A combined power generation system based on geothermal energy and solar energy in the prior art employs carbon dioxide as a working fluid and improves thermal efficiency of the system by extraction, recompression and regeneration, because the specific heat of carbon dioxide varies greatly with pressure. For example, within 200 degrees centigrade, the specific heat of carbon dioxide at a pressure of 15 MPa is greatly different from that of carbon dioxide at a pressure of 8 MPa. Consequently, the temperature difference of heat transfer in regenerator increases and thus the irreversibility of heat transfer of the system increases, since the heat capacity (a product of mass flow rate and specific heat) of the high-pressure and low-temperature carbon dioxide is different from that of the low-pressure and high-temperature carbon dioxide in a simple regenerative cycle. The heat capacity at the high-pressure side may be reduced to match the heat capacity of the carbon dioxide at the low-pressure side by recompression and partial cooling. The temperature difference of heat transfer can decrease in the regenerator, so as to make full recover of the heat energy in the exhaust gas at a turbine outlet.
However, a compressor operating efficiently under high-temperature and high-pressure is needed to ensure the recompression and partial cooling cycle implement. Specifically, the working circumstance of the compressor used for supercritical CO2 recompression Brayton cycleis adverse. Although the pressure ratio is not great, the ratio of specific heat of carbon dioxide is large, and the carbon dioxide at the final state of recompression may have a temperature up to 200 degrees centigrade and an operation pressure over 20 MPa, which will cause many problems for compressor designing and manufacturing.
Currently, the compressor operating efficiently under high-temperature and high-pressure has not been commercialized yet, so the method for improving thermal efficiency of the existing combined power generation system based on geothermal energy and solar energy by recompression and partial cooling still remains in the test program. Even though the compressor operating efficiently under high-temperature and high-pressure can be created in future, the manufacture difficulty, the manufacture cost and the operation cost of the combined power generation system based on geothermal energy and solar energy will be increased considerably.