Thermodynamic and Economic Analysis of Geothermal Combined Heat and Power Based on a Double-stage Organic Rankine Cycle

orc2019 Tracking Number 110

Geothermal energy is a suitable low-carbon energy resource for heat and power production. The techno-ecological potential for pure power production in Germany is estimated to 8.5 GW or 63.75 TWh/a. For low enthalpy reservoirs, the thermal power of the geothermal fluid is usually converted to electricity by binary power plants like Organic Rankine Cycle (ORC) power systems. Previous investigations show that the efficiency and the profitability of these power plants can be increased by an additional heat supply. The aim of this study is to investigate different concepts based on a double-stage Organic Rankine Cycle for geothermal combined heat and power (CHP) production. The evaluation is based on annual return simulations, which are performed according to VDI 4655 [1]. For modelling and simulation the software Dymola [2] is used. The built-up model is based on the ThermoCycle library [3] and the fluid properties are calculated by the software CoolProp [4]. The operation mode of the combined heat and power plant is heat-driven, which means that the heat demand must be covered at any time by the geothermal resource. In this context, heat demand profiles are developed based on operational data of a real geothermal heat plant. Regarding the heat supply, a parallel configuration of ORC and district heating network (DHN) is considered. For the DHN, a supply temperature of 90 °C and a return temperature of 60 °C is assumed. For the peak load of the DHN a sensitivity analysis is conducted. The results show that the efficiency and the profitability of the power plant can be significantly increased by supplying heat to a DHN with a peak load of 5 MW compared to the pure electricity production. By increasing the peak load of the DHN to 10 MW the average annual exergetic efficiency of the energy system can be further increased by 4.3 % to 51.7 %. In addition, the annual return is 1.6 million Euro higher than for a 5 MW DHN and 3.5 million Euro higher than for power production. In ongoing work, the effect of potential future scenarios, like revision of feed-in tariffs, are economically evaluated.