The following points highlight the two types of collectors of solar energy. The types are: 1. Flat Plate Collectors 2. Focusing or Concentrating Collectors.
Since solar energy is widely spread, there is a need to make it available in concentrated form for which solar collectors are required. A solar collector absorbs the incident solar radiation and converts it into useful heat energy which is used for heating a collector fluid such as water, oil or air. The surface of a solar collector is designed for high absorption and low emission. Solar collectors are of two types namely flat plate type (or non-concentrating type) and focusing (concentrating) collectors.
Flat plate collectors are used where temperatures below 100°C are required. These collectors could be of liquid heating or air heating types. The liquid heating collectors are often used for heating water whereas air heating collectors are used for drying of agricultural products, heating of green-houses etc. Concentrating collectors use optical systems in the form of reflectors or refractor to concentrate the energy of solar direct radiation on the absorbing surface. The reflectors may be flat mirrors or in the shape of a parabolic trough or paraboloidal dish.
Type # 1. Flat Plate Collectors:
The important parts of a typical flat plate collector are shown in Fig. 7.24. The absorber plate with several parallel tubes is fabricated from copper tube and sheet by soft soldering. The plate is blackened in order to absorb maximum amount of sunlight. The plate is enclosed in a box to insulate it on the sides and bottom so as to prevent losses and thereby attain high temperatures. Also one or more transparent glass or plastic sheets are placed on the top of the blackened sheet so as to avoid heat losses by re-radiation.
The complete structure is placed at a proper inclination to the sun so as to absorb solar radiations. The heat absorbed by plate is removed by circulating water or fluid or air in contact with it or through the tubes. The flat plate collector gives a temperature of only about 60°C above ambient, i.e., less than 100°C with an efficiency of 30-50%. So flat plate collector is mainly used for water heating, space heating, space cooling and
drying. However, by evaporating a low boiling organic fluid, a vapour turbine can be used to produce electric power
The collection efficiency of flat plate collector varies from 40 to 60% for a temperature rise of about 15°C but drops to 30% or even less for a 50°C temperature rise. However, the absorbing efficiency of a flat plate collector can be increased by applying a selecting coating to the collecting surface, instead of flat black paint. The efficiency can also be improved by applying anti-reflective coatings on the transparent covers.
The flat plate collector absorbs both diffuse and direct radiations. However, a tracking or a tilting arrangement is usually provided to track the sun from morning to sunset. Here the heat absorbed is utilised to evaporate some working medium, butane, in a heat exchanger. The vapours are then expanded in a vapour turbine to get the output.
Different types of flat plate collectors are shown in Fig. 7.26. Pipe and fin type collectors are expensive but have good corrosion resistance. In this type, liquid flows only in pipe and hence have low wetted area and liquid capacity. In sandwich type both the wetted area and capacity of water are high. They are cheap and light. These are used for low temperature requirement, such as swimming pool.
The properties of the materials used for collectors can be classified by thermo physical properties (such as thermal conductivity, heat capacity etc.); physical properties (like density, tensile strength, melting point etc.) and environmental properties (such as moisture penetration, corrosion resistance and degradation due to pollutants in atmosphere.)
The material for absorber plate should have high thermal conductivity, adequate tensile strength and good corrosion resistance. The most common material used for absorber plate is copper because of high conductivity and resistance to corrosion. Other materials used for absorber plate are aluminium, iron, brass, silver, tin and zinc.
The material for thermal insulation should have low thermal conductivity, should be stable at high temperature. Some commonly used materials are crown white wool, glass wool, calcium silicate, cellular form etc.
For cover plate tempered glass is most commonly used material. Transparent plastic materials such as acrylic polycarbonate plastic, polyvinyl fluoride are used for cover plate.
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For higher temperature requirement, such as in domestic applications or for industrial use pipe and fin type collectors are used.
Type # 2. Focusing or Concentrating Collectors:
Inspite if the methods of reducing heat losses, the maximum temperature at which a flat plate collector operates is quite low, about 100°C in summer and 40°C in winter. So for higher temperature focusing or concentrating collectors are used. Such collectors are more effective but very costly.
In focusing collectors, a parabolic or a fresnel mirror is used. Sun rays are focused on the focal point of the mirror by reflection from its surface. A tube may be placed along the focal line of the mirror and a fluid circulated through it to absorb the heat. With these collectors, temperatures of 200- 300°C or above may be obtained. Some mechanism should be provided to track the sun seasonally.
The focusing collectors can have two arrangements:
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(a) Cylindrical Parabolic Concentrator:
This is a medium range temperature concentrator. It can give temperature in the range of 100-200°C. This type of arrangement is generally employed for vapour engines and turbines, process heating, cooking etc.
(b) Paraboloids, Mirror Arrays:
This is high temperature arrangement (gives temperature above 200°C). It can be used for steam engines and turbines, sterling engines and thermoelectric generator.
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Mirrors and Fresnel lenses are imaging concentrators which are capable of focusing only beam radiation. In order to continuously collect beam radiation these concentrators have to be rotated by a tracking mechanism about one axis (linear concentrators) or two axes (point-focus concentrators) to follow the motion of the sun. High operating temperatures such as 1,000°C or even higher can be achieved when point focus concentrators with high concentration ratio are used.
Various types of concentrator solar collectors are shown in Fig. 7.27.
Figure 7.27(a) shows a paraboloidal reflector. Its polar axis is aligned E.W. A collecting tube receives a useful fraction of energy falling on the reflector.
Figure 7.27(b) shows a parabolic trough concentrator. This system has a parabolic mirror and receives at its focal point. The concentration ratio is very high, and therefore, can be used where high temperatures are required.
Fresnel lens concentrator is shown in Fig. 7.27(c). It is a linear Fresnel lens solar concentrator. It consists of linear grooves on one surface of the refracting material. This concentrator may be installed with either the grooves facing the sun or the grooves facing downwards. Both glass and plastic can be used as refracting materials for Fresnel lenses.
In Fig. 7.27(d), the central stationary receiver, receives the solar radiations on a central receiver, which is stationary, concentration ratio is high, about 3,000. A combination of heliostat and paraboloidal reflector may also be used. The heliostat can be adjusted to account for the change in sun’s position.
In Figure 7.26 (e)heliostats have been found to be very suitable as collectors for solar power plants. The absorbed energy can be extracted from the receiver and delivered at a temperature and pressure suitable for driving turbines for power generation.
Figure 7.27(f) shows the compound parabolic concentrator. It was designed by Winston and Baranov. It consists of two parabolic segments, oriented such that focus of one is located at the bottom end point of the other and vice versa. The receiver is a flat surface parallel to the aperture joining of two foci of the reflecting surfaces.
Flat collectors have the following advantages over concentrator collectors:
1. Absorbs the diffuse, direct and reflected components of radiation.
2. Comparatively easy to fabricate and cheaper; and
3. Since these are usually fixed in tilt and orientation, tracking is not required—this makes them maintenance free, except for surface cleaning.
Expensive research is being done in the various methods of converting solar energy into electrical energy. Broadly speaking there are two methods for converting solar energy into electrical energy namely direct conversion method and conventional boiler method.
Conventional Boiler Method:
Basic elements of a solar power plant are shown in Fig. 7.28. Large parabolic collectors are employed for collecting solar energy, which is used to heat a fluid (usually water). This heat energy is finally transferred to feed water which is converted into steam. This steam is utilised to run a prime mover (steam turbine) coupled to an electric generator, which generates electric power. Steam is condensed in the condenser and feed water returns to the boiler for re-use. The heat of the cooling water of the condenser may be utilized for some other purposes.
Regarding the size of generating units some engineers prefer use of small units located at the place of use whereas others propose large centralised solar thermal plants.
Small generating units are best suited for India. According to the report of the National Council of Science and Technology (NCST) solar energy panel, 50 kW electrical generation plants are required to meet the energy needs of a village of 500 people. Such villages constitute almost 80% of the settlements in the country.
Solar energy is the only possible energy source which can be efficiently used for inland areas remote from the existing power grids. For Indian conditions, flat plate collectors with mirror boosters and nonselective absorbers which yield low pressure steam at 150°C, are best. An efficiency of 15 to 18% can be aimed in such systems.
Collector areas required would be 20 m2/kW of peak generation and about 40 m2 for the rated capacity. Small vapour turbines to generate electrical energy have been developed in Israel by Harry Tabor. According to him, monochlorobenzene is the best working fluid for use in small turbines operating at 150°C and 18,000 rpm. Other vapour engines using SO2, NH3 etc. have also been built. A miniature solar power plant in Senegal is already in operation.
Under the large centralised units for power generation there are two types of systems used namely solar furnace system and solar farm system. In solar furnace system the sun rays reflected from many different heliostats are concentrated on a single heat exchanger whereas in solar farm system a large number of linear reflectors focus sunrays on long pipes which collect heat.
There are two designs which can be used in solar furnace system and these are shown in Figs. 7.29 and 7.30. Figure 7.29 illustrates the tower concept for power generation. A boiler is mounted on the top of the tower located near the centre of the field of large mirrors called the heliostats. Sun rays after getting reflected from the heliostats are focused on the boiler.
Thus boiler generates high temperature steam which is used to drive a steam turbine coupled to an electric generator. A 50 kW power plant based on this design has been built and operated in Italy. However, the world’s largest power plant of 10 MW has been commissioned in 1982 in California (U.S.A.).
In another design of solar furnace system, shown in Fig. 7.30, arrays of heliostat mirrors to focus sun rays into a cavity type boiler are used. Sun rays striking the heliostat mirrors are reflected on to a parabolic reflector. From the parabolic reflector, the sun rays get reflected and concentrated in the cavity of a heat exchanger. A 1,000 kW solar furnace power plant, based on this principle, is being operated at Odeillo (France).
In a solar farm system the sun’s heat is to be trapped in extensive arrays of steel pipes spread out in panels.
For focusing the sun rays, cylindrical concave mirrors with parabolic cross section are used. The tubes are placed along the focal line of the mirrors. Heliostats are pivoted and are slowly rotated at a rate of 15° per hour to match the daily course of sun. During all these movements of mirrors, the tubes remain fixed.
The tubes are coated with selective coatings (mixture of silicon and silver) and are enclosed in an evacuated glass chamber so that only a small proportion of the absorbed energy is emitted and conductive and convective heat losses are reduced. Nitrogen gas is pumped through the tubes at about 4 m/s to transfer the heat from the collectors to a central storage unit.
The neat storage medium is a eutectic mixture of salts, mostly sodium nitrate, and the heat would be used to raise steam. An overall efficiency of about 6% has been obtained at present. According to Dr. Menial of University of Arizona, 20 km2 of cloud free land using this technique could produce a million kW of clean, pollution free power.
India’s first solar thermal power plant based upon solar farm system was commissioned in May 1989 at Gawal Pahari, district Gurgaon (Haryana). The total system consists of a solar collector field, an oil storage tank, a steam generator, air cooled steam condenser and a demineralised water plant. The capacity of the plant is 50 kW.
Solar Trough System:
Trough systems predominate among today’s commercial solar power plants. Trough systems convert the heat from the sun into electricity. Because of their parabolic shape, troughs can focus the sun at 30 to 60 times its normal intensity on a receiver pipe located along the focal line of the trough. Synthetic oil captures this heat as the oil circulates through the pipe, reaching temperatures as high as 390°C (735°F). The hot oil is pumped to a generating station and routed through a heat exchanger to produce steam. Finally, electricity is produced in a conventional steam turbine.
Solar Dish/Engine Systems:
These systems, with net solar-to-electric conversion efficiencies reaching 30% can operate as stand-alone units in remote locations or can be linked together in groups to provide utility-scale power.
Solar dish/engine systems convert the heat energy of the sun into electricity at a very high efficiency. Using a mirror array formed into the shape of a dish, the solar dish focuses the sun’s rays onto a receiver. The receiver transmits the energy to an engine, typically a kinematic sterling engine (although Brayton-cycle engines are also being considered), that generates electrical energy.
Because of high concentration ratios achievable with parabolic dishes and the small size of the receiver, solar dishes are efficient for collecting solar energy at very high temperatures. Tests of prototype systems and components at locations throughout the United States have demonstrated net solar-to-electric conversion efficiencies as high as 30%. This is significantly higher than any other solar technology.
Application of Flat Plate Collectors in Power Generation:
Low temperature power generation using flat plate collectors is shown in Fig. 7.33. The solar radiations are received by flat plate collectors. The heat energy is collected by water. The hot water is stored in insulated storage tank. From here, it flows through vapour generator through which the working fluid of Rankine cycle is also passed. The working fluid has a low boiling point. Vapour at 90°C leaves the vapour generator.
The vapour then executes a regular Rankine cycle by flowing through prime mover condenser and pump.
I. Solar Chimney Power Plant:
Solar chimney power plant is shown in Fig. 7.34. The main components of such a plant are air heater of large area, a high chimney and air turbine.
The air stream is heated by solar radiation absorbed by the ground under a transparent cover of large size, flows through the air turbine rotor installed in the chimney. The driving force causing the upward flow of the air is determined by the density difference of cold and hot air and the height of the chimney. For 1 m2 cross-sectional area of chimney, the driving force is equal to the pressure difference of the air stream.
The world’s only chimney solar power plant was installed in 1981 at Manzanares in South Spain.
II. Solar Ponds:
Solar ponds are also called solar salt ponds. Natural ponds convert solar radiation into heat, but the heat is quickly lost through convection in the pond and evaporation from its surface. A solar pond is designed to reduce convective and evaporative heat losses by reversing the temperature gradient with the help of non-uniform vertical concentration of salts. A greater salt concentration at the bottom than at top causes bottom water to have greater density and remains at the bottom and is also hotter. The solar energy is absorbed in deep layers and is usefully trapped.
The solar ponds are useful in two ways:
1. The conversion of solar energy to useful work as a result of the temperature difference between bottom and top layers of the pond.
2. The use of pond as a thermal storage medium.
A solar pond is a combination of solar collector and storage medium.
A schematic of artificial solar pond and conversion system is shown in Fig. 7.35 below:
The pond is divided into three layers:
1. Top Layer:
Low density convective layer.
2. Central Layer:
Non-convective layer and contains the required salinity and density gradient with salinity greatest at the bottom.
3. Bottom Layer:
High density-salinity convective storage layer.
The bottom warm water is used as the heat source and top cool water as the heat sink. The hot bottom water is pumped through an evaporator and back to the bottom. An organic fluid is evaporated and drives a turbo-generator. The turbine exhaust is condensed and the condensate is fed back to evaporator. The cooling water for condenser is taken from the top cool layer of the pond.
A leak-proof pond is constructed 330 kg of salt per square metre of pond area is added. The bottom layer can achieve a temperature of 93°C. The salinity gradient is maintained in the pond by injecting salt to the bottom layer and flushing the top layer with fresh water periodically.
The efficiency is very low but these plants have large potential for tapping solar energy.
The main applications of solar ponds are:
(i) Power generation,
(ii) Space heating and cooling,
(iii) Crop drying,
(iv) Desalination, and
(v) Process heat.
The main limitations are:
(i) Sunny climate,
(ii) Requirement of large land area,
(iii) Availability of water, and
(iv) Availability of salt (brackish water).
A solar pond power plant of 5 MW capacity is in operation in Dead sea, Israel.
III. Solar Lighting:
Day-lighting systems collect and distribute sunlight to provide interior illumination. This passive technology directly offsets energy use by replacing artificial lighting and indirectly offsets non-solar energy use by reducing the requirement for air-conditioning. Although difficult to quantify, the use of natural lighting also offers physiological and psychological benefits compared to artificial lighting. Day lighting design implies careful selection of window types, sizes and orientation; exterior shading devices may be considered as well. On implementation of day lighting features properly, light-related energy requirements can be reduced by 25%.
Hybrid solar lighting (HSL) is an active solar method of providing interior illumination. HSL systems collect sunlight using focus mirrors that track the Sun and use optical fibers for its transmission to inside the building for supplementing conventional lighting.
Solar Home Lighting System:
In this system, solar cells are employed for supply of power. The electrical energy is stored in batteries and used for the lighting purpose when needed. Such systems are useful in non-electrified rural areas and as reliable emergency lighting system for important domestic, commercial and industrial applications. Solar photovoltaic systems have found important use in the dairy industry for lighting milk collection/chilling centers mostly located in rural areas.
The solar home lighting system is a fixed installation for domestic application. It comprises of solar PV module (solar cells), charge controller, battery and lighting system (lamps and fans). The solar module is installed in the open on roof/terrace-exposed to sunlight and the charge controller and battery are kept inside a protected place in the house. The solar module needs periodic dusting for effective performance. The systems are designed to provide a daily working time of 3-4 hours with a fully charged battery. The system provides for buffer storage for 1-2 non-sunny/cloudy days.
IV. Solar-Powered Pump:
A solar powered pump is a pump running on the solar energy. It can be more environmentally friendly and economical in its operation. It consists of three parts – water pump, engine and energy source being powered by the sun.
The solar water pumping system is a stand-alone system operating on power generated by use of PV system. The energy produced by solar cells is used in operation of dc surface centrifugal mono-block pump set for lifting water from bore/ open well or water reservoir for minor irrigation and drinking water purpose.
The system needs a shadow-free area for installation of the solar panel. The system is provided with 1,800 W solar PV panel (24 nos × 75 W power) and 2 hp centrifugal dc mono-block/ac submersible with inverter. The average water delivery of such a pump will be about 1.38-1.40 lakh litres per day, for a suction head of 6 m and dynamic head of 10 m.
The size of suction and delivery lines is about 62.5 mm. Solar water pump sets have the advantages of zero fuel cost, nil conventional grid electricity requirement, long operating life, highly reliable and durable performance, easy operation and maintenance, eco-friendship and saving of conventional diesel fuel.
These systems are employed for irrigation and drinking water in India. The majority of the pumps fitted with 200-3,000 watt motors are powered with 1,800 W power PV arrays that can deliver about 140,000 litres of water/day from a total head of 10 m.