Preliminary Analysis of an Electro-Thermal Energy Storage System for Geothermal Applications

The shift towards renewable energy sources, particularly solar and wind, is reshaping the power generation sector. However, the intermittent nature of these sources necessitates their integration with flexible and dispatchable power generation systems and efficient energy storage solutions, to ensure grid stability and support Transmission System Operators (TSOs), enhancing both reliability and profitability. In response several different storage technologies are currently undergoing active research and development. The Horizon Europe SEHRENE project investigates electro-thermal energy storage (ETES) through the development of an innovative Carnot Battery system. During the charging phase, a high-temperature heat pump is used to store heat exploiting renewable excess electricity, while during the discharging phase an Organic Rankine Cycle (ORC) exploits the stored heat to flexibly produce electricity and respond to load variations. These two thermodynamic cycles are coupled by means of an advanced Thermal Energy Storage (TES) configuration featuring a single heat exchanger that utilizes a Phase Change Material (PCM) to achieve high energy density, integrating Kelvin cell structures to further enhance heat transfer efficiency. The project aims to demonstrate the ETES concept across three different use cases: (i) the Kızıldere II geothermal power plant in Turkey, (ii) a ceramic production facility in Alcora, Spain, and (iii) a Distribution System Operator (DSO) substation supporting an industrial park with photovoltaic energy production near Granada, Spain. The optimal system configuration is determined for each application according to the specific heat source availability and use-case requirements. This work focuses on the Kızıldere II geothermal power plant, presenting a preliminary ETES system design and detailing the selection criteria for the ETES working fluid. In the first part, aimed at working fluid selection, a simplified approach for PCM heat exchanger sizing is adopted. This study evaluates system performance, particularly focusing on the off-design of PCM heat exchanger and its influence on round trip efficiency (RTE). In the second part of the work, to improve calculation accuracy, the detailed design of the PCM heat exchanger is carried out considering the preselected working fluid, and RTE is recalculated. The findings of this work aim to numerically validate the feasibility of PCM heat exchanger technology and the potential of ETES systems for geothermal applications, also providing some valuable insights for optimizing similar systems in different industrial contexts.