SIZING AND PARAMETRIC STUDY OF A 10kWel SOLAR ORGANIC RANKINE CYCLE FOR BRAZILIAN CONDITIONS.

SIZING AND PARAMETRIC STUDY OF A 10kWel SOLAR ORGANIC RANKINE CYCLE FOR BRAZILIAN CONDITIONS. Marcel Senaubar Alves, [email protected] 1 Electo Eduardo Silva Lora, [email protected] José Carlos Escobar Palacio, [email protected] 1 1

UNIFEI – Federal University of Itajubá Av. BPS, 1303, District Pinheirinho, Itajubá – Minas Gerais

 Abstract: This paper presents a technical analysis of a stand-alone ORC Solar power system design of 10kWel for   Brazilian conditions. The analysis are based on the direct solar irradiance of Itajubá - MG, price criteria of  commercial suppliers, simulation of temperature and evaporation pressure influences and a comparative analyses cost  with photovoltaic and Diesel generators. The main objective is to design this equipment using available commercial systems in the market (PTC + ORC) for an implantation on the city of Itajubá - MG. Today a system can be built from 9.34  –  15.03 U$/W, the double from large CSP plants using Rankine Cycle, normally in the range 4.2 - 8.4 U$/W. The ORC Solar power system simulated for Brazilians conditions is able to produce 2.63kWh/year on average, producing electrical energy with a cost range of 0.081  –  0.131 U$/kW, also producing hot water, unfortunately the cost of  electricity is a bit more expensive than photovoltaic technology in the range 0.058  –   –  0.081 U$/kW.  Key Words: Words: Solar, Organic Rankine Cycle, Parabolic trough collector, Renewable energy.

1. INTRODUCTION

Energy is fundamental to any human being, used to meeting our basic needs, such as transportation, food and the own maintenance of life, dependent on energy in its various forms, throughout history we have developed, machines and ways of living that always need more and more energy, particularly electrical one. Nowadays, there is already a scientific consensus that climate has changed, it is already a reality and its main causes are human activities, mainly burning fossil fuels. Based on science, if we do not do something to reduce the emissions, the consequences will be catastrophic, says the Intergovernmental Panel on Climate Change (IPCC). Renewable energy could be a solution to the existing environmental problems. Almost every renewable source of  energy  –  hydraulic, biomass, wind, fossil fuels and energy from the oceans –  are indirectly solar energy (ANEEL, 2002). Analyzing the commercially renewable energy options available today, it is easy to see that solar is far more abundant and it has greater potential than all other forms of renewable energy. The use of renewable energy sources such as solar power aims to ensure the future and prosperity of the energy sector in a sustainable way. In other words, by using directly solar energy it is like to take a shortcut, in a certain way be more efficient. Most forms of renewable energy are limited in their quantity – solar  –  solar is not. In addition, as a natural resource, the sun’s energy is more abundant and geographically spread compared to other renewable resources (Solar Potential, 2009). The sun’s potential for  power generation, occult all other renewable energy sources, providing available amount energy on earth of 1410W/m². This means that every hour it floods the earth with thermal energy equal to 21 billion tons of coal. The enormous output of solar energy is almost impossible to conceive, but for now solar barely registers in the world’s energy portfolio accounting for only a small fraction of total electrical output (Solar Potential, 2009). Until today, the use of solar energy to produce electricity gets more compliments than investments. From a technical standpoint, it is capable of producing clean energy in an unproductive area or in land that would hardly be used for any other economic development purpose. Also it has a reduced and relatively low risk management and low environmental impact, situation that turns to be quite different from what occurs with the use of rivers for hydroelectric and nuclear generation projects. This is a clean and endless source that does not emit waste, does not require deforestation, flooding areas or modify the course of rivers or be frightened by the possibility of radiation leak. Electricity production in the world is heavily dependent on fossil fuels (Figure 1), but this has been changed during past few years. In 2008 and 2009 were installed worldwide more than 300GW of power plants and 140GW (47%) of them use renewable energy (REN21, 2010).

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Figure 1. Worldwide electricity supply by source (2008), with global consumption of 132 PWh (REN21, 2010)

While the industrialized world needs to urgently rethink its energy strategy, the developing world should learn from the past mistakes and build their economies from the beginning on a solid basis for a sustainable energy supply. A new infrastructure must be constructed to allow that to happen. Brazil gives an good example of sustainability, most of the energy supply is renewable (over 86%), with a hydraulic major part (Figure 2), even though with this incredible infrastructure we cannot meet all the future energy needs, of the habitants and industries, or by insufficient production, deficiencies in the transmission and distribution systems, geographical issues, or even by the high cost of producing energy (Brazilian energy balance report, 2011).

Figure 2. Domestic electricity supply by source (2010), with a global consumption of 455.7 TWh according to the Brazilian energy balance report (2011).

Currently in Brazil there are about 600 thousand houses without electricity. The federal program “Luz para todos” helped much in the past eight years taking over more than 14.3 million Brazilians out of darkness (Programa Luz para Todos, 2011). Therefore, this paper presents a structured technical analysis of a stand-alone ORC Solar power system design of 10kWel for Brazilian conditions. There are many ways to converted solar energy in to electricity, one of the most promising solutions nowadays is the small-scale and small concentration of solar energy and even recent analysis suggests that ORC systems can compete in the costs of generating electricity with photovoltaic and diesel generators (Quoilin, 2011). The analysis are based on data about the direct solar irradiance of Itajubá - MG, price criteria of commercial suppliers, influence of temperature and evaporation pressure and a comparative analyses cost with photovoltaic and Diesel generators. 2.

HARVESTING THE SUN

The ways they capture, convert and distribute sunlight are broadly characterized as either direct or indirect, depending on solar technologies adopted. Direct means with only one transformation sunlight is transformed in useful energy. As an example, when the sunlight hits a photovoltaic cell creating electricity – although the electricity generated

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will require others conversion. Also include solar techniques, which selecting materials with favorable thermal properties, designing spaces for natural air flow, and referencing the position of a building to better use the light or mechanical energy. Indirect means that need to have more than one transformation to come up with usable energy. Solar energy collectors are special kind of heat exchangers that transform solar radiation energy to thermal energy and transport it in a fluid. The major component of any solar system is the solar collector. There are basically two types of solar collectors: non-concentrating or stationary and concentrating. A non-concentrating collector has the same area for intercepting and for absorbing solar radiation, whereas a solar collector with sun-tracking usually has concave reflecting surfaces to intercept and focus the sun’s beam radiation to a smaller receiving area, thereby increasing the radiation flux. A large number of solar collectors are available in the market; therefor in Table 1 they are better explained and some are showed in Figure 3. Table 1. Solar energy collectors, Kalogirou (2004).

Stationary collector systems normally are used for water heating, and other fluid´s at low range temperature. Even though, a compound parabolic collector (CPC) was used in Japan to generate electrical power (350W) for a residential scale (Saitoh, 2007). Regarding the temperature range it´s useful for electrical power generation and hot water, but the space required and the specific cost are higher them the small PTC system (Kalogirou, 2004). Parabolic trough collectors (PTC) concentrate the solar energy by parabolic mirrors onto vacuum tubes containing oil or other heat transfer medium. Normally, the heated medium is used to generate steam and drive a steam turbine generator (Rankine cycle). The global market has been dominated by parabolic-trough plants, which account for 90% of  CSP plants (Solar Potential, 2009). Linear Fresnel reflectors (LFR), some says that is the next stage on from the parabolic trough technology (Ausra, 2011). Solar energy is concentrated by several rows horizontally arranged of flat mirrors onto a water filled collector pipe doing the same job as a parabolic trough. Heliostat field collector (HFC) or known as “solar towers” have huge arrays of flat mirrors that concentrate all the sunlight towards a receiver at the top of a tower. This allows achieve very high temperatures and as a result, they are highly efficient. Parabolic dish reflectors (PDR) concentrate the sunlight onto a Stirling engine. These comparatively small units can be used individually for decentralized power supply, or whole arrays can produce electricity on a large scale. However, heat storage tanks cannot be used with these systems.

Figure 3. From left to right, examples of collectors  – PTC, LFR, HFC and PDR. (Solar Potential, 2009)

Known as well as Concentrated Solar Power technology (CSP), widely commercialized the generating capacity of  CSP market had grew about 740 MW between the periods of 2007 until the end of 2010. More than half of this (about 478 MW) was installed during 2010, bringing the global total to 1095MW (at the end of 2010). The decreasing cost of CSP production is due to the investments on technological development during the last 25 years, allowing nowadays a range of 4.2 U$/W up to 8.4 U$/W. It is expected a reduction on the construction´s cost of  new CSP plants, depending on the direct normal irradiation (DNI) on the site, we can reach from 75% up to 84% of cost reduction by the year 2050 (IEA, 2010).

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As showed in the figure 4, CSP growth is expected to continue at large steps; only in Spain another 946 MW of  capacity is under construction with total new capacity of 1,789 MW expected to be in operation by the end of 2013. Interest is also remarkable in North Africa and the Middle East, as well as India and China (REN21, 2010).

Figure 4. Evolutional growth of CSP technology. (Solar Potential, 2009)

Brazil undergoes trough some changes in the energy sector, concerned with environmental factors and their impacts. The Brazilian energy matrix is changing, created in 2003 the Incentive Program for Alternative Sources for Electric Generation (PROINFA), encourage the creation of primary energy supply from renewable sources. Most of the financial resources are designed for biomass, wind and SHP (small hydropower), unfortunately there are no specific resource to reinforce the usage of solar energy. Which is a shame, the solar resource in Brazil is enormous, as it is showed in the figure 5.

Figure 5. Annual average direct normal solar irradiation (SWERA, 2005).

Actually in Brazil, solar energy appears in symbolic form, at the Tauá plant, in Ceará (Eike Batista's work) with capacity of 1MW is the largest solar power plant, using photovoltaic technology in Brazil. But this story is about to

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change, in the “desert” of Paraiba, in Coremas, which has an annual average direct irradiation of 5.5kWh/m².day; will be built the first CSP solar power plant in Brazil, providing an initial capacity of 50MW by the end of 2015, and possibly expanded to 150MW, placing Brazil in map of solar power generation on large scale. (Época, 2011) 3.

CONVERTION METHOD

Currently there are several types of technologies that allow converting thermal energy, in an efficient way, to mechanical work with a suitable heat engine based on a properly chosen appropriate thermodynamic cycle that is matched with the heat source temperature and capacity (Figure 6). For use of renewable sources the majority are in rapid developments (but not all are commercial), which are internal combustion engines burning syngas (gasification), Stirling engines, external fired gas turbine (EFGT), organic Rankine cycle and others.

Figure 6. Comparative chart of heat engines with applicable temperature range (Tarique, 2011).

For instance, depending on the type of organic fluid used, wasteheat temperatures as low as 70-80°C can be used in an ORC to generate electricity. At these low temperatures a steam cycle would be inefficient, due to enormous volumes of low pressure steam, causing very voluminous and costly plants. The Organic Rankine Cycle (ORC), units actually have higher cycle efficiency at these low temperatures. The solar collector has also a low temperature limitation, the most suitable option for small scale solar application, is the ORC system, which can be technically and economically reasonable for distributed electricity generation, applied with low temperature range (under 200°C), associate or not with cogeneration processes (Tarique, 2011). The Organic Rankine Cycle's (ORC) principle is based on a turbo-generator working as a normal steam turbine to transform thermal energy into mechanical energy and finally into electricity through an electric generator. Instead of the water steam, the ORC system vaporizes an organic fluid, characterized by a molecular mass higher than water, resulting in an enthalpy differences significantly lower for organic substances compared with water. This implies higher mass flows for the same power output, leading to a slower rotation of the turbine and lower pressure and erosion of the metallic parts and blades. The success of the ORC technology can be partly explained by its modular feature: a similar ORC system can be used, with little modifications, in conjunction with various heat sources, reinforced by the high technological maturity of most of its components, due to their extensive use similar unit technologies in refrigeration applications. Moreover, unlike with conventional power cycles, local and small scale power generation is made possible by this technology. ORC technology is spread in over 23 countries, generating about 2,1GWel, normally divided in four basics applications (Figure 7): biomass, waste heat recovery (WHR), geothermal and solar power.

Figure 7. Most common ORC applications (adapted from Enertime, 2009).

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In the selection of organic fluid for use with low temperature heat sources in ORC heat engines, attention should be given to obtain higher cycle efficiency (Figure 8). Simultaneously safety criteria, environmental impact, cost and availability should also be considered. The important parameters are as follows (Tarique, 2011): If an ORC operates with a low temperature heat source, working fluid with low boiling point is preferred. However, a very low boiling point at atmospheric pressure may require a low condensing temperature; A lower freezing point below the heat sink temperature is desired to prevent freezing of the working fluid; At high pressure and temperature organic fluids can suffer from chemical deteriorations and decompositions. A working fluid with a high latent heat of vaporization can absorb more heat during evaporation. Therefore a fluid with high latent heat of vaporization is preferred to increase the efficiency of the system; In selection of a working fluid, Ozone Depletion Potential (ODP), Global Warming Potential (GWP) and toxicity rating of the working fluids should be considered; 

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Figure 8. Efficiency of Rakine cycle vs. boiling temperature (Aoun, 2008).

In an ORC heat engine, the expansion device is the most important component of the cycle. The performance and efficiency of the cycle strongly depend on the expander (Tarique, 2011). Consequently, this correlates with working fluid, operating conditions, and heat source characteristics, also the system power output. Expanders are broadly divided into two categories: turbo-machines and positive displacement machines, demonstrating that the uses of positive displacement expanders (screw and scroll machines) were advantageous compared t o turbomachines (radial turbine) for use in low temperature and low capacity applications (Quoilin, 2010). 4.

SOLAR ORGANIC RANKINE CYCLE

Here is a model of the solar organic Rankine cycle for better comprehension (Figure 9), starting with the solar field (PTC) and the thermal storage in a primary circuit, the ORC unit and the condenser system.

Figure 9. Schematic model from an ORC Solar system (Quoilin, 2011).

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4.1. Sizing

The first decision is to specify the maximum power of the ORC equipment. In a small village (up to 20 houses) or a small industry a 10kWel can supply enough energy. There are a few suppliers for this type of range with a specific cost of 3270 – 5000 U$/kW, this difference can be explained by the type of the turbine (scroll, screw or radial turbine), the temperature (from 80°C to 150°C) and the global efficiency of the machine, around 7% to 10%, but all of them uses as fluid R245fa, which performs the highest thermal and exergy efficiency (Wang, 2011). That means, in the worst case, to produce 10kW of electrical power is need at the maximum 140kW of thermal energy. Some information about the specific irradiation is needed for sizing the solar field, the system is designed for Itajubá  –  Minas Gerais conditions. As showed in the table 2 some data about 3 other city’s nearby (less than 50km radius). The lowest average irradiation, is in June (2011), when reaches 3kWh/m².day, the lowest average peak is inferior than 600W/m² (Figure 10). The best scenario is normally in January (2011), when the average peak is near 950W/m² (figure 11) with an irradiation superior than 5kWh/m².day. Table 2. Annual average direct normal irradiation in kWh/m².day by month in 50km range (Cresesb, 2011)

City/Month Campos do Jordão Lorena Passa Quatro

Jan 4,75 5,35 5,19

Feb 4,83 5,23 5,25

Mar 4,64 4,65 5,22

Apr 4,00 4,07 4,47

Figure 10. Average irradiation during June 2011 in Itajubá (UNIFEI meteorological station).

Mai 3,83 3,49 4,11

Jun 3,33 3,02 3,53

Jul 3,97 3,49 3,97

Aug 4,28 3,95 4,58

Sep 4,33 3,95 4,33

Oct 4,97 4,65 5,17

Nov 5,06 5,35 5,61

Dec 4,81 5,12 5,39

Figure 11. Average irradiation during January 2011 in Itajubá (UNIFEI meteorological station).

The system will be set to work with a nominal solar input from 500 - 650W/m², equal to 6 hours (minimum) per day of sun light in the worst case (June). The system used to concentrated the sun is a parabolic trough collector, with an efficiency estimated on the range of 50% to 70% depending on the work temperature, with a specific cost of 152 –  210 U$/m². Changing the ORC unit and the collector efficiency, sizing the solar field for a specific average irradiation of 3kWh/m².day, the result is the overall price system with the solar field area (Figure 12).

Figure 12. ORC Solar overalls cost.

The overall specific cost remains on the range of 9.34 – 15.03 U$/W for a maximal coverture during the year, using an area from 286 m² up to 560 m². The sunlight is intermittent, for this reason it is a challenge to integrate directly the renewable energy in to the grid. The ORC system only runs with a minimum input power, normally 35% of the nominal power, which results in a minimal irradiation of 200W/m² (with 286m² of solar field), for that a parametric study is needed.

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4.2. Parametric study

Through a simulation of the system (in Aspen HYSYS), using the basics parameters presents in the figure 13, the first simulation analyses the influence of the pressure on the system (Figure 14), starting from an initial evaporation pressure of 550kPa using an increase rate of 64.2kPa, (using an constant temperature) increases the system efficiency, but unlike the pressure, efficiency does not grow in the same way. Within the range of 550kPa to 1000kPa efficiency increases with an intervals of (in order) 0.82, 0.72, 0.64, 0.57, 0.52, 0.47 and 0.43%. If this progression is continued it will reach a limit pressure of 1400kPa, from that point the increase of pressure would have a negative influence on the system. The second simulation analyze is whether the fluid is superheated (Figure 15), unlike the pressure, the temperature and efficiency changed linearly, with increase of 4.7°C in the system causes a decrease in the efficiency of 0.02%, as conclusion there is no benefit associated with the increased degree of superheating over certain value. If one fixes the intake and discharge pressures for an expansion process, the enthalpy difference between the high pressure and low pressure states becomes lower as the degree of superheating increases resulting in a reduction in the specific work, in another words, for low temperatures the superheating for dry fluids do not make sense (Tarique, 2011).

Figure 13. Basic parameters used to simulate the ORC system using Aspen HYSYS.

Figure 14. Influence of the pressure increases during the heating up of the system.

Figure 15. Influence of the fluid superheating on the system.

4.3. Economic study

The work from Orosz et. Al. (2010), which they built a small scale ORC solar system, adapting a vehicle airconditioning compressor to work as a turbine (scroll), reaching a specific cost of about 6 U$/W, with this cost a small scale CSP system can compete with photovoltaic (PV) and Diesel generator (cost data from Alibaba.com, 2012), we can synthetize all cost in the table 3, to make a technical analysis, here some considerations: Annual average irradiation of 600W/m², working 7 hours a day (4kWh/m².day); Overall Equipment Efficiency (OEE): o No Diesel engine maintenance; Efficiency of photovoltaic system according to average irradiation (Kyocera, 2012); o Heating up process for CSP, without buffer/storage system. o Pay-back time of 5 years; Diesel cost 1,2 U$/L and consumption of 0.3L/kWh (Geradores rio preto, 2012);  

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Table 3. Specific cost evaluation between 10kW el stand-alone generators.

Installation cost in Brazil [U$/W] OEE* Average production [kWh/year] Final cost [U$/kWh]

CSP 9.34 – 15.03 90% 2.63 0.081 – 0.131

PV Diesel 4.67 – 6.5 0.3 – 0.5 60% 100% 1.82 2.92 0.058 – 0.081 0.383 – 0.399

The result is very close with the predicted, but this technology is new in the market with few options for suppliers. In a future possibility of market expansion, mass production and cost reduction, we will reach the same result (Figure 16) near to 6 U$/W, which is also close from large solar plants (from 4.2 U$/W up to 8.4 U$/W). Also we have another solution, changing the equipment for other location, with a higher average direct irradiation. To make this consideration only the CSP and PV will be used, hence Diesel generator does not depend on the location nether the solar irradiance. There are two possible scenarios: The present scenario – Using available technology (Figure 17). The future scenario – Considering a cost reduction of 50% on the CSP and ORC technology (Figure 18).  

Figure 16. Influence of cost reduction on ORC solar system in electricity cost.

Figure 17. Influence of solar irradiation on the system output and electricity cost.

Figure 18. Influence of solar irradiation on the system output and electricity cost, with a reduction of 50% in CSP cost.

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CONCLUSION

The available technologies commercial in the market is capable to built a system from 9.34 to 15.03 U$/W, almost twice more than a large CSP plants using Rankine Cycle, normally in the range 4.2 to 8.4 U$/W. Designed for the minimal irradiation of 3kWh/m².day, the ORC Solar system is able to produce 2.63kWh/year on average, producing electricity with a cost range of 0.081 to 0.131 U$/kW, also produces hot water (at least 80kW thermal), unfortunately the cost of electricity is more expensive than photovoltaic technology, in the range 0.058 to 0.081 U$/kW. Therefor the need of hot water (thermal power) could qualify the system to work in better conditions of cost than the PV technology. Diesel generators are the smallest if compared against CSP technology, being the larger one, and PV is normally more than half of the size of CSP. Another remarkable possibility over the PV technology is the capacity to improve the system, by using thermal storage (buffer) or cogeneration processes during the solar irradiations peaks, hence the superheating for low temperature are baneful for the system efficiency. The thermal storage allows the solar system work without sun, during a programed period of time, which could be from a passage of cloud until work all night long. The expected cost reduction on CSP technology, from 75% to 84%, will also reflect on small-scale system, which will possible reduce the cost of this technology. Also the governments could even give fin ancial incentives, allowing the commercialization in large-scale or by developing the national industry for this type of technology (ORC and CSP). 6.

ACKNOWLEDGEMENTS

The authors want to thank to CAPES, CNPq, FAPEMIG, ANEEL and CPFL for their collaboration and financial support in the development of this work. 7.

REFERENCES

ANEEL, (2002) – Atlas da Energia Solar. Available < http://www.aneel.gov.br/aplicacoes/Atlas/download.htm> AOUN, B. (2008) Micro-Cogeration pour les batiments residentiels fonctionnant avec des energies r enouvelables. 13 de novembro 2008. 186p. Tese – Ecole Nationale Superieure des Mines de Paris. AOUN, B., AND D. CLODIC. (2008). "Theoretical and experimental study of an oil-free scroll type vapor expander", Proceedings of the International Compressor Engineering Conference at Purdue: paper 1188. Ausra (2012). Available: < http://www.areva.com/EN/solar-220/areva-solar.html> Brazilian Energy Balance (2011). EPE – Empresa de pesquisa energética. Available . CRESESB (2011) - Centro de Referência para Energia Solar e Eólica Sérgio Brito. Available < http://www.cresesb. cepel.br/ > ENERTIME, (2009) – Available < http://www.cycle-organique-rankine.com/ > Época (2011). Energia extraída do sol da caatinga. Available: < http://revistaepoca.globo.com/Ciencia-etecnologia/noticia/2011/09/energia-extraida-do-sol-da-caatinga.html > Geradores rio preto. (2012). Available IEA (International Energy Agency) (2010). Technology Roadmap - Concentrating Solar Power, OECD/IEA, Paris. KALOGIROU, S.A. (2004). Solar thermal collectors and applications. Progress in energy and combustion science. Volume 30, Páginas 231-295, Fevereiro 2004. Kyocera (2012). Product description. Available OROSZ, M. S.; QUOILIN, S.; HEMOND, H. (2010). SORCE: A Design Tool for Solar Organic Rankine Cycle Systems in Distributed Generation Applications. EuroSun 2010: International Conference on Solar Heating, Cooling and Buildings. Programa Luz para Todos, (2011). QUOILIN, S. OROSZ, M. LEMORT, V. Modeling and experimental investigation of an Organic Rankine Cycle using scroll expander for small scale solar applications. In: International Congress on Heating, Cooling, and Buildings, 1. 2008. Lisboa, Portugal QUOILIN, S.; OROSZ, M. S.; HEMOND, H. (2011). Performance and design optimization of a low-cost solar organic Rankine cycle for remote power generation. Solar Energy 85, 955 – 966 REN21. 2010. Renewables 2010 Global Status Report (Paris: REN21 Secretariat). SAITOH, T.; YAMADA, N.; WAKASHIMA, S. (2007). Solar Rakine cycle system using scroll expander. Journal of  environment and engineering. Vol. 2, No 4. Solar Potential (2009). The climate group, Price Water House Coopers. SWERA. (2005) - Solar and Wind Energy Resource Assessment. Available TARIQUE, M. A. (2011). Experimental Investigation of Scroll Based Organic Rankine Systems. Abril 2011. Pg. 167. The Faculty of Engineering and Applied Science - University of Ontario Institute of Technology. WANG, D. Y.; PEI, G.; LI, J.; JI, J. (2010). Analysis of working fluid for organic Rankine Cycle. University of science and technology of China. 8.

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