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Q. Describe a flat plate solar collector. Give its advantages and disadvantages.
Ans. When temperatures up to 90°C are adequate as for space and service water heating. Flat plate collectors are used. They are made in rectangular panels, from 1.7-2.9 m² in area, and are relatively simple to construct and erect. They can absorb both direct and scattered solar radiation, therefore they are partially effective even on cloudy days, when there is no direct radiation. Basic principle of a typical flat plate collector (liquid type) is shown in fig. 1.3.
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(ii) A transparent cover
(iii) A heat transfer fluid
(iv) A heat insulating backing.
The solar rays pass through transparent covers and fall on absorbing surface of flat plate collectors. The absorbing surface is made of copper, aluminum, or steel coated with a heat resistant black paint or a thin layer of a black oxide (e.g.. copper oxide on copper) to absorb the solar radiation energy. Radiation energy is converted into heat, and water flowing through the tubes gets heated.
The function of insulation is to prevent the heat loss from the absorber and the heat transferring fluid. Commonly used insulating materials are fiberglass and Styrofoam.
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 as compared to focusing collectors which requires concentrating reflectors, absorbing surfaces and orientation devices.
(iv) They require little maintenance.
Disadvantages -
(i) Low thermal efficiency.
(ii) Requires larger collecting area.
(iii) Energy storage cost is more.
(iv) An anti-freeze solution has to be used to prevent the freezing of heat transferring fluid.
Disadvantages -
(i) Low thermal efficiency.
(ii) Requires larger collecting area.
(iii) Energy storage cost is more.
(iv) An anti-freeze solution has to be used to prevent the freezing of heat transferring fluid.
Q. How solar energy can be used for electricity generation and water heating? (R.G.P.V., Dec. 2013)
Ans. Use of Solar Energy for -
(i) Electricity Generation - Electricity from solar energy can be produced in two ways -
(a) By solar thermal power production system. (b) By solar photovoltaic cells.
In a solar thermal power production system, solar energy is first collected by using a solar pond, a flat plate collector, focusing collector or heliostats (tunable mirrors). This energy is used to increase the internal energy or temperature of a fluid. This heated fluid may be used directly in any of the common cycles such as Rankine, or it may be used to heat another fluid by means of a heat exchanger, which in turn can be used in cycle to produce mechanical power, from which electricity can be produced.
In second case solar photovoltaic cells can be used to convert solar energy directly into electricity by means of photovoltaic effect. Photovoltaic effect can be defined as the generation of an electromotive force due to absorption of ionizing radiation.
Photovoltaic cells are made of semiconductors that generates electricity when they absorb light. When photons from sun are absorbed in a semiconductor, they create free electrons of energy higher than the electrons which provides the bonding in the base crystal. These free electrons can be collected on the contacts applied at the surfaces of semiconductor. These devices have theoretical efficiencies of the order of 25%, while actual efficiencies can be only 10 to 12%. The best known application of photovoltaic cell is electric power generation in spacecrafts. They can also be used for energization of pump sets for irrigation, drinking water supply and rural electrification.
(ii) Water Heating :- The basic elements of a solar water heater are flat plate collector and an insulated storage tank. A typical flat plate collector consists of a blackened metal plate absorber containing metal tubing for water flow. It is usually provided with a glass cover (one or more layers of glass) and a layer of insulation beneath the plate. The collector is placed to face the direction of sun as shown in fig. 1.4. The collector tubing is connected to an insulated tank, that stores hot water, when it is not required. The collector absorbs the solar radiation.
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Q. Explain the uses of solar energy for the purpose of- (i) Solar water heating (ii) Solar air conditioning.
Ans. (i) Solar Water Heating - The basic elements of a solar water heater are flat plate collector and an insulated storage tank. A typical flat plate collector consists of a blackened metal plate absorber containing metal tubing for water flow. It is usually provided with a glass cover (one or more layers of glass) and a layer of insulation beneath the plate. The collector is placed to face the direction of sun as shown in fig. 1.4. The collector tubing is connected to an insulated tank, that stores hot water, when it is not required. The collector absorbs the solar radiation.
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and transfer this heat to the water circulating through the tubes either by gravity or by a pump. A solar collector, area of 3-4 m² in combination with an insulated tank of 200-400 liter capacity can provide 200-300 liters of hot water at an average temperature of 60°C on a sunny day. Solar water heating systems can be used for domestic, commercial and industrial applications. Commercial uses may be for hotels, hospitals, guest houses, hostels, canteen, etc. Industrial applications includes for sugar, textile, chemical, pharmaceutical, dairy and food processing industries.
(ii) Solar Air Conditioning- Solar air conditioning refers to the cooling of buildings by using solar energy. Two common techniques used for cooling are vapor compression and vapor absorption. For solar assisted vapor compression cooling, the solar energy is converted into mechanical energy which is then used for compression of vapor. The vapor is then cooled and condensed into a liquid. The condensed liquid is then vaporized by using the heat from the air to be cooled. This process has limited application due to the high cost of conversion of solar energy into mechanical energy. In absorption cooling systems two working fluids, a refrigerant and an absorbent- refrigerant solution, are used. The absorbent cooling is based on the principle that the refrigerant can be bound by a liquid or solid solvent, known as the absorbent to release heat during absorption, while it absorbs heat during evaporation (and thus produces a cooling effect). The two most common combinations are LiBr-water (LiBr as absorbent and water as refrigerant) and aqua-ammonia (water as absorbent and ammonia as refrigerant).
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A simple solar operated absorption cooling system is shown in fig. 1.6. Water is heated in a flat plate collector array and is passed through a heat exchanger called the generator. The whole system consists of four units - generator, condenser, evaporator and absorber. The generator contains a solution mixture of absorbent and refrigerant, and this mixture gets heated with solar energy. Refrigerant vapor is boiled off at a high pressure and flows into the condenser, where it gets condensed rejecting heat and becomes liquid at high pressure. Refrigerant then passes through the expansion valve and evaporates in the evaporator. The refrigerant vapor thus obtained absorbed into a solution mixture taken from the generator in which the refrigerant concentration is quite low. The rich solution thus prepared is pumped back to the generator at a high pressure to complete the cycle. A heat exchanger is employed to transfer heat between solutions flowing between the absorber and the generator.
(ii) Solar Air Conditioning- Solar air conditioning refers to the cooling of buildings by using solar energy. Two common techniques used for cooling are vapor compression and vapor absorption. For solar assisted vapor compression cooling, the solar energy is converted into mechanical energy which is then used for compression of vapor. The vapor is then cooled and condensed into a liquid. The condensed liquid is then vaporized by using the heat from the air to be cooled. This process has limited application due to the high cost of conversion of solar energy into mechanical energy. In absorption cooling systems two working fluids, a refrigerant and an absorbent- refrigerant solution, are used. The absorbent cooling is based on the principle that the refrigerant can be bound by a liquid or solid solvent, known as the absorbent to release heat during absorption, while it absorbs heat during evaporation (and thus produces a cooling effect). The two most common combinations are LiBr-water (LiBr as absorbent and water as refrigerant) and aqua-ammonia (water as absorbent and ammonia as refrigerant).
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A simple solar operated absorption cooling system is shown in fig. 1.6. Water is heated in a flat plate collector array and is passed through a heat exchanger called the generator. The whole system consists of four units - generator, condenser, evaporator and absorber. The generator contains a solution mixture of absorbent and refrigerant, and this mixture gets heated with solar energy. Refrigerant vapor is boiled off at a high pressure and flows into the condenser, where it gets condensed rejecting heat and becomes liquid at high pressure. Refrigerant then passes through the expansion valve and evaporates in the evaporator. The refrigerant vapor thus obtained absorbed into a solution mixture taken from the generator in which the refrigerant concentration is quite low. The rich solution thus prepared is pumped back to the generator at a high pressure to complete the cycle. A heat exchanger is employed to transfer heat between solutions flowing between the absorber and the generator.
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