SOLAR ENERGY CONVERSION AND USES OF SOLAR ENERGY :
Energy from the sun is solar energy. It is an inexhaustible source of energy. Energy emitted by sun within three minutes is equivalent to world's energy consumption per year. Most of the solar energy reaches earth as electromagnetic waves of about 3 micron wave length. About half of this radiation is visible as Light and the rest is infrared which accounts for Heat.
Sun radiates 3.8 x 10^26 Joules of energy per second. But only 1 / 2 x 10^9 th part i.e., 1.9 x 10^17 joules of energy per second is falling on earth. 30% of this energy is re-radiated. Considering such losses, quantum of solar energy falling on 1 square kilometer area is about 13,80,000 kW. Intensity of solar energy on a sunny day is 1.12 kW/m^2. It would be highly beneficial if such a large quantum of energy is harnessed.
Technologically there are two types of conversion of solar energy. They are
1. Solar Thermal Conversion and
2. Direct Electrical Conversion (Photo Voltaic techniques)
Solar Thermal conversion technique involves conversion of solar energy into electricity via thermo-electric power system. This method has number of terrestrial applications such as
► Solar hot water systems
► Solar space heating systems
► Solar refrigeration and air conditioning systems
► Solar electric power generation etc.,
Basic component of any such system is solar collector.
Thus main uses of solar energy can be summarized as
►water heating
►space heating
►agricultural crop drying
►electricity generation using solar cells (photovoltaic cells)
►thermal electric power generation
►refrigeration and air conditioning through photovoltaic, (solar) cells
THERMAL CONVERSION :
Solar collector is a device used to collect solar radiation (INSOLATION) in the form of light and heat and convert it to a usable form of thermal energy. Thermodynamically, solar collector behaves like a black body absorber. It absorbs energy from Sun and transmits it to a chosen heat transfer fluid. Energy may be stored or utilized directly.
Depending upon the temperature of operating cycle of energy conversion; the collectors are classified as
(i) low temperature operated collectors (60°C to 100°C)
(ii) medium temperature operated collectors (100°C to 250°C)
and (iii) high temperature operated collectors (250°C to 425°C)
Depending on optical concentration of energy, the solar collectors are classified as
(i) Flat plate collectors
Ex : Tube and sheet collectors
Bond duct parallel collectors
Honeycomb structured collectors
(ii) Line concentrating collectors
Ex : Parabolic trough collectors
Winston type collectors and
(iii) Point Focussing collectors (Dish type)
FLAT PLATE COLLECTOR:
The flat plate collector is the most important type of solar collector because it is simple in design and has no moving parts and it requires little maintenance.
Figure shows a liquid flat plate solar collector.
It consists of an absorber plate (5) on which the solar radiation falls. The absorber plate is coated with black coke powder and it acts as a black body absorber. The absorber plate is covered by one or more transparent glass plates (3 and 4). The sides and bottom of the box are insulated. The sunlight (insolation) passes through the glass plates and is absorbed by the black surface.
1. Casing
2. Thermal Insulation
3, 4. Transparent (Glass) Covers
5. Absorber Plate (Black Coated)
6. Fluid Passage (Tubes)
7. Fluid Inlet
8. Fluid Outlet
A. Direct Beam (Solar) Radiation
B. Diffuse Radiation
The absorbed radiation is partly transferred to a liquid flowing through the tubes which are fixed to the absorber plate. The rest of radiation is lost by convection and re-radiation to the surroundings form the top surface, and by conduction through the edges of box. However, the transparent covers help in reducing the losses by convection and re-radiation, while thermal insulation on the back side and the edges helps in reducing conduction heat loss. The liquid most commonly used is water since temperature involved is below 100°C. The collector is usually held tilted in a fixed position on a supporting structure, facing north if located in a southern hemisphere.