FUNDAMENTALS OF SOLAR ENERGY Prof. (Dr.) H.P. Garg IREDA Chair Emeritus Professor Centre for Energy Studies Indian Institute of Technology, Hauz Khas, New Delhi-110016, India Tel. No. 91-11-2659 1249 (office) 91-11-2508 7744 (res.) Mob. 98180 00984 Fax: 91-11-2659 1249 / 2658 1121 E-mail: [email protected] [email protected]
Energy related issues for India? I S S U E S
• Wider access to electricity • Significant investment needs • Choice of current & emerging technology that address • Environmental protection: climate concerns & increasing CO2 emissions • Energy security • Economic growth • Institutional, financial and technological barriers • Affordability issues
The SUN Source of all Energy Produces Energy from H2
What is Solar Energy? Originates with the
thermonuclear fusion reactions occurring in the sun. • Represents the entire electromagnetic spectrum (visible light, infrared, ultraviolet, x-rays, and radio waves). • The sun is, in effect, a continuous fusion reactor with its constituent gases as the ‘containing vessel’ retained by gravitational forces. The fusion reaction in which hydrogen (i.e. four protons) combines to form helium (i.e. one helium nucleus) accompanied by a 0.7 percent loss of mass and converted to energy is the source of energy in the SUN. 1 4 +
4H →He + 2β + 2ν + 25 MeV E = mc2
Global Solar Energy Balance (TeraWatts) Solar Energy Input
Reflected to Space Immediately
Absorbed and Then Reflected as Heat
Used to Evaporate Water (Weather)
Captured by Plant Photosynthesis Total Energy Used by Human Society Solar Energy Potential (Theoretical) Solar Energy Potential (Practical)
100 13 120,000 502000
• We are concerned about the portion of the electromagnetic radiation emitted from the run in the wavelength range of 0.25 – 3.0 µm (micron). • We are also concerned about the solar geometry i.e. sun and its position in the sky, the direction of direct (beam radiation) on variously inclined and oriented surfaces. • We are also concerned about the extraterrestrial radiation on a horizontal surface which is the limit of the solar radiation on the surface of the earth. • We are also concerned about the earth; its motion, orientation and tilt with respect to the sun effecting the availability of solar radiation. • We are also concerned about the earth’s atmosphere responsible for the reduction due to absorption, scattering and reflection of solar radiation.
The Sun’s Structure
The sun is a sphere of Intensely hot gaseous matter with a diameter of 1.39 × 109 m and is, on an average, 1.5 × 1011m from the earth.
The sun has an effective blackbody temperature of 5777k. The temperature in the central interior region is variously estimated at 8 × 106 to 40× 106 K and the density is estimated to be about 100 times water. that The of sun is, in effect, a continuous fusion reactor with its constituent gases as the ‘containing vessel’ retained by gravitational forces. The most accepted fusion reaction is in which hydrogen (i.e. four protons) combines to form helium (i.e. one helium nucleus); the mass of the helium nucleus is less that of the four protons, mass having
The Sun’s Structure • The Sun is 333,400 times more massive than the Earth and contains 99.86% of the mass of the entire solar system • It consist of 78% Hydrogen, 20% Helium and 2% of other elements • It is estimated that 90% of the energy is generated in the region of 0 to 0.23 R (where R is the radius of the sun), which contains 40% of the mass of the sun and density is about 105 kg/m3. • At a distance 0.7 R from the centre, the temperature drops to about 130,000 K and density drops to 70 kg / m3; and the zone from 0.7 to 1.0 R is known as convective zone, where temperature drops to about 6000 K and density to about 10-5 kg/m3
The Sun’s Structure • The outer layer of the convective zone is called the photosphere, whose edge is sharply defined, opaque, gases here are strongly ionized and is the source of most radiation. • The emitted solar radiation is the composite result of several layers that emit and absorb radiation of various wavelengths.
The Sun’s Structure • Outside the photosphere is a layer of cooler gases several hundred kilometers deep called the reversing layer and after this 10,000 km deep layer called Chromosphere. • Further there is Corona with very low density and of very high temperature.
• The earth is shaped as an oblate spheroid – a sphere flattened at the poles and bulging in the plane normal to the poles. For most practical purposes we consider the earth as a sphere with a diameter of about 12,800 km and a mean density of about 5.517 g/cm3. • Earth has a central core of about 2560 km in diameter which is more rigid than steel. Beyond Central Core is the mantle, which forms about 70 percent of the earth’s mass, and beyond this is the outer crust which forms about 1 per cent of the mass. • The earth describes an ellipse round the sun, with the later at one of the foci. The apparent path of the sun as seen from the earth is known as the ecliptic. • The eccentricity of the earth’s orbit is very small (e=0.01673), so that the orbit is in fact very nearly circular. The shortest distance is Rp = a(1-e)=147.10×106 km and longest distance is Ra = a (1+e) = 152.10 × 106 km Where ‘a’ is the semi-measure axis of the earth’s orbit. • The mean earth – sun distance is the mean of Rp and Ra and its numerical value is 149.5985 × 106 km. • On January 1, the earth is closest to the sun and on July 1 the earth is most remote to the sun.
The Earth (contd.)
• The earth makes one rotation about its axis every 24 hrs and completes a revolution around the sun in a period of 365.25 days approx. • The earth’s axis of rotation is tilted 23.5 deg. with respect to its orbit about the sun. In its orbital movement, the earth keeps its axis oriented in the same direction. • This tilted position of the earth, alongwith the earth’s daily rotation and yearly revolution, accounts for the distribution of solar radiation over the earth surface, the changing length of hours of daylight and night length, and the changing of the seasons.
Earth Data Mean distance from the Sun:
1.496 x 108 km
Maximum distance from the Sun:
1.521 x 108 km
Minimum distance from the Sun:
1.471 x 108 km
Mean orbital velocity:
Inclination of equator to orbit: Diameter (equatorial): Mass:
230 26’ 12,756 km 5.976 x 1024 kg
Surface temperature range:
Maximum: 60 0C Mean: 20 0C Minimum: - 90 0C
Internal Structure of the Solid Earth
The Solar Constant • The geometry of the sun - earth relationship is schematically shown in the figure. • The eccentricity of the earth’s orbit is such that the distance between the sun and earth (1.495 × 1011m) varies by 1.7 per cent. • The sun substends an angle of 32' at the earth because of its large size and distance. • The radiation emitted by the sun reaches unattenuated upto the outside of the atmosphere and thus is a fixed intensity. • The solar constant (Ion) is the energy received from the sun, per unit time, on a unit area of surface perpendicular to the direction of radiation, at a mean earth-sun distance, outside the earth atmosphere. • The latest value of solar constant is 1366.8 ± 4.2 watts/m2 or 433 Btu/ft2 hr or 4.921 MJ/m2 hr or 1.960 cal/cm2 min.
The Solar Constant (contd.)
• In olden days when rocket or space craft facilities were not available, solar radiation measurements were made on ground and at different heights of mountains and extrapolations and corrections for attenuations produced by different constituents of the atmosphere for different portions of the solar spectrum were made and value of solar constant was determined. • Pioneering studies were done by C.G. Abbot in Smithsonian Laboratories who gave a value of 1322 W/m2 which got revised by F.S. Johnson (1954) to 1395 W/m2. • Later with the availability of very high altitude aircraft, balloons, and space craft, direct measurement of solar radiation outside the earth atmosphere was made and reported by several scientists like A.J. Drummond, M.P. Thekaekara, C.Frohlick etc. and gave a value of 1353 W/m2 with an error of ± 1.5 per cent. • Later C. Frohlick reexamined the value of 1353 W/m2 in view of new pyrheliometric scale and with some additional space craft measurements and with three rocket flights the World Radiation Centre (WRC) adopted a new value of solar constant as 1367 W/m2.
Spectral Distribution of Extraterrestrial Radiation • In addition to the total energy in the solar spectrum (i.e. the solar constant), it is useful to know the spectral distribution of the extraterrestrial solar radiation, that is, the solar radiation that would be received in the absence of the atmosphere. • A standard spectral irradiance curve based on high altitude and space measurements is shown here which is found to be similar to the 5777K blackbody spectrum. • From this figure following observations are made: – The peak solar intensity is 2028.8 w/m2 at a wavelength of 0.48 µm. – The solar spectrum varies from 0.2 – 3.0 µm, – The energy in various spectral ranges is as follows:
Wavelength Energy (W/m2) Percent
0.2 – 0.38µm) 88 6
(0.38 – 0.78 µm) 656 48
(0.78 – 3.0 µm) 623 46
The WRC standard spectral irradiance curve at mean earth-sun distance
Solar Radiation Spectrum
Variation of Distribution of Extraterristrial Radiation • There is a very small variation in the extraterrestrial solar radiation with different periodicities and variation related to sunspot activities. For practical and engineering applications and due to variability of atmospheric transmission, the energy emitted by the sun can be considered as fixed. • However due to variation in the earth-sun distance there is a variation of ±3 percent in the extraterristrial radiation flux and the same is shown in figure with time of year and can also be calculated from the following equation. 360n I on = I sc (1 + 0.033 cos 365 Where Ion is the entraterristrial radiation measured on the plane normal to the radiation on the nth day of the year and Isc is the solar
Global Radiation Budget
Solar radiation passing through earth's atmosphere is scattered by gases, aerosols, and dust. At the horizon sunlight passes through more scatterers, leaving longer wavelengths and redder colors revealed.
Scattering of Light
Depletion of Solar Radiation by the Atmosphere • The earth is surrounded by an atmosphere containing various gases, dust and other suspended particles, water vapour and clouds of various types. The solar radiation during its passage in the atmosphere gets partly absorbed, scattered and reflected in different wavelength bands selectively. • Radiation gets absorbed in water vapor, Ozone, CO2 , O2 in certain wavelengths. • Radiation gets scattered by molecules of different gases and small dust particles known as Rayleigh scattering where the intensity is inversely proportional to the fourth power of wavelength of light (l α 1/λ 4). • If the size of the particles are larger than the wavelength of light then Mie Scattering will takes place. • There will be a reflection of radiation due to clouds, particles of larger size and other material in the atmosphere. • Considerable amount of solar radiation also gets absorbed by clouds which are of several types.
Depletion of solar radiation by the atmosphere
• Some radiation gets reflected back in the atmosphere due to reflection from the ground, from the clouds, and scattering. This fraction of radiation reflected back is called albedo of the ground and on an average the albedo is 0.3. • The solar radiation which reaches on the earth surface unattenuated (after scattering, reflection and absorption) is called direct radiation or beam radiation. • The radiation which gets reflected, absorbed or scattered is not completely lost in the atmosphere and comes back on the surface of the earth in the short wavelength region and called sky or diffuse solar radiation. • The sum of the diffuse and direct radiation on the surface of the earth is called global or total solar radiation.
The distance travelled by the sunbeam in the earth’s atmosphere is responsible for the amount of scattering, absorption and reflection of solar radiation. The shortest distance travelled by the sunbeam in the atmosphere is when the sun is at the Zenith and is longest when the sun is rising or setting. Airmass ‘m’ is defined as :
actual path length travelled AB m= = vertical depth of the atmosphere AC = cosec α = Sec φ Z m = 0 when outside the earth atmosphere m = 1 when sun is at the Zenith m = 2 when Zenith angle is 60°
Depletion of solar radiation by the atmosphere • Moon (1940) has proposed standard curves for calculating transmittence. • For Indian conditions a standard atmosphere composed of following conditions is defined as: Standard atmosphere : p =760 mm ω =20 mm d =300 / cm3 ozone = 2.8 mm For m = 0 to 5 for Indian atmosphere
1246 = w / m2 1 + (0.3135)m
This equation in India is used extensively for computing direct radiation at normal incidence for several stations.
Basic Earth – Sun Angles • For calculating solar radiation and designing solar devices, the knowledge of sun’s path in the sky, on various days in a year at a particular place is a pre-requisite. • Solar altitude angle (α) and solar azimuth angle (Az) are the two coordinates locating the sun in the sky. • The apparent solar path on a particular day is shown in the figure thereby showing sun’s zenith angle (θ z), altitude (α) and azimuth angle (Az) at a particular position of the sun. • The altitude angle of the sun (α) is defined as the angle in a vertical plane between the sun’s rays and the horizontal projection of the sun rays. • The azimuth angle (Az) is the angle in the horizontal plane measured from the south (northern hemisphere) to the horizontal projection of the sun rays. Displacements east of south are negative and west of south are positive. • The zenith angle (θ z) is the angle between sun’s rays and the line perpendicular to the horizontal plane i.e. the angle of incidence of beam radiation on a horizontal surface (α + θ z = π/2)
Solar zenith, altitude and azimuth angles (northern hemisphere), θ z = zenith angle, α=solar altitude, Az=solar azimuth
Basic Earth – Sun Angles • To specify the location of a place on the earth, two angles the latitude (L) and longitude angle (φ) are r eq uir ed. • To understand L and φ, please see the figure in which, the polar axis is shown by NOS, the earth’s centre being at 0. The great circle ABDA, normal to the polar axis, is known as equator. • Latitude (L) of a place (say C in figure) is the angle between the lines joining the place with the centre of the earth and the equator with the centre of earth or it is the angular displacement of the place north or south of the equator, north positive, -90°≤ L ≤90°. • The angle between the prime meridian (a semicircle passing through the poles and observatory at Greenwich, UK) and the meridian (a similar semicircle passing through the place, C, and the poles) is called longitude, φ, of that place. In the figure NGJS represents the prime meridian and NCBS represents the meridian of the place. The prime meridian has zero longitude. In the figure the longitude of the point C is φ° 1, east and that of point D is φ° 2 west and written as φ 1°E and φ° 2W respectively.
Latitude and longitude
Basic Earth – Sun Angles (contd.) • From this figure it can be seen that solar declinations (defined as the angular displacement of the sun from the plane of the earth’s equator), vary from +23.5° on June 22 to 0° at the equinoxes (March 21 and September 24) to -23.5° on December 22. • The values of sun’s declaration, δ, can be found out from the table orfigure as shownhere and given as: 284 +n δ = 23.45 sin 360 365 • Where n is the day of the year. The exact value of δ for a particular day can be read from Nautical Almanak since the declination varies slightly to the same day from year to year. -23.45°≤δ≤+23.45° For a day declination may be assumed constant and for practical purposes the values as shown graphically can be conveniently used.
Basic Earth – Sun Angles
• The position of a point P on the earth’s surface with respect to the sun’s rays can be determined at any instant if the latitude of the place L, hour angle w and the sun’s declination δ are known as shown in the figure. • Point P in the figure represents a place in the northern hemisphere. The hour angle is the angular displacement of the sun east or west of the local meridian due to rotation of the earth on its axis at 15° per hour, morning negative and afternoon positive. • At solar noon the sun is highest in the sky and at that time hour angle is zero. The hour angle express the time of day with respect to solar noon. One hour of time equals 15° of hour angle.
SOLAR TIME AND EQUATION OF TIME •
Solar time is the time used in all sun-angle relationship and it does not coincide with local clock (standard time) time. Two corrections are required to convert standard time to solar time. The first correction is due to difference in longitude (L) between observer’s meridian (longitude, φ loc) and the meridian on which the local standard time is based (φ st). The sun takes 4 minutes to traverse 1 deg. of longitude. The second correction is due to equation of time (E in minutes), which takes into account the perturbations in the earth’s rate of rotation which affect the time the sun crosses the observer’s meridian. The difference in minutes between solar time and standard time is : Solar time – Standard time = 4 (φ st - φ loc) + E
φloc − φst 12 − −E Solar noon = 15
For India φ st = longitude of standard meridian = Allahabad = 82.5° •
Equation of time as shown in the figure can be represented as : E = 9.87 Sin 2B – 7.53 cos B - 1.5 Sin B where B = 360 (n-81) / 364
Angle of Incidence on Horizontal and Inclined Planes •
Since, most solar equipments (e.g. flat-plate collectors) for absorbing radiation are tilted at an angle to the horizontal, it becomes necessary to calculate the solar flux that falls on a tilted surface. This flux is the sum of the beam and diffuse radiations falling directly on the surface and the radiation reflected on the surface from the surroundings. • Although the earth's path around the sun is elliptical and the solar day is not 24 hours, the position of the sun at any instant relative to a place on the spinning earth can be easily determined in terms of various angles as described below. Some angles used are: L = latitude of place north or south of equator (north positive) δ = declination of sun (north positive) ω = hour angle from solar noon (morning positive and afternoon negative) θ z = zenith angle α= altitude of sun β = tilt of plane from horizontal φ = longitude of place Az= azimuth of sun from south Azs= azimuth of surface from south, east positive and west negative θ i = angle of incidence of beam or direct radiation on a surface.
Angle of Incidence on Horizontal and Inclined Planes (contd.) From the figure one can easily calculate the altitude (α) of the sun at any given point of time, place and day as given below: sin α = cos L cos δ cos ω + sin L sin δ
It is also seen in the figure that a surface located at the latitude L, tilted towards the equator at an angle β from the horizontal surface is parallel to a horizontal surface at the latitude (L-β) degrees. Thus Eq.(1) can be written as: cos θt = cos(L-β) cos δ cos ω + sin (L-β) sin δ
Where θ t is the angle of incidence on an
Angle of Incidence on Horizontal and Inclined Planes (contd.) • At the time of solar noon, the altitude of the sun, α n, can be determined by putting ω=0 in eq. (1): α n = 90° - (L-δ) (3) • Sunrise hour angle or sunset hour angle, ω s, can also be determined from Eq.(1) by putting α =0. Cos ω s = - tan L tan δ (4) • Day length or possible sunshine hours, N, is given by 2ωs 2 N= = Cos −1 (− tan L tan δ ) 15 15
Angle of Incidence on Horizontal and Inclined Planes (contd.) For an inclined plane cos ω’s = - tan (L-β) tan δ, where ω’s is the sunrise or sunset hour angle for an inclined plane. As we have derived the expression for sin α, similarly an expression for cos AZ can also be derived: cos AZ cos α = sin L cos δ ω - cos L sin δ
and also sin AZ cos α = cos δ sin ω and also,
sin L cos ω − cos L tan δ cot AZ = sin ω
Angle of incidence on horizontal and inclined Planes (contd.) The general expression for angle of incidence (θi) of the sun’s rays on any surface can be derived and is given as: cos θi = (cos L cos β + sin L sin β cos Azs) cos δ cos ω + cos δ sin ω sin β sin Azs + sin δ (sin L cos β - cos L sin β cos Azs) (9) Now the intensity It incident on a given plane is given by It = IN cos θi or It = IN [(cos L cos β + sin L sin β cos Azs) cos δ cos ω + cos δ sin ω sin β Azs)] + sin δ (Sin L Cos β - Cos L Sin β Cos Azs (10) The intensities and the angle of incidence on horizontal and vertical surfaces can be obtained by putting β = 0 (for horizontal) and β = 90
Factors Governing availability of solar energy on the earth • Earth sun distance • Tilt of the earth’s axis • Atmospheric Attenuation
Factors Affecting Solar Energy availability on a Collector Surface • • • • • • •
Geographic location Site location of collector Collector orientation and tilt Time of day Time of year Atmospheric conditions Type of collector
Radiation Instruments • • • • •
Pyranometer Pyrheliometer Pyrgeometer Net Radiometer Sunshine Recorder These instruments are classified either as first class or second class or third class depending on their sensitivity, stability and accuracy.
Solar Radiation Components • DIRECT RADIATION Direct transmission of solar radiation to earth surface • DIFFUSE SOLAR RADIATION Scattered by molecules and aerosols on entering the earth’s atmosphere • GLOBAL SOLAR RADIATION = DIRECT RADIATION + DIFFUSE SOLAR RADIATION Concentrators use Direct Radiation plus a Small Portion of Scattered Radiation Flat Plate collectors use Direct and Diffuse Solar Radiation and also reflected Radiation
INSTRUMENTS USED • GLOBAL SOLAR RADIATION: Direct + diffuse radiation on horizontal surface PYRANOMETER • DIFFUSE SOLAR RADIATION: Short wave radiation from entire hemispherical sky PYRANOMETER WITH SHADING RING • DIRECT RADIATION Direct radiation from sun PYRHELIOMETER • REFLECTED SOLAR RADIATION Short wave radiation reflected from ground PYRANOMETER FACING DOWNWARDS • LONGWAVE RADIATION (i) Emitted from ground (upward direction) (ij) Atmospheric radiation (Downward direction) PYRGEOMETER & NET PYRADIOMETER
DETECTORS FOR RADIATION MEASUREMENT CALORIMETRIC SENSORS • The radiant energy is incident on a high conductivity metal coated with a nonselective black paint of high absorptance. THERMOMECHANICAL SENSORS • The radiant flux is measured through bendings of a bimetallic strip. THERMOELECTRIC SENSORS • Consists of two dissimilar metallic wires with their ends connected. PHOTOELECTRIC SENSORS • Photovoltaic instruments are most numerous in the field of solar radiation measurement. A photovoltaic device is made of a semiconducting material such as silicon.
Radiation Measurement in India All Instruments should be periodically calibrated 1. Systematic measurement of solar and terrestrial radiation in India started during IGY 1957-58 2. National Radiation Centre, POONA has absolute cavity radiometer which is used as primary standard. 3. IMD National Radiation Centre, POONA not only serves as National Radiation Centre but also as a WMO Regional Radiation Centre for Asia. 4. IMD National Radiation Centre maintains a set of reference, working and travelling standard instruments for ensuring the accuracy of radiation measurements on a National and Regional level.
MEASUREMENT OF DIFFUSE RADIATION • Same Instrument as used for the Measurement of Total or Global Radiation • A Suitable Device (Disc or Shadow Ring) is used to prevent Direct Solar Radiation from reaching the receiver (Pyranorneter). Factors Affecting the Accuracy are given below: • Multiple Reflection within the Glass Cover Affects the Accuracy of the Measurement. • In Calculating the Correction Factor, it is Assumed that the Sky is Isotropic. • A Part of the Circumsolar Radiation is also prevented from reaching the receiver by the Shading Device. • The Dimensions of the Receivers are not Adequately Standardized.
PARAMETERS OF PYRANOMETERS Important parameters associated with a pyranometer includes the following: • SENSITIVITY – Sensitivity, R is Ratio of Output Signal, ‘S’, to the received irradiance, I. R = S/I, UNIT : mV / W/m2
• TEMP. COEFFICIENT OF SENSITIVITY ∆R / R C= ×100 ∆T
UNIT : °C-1
• COSINE ERROR Actual reading of Pyranometer × 100 Cosine Error = l η cos θ
PARAMETERS OF PYRANOMETERS (contd.) • AZIMUTHAL ERROR Variation in output of the pyranometer as Azimuthal Angle alone is changed. • LINEARITY Output of the Pyranometer should be Proportional to the intensity of the Irradiance but it is not so in the true sense.
PARAMETERS OF PYRANOMETERS (contd.) •
• • • •
TILT ERROR Calibration Factor Changes if the tilt of the Instrument is changed from 0° to any other value. Eppley PSP model shows no tilt error. SPIRIT LEVEL If the detector is not horiozntal, it will record the radiation higher or lower than the actual value. Horizontality is assured by spirit level. TIME CONSTANT Reponse of pyranometer to a step function. STABILITY Variations of calibration factor with time. Coating peels off, with time. SPECIAL RESPONSE Response should be uniform over 0.3 to 3.0 µm range. RELATED SITUATIONS MOISTURE Silica Gel DEPOSITION Frost, Dew, Bird NEGATIVE VALUES Detector irradiates at night READING EXCEEDS(Ion) Deflection from cloud or building
General characteristics of sensors for radiant energy measurements Effect used
Wave length (µm)
0.4 – 0.75
Classification of pyrheliometers •
STANDARD PYRHELIOMETERS Absolute cavity radiometer Angstrom electrical compensation pyrheliometer Abbot silver – disk pyrheliometer
FIRST – CLASS PYRHELIOMETER Michelson bimetallic pyrheliometer Linke – Feussner iron – clad pyrheliometer New eppley pyrheliometer (temperature compensated) Yanishevsky thermoelectric pyrheliometer
SECOND CLASS PYRHELIOMETERS Moll – Gorczynski pyrheliometer Old Eppley pyrheliometer (not temperature compensated) The smithsonian water – flow pyrheliometer was omitted from the list of standard instrument, but it has been one of the primary standard of the United States.
A PYRANOMETER SHOULD HAVE THE FOLLOWING CHARACTERSTICS The calibration factor must be independent of temperature It should not be wavelength-selective Absence of zero drift Calibration factor must be independent of the intensity Response time should be as small as possible Calibration Factor must be independent of time Temperature response should be minimum Cosine and azimuthal response or spatial variation in the sensitivity of the detector should be minimum Sensitivity should be as large as possible
Typical thermopile used in pyranometers
Measurement of global and diffuse solar radiation on horizontal surface
Measurement of Direct radiation at normal incidence
Eppley Precision Pyranometer
NORMAL INCIDENCE PYRHELIOMETER
Global radiation availability in India
Geographical parameters for four typical Indian Stations
Height above sea level (m)
New Delhi Kokatta Pune Chennai
28.63 22.60 18.48 12.13
77.33 88.45 73.85 80.30
216 6 559 16
Daily global radiation on horizontal surface and on optimum tilt for four different Indian Stations (Unit: MJ m-2 day-1)
New Delhi Kolkatta Pune Chennai
H = daily global radiation on horizontal surface HT = daily global radiation at annual optimum tilt
Annual mean daily solar radiation (M J / m2 day1)
New Delhi Pune Kolkat ta Chenn ai Tilt of Surface (degrees)
Variation of Radiation with Tilt for a South Facing Surface
DURATION OF SUNSHINE HOURS •
A knowledge of the daily and hourly records of the amount of sunshine is necessary for estimating global solar radiation values using regression equations and for optimizing the design of a particular solar collector. This measurement is simpler and sunshine recorders are far less expensive than solar radiation measuring equipments. • The sunshine hours are extensively measured all over the world using Campbell Stokes sunshine recorders. It consists essentially of a glass sphere about 10 cm in diameter with an axis mounted in a section of a spherical bowl parallel to that of the earth, the diameter of which is such that the Sun's rays are focused sharply on a card held in grooves in the bowl. • The sphere acts as a lens and the focused image moves on a specially prepared paper bearing a time scale. Bright sunshine burn a path along this paper. The method of supporting the sphere differs according to whether the instrument is required for operation in polar, temperate or tropical latitudes. • Three overlapping pairs of grooves are provided in the spherical segment to take cards suitable for different seasons of the year. The chief requirement of the sphere is that it should be of uniform, well annealed and colourless glass.
The Campbell-Stokes sunshine recorder
Estimation of Average daily global solar radiation Angstrom proposed the following empirical correlation for computing the average daily global radiation on a horizontal surface: S H = a '+b' a (1) Hc Sp where H = monthly average daily radiation on a horizontal surface,
= average clear sky daily radiation for the location and month in question,
a' , b' = empirical constants,
= monthly average daily actual hours of sunshine,
S p = monthly average daily possible sunshine hours There is an ambiguity in defining clear day and hence to get H c , the above formula was modified using extraterristrial solar radiation, H o
S H = a+b a Ho Sp
Estimation of Average daily global solar radiation (Contd ... ) where Ho is the extraterristrial solar radiation on a horizontal surface and can be calculated as:
24 360 cos L cos δ sin W + πWs sin L sin δ H 0 = lon1 + 0.33 cos n s 180 π 365
where Ws in the sunset hour angle in degrees, n is the average day for the whole month and π is in radians
S a is measured value of actual sunshine hours and measured using Campbell Stokes sunshine recorder. The possible sunshine hours, Sp, can be calculated for a place using the formula
2Ws 2 Sp = = cos −1 (− tan L tan δ ) 15 15
Estimation of Average daily global solar radiation (Contd ... ) Equation (2) can be used for calculating average daily global radiation at a location when data on actual sunshine hours, Sa , possible sunshine hours, Sp , extraterrestrial solar radiation, H0 and values of a and b are known for a nearby location with a similar climate. The constants a and b for a place is found out by plotting a graph between known values of H / H0 and Sa / Sp, as follows:
Slope b a
Sa / S p
Estimation of Average daily global solar radiation (Contd ... ) The regression constants a and b for few Indian stations are:
New Delhi Pune Calcutta Chennai
0.25 0.31 0.28 0.30
0.57 0.43 0.42 0.44
Uses of Solar Energy
• Heating of Water • Heating of Houses (active systems) • Distillation of Water • Cooking of Food • Greenhouse Heating • Drying of Food • Power Generation • Refrigeration and Airconditioning • Passive Heating and Cooling • Production of Very High Temperatures • Industrial Process Heat Systems • Pumping of Water • Direct Conversion of Electricity (PV)
FLAT PLATE COLLECTORS • The flat plate collector forms the heart of any solar energy collection system and can be employed to heat fluid (liquid or air) from ambient to near 100°C. • The term ‘flat plate’ is slightly misleading since the absorbing surface may not necessarily be flat but may be grooved and other shapes. • Flat plate collectors are under investigation for the last 300 years. The first reported flat plate collector was demonstrated by Mr. H.B. Saussure, a Swiss scientist during the second half of the seventeenth century. • During the last six decades scientists in several countries mainly in USA, UK, Australia, Israel, Germany, South Africa, China and India have built, tested, studied and optimized different types of flat plate collectors mainly liquid heating flat plate collector.
FLAT PLATE COLLECTORS Pioneering work on solar flat-plate collectors have been done by Hottel, Whillier and Bliss in USA who mathematically modelled the collector and gave Hottel-Whillier-Bliss equations to understand the collectors. Later Prof. H.Tabor in Israel has done significant work on understanding the behaviour of collectors and gave several original ideas like convectionsuppression, selective black coatings and evacuated collectors. Significant work on flat-plate collectors was done by Prof. H.P.Garg in India and gave methodology for optimizing the collector configuration, designing the collector, thermal rating procedure of collectors, thermal loss optimization, collector tilt optimization and dirt correction factor, etc.
FLAT PLATE COLLECTORS • Flat plate collectors are of two type: liquid heating type and air heating type, • The most obvious difference between the two is the mode of heat transfer between the absorber plate and the heated fluid, • In the best type of liquid – plate collector, which generally makes use of a fin-tube construction, heat absorbed is transferred to the tubes by conduction, • In a conventional flat-plate air heater there is a duct (passage) between the absorbing plate and rear plate. Thus the difference being in the heat transfer exchanger design. • Other components like glazing, insulation, casing, orientation, tilt, exposure, etc. remain the same.
Schematic cross-section of a typical flat plate solar collector illustrating the major functional parts
Flat Plate Collectors • The main purpose of the collector is to absorb the sun’s energy and transfer this energy efficiently to the liquid flowing in it. There is a great variety of flat plate collectors, but a tube in plate type of collector, is widely used. The collector can be all metallic or plastic, single glazed or double glazed, selectively coated or ordinary black painted depending on the temperature of operation and outside climatic conditions. • As is seen earlier, a flat plate collector has the following components: – A blackened or selectively coated flat – absorbing plate, normally metallic, which absorbs the incident solar radiation, convert it into heat and conducts the heat to the fluid passages. – Tubes, channels or passages attached to the collector absorber plate to circulate the fluid required to remove the thermal energy from the plate.
COMPONENTS OF FLAT PLATE COLLECTOR (contd.) • Insulation material provided at the back and sides of the absorber plate whose principal function is to reduce heat loss from the back and sides of the absorber plate. • A transparent or translucent cover or covers whose principal functions are to reduce the upward heat losses and to provide weather proofing. • An enclosing box whose principal functions are to hold the other components of the collector and to protect the collector plate and insulation material from the weather. Collectors generally available in the market, although confirming to the above general design, have some differences between them. The components most often changed are the absorber plate configuration, the black coating on the absorber plate, and the glazing.
Improving Efficiency of a Flat-Plat Collector The efficiency can be improved by: • Improving transmittance - absortance product, • Reducing thermal losses (conduction, convection and radiation), • Improving heat transfer coefficient from absorbing plate to the working fluid, • Optimizing collector configuration for better heat exchanger efficiency, • Optimizing tilt, orientation and exposure of collector
Transparent Cover Plate The function of cover plates are: • • •
Transmit maximum solar radiation, Minimize upward heat loss from absorber plate to the environment, Protecting the absorber plate from weather.
The most critical factors for the cover plate materials are: – – – – –
Strength Durability Non-degradability Cost Solar-energy and thermal energy transmittance
Tempered glass is the most common cover material for collectors because of its proven durability and stability against UV radiation. Tempered glass cover, if properly mounted, is highly resistant to breakage both from thermal cycling and natural events.
Antireflective coatings •
All transparent materials (like glass) reflect some light from their surfaces. By using a thin film having a refractive index between that of air and transparent medium, the reflectance of the interfaces can be changed. For normal incidence, the fraction of light reflected is given by:
n2 − n1 R = n2 + n1
Where n2 and n1 are the refractive indices of the transparent sheet and the medium respectively. Coating the surface with a non-absorbing film will reduce the reflectance.
Insulation materials for Flat-Plate Collectors
• Several thermal insulating materials which can be used to reduce heat losses from the absorbing plate and pipes are commonly available. • The desired characteristics of an insulating material are: – – – – – – –
Low thermal conductivity, Stability at high temperature (upto 200°C), No degassing upto around 200°C, Self-supporting feature without tendency to settle, Ease of application, No contribution in corrosion, and Low cost.
• Some of the good insulating materials are: glass wool, fibre glass, rock wool, polyurethane, cork etc.
SELECTIVE BLACK COATINGS • For efficient collection of solar radiation, the absorber surface should absorb more solar radiation and emit less thermal radiation. • This selective behavior is possible since solar radiation is in the wavelength range of 0.2 – 2.5 µm while thermal radiations emitted from a surface at temperature more than 100°C is above 5.0 µm. • An ideal selective coating would be one with absorptance (α) = 1 in the range of 0.2 – 2.5 µm and emittance (ε)=0 in the operating temperature range (above 100°C or 3.0 – 7.0 µ m wavelength range). • Practical selective black coating will have α/ε as high as possible.
SELECTIVE BLACK COATINGS (contd.)
• There are four principal types of selective surface (opague). • The first is one which absorb and emit as much radiation as possible at all wavelengths and is known as black body. • The second surface will absorb more solar radiation and emit less radiation. The example is nickel black on a polished substrate. • The third surface will absorb less solar radiation and emit more radiation. The example is white paint on a metal sheet. • The fourth surface will absorb less solar radiation and emit less radiation. The example is aluminium foil.
Reflectance of selective coatings
Collector – Plate configuration 1. Integral construction • •
Tube wall should be thick to withstand fluid pressure and prevent corrosion. Here tube thickness is one half the plate thickness resulting in an ultra thick weight and costs 50% more than tube and fin absorber.
2. Tube and Fin construction (Mechanical Jointing) • •
Simple construction but shows poor bonding resulting in poor heat transfer. Therefore the contact area should be large and joint should be uniformly tight.
3. Tube and Fin construction (Adhesive or soldered bonding) • • •
This type of jointing is better than mechanical jointing but suffers from low thermal conductivity. For better heat flow large contact area, and thin and continuous layer of bonding material are necessary. The bonding material may deteriorate with aging and thermal cycling.
4. Tube and Fin construction (metallurgical bond) • •
A good joint from mechanical strength point of view but shows low thermal conductivity compared to solder bonding. High plate thickness required.
5. Tube and Fin construction (Forge welding ) • •
Tube and Fin of different materials can be used. High thermal conductivity.
ENERGY BALANCE ON A FLAT PLATE COLLECTOR The useful energy derived from a flat plate collector is the difference between the energy absorbed and the energy lost from the collector. For a flat plate collector of area Ac the energy balance equation is written as :
I Tt (Tα ) e = qu + q1 + Where
d ic = qa dτ
(Tα)e = effective transmittance-absorptance product of the absorber given as
τα 1 − (1 −α) ρd
The flat plate collectors are always oriented and tilted (fixed) so that they receive maximum solar radiation during the desired season of use. But the solar radiation is generally measured on the horizontal surfaces so these values require conversion to use on tilted surfaces. In unit time, an unit area of the absorber will absorb energy qa given by
qa = [ I Th − I dh ) RDτ Dα D + I dh Rdτ d α d + I Th RR Rτ Rα R ]DS
ENERGY BALANCE ON A FLAT PLATE COLLECTOR •
Under steady state conditions, the heat balance of the absorber is given by the simple equation:
(useful heat collected) = (heat absorbed by the plate) - (heat losses) qu = ITt(τα)e - UL(Tp - Ta)
Usually the plate temperature Tp given in equation (3) is not known and is difficult to calculate or measure since it is a function of several parameters discussed earlier.
More useful for design is a relation in which Tp is replaced by the inlet fluid temperature Ti and the whole right hand side is multiplied by a term FR, the heat removal efficiency factor, which depends on collector design details and fluid flow rate. qu = FR[ITt(τα)e - UL(Ti - Ta)]
The three design factors, FR, (τα)e and UL are measures of thermal performance and combine to yield overall collector efficiency in terms of the operating variables of temperature and insolation.
ENERGY BALANCE ON A FLAT PLATE COLLECTOR
The instantaneous efficiency of a collector, η c is simply the ratio of the useful energy derived to the total solar energy falling on the collector, or q
Ac I Tt
Usually, the efficiency is computed over a finite time period, τ, and therefore the expression for average efficiency is as follows: τ
Ac I Tt dτ
where τ is the time period over which the performance is averaged. Thus instantaneous efficiency using equation 4 & 5 of the flat plate collector is given as:
η = FR (τα ) e − FRU L
(Ti − Ta ) IT
ENERGY BALANCE ON A FLAT PLATE COLLECTOR
Indicating that if η is plotted against (Ti – Ta)/IT a straight line will result, with a slope of FRUL and y- intercept of FR(τα)e. This is the way actual performance data for solar collectors are presented. The collector heat removal factor may be calculated from the following equation :
U L Ac Fp m C p FR = 1 − exp − U L Ac m C p Fp =
actual useful energy collected useful energy collected if the entire absorber surface is at the local fluid temperature
Where, Fp= collector plate efficiency factor.
The Eq. (3) can now be written as:
qu = Fp [ I T (ατ ) e − U L (Tm − Ta ) Ac Where, Tm is the average fluid temperature
ENERGY BALANCE ON A FLAT PLATE COLLECTOR
(Cont.) The plate efficiency factor (Fp) for a tube in plate type of collector may be calculated from the following equation:
I /UL 1 mt 1 1 w + + + π Dh π DK C U [ D + ( W − D ) F fi t b L
Where w = centre-to-centre tube spacing D = outside diameter of the tube hfi= tube-to-fluid (film) heat transfer coefficient Kt = thermal conductivity of tube Cb = bond conductance ( = Kb b/t) Kb = bond material thermal conductivity b = bond width t = bond thickness mt = tube thickness F = fin efficiency factor given as:
tanh[a ( w − D) / 2] F= a( w − D) / 2
ENERGY BALANCE ON A FLAT PLATE COLLECTOR where
wU L = heat transfer resistance from inner surface of tube to πdhc the fluid, wU L mt = conduction of heat from outside wall to inside wall of tube, πdK t wU L = Cb
conduction of heat from the fin to the tube through the tube fin bond,
wU L = conduction of heat along the fin towards the U L (b + F ' ( w − b)) pipe,
LOSS COEFFICIENT OF FLAT PLATE COLLECTORS The overall heat loss coefficient UL =Ql/(Tp-Ta) is made up of three components – top loss coefficient Ut, the bottom loss coefficient Ub, and the edge loss coefficient Ue: UL = Ut + Ub + Ue
The bottom loss coefficient, Ub, is simply the ratio of the thermal conductivity of the insulation (Ki) beneath the absorber plate to the thickness li: Ub = Ki / li Ac
Likewise, the edge loss coefficient is the ratio of the thermal conductivity of the insulation at the edge to the thickness, times the ratio of the area of edge Ae to the collectorty effective aperture thermal conductivi of insulation at area edgeAc:
thickness of insulation at the edge
Ae × Ac
LOSS COEFFICIENT OF FLAT PLATE COLLECTORS (Cont.)
The modified equation as given by Garg for Ut is : 1
N 1 + 0.252 3 (204.429 / Tp ) L cos β (Tp − Ta ) /( N + f ) / L hw σ (Tp 2 + Ta 2 )(Tp + Ta ) + w / m 2 OC 1 2N + f − 1 + − N εg ε p + 0.0425 N (1 − ε p )
where f = (9/hω - 30/h2ω ) (Ta/316.9) (1+0.091 N) where Tp = absorber plate temperature (k)
LOSS COEFFICIENT OF FLAT PLATE COLLECTORS
Ta = ambient temperature (k) N = number of transparent cover plates ε p = thermal emissivity of absorver plate surface ε g= thermal emissivity of the cover plate (for glass, ε g= 0.88) β = Collector slope (degrees) σ = Stefan – Boltzman constant = 5.67 ×10-8 W/m2 k4 hw = convective heat transfer coefficient due to wind (w/m2 °C) = 2.8 + 3.0 V V = Wind speed (m/sec)
COLLECTOR CONFIGURATION • The collector system considered here is of the pipe and fin type as shown below:
• Which is supposed to be the best choice for domestic as well as industrial water heating requirements. The possible materials of the fin (Kp) may be copper, aluminum, steel or galvanized iron of thickness (mp) 0.091 cm, 0.071 cm, 0.056 cm, 0.046 cm and 0.038 cm. Similarly the pipe may be of copper, aluminum, steel or galvanized iron of inner diameter (d) as 1.27 cm, 1.91 cm and 2.54 cm, spaced (w) at 2.5, 5.0, 7.5, 10.0, 12.5, 15.0, 17.5, or 20.0 cm. The bond conductance is taken as 10, 20, 30, 40 (W/m°C).
COLLECTOR CONFIGURATION Thus from all above description we conclude that the tube spacing, its diameter, its material; fin material and its thickness; heat transfer coefficient; bond conductance; heat loss coefficient are all directly related to the system performance. Therefore the aim of the designer should be the best cost effectiveness which is a function of efficiency and cost. The main scope for reducing the cost lies in selecting the optimum combination of pipe spacing and fin thickness for a particular material of pipe and fin. Material cost will be reduced by increasing the spacing between pipes and by making the plate thinner. However this leads to a reduction in fin efficiency, plate efficiency factor and overall system performance. Therefore the aim should be to determine the combination of pipe spacing and plate thickness, which will minimize the ratio of cost to useful energy collected by the system
Optimization of collector configuration • Optimization of collector configuration means the selection of best combination of plate and pipe materials pipe to give maximum efficiency at minimum cost. • Several parameters and combinations of material that can be used for a flat-plate collector as shown in the equation of plate efficiency factor have been used along with the associated cost of each combination and minimum value of C/Fp (cost/efficiency) for each geometry calculated. • The optimized configuration for a minimum value of cost/efficiency is for the following specifications of flat-plate collector: Plate material : Aluminum Thickness of plate : 28 SWG Tube material : Galvanised Iron Tube diameter : 19 mm Tube to tube spacing : 10 cm
The photograph of an optimised collector plate
Optimization of Collector Tilt and Orientation • •
A flat-plate collector is always titled and oriented (fixed) in such a way that it receives maximum solar radiation during the desired season of use. Since in northern hemisphere such as in India, sun appears to be moving from east to west via south, the collector should face exactly towards the south. Deviation of 5-10 degrees from south towards east or west will not effect the performance much. The exact south at a place can be determined at solar time using plumb line. A detailed scientific analysis for finding out optimum tilt for flat plate collectors was conducted by Prof. H.P.Garg considering, direct and diffuse solar radiation separately, transmittance of glass cover with angle of incidence; place(L), date(δ) and time of day(ω) and derived an expression of optimum tilt(βopt). Based on this equation and curves developed for different Indian stations, following thumb rules are derived for collector tilt: – For Winter performance (November-February), the collector tilt can be latitude of the place plus 15 degrees (L+150), – For summer performance (March-October), the collector tilt can be latitude of the place minus 15 degrees (L-150), – For year round performance (January-December), the collector tilt can be 0.9 times the latitude(0.9L0).
THERMAL TESTING OF SOLAR COLLECTORS There are variety of solar collectors and each behave differently under different climatic conditions, operating parameters and design variables. Hence there was a need of unified approach for thermally rating the collectors for finding out instantaneous efficiency, effect of angle of incidence of solar radiation and determination of collector time constant (a measure of effective heat capacity). National Bureau of Standards (NBS) of USA in 1974 developed the first procedure for testing and thermal rating of collectors (as proposed earlier by Garg & Gupta) which was later modified by ASHRAE in 1977 and is known as ASHRAE Standard 93-77. The ASHRAE 93-77 was adopted with some minor changes in many countries of the world including India.
THERMAL TESTING OF SOLAR COLLECTORS (contd.)
The collector performance equation as discussed earlier are:
Qu = m C p (To − Ti )
Qu = Ac FR [ITt (τα)e – UL (Ti-Ta)]
Qu FRU L (Ti − Ta ) ηi = = FR (τα ) e − Ac I Tt I Tt
m C p (To − Ti ) Ac I Tt
These equations are the basis of the standard test procedures.
THERMAL TESTING OF SOLAR COLLECTORS (contd.)
The general test procedure is to operate the collector in the test facility under nearly steady conditions, measure the data to determine Qu from Equation (1), and measure ITt, Ti, and Ta which are needed for analysis based on Equation 3. Of necessity, this means outdoor tests are done in the midday hours on clear days when the beam radiation is high and usually with the beam radiation nearly normal to the collector. Thus the transmittance – absorptance product for these test conditions is approximately the normal incidence value and is written as (τα)n. Tests are made with a range of inlet temperature conditions. To minimize effects of heat capacity of collectors, tests are usually made in nearly symmetrical pairs, one before and one after solar noon, with results of the pairs averaged. Instantaneous efficiencies are determined from ηi=mCp(To)/AcITt for the averaged pairs, and are plotted as a function of (Ti-Ta)/ITt). A sample plot of data taken at five test sites under conditions meeting ASHRAE 93-97 specifications, is shown in figure.
THERMAL TESTING OF SOLAR COLLECTORS (contd.)
If UL, FR, and (τα)n were all constant, the plots of ηi versus (Ti-Ta)/ITt would be straight lines with intercept FR (τα)n and slope – FR UL. However, they are not, and the data scatter. We know that UL is a function of temperature and wind speed, with decreasing dependence as the number of covers increases. Also, FR is a weak function of temperature. And some variations of the relative proportions of beam, diffuse, and ground-reflected components of solar radiation will occur. Thus scatter in the data are to be expected, because of temperature dependence, wind effects, and angle of incidence variations. In spite of these difficulties, long time performance estimates of many solar heating systems, collectors can be characterized by the intercept and slope [i.e.
Performance curve of a solar collector
Longterm Average Performance of Flat-Plate Collectors •
Generally the performance of solar collectors is given by instantaneous efficiency on clear days.
The true performance of solar collector will depend on cloudiness of atmosphere and varying angle of incidence.
Longterm performance can help in optimizing the design and evaluation of economics.
Two methods are generally employed for longterm performance: ix) Computer simulation method using longterm weather data ii) Utilizability (Φ) method as given by Liu and Jordan using monthly average hourly radiation and temperature data •
Using Hottel-Whillier-Bliss equations and longterm monthly average solar radiation and ambient temperature data, utilizability curves were produced for various cloudiness indices or cities of USA.
Using the same analogy design curves of several Indian stations both for summer months and winter months were produced by Garg for flatplate liquid heating collectors.
for winter use
for summer use
Design curves for Flat Plate Collector
A typical air-heating solar collector
Flat plate air heating collectors • A Conventional air heater is typically a flat passage between two parallel plates. One of the plates is blackened to absorb incident solar radiation. One or more transparent covers are located above the absorbing surface. Insulation around the sides and base of the unit is necessary to keep heat losses to a minimum. • There are eight variables that a designer concerns himself with in the construction of an air heater; – Heater configuration is the aspect ratio of the duct and the length of the duct through which the air passes. – Airflow: Air must be pumped through the heater; increasing the air velocity results in higher collection efficiencies, but also in increased operating costs. – The type and number of layers of glazing must be considered and spectral transmittance properties must be examined.
Flat plate air heating collectors (contd.)
– Absorber plate material: although selective surfaces can significantly improve the performance of solar air heaters by increasing the collector efficiency, blackpainted solar heaters are commonly used due to the cost of selective surfaces. The absorber need not be metal, since the air to be heated is in contact with the entire absorbing surface This means that the thermal conductivity of the absorber plate is relatively unimportant. – Natural convection barriers: a stagnant air gap interposes a high impedance to convective heat flow between the absorber plate and the ambient air. The losses, both of radiation and convection, can be reduced to low values by the use of multiple covers or honeycombs, but the consequent reduction in transmission of solar radiation makes more than one air gap of doubtful value.
Flat plate air heating collectors (contd.) – Plate-to-air heat transfer coefficient: the absorber can be roughened and coated to increase the effective coefficient of heat transfer between the air and the plate. The roughness ensures a high level of turbulence in the boundary layer of the flowing air steam. For this reason, crumpled or corrugated sheets and wire screens are attractive as absorbing materials. – Insulation is required at the absorber base to minimize heat losses through the underside of the heater. – Solar radiation data corresponding to the site are needed to evaluate heater performance.
TUBULAR SOLAR ENERGY COLLECTORS There are two methods for improving the performance of solar collectors. The first method increases solar flux incident on the absorber by using some type of concentrators. The second method involves the reduction of heat loss from the absorbing surface. Tubular collectors or evacuated tube collectors (ETC) with their inherently high compressive strength and resistance to implosion, are the most practical means for eliminating convection losses by surrounding the absorber with a vacuum of the order of 10-4 mm of Hg.
TUBULAR SOLAR ENERGY COLLECTORS (contd.) • Tubular collectors have several advantages. They may be used to get small concentration ratio (1.5-2.0) by forming a mirror from part of the internal concave surface of a glass tube. This reflector can focus radiation on to the absorber inside the tube. • Performance may also be improved by filling the envelope with high-molecularweight noble gases. External concentrators of radiation are generally used in an evacuated receiver for improvement of its performance.
TUBULAR SOLAR ENERGY COLLECTORS (contd.)
Several versions of evacuated tube collectors are manufactured by industries such as Philips in Holland and Sanyo in Japan. With the recent advances in vacuum technology. evacuated tube collectors are reliably mass produced mainly in China. Their high temperature effectiveness is essential for the efficient operation of solar air-conditioning systems and process heat systems and now even for water heating.
Schematic diagram of concentric-tube collector optics; (b) cut-way view of evacuated tube solar collector manufactured by Owens-Illinois, Inc., USA
Chinese Solar tube collector
Chinese Solar Tubes Borosilicate Glass (3.3) Glass-glass seal (not metal to glass) Selective absorber coating (sputtered) Thermal absorption of 92% Excellent thermal insulation = performance Passively track sun throughout the day Silver (barium getter) vacuum indicator Strong (excellent hail resistance) Long lasting performance Cheap and easy to replace if damaged
SOLAR POND •
A solar pond is a body of water that collects and stores solar energy. Solar energy will warm a body of water (that is exposed to the sun), but the water loses its heat unless some method is used to trap it. Water warmed by the sun expands and rises as it becomes less dense. Once it reaches the surface, the water loses its heat to the air through convection, or evaporates, taking heat with it. The colder water, which is heavier, moves down to replace the warm water, creating a natural convective circulation that mixes the water and dissipates the heat. The design of solar ponds reduces either convection or evaporation in order to store the heat collected by the pond. A solar pond can store solar heat much more efficiently than a body of water of the same size because the salinity gradient prevents convection currents. Solar radiation entering the pond penetrates through to the lower layer, which contains concentrated salt solution. The temperature in this layer rises since the heat it absorbs from the sunlight is unable to move upwards to the surface by convection. Solar heat is thus stored in the lower layer of the pond. The solar pond works on a very simple principle. It is well-known that water or air is heated they become lighter and rise upward. Similarly, in an ordinary pond, the sun’s rays heat the water and the heated water from within the pond rises and reaches the top but loses the heat into the atmosphere. The net result is that the pond water remains at the atmospheric temperature. The solar pond restricts this tendency by dissolving salt in the bottom layer of the pond making it too heavy to rise. A shematic view of a solar pond is given in Figure.
Salt gradient solar pond with heat exchanger
MAJOR SALT – GRADIENT SOLAR PONDS (in India) Location
Depth (m) 1.2
Operating experience and behaviour of materials
Max. Temp. 800C in 1972. Worked for two years.
Operating experience and applications for power production.
Getting heated, designed to supply 20 KW. Rankine cycle turbines.
Experience, material behaviour, monitoring & modeling.
Built in 1980. Problems like leaking, algae growth & mineral impurities were observed.
Operating Supplying process experience, material heat to a dairy behaviour and possible applications
Asia’s largest solar pond of 6000 m2 area at Bhuj, Gujarat in 1990/91
Solar Concentrators • Solar concentrators are optical devices which increase the flux on the absorber surface as compared to the flux incident on the concentrator aperture. Optical concentration is achieved by the use of reflecting or refracting elements positioned to concentrate the incident flux onto a desired absorber surface. • A solar concentrator usually consists of (i) an optical device to focus solar radiation (ii) a blackened metaliic absorber provided with a transparent cover, and (iii) a tracking device for continuously following the sun. • Temperatures as high as 3000°C can be achieved with such devices and they find applications in both photothermal and photovoltaic conversion of solar energy.
Solar Concentrators (contd.) Classifications
• Solar concentrators may be broadly classified into three categories, namely, (i) point focusing (ii) line focusing, and (iii) line focusing of limited extent Point focusing concentrators have circular symmetry and are generally used when high concentration is required. These systems requiring two axis tracking can generate temperature in the range 800-3000°C. Point focusing concentrators are being used for solar thermal power generation purposes. Line focus concentrators have cylindrical symmetry and are generally used when intermediate concentration is required to meet the demand of a desired task. Temperatures in the range of 100-350°C can be generated using line focus concentrators. These systems can be utilised for solar thermal power generation as well as for industrial process heat applications.
Schematic diagrams of different solar concentrators (a) Flat absorber with flat reflectors, (b) Parabolic cencentrator, (c) Compound parabolic concentrator, (d) Fresnel lens, (e) Cylindrical parabolic concentrator
Solar Concentrators (contd.) THERMODYNAMIC LIMITS TO CONCENTRATION The concentration has an upper limit that depends on whether the concentrator is a point focus (three dimensional geometry) or line focus (two dimensional geometry) type. The maximum possible concentration achievable with a concentrator that only accepts all the incident sunlight within an acceptance half angle Qm is given by
1 Sin 2 θ m 1 = Sin θ m
Cmax ( 3 D ) =
Cmax ( 2 D )
Where θ m is the half of the angular substance of the sun at any point on the earth ( = 16' ). The maximum achievable concentration for these two types of concentrators are about. 45,000 and 215 respectively. In practice, however, these levels of concentration are not achievable because of tracking errors and presence of surface imperfections in the surface of reflecting or refracting element.
POINT FOCUSING CONCENTRATIONS To achieve high efficiencies at high temperatures one needs concentrations producing point focus. These concentrations require two axis tracking. Concentrator designs which fall in this category are – a paraboloid of revolution, central tower receiver system and circular freshnel lens etc. Paraboloid of Revolution The surface produced by rotating a parabola about its optical axis is called a paraboloid. With perfect optical surfaces, a parallel beam of light produces a point focus. However, a somewhat enlarged focal point or image is produced due to finite angular substance of the sun. The concentration ratio for a paraboloid can be determined easily from basic geometry but depends on the shape of the absorber. For a spherical absorber it is given by
C sph =
Sin 2 θr
4 Sin 2 ξ0
Where θr is the rim angle of the parabola. Maximum concentration is achieved for
Parabolic Trough Concentrator • Linear concentrators with parabolic cross section have been studied extensively both analytically and experimentally, and have been proposed and used for applications requiring intermediate concentration ratios and temperatures in the range of 100 to 500°C. Figure shows a collector of this type which is part of a power generation system in California. The receiver used with this concentrator is cylindrical and is enclosed in an evacuated tubular cover ; flat receivers have also been used with reflectors of this type. • Designed in a power range of 30 – 150 MW. • Working Principle: – Solar Receiver consists of a large array of parabolic trough reflectors that reflects the sunlight to a receiver tube located along the trough’s focal line. Heat transfer fluid (HTF) flowing in the tube is heated and then transported to a heat exchanger / evaporator for steam and power generation.
Parabolic Trough Concentrator Tracking System
Reflector (Parabolic Trough)
Aperture Edge Angle Focal Length Absorber Diameter
Improvements in the parabolic trough concentrators and systems since 1982 Feature
Turbine Cycle efficiency
Large Area Solar Dish at Milk Dairy at Latur, Maharashtra
A large area solar dish has been developed to provide process heat for milk pasteurization at a dairy of Maharashtra Rajya Sahakari Dugdh Mahasangh Maryadit (MRSDMM), Maharashtra under a R&D project sponsored by MNRE to IIT Bombay jointly with M/ s. Clique Developments Pvt. Ltd. (CDPL), Mumbai. The solar dish has been installed and commissioned. The technical specifications of the solar system are Aperture Area 160 m2 Reflector area 123 m2 Thermal power (annual average) 50-70 kWth Annual operating hours 3200-3350 hours/ year Annual fuel savings (Furnace oil) 16 to 24 kilo litre/ year Operating wind speed up to 54 kmph Survival wind speed up to 140 kmph Aerial clear space required for the dish 25 m x 20 m x 18 m height Clear area required on ground / roof 3 m x 3 m Tracking power 500 W
Solar Water Heating • Solar Water Heaters (SWH) have been extensively used for the last more than 8 decades. • The countries where these are extensively studied are USA, Australia, U.K., Israel, South Africa and India. • The countries in which Solar Water Heaters are extensively used are : USA, Australia, U.K., Germany, India, Jordan, Israel, Cyprus, China, Greece, Japan, Sweden and several other countries. • In recent years considerable knowledge has been developed about solar hot water systems. • Basically solar water heaters are either for domestic applications, large applications or swimming pool water heating applications.
TYPES OF SOLAR WATER HEATER •
Built-in-storage type Solar Water Heater (Integrated – collector storage type)
Domestic Solar Water Heaters (Natural Circulation type / thermosyphon type)
Large Size Solar Water Heater (Industrial type)
Swimming Pool Water Heater
Domestic Solar Water Heaters Many different designs of solar water heaters are possible and they may be classified in many ways. Each type has its own advantages and disadvantages, and depending on the situation a particular design is recommended. Some of the solar water heating configurations are as follows : A direct natural circulation solar water heater. An indirect natural circulation solar water heater. An indirect forced circulation type solar water heater. A single cylinder indirect forced circulation solar water heater. An indirect system with air heat collectors. In general it can be said that a solar water heating system consists of the following components : Flat plate collectors Storage tank Heat exchanger Automatic control Pumps, pipe work, valves and fittings
Conventional Domestic Solar Water Heater
Working Principle of Solar Water Heating System
Natural circulation type solar water heater (Schematic)
Simple model for Natural Circulation Type SWH It has been experimentally observed that in a SWH, the inlet (Ti) and outlet (To) water temperature rise for a collector is nearly constant and generally it is about 10°C. Thus (To-Ti) = 10°C Thus we can calculate the natural flow rate (m ) using collector equation
Qu = FR AC [ H (τα )e − U L (Ti − Ta )]
Cp (To − Ti ) = m Cp∆Tf Qu = m
FR AC [ H (τα )e − U L (Ti − Ta ) = m Cp∆Tf
Substituting the values of FR we get,
m = 40 litres /m2 hr
Collector inlet (Ti) and outlet (To) temperature for a natural circulation water heater
THE STORAGE TANK
o o o o
The storage tank stores the heat collected during the day for use when needed. For the storage of hot water, copper, steel, galvanized iron, aluminium, concrete, plastic, and sometimes wooden tanks are used. The tank should be sized to hold between 1.5 and 2 days supply of hot water. The auxiliary heating arrangements may be electric or gas booster and thermostat should be fitted in the central part of the tank and not in the bottom of the tank. For domestic purposes, the thermostat setting is done between 50-60°C. There are many variations in the tank design and a few are listed below : Vertical or horizontal type Pressure or non-pressure type Gas, electric or solid fuel booster, off-peak or continuous tariff, or Internally or externally mounted. There is very little information available on system performance for the above storage types.
Some Common Liquid to Liquid Heat Exchanger Designs for Solar Energy Use
Recommended Way of Connecting Bank of Collectors
Positioning of Differential Controller
Schematic of forced circulation solar hot water system with 3 different schemes for supplying auxiliary energy
Solar Water Heaters • Hot water at 60-80oC for hotels, hospitals, restaurants, dairies, industry and domestic use. • System comprises one or more collectors, storage tank, piping etc. Heat exchanger and pumps added, if necessary. • About 2.15 million sq.m. collector area installed. • BIS standard for collectors introduced in 1990/1992. Standards updated recently. • 60 BIS approved manufacturers with production capacity of over 300,000 sq. m. collector area per annum.
120,000 LPD CAPACITY SOLAR WATER HEATER AT GODAVARI FERTILISER & CHEMICALS LTD.
As boiler feed water for steam generation Godavari Fertilizers & Chemicals Ltd., Kakinada Quinn India Ltd., Hyderabad Shivamrut Dudh Utpadak Sahakari Sangh Ltd., Akluj
: 1,20,000 lpd : 75,000 lpd : 30,000 lpd
Hot water for multistoried residential complex DS Kulkarni Developers Ltd., Pune : 56,400 lpd at 60o C
SOLAR DOMESTIC HOT WATER SYSTEMS IN Pune (India)
SOLAR DOMESTIC HOT WATER SYSTEMS IN ISRAEL
Why Solar Cookers ? • High cost or Unavailability of commercial fuels – Kerosene, Coal, Gas, Electricity • Deforestation caused by Increasing Firewood Consumption • Use of Dung and Agricultural Waste as Fuels Instead of for Soil Enrichment • Diversion of Human Resource for Fuel Gathering
Types of Solar Cookers • Direct or focusing type solar cooker – In these cookers some kind of single or multifacet solar energy concentrator (parabolic, spherical, cylindrical, fresnel) is used which when directed towards the sun focus the solar radiation on a focal point or area where a cooking pot or frying pan is placed. In these cookers the convection heat loss from cooking vessel is large and the cooker utilizes only the direct solar radiation.
• Indirect or Box type Solar Cooker – In these cookers an insulated hot box (square, rectangular, cylindrical) painted black from inside and insulated from all sides except window side which is double glazed is used. Single plane or multiple plane reflectors are used. Some times these are also known as oven type solar cookers. These can be electrical cum solar cookers and some cookers utilize a kind of latent heat storage material.
• Advanced type Solar Cooker – In these cookers, the problem of cooking outdoors is avoided to some extent. The cookers use either a flat plate collector, cylindrical (PTC) concentrator, or a multifacet or large parabolic (mosaic type) concentrator which collect or focuses the solar heat and transfers or reflect from a secondary reflector to the cooking vessel. The cooking in some cases can either be done with stored heat or the solar heat is directly transferred to the cooking vessel in the kitchen.
BOX SOLAR COOKER
Solar Box – type Cooker : Design Details Component
Thickness / size
Requirements / Remarks
•Galvanished iron •Aluminium •FRP
0.48 mm thick (60 x 60 x 17 cm) 0.56 mm thick (60 x 60 x 17 cm) 2 mm thick (60 x 60 x 17 cm)
Resistant to ultraviolet radiation and atmospheric variations
Inner Cooking Box
0.56 mm thick (50 x 50 x 10 cm)
Painted dull black Should not touch outer body
Insulation (Back and side)
Glass fibres in the form of pads
5 cm or more thick k = 0.052 W/m K
Free from resin binders Stable upto 250°C
Glazing (Double glass lid)
Water white glass (Temperated / toughned)
3-4 mm thick 50 x 50 cm size spacing between sheets 1 cm
Double glass system must be air tight Transmittance > 85%
Silvered or Glass aluminized
4 mm thick 54 x 54 cm
Reflectivity > 85% Scratch resistant Resistant to solar radiation and atmospheric variation
•Aluminium alloy 1.2 mm thick sheet Two pots – dia 200 m •Stainless Steel sheet Two pots – dia 150 mm Depth of pots – 67 mm
Dull black painted stable upto 250°C Very good adhesive characteristics
Solar Box-type Cooker : Cooking Time for Recipes It takes about 2 – 2.25 hours for cooking depending upon the kind of food and season. Different items like dal, rice, vegetables etc. are normally cooked simultaneously in separate containers. The time taken for cooking is less in summer than in winter.
SK - Type Solar Cookers (SK-10, SK-12, SK-14, SK-98) • SK – Solar Cooker uses parabolic reflector made of thin, hard aluminium sheets with protected, high reflecting surface mounted at a rigid basket structure. • Reflector with short focal distance for safety reasons, long tracking intervals and high efficiency. • Cooking pot in a standard 12 – litres pot of black enameled steel with a diameter of 28 cm. • Tracking is done by moving the whole cooker (azimuth) and by turning the reflector around the horizontal axis (elevation), adjustment of the reflector to the sun by use of a shadow indicator.
Technical Data (SK Type Solar Cooker) • • • • • •
Reflector diameter : Nominal effective power : Pot capacity : Pot diameter : Max. temperature : Capacity :
• Tracking • Cost • Cooking Food
: : :
140 cm 0.6 kW 12 litres 28 cm 200°C Boils 48 litre of water in a day Manual INR Rs. 6000/10-15 people at a time
Parabolic solar cooker, not only for cooking …
… but also for baking, frying, conserving, and much more …
Parabolic Domestic Solar Cooker (SK 14)
World's Largest Solar Steam Cooking System at Tirupati, Andhra Pradesh
Location • Installed at the temple town of Tirumala, Andhra Pradesh with nearly 50 percent funding from MNRE. System • Employs automatic tracking solar dish concentrators to convert water into high pressure steam which Is used for cooking purpose in the community kitchen. Technical Details • Solar dish concentrators (106 Nos) with total reflector area of about 1000m2. • Modular in nature and consists of several units (parallel & series) connected to central pipe-line system. • Each dish consists of scheffler mirrors with an aperture area of 9.4 sq.m. • Generates 4,000 kg of steam per day at 180°C and 10 Kg/cm2. • Cook meals for around 15,000 persons per day. • The cooker saves about 1,20,000 litres of diesel per year. • The total cost of the system is about Rs. 110 lakh. Implementing Agency • Ministry of New & Renewable Energy (MNRE).
World’s Largest Solar Steam Cooking System
WORLD’S LARGEST SOLAR STEAM COOKING SYSTEM AT TIRUPATI
Solar Steam Cooking System at Army Mess, Ladakh, Jammu & Kashmir (Installation 12.04.05)
Solar Bowl Cooking Concentrator • Developed at Centre for Scientific Research, Auroville • Capable of Cooking food for 1000 people. • System consists of : – 15 m. diameter non-tracking solar Bowl concentrator – Automatic tracking receiver – Use of thermic fluid to transfer energy collected by receiver for generating steam – Heat storage tank with heat exchanger – Double jacketed cooking pots
Bowl Concentrator (15 m dia) for Community Cooking (1000 people) at Auroville, Pondicherry
Reasons for the non-acceptance of the solar cookers • • • • • • • • • • • • • •
Too expensive for individual family ownership Incompatible with traditional cooking practices too complicated to handle cooking can be done only in the direct sun can not cook at night can not cook in cloudy weather can not cook indoors danger of getting burned or eye damage are not locally available less durable; needs repair or replacement of parts which are not easily available The cooker needs frequent adjustment towards the sun and exposure of the cooking pot to the blowing dust and sand effected the food taste Easy availability of alternative cooking fuels like wood and fuel wood There is no provision of storing the heat therefore cooking of food was not possible where there are clouds or sun is not strong No proper education, training and involvement of women folk
Technical issues need attention for the wider use of solar cookers • • • • • • • • •
Reliability Efficiency Quality Durability Utility Maintenance Weight Servicing Affordability
• • • • • • • • • •
Cost effectiveness Compatibility with food habits Training and education Micro level financing Marketing strategy Local availability Involvement of rural folk Dedication and commitments Provision of storage material Cooking indoors
Solar Buildings • •
• • •
• • •
The function of a Building or a house is to provide shelter to its occupants from weather. Since weather conditions vary from one place to another and vary widely over the year, and humans feel comfortable within certain range of temperatures and humidities, the house are made to provide everyday living comfort. The heating of house in winter and cooling in summer to provide comfort using solar energy or other natural concepts is an ancient concept and is in use since men started to build habitations. Basically solar heating or cooling systems are of two types : Passive heating and cooling and active heating & cooling. Passive systems do not need any mechanical system and are designed such as the glazed area, walls and roofs are made use of collecting, storing and distributing the heat indoors by natural processes of convection, conduction and radiation. Five basic concepts of passive heating are : direct gains, collector storage wall, sunspace collector - storage roof and convective zone. Components of active heating system are : (I) solar collector, (ii) storage device, (iii) auxiliary heating system (iv) Distribution system including fan, duct and controls. To provide near comfort conditions the most cost effective method is to Judiciously make use of both passive and active systems.
Solar Buildings (contd.) • • • •
Everybody needs a comfortable house where activities like sitting, sleeping, dinning, food preparation, storing, studying, recreation, bathing, hobbies, etc. can be conducted. Building site and location is very important. The natural topography and micro climate may significantly effect the performance. The three thermo physical properties, the thermal resistance, heat capacity and solar absorption of surface are very important. There is no fixed thumb rule to find out the optimum combination of various requirements or features. This can be done by using economic methodologies, and performance prediction methods using computer simulation. Several climatic parameters effecting the performance of the building are solar radiation, air temperature its diurnal variation and extreme, air humidity, precipitation its quantity and distribution, wind its speed and direction, incoming and outgoing radiation, sky temperature and sky conditions, sunshine duration, day length and night length. There are several factors which are responsible for thermal comfort such as air temperature, mean radiant temperature, air humidity, air motion, clothing and activity level. Apart from Climatic parameters and thermophysical properties of materials used in the buildings, the Building site, shape, location, orientation, plan, elevation, topogtaphy, microclimate, etc. significantly effect the performance.
Solar Passive Building of Solar Energy Centre
The Solar Passive Building of Punjab Energy Development Agency at
SOLAR PASSIVE BUILDING STATE BANK OF PATIALA, SHIMLA
PARAMETERS FOR SOLAR DRYING •
The drying of product depends on external variables like temperature, humidity and velocity of air stream and internal variables which is a function of drying material and depends on parameters like surface characteristics (rough or smooth surface), Chemical composition (sugar, starch, etc.), physical structure (porosity, density, etc.). and size and shape of the product. The rate of moisture movement from the product inside to the air outside differ from one product to another and very much depends weather the material is hygroscopic or non-hygroscopic. Non- hygroscopic materials can be dried to zero moisture level while the hygroscopic materials like most of the food products will always have a residual moisture content.
The design of a solar dryer depends on : solar radiation, temperature of air, relative humidity of air, moisture content of the product, amount of product to be dried, time required for drying, availability of auxiliary energy, material of construction of dryer and the resource availability.
PHYSICS OF SOLAR DRYING • Heat by convection and radiation to Surface of product → Goes to interior of product • Increase in temperature • Formation of water vapour → Evaporation of moisture from Surface Drying can be accelerated by: • Increasing flow rate of air • Increasing temperature of drying air • Initial Drying - Surface drying, later on drying depends on type of materials. • Non hygroscopic- drying possible upto zero moisture content. • Hygroscopic - grains, fruit, food stuff have residual moisture.
RATIONALE FOR CONTROLLED DRYING 1. Grain • Improves product quality, • Improves storage capability, • Reduces time and space requirement for drying, • Facilitates quick preparation of fields for next cropping, • Facilitates wet season harvesting and storage, • Improves drying hygene. 2. Timber • Improves product quality, • Reduces period capitoltied up in drying stock, • Improves low expertise, low capital, improved drying options, • Expands range of usable timber species, • Improves attainable drying level. 3. Fruits, Vegetables & Fish • Reduces product seasonability, • Improves marketing control of farmer, • Reduces spoilage, • Improves drying hygene, • Improves storage capability, • Reduces nutritional fluctuations.
CLASSIFICATION OF SOLAR DRYERS • DIRECT TYPE DRYERS: In direct or natural convection type dryers, the agricultural product is placed in shallow layers in a blackened enclosure with a transparent cover. The solar radiations are directly absorbed by the product itself. The food product is heated up and the moisture from the product evaporates and goes out by the natural convection. • INDIRECT TYPE DRYERS: In these dryers the food product is placed in a drying chamber. The air is heated in solar air heaters and then blown through the drying chamber. In some of the designs, dryers receive direct solar radiations and also heated air from solar air heaters. In these dryers manipulation of temperature, humidity and drying rate is possible to some extent. • FORCED CIRCULATION TYPE DRYERS: In these dryers, hot air is continuously blown over the food product. The food product itself is loaded or unload continuously or periodically. These kind of dryers are comparatively thermodynamically efficient, faster and can be used for drying large agricultural product. These dryers can be of cross-flow type, concurrent flow type or counter-flow type.
(a) Direct type solar dryers
(b) Indirect type solar dryers
(c) Forced circulation type solar dryers
TYPE OF SOLAR DRYERS
Details of few Solar Drying Systems for Tea Drying in India Location Manjolai Tea Factory, Tirunelveli, TN Golden Hills Tea Factory, Near Coonoor, TN UPASI Demonstration Tea Factory, Coonoor, TN Kavukal Tea Factory, Kothagiri, TN Kilkothagiri Tea Factory, Milkothagiri, TN Parkside Tea Factory, Near Coonoor, TN Pandiar Tea Factory, Near Gudalur, TN Guernesy Tea Factory, Brookland, Coonoor, TN Glendale Tea Factory, Coonoor, TN
Collector area (m2) 130 112 100 220 250 320 320 390 585
Details of a Roof Integrated Solar Air Heating System Installed at Coornoor, Tamil Nadu
Site Latitude Longitude Altitude
Solar Collector Total Area Type Absorber Glazing Air Flow • • •
11°N 77°E 1950 m 212 m2 (Glazed) + 424 m2 (unglazed) Flat Plate Galvanized Iron with black paint 4 mm thick tempered glass 5 – 5.5 kg s-1
In the period 1991-95 nine such units, having a total collector area of about 2700 m2, were installed in South Indian Tea Factories. It is possible to save annually an average of 25% of the fossil fuel used in the tea factories. The payback period for the system is less than 2 years
Leather Dryer with Roof mounted Solar Air Heaters (4 x 167m2 area) at M.A. Khizar Hussain & Sons, Ranipet, Chennai
LEATHER DRIER WITH SOLAR HOT AIR DUCTS AT M/S M.A. KHIZAR HUSSAIN & SONS, RANIPET
Important Conclusions • Experience over the past four decades has shown that inspite of high potential of solar drying it has not taken off. Some of the reasons are; • Systematic work on solar dryer has been done only in few countries. • Solar dryer has not been developed as a system. • In industralized countries, there is great interest towards solar drying. However, neither the temperature nor the heat requirement can be achieved with solar collector. • Solar drying is considered more applicable to low temperature in-storage type drying in tropical and subtropical countries. • Pre-healing of drying air in batch dryers has been demonstrated to be techno-economically viable. • Solar drying should be disseminated for medium and low scale farmers for drying cash crops. • To popularise solar drying, pilot demonstration followed by training and workshop will have to be intensified.
SOLAR DESALINATION TECHNIQUES Potable Water
Less than 550 ppm
Domestic, Industries and Agriculture Rivers, Lakes, Ponds, Wells etc.
Sources of Potable Water Demand of Potable 15-25 litres / person / day Water (OLD) 100-125 litres / person / day (NEW) Underground 2,000 – 2,500 ppm Saline Water Sea Water 30,000 – 50,000 ppm
WATER DESALINATION TECHNOLOGY • Potable water (fresh water) suitable for human consumption should not contain dissolved salts more than 500 ppm. • For agricultural purposes, water containing salt content of 1000 ppm is considered as the upper limit. • Potable water is required for domestic, agriculture and industries. • Some applications in industries like cooling purposes, sea water is feasible despite the corrosion problems while other industries use higher quality water than is acceptable for drinking water. Modern steam power generation plant need water with less than 10 ppm. • Potable/fresh water is available from rivers, lakes, ponds, wells, etc. • Underground saline/brackish water contains dissolved salts of about 2,000-2,500 ppm.
METHODS OF CONVERTING BRACKISH WATER INTO POTABLE WATER • DESALINATION: The saline water is evaporated using thermal energy and the resulting steam is collected and condensed as final product. • VAPOR COMPRESSION: Here water vapour from boiling water is compressed adiabatically and vapour gets superheated. The superheated vapor is first cooled to saturation temperature and then condensed at constant pressure. This process is derived by mechanical energy. • REVERSE OSMOSIS: Here saline water is pushed at high pressure through special membranes allowing water molecules pass selectively and not the dissolved salts. • ELECTRODIALYSIS: Here a pair of special membranes, perpendicular to which there is an electric field are used and water is passed through them. Water does not pass through the membranes while dissolved salts pass selectively. In distillation; thermal energy is used while in vapour compression, reverse osmosis, electrodialysis, etc. some mechanical and electrical energy is used.
Types of Solar Still • • • • • • • • • • • •
Single Effect Basin Solar Still Tilted Tray Solar Still Multibasin Stepped Solar Still Regeneration Inclined Step Solar Still Wick Type Solar Still Multiple Effect Diffusion Solar Still Chimney Type Solar Still Multi-Tray Multiple Effect Solar Still Double Basin Solar Still Humidification Dumidification Distiller Multistage Flash Distiller Solar – Assisted wiped film Multistage Flash Distiller
COMPONENTS OF SINGLE EFFECT SOLAR STILL • • • • • • •
Basin Black Liner Transparent Cover Condensate Channel Sealant Insulation Supply and Delivery System
BASIC REQUIREMENTS OF A GOOD SOLAR STILL • Be easily assembled in the field,' • Be constructed with locally available materials, • Be light weight for ease of handling and transportation, • Have an effective life of 10 to 20 Yrs. • No requirement of any external power sources, • Can also serve as a rainfall catchment surface, • Is able to withstand prevailing winds, • Materials used should not contaminate the distillate, • Meet standard civil and structural engineering standards, and, • Should be low in cost.
Double sloped experimental solar still
SOLAR STILL OUTPUT DEPENDS ON MANY PARAMETERS
1. Climatic Parameters • • • • •
Solar Radiation Ambient Temperature Wind Speed Outside Humidity Sky Conditions
2. Design Parameters • • • • • • • •
Single slope or double slope Glazing material Water depth in Basin Bottom insulation Orientation of still Inclination of glazing Spacing between water and glazing Type of solar still
SOLAR STILL OUTPUT DEPENDS ON Contd… MANY PARAMETERS 1.
Operational parameters • • • • • •
Water Depth Preheating of Water Colouring of Water Salinity of Water Rate of Algae Growth Input Water supply arrangement (continuously or in batches)
Main Problems of Solar Still • • • •
Low distillate output per unit area Leakage of vapour through joints High maintenance Productivity decreases with time for a variety of reasons • Cost per unit output is very high
CONCLUSIONS ON BASIN- TYPE SOLAR STILL
The solar still output (distillate) is a strong function of solar radiation on a horizontal surface. The distillate output increases linearly with the solar insolation for a given ambient temperature. If the ambient temperature increases or the wind velocity decreases, the heat loss from solar still decreases resulting in higher distillation rate. It is observed for each 10°C rise in ambient temperature the output increases by 10 percent.
The depth of water in the basin also effects the performance considerably. At lower basin depths, the thermal capacity will be lower and hence the increase in water temperature will be large resulting in higher output. However, it all depends on the insulation of the still. If there is no lnsulatlon, increase in water temperature will also increase the bottom heat loss. It has been observed that if the water depth increases from 1.2 cm to 30 cm the output of still decreases by 30 percent.
CONCLUSIONS ON BASIN- TYPE SOLAR STILL (contd.)
Number of transparent covers in a solar still do not increase the output since it increases the temperature of the inner cover resulting in lower condensation of water vapour. Lower cover slope increases the output. From practical considerations a minimum cover slope of 10 deg. is suggested. The maximum possible efficiency of a single basin solar still is about 60 percent. For higher receipt of solar radiation and therefore the higher yield the long axis of the solar still should be placed in the East-West direction if the still is installed at a high latitude station. At low latitude stations the orientation has no effect on solar radiation receipt.
ADDITIONAL CONCLUSIONS DRAWN FROM EXPERIMENTAL STUDIES ON SOLAR STILLS •
• • •
The main problem in a solar still Is the salt deposition of calcium carbonate and calcium sulphate on the basin liner which are white and insoluble and reflect solar radiation from basin water and basin liner and thereby lowering the still output. It is difficult to stop the salt deposition. The physical methods suggested to prevent the salt deposition are Frequent flushing of the stills with complete drainage & Refilling or continuous agitation of the still water by circulating it with a small pump. Once the salt gets deposited then the only way is completely draining the still and then scrubbing the sides and basin liner and then refilling the still. Another serious observation made in Australia is the crystalline salt growth which takes place on the sides of the basin and into the distillate trough effecting the purity of distilled water. Some success in preventing the crystalline salt growth is achieved in Australia by pre-treating the feed water with a complex phosphate compound which reduces the rate of nucleation of salt crystals.
ADDITIONAL CONCLUSIONS DRAWN FROM EXPERIMENTAL STUDIES ON SOLAR STILLS • •
Saline water in the still can be supplied either continuously or in batches. In Australia continuous supply of saline water in the solar still is preferred at a rate of about 1.70 I/sq.m hr which Is twice the maximum distillate rate. This helps in reducing the salt deposition from the salt solution. From thermal efficiency point of view, batch filling i.e. filling of saline water when the basin water is coolest (early morning) is the best but it involves greater labour costs and special plumbing arrangements. Algae growth within the solar still also effects the performance to a little extent but its growth must be checked since its growth is unsightly and may finally block the basin and contaminate the distillation troughs. The algae growth can be checked by adding copper sulphate and chlorine compounds in the saline water in the still.
Multi-Effect Humidification – Dehumidification Solar Distillation • Conventional methods of water desalination based on MSF, ME, RO use high energy sources and require heavy investment and infrastructure. • A simple multi-effect humidification – dehumidification solar distillation unit is schematically shown in the figure. A photograph of the unit is also shown. • The complete unit consists of two parts: (i) solar flat-place collectors and storage tank, and (ii) Distillation chamber (in two parts: evaporator and condenser). • Solar array consist of 5 flat – plate collectors each of 2m2 collector area and a hot water storage tank of 500 litres capacity. •
The distillation chamber consists of evaporation (1.50 x 0.5 x 1.0 m) and condensation tower (1.50 x 0.50 x 0.5 m).
Multi-Effect Humidification – Dehumidification Solar Distillation (contd.) • In the solar collector storage tank loop, water gets heated and stored in the storage tank. The circulation is by natural circulation of water. • In the distillation loop, the saline water is preheated as it passes through the coil of the condenser tower and further heated through the coil (heat exchanger) in the hot water storage tank. This hot water gets automatically sprayed from the top in the evaporator.
Multi-Effect Humidification – Dehumidification Solar Distillation (contd.) • The partially humidified air from the condenser side moves through the evaporator (packed bed) in the upward direction, gets fully humidified and goes to the condenser, water from air gets condensed on the outer side of the coil (heat exchanger) in the condenser. • The pure (distilled) water gets collected in the bottom of the condenser and brine from the bottom of the evaporator. • The system is fully automatic and no other energy except the solar energy is used in the process.
Multi-effect Solar Distillation System
Photograph of the multi effect H-D Solar Distillation unit at IIT Delhi
Types of Solar Green Houses • Passive Solar Green Houses: The green houses where the energy is stored directly in heavy brickwalls or rockwalls and/or water pools or water containers exposed to solar radiation and heat is distributed inside the green house by natural means, are known as passive greenhouses. • Active Solar Green Houses: In houses where solar energy is collected and stored and distributed and where some auxiliary energy is employed either for circulation or for distribution or for both. Generally a combination of both active and passive features are employed in solar green house with an objective to minimize the use of auxiliary energy either for heating the green house or for collection – storage-distribution system.
Photograph of the Pipe Frame Solar Greenhouse
Solar Greenhouse in Leh
DIRECT CONVERSION 1 Photovoltaic 2 Photoemissive 3 Photogalvanic 4 Photomagnetic
3. 4. 5. 6. 7.
THERMAL ENERGY Thermoelectric Thermionic Ferroelectricity Magnetohydrodynamics Electrogasdynamics
Few schemes of converting solar energy into electricity
Solar Thermal Power Generation
Solar Thermal Technologies for Power Generation: Global Trends
• Basic Systems – – – –
Collector Receiver Transport Storage Power Conversion
• Major Varieties – Parabolic Trough Solar Energy Generating Systems (SEGS) – Central Receiver Power Plants – Solar Chimney Power Plants – Dish Sterling Systems – Solar Pond Power Plants
Solar Technologies :Thermal
Solar chimney has a large potential for power generation and a number of technological and physical advantages
• It makes use of both direct and diffuse solar radiation. • The natural storage medium – the ground operates as a storage medium. Large ground area will operate the system throughout the 24 hours. • No moving parts except turbine and generator. • No water is required to cool mechanical parts. • It features a simple, low-cost design utilizing know-how and materials locally available almost everywhere. • Most of the cost is labour oriented and would benefit the local labour market while at the same time helping to keep overall costs down.
Principle of operation of solar chimney
Solar Technologies :Thermal Solar Chimney • One 200 MW plant is planned in Australia • Internal estimates for 1 MW plant • Collection diameter of solar glass roof : 1153 m • Chimney height
: 250 m
• Installed Cost
: Rs. 30.00 Cr.
: Rs. 6.20 / kWh
Solar Trough Technology
Reflector (Parabolic Trough)
Aperture Edge Angle Focal Length Absorber Diameter
Solar Technologies :Thermal Solar Trough •
Light Concentration through parabolic troughs- Steel, Al or mirrors
Trough being very large- only single axis tracking possible
Hence low collection efficiency – about 55%
Also max. temp : 450 Deg C
Runs a steam turbine – Overall system efficiency : about 15%
To improve plant use, hybridization with Fossil Fuels possible
Solar Technologies :Thermal Solar Trough •
The earlier systems used ‘heat transfer fluids’ to collect heat
Steam generation through heat exchangers
Latest Advancements: •
Direct steam generation : To reduce cost and increase efficiency
The three basic DSG processes (once-through, injection and recirculation)
Proven Technology: SEGS plants SEGS: Solar Electric Generating System 9 plants at three locations (Daggett, Kramer Junction, Harper Lake), Mojave desert (CA) Individual unit size: 14 to 80 MWe Installed between 1984 and 1990 Total size: 354 Mwe Still operating: actual operator of units III to IX is FPL Energy; electricity customer is SCE
Solar Technologies :Thermal Solar Trough •
Cheapest solar technology
Well established, mainly as hybrid
Leading commercial installations : in US, also planned in Mathania, Rajasthan SEGS Plant
Ist Year of Opn
Net Output (MW)
Solar Field Output Temp (Deg C)
Solar Field Area (Sq m)
Solar Turbine Eff (%)
Fossil Turbine Eff (%)
Annual Output (MWh)
III & IV
140 MW Largest Integrated Solar Combined Cycle (ISCC) Power Plant • Status of the Project Very Recently Approved from Cabinet Committee on Economic Affair (CCEA) for an Investment of Rs. 871 crore. Techno-Economic Clearance (TEC) by Central Electricity Authority (CEA) on 27 August 1999. Completion of ground work for appraisal by Kfw, project has entered into Preparation Phase. Clearance from State Pollution Control Board. 609 bighas land have been acquired.
• Financial Assisting Agencies
Govt. of India (Grant of Rs. 50 crore) Govt. of Rajasthan (Grant of Rs. 50 crore) Global Environment Facility (GEF) (Grant of US$ 49 million) Govt. of Germany through KfW (Loan of DM 250 million)
• Implementing Agency Rajasthan State Power Corporation Limited (RSPCL) with assistance from GEF, kfW and Govt. of India.
Solar Refrigeration and Air conditioning •
Solar energy can also be used for cooling buildings (generally known as air-conditioning) or for refrigeration required for other applications. Solar cooling appears to be attractive proposition due to the fact that when the cooling demand is more, the sunshine is strongest. Although, considerable work on solar cooling systems has been done in the last five decades, due to its complexity, both in concept and in construction, the utilization and commercialization of solar cooling is not as widespread as other solar energy applications. However, if solar cooling of buildings is combined with the solar heating then the combined solar cooling and heating systems can become economical. Similarly, solar refrigerators or cooled space (cold storage) will be a boon and economical in isolated locations for preserving essential drugs and food.
Solar Refrigeration and Air conditioning (contd.) There are several ways of using solar energy for cooling such as: • • • • • •
Using the absorption cycle with liquid absorbents such as LiBr – H2O, NH3, LiCI – H2O, NH3 – LinNO3, R22 – DMF, NH3 – NaSCN. Using the absorption cycle with solid absorbents such as: CaCl2 NH3 Using adsorption cycle with solid absorbents such as: Silicagel H2O, Zeolites – H2O. Using the vapor compression cycle employing a solar powered Rankine engine. Using the vapor compression .cycle with the compressor driven by electricity from photovoltaic panels. Nocturnal passive cooling. Several prototype systems based on some of the above principles have already been made and demonstrated but these are still under development to be dependable and commercial. The choice of a particular system not only depends on its economics but also on local factors such as climate, availability of cooling water, auxiliary energy source, and the type of collector available.
Solar Refrigeration and Air conditioning (contd.) • A solar air conditioning system is complicated and will consist of many components, the major ones being the solar collector field, a heat storage device, a solar cooling device (based on absorption or Rankine cycle) a cold storage device, a heat rejection device, air handling system, etc. as shown schematically in Figure. • A simple flat-plate collector or evacuated tube collector or concentrating collector depending on the temperature requirement, can be employed to heat a fluid which is used to operate the cooling device. • A part of the heat can be stored in the storage unit. The heat collected from the building is rejected to the atmosphere using a cooling tower or any other suitable heat rejecting device. If air is cooled by the cooling device then it is directly supplied to the building to be cooled or if chilled water is produced then it is circulated through fan coil units and a part of a chilled water is stored for use when the cooling device is not in operation.
Solar Refrigeration and Air conditioning (contd.) •
The performance of a cooling process is judged from its COP (coefficient of performance), which is the ratio of the amount of cooling produced to the energy input. The overall COP for a Rankine cycle operated solar cooling system is of about 0.3 to 0.4 which very much depends on the solar collector efficiency. The main advantage of the solar Rankine vapour compression cooling process is that it can be used in the heat pump mode, and also for electricity generation when cooling is not required. Moreover, the system may be designed for any operating range of temperatures with minimum pumping operations. The vapour compression cooling process operated by photovoltaic panels gives a COP in the range of 0.25 to 0.35 due to lower solar cell efficiency. This system can also be used in the heat pump mode, and the electricity can be used for other applications when cooling is not required. Here no auxiliary pumps are required. The cooling system based on closed absorption cycle gives a COP of about 0.10 to 0.20 depending on the collector efficiency. The advantage of this system is that it can be used with low grade heat (even waste heat) and is very quiet in operation. In this system some auxiliary power is required to drive fans and pumps. Cooling systems based on adsorption cycle are simple, quiet in operation, and operate at a COP of about 0.2. Here also auxiliary power is required to drive fans and pumps. Some experimental systems based on adsorption cycle are made but considerable research and development is required to improve performance and reliability.
Intermittent Absorption Refrigeration System •
Intermittent type of solar absorption refrigeration systems are studied by many investigators because of its suitability in areas where there is no electricity, and because of the intermittent nature of solar radiation. The solar intermittent refrigerator may be used for making ice, or as cold storage for food or vaccine in remote areas or small islands. The overall performance of solar intermittent refrigerators so far produced is low, since they operate at low efficiency. The thermodynamic processes of intermittent operation are not reversible and its operation depends on the absorbent refrigerant combination and concentration of refrigerant. Some of the most promising refrigerant-absorbent pairs are given in the table.
Refrigerant-absorbent pairs for intermittent cycle Refrigerant
Water Sodium Thiocyanide (NaSCN) Lithium nitrate (LiNO3) Calcium chloride (CaCl2) Strontium chloride (SrCl2) Dimethyl formamide (DMF)
Schematic diagram of intermittent solar refrigerator and thermodynamic cycle
Intermittent Absorption Refrigeration System (contd.)
• The solar intermittent refrigerator as described by Exell and Kornsakoo in Thailand is described here. The flow diagram is shown in figure. Flatplate collectors of 5.0 m2 area with plane mirror boosters containing about 67 kg of 46% NH3-H2O solution are used as generators from where ammonia gets vaporized by solar heating during the day with control valve A open while valves B and C closed. The ammonia is condensed and stored in a water cooled receiver. In the evening the solution is allowed to cool by opening the collector glass panes and closing the valve A. The valves B and C are then opened to produce refrigeration in the evaporator by the evaporation of ammonia passing through the expansion valve B. • The ammonia vapour goes through the bottom of the collector and gets reabsorbed in the solution; the heat of absorption escapes from the collector.
• On average bright days about 14 kg of ammonia was distilled and about 25 kg of ice was produced on the following night from water at 28°C. • A similar solar intermittent refrigerator was much earlier developed at IIT Delhi and the same is shown in the photograph.
An experimental solar intermittent refrigerator at IIT Delhi
Global Scenario of Solar Thermal Energy • Solar thermal installations worldwide had reached 135 million sq.m. of collector area at the end of 2007. • Majority of the installations are of domestic water heaters, with around 48 million homes with solar thermal installations. • China accounts for 63% of the installed capacity, followed by EU with 12.7%, Turkey (6.5%), Japan (6%), Israel (3.8%), Brazil (1.8%), USA (1.8%), Australia (1.4%), and India (1.3%). • The Chinese market is expected to grow at 15% per year, with annual production reaching 20 million sq.m. by 2010 and cumulative installations to 300 million sq.m. by 2020. • Evacuated tube collectors are very popular in China, which account for more than 85% of the market. • There are more than 1000 industries/dealers producing /selling solar water heaters in China.
China catches up on technology •
Chinese firms are the global leaders in producing evacuated tubes; and they have tube thermosiphon systems under control. Yet when it comes to pumped solar heating systems, which are pre-dominant in Europe, and necessary for tasks such as solar cooling, there are certain deficits; for example with collector hydraulics or insulation. However, collector manufacturers are catching up on these points.
China pins its hopes on solar heat like almost no other country in the world. By the year 2015, the total installed collector surface area should, ac-cording to the Huichong Research Institute, which works on behalf of the government, reach 2.32 billion m2 (the equivalent of 1,624 GWth of thermal output). That would be more than 20 times as much as today. 20 to 30 % of the Chinese population will then be using the sun to heat their domestic water.
Solar thermal energy use in China The annually installed output has grown continually, reaching 12.6 GWth in 2006. However, there are also contradictions between the sources here for 2006, as with the total production. The cumulative installed collector surface area stands at more than 100 million m2. If China wishes to reach a total installed surface area of 2.23 billion m2 by 2015, it requires significantly faster rates of growth than in past years.
The Chinese collector manufacturers’ exports grew from around 200,000 m2 in 1999 to 600,000 m2 in 2003. After having paused on 500,000 m2, they rocketed straight up to 2 million m2. in 2006
Israel is still at the solar top • • •
• • •
Water heating in Israel means heating water with solar. Nine out of ten Israeli households take showers using solar energy. The solar miracle in Israel had happened due to Prof. Harry Tabor, a physicist who migrated from Great Britain to Israel in 1948, developed and demonstrated the first advanced level solar water heater using nickel black selective coating and exhibited the same in the first world congress on Applied Solar Energy in Arizona, USA in 1955. Israel was ranked second in the collector statistics(498 Wth cumulative installed collector power per 1000 inhabitants) of the International Energy Agency (IEA) in 2005, behind cyprus on 657 Wth. In third, fourth and fifth places come Austria with 205 Wth, Barbados with 200 Wth and Greece with 192 Wth. Solar heating makes up 3 percent of primary energy consumption in Israel. According to government figures this saves Israel 8 percent of its electricity consumption. In order to protect the consumer from bad quality, the state introduced norms for collectors and storage tanks, as well as a compulsory guarantee period of 5 years. Families can buy a typical thermosiphon system with 2.5m2 of collector area and a 150 litre tank for 2000 Israeli New Shekel.
A Step towards achieving the Vision
The Delhi Government has decided to make use of solar power compulsory for lighting up hoardings and for street lighting and solar water heating systems in several categories of buildings/residences.
Renewable Energy: Basic Issues to be Addressed 1.
Initial cost, financial viability and financing mechanisms High initial (capital) cost Site and application / case specific financial viability. Unavailability of attractive financing mechanisms.
Restricted availability of different type(s), size(s) of renewable energy devices to suit user taste / need / demand.
Reliability, Durability, Repair and Maintenance, standardization (Quality Assurance) and Credibility Related issues – Usually unreliable (problem(s) of resource availability and technological appropriateness). Technologies being disseminated are not durable (wrong choice of materials, design) Absence of maintenance infrastructure, after sales service.
Renewable Energy: Basic Issues to (Contd..) be Addressed 1.
Relevant Environmental Issues Potential of reduction in greehouse gas emissions. Clean Development Mechanism
Policy Measures, absence of level planning field for RET’s Prioritization Pricing of fossil fuels vs. promotional measures for renewables Some
Identification of Niche Areas for Each Technology and formation and implementation of specific measures.
Renewable Energy: Basic Issues to (Contd..) be Addressed 7. Lack of awareness, education and training, Human Resource Development – Mass level awareness programmes Education of policy makers, administrator Education of technicians, mechanics Employment related issues. 8. Support to Research and Development to develop and disseminate appropriate renewable energy technologies – Identification of problems and provide solutions / remedial measures.
“ By the year 2030, India should achieve Energy Independence through solar power and other forms of renewable energy ” Dr. A. P. J. Abdul Kalam President of India Independence Day Speech, 2005