Experimental investigation of the power production cycle in a co2 based electrothermal energy and geological storage system

* corresponding author(s) 1 6th Edition of the European Conference on Supercritical CO₂ (sCO₂) for Energy Systems April 9-11, 2025, Delft, Netherlands 2025-sCO2.eu-211 EXPERIMENTAL INVESTIGATION OF THE POWER PRODUCTION CYCLE IN A CO2 BASED ELECTROTHERMAL ENERGY AND GEOLOGICAL STORAGE SYSTEM Sebastian Unger* Helmholtz-Zentrum Dresden-Rossendorf Dresden, Germany Stefan Fogel Helmholtz-Zentrum Dresden-Rossendorf Dresden, Germany Peter Schütz Helmholtz-Zentrum Dresden-Rossendorf Dresden, Germany Ricardo Chacartegui Universidad de Sevilla, Sevilla, Spain Andrés Carro Universidad de Sevilla, Sevilla, Spain Márton Pál Farkas GFZ German Research Centre for Geosciences, Potsdam, Germany Cornelia SchmidtHattenberger GFZ German Research Centre for Geosciences, Potsdam, Germany Uwe Hampel Helmholtz-Zentrum Dresden-Rossendorf; Dresden University of Technology Dresden, Germany ABSTRACT The European Commission aims at achieving a net-zero greenhouse gas emissions economy until 2050. For this reason, renewable electricity generation is expected to increase up to 69% by 2030. However, the intermittent nature of solar and wind power generation requires sustainable storage systems, in order to compensate for the mismatch between energy supply and demand. Large-scale thermal energy storage in combination with supercritical carbon dioxide (sCO2) power cycles is a promising solution to address this issue. The EU-project CEEGS (Novel CO2-based Electrothermal Energy and Geological Storage System) aims to develop a highly efficient, cost-effective and scalable energy storage technology. When excess renewable electricity is available, a compressor drives a heat pump cycle to increase temperature and pressure. The hot CO2 heats a hot water storage and cools down before entering an expansion turbine. The low-temperature CO2 cools a cold-water storage tank before entering the compressor. In the discharge cycle, CO2 is pumped from the geological reservoir into the surface components and gets heated by the stored thermal energy through a heat exchanger, before entering the turbine to generate electricity. The low-pressure CO2 is liquified by entering a condenser, which is cooled by the cold-water reservoir, and pumped back into the subsurface reservoir through an injection well to extract heat from the subsurface. In order to demonstrate the transcritical cycle of the CEEGS concept and to validate the surface components, a 20 kW demonstrator was designed, built and operated at the HelmholtzZentrum Dresden-Rossendorf. In the present contribution, the design of the facility and the operation of the discharge cycle at CO2 temperature and pressure of up to 250 °C and 235 bar will be presented and discussed. The safety and measurement concept of the facility is presented. The design of the main components, such as the high-temperature heat exchanger (HXW), the lowtemperature heat exchanger (HXI) and the CO2-pump, as well as the cycle behavior are presented. The experimental results show a lower Reynolds number at higher cycle pressures, due to the more closed valve position. Particular attention is paid to the performance of the high-temperature heat exchanger in terms of overall heat transfer coefficient. Convective heat transfer on the CO2 side plays a dominant role in the thermal resistance of the heat exchanger. The thermal efficiency of the cycle during discharge is strongly influenced by the maximum cycle pressure. In fact, at higher pressures, higher cycle efficiency is achieved due to greater expansion work. The experimental results will be used by the project partners to validate the numerical models. Future experimental campaigns will investigate the dynamic operation of the facility. Keywords: transcritical CO2, electrothermal energy storage system, geological storage cycle, experimental investigation INTRODUCTION AND STORAGE CONCEPT The production of electrical energy from renewable sources is gaining increased attention as a key strategy to reduce greenhouse gas emissions. However, the intermittent nature of electricity generation from these sources often leads to a mismatch between energy supply and demand. To improve reliability and prevent potential grid instabilities, energy storage systems are essential. Large-scale thermal energy storage (TES) systems are considered a promising solution for enabling a sustainable energy system powered by renewables. A TES system converts excess electrical energy into thermal energy, This work may be used under a Creative Commons Attribution 4.0 License. DOI: 10.17185/duepublico/83305 2 stores it, and reconverts it into electricity during periods of high demand and lack of renewable production using, for example, a supercritical carbon dioxide (sCO2) power cycle. TES systems present several advantages, including simplicity, cost-effectiveness, and reliability, compared to alternative storage technologies. Meanwhile, sCO2 power cycles offer unique benefits such as high conversion efficiency, compact turbomachinery, and a temperature profile that aligns well with sensible thermal energy storage systems. Additionally, sCO2 is non-toxic, chemically stable, and significantly less expensive, costing only 10 % of the price of helium and 14.3 % of the price of the refrigerant R134a [1]. These cycles can also utilize heat across various temperature ranges, with particularly high efficiencies achieved from high-temperature heat sources [2]. With growing interest in sCO2 power cycles from both academia and industry, some review articles have delved into specific aspects of their applications. Wu et al. [3] explored sCO2 Brayton cycles in nuclear engineering, while White et al. [4] studied the specifics of sCO2 turbomachinery. A recent bibliometric analysis by Yu et al. [5] highlighted research trends and identified key countries driving sCO2 technology forward, including the United States, China, South Korea, Australia, and India. In contrast, European research in this field remains less developed, with experimental facilities operating at a relatively small scale. Few studies have focused on operational experiences with sCO2 facilities. Initial insights into closed-loop sCO2 cycles at Cranfield University are provided in [6] and [7], while other researchers reported on the commissioning of facilities producing up to 1 MW of power [8]. Lee and Lee [9] investigated transient CO2 cycles for solar thermal applications using a smallscale experimental setup, studying the effects of maximum system temperature and facility insulation on cycle efficiency. Illyés et al. [10] extensively documented the evolution of a sCO2 facility developed through various scientific and industrial research projects. A related concept, a CO2-based tri-generation energy storage system operating as both a heat pump (during charging) and a heat engine (during discharging), was presented by Garcia et al. [11]. To further develop and evaluate TES systems integrated with sCO2 power cycles, the EU-funded project CEEGS (CO2based Electrothermal Energy and Geological Storage) has been launched. This initiative aims to create an efficient, costeffective, and scalable energy storage technology using transcritical CO2 as the working fluid. Transcritical CO2 systems are characterised by the fact that their working fluids undergo subcritical and supercritical states. The concept combines underground energy storage with a sCO2 heat pump and power cycle, offering low-cost storage, long-term CO2 sequestration, and the potential for geothermal heat extraction. The CEEGS system (CO2-based Electrothermal Energy and Geological Storage) integrates a transcritical CO2 reversible heat pump with the storage and recovery of CO2 in underground geological formations, operating as a closed-loop system for energy storage and partial CO2 sequestration [12]. By using CO2 as the working fluid and leveraging the temperature and pressure conditions on the high-temperature side of the cycle, this system allows for efficient integration of the Carnot battery concept with geological storage. This integration incurs no additional energy costs while offering enhanced energy storage capacity. The transcritical CO2 reversible heat pump operates in two distinct phases: the charging phase, functioning as a heat pump (cycle 12-3-4), and the discharge phase, functioning as a thermal engine (cycle 5-6-7-8) as displayed in the schematic concept of the CEEGS cycle presented in Figure 1. Figure 1: Schematic concept of the CEEGS cycles, the charging cycle (1-2-3-4) and the discharging cycle (5-6-7-8). During the charging phase, renewable electrical energy (e.g., from solar or wind) is converted into thermal energy stored at two temperature levels: high-temperature thermal energy storage (HT-TES) and low-temperature thermal energy storage (LT-TES). In the discharge phase, the stored thermal energy is converted back into electricity for grid dispatch. The efficiency of this electrothermal energy storage system heavily depends on the precise integration of temperature profiles between heat exchangers and storage tanks during the charge and discharge processes [12], which is critical for optimal system design. Both the charging and discharging phases of the transcritical CO2 cycle involve sensible heat transfer on the high-temperature side and latent heat transfer on the low-temperature side. Since CO2 has a critical temperature of approximately 31 °C, water is an ideal storage medium for both hot and cold storage. Water can store sensible heat as hot water (potentially pressurized) and latent heat through the phase change of ice [13], making it environmentally friendly, widely available, and cost-effective. The feasibility of the integration of geological storage to the thermo-electric energy storage system depends on the geological characteristics of the site, with potential scenarios including porous formations such as deep saline aquifers [14] or salt cavities [15]. In an open cycle operation, CO2 from a capture plant is directed through the electrothermal system during the charging phase (cycle 4-1-2-3 in Figure 1) and subsequently injected into the geological formation. During the discharge phase, CO2 extracted from the geological formation passes back through the system (cycle 6-7-8-5 in Figure 1) and is re-injected. This operational mode enables partial CO2 sequestration. The geological subsurface reservoir has three main functions: it enables fluid circulation, i.e., injection and backproduction of CO2, serves as an energy store for the mechanical DOI: 10.17185/duepublico/83305 3 energy (via pressure change through compression and injection of CO2) and contributes to permanent CO2 storage. In contrast to geothermal plants, the CEEGS concept can profit from the following typical CO2 retention mechanisms occurring as losses in the underground: (i) structural trapping, which keeps the vertically rising CO2 stratigraphically trapped by impermeable rock layers (caprocks) which serve as sealant, (ii) residual trapping, where CO2 is immobilized in the pore spaces of a rock formation after being injected, induced by capillary forces due to surface tension effects at the interface between CO2 and brine, (iii) solubility trapping, where CO2 is solved in the formation water present in the pore spaces, and (iv) mineral trapping, where dissolved CO2 reacts with rock minerals and formation water and precipitates as stable carbonate minerals such as calcite or magnesite. All these trapping processes take place in the CEEGS concept, assuming a suitable geological subsurface is available, and cause an emission reduction contribution through the permanent retention of CO2 within the geological formation. The CEEGS storage concept provides several key benefits. It enables large-capacity electrothermal energy storage with high operational flexibility in both open and closed cycles. The system achieves round-trip efficiencies exceeding 50 %, contingent on optimal integration design. Alongside dispatchable electricity generation, it allows the integration of thermal energy applications for heating and cooling via its thermal energy storage tanks (trigeneration). Partial CO2 sequestration offers the possibility of incorporating previously captured CO2 from industrial sources [12], or directly captured CO2 from the atmosphere which counts even as negative emission. This energy storage system can utilize various renewable energy resources — such as photovoltaic (PV) and wind power for the heat pump and biomass or concentrated solar power for the hot storage tank. Its scalability allows for development across a wide range of power capacities. Currently, the concept is at a low Technology Readiness Level (TRL 2). The CEEGS project aims to address key challenges to advance to higher TRLs, including the integration of surface and subsurface components and the impact of fluid streams on component performance. DESIGN OF THE CEEGS FACILITY The experimental setup is partially housed outdoors and partially located within an experimental hall, with total dimensions of approximately 6.0 m × 4.3 m × 4.8 m. To minimize heat losses and condensation, the facility’s piping and equipment are thermally insulated within the indoor area. The cold water for the HXI is supplied from a water vessel, with flow rate adjustments achieved by regulating the rotational speed of the water pump. The maximum pressure, temperature, and mass flow rate for the cold-water supply are 6.5 bar, 60 °C, and 3 kg/s, respectively, with typical operational parameters set at 2 bar, 15 °C, and 1 kg/s. The CEEGS facility can be seen in Figure 2. Figure 2: CEEGS facility. In the present investigation the discharging cycle within the CEEGS framework is presented. Thus, the CO2 is extracted from the bottom of the separator vessel by a CO2 pump. This process raises the pressure to a maximum of 220 bar, while the temperature stays between 10 to 25 °C. To facilitate the inflow or outflow of CO2 to or from a geological subsurface reservoir, a three-way valve has been integrated into the setup. However, this inflow or outflow interaction with a geological reservoir was not part of the experiments conducted and remains a subject for future studies. The CO2 stream then flows through the HXW, a highpressure heat exchanger, where it absorbs heat from a hot water circuit during the discharging cycle. Consequently, the CO2 temperature is increased up to 250 °C, depending on the operational mode. Following this, the CO2 expands through an expansion device, lowering its pressure to 55 bar, which mimics the expansion process through a turbine. Then, the low-pressure CO2 enters the HXI, a low-pressure heat exchanger, where the CO2 is cooled by the cold-water circuit and undergoes condensation. The resulting liquefied CO2 is returned to the separator vessel, completing the cycle. For the hot-water supply, a separate pump is installed with rotational speed control, and a bypass circuit around the HXW allows for fine-tuning of the water mass flow rate through the heat exchanger. During experiments, hot water is pumped from an electric heating circuit before entering the HXW. The maximum hot-water supply parameters are 69 bar, 286 °C, and 1.5 kg/s for heating the CO2. After leaving the heat exchangers, the water flows into a blow-off tank before recirculating into the water supply system. It is worth noting, that hot-water and coldwater storage tanks were not included in the current stage of the experimental facility's development. A schematic representation of the installed test rig is shown in Figure 3. DOI: 10.17185/duepublico/83305 4 Figure 3: Scheme of the instrumentation and components of the CEEGS facility. COMMISSIONING AND MEASUREMENTS Before Commissioning, a high-pressure test was performed by filling the facility with water and maintaining the pressure for one hour before releasing the water. Multiple alternating cycles of vacuum establishment and flushing with compressed air were conducted. This process effectively removed residual impurities, humidity, and any remaining liquids or gases. Following this, a vacuum pump was operated for several hours to create a sustained vacuum prior to CO2 introduction to verify the system’s leak-tightness. Once this was achieved, CO2 bottle bundles were connected to the facility, and CO2 was gradually introduced into the separator vessel and the rest of the system via a valve. After pressure equalization was reached, the connection to the vessel was closed, and a direct link to a filling compressor was established. The compressor then extracted CO2 from the cylinders to further fill the circuit until its lower suction limit was attained. Subsequent adjustments to the system pressure were managed by modulating the distribution of CO2 mass among the bottle bundles, the pressure vessel, and the facility's remaining components. Temperature measurements are performed using multiple type-K thermocouples with a thickness of 1.5 mm. All thermocouples installed in the system are calibrated to ensure a measurement accuracy of ±0.3 K. Pressure is monitored using transducers with a measurement range of 0 to 600 bar and an accuracy of 0.1% of the full-scale range. The pressure drop across the HXW is recorded for each fluid. Differential pressure transmitters are used for this purpose, with a range of 0 to 1.5 bar and an accuracy of 0.01% of the full-scale range on the CO2 side, and 0 to 2.5 bar with an accuracy of 0.8% on the water side. The mass flow rates for the pump are measured using a Coriolis flow meter. This has a measurement range of up to 2.26 kg/s, with an accuracy of 0.10% of the full-scale range. The locations of the sensors for temperature, pressure, and flow measurements are illustrated in Figure 3. To manage emergency shutdowns in cases of excessive temperature or pressure, eight safety temperature limiters are installed at four distinct locations, and four safety pressure limiters are installed at two locations within the system. Measurements are recorded for 100 seconds at a rate of 1 Hz for each sensor once a steady-state condition is achieved. The pump power is controlled using a programmable logic controller (Phoenix-Contact™). A moisture sensor is installed at the outlet of the vacuum pump to monitor residual humidity in the system. The moisture analyzer has a measurement range of 0% to 97% relative humidity, with a calibrated accuracy of ±1.0%. An overview of the instrumentation of the experimental setup can be seen in Table 1. Table 1: Measurements and accuracy of the experimental setup. Measurement accuracy Temperature 1.5 mm, type-K ± 0.3 K Pressure (absolute) 0 to 600 bar 0.1 % Pressure difference CO2 0 to 1.5 bar 0.01 % Pressure difference H2O 0 to 2.5 bar 0.08 % Coriolis flow meter 0 to 2.26 kg/s 0.1 % During the design phase of the CEEGS facility, a comprehensive hazard and risk analysis was conducted, adhering to the guidelines outlined in “A Guide to Hazard and Operability Studies” by the Chemical Industry Safety and Health Council of the Chemical Industries Association. This analysis addressed potential risks and necessary precautions and safety measures were incorporated into the design. To monitor safety conditions, four gas sensors are installed in the experimental hall to track CO2 and O2 concentrations around the facility. Two CO2 sensors are located near the bottom of the setup, while two O2 sensors are positioned in the upper area. If CO2 levels rise or O2 levels drop beyond safe limits, both acoustic and visual alarms are triggered. The facility is operated remotely from a dedicated control room, which houses the process control system. The setup is also monitored through several cameras for optical observation. COMPONENTS OF THE FACILITY The pump is capable of conveying up to 180 kg/h of liquid CO2, raising the pressure to a maximum of 250 bar. The pistons of the pump and the suction line of the pump are cooled by cold pressurized air, to reduce the piston temperature and to ensure liquid suction at pump inlet. An image of the pump can be seen in Figure 4. DOI: 10.17185/duepublico/83305 5 Figure 4: CO2 pump of the CEEGS facility. The expansion device consists of a stainless steel DN 25 PN400 control valves, as illustrated in Figure 5. These valves are actuated using compressed air, and their folding bellows are constructed from Inconel alloy 625. The position of the expansion valves determines the cycle's maximum pressure and mass flow rate. A more closed valve position results in higher maximum pressure and lower mass flow rate, while a more open position decreases the pressure and increases the flow rate. Figure 5: Expansion device of the CEEGS facility. There are two heat exchangers involved in the CEEGS power generation cycle or discharge cycle, the HXW and the HXI, to heat and to cool the CO2 respectively. The heating and cooling of CO2 in the heat exchangers are achieved using water supplied by the neighboring hot water facility. During the charging cycle, the HXW utilizes hot water to heat the CO2 to temperatures of up to 250 °C, while the HXI cools the CO2 to low temperatures as low as 10 °C. The HXW is a printed circuit heat exchanger (PCHE) composed of approximately 3,000 semicircular channels, each with a diameter of 1.4 mm. It features multiple stacked plates arranged alternately to guide CO2 and water in a countercurrent flow configuration. The HXI, in contrast, is a rod bundle heat exchanger made up of 61 tubes with an outer diameter of 6 mm. Water flows through the shell side, equipped with buffer plates, while CO2 flows through the tube side. The mass flow rate of the cooling water can be adjusted via the water supply system to regulate the heat transfer rate. Figure 6 provides an illustration of both the HXW and the HXI. Figure 6: Water-CO2 heater HXW (left) and the water-CO2 cooler HXI (right). DATA ACQUISITION AND PROCESSING To analyze the installed components and the CO2 circuit, several parameters were calculated. The dimensionless Reynolds number was derived using the fluid properties and a characteristic length, defined here as the inner tube diameter of the HXI or the channel diameter for the HXW. The Reynolds number is given by ReCO2=ρ∙u∙dt μ, (1) were ρ is the density, u is the bulk flow velocity, dt is the characteristic length and μ is the dynamic viscosity of CO2. To evaluate the heat exchanger’s performance, the overall heat transfer coefficient was calculated. This accounts for the convective heat transfer of CO2, conduction through the steel walls, and the convective heat transfer of water. The equation for overall heat transfer coefficient is htctotal=Q󰇗CO2 AHX ∙∆Tlog , (2) where the heat flux is determined by the inlet and outlet enthalpy of CO2 in the HXI and the HXW. The AHX denotes the heat transfer surface area between the water and CO2, and ∆Tlog represents the logarithmic mean temperature difference, calculated as ∆Tlog=∆Tmax-∆Tmin ln(∆Tmax ∆Tmin). (3) DOI: 10.17185/duepublico/83305 6 Since this study focused solely on the discharging cycle, the thermal efficiency of the cycle ηthermal was calculated for the present study. Here, ηthermal was defined as the ratio of the heat input by the HXW and the compression work by the pump, to the expansion work through the expansion device. The expansion work is reduced by the mechanical losses in the shaft. The injected heat flux is based on the inlet and outlet temperatures of CO2 in the HXW. The thermal efficiency is expressed as ηthermal=Pnet Q󰇗HXW =Pexp-PPump Q󰇗HXW , (4) An uncertainty propagation analysis was conducted to assess how measurement uncertainties affected the calculated parameters. The maximum relative deviations were 3.5% for the Reynolds number, 3.6% for the overall heat transfer coefficient, and 3.7% for the thermal efficiency. However, most deviations were significantly smaller, and the resulting error bars were negligible in the plotted results. Consequently, the generated plots are presented without error bars. OBJECTIVES OF THE PRESENT INVESTIGATION One of the key challenges in advancing the Technology Readiness Level (TRL) of the CEEGS concept lies in operating an energy storage cycle that incorporates all major components. The CEEGS facility has been designed and is being operated to address these challenges. The current objective is to demonstrate the functionality of the surface part of the CEEGS cycle, including its control system. This involves evaluating the performance of critical equipment such as pressure vessels, heat exchangers, pump and auxiliary equipment. To ensure grid stability, it is essential to conduct tests under various operating conditions, including startup, partial load, and shutdown scenarios. The technical impact of integrating subsurface CO2 into the cycle will also be examined in future investigations. During discharge, the extracted fluid at the production wellhead will primarily consist of CO2 with a small fraction of brine. This brine content is unavoidable due to residual saturation in the reservoir rock formation, as it is impossible to completely displace the formation water surrounding the production well. As a result, a CO2/brine separation process will need to be implemented to restore the CO2 stream to the purity levels required for surface equipment. Finally, the experimental facility’s measurements will allow validation of the models developed by the CEEGS project partners. EXPERIMENTAL RESULTS This chapter presents the results of the experiments on the discharge cycle of the CEEGS process. The relation between the pressure ratio of the system, which is the ratio of the upper pressure at points 6 and 7 to the lower pressure at points 8 and 5, and the Reynolds number in the HXW is shown in Figure 7. It becomes clear, that the pressure drop across the expansion device and the mass flow rate are related via the pump characteristics. The higher the cycle pressure, the lower the mass flow rate and the Reynolds number in the HXW. The highest pressure during operation was at around 235 bar, giving in a pressure ratio of around 5. The results are similar as expected, as the Reynolds number decreases due to the lower mass flow rate resulting from the more closed valve. On the other hand, the more closed valve is the reason for the higher cycle pressure. Figure 7: Relation between pressure ratio and Reynolds number of the flow in the HXW. Figure 8 shows the relation between the heat flow in the HXW and the maximum cycle pressure. The maximum cycle pressures are grouped into four categories to illustrate the behavior more clearly. For higher maximum pressures the heat flow in the HXW decreases, because the mass flow rate and thus the convective heat transfer on the CO2 side decreases. It is worth mentioning, that the CO2 enters the HXE in the liquid phase and transitions to the supercritical phase within the HXW. The performance of the heat exchanger HXW, based on the experimental measurements, shows the expected behavior, i.e. high convective heat transfer and heat flow at higher mass flow rates. The mass flow rate is higher at lower pressures, due to the more closed valve, as discussed for figure 7. DOI: 10.17185/duepublico/83305 7 Figure 8: Relation between the heat flow in the HXW and maximum cycle pressure. The total heat transfer coefficient is comprised of the convective heat transfer on the hot water side, the conduction through the steel wall and the convective heat transfer on the CO2 side. The total heat transfer coefficient of the HXW is depicted in Figure 9 for the respective Reynolds numbers and maximum cycle pressures. The typical behavior for convective heat transfer can be observed, as the heat transfer coefficient increases with higher Reynolds numbers. Since the convective heat transfer on the CO2 side is the most relevant thermal resistance, it has the greatest impact on the heat flow of the HXW. The strong influence of convective heat transfer on the CO2 side was not expected. Therefore, from a heat transfer point of view of, future experimental campaigns should be carried out at higher mass flows on the CO2 side, to enhance the convective heat transfer. These data can be used to describe the HXW in the CEEGS model, especially for low mass flow rates. Figure 9: Heat transfer coefficient of the HXW for different Reynolds numbers and maximum cycle pressures. As the HXW acts as a heater in the CEEGS cycle, the temperatures vary between 150 °C and 250 °C as well as pressures between 105 bar and 235 bar. For the CEEGS cycle high temperatures above 200 °C are required, when the pressure is higher than 200 bar. Therefore, the experimental campaigns have been adapted to this combination. As a result, the highpressure experiments were carried out for high temperatures only. Figure 10 shows the heat transfer coefficient of the HXW for different maximum cycle temperatures and pressures. It can be seen, that the influence of temperature on the heat transfer coefficient is low. However, the influence of the cycle pressure is significant, as the heat transfer coefficient decreases for higher cycle pressures - especially for pressures above 150 bar. Thus, the highest cycle temperature should be determined by the thermal efficiency of the cycle, as the temperature has little effect on heat exchanger performance. Figure 10: Heat transfer coefficient of the HXW for different maximum cycle temperatures and pressures. Based on the heat flow in the HXW, the pump power and the expansion power through the expansion device, the thermal cycle efficiency was calculated for different maximum cycle pressures and temperatures (Figure 11). A clear trend of the influence of the temperature on the cycle efficiency cannot yet be determined and further experiments are required. However, for the 250 °C experiments, the influence of the pressure on the cycle performance can be seen. Higher pressure results in high cycle efficiency for the 250 °C experiments, indicating an enhanced operation. These experimental results were partly expected, as a higher cycle pressure means a high expansion work. As liquid CO2 is an incompressible fluid, the higher compression work of the pump is insignificant and the overall efficiency increases. However, the thermal efficiency for the 200 °C case at 200 bar is higher compared to the 250 °C case at 200 bar. This behavior was not expected and additional experimental campaigns need to be carried out to ensure the reproducibility of the results. DOI: 10.17185/duepublico/83305 8 Figure 11: Thermal cycle efficiency for different maximum cycle temperatures and pressures. SUMMARY AND OUTLOOK The performance of a transcritical CO2 cycle representing the CEEGS concept was investigated experimentally to validate the surface components involved during the discharge cycle. A demonstrator (for up to 235 bar and 250 °C) was designed, built and operated. Measurements from the experiments were used to determine the performance of key components, such as the hightemperature heat exchanger, the pressure equipment and the cycle behavior. OUTLOOK From the measurement data of the experimental campaign the performance of the components can be described in more detail and the data will be used by the project partner to validate the models. Further operation of the facility will be performed, to obtain more information on the dynamic behavior of the transcritical CO2 cycle. The implementation of geological reservoir component will provide the variation in pressure and temperature at given CO2 mass flow rate during the energy storage cycles in future research activities. The component will be represented by a large rock sample with boreholes. First, pure CO2 will be stored and backproduced followed by adding brine to mimic co-production of residual water. At a later stage, adding impurities to the stream mimicking the capture of CO2 from a point source, such as a cement factory. The inclusion of brine and/or gas impurities allows for an investigation of their impact on the system efficiency. An efficiency decrease may arise from the geochemical interaction between (impure) CO2, brine and the rock matrix. The reactions can result in salt precipitation in the wells eventually decreasing their injectivity or productivity, and they may also affect the composition of the back-produced gas [16]. The injection of (impure) CO2 into rock samples also allows for a retrieval of implications with respect to storage security, due to cyclic change in reservoir pressure during injection and back-production, as well as for a characterization of the involved fluid dynamics, such as the spatial-temporal distribution of the CO2 plume and inevitable trapping of CO2. NOMENCLATURE 𝐴𝐻𝑋: Heat transfer surface [m²] ∆𝑇𝑙𝑜𝑔: logarithmic mean temperature difference [K] 𝐶𝑂𝑃: coefficient of performance ∆𝑇𝑚𝑎𝑥: maximum temperature difference [K] 𝑑𝑡: characteristic length [m] ∆𝑇𝑚𝑖𝑛: minimum temperature difference [K] ℎ𝑡𝑐𝑡𝑜𝑡𝑎𝑙: overall heat transfer coefficient [W/m²K] 𝑅𝑒𝐶𝑂2: Reynolds number Pexp: expansion work [W] 𝑢: fluid velocity [m/2] Ppump: compression work by pump [W] ηthermal: thermal efficiency of the cycle [%] 𝑄󰇗𝐶𝑂2: heat flux [W] μ: dynamic viscosity [Pa s] 𝑄󰇗𝐻𝑋𝑊: heat flux in the HXW [W] 𝜌: density [kg/m³] ACKNOWLEDGEMENTS This work has been partially funded by the project 'CEEGS: Novel CO2-based Electrothermal Energy and Geological Storage', from the European Commission through Horizon, the EU Framework Program for Research and Innovation, under Grant Agreement No. 101084376. REFERENCES [1] Y. Liu et al., "Supercritical CO2 brayton cycle: a state-ofthe-art review," Energy, p. 115900, 2019. [2] M. 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