {12:13:10pm > Depth of contours:
6.3500}
msg {12:13:10pm > Part interior depth:
1.0160}
msg {12:13:10pm > Part raster width:
0.5080}
msg {12:13:10pm > Application:
Catalyst 3.6.1, Build version: 2160}
msg {12:13:10pm > Saved job: C:/Documents and Settings/sean.keane/Desktop/robert/ssys_scaled blade for rapid prototype/scaled blade for rapid prototype.sjb, approximate build time 4 hr 51 min } msg {12:13:10pm > Saved job: C:/Documents and Settings/sean.keane/Desktop/robert/ssys_scaled blade for rapid prototype/scaled blade for rapid prototype.sjb} session {12/9 12:13:51pm > sean.keane closed job.}
94
session {12/9 12:19:15pm > sean.keane opened job C:/Documents and Settings/sean.keane/Desktop/robert/ssys_scaled blade for rapid prototype/scaled blade for rapid prototype.sjb. Catalyst 3.6.1 (2160).} msg {12:19:15pm > C:/Documents and Settings/sean.keane/Desktop/robert/ssys_scaled blade for rapid prototype/scaled blade for rapid prototype.sbs loaded...} msg {12:19:17pm > Done reading boundary curves after 2 sec.} msg {12:19:21pm > Saved job: C:/Documents and Settings/sean.keane/Desktop/robert/ssys_scaled blade for rapid prototype/scaled blade for rapid prototype.sjb} msg {12:19:21pm > Part fill style: Perimeter / rasters} msg {12:19:21pm > Part interior style:
Solid - normal}
msg {12:19:21pm > Contour width:
0.5080}
msg {12:19:21pm > Part raster width:
0.5080}
msg {12:19:24pm > Done generating toolpaths after 3 sec.} msg {12:19:25pm > Part start (-0.006986, -0.083297) was outside of modeling envelope; CMB moved to origin.} msg {12:19:30pm > Toolpath Information} msg {12:19:30pm > Est. build time:
4 hr 51 min }
msg {12:19:30pm > Model material:
41.00 cm³}
msg {12:19:30pm > Support material:
4.39 cm³}
msg {12:19:31pm > Done writing CMB file after 6 sec.} msg {12:19:31pm > Job Summary} msg {12:19:31pm > Full job path: C:/Documents and Settings/sean.keane/Desktop/robert/ssys_scaled blade for rapid prototype/scaled blade for rapid prototype.sjb} msg {12:19:31pm > Modeler:
Dimension}
msg {12:19:31pm > Est. build time:
4 hr 51 min }
msg {12:19:31pm > Model material:
41.00 cm³,
T12,
ABS P400}
msg {12:19:31pm > Support material:
4.39 cm³,
T12,
ABS P400R}
msg {12:19:31pm > Slice height: 0.2540} 95
msg {12:19:31pm > Part fill style: Perimeter / rasters} msg {12:19:31pm > Part interior style:
Solid - normal}
msg {12:19:31pm > Contour width:
0.5080}
msg {12:19:31pm > Depth of contours:
6.3500}
msg {12:19:31pm > Part interior depth:
1.0160}
msg {12:19:31pm > Part raster width:
0.5080}
msg {12:19:31pm > Application:
Catalyst 3.6.1, Build version: 2160}
msg {12:19:31pm > Saved job: C:/Documents and Settings/sean.keane/Desktop/robert/ssys_scaled blade for rapid prototype/scaled blade for rapid prototype.sjb, approximate build time 4 hr 51 min } msg {12:19:31pm > Saved job: C:/Documents and Settings/sean.keane/Desktop/robert/ssys_scaled blade for rapid prototype/scaled blade for rapid prototype.sjb}
96
Appendix B: MPB Specifications
97
98
99
100
101
102
Appendix C: Engineering Drawings
103
104
105
106
107
Appendix D: Bearing Specifications
108
109
Appendix E: Wind Tunnel Testing Log
110
Wind Tunnel Testing Log Test No.
1
Test type
Prony Brake
Wind Speed (m/s)
Description
24.2
Prony brake apparatus was tested, wind tunnel was run at full speed to get the maximum possible speed from the VAWT.
24th February 2012 2
Prony Brake
22.6
3 4 5
Prony Brake Prony Brake Prony Brake
22.7 19.5 15.3 13
6
Prony Brake
7
Prony Brake
Test carried out to investigate repeatability of experiment
M.P.B
22
A short test was carried out to see if the VAWT had a high enough torque to drive the magnetic particle brake, using a basic flexible coupling.
22.1
Replaced Central shaft with a straighter shaft. Achieved self starting at certain positions. Bearings were cleaned out using compressed air and the turbine achieved a max rpm of over 1000rpm. Also a significant increase in the torques. Self start was also drastically improved.
29th February 2012
22.2
1st March 2012
2nd March 2012
System losses were reduced using various alignment techniques, producing almost twice the rpm. Test ran successfully Test ran successfully Test ran successfully Wind speed was too low to get substantial amount of data points from the apparatus
8
9
Prony Brake
10
Prony Brake
22.5
11
Prony Brake
20.3
12
Prony Brake
15.5
13
Prony Brake
12.1
7th March 2012
111
Speed Quite low, difficult to get many data points.
112
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