MODULE 2-renewable energy sources-Final-new.pptx

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1. MODULE 2a SOLAR ENERGY

2. INTRODUCTION Sun’s energy is the primary source of energy for all the life on earth. The sun radiates energy uniformly in all directions in the form of electromagnetic waves. The emission rate of this energy is equivalent to the energy produced in a furnace at a temperature of about 6,000K. Solar energy can be utilised in 2 ways. Solar thermal and solar photovoltaic system.

3. SOLAR RADIATION From point of view of utilisation of solar energy we are more interested in the energy received at the earth’s surface than in the extra terrestrial energy Solar radiation received at the surface of earth is entirely different due to various reasons The solar radiation that penetrates the earth’s atmosphere and reaches the surface differs in both amount and character from the radiation at the top of the atmosphere

4. Part of radiation is reflected back into space by the clouds. Radiation entering the atmosphere is partly absorbed by the molecules in the air. Oxygen and ozone absorb nearly all the UV radiation and water vapour and CO2 absorb some of the energy in the infrared range. Part of the solar radiation is scattered by droplets in clouds by atmosphere molecules and by dust particles.

5. Beam and Diffuse Solar radiation Beam Radiation: Solar radiation that has not been absorbed or scattered and reaches the ground directly from the sun is called direct or beam radiation. This radiation produces a shadow when interrupted by opaque object.

6. Diffuse Radiation : Is that solar radiation received from the sun after its direction has been changed by reflection and scattered by the atmosphere. Because of the solar radiation is scattered in all directions in the atmosphere diffuse radiation comes to the earth from all parts of the sky.

7. Total radiation received at any point on the earth’s surface is the sum of direct and diffuse radiation. This is referred to in a general sense as the insolation at that point. Insolation is defined as the total solar radiation energy received on a horizontal surface of unit area on the ground in unit time.

8. Absorption As solar radiation passes through the earth’s atmosphere the short wave UV rays are absorbed by the ozone in the atmosphere and long wave infrared waves are absorbed by the CO2 and moisture in the atmosphere. This results in the narrowing of the bandwidth

9. Scattering: As solar radiation passes through the earth’s atmosphere the components of the atmosphere such as water vapour and dust scatter a portion of the radiation. A portion of this scattered radiation always reaches the earth’s surface as diffuse radiation. Thus the radiation finally received at the earth’s surface consists partly of beam radiation and partly of diffuse radiation.

10. SOLAR RADIATION DATA The radiation data are mostly measured on a horizontal surface.

11. Typical records of global and defuse radiation versus solar time on a horizontal surface for a clear day and partially cloudy day are shown in Fig. Daily radiant energy is obtained from the area under the corresponding curve. Monthly average of the daily radiation is obtained by averaging over a span of the corresponding month and expressed in kJ/m 2 . An alternative unit for expressing solar radiation is Langley per unit time, where one Langley is equal to 1 calorie/cm 2 .

12. Thus, solar radiation data are presented in three ways: ( i ) Flow of energy per unit area per second, ( kJ/m 2 /s) (ii) Flow of energy per unit area per hour, (kJ/m 2 /h) (iii) Flow of energy per unit area per day, (kJ/m 2 /day) The incident solar radiation is also a function of the orientation (or tilt, due south in northern hemisphere) of solar collector from horizontal. The radiation pattern indicates favoring of certain tilt during certain periods of the year. Therefore, seasonal adjustment of tilt angle may result in enhanced radiation collection. However, overall strategy changes from place to place and also on the type of application.

13. ( i ) Pyrheliometer: An instrument that measures beam radiation by using a long and narrow tube to collect only beam radiation from the sun . Types: a)Angstrom pyrheliometer b)Abbot silver disc pyrheliometer c)Eppley pyrheliometer. MEASUREMENT OF SOLAR RADIATION

14. i )Angstrom Pyrheliometer

15. A thick blackened shaded manganin strip is heated electrically until it is at the same temperature as a similar strip which is exposed to solar radiation. Under steady state condition(both strips are identical temperature) the energy used for heating is equal to the absorbed solar energy. The thermocouple on the back of each strip, is connected in opposition through a sensing galvanometer. Galvanometer is used to test the equality of temperature.

16. The energy H of direct radiation is calculated by, H DN = Ki 2 where H DN =Direct radiation incident on area normal to sun’s rays. i =heating current in amperes, K is a dimension or instrument constant. K= ; R is the resistance per unit length of the absorbing strip( Ω /cm) W=mean width of the absorbing strip α= absorbing coefficient of the absorbing strip.  

17. ii)Abbot silver disc Pyrheliometer

18. Abbot silver disc Pyrheliometer The Pyrheliometer, shown in Fig., uses a long collimator tube to collect beam radiation. The inside of the tube is blackened to absorb any radiation incident at angles outside the collection solid angle. A blackened silver disc positioned at the lower end of the tube with diaphragms. A mercury in glass thermometer used to measure the temperature at the disk. A shutter made up of three polished metal leaves is provided at the upper end of the tube to allow solar radiation to fall on the disc at regular intervals and corresponding changes in temperature are measured. The tube is sealed with dry air to eliminate absorption of beam radiation within the tube by water vapor. A tracker is needed if continuous readings are desired.

19. iii)Eppley Pyrheliometer Sensitive element is a temperature compensated 15 junction bismuth silver thermophile mounted at the base of a brass tube. Diaphragms subtend an angle of 5.7º. A thermophile is a series arrangement of thermocouples used to develop a much greater voltage than in possible using only one. The tube is filled with dry air and sealed with a crystal quartz window which is removable.

20. MEASUREMENT OF SOLAR RADIATION (ii) Pyranometer: A pyranometer is designed to measure global radiation, usually on a horizontal surface but can also be used on an inclined surface. When shaded from beam radiation by using a shading ring, it measures diffuse radiation only. The sun’s radiation is allowed to fall on a black surface to which hot junction of thermopile are attached. The cold junctions of the thermopile are located in such a way that they do not receive radiation. As a result an e.m.f proportional to the solar radiation is generated.

21. Eppley Pyranometer

22. Eppley Pyranometer A precision pyranometer is designed to respond to radiation of all wavelengths and hence measures accurately the total power in the incident spectrum. It contains a thermopile whose sensitive surface consists of circular, blackened, hot junctions, exposed to the sun and cold junctions are completely shaded. The temperature difference between the hot and cold junctions is the function of radiation falling on the sensitive surface. The sensing element is covered by two concentric hemispherical glass domes to shield it from wind and rain. This also reduces the convection currents. A radiation shield surrounding the outer dome and coplanar with the sensing element, prevents direct solar radiation from heating the base of the instrument. The instrument has a voltage output of approximately 9 μV /W/m 2 and has an output impedance of 650 W. The pyranometer, when provided with a shadow band (or occulting disc) to prevent beam radiation from reaching the sensing element, measures the diffuse radiation only.

23. ii) Yellot Solarimeter(Photovoltaic Solar Cell) Pyranometer have been used in solar cell detectors. Silicon cells are most common for solar energy. Silicon solar cells have the property that their light current is a linear function of incident solar radiation.

24. Sunshine Recorder(Important)

25. Sunshine Recorder This instrument measures the duration in hours, of bright sunshine during the course of the day. It essentially consists of glass sphere (about 10 cm in diameter) mounted on its axis parallel to that of earth, within a spherical section (bowl) as shown in Fig. The bowl and glass sphere is arranged in such a way that sun’s rays are focused sharply at a spot on a card held in a groove in the bowl. The card is prepared from special paper bearing a time scale. As the sun moves, the focused bright sunshine burns a path along this paper. The length of the trace thus obtained on the paper is the measure of the duration of the bright sunshine. Three overlapping pairs of grooves are provided in the spherical segment to take care of the different seasons of the year.

26. SOLAR THERMAL SYSTEMS Solar thermal systems provide thermal energy for various processes. In cold climate regions, large amount of low-grade thermal energy is required for heating air for comfort and hot water for washing, cleaning and other domestic and industrial needs. Various industrial surveys show that up to 24 per cent of all industrial heat is consumed for heating fluids to a low temperature. Solar energy is best suited for low-grade thermal applications.

27. Even in high temperature heating applications a significant amount of fuel can be saved by using solar energy for preheating (up to about 180 °C). Due to this reason, manufacturing of solar water heaters has become a thriving industry in several countries, especially Australia, Israel, USA and Japan. Solar thermal energy is also being utilized in drying and process industries. It can also be converted and utilized as mechanical and electrical energy in the same way as in any conventional thermal system.

28. SOLAR COLLECTORS Solar Power has low density per unit area. Hence it is to be collected by covering large ground area by solar thermal collectors. Solar thermal collector essentially forms the first unit in a solar thermal system. It absorbs solar energy as heat and then transfers it to heat transport fluid efficiently. The heat transport fluid delivers this heat to thermal storage tank / boiler / heat exchanger, etc., to be utilized in the subsequent stages of the system.

29. CLASSIFICATION OF SOLAR COLLECTORS

30. CLASSIFICATION OF SOLAR COLLECTORS 1. Concentrating Type Solar collectors In concentrating type solar collectors, solar radiation is converged from large area into smaller area using optical means. Beam radiation, which has a unique direction and travels in a straight line, can be converged by reflection or refraction techniques. Diffuse radiation however, has no unique direction and so does not obey optical principles. Therefore, diffuse component cannot be concentrated. Thus concentrating type solar collectors mainly make use of beam radiation Main advantage of concentrating type collectors is that high temperatures can be attained due to concentration of radiation.

31. 2. Non Concentrating solar collector (Flat plate collector) A flat plate collector is simple in construction and does not require solar tracking. Therefore, it can be properly secured on a rigid platform and thus becomes mechanically stronger than those requiring flexibility for tracking purpose. As the collector is installed outdoors and exposed to atmospheric disturbances (rain, storm, etc.), the flat plate type is more likely to withstand harsh outdoor conditions. Also because of simple stationary design, a flat plate collector requires little maintenance.

32. The principal disadvantage of flat plate collector is that because of absence of optical concentration, the area from which heat is lost is large. Also due to same reason high temperatures cannot be attained.

33. LIQUID FLAT PLATE COLLECTOR The basic elements in a majority of these collectors are: i . Transparent cover (one or two sheets) of glass or plastic ii. Blackened absorber plate usually of copper, aluminum or steel, iii. Tubes, channels or passages, in thermal contact with the absorber plate. In some designs, the tubes form integral part of absorber plate. iv. Weather tight, insulated container to enclose the above components

34. A liquid, most commonly, water is used as heat transport medium from collector to next stage of the system. However, sometimes mixture of water and ethylene glycol (antifreeze mixture) are also used if the ambient temperatures are likely to drop below 0°C during nights. As solar radiation strikes on specially treated metallic absorber plate, it is absorbed and raises its temperature. The absorber plate is usually made from a metal sheet ranging in thickness from 0.2 to 1 mm.

35. The heat is transferred to heat transfer liquid circulating in the tube (or channels), beneath the absorber plate and in intimate contact with it. The metallic tubes range in diameter from 1 to 1.5 cm. In some designs, the tubes are bonded to the top of absorber plate or in line with and integral to absorber plate.

36. Header pipes which are of slightly larger diameter, typically 2 to 2.5 cm, lead the water in and out of the collector and distributed to tubes. The metal most commonly used, both for absorber plate, the tubes and the header pipes is copper, but other metals and plastics have also been tried. In the bottom and along the sidewalls, thermal insulation, provided by 2.5 to 8-cm thick layer of glass wool, prevents heat loss from the rear surface and sides of the collector. The glass-cover permits the entry of solar radiation as it is transparent for incoming short wavelengths but is largely opaque to the longer infrared radiation reflected from the absorber.

37. As a result, heat remains trapped in the airspace between the absorber plate and glass cover in a manner similar to green house. The glass cover also prevents heat loss due to convection by keeping the air stagnant. The glass cover may reflect some 15 per cent of incoming solar radiation, which can be reduced by applying anti reflective coating on the outer surface of the glass. The usual practice is to have one to two covers with spacing ranging from 1.5 to 3 cm.

38. Plain or toughened glass of 4 to 5 mm thickness is most favored material. Transparent plastics may also be used in place of glass but they often offer inferior performance as compared to glass. Most plastics are not as opaque to infrared radiation as glass. Also their transparency for incoming solar radiation decreases with aging. The life of plastic material is short when exposed to sun rays as it breaks down and cracks are developed over a span of time.

39. Advantages and Disadvantages of Solar Flat Plate Collectors Advantages - ( i ) They can use both direct and diffuse solar radiation. (ii) They do not require orientation towards the sun. (iii) They are simpler in construction and requires less maintenance. (iv) Mechanically strong to withstand atmospheric condition. Disadvantages - ( i ) Low thermal efficiency. (ii) Requires larger collecting area. (iii) Corrosion of the metal tubes by water. (iv) An anti-freeze solution has to be used to prevent the freezing of heat transferring fluid.

40. EFFECT OF VARIOUS PARAMETERS ON PERFORMANCE a)Selective Surface: Absorber plate surfaces which exhibit the characteristics of high value of absorptivity for incoming solar radiation and low value of emissivity for outgoing re-radiation are called selective surfaces. Such surfaces are desirable because they maximize the net energy collection. Some examples of selective surface layers are copper oxide, nickel black and black chrome.

41. b)Number of Covers: With increase in the number of covers, the values of both transmissivity-absorptivity product for beam radiation and diffused radiation decreases and thus the flux absorbed by absorber plate decreases. The value of heat loss from the absorber plate also decreases. However, the amount of decrease is not the same for each cover. Maximum efficiency is obtained with one or two covers.

42. c)Spacing Heat loss varies with spacing between two covers. Since collectors are designed to operate at different locations with varying tilts and under varying service conditions, an optimum value of spacing is difficult to specify. Spacing in range from 4 to 8 cm is normally suggested.

43. d)Dust on the Top of the Cover When a collector is deployed in a practical system, dust gets accumulated over it, reducing transmitted flux through the cover. This requires continuous cleaning of the cover, which is not possible in practical situation. Cleaning is generally done once in few days. For this reason it is recommended that the incident flux be multiplied by a correction factor which accounts for the reduction in intensity because of accumulation of dust.

44. SOLAR DISTILLATION(DESALINATION OF WATER) Potable or fresh water (water with less than 500-ppm salt content) is one of the fundamental necessities of life for a man. Industries and agriculture also require fresh water without which they cannot thrive. Man has been dependent on rivers, lakes and underground water reservoir to fulfill his need of fresh water. Because of rapid industrialization and population explosion the demand of fresh water has been increasing enormously. With the standard of living, the average per capita consumption of water has also increased.

45. Due to climate changes and less rainfall in many part of the world fresh water, which was available in abundance from rivers, lakes and ponds, is becoming scarce. Also the available resources are getting polluted due to discharge of industrial effluents and sewage in large quantities. According to one estimate, about 79 percent of water available on earth is salty, 20 percent is brackish (less salty water from well) and only one percent is fresh.

46. Therefore, conversion of brackish or saline water to fresh water through distillation process using solar energy is a good idea for places where plenty of saline water and sun are available The use of solar energy for desalting seawater and brackish well water has been demonstrated in several moderate sized pilot plants in the Unites States, Greece, Australia and several other countries. The idea was first applied in 1872 at Las Salinas, Chile, in a plant supplying drinking water for animals.

47. BASIN TYPE SOLAR STILL

48. A simple basin type solar still consists of a shallow blackened basin filled with saline or brackish water to be distilled. The depth of water is kept about 5–10 cm. It is covered with sloppy transparent roof. Solar radiation, after passing through the roof is absorbed by the blackened surface of the basin and thus increasing the temperature of water. The evaporated water increases the moisture content, which gets condensed on the cooler underside of the glass. The condensed water slips down the slope and is collected through the condensate channel attached to the glass

49. The still is erected in open area facing East-West direction. The still can be fed with saline water either continuously or intermittently. The supply is generally kept at twice the rate at which the fresh water is produced but may vary depending on the initial salinity of input water. The output of a typical solar still in Indian climate varies from 5.3 l/m 2 day (in summer) to 0.9 l/m 2 day (in winter).

50. SOLAR POND A natural or artificial body of water for collecting and absorbing solar radiation energy and storing it as heat. Thus a solar pond combines solar energy collection and sensible heat storage. The simplest type of solar pond is very shallow, about 5m to 10m deep with a radiation absorbing bottom. A bed of insulating material under the pond minimises loss of heat to the ground. A curved cover made of transparent fibre glass over the pond permits entry of solar radiation but reduces losses by radiation and convection

51. SOLAR POND ELECTRIC POWER PLANT

52. A non-convective solar pond serves the purpose of a large flat plate collector as well as long term thermal storage and can provide sufficient heat for the entire year. The black bottom serves as absorber and the layer of still water above it is used as insulator rather than normal glazing and air space. In a large area pond approximately 1–2 m deep, vertical gradient of salt concentration is maintained such that most concentrated and dense solutions are at the bottom. The salt concentration varies from 20–30 per cent at the bottom to almost zero at the top. Left to itself, the salt concentration gradient will disappear over a period of time because of upward diffusion of the salt.

53. In order to maintain it, fresh water is added at the top of the pond through a horizontal diffuser, while slightly saline water is run off. At the same time concentrated brine is added at the bottom of the pond. The amount of salt required for this purpose is about 50 kg/m2 -day, which is a large quantity when considered on an annual basis. For this reason, normally the salt is recycled by evaporating the saline water run-off from the surface in an adjoining evaporation tank.

54. Because of movement and mixing of the fluid both at the top and at the bottom, the solar pond is characterized by three zones: ( i ) a surface convective zone, (ii) a non convective concentration gradient zone and a (iii) lower convective zone.

55. The surface convective zone usually has a small thickness, around 10–20 cm. It has a uniform temperature, close to ambient air temperature and low, uniform concentration close to zero. The non-convective zone is much thicker and occupies more than half the depth of the pond. In this zone, both temperature as well as concentration increase with depth. It principally serves as an insulating layer and reduces heat loss in the upward direction.

56. It also serves as part of heat storage as some of the heat collection takes place in this zone also. The lower convective zone is comparable in thickness to the non-convective zone. Both the temperature and concentration are nearly constant in this zone. It serves as the main heat-collection as well as thermal storage medium. This zone is also referred to as storage zone.

57. Solar radiation penetrates through water up to the blackened bottom, where it gets absorbed and increases the temperature near bottom. The bottom layers of the brine reach 70–85 °C while the top remains at 25 °C. The hot brine from the bottom is slowly withdrawn in a laminar flow pattern from the pond and used to evaporate an organic working fluid in a heat exchanger and returned to the pond

58. Working of Solar pond Electric Power plant Thermal energy from solar pond is used to drive a rankine cycle heat engine. Hot water from the buttom level of the pond is pumped to the the evaporator where the organic fluid is to be vapourized . Vapour flows from high pressure to the turbine and thereby expanding through the turbine wheel and electric generator linked to it. The vapour then travels to the condenser where cold water from the cooling tower condenses the vapour back into the liquid. The liquid is pumped back to the evaporator where the cycle is repeated.

59. Alternatively, heat is extracted by water flowing through a heat exchanger coil submerged at the bottom. The organic working fluid produces mechanical power in a Rankine cycle, which in turn generates electrical power using alternator. The annual collection efficiency generally ranges between 15 to 20 percent. These values are lower than those obtained for a flat plate collector. Nevertheless, solar ponds are more cost effective, since their cost per square meter is much less than that for a liquid flat plate collector

60. The first experimental solar pond was constructed in Israel to demonstrate its principle of working. So far, more than sixty solar ponds have been built all around the world for a variety of applications. A 2000 sq. m. solar pond equipped with a 20 kW turbine has been constructed in Australia. The largest solar pond built so far is a 2,50,000-m2 pond at Bet Ha Arava in Israel, to generate 5 MW electric power using an organic fluid Rankine cycle. First and largest solar pond in India is located at Bhuj, Gujarat.

61. ADVANTAGES OF SOLAR POND Has low cost per unit area of collection and has an inherent capacity for storage purpose. Provides many environmental advantages when compared to the use of fossil fuels in generation of electricity. Conventional energy sources are conserved.