Chapter 3 - Organic Rankine cycle driven by geothermal heat source: life cycle techno-economic–environmental analysis

Abstract

Geothermal energy as a kind of clean energy has drawn global attention and shown extensive application prospects. Improving the utilization amount and efficiency of geothermal energy is very important to change the structural framework based on fossil energy. Among the different power generation systems, the organic Rankine cycle (ORC) has been proved to be a reliable technology for low-grade heat generation. In this study a basic ORC driven by a low-grade geothermal source is evaluated from thermoeconomic, exergoeconomic, and environmental viewpoints. Six different working fluids are selected as candidates. Furthermore a comparison between different layouts of ORCs, namely the basic ORC, ORC with internal heat exchanger, and dual-pressure ORC, is carried out. The results show that the basic ORC with R1234yf has the optimal thermoeconomic and exergoeconomic performances while the ORC with R600a brings the most environmental benefit. The thermoeconomic evaluation reveals that more than 50% of the capital cost of the basic ORC comes from the turbine, followed by the heat exchangers. Environmental analysis shows that for an ORC with a low global warming potential (GWP) fluid, at least 75% of CO2,eq emissions are from the construction phase. For a basic ORC with a high GWP fluid such as R134a, the main source of CO2,eq emissions is the leak of the working fluid. The cycles are optimized to reach the minimum electricity production cost and maximum emission reduction. The optimization results show that the basic ORC with R1234yf has the optimal thermoeconomic and exergoeconomic performances whereas the ORC with an internal heat exchanger (IHX-ORC) with R1234yf is the most suitable candidate for maximum emission reduction. Sensitivity analysis indicates that the thermoeconomic, exergoeconomic, and environmental performances of the basic ORC can be improved by increasing the inlet temperature of the geothermal source.