Module 2

Solar Energy collectors: Solar thermal collectors -Flat plate collectors –Solar concentrators (parabolic trough, parabolic dish, Central Tower Collector) –Solar Air Heaters

Solar thermal electric power generation -Thermal Energy storage, sensible heat storage, latent heat storage , Thermo chemical storage , photovoltaic system for power generation , Solar pond -Solar Cells-Types of solar cells , principle of working and performance characteristics, Production process- Block diagram only

Applications- Solar space heating and cooling of buildings, solar pumping, solar cooker, solar still, solar drier, solar refrigeration and air-conditioning, heliostat, solar furnace

Solar thermal energy collectors

• A solar thermal energy collector is an equipment in which solar energy is collected by absorbing radiation in an absorber and then transferring to a fluid.
• Two types of collectors
• Flat plate solar collector
• Concentrating type solar collector

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Flat plate solar collector

• No optical concentrator
• The collector area and absorber area are numerically the same, the efficiency is low, and temperatures of the working fluid can be raised only up to 100^oC.

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Concentrating type solar collector

• The area receiving the solar radiation is several times greater than the absorber area and the efficiency is high.
• Mirrors and lenses are used to concentrate the sun’s rays on the absorber, and the fluid temperature can be raised up to 500^oC.
• For better performance, the collector is mounted on a tracking equipment to face the sun always with its changing position.

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Flat plate collector

Flat plate collector

• A metallic flat absorber plate of high thermal conductivity made of copper, steel or aluminium, and having black surface. The thickness of the metal sheet ranges from 0.5 mm to 1 mm.
• Tubes or channels are soldered to the absorber plate.
• Water flowing through these tubes takes away the heat from the absorber plate.
• The diameter of the 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.

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Flat plate collector

• A transparent toughened glass sheet of 5 mm thickness is provided as the cover plate. It reduces convection losses through a stagnant air layer between the absorber plate and the glass.
• Fibre glass insulation of thickness 2.5 cm to 8 cm is provided at the bottom and on the sides in order to minimize heat loss.
• A container encloses the whole assembly in a box made of metallic sheet or fibre glass.

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Solar collector with air as the heat transfer fluid

Effect of design parameters on performance

• The parameters that affect the performance of a flat plate collector are
• Heat transport system
• Selective surfaces
• Number of covers
• Spacing

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Effect of design parameters on performance

1. Heat transport system

• Heat from the absorber plate is removed by continuous flow of a heat transport medium.
• When water is used, it flows through metal tubes that are welded to the absorber plate for effective heat transfer.
• Cold water enters the bottom header, flows upwards and gets warmed by the absorber. The hot water then flows out through top header.

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Effect of design parameters on performance

• When air is used as the heat transfer fluid, an air stream flows at the rear side of the collector plate.
• Fins welded to the plate increase the contact surface area.
• The rear side of the air passages is insulated with mineral wool.
• Solar air heaters are used for drying agricultural products, space heating and seasoning of timber.

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Effect of design parameters on performance

2. Selective surfaces

• The absorber plate surfaces which provide high absorptivity for incoming solar radiation and low emissivity for outgoing radiation are termed selective surfaces.
• A selective surface is composed of a thin black metallic oxide coated on a bright metal base.
• These are important for low concentration solar equipment operating at high temperatures.

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Effect of design parameters on performance

3. Number of covers

• To minimize convection and radiation loss, a solar collector is provided with a transparent glass sheet over the absorber plate.
• Solar radiation incident on a glass sheet passes through the glass cover.
• By using two glass covers, heat loss can be reduced further.

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Effect of design parameters on performance

Spacing

• The spacing between the absorber plate and the cover or between two covers also influences the performance of a flat plate collector.
• The operating performance varies with the spacing as well as with tilt and service conditions and hence there is no way to specify the exact optimum spacing.

Solar concentrating collectors

• Used in medium temperature and high temperature energy conversion cycles.
• An optical system of mirrors or lenses projects the radiation on to an absorber of smaller area.
• Concentrator is the optical subsystem that projects solar radiation on to the absorber.
• Receiver shall be used to represent the sub- system that includes the absorber, its cover and accessories.

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• Aperture (W) is the opening of the concentrator through which solar radiation passes.
• Acceptance angle (2θd) is the angle across which beam radiation may deviate from the normal to the aperture plane and then reach the absorber.
• Concentration ratio (CR) is the ratio of the effective area of the aperture to the surface area of the absorber.
• The value of CR varies from unity (for flat plate collectors) to a thousand (for parabolic dish collectors).

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Types of concentrating collectors

• Plane receiver with plane collectors
• Compound parabolic collector with Plane receiver
• Cylindrical parabolic collector
• Collector with a fixed circular concentrator and moving receiver
• Frensel lens collector
• Parabolic dish collector
• Central receiver with heliostat

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Types of concentrating collectors

• 1. Plane receiver with plane collectors
• It is a simple concentrating collector, having up to four adjustable reflectors all around, with a single collector.
• CR varies from 1 to 4 and operating temperature can go up to 140oC.

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Plane receiver with plane collectors

Compound parabolic collector with Plane receiver

• Reflectors are curved segments that are parts of two parabolas. The CR varies from 3 to 10.
• For a CR of 10, the acceptance angle is 11.5^o and tracking adjustment is required after a few days to ensure collection of 8 hours a day.

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Cylindrical parabolic collector

• The reflector is in the form of trough with a parabolic cross section in which the image is formed on the focus of the parabola along a line.
• The basic parts are
• (i) an absorber tube with a selective coating located at the focal axis through which the liquid to be heated flows
• (ii) a parabolic concentrator, and
• (iii) a concentric transparent cover.

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Collector with a fixed circular concentrator and moving receiver

• It consists of an array of long, narrow, flat mirror strips fixed over a cylindrical surface.
• The mirror strips create a narrow line image that follows a circular path as the sun moves across the sky. The CR varies from 10 to 100.

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Frensel lens collector

• Frensel lens refraction type focusing collector is made of an acrylic plastic sheet, flat on one side, with fine longitudinal grooves.
• The angles of grooves are designed to bring radiation to a line focus.
• The CR changes between 10 and 80 with temperature varying between 150^oC and 400^oC.

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Frensel lens collector

Parabolic dish collector

• To achieve high CRs and temperature, it is required to build a point focusing collector.
• A paraboloid dish collector is of point focusing type as the receiver is placed at the focus of the paraboloid reflector.

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Parabolic dish collector

Central receiver with heliostat�

• To collect large amounts of heat energy at one point, the central receiver concept is followed.
• Solar radiation is reflected from a field of heliostats (an array of mirrors) to a centrally located receiver on a tower.
• Heliostat follow the sun to harness maximum solar heat.
• Water flowing through the receiver absorbs heat to produce steam which operates a Rankine cycle turbo generator to generate electrical energy.

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Rankine cycle

Solar air heaters

• A solar air heater constitutes a flat plate collector with an absorber plate, transparent cover at the top, a passage through which air flows and insulation at the bottom and sides.
• Air passages are only a parallel plate duct.
• Air to be heated flows between the cover and the absorber plate which is fabricated from a metal sheet 1 mm thickness.
• Cover is either made of glass or plastics of 4 mm to 5 mm thickness, glass wool of thickness 5 cm to 8 cm is used for bottom and side insulation.

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Solar air heaters

Solar air heaters

• Full assembly is encased in a sheet metal box and kept inclined at a suitable angle.
• The value of heat transfer coefficient between the absorber plate and air is low and the operating efficiency of a simple air heater is also low.
• To boost heat transfer, the contact area of air with the absorber plate is increased either by adopting a V-shaped absorber plate or by designing two pass air heaters.

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Solar air heater

V shaped absorber plate

Two pass solar air heater

Solar air heater

• The two pass solar air heater carries two glass cover sheets, separated by an air gap which reduces heat losses.
• In matrix air heater, air flows through a porous metallic matrix which receives and absorbs solar radiation directly.
• Solar air heaters have major applications like drying of agricultural products, seasoning of timber, space heating.

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Solar air heater

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.
• There are three important methods for storing solar thermal energy.

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Methods of storing solar thermal energy

• Sensible heat storage
• Latent heat storage (Phase change heat storage)
• Thermochemical storage

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Sensible Heat Storage (SHS)

• Mechanism: Sensible heat storage involves storing energy by raising the temperature of a material. The amount of energy stored is directly proportional to the temperature change and the heat capacity of the storage material.
• Materials: Common materials used include water, rock, and molten salts.
• Advantages: Simple technology, relatively inexpensive, and can use a variety of materials.
• Applications: Widely used in solar thermal power plants, building heating systems, and industrial waste heat recovery.
• Challenges: Large volumes are often required due to the relatively low energy density of sensible heat storage.

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Latent Heat Storage (Phase Change Heat Storage)

• Mechanism: Latent heat storage utilizes the phase change of materials (e.g., solid to liquid or liquid to gas) to store and release energy. The energy is stored during the phase change process at a constant temperature, known as the phase transition temperature.
• Materials: Phase Change Materials (PCMs) such as paraffin, salt hydrates, and certain organic compounds.
• Advantages: High energy storage density, ability to store energy at a nearly constant temperature, making it efficient for temperature control applications.
• Applications: Thermal regulation in buildings, solar energy systems, and temperature-sensitive environments (e.g., food storage, medical supplies).
• Challenges: High cost of PCMs, limited number of suitable phase change materials, and potential issues with material degradation over repeated cycles.

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Thermochemical Storage

• Mechanism: Thermochemical storage involves storing energy through reversible chemical reactions. When heat is supplied, a chemical reaction occurs, storing energy in the form of chemical potential. The energy is released when the reverse reaction occurs.
• Materials: Reversible chemical systems such as metal hydrides, hydroxides, and carbonates.
• Advantages: Very high energy density, long-term storage capability, and can store energy without heat loss over long periods.
• Applications: Solar thermal energy storage, seasonal energy storage, and industrial applications requiring high-temperature heat.
• Challenges: Complexity of system design, cost of materials, and the need for precise control of the chemical reaction conditions.

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Solar thermal energy storage

• 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 T1 and T2 represents lower and higher temperature, V the volume and ρ the density of the storage material, and cp the specific heat, the energy stored Q is given by

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Solar thermal energy storage

• For a sensible heat storage system, energy is stored by heating a liquid or a solid.
• Materials that are used in such 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 utilisation.
• Water is used for temperature below 100^oC whereas refractory bricks can be used for temperature up to 1000^oC.

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Solar thermal energy storage

• 2. Latent heat storage (Phase change heat storage)
• 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 economical.

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• 2. Latent heat storage (Phase change heat storage)
• The phase change materials like paraffin wax and fatty acids, hydrated salts such as calcium chloride hexo hydrate and sodium sulphate deca hydrate and inorganic materials like ice (H2O), sodium nitrate and soidum hydroxide can be used.
• Phase change materials such as sodium sulphate decahydrate melt at 32^oC, with a heat of fusion of 241 kJ/kg.
• Paraffin wax possess a high latent heat of fusion and is known to freeze without supercooling.

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Latent heat storage (Phase change heat storage)

Solar thermal energy storage

• 3. Thermochemical storage
• In this solar 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.
• The thermochemical thermal energy is the binding energy of reversible chemical reactions.

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Schematic representation of thermochemical storage reaction

3. Thermochemical storage

• Chemicals A and B react with solar heat and through forward reaction are converted into products C and D.
• The new products are stored at ambient temperature.
• When energy is required, the reverse reaction is allowed to take place at a lower temperature where products C and D react to form A and B.
• During the reaction, heat is released and utilized.

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Chemical energy storage reactions

Solar pond

• The concept of solar pond was derived from the natural lakes where the temperature rises towards the bottom.
• It happens due to natural salt gradient in these lakes where water at the bottom is denser.
• In salt concentration lakes, convection does not occur and heat loss from hot water takes place only by conduction.

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Solar pond

• This technique is utilised for collecting and storing solar energy.
• An artificially designed pond filled with salty water maintaining a definite concentration gradient is called solar pond.
• The top layers remain at ambient temperature while the bottom layer attains a maximum steady state temperature of about 60^oC – 85^oC.

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Solar pond

• For extracting heat energy from the pond, hot water is taken out continuously from the bottom and returned after passing through a heat exchanger.
• Alternately, heat is extracted by water flowing through a submerged heat exchanger coil.

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Schematic diagram of a solar pond

Solar pond

• As a result of continuous movement and mixing of salty water at the top and bottom, the solar pond can have three zones.
• Surface Convective Zone (SCZ) having a thickness of about 10 cm – 20 cm with a low uniform concentration at nearly the ambient air temperature.
• Non-convective Zone (NCZ) occupying more than half the depth of the pond and serves as the insulating layer from heat losses in the upward direction.
• Lower convective zone (LCZ) having thickness nearly equal to NCZ. Characterised by constant temperature and concentration.

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Solar pond

• The largest solar pond so far built is the 2,50,000 m² pond at Bet Ha Arava in Israel.
• Based on Rankine cycle principle, this pond is used to generate 5 MWe of electrical power with an organic fluid.

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Solar Pond in Bhuj, India

• One of the well-known examples of a solar pond is the Bhuj solar pond in India:
• Location: Bhuj, Gujarat, India.
• Size: Approximately 6000 square meters.
• Application: The pond was used to supply process heat to a nearby salt manufacturing plant.
• Performance: The LCZ reached temperatures of up to 80°C, providing a reliable source of thermal energy.

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Photovoltaic system for power generation

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

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Photovoltaic system for power generation

• In semiconductors, atoms carry four electrons in the outer valence shell, some of which can be dislodged to move freely in the materials if extra energy is supplied.
• Then, a semiconductor attains the property to conduct the current. This is the basic principle on which the solar cell works and generates power.

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Semiconductor materials and Doping

• A few semiconductor materials such as silicon (Si), cadmium suphide (CdS) and gallium arsenide (GaAs) can be used to fabricate solar cells.
• Semiconductors are divided into two categories – intrinsic (pure) and extrinsic.
• An intrinsic semiconductor has negligible conductivity, which is of little use.

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• To increase the conductivity of an intrinsic semiconductor, a controlled quantity of selected impurity atoms is added to it to obatin an extrinsic semiconductor. The process of adding the impurity atoms is called doping.
• In a pure semi conductor, electrons can stay in one of the two energy bands – the conduction band and the valence band.

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• The conduction band has electrons at the higher energy level and is not fully occupied, while the valence band possesses electrons at a lower energy level but is fully occupied.
• The energy level of the electrons differs between the two bands and this difference is called the band gap energy, Eg.
• The photons of solar radiation possessing energy E higher than the band gap energy Eg when absorbed by a semiconductor material, dislodge some of the electrons.

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• These electrons possess enough energy jump over the band gap from the valence band into the conduction band.
• In this process, vacant electron positions or holes are left behind in valence band.
• These holes act as positive charges and can move if a neighbouring electron leaves its position to fill the hole site.
• Mobile electrons and holes can thus enable a current flow through an external circuit if a potential gradient exists in the cell material.

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Semiconductor diode band structure

• Photon Energy
• Sunlight is composed of tiny energy capsules called photons.
• The number of photons present in solar radiation depend upon the intensity of solar radiation and their energy content on the wavelength band.

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Factors limiting the efficiency of solar cells

• In a solar cell lot of energy gets lost, hence efficiency drops to about 20%.
• Reflection losses
• Incomplete absorption
• Partial utilization of photon energy
• Collection losses
• High temperature loss
• Series resistance losses
• Thickness of the cell
• Location of p-n junction

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• Reflection losses-
• Nearly 30% of the incident radiation is lost through reflection from the surface of the cell.
• Incomplete absorption-
• Cell should be manufactured from material that is suitable for absorbing the energy of photons from solar radiation.
• Photons with less energy than energy gap will generate heat in the cell, which is about 33%. The difference between conduction and valence band is called the band gap energy.

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• Partial utilization of photon energy
• Many photons in solar radiation generate electron hole pair having more energy than binding energy, which is likely to ejected as dissipated heat.
• Collection losses
• Only those electron hole pair carrier that reach the junction before recommending are absorbed and contribute to the output current while others only generate heat.

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• High temperature loss
• PV cells are exposed directly to the sun, as the temperature rises, leakage across the cell increases. This causes reduction in power output, for silicon the output decreases 0.5% per oC.
• Series resistance losses
• Electric current generated flows out of the top surface by a mesh of metal contacts provided to reduce series resistance losses.

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• Thickness of the cell –
• The cell must be thin in the 60 to 100 μm range.
• Location of p-n junction
• For higher efficiency, the p-n junction should be located near to the top surface.

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Solar cells

• Solar cells are fabricated from semiconductor materials prepared in three physical states –
• Single crystal,
• Many small crystals (polycrystalline)
• and amorphous (non crystalline).

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Solar cell

• 1. Single crystal silicon
• Silicon solar cells are commonly used for both terrestrial and space applications.
• The basic raw material is sand (SiO2) from which silica (Si) is extracted and purified repeatedly to obtain the metallurgical grade silicon.
• A single crystal ingot is a long cylindrical block about 6 cm to 15 cm in diameter.

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Solar cell, module, array and array field

• Crystalline cells basically require 300 μm to 400 μm of absorber material.
• The ingot is sliced in wafers of 300 μm thickness.
• The wafers are starting material for a series of process steps such as surface preparation, dopants diffusion, anti-reflection coating, contact grid on the surface and base contact on the upper surface and on the lower one.

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• Solar cells are fixed on a board and connected in series and parallel combination to provide the required voltage and power to form a PV module.
• To protect solar cells from damage a module is hermetically sealed between a plate of toughened glass and layers of Ethyl Vinyl Acetate (EVA).
• A terminal box is attached to the back of the module where the two ends of the solar string are soldered to the load.

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• When the PV module is in use, terminals are connected directly to the load.
• Single PV modules of capacities ranging from 10 Wp (peak Watt) to 120 Wp can provide power for different loads.
• Several panels of modules constitute an array which is rated according to peak wattage it delivers at noon on a clear day.

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• The size of an individual cell varies from 10 cm2 to 100 cm2 and a module contains about 20 cells to 40 cells.
• A standard module constituting 30 cells, each , of 7.5 cm diameter, can provide electrical parameters of 12 V, 1.2 A and 18 Wp power.

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• 2. Polycrystalline Silicon Cells
• The production cost of a single crystal cell is quite high compared to the polycrystalline silicon cell.
• Polysilicon can be obtained in thin ribbons draw from molten silicon bath and cooled very slowly to obtain large size crystallites.
• The cells are made with care so that the grain boundaries cause no major interference with the flow of electrons and grains are larger in size than the thickness of the cell.

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• The polycrytalline silicon solar cells can be fabricated in three designs
• (i) p-n junction cells,
• (ii) Metal Insulator Seminconductor (MIS) cells,
• (iii) Conducting oxide insulator semiconductor cells.

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Cross section of a polycrystalline silicon cell

• 3. Amorphous Silicon Cells (Thin Film Cells)
• These are developed recently using thin film cells.
• Amorphous silicon is a pure silicon with no crystal properties.
• It is highly light absorbent and requires only 1 μm to 2 μm of material to absorb photons of incident light.
• Thin amorphous layers can be deposited on cheap substrates like steel, glass and plastic.

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• Hydrogenated amorphous silicon (a-Si: H) is a suitable material for thin film solar cells, mainly due to its high photo-conductivity, high optical absorption of visible light with optical band gap of 1.55 eV.

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Principle of working of Solar cell

• Working of a solar cell depend on two steps
• Creation of pairs of negative and positive charges (known as electron hole pairs) in the solar cell by absorbing solar radiation.
• Separation of the negative and positive charges by creating potential gradient in the cell.

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• For achieving the first step, the cell to be manufactured of a material which can absorb the photons energy of sunlight.
• The photon energy E has relation with Wavelength (λ) as
• E= HV/ λ
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• H= Planck’s constant = 6.62 x 10^-34 Joules second
• V= Velocity of light = 3x 10⁸ m/s
• λ = wavelength of photons in meters

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• The energy in a photon must be higher than the semiconductor band gap energy Eg in order to get absorbed in the cell material and create an electron hole pair to generate potential gradient in the cell.
• A variety of compound semiconductors can be used to manufacture thin film solar cells.
• Cadmium Sulphide (CdS), Cadmium Telluride (CdTe), Gallium Arsenide (GaAs).

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Solar cell- Manufacturing Process

• A variety of compound semiconductors can be used to manufacture thin film solar cells.
• Cadmium Sulphide (CdS),
• Cadmium Telluride (CdTe),
• Gallium Arsenide (GaAs).

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• (a) Gallium Arsenide cell:
• Ordinary GaAs cells have thin films of n type and p type grown on a designed substrate.
• A GaAs has a direct band gap of 1.43 V which makes it suitable for PV thin film material.
• Single cell open circuit voltage is 0.8 V to 0.9 V.
• The efficiency is more than 20% which can be increased using more expensive GaAs subtracts.
• Due to high cost of production this cell is used for space crafts.
• (b) Cadmium Telluride (CdTe)
• Commercially used in Japan
• Thin film heterogeneous junction with n-type CdS and p-type CdTe is fabricated.

Magnified structure of CdTe cell

• CdTe has a direct band gap of 1.44 eV which is favourable to make a thin film cell.
• Cell temperature
• The temperature of a PV cell is obtained by an energy balance.
• The solar energy absorbed by a cell/ mdule is converted potentiallay into electrical energy and the remaining into thermal energy. The electrical energy is used in external circuits while thermal energy is dissipated.

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Solar Photovoltaic System (SPS)

• 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 module are collectively known as Balance of System (BOS) which includes storage batteries, an electronic charge controller and an inverter.

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Block diagram of SPS

Solar Photovoltaic System (SPS)

• Storage batteries with charge regulators are provided for back up power supply during periods of cloudy day and during nights.
• Batteries are charged during the day and supply power to the loads.
• The capacity of a battery is expressed in ampere-hours (Ah) and each cell of the lead acid type battery is of 2 volts.
• Batteries are installed with a microprocessor based charge regulator to monitor the voltage and temperature and to regulate the input and the output currents to obviate overcharging and excessive discharge.

Solar Photovoltaic System (SPS)

• An inverter is provided for converting dc power from battery or PV array to ac power.
• It needs to have an automatic switch off in case the output voltage from the array is too low or too high.

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Performance characteristics

• The electrical characteristics of a solar cell are expressed by the current voltage curves plotted under a given illumination temperature conditions.
• The significant points of the curve are short circuit current I_sc and open circuit voltage V_oc.
• The maximum useful power of the cell is represented by the rectangle with the largest area.
• When the cell yields maximum power, the current and voltage are represented by the symbols Im and Vm respectively.

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Current (I)-Voltage (V) Characteristics of a solar cell

Cell quality is maximum when the value of fill factor approaches unity where the fill factor (FF) is given by

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• The maximum efficiency of a solar cell is defined as the ratio of maximum electric power output to the incident solar radiation

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• Is- Incident solar flux
• Ac – Cell’s area

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Solar thermal electric power plant

• Solar thermal power generation involves the collection of solar heat which is utilised to increase the temperature of a fluid in a turbine operating on a cycle such as Rankine or Brayton.
• Hot fluid is allowed to pass through a heat exchanger to evaporate a working fluid that operates a turbine coupled with a generator.

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Solar thermal electric power plant

• Solar thermal power plants can be classified as low, medium and high temperature cycles.
• Low temperature cycle operate at about 100^oC,
• medium temperature cycles up to 400^oC
• while high temperature cycle work above 500^oC.

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Solar thermal electric power plant

1. Low temperature Solar Power plant

• It uses flat plate collector arrays.
• Hot water (above 90^oC) is collected in an air insulated tank.
• It flows through a heat exchanger, through which the working fluid of energy conversion cycle is also circulated.
• The working fluid is either methyl chloride or butane having a low boiling temperature upto 90^oC.

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Low temperature solar power plant

Low temperature solar power plant

• Vapours so formed operate a regular Rankine cycle by flowing through a turbine, a condenser and a liquid pump.
• As the temperature difference between the turbine outlet and the condensed liquid flowing out is small, about 50^oC, the overall efficiency of the generating system is about 2%.
• Finally, the organic fluid is pumped back to the evaporator for repeating the whole cycle.

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Technical parameters of a 80 MW plant

2. Medium Temperature Solar Power plant

• It uses line focusing parabolic collector for heating a synthetic oil flowing in the absorber tube.
• A suitable sun tracking arrangement is made to ensure that maximum quantity of solar radiation is focused on the absorber pipeline.
• Preheater and superheater are used to increase the inlet steam temperature for the High pressure turbine.

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Medium temperature solar power plant

• Reheaters are used to raise the steam temperature for Low pressure turbine.
• The system generates superheated high pressure steam to operate a Rankine cycle with maximum efficiency.

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3. High temperature Solar Thermal Power Generator

• For efficient conversion of solar heat into electrical energy, the working fluid needs to be delivered into turbine at a high temperature.
• Two possible systems-
• the paraboloidal dish and central receiver to achieve high temperature.

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Central receiver power plants

• Solar radiations are reflected from arrays of mirrors (heliostat) installed in circular arcs around the central tower.
• Reflected radiations concentrate on to the receiver.
• The arrays are provided with a tracking control system that focuses beam radiation towards the receiver.
• Water is converted into steam in the receiver itself that operates a turbine coupled with a generator.

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Central receiver power plant

Solar pumping system

• Water pumps can be driven directly by solar heated water or fluid which operates either a heat engine or a turbine.
• For low heads, the pump driven by vapour of a low boiling point liquid heated by a flat plate collector is used.
• For larger heads, a parabolic trough concentrator or a parabolic bowl concentrator is installed to drive a steam turbine.

Solar pumping system

• Solar flat plate collector arrays are installed to heat water or an organic fluid.
• Hot fluid then flows to a mixing tank/storage tank and then to a heat exchanger to convert the working fluid of the heat engine from liquid to vapour.
• R-115 can be used as working fluid.
• Hot transport fluid or water is fed again into the collector circuit by a circulating pump.

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Solar pumping system

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• With heat engine cycle, discharged vapour from the turbine flows into the condenser where the vapour gets condensed.
• Working liquid is fed into the heat exchanger by a feed pump to complete the cycle.
• Pumped water is used as coolant in the turbine condenser.
• As higher temperature in a heat exchanger or boiler, provides a high engine efficiency.

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• An optimum range of operating temperature is used for a solar pumping system to attain maximum efficiency.

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Solar cooker

• Cooking is a common application of solar energy in India.

Types of solar cookers for different requirements are

1. Box solar cooker
2. Dish solar cooker
3. Community solar cooker for indoor cooking

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1. Box solar cooker

• It consists of an outer box made of either fibre glass or aluminium sheet, a blackened aluminium tray, a double glass lid, a reflector, insulation and cooking pot.
• A blackened aluminium tray is fixed inside the box, and sides are covered with an insulating material to prevent heat loss.
• A reflecting mirror provided on the box cover increases the solar energy input.

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• Metallic cooking pots are painted in black on the outer side.
• Food to be cooked is placed in cooking pots and the cooker is kept facing the sun to cook the food.
• An electric heater may also be installed to serve as a back up during non-sunshine hours.

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Box solar cooker

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2. Dish solar cooker

• A dish solar cooker uses a

parabolic dish to

concentrate the incident solar radiation.

• A typical dish solar cooker has an aperture of diameter 1.4 m with focal length of 0.8 m.
• The reflecting material is an anodized aluminium sheet having reflectivity of over 80%.
• The cooker needs to track the sun to deliver power of about 0.6 kW.
• The temperature at the bottom of the vessel may reach up to 400^oC which is sufficient for roasting, frying and boiling.

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Dish solar cooker

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3. Community solar cooker for indoor cooking

• Like dish solar cooker, the community solar cooker is a parabolic reflector cooker.
• It has large reflector ranging from 7 m² to 12 m² of aperture area.
• The reflector is placed outside the kitchen so as to reflect solar rays into the kitchen.
• A secondary reflector further concentrates the rays on to the bottom of the cooking pot painted black.
• Temperature can reach up to 400^oC and food can be cooked quickly for 50 persons.

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Community solar cooker

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Solar still

• Safe drinking water is scarce in arid, semiarid and coastal areas.
• At such places, saline water is available underground or in the ocean. This water can be distilled utilising the abundant solar insolation available in that area.
• A device which produces potable water by utilizing solar heat energy is called solar water still.

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Cross section of a solar still

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Solar still

• A solar still consists of a basin with black bottom having trays for saline water with shallow depth.
• A transparent air tight glass or a plastic slanting cover encloses completely the space above the basin.
• Incident solar radiation passes through the transparent cover and is absorbed by the black surface of the still.

• Brackish condense

water over

is then heated and water vapours the cool interior surface of the

transparent cover.

• The condensate flows down the glass and gets collected in troughs installed as outer frame of the solar still.

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Solar still

• Distilled water then transferred into a storage tank.
• This system is capable of purifying sea water with salinity of about 30,000 mg/litre.
• The production rate is about 3 litres/m²/day in a well designed still on a good sunny day.
• The performance of solar still is expressed as the quantity of water produced by each unit of basin area per day.
• The production rate depends on the intensity of solar radiation, the ambient air temperature, wind speed and the sky condition.

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Solar still

• Design parameters that affect the production of drinking water include orientation of still, inclination of glass cover and insulation of the base.

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Large solar stills in India

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Solar drier

• Solar energy is effectively utilised for controlled drying of agricultural products to avoid food losses between harvesting and consumption.
• High moisture crops are prone to fungus infection, attack by insects and pests.
• Solar dryers remove moisture with no ingress of dust, and the product can be preserved for a longer period of time.

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Solar drier

• Openings are provided at the bottom and top of the enclosure for natural circulation.
• The material to be dried, is spread on perforated trays.
• Solar radiation enters the enclosure, is absorbed by the material and internal surfaces of the enclosed.
• Consequently, moisture from the product evaporates, the air inside is heated and natural air circulation starts.

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Cross section of a cabinet Solar drier

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Solar drier

• The temperature inside the cabinet ranges from 50^oC to 75^oC and the drying time for products like dates, grapes, apricots, cashew nuts and chillies varies from 2 to 4 days.
• For large scale drying like seasoning of timber, corn drying, tea processing, tobacco curing, fish and fruit drying solar kilns are used.

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Solar Kilns

• Heating and drying on a large scale using solar energy.
• It operates on the principle that a transparent glass sheet or polythene sheathing allows solar radiation to pass through into the kiln and blocks long wavelength radiation emitted by products like timber back into the atmosphere.

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Side view of timber solar kiln

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Solar Kilns

Factors affecting the drying process are:

• Relative humidity and temperature of air
• Air flow rate
• Initial moisture content of the product
• Final desired moisture content of the product
• Solar kiln used for seasoning of timber consists of three major parts: (i) a wood seasoning chamber, (ii) a flat plate collector, and (iii) a chimney seasoning chamber which is placed on a raised masonry platform.

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Solar Kilns

• The chimney creates a natural draught in the seasoning chamber, causing hot air circulation around stacked wood.
• Circulating air carries heat from the solar absorbing plate to timber logs and evaporates moisture.
• Drying is basically a heat and mass transfer process, the moisture from surface and inside the product is vaporized and removed by circulating hot air.

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Solar air-conditioning and refrigeration

• One of the thermal applications of solar energy is for cooling buildings (air-conditioning) or for refrigeration needed for preserving food.
• Three modes of solar energy cooling
• Evaporative cooling
• Absorption cooling
• Passive desiccant cooling

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1. Evaporative cooling

• It is a passive cooking technique.
• It works on the principle that when warm air is used to evaporate water, the air itself becomes cool and then t cools the living space of the building.
• Common techniques used for cooling are vapour absorption and vapour compression.

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Absorption cooling system

• Water is heated in a flat plate collector array and is passed through a heat exchanger called the generator.
• Suitable chemical solutions for absorption cooling are (i) NH₃- H₂O where NH₃ is used as the working fluid and (ii) LiBr- H₂O soultion where H₂O operates as the working fluid.

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Solar absorption cooling system

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Absorption cooling system

• The whole system consists of following four units: generator, condenser, evaporator and absorber.
• The generator contains a solution mixture of absorbent and refrigerant, and this mixture is heated with solar energy.
• Refrigerant vapour is boiled off at a high temperature and flows into condenser, where it gets condensed rejecting heat and becomes liquid at high pressure.

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Absorption cooling system

• Refrigerant then passes through the expansion valve and evaporates in the evaporator.
• The refrigerant vapour is then absorbed into a solution mixture taken from the generator in which the refrigerant concentration is quite low.
• A heat exchanger is provided to transfer heat between solutions flowing between the absorber and generator.

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• Passive Desiccant Cooling
• The passive desiccant cooling method is effective in a warm and humid climate.

• Natural cooling of human sweating does not occur in

body highly

through humid

conditions.

• The removal of moisture from the room air using either the absorbent, followed by evaporative cooling of air, is a workable air conditioning method for use in a hot and humid climate.

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Solar dehumidification and evaporative cooling

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• Desiccant materials have a high affinity for water vapour which are used to dehumidify moisture.
• In solar airconditioning, silica gel, molecular sieve and triethylene glycol are used as desiccant materials.
• In desiccant cooling, the hot and humid air from rooms is first dehumidified with a solid or liquid desiccant, then cooled by exchange of sensible heat and finally it is evaporately cooled.

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• The triethylene glycol (TEG) is atomised in order to cause rapid absorption of water vapour in the absorption chamber.
• The TEG is then pumped through a heat exchanger to a stripping chamber (regenerator), sprayed counter-currently to solar heated air from solar collectors.
• Hot air takes a part of moisture from glycol solution and is exhausted to the atmosphere.

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• Hot concentrated glycol is pumped back through a heat exchanger to the absorption chamber (dehumidifier).
• Dehydrated air from the absorption chamber passes through the evaporative cooler for further supply into the air conditioned room.

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Solar furnace

• Solar furnace is an optical equipment which concentrates solar radiation on a small area for creating a high temperature.
• To make a concentration radiation in a small area from a large area receiving solar radiation, there are two ways:
• (i) refraction from a big single lens or multiple lenses
• (ii) paraboloidal reflector either single or heliostat type.

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Major components of a solar furnace

• Concentrator- Solar furnaces use either a paraboloidal reflector concentrator or a spherical reflector concentrator.

• The paraboloidal reflector

superior due to unacceptable

is considered

spherical

aberration in a spherical reflector.

• An electronically polished aluminium sheet, finished with anodization, provides a better reflecting surface.

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Heliostat

Heliostats in a solar furnace serve to orient solar radiation parallel to the optical axis of the concentrator.

The shape and size of a heliostat is guided by the aperture of the concentrator as the heliostat has to reflect solar radiation over the full aperture of the concentrator, considering the latitude of location, the solar declination and the angular width of diverted rays from the heliostat.

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Solar furnaces- (a) Multiple lens type, (b) Heliostat type (optical axis horizontal), (c) Heliostat type (optical axis vertical)

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• Sun tracking
• For optimum functioning of a solar furnace, heliostats need to follow the sun from morning till evening.
• Using a servo system, small deviation of solar

concentrator is

radiation incident monitored by

photo

activate the azimuth

on the cells, and

elevation of

which in turn

the

operating system and adjust the solar radiation to make it parallel to the optical axis of the concentrator.

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1000 kW solar furnace with multiple heliostat

​

• The first 1000 kW solar furnace started operation in 1973, France.

• It consisted of 63 heliostats

installed at 8

elevations which reflected sun rays to the concentrator parallel to its optical axis.

• The paraboloidal concentrator was 40 m high with a focal length of 18 m, effective mirror area 1920 m², aperture ratio nearly 2.8, with input solar energy of 1800 kW.

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1000 kW solar furnace with heliostat

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