SOLAR ENERGY SYSTEMS

SOLAR ENERGY SYSTEMS

The sun is the source of the vast majority of the energy we use on earth. Most of the energy we use has undergone various transformations before it is finally utilized, but it is also possible to tap this source of solar energy as it arrives on the earth’s surface. There are many applications for the direct use of solar thermal energy, space heating and cooling, water heating, crop drying and solar cooking. It is a technology, which is well understood and widely used in many countries throughout the world. Most solar thermal technologies have been in existence in one form or another for centuries and have a well established manufacturing base in most sun-rich developed countries.

The most common use for solar thermal technology is for domestic water heating. Hundreds of thousands of domestic hot water systems are in use throughout the world, especially in areas such as the Mediterranean and Australia where there is high solar insulation (the total energy per unit area received from the sun). As world oil prices vary, it is a technology, which is rapidly gaining acceptance as an energy saving measure in both domestic and commercial water heating applications. Presently, domestic water heaters are usually only found among-st wealthier sections of the community in developing countries. Other technologies exist which take advantage of the free energy provided by the sun. Water heating technologies are usually referred to as active solar technologies, whereas other technologies, such as space heating or cooling, which passively absorb the energy of the sun and have no moving components, are referred to as passive solar technologies. More sophisticated solar technologies exist for providing power for electricity generation. We will look at these briefly later in this fact sheet.

Sun is the source of many forms of energy available to us. Do you know how energy is obtained from the sun? The most abundant element in sun is hydrogen. It is in a plasma state. This hydrogen at high temperature, high pressure and high density undergoes nuclear fusion and hence releases an enormous amount of energy. This energy is emitted as radiations of different forms in the electromagnetic spectrum.

Out of these X-rays, gamma rays and most of ultraviolet rays do not pass through the earth's atmosphere. But heat energy and light energy are the main radiations that reach the earth. This energy is the basis for the existence of life on earth.

Sun is a sphere of intensely hot gaseous matter with a diameter of 1.39e⁹ m and 1.5e¹¹ m away from earth. Sun has an effective black body temperature of 5762 K and has a temperature of 8e⁶ K to 40e6⁶K. The sun is a continuous fusion reactor in which hydrogen (4 protons) combines to form helium (one He nucleus). The mass of the He nucleus is less than that of the four protons, mass having been lost in the reaction and converted to energy. The energy received from the sun on a unit area perpendicular to the direction of propagation of radiation outside atmosphere is called solar constant, and has a value 1353 Wm– 2. This radiation when received on the earth has a typical value of 1100 Wm– 2 and is variable. The wavelength range is 0.29 to 2.5 micro meters. This energy is typically converted into usual energy form through natural and man-made processes. Natural processes include wind and biomass. Man-made processes include conversion into heat and electricity.

SOLAR RADIATIONS:-

Radiation from sun on entering the earth’s atmosphere gets scattered by the atmospheric gas molecules and dust particles and received on earth from all directions and is called diffuse radiation. The portion of radiation received on earth from sun without change in original quality is called beam or direct radiation.

The earth revolves about the sun in an approximately circular path, with the sun located slightly off center of the circle. The earth’s axis of rotation is tilted 23.5 degrees with respect to its pane of revolution about the sun, the position of the earth relative to the sun’s rays at the time of winter solstice when the North Pole is inclined 23.5 degree away from the sun. All points on the earth’s surface north of 66.5 N latitude are in total darkness while all regions within 23.5 degree of the South Pole receive continuous sunlight. At the time of the summer solstice, the situation is reversed. At the time of the two equinoxes, both poles are equidistant from the sun and all points on the earth's surface have 12 hours of daylight and 12 hours of darkness. The sun’s ray passing through the center of the earth lies in the equatorial plane at the time of equinoxes. From vernal equinox to autumnal equinox, the rays lie north of the equatorial plane. From autumnal equinox to vernal equinox, the rays lie south of the equatorial plane. The average direction of the sun’s rays for the entire year lies in the equatorial plane. Accordingly to intercept maximum amount of solar energy over the whole year, a solar collector in the northern hemisphere should be tilted and face due south.

The Nature and Availability of Solar Radiation:- Solar radiation arrives on the surface of the earth at a maximum power density of approximately 1 kilowatt per meter squared (kWm⁻²). The actual usable radiation component varies depending on geographical location, cloud cover, hours of sunlight each day, etc. In reality, the solar flux density (same as power density) varies between 250 and 2500 kilowatt hours per meter squared per year (kWhm⁻² per year). As might be expected the total solar radiation is highest at the equator, especially in sunny, desert areas. Solar radiation arrives at the earth's outer atmosphere in the form of a direct beam. This light is then partially scattered by cloud, smog, dust or other atmospheric phenomenon. We therefore receive solar radiation either as direct radiation or scattered or diffuse radiation, the ratio depending on the atmospheric conditions. Both direct and diffuse components of radiation are useful, the only distinction between the two being that diffuse radiation cannot be concentrated for use.

Solar radiation arriving from the sun reaches the earth’s surface as short wave radiation. All of the energy arriving from the sun is eventually re-radiated into deep space otherwise the temperature of the earth would be constantly increasing. This heat is radiated away from the earth as long-wave radiation. The art of extracting the power from the solar energy source is based around the principle of capturing the short wave radiation and preventing it from being re-radiated directly to the atmosphere. Glass and other selective surfaces are used to achieve this. Glass has the ability to allow the passage of short wave radiation whilst preventing heat from being radiated in the form of long wave radiation. For storage of this trapped heat, a liquid or solid with a high thermal mass is employed. In a water heating system this will be the fluid that runs through the collector, whereas in a building the walls will act as the thermal mass. Pools or lakes are sometimes used for seasonal storage of heat.

SOLAR THERMAL POWER PLANT:-

In the solar power plant, solar energy is used to generate electricity. Sun rays are focused using concave reflectors on to copper tubes filled with water and painted black outside. The water in the tubes then boils and become steam. This steam is used to drive steam turbine, which in turn causes the generator to work. A plant using this principle is working on experimental basis in Gurgaon in Haryana. Its capacity is 500 kilowatt. Another plant of similar type is being constructed in Jodhpur in Rajastan.

Many power plants today use fossil fuels as a heat source to boil water. The steam from the boiling water rotates a large turbine, which activates a generator that produces electricity. However, a new generation of power plants, with concentrating solar power systems, uses the sun as a heat source. There are three main types of concentrating solar power systems: parabolic-trough, dish/engine, and power tower.

Parabolic-trough systems concentrate the sun’s energy through long rectangular, curved (U-shaped) mirrors. The mirrors are tilted toward the sun, focusing sunlight on a pipe that runs down the center of the trough. This heats the oil flowing through the pipe. The hot oil then is used to boil water in a conventional steam generator to produce electricity.

A dish/engine system uses a mirrored dish (similar to a very large satellite dish). The dish shaped surface collects and concentrates the sun's heat onto a receiver, which absorbs the heat and transfers it to fluid within the engine. The heat causes the fluid to expand against a piston or turbine to produce mechanical power. The mechanical power is then used to run a generator or alternator to produce electricity.

A power tower system uses a large field of mirrors to concentrate sunlight onto the top of a tower, where a receiver sits. This heats molten salt flowing through the receiver. Then, the salt’s heat is used to generate electricity through a conventional steam generator. Molten salt retains heat efficiently, so it can be stored for days before being converted into electricity. That means electricity can be produced on cloudy days or even several hours after sunset.

‘Solar Power Tower’ Power Plant The first is the 'Solar Power Tower' design which uses thousands of sun-tracking reflectors or heliostats to direct and concentrate solar radiation onto a boiler located atop a tower. The temperature in the boiler rises to 500 – 7000°C and the steam raised can be used to drive a turbine, which in turn drives an electricity producing turbine. There are also called central Receiver Solar Power Plants.

It can be divided into solar plant and conventional steam power plant. The flow diagram is given in the Fig.

A heliostat field consists of a large number of flat mirrors of 25 to 150 m² area which reflects the beam radiations onto a central receiver mounted on a tower. Each mirror is tracked on two axis. The absorber surface temperature may be 400 to 1000°C. The concentration ratio (total mirror area divided by receiver area) may be 1500. Steam, air or liquid metal may be used as working fluid. Steam is raised for the conventional steam power plant.

‘Distributed (Parabolic) Collector System’ Power Plant:- The second type is the distributed collector system. It is also called solar farm power plant as a number of solar modules consisting of parabolic trough solar collectors are interconnected. This system uses a series of specially designed ‘Trough’ collectors which have an absorber tube running along their length. Large arrays of these collectors are coupled to provide high temperature water for driving a steam turbine. Such power stations can produce many megawatts (mW) of electricity, but are confined to areas where there is ample solar insulation.

Every module consists of a collector as shown in Figs. 2.13 and 2.14. It is rotated about one axis by a sun tracking mechanism. Thermo-oil is mostly used as heating fluid as it has very high boiling point.

Water/steam working fluid can also be used. The tubes have evacuated glass enclosure to reduce the losses. The concentration ratio is between 40 and 100. The maximum oil temperature is limited to 400°C as oil degrades above this temperature. Alternately steam at 550°C can be directly generated in

the absorber tube.

These are commercially under operation. Fig. 2.14. shows a flow diagram of parabolic trough solar power plant. The working fluid is heated in collectors and collected in hot storage tank (2). The hot thermo-oil is used in boiler (5) to raise steam for the steam power plant. The boiler also is provided with a back-up unit (6) fired with natural gas. The cooled oil is stored in tank (3) and pumped (4) back to collector (1). Solar thermal power plants with a generating capacity of 80 MW are functioning in the USA.

Solar Chimney Power Plant:- The air stream is heated by solar radiation absorbed by the ground and covered by a transparent cover. The hot air flow through or chimney which gives the air a certain velocity due to pressure drop caused by the chimney effect. The hot air flows through an air turbine to generate power.

SOLAR ENERGY STORAGE:-

It is well known that human beings have been using solar energy for different uses, from ancient

days. Find examples of these uses and add to the list given below.

1. To get salt from sea water.
2. To dry wet clothes.
3. To dry firewood.
4. To dry cereals.
5. To dry fish.
6. To dry leather.

We now use several appliances which work using solar energy. Appliances like solar cooker and solar heater absorb solar radiations and convert it into heat. Then what about a solar cell? Solar energy is converted into electrical energy and it is directly used or stored in a battery.

There are eight possible pathways for conversion of solar radiation to useful energy. Solar thermal conversion method converts radiation to heat using solar flat collectors. Solar thermo chemical conversion method converts radiation to heat and produce steam then to kinetic energy using a pump or turbine. Solar thermal electric conversion method converts radiation to steam and to kinetic and electrical energy through a turbine and generator to electrical energy. The above route through a further electrolysis process gives chemical energy (H₂ fuel). A high temperature catalytic conversion process produces chemical energy (H₂ fuel) directly. Photo-voltaic conversion of solar radiation gives direct electrical energy. Photosynthesis process produces chemical energy directly from radiation. Chemical energy (H₂ fuel) is directly produced from solar radiation using the electricity produced by the photo-voltaic method. A few of these methods are dealt in detail further.

Commercial and industrial buildings may use the same solar technologies photo-voltaic, passive heating, day lighting, and water heating that are used for residential buildings. These nonresidential buildings can also use solar energy technologies that would be impractical for a home. These technologies include ventilation air preheating, solar process heating and solar cooling.

Many large buildings need ventilated air to maintain indoor air quality. In cold climates, heating this air can use large amounts of energy. A solar ventilation system can preheat the air, saving both energy and money. This type of system typically uses a transpired collector, which consists of a thin, black metal panel mounted on a south-facing wall to absorb the sun’s heat. Air passes through the many small holes in the panel. A space behind the perforated wall allows the air streams from the holes to mix together. The heated air is then sucked out from the top of the space into the ventilation system.

Solar process heating systems are designed to provide large quantities of hot water or space heating for nonresidential buildings. A typical system includes solar collectors that work along with a pump, a heat exchanger, and/or one or more large storage tanks. The two main types of solar collectors used an evacuated tube collector and a parabolic trough collector can operate at high temperatures with high efficiency. An evacuated-tube collector is a shallow box full of many glass, double-walled tubes and reflectors to heat the fluid inside the tubes. A vacuum between the two walls insulates the inner tube, holding in the heat. Parabolic troughs are long, rectangular, curved (U-shaped) mirrors tilted to focus sunlight on a tube, which runs down the center of the trough. This heats the fluid within the tube.

The heat from a solar collector can also be used to cool a building. It may seem impossible to use heat to cool a building, but it makes more sense if you just think of the solar heat as an energy source. Your familiar home air conditioner uses an energy source, electricity, to create cool air. Solar absorption coolers use a similar approach, combined with some very complex chemistry tricks, to create cool air from solar energy. Solar energy can also be used with evaporative coolers (also called “swamp coolers”) to extend their usefulness to more humid climates, using another chemistry trick called desiccant cooling.

SPACE HEATING:-

In colder areas of the world (including high altitude areas within the tropics) space heating is often required during the winter months. Vast quantities of energy can be used to achieve this. If buildings are carefully designed to take full advantage of the solar isolation which they receive then much of the heating requirement can be met by solar gain alone. By incorporating certain simple design principles a new dwelling can be made to be fuel efficient and comfortable for habitation. The bulk of

these technologies are architecture based and passive in nature. The use of building materials with a high thermal mass (which stores heat), good insulation and large glazed areas can increase a buildings capacity to capture and store heat from the sun. Many technologies exist to assist with diurnal heating needs but seasonal storage is more difficult and costly.

For passive solar design to be effective certain guidelines should be followed:

1. A building should have large areas of glazing facing the sun to maximize solar gain.
2. Features should be included to regulate heat intake to prevent the building from overheating.
3. A building should be of sufficient mass to allow heat storage for the required period.
4. Contain features which promote the even distribution of heat throughout the building.

One example of a simple passive space heating technology is the Trombe wall. A massive black painted wall has a double glazed skin to prevent captured heat from escaping. The wall is vented to allow the warm air to enter the room at high level and cool air to enter the cavity between the wall and the glazing. Heat stored during the wall during the day is radiated into the room during the night. This type of technology is useful in areas where the nights are cold but the days are warm and sunny.

SPACE COOLING:-

The majority of the worlds developing countries, however, lie within the tropics and have little need of space heating. There is a demand, however, for space cooling. The majority of the worlds warm-climate cultures have again developed traditional, simple, elegant techniques for cooling their dwellings, often using effects promoted by passive solar phenomenon. There are many methods for minimizing heat gain. These include siting a building in shade or near water, using vegetation or landscaping to direct wind into the building, good town planning to optimize the prevailing wind and available shade. Buildings can be designed for a given climate domed roofs and thermally massive structures in hot arid climates, shuttered and shaded windows to prevent heat gain, open structure bamboo housing in warm, humid areas. In some countries dwellings are constructed underground and take advantage of the relatively low and stable temperature of the surrounding ground. There are as many options as there are people.

RECENT DEVELOPMENTS IN SOLAR POWER PLANTS:-

Solar Thermal Applications:- The applications include water heating for domestic, commercial and industrial use, space heating and drying, solar distillation, solar cooling through absorption & adsorption cycles, solar water pumping and solar power generation.

Solar Photo-voltaic:- Photo-voltaic (PV) or solar cells refers to the creation of voltage from light. A solar cell is a converter; it changes the light energy into electrical energy. A cell does not store any energy, so when the source of light (typically the sun) is removed, there is no electrical current from the cell. If electricity is needed in the night, a battery must be included in the circuit. There are many materials that can be used to make solar cells, but the most common is the element silicon. A typical solar cell is 3-6 inches in diameter and are now available in various shapes like circular, square, etc. The conversion processes occurs instantly whenever there is light falling on the surface of a cell. And the output of the cell is proportional to the input light.