Solar Thermal Power Plants

In case of solar thermal plants, the solar energy is first collected by using solar ponds, solar flat plate collector, focusing type solar collectors, heliostats etc : It requires very large collection area covering few kilometres of area. It also needs a complex and costly sun tracking system. The collector solar energy is transferred either directly to the working fluid or though the heat exchanger to operate a power plant. Commonly used cycles for solar thermal power plants are :

1. Stirling cycle
2. Brayton cycle
3. Rankine cycle

• The Stirling cycle consists of two constant volume processes and two isothermal processes during which the heat addition and heat rejection processes take place. It needs large regenerators due to which the Stirling cycle is not used in case of solar thermal power plants.
• The Brayton cycle consists of two constant pressure processes (for heat addition and heat rejection) and two adiabatic processes. The working fluid is air.
• The air temperatures needed are beyond 500°C for its efficient operation. In this cycle, the air is compressed in a compressor, then the solar heat is transferred to air in a heat exchanger. These high temperature gases at high pressure are allowed to expand in an air turbine to produce mechanical power and finally the heat is rejected at constant pressure to surroundings.
• The efficiency of such a Brayton cycle is low. However, its efficiency can be further improved by using a regenerator where the energy of turbine exhaust is utilized to heat the compressed air.
• The Rankine cycle used in conventional thermal power plants can be applied to thermal power plants. The system is most commonly used.
• The efficiency of power plants based on Brayton cycle and Rankine cycle is lower than Stirling cycle. Presently, the cost of power generation using solar energy is very high compared to cost of a conventional power plant using fossil fuels.
• However, due to rising cost and depleting fossil fuels, the solar energy utilization for thermal power plants is a viable option particularly in remote areas where favourable sun shine conditions exist.