Combined Heat and Power Generation by Enhanced Geothermal Systems: Comparison of Direct and Indirect Concepts for Water and Supercritical CO2 as Heat Carriers

orc2019 Tracking Number 118

While the utilization of hydrothermal resources is limited to certain geological regions, Enhanced Geothermal Systems (EGS) have a tremendous technical potential without major regional restrictions [1]. In the last years, an increasing focus is laid on the utilization of EGS with supercritical CO2 (sCO2) instead of water due to its beneficial fluid characteristics [2]. First studies discuss several different concepts for power generation from sCO2 EGS. Next to the direct expansion of the sCO2 in a turbine [2], also the application of the sCO2 solely as heat carrier for an ORC system as well as the combination of a direct expansion and an ORC system is investigated [3,4]. Concerning the utilization of EGS by water, several potential plant layouts are discussed [5]. Direct expansion by single or double flash plants as well as their combination with an additional ORC as binary cycle are possible. Depending on the achievable temperature level of the brine and as well avoiding potential corrosion issues within the components in case of direct utilization, also the sole application of an ORC as a binary power plant may be advantageous [6]. However, the current development of the geothermal sector in Central Europe increasingly emphasizes the combined heat and power generation (CHP). Consequently, the annual available heat flow for power generation might differ significantly due to the varying heat demand. Thus, an assessment of the potential different power plant types for EGS should consider the part load characteristic of the plant layout, instead of assuming constant full load operation. Based on the heat demand characteristic of an actual district heating network, this study investigates the performance of the above-ground plant, comparing sCO2 and water as heat carrier for the utilization of the EGS. The performance of both options is evaluated with respect to different temperatures of the geothermal reservoir. The results present a recommendation regarding the most suitable plant layout and heat carrier medium for EGS projects in case of CHP operation. The work provides a valuable contribution to the debate about suitable plant concepts for EGS projects against the background of combined heat and power generation. References: [1] Chamorro et al. Enhanced geothermal systems in Europe: An estimation and comparison of the technical and sustainable potentials. Energy (2014) [2] Garapati et al. Brine displacement by CO2, energy extraction rates, and lifespan of aCO2-limited CO2-Plume Geothermal (CPG) system with a horizontal lproduction well. Geothermics (2015) [3] Wang et al. Working fluid selection for organic Rankine cycle power generation using hot produced supercritical CO2 from geothermal reservoirs. Applied Thermal Engineering 149 (2019) [4] Ruiz-Casanove et al. Use of supercritical CO2 heated with geothermal energy for power production through direct expansion and heat supply to an ORC cycle. Proceedings of the ASME 2017 International Mechanical Engineering Congress and Exposition (2017) [5] Hu et al. A Selection Method for Power Generation Plants Used for Enhanced Geothermal Systems (EGS). Energies (2016) [6] Lu, S.-M., A global review of enhanced geothermal system (EGS). Renewable and Sustainable Energy Reviews (2017)