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There are mainly three ways in which solar energy can be converted into electrical energy.
- The first one is through the Thermal mode, where heat is used for generating electricity.
- The second one is through Photo voltaic where solar energy is converted directly into electricity that could be used for a variety of applications like lighting, moving or pumping,
- The third method involves use of thermocouples.
Solar Photo voltaic
Solar Photo voltaic cells work by converting sunlight into electric current. An Solar Photovoltaic cell is a semi-conductor system made of silicon or similar materials. The system generates electricity when it is exposed to sunlight. Power is generated by connecting thousands of tiny solar cells which forms modules. The generated electricity will be in dc form which can be converted into Alternating Current form sing DC to AC converters called inverters.
There are different types of solar PV cells based on the choice of materials used. Crystalline Silicon solar cells are of two types – polycrystalline and monocrystalline. Thin-film solar PV cells can be made of Amorphous Silicon, CdTe e.t.c. solar photovoltaic cells directly convert light energy into electrical energy with lab efficiencies reaching 44 % using solar cells manufactured by nano technology.
Solar electric conversion through thermocouples
Thermocouples are devices consisting of two dissimilar metals that meet each other at one or more junctions. When one of the junctions is kept at a reference temperature and the other is maintained at a temperature different from the reference temperature, voltage is generated across the two junctions and current will start flowing in the loop. This is thermoelectric effect or seebeck effect named after German–Estonian physicist Thomas Johann Seebeck who discovered this effect. The voltage developed across the junctions of dissimilar metals is called as seebeck voltage. For small changes in temperature the Seebeck voltage is linearly proportional to temperature:
∆eSV = α∆T
Where α is the Seebeck coefficient, is the constant of proportionality,
∆T is the temperature difference between cold and hot junctions.
The hot junction is maintained at higher temperature compared to cold junction temperature by a heat source (which is the solar energy in this case). A continuous transfer of heat through solar energy is given to hot junction to maintain the electric potential. A continuous transfer of heat is necessary because the current flowing through the thermocouple tends to cause the hot side to cool down and the cold side to heat up which is called as Peltier effect. Thermocouples can be connected in series or parallel to form a electronic device called thermopile optimized to convert thermal energy to electrical energy. The output is the sum of the voltages across the individual junctions, giving larger voltage and power output. The efficiencies of thermocouples are very poor approximately of the order of 10 %.
The next type of technology in solar energy, solar thermal power systems work by concentrating sunlight for producing electricity through the thermal mode. Also referred to as high temperature solar power systems, the collectors are of 3 main types – Parabolic Dish Systems, Power Tower System and Parabolic Trough System.
A solar thermal electric conversion system is shown. The light energy from sun is concentrated on a photo to thermal energy converters by heliostats which track the sun from sunrise to sunset. A large number of mirrors are placed to focus light at the transducers to ale to generate sufficient heat to convert water into steam. The boiler is kept at the focal points of the mirrors. Another feature 0f this system is the energy storage. The storage is placed before and after the electrical energy is produced. The heat energy available will be converting the water pumped into steam. This steam is used to rotate turbines by which electricity is produced. This form of energy conversion is potentially more convenient storage of electricity generation and storage. The thermal reserves are adequately stores to keep the turbine spinning during periods of cloud cover. The efficiency of entire plant depends on so many factors such as mirrors focussing efficiency, photo to thermal energy conversion efficiency, heat storage losses to ambient, turbine conversion efficiency, generator losses e.t.c Hence the efficiency of plant
ξplant = ξptoe conversion*ξturbine efficiency*ξgenerator efficiency* ξmirrors
Where ξptoe conversion = Thermal energy generated/ incident light energy,
(ξturbine efficiency)max = mechanical energy of the shaft/energy of steam = 1-(Tc/Th) where Tc is temperature of water pumped,
Th is the temperature of steam at the turbine inlet. The turbine efficiency practically will be still lower than this value.
ξgenerator efficiency = electrical energy generated / mechanical energy of shaft at rotor .
ξmirrors = light energy at the boiler/Incident light energy on mirrors .
Here it is assumed that the mirrors exactly track the sun at all times otherwise the output energy still decreases. This is not efficient as direct conversion of solar energy into electrical energy as is the case with photovoltaic cells. The overall efficiencies are of the order of 15 -20 %.