Thermodynamic evaluation and comparison of direct geothermal power systems and their expanders

Direct geothermal power system is an effective method to utilize medium-high temperature geothermal resources employing geothermal steam as working fluid. The most widely applied technology are single flash and double flash system. Recently, full flow system has been introduced into geothermal fields and developed to work with two-phase expansion in several geothermal fields in China. To theoretically understand the thermodynamic characteristics of these three direct geothermal power systems and their expanders, a comparative study and thermodynamic evaluation were performed in this paper. A thermodynamic model, taking into account the power consumption of cooling water pump and fan, was developed to simulate the performance of direct geothermal power systems and their expansion equipment. The effects of operating parameters were discussed and analyzed, including inlet dryness, inlet temperature and outlet temperature. Results showed that full flow system has larger output power and thermal efficiency than flashing systems, especially at lower inlet dryness. When inlet dryness is 0.1, the thermal efficiency of full flow system is 38% higher than that of single flash system. Moreover, the output power increases with inlet dryness under constant mass flowrate, but shows opposite variation trend under constant volume flowrate. Higher inlet temperature and lower outlet temperature lead to higher output power and thermal efficiency, however it is limited by the expansion ratio and volume flowrate of expanders. The full flow system is more competitive than flash systems when the isentropic efficiency of full flow expander can reach 50%–70%.