1. 1 Solar Energy Resource and Technologies Dr. Ram Chandra DBT’s Energy Bioscience Overseas Fellow Centre for Rural Development and Technology Indian Institute of Technology Delhi Hauz Khas, New Delhi – 110 016

2. Solar Energy The energy radiated from the sun is electromagnetic waves reaching the planet earth in three spectral regions; (i) Ultraviolet 6.4 % ( ƛ ≤ 0.38 µm), (ii) Visible 48.0 % (0.38 µm < ƛ < 0.78 µm), and (iii) Infrared 45.6 % ( ƛ ≥ 0.78 µm) of total energy. Due to the large distance between the sun and the earth (1.495 × 10 8 km) the beam radiation received from the sun on the earth is almost parallel.

3. The energy flux received from the sun before entering the earth’s atmosphere is a constant quantity. The solar constant I sc is the energy from the sun received on a unit area perpendicular to the sun rays at the mean distance from the sun outside the atmosphere. The standard value of the solar constant is 1367 W/m 2 or 1.958 langley per minute (1 langley/min = 1 cal/cm 2 /min). In the term of other units, I sc = 432 Btu/ft 2 /h or 4.921 MJ/m 2 /h.

4. The distance between the sun and the earth varies due to the elliptical motion of the earth. Accordingly, the extraterrestrial flux also varies, which can be calculated (on any day) by the equation: Where n is the day of the year counted from the first day of January.

5. Irradiance. The rate of solar radiation falling on a given area at a moment in time. Irradiance is measured in units of kW/m 2. Irradiation. The amount of solar energy over time. Irradiation is measured in units of kWh/m 2 /day.

7. Direct Radiation: Solar radiation that reaches to the surface of earth without being diffused is called direct beam radiation. Diffused Radiation: As sunlight passes through the atmosphere, some of it is absorbed, scattered and reflected by air molecules, water vapour, cloud, dust, and pollutants from power plants, forest fires, and volcanoes. This is called diffused radiation. Global Solar Radiation: The sum of diffuse and direct solar radiation is called global solar radiation.

8. India: 4 to 5 kWh/day

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11. Solar Thermal 11

12. Solar Thermal Energy Collector Solar Thermal Energy Collector: Solar thermal energy collector is an equipment in which solar energy is collected by absorbing the radiation in an absorber and then transferring to a fluid. There are two type of collectors; Flat Plate solar Collector: It has no optical concentrator. Here the collector area and the absorber area are numerically same. The efficiency of flat plate collector is low and temperature of working fluid can be raised only up to 100 0 C.

13. Schematic Cross Section of a Flat Plate Collector

14. The flat plate collector consists of five major parts as given below: 1: A metallic flat absorber plate: It is made of copper, steel or aluminium (having high thermal conductivity) and having black surface. The thickness of the metal sheet ranges from 0.5 to 1.0 mm. 2: Tubes or channels: they are soldered to the absorber plate. Water flowing through these tubes takes away the heat from the absorber plate. The diameter of tubes is around 1.25 cm, while that of the header pipe which leads water in and out of the collector and distributes it to absorber tubes is 2.5 cm.

15. 3: A transparent toughened glass sheet: of 5 mm thickness is provided as the cover plate. It reduces convection heat losses through a stagnant air layer between the absorber plate and the glass. Radiation loss are also reduced as the spectral transmissivity of glass is such that it transparent to short wave radiation and nearly opaque to long wave thermal radiation emitted by interior collector walls and absorbing plate. 4: Fibre glass insulation: of 2.5 to 8.0 cm thickness is provided at the bottom and on the sides in order to minimize the heat loss.

16. 5: A container encloses the whole assembly in a box made of metallic sheet or fibre glass. The commercially available collector have a face area of 2 m 2. The whole assembly is fixed on a supporting structure that is installed on a tilted position at a suitable angle facing south in northern hemisphere. For the whole year, the optimum tilt angle of collector is equal to the latitude of its location. During winter the tilt angle is kept 10-15 0 more than the latitude of the location, while in summer it should be 10-15 0 less than the latitude.

17. Concentrating Type solar Collector: Here the receiving area of solar radiation is several times greater than the absorber area and the efficiency is high. Mirrors and lenses are used to concentrate sun rays on the absorber. The temperature of working fluid can be raised only up to 500 0 C. For better performance, the collector is mounted on a tracking equipment to always face the sun with its changing position

18. Natural Circulation Solar Water Heater

19. Forced Circulation Solar Water Heater

20. Solar Thermal Energy Storage Solar energy is available only during the sunshine hours. Consumer energy demands follow their own time pattern and the solar energy does not fully match the demand. As a result, energy storage is a must to meet the consumer requirement. There are three important methods for storing solar thermal energy.

21. 1. Sensible Heat Storage Heating a liquid or solid which does not change phase comes under this category. The quantity of heat stored is proportional to the temperature rise of the material. If T 1 and T 2 represent the lower and higher temperature, V the volume and ρ the density of the storage material, and C p the specific heat, then the energy stored Q is given by:

22. For a sensible heat storage system, energy is stored by heating a liquid or a solid. Materials that are used in such a system include liquids like water, inorganic molten salts and solids like rock, gravel and refractories. The choice of the material used depends on the temperature level of its utilization. Water is used for temperature below 100 0 C whereas refractory bricks can be used for temperature upto 1000 0 C.

23. 2. Latent Heat Storage (Phase change heat storage) In this system, heat is stored in a material when it melts, and heat is extracted from the material when it freezes. Heat can also be stored when a liquid changes to gaseous state, but as the volume change is large, such a system is not economic.

24. 3. Thermo-chemical Storage With a thermo-chemical storage system, solar heat energy can start an endothermic chemical reaction and new products of reactions remain intact. To extract energy, a reverse exothermic reaction is allowed to take place. Actually, thermo-chemical thermal energy is the binding energy of reversible chemical reactions.

25. Photovoltaic System

26. Photon An elementary particle that is the carrier of electromagnetic radiation of all wavelengths. The photon is different from many other elementary particles, such as the electron, because it has no mass. That means that it travels (in vacuum) at the speed of light.

27. Photoelectric Effect When photons of light hit electrons in the silicon lattice and provide energy to flow. Introducing dopants such as boron and phosphorus into the silicon lattice provides a direction for the electrons to flow. Finally, electrons flowing from one cell into the next cell in a module gain about 1/2 volt from each cell

28. Solar Photovoltaic System Photovoltaic power generation is a method of producing electricity using solar cell. A solar cell converts solar optical energy directly into electrical energy. A solar cell is essentially a semiconductor device fabricated in a manner which generates a voltage when solar radiation falls on it.

29. Solar cells are fixed on a board and connected in series and parallel combinations to provide the required voltage and power to form a photovoltaic (PV) module. To protect the cells from damage the module is hermetically sealed between a plate of toughened glass and layers of Ethyl Vinyl Acetate. A terminal box is attached to the back of module where the two ends of solar string are shouldered to the terminals. When the PV module is in use, terminals are connected directly to the load.

30. Single PV modules of capacities ranging from 10 Wp (peak watt) to 120 Wp can provide power for different loads. Several panels of module constitute an array which is rated according to peak wattage it delivers at noon on a clear day. For a higher output an array field is created. The size of an individual cell varies from 10 - 100 cm 2 and a module contains about 20 to 40 cells. A standard modules constituting 30 cells, each of 7.5 cm diameter, can provide 12 V, 1.2 A and 18 Wp.

31. Structure of Solar Cell

32. Solar Photovoltaic System A PV module produces DC power. To operate electrical appliances used in households, inverters are used to convert DC power into 220 V, 50 Hz AC power. Components other than PV modules are collectively known as balance of system (BOS) which includes storage batteries, an electronic charge controller and an inverter.

33. Application of PV Systems Solar PV power systems are categories into four classes. 1: Standalone 2: PV hybrid 3: Grid connected, and 4: Solar power satellite

34. T hank Y ou