Surface processes optimisation in a novel CO2-based electrothermal energy and geological storage trigeneration system

Surface processes optimisation in a novel CO 2 -based electrothermal energy and geological storage trigeneration system ☆ A. Carro a,* , C. Ortiz b , S. Unger c , A. Stoikos d,e , A.-S. Kyriakides d , I.N. Tsimpanogiannis d , J.A. Becerra a,f , S. Voutetakis d , U. Hampel c , R. Chacartegui a,f a Dpto. Ingeniería Energ´ etica, Universidad de Sevilla, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain b Materials and Sustainability Group, Department of Engineering, Universidad Loyola Andalucía, Avda. De las Universidades s/n, 41704 Dos Hermanas, Seville, Spain c Helmholtz-Zentrum Dresden-Rossendorf, Institute of Fluid Dynamics, Bautzner Landstr. 400, 01328 Dresden, Germany d Chemical Process and Energy Resources Institute, Centre for Research and Technology-Hellas, 60361, 57001 Thermi, Thessaloniki, Greece e Department of Mechanical Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece f Universidad de Sevilla, Laboratory of Engineering for Energy and Environmental Sustainability, 41092 Seville, Spain HIGHLIGHTS •Optimised CO2 and energy storage system enhances efficiency and extends applications. •Sequential heat transfer improves thermal integration and broadens demand coverage. •Reducing phase change temperature to −40 ◦C increases efficiency range to 56.6–67.7 %. •Recuperative cycle and multi-stage processes improve efficiency & reduce requirements. •Alternative configurations enable system to cover thermal demands from −50 to 270 ◦C. ARTICLE INFO Keywords: Renewable energy storage Electrothermal energy storage Transcritical CO 2 Geological storage CO 2 Trigeneration ABSTRACT Electrothermal energy storage is a promising technology for high penetration of renewable energy. In recent years, the integration of this energy storage system with geological CO 2 storage has been introduced. The system consists of a reversible heat pump formed by transcritical CO 2 cycles with thermal storage at two temperature levels, enabling the simultaneous operation of geological CO 2 storage and the storage/production of renewable electrical energy. This work focuses on studying high and low-temperature thermal energy storage. Step heating on the high-temperature side allows for better integration of the supercritical and subcritical temperature profiles of the CO 2 and the thermal storage fluid. Thermal storage at different temperature levels provides a higher turbine inlet temperature, improving the efficiency of the power production cycle and increasing heating applications such as district heating or domestic hot water. Considering four high-temperature tanks, round-trip efficiency increases from 52.8 to 55.4 %. It presents a thermal demand coverage range of about 20–150 ◦C, with temperature increases of approximately 30 ◦C. The phase change temperature shift on the low-temperature side directly impacts electric power production and enables new cooling applications. The system's efficiency increases as the low-temperature phase change temperature decreases, reaching 58.7 % at −30 ◦C. Using alternative configurations in the transcritical CO 2 cycle, such as the recuperative cycle and multi-stage compression and expansion, high-efficiency values can be maintained with lower system requirements. 1. Introduction The transition towards 100 % renewable energy systems has reached advanced levels in different countries around the world [1,2]. The increase in renewable electricity global capacity reached 507 GW in 2023, nearly 50 % more than in 2022, marking a significant shift in the global growth trend [3,4]. In 2024–2030, renewable energy capacity ☆ This article is part of a Special issue entitled: ‘SDEWES2024(B.N.)’ published in Applied Energy. * Corresponding author. E-mail address: [email protected] (A. Carro). Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy https://doi.org/10.1016/j.apenergy.2025.126165 Received 9 January 2025; Received in revised form 27 April 2025; Accepted 16 May 2025 Applied Energy 395 (2025) 126165 Available online 31 May 2025 0306-2619/© 2025 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). increments will continue to rise, with generation costs lower than those of fossil and non-fossil alternatives in most countries, supported by political initiatives [3]. The increase in renewable energy's share of the electricity demand presents challenges in energy systems, depending on the extent of the electricity demand coverage [4]. In the European Union, it is projected that the potential penetration of renewable energy in 2028 will exceed 50 % in nine countries, with over 90 % of Denmark's electricity generation coming from wind and solar photovoltaic systems [5]. One phase behind are the UK, Spain, Germany, and Ireland, where variable renewable energy (VRE) meets nearly all demand in some periods [6,7]. The increasing proportion of renewable energies, especially in countries with high penetration of wind and photovoltaic solar energy, poses significant challenges related to energy storage [8,9]. Electricity storage solutions can mitigate renewable energy integration issues into the grid [10]. Currently, two complementary technologies are capable of providing cost-effective storage: stationary batteries for 1–4 h [11] and pumped hydroelectric power plants (PHS) for 4–15 h [12], depending on reservoir size. Supportive policies and the decline in stationary battery costs over the last decade have spurred investment between 2018 and 2023 in Europe, the United States, China, and Australia [6]. They aim to capitalise on market arbitrage opportunities during peak hours but cannot cover the daily or seasonal mismatch between renewable resource availability and energy demand [13]. Long-term storage is key to achieving effective renewable deployment in energy systems [14]. Its deployment remains limited compared to stationary batteries, especially where it is most needed: markets forecasted to exceed a 50 % share of renewable-origin electricity by 2028. Although existing long-term storage capacity in Europe, the United States, and Japan will contribute to system integration, long-term storage needs are growing rapidly, requiring investment decisions today for plants that will be needed in 6–10 years [15]. Effective integration of renewables also requires consideration of the coverage of thermal energy demand [16]. Heat accounted for nearly half of total final energy consumption and 38 % of energy-related CO 2 emissions in 2022 [6]. Urban heating networks offer considerable potential for renewable heat integration. They covered nearly 7 % of global heat demand from the building and industry sectors in 2022 [17]. Lower operating temperatures, integrated thermal storage, advanced measurement, control, and optimisation strategies for fourth and fifthgeneration urban heating and cooling networks and local ambient heat circuits can further facilitate renewable energy integration [18]. In the building sector, the installation of heat pumps has played a crucial role in meeting thermal demand, resulting in increased consumption of both electricity and heat for space and water heating [19]. Large-scale heat pump technologies and solar thermal systems are generating increasing interest in China and several European countries, with 400–500 MW th projects under development in 2023 [20]. The increased electricity use for process heat raises renewable heat consumption but is insufficient to curb fossil fuel use. According to IEA [7] data, the global industrial heat demand is projected to increase by 16 % between 2023 and 2028. If fossil fuel use is not contained, the heating sector alone could consume over one-fifth of the remaining carbon budget to limit global warming to 1.5 ◦C (86 Gt of CO 2 ) in the period 2023–2028 [6]. In this context of an urgent need to develop new photovoltaic and wind energy storage systems and coverage of electricity and heat demands through renewable energy, electrothermal energy storage (ETES) stands out. Recent advancements in pumped thermal energy storage (PTES) and liquid air energy storage (LAES) offer cost-effective, scalable alternatives to PHS and CAES without geographical limits [21]. As Carnot batteries (CB), they involve a reversible conversion of electrical energy into thermal energy [22], and cover a broader range of applications, which are not accessible to batteries and pumped hydro, such as integration into heating and cooling networks, opening up more potentially favourable business cases [23]. A review of CB projects and literature [23] reveals differences in layouts and scales, especially for Rankine and Brayton PTES. PTES (in both Rankine and Brayton configurations) seem to achieve higher efficiency than LAES, but Brayton PTES also have the highest specific cost. They focus on different installation sizes: Rankine PTES at 1–10 MW, Brayton 5–50 MW and LAES for more than 50 MW. Rankine-based PTES studies include steam cycles (and steam-ammonia cascade cycles) [24], organic Rankine cycles (ORC) [25] and transcritical CO 2 cycles [26]. The round-trip efficiency of the Rankine-based PTES cycle usually ranges from [25] to 70 % [24]. Mercang¨ oz and Morandin [26,27] set the basis for electrothermal energy storage systems using transcritical CO 2 cycles, demonstrating around 50–60 % electric-to-electric conversion efficiencies. This allows for flexible electricity production and direct coverage of thermal energy demands (heating and cooling) by storing thermal energy at two temperature levels and levelised cost of electricity (LCOE) of 70–140 USD/ MWh [28]. In these systems, transcritical CO 2 cycles and thermal energy storage in water and ice (low-cost materials with high availability and easy access, low environmental impact, and good thermodynamic properties) play a prominent role [26]. One of the most interesting applications of transcritical CO 2 cycles as electrothermal energy storage systems includes CO 2 storage within underground geological formations, developed by Carro et al. [29,30]. The use of CO 2 as a working fluid and the operating conditions of transcritical cycles allow the system to integrate the storage of CO 2 in Nomenclature CAES Compressed Air Energy Storage CB Carnot Batteries CC Carbon Capture CCS Carbon Capture and Storage CEEGS Novel CO 2 -based Electrothermal Energy and Geological Storage CO 2 Carbon dioxide ECE Electric-to-electric Conversion Efficiency ETES Electrothermal Energy Storage EOR Enhanced Oil Recovery GS Geological Storage h Specific enthalpy (kJ/kg) HE Heat Engine HP Heat Pump HT High Temperature hx Heat exchanger IEA International Energy Agency LAES Liquid Air Energy Storage LCOE Levelised Cost of Electricity LT Low Temperature Mtpa Mega-tonnes per annum ORC Organic Rankine Cycle P Pressure (bar) PHS Pumped Hydro Storage PTES Pumped Thermal Energy Storage Q Heat Rec Recuperative s Specific entropy (kJ/kg-K) T Temperature (◦C) TIT Turbine Inlet Temperature TRL Technology Readiness Level TES Thermal Energy Storage VRE Variable Renewable Energy A. Carro et al. Applied Energy 395 (2025) 126165 2 geological formations such as porous media (saline aquifers, depleted oil/gas reservoirs) or salt caverns [31,32]. The benefits of including geological storage in the ETES system include thermal and mechanical energy storage, geothermal gains, lateral migration, or partial sequestration of stored CO 2 [33]. With electric-to-electric conversion efficiencies exceeding 50 %, the system could achieve +1 Mtpa CO 2 considering 20 MW plants [29]. These studies formed the basis for the “Novel CO 2 -based Electrothermal Energy and Geological Storage System” (CEEGS) project [34], currently in progress, funded by the European Commission through Horizon, the EU Framework Programme for Research and Innovation, under Grant Agreement No. 101084376. At Technology Readiness Level (TRL) 2, the system still has significant development gaps, such as the optimal integration between the transcritical surface cycles and the underground CO 2 storage. This work presents a novel study of the processes involved in the surface energy conversion cycles of the CEEGS system, focusing on optimising thermal storage at both temperature levels and approaching alternative configurations of the heat pump and the power production cycles. The analysis of new scenarios is presented, such as the recuperative configuration of the charging phase cycle and the use of staged compression and expansion with heat recovery. The study of staged sensible heat transfer on the HT side is introduced, providing better integration of supercritical and subcritical temperature profiles of the CO 2 and the thermal storage fluid and establishing different temperature levels for thermal energy demand coverage. The impact of phase change temperature on the LT-TES side is also investigated. The phase change temperature influences the shape of transcritical CO 2 cycles and is the most important factor in system cooling applications. This study demonstrates how renewable energy storage and flexible power generation systems, with the storage of CO 2 captured in geological formations, acquire the potential for flexible coverage of many highand lowtemperature thermal energy demands. The CEEGS system expands the temperature range of thermal energy in the form of sensible heat, from a single temperature limited to about 140–150 ◦C [29,30] to various temperatures, ranging from −30 ◦C to 275 ◦C. The availability of thermal energy in the form of latent heat also expands its possibilities, allowing coverage in a range of −50 ◦C to 20 ◦C, depending on the phase change temperature. The electric-to-electric efficiency is used as a reference indicator, ranging from 58.54 to 71.09 % when the phase change temperature is −50 ◦C. 2. Methods 2.1. Resources For the study, numerical modelling was conducted using the Python programming language along with various specialised libraries. In implementing the model, the CoolProp library [35] was used to calculate the different thermodynamic properties of the substances involved in the study [36]. This combination of tools enabled a detailed and accurate system representation, facilitating the analysis of its performance under various operational conditions. 2.2. Novel CO 2 -based electrothermal energy and geological storage system as trigeneration technology The system under study is the novel integrated electrothermal energy storage and CO 2 storage system in geological formations. The concept originates from the combination of two technologies with different objectives. On the one hand, a reversible heat pump or Carnot battery is used for renewable energy storage and subsequent electricity production through electrical-to-thermal and vice versa energy conversion [37]. On the other hand, the injection and production of CO 2 into geological formations for CO 2 storage [38] from capture plants (CCS) or for enhanced oil recovery (EOR) [39], among other applications. The common denominator is using CO 2 as the working fluid [40]. The Carnot battery, composed of transcritical CO 2 cycles, offers integration possibilities with geological storage. Favourable conditions for CO 2 integration with injection and production processes from geological formations occur during the cycles. The concept was first presented in 2020–2021 by Carro et al. [29,30] and continues to advance in its development phases, now propelled by the CEEGS project [34]. The CEEGS system offers a wide range of potential applications. Favourable CO 2 injection or production conditions in geological storage processes allow the CEEGS system joint integrations with CO 2 capture plants. Geological storage provides several potential advantages, such as geothermal gains or residual and/or dissolved CO 2 trapping underground in the short term, while chemical reactions can trap CO 2 in the form of minerals in the long term. As a trigeneration system, it adds heating or cooling applications to flexible electricity generation. The temperature range of the transcritical CO 2 cycle allows for the use of hot water as a thermal storage medium on the high-temperature (HT) side or ice slurry on the low-temperature (LT) side [27,29]. 2.2.1. Principle of operation. Energy and geological storage As a reversible heat pump or Carnot battery, the electrothermal energy storage system is based on converting electrical energy to thermal energy, storage in the form of thermal energy, and subsequent conversion back to electrical energy. The system stores thermal energy at two temperature levels, thus gaining the ability to meet thermal demand directly. The reversible heat pump utilises transcritical cycles, with CO 2 as the working fluid. The conditions of transcritical CO 2 cycles are compatible with those necessary for CO 2 injection/production in underground geological formations. It allows the electrothermal energy storage system to include geological CO 2 storage, expanding integration possibilities with CO 2 capture, utilisation, and storage applications. See Fig. 1. During the charging phase, where electrical (renewable) energy is converted into thermal energy, the system can inject a certain flow rate of previously captured CO 2 into an underground geological formation. The thermal energy stored at two temperature levels could be used directly in heating/cooling applications. During the discharge phase, the stored thermal energy would be flexibly converted back into electrical energy. The discharge cycle can be coupled with the processes of CO 2 production from underground geological formation and subsequent reinjection, allowing for exploiting phenomena such as geothermal gains. 2.2.2. Thermal energy storage and power production. Electric-to-electric conversion Fig. 2 shows a schematic of the components and processes of the system that perform renewable energy (photovoltaic or wind) storage and flexible electricity production. During the charging phase (1-2-3-4), a compressor is used as the main equipment of the transcritical CO 2 cycle. It transfers heat under supercritical conditions to the HT storage and receives thermal energy to complete its evaporation from the LT storage. During the discharge phase (5-6-7-8), the thermal energy stored at high and low temperatures is used to heat the CO 2 under supercritical conditions and condense it after the turbine's expansion. One critical aspect of transcritical CO 2 cycles as an electrothermal energy storage system is the integration into the temperature profiles of heat exchanges with thermal storage. Proper integration of temperature profiles allows the charging and discharging cycles to have a similar shape, increasing electrical conversion efficiency. Due to the characteristics of transcritical CO 2 cycles, with supercritical heating/cooling processes on the HT side and evaporation/condensation on the LT side, good integration requires sensible heat transfer on the HT side and latent heat transfer on the LT side. The temperature-entropy CO 2 cycle evolution constrains the heat transferred to the thermal storage tanks in the charging and discharging phases. The system may require balancing depending on the ratio between the heat transferred and the heat required at each temperature level. Balancing would be aimed at utilising the thermal energy stored in A. Carro et al. Applied Energy 395 (2025) 126165 3 the surplus reservoir when the limiting reservoir becomes depleted. Fig. 2 shows an additional simple compression refrigeration cycle connected to LT-TES, which would allow balancing of the simultaneous discharge using renewable energy surplus during the charging phase. 2.2.3. CO 2 storage in geological formations The temperature and pressure conditions of CO 2 in the transcritical cycles of the electrothermal energy storage system are compatible with those of CO 2 injection and production in geological formations. The technique has already been used in geological CO 2 storage applications to reduce CO 2 emissions into the atmosphere and combat climate change [41]. This technique has been traditionally used in other applications, such as Enhanced Oil Recovery (EOR) [42]. It involves injecting CO 2 into oil fields to increase pressure and, in a subsequent step, facilitating additional hydrocarbon extraction via tertiary miscible oil displacement. Once the oil is recovered and the dissolved CO 2 is separated from the oil, the CO 2 can be reinjected into the geological formation for further use as an enhanced oil recovery agent. Worldwide, the mass of CO 2 injected annually exceeds 40 Mton [43]. CO 2 is injected into suitable underground geological formations for long-term storage. In CCS, the shape and spread of the plumes are of concern for safety reasons. Common geological formations already used for this purpose include deep saline aquifers, depleted oil and gas fields, and coal seams. These formations must have specific geological characteristics to ensure CO 2 retention capacity, such as adequate porosity and permeability [44]. Identifying the characteristics of reservoirs that favour the early development of a sufficiently large plume is important for understanding the limitations imposed by geological environments on the CEEGS technology. Currently, through the CEEGS project, porous media such as deep saline aquifers, depleted hydrocarbon reservoirs, and salt caverns [33] are being investigated. Porous media are characterised by their high storage capacity and associated CO 2 fixation mechanisms, such as lateral migration, residual trapping, dissolution or mineralisation [32]. Salt cavities, generally with a lower level of storage capacity and associated fixation mechanisms, present better capabilities for mechanical energy storage, as the allowable pressures are considerably higher than those of porous media [33]. 2.2.4. Integrated energy and CO 2 storage system The CEEGS system features two distinct operating modes: closedloop operation and open-loop operation, with a charging and discharging phase in each. In the closed-loop operation mode, CO 2 circulates only through the power equipment, converting electrical energy to thermal energy during the charging phase and thermal energy to electrical energy during the discharging phase in a closed circuit. See Fig. 3a. This mode of operation corresponds to that of the traditional electrothermal storage system, in which the CEEGS system acts only as an electrothermal energy storage/production system. In the open-loop operation mode, geological storage capacity is included. A stream of CO 2 from a capture plant (CC) is introduced into the transcritical cycle during the charging phase at the inlet of the evaporator (LT-hx) on the low-temperature side. After passing through the electrical-to-thermal energy conversion process equipment, the CO 2 in a supercritical state with a relatively low temperature at the high-temperature side heat exchanger (HT-hx) outlet is injected and stored within the geological formation (GS). The entire process corresponds to the sequence CC-4-12-3-GS in Fig. 3b. In the discharging phase, after the production process from the geological formation, the CO 2 is introduced into the power production cycle, receiving thermal energy from HT-TES and using LTTES as a condenser, to be finally reinjected in a second injection process sequence GS-6-7-8-5-GS in Fig. 3b. In the open-cycle operation mode, the CEEGS system acts simultaneously as an electricity storage/production system and a CO 2 geological storage system. Depending on the Fig. 1. Operating principle of the CEEGS trigeneration system. Fig. 2. Scheme of the CEEGS system. A. Carro et al. Applied Energy 395 (2025) 126165 4 characteristics of the geological formation, such as depth, terrain, and the geothermal gradient, additional equipment is required to perfectly adapt to the CO 2 conditions in the injection/production processes and the transcritical cycle, such as auxiliary pumps and condensers or cleaning equipment. The pressure (P) and temperature (T) conditions in the reservoir and the variation of P-T between wellheads and bottom holes affect the efficiency of the CEEGS system with underground injection and CO 2 retroproduction. These conditions depend primarily on the flow rates and diameters of the wells and their length, i.e., the depth of the reservoir. PT changes within the well are especially important for CO 2 retroproduction from the reservoir to ensure that the CO 2 saturation line is not reached and that no two-phase flow occurs in the producing well. These additional processes derived from CO 2 injection/production in geological formations potentially benefit the already high relative efficiency of the electrothermal storage system. Considering the two modes of operation, the energy storage/production capacity of the CEEGS system is independent of the captured CO 2 storage requirements in the geological formation. The most critical point in the efficiency of the CEEGS system is the integration between transcritical CO 2 cycles and thermal storage, both at high and low temperatures. The energy storage processes of the charging phase and power production of the discharging phase are present in both operating modes. By improving the integration of temperature profiles and exploring all options offered by thermal storage, the round-trip efficiency of electrothermal conversion can be improved, and additional applications in the trigeneration system can also be found. 2.3. Main assumptions and indicators The components are modelled using a lumped volume approach, evaluating inlet and outlet streams enthalpies. Heat transfer in heat exchangers ( ˙ Qhx) and power exchange ( ˙ W) in compressors, pumps and turbines are evaluated through the general Eqs. (1)–(2). ˙ Qhx =˙ mhot •(hhot in −hhot out)= − ˙ mcold •(hcold in −hcold out )(1) ˙ W=˙ m• (hin −hout) = ˙ m•Δh(2) Being ( ˙ m) the mass flow rate and the subscripts ‘in’ and ‘out’ the inlet and outlet conditions that define the specific enthalpy. Evolutions in turbines, compressors and pumps are closed through their isentropic efficiencies, Eqs. (3)–(4), using reference values from similar studies [27,29]. η exp s=hin −hout hin −hs out (3) η comp s=hs out −hin hout −hin (4) where η s is isentropic efficiency, ‘exp’ and ‘comp’ refer to expansion and compression processes, h is the specific enthalpy and hs is the enthalpy in an isentropic process, at the inlet (‘in’) and outlet (‘out’) of the equipment under consideration. Heat exchangers are modelled by setting a minimum temperature difference (dTmin) between the temperature profiles. Pressure losses in heat exchangers are neglected in this study, considering they will be comparatively small compared to the absolute pressure and effects on the injection/extraction processes. The main assumptions are summarised in Table 1. The CO 2 pressure on the LT side depends on the phase change temperature of CO 2 (evaporation during charging and condensation during discharging), according to the phase change temperature of the LT-TES. See Eqs. (5)–(6). The inlet conditions in the compressor (saturated gas) and the pump (saturated liquid) at LT side pressure establish the first reference point in constructing the transcritical CO 2 cycles of the charging and discharging phases, respectively. The outlet conditions in the compression processes are established from the isentropic efficiency of each unit and the pressure on the HT side. The compression line during charging, from saturated gas conditions to the pressure on the HT side, marks the highest temperature of the system. The evaporation, also Fig. 3. Operating modes of the CEEGS system: a) Closed-cycle operation, b) Open-cycle operation. Table 1 Main starting assumptions in the modelling of the system. Reference Equipment Value Unit Efficiency CO 2 compressor 0.89 – CO 2 turbine 0.91 – CO 2 pump 0.86 – CO 2 expander 0.88 – Minimum temperature difference HT-hx 4 ◦C LT-hx 4 ◦C Phase change temperature LT-TES 0 ◦C Number of storage tanks HT-TES 1 – Charging cycle CO 2 HT-Pressure 200 bar Discharging cycle CO 2 HT-Pressure 200 bar CO 2 flow rate Cycle 30 kg/s A. Carro et al. Applied Energy 395 (2025) 126165 5 from the charging phase, establishes the lowest temperature. These factors directly influence the heating and cooling applications of the trigeneration system. PCO2 charge,LT =PCO2 sat (T=TLT−TES phase −dTLT−hx min )(5) PCO2 discharge,LT =PCO2 sat (T=TLT−TES phase +dTLT−hx min )(6) The outlet temperature of the compression processes, pump and compressor, and the minimum temperature difference, set the limits of the temperature range in HT-TES. The outlet temperature of the compressor conditions the upper limit. The pump outlet temperature conditions the lower limit. The heat transfer is modelled in sequential mode: first, the cooling of the charging phase and then the heating of the discharging phase. The pinch point of the supercritical cooling is placed at the CO 2 outlet, setting the inlet temperature at the expansion of the charging phase. The temperature profile of the supercritical CO 2 cooling is calculated as a constant pressure cooling between the compressor's outlet temperature and the expansion's inlet temperature. The heat exchange is optimised by adjusting the slope of the temperature profile (straight) of the subcritical heating of the HT-TES fluid. In this reference case, pressurised water. This is achieved through the mass flow rate of the water in the charging phase. See Eq. (1). The HT-hx outlet temperature depends on the integration between the supercritical profile's curvature and the subcritical profile's straight line. In the supercritical CO 2 heating of the discharging phase, the temperature profile of the HTTES fluid is the same as in the charging phase, with the pinch point at the CO 2 inlet. On this occasion, the supercritical temperature profile is adjusted to the straight line of the subcritical profile, optimising the second heat exchange. This adjustment is conditioned by the mass flow rate of water in the discharging phase (Eq. (1)). The CO 2 outlet temperature sets the turbine inlet temperature. The turbine inlet temperature depends on the integration between the profiles of supercritical heating and cooling of CO 2 on the HT side and the temperature change profile of the HT-TES. Optimising the integration of temperature profiles increases the turbine inlet temperature and system's efficiency. The outlet conditions of the expansion equipment, expander in charging and turbine in discharging, are determined from the inlet conditions and the isentropic efficiency (Eqs. (3)–(4)) of each equipment. The outlet conditions of the expansion are the inlet conditions at LT-hx, charging, and discharging. The heat exchanges in LT-hx close the cycles, and are modelled as heat transfers at constant pressure that condition the mass flow rate of the storage medium in LT-TES, in this reference case, ice slurry. The turbine inlet temperature and the condensation conditions on the LT side mark the increase in enthalpy during expansion. Enthalpy increases will be negative for compression equipment and heat transfer in heat exchanges. This increases in enthalpy determine the efficiency of the system, defined based on the indicators of Eqs. (7)–(8). η ele LT =(ΔhT HE +ΔhP HE)•ΔhLT−TES HP (ΔhC HP +ΔhHydT HP )•ΔhLT−TES HE (7) η ele HT =(ΔhT HE +ΔhP HE)•ΔhHT−TES HP (ΔhC HP +ΔhHydT HP )•ΔhHT−TES HE (8) Eqs. (7)–(8) indicate the electric-to-electric conversion efficiency (ECE), differentiating between the HT and LT sides. Where ΔhTES HP and Δ hTES HE refer to the enthalpy drop in the high or low-temperature exchanges for each phase, charging and discharging. ΔhX HP,HE is the enthalpy drop developed by the compression and expansion equipment, compressor (C), pump (P), hydraulic turbine (HydT) and gas turbine (T) in charging (HP) or discharging (HE) phases. This difference in efficiency in the overall conversion process establishes a range depending on the thermal energy stored at each temperature level. ECE quantifies the conversion of electrical to electrical power from the point of view of the two temperature levels of TES. The indicator is based on the ratio between the electrical energy generated in the discharge phase and the electrical energy consumed during the charging phase. The development of the efficiency indicator, which depends on enthalpy drops, has been made possible by considering that all the energy stored during the charging phase is used in the power production of the discharging phase [∅HT,LT−TES HP =∅HT,LT−TES HE ], at each temperature level. ECE depends only on the shape of the CO 2 transcritical chargedischarge cycles. This hypothesis implies establishing a range in which the system efficiency is located, a limiting reservoir and an excess reservoir. The tank on the side (LT or HT) that marks the lower limit of the efficiency range will act as the ‘limiting’ tank and will be discharged before the other, which will mark the upper limit. The system's roundtrip efficiency will ultimately depend on using this surplus energy. The surplus thermal energy in HT could be used in power production when heat is given up to the environment in the condensation of the discharge phase, or in an absorption cooling process, transferring energy from HT to LT, bringing the efficiency close to the upper limit. In the case of using an additional simple compression refrigeration cycle during the charging phase, the roundtrip efficiency (RdEff), where the system returns to its initial state, would also depend on the compressor power of the refrigeration cycle and the additional thermal power on the LT-side, as shown in the Eq. (9). RdEff =(˙ WT HE +˙ WP HE)•(˙ QLT−TES HP + ˙ QLT−TES Ref ) (˙ WC HP + ˙ WHydT HP + ˙ WC Ref )• ˙ QLT−TES HE =(˙ WT HE +˙ WP HE)• ˙ QHT−TES HP (˙ WC HP + ˙ WHydT HP + ˙ WC Ref )• ˙ QHT−TES HE (9) where ˙ QLT−TES Ref represents the additional thermal power in LT-TES during the charging phase, and ˙ WC Ref is compressor power of the additional refrigeration cycle. Considering the temperature range required in the evaporator of the compression refrigeration cycle, imposed by LT-TES, and that of the condenser, which evacuates heat to the environment, an ammonia-based cycle suits the temperature profiles. This option was considered in [27], in a heat pump under similar conditions. The ammonia compression refrigeration cycle would operate between 3.67 bar (evaporator) and 11.32 bar (condenser), when the CEEGS system operates at the conditions in the Table 1. Under these design conditions, it would have a COP of 6.4. The cycle is sized based on the heat exchange in the evaporator ( ˙ QLT−TES Ref ), which will operate during the charging phase, balancing the two phases. The required thermal power depends on the thermal power of the LT and HT exchanges in the charging and discharging phases, and is defined by Eq. (10). ˙ QLT−TES Ref = ˙ QLT−TES HP − ˙ QLT−TES HE • ˙ QHT−TES HP ˙ QHT−TES HE (10) The geological formations considered in the model are porous media and salt cavities. In porous media, the downhole requirements are characterised by the hydrostatic pressure [Phydro = ρ water •g•z], defined by the pressure exerted by the water column (Pa) and where ρ is the density of water (kg/m 3 ), g is the gravity constant (m/s 2 ), and z is the reservoir depth (m) [29]. The limit is considered to be 20 % of the accumulated pressure. In salt caverns, the maximum storage pressure is limited by the minimum principal stress in the salt rock acting on the cavern roof, the lithostatic pressure [45]. The lithostatic pressure (MPa) is calculated as a function of the depth (m) of the top as [Plith = 0.022 •z], and the allowable range follows the maximum and minimum A. Carro et al. Applied Energy 395 (2025) 126165 6 pressure values used in the CAES industry [31,45]: 30–80 % of the lithostatic pressure. The geological formation depth range is between the minimum value required to maintain the supercritical state of the stored CO 2 , and the maximum recommended for CAES or natural gas storage, which is 2500 [31]. The required injection conditions at the wellhead are determined using the wellbore flow model of Adams et al. [33,46], in which the average values within the reservoir and the pressure loss in the producing well are determined. The flow rate and the diameter of the well play an important role in the temperatures and pressures required at the injection wellhead. 2.4. Validation methods Methods and resources such as the CoolProp library and the Python programming language have been used and validated in numerical modelling. The open-access CoolProp library has been validated [35] and is increasingly used in a variety of technical fields. The main limitation of this library is that it does not include thermophysical properties of mixtures, which does not influence this study. The thermodynamic cycles comprise basic compression/expansion processes and heat exchanges, solved with mass and energy balances, similar to those used in other related works [27,29]. The isentropic efficiency considered in the equipment and the temperature differences in the heat exchanges are also identical to those used in other studies of the same type. Fig. 4a shows the T-s diagrams of the transcritical cycles during the charging and discharging of the CEEGS system, and Fig. 4b represents the T-s diagrams of the additional Ammonia compression refrigeration cycle, according to the initial conditions listed in Table 1. The phase change temperature of LT-TES and the minimum established temperature condition of the pressures of CO 2 on the LT side determine the characteristics of evaporation during charging and condensation during discharging, see Fig. 4c. On the HT side, the pressures of the transcritical cycles and the integration in the temperature profiles in the heat exchanger determine the evolution in HT-TES, see Fig. 4d. The evolution of temperature profiles in the heat exchange diagrams T-Q corresponds to those of the T-s and P-h diagrams of CO 2 . Heat transfer is consistent at both temperature levels. The minimum temperature difference is established in LT-hx. In the case of HT-hx, it is slightly exceeded due to adjustment with the transcritical cycles, but no crossings occur in any case. The ECE range of 48.3–56.7 % is also consistent with similar systems studied in the works mentioned above [27,29]. The roundtrip efficiency using the additional ammonia refrigeration cycle would be 52.76 %. Fig. 4. Diagrams of the CEEGS system operating in closed-cycle mode: a) T-s CO 2 transcritical cycles, b) T-s ammonia refrigeration cycle, c) T-Q of the exchange with LT-TES, and d) T-Q of the exchange with HT-TES. A. Carro et al. Applied Energy 395 (2025) 126165 7 2.5. Scenarios under analysis The study on the optimisation of surface processes and alternative cycle configurations in the CEEGS system performs several analyses, ranging from the adaptation of CO 2 transcritical heating/cooling profiles with HT-TES and the impact of phase change temperature in LTTES, recuperative cycles, multi-compression, heat recovery in the intermediate reheat and multi-expansion. The system uses water as the thermal storage medium in the reference case. The temperature ranges of the initial conditions allow for the use of water as a storage medium at both temperature levels. In the case of HT-TES, the 22–140 ◦C range could be covered with slightly pressurised liquid water (less than 8 bar). On the low-temperature side, LTTES could utilise solid-liquid phase change, employing ice slurry technology to facilitate heat transfer. The different optimised configurations and processes are compared with the efficiency of electrical energy conversion, and the new heating/cooling options that the system can offer are described qualitatively. 2.5.1. Step heating/cooling on the HT-TES First, the benefits of integrating a stepped sensible heat transfer on the HT-TES side are evaluated. It aims to integrate better the supercritical and subcritical temperature profiles of CO 2 and the thermal storage fluid. See Fig. 5. A greater number of storage tanks on the HT-TES side and better integration in temperature profiles allow for the expansion of the possibility of meeting thermal demand. This would offer the opportunity to cover various temperature ranges, different from the maximum and minimum temperatures provided by the single-step sensible heat transfer configuration. The number of stages increases with the number of tanks. In order to have a similar heat transfer in each stage, the division is made according to the heat transferred, and the slope of the subcritical temperature profile is adjusted to that of the supercritical temperature profile. This requires slightly varying the flow rate in each heat exchange section of the thermal storage fluid, water in this case, countering the effects of the variable heat capacity of CO 2 at high pressure. Increasing the flow rate can achieve a lower slope in the temperature profile at subcritical heating/cooling and vice versa. The additional thermal storage tanks manage the flow variations in the different sections, so the size is much smaller than that of the main tanks. Furthermore, the stored fluid in each tank could have different characteristics. A study range of 1–10 stepwise heat transfer configurations is considered. 2.5.2. Phase change temperature change on the LT-TES The phase change temperature on the LT-TES side is the starting point for constructing the transcritical cycles, establishing the lower limit on the enthalpy increments of the expansion/compression processes. The LT-TES phase change temperature ultimately determines the shape of the transcritical cycles and, therefore, the system's efficiency. The LT-TES phase change temperature is the key factor in the system's cooling applications. This study considers a temperature range from −40 to 20 ◦C, covering cooling options above and below the 0 ◦C initial assumptions. Once the impact of the phase change temperature in LT-TES is considered, different integration possibilities with fluids that can provide the required conditions are also considered. 2.5.3. Recuperative cycle and multi-stage compression There are different opportunities for optimisation in the processes that form the transcritical CO 2 cycles. In the charging cycle, the extreme temperature conditions in the system are reached: the highest temperature after compression and the lowest temperature after expansion. Due to the characteristics of the transcritical cycle, the compressor inlet temperature is the evaporation temperature below the expander inlet temperature. A recuperative cycle configuration seems appropriate in this case. In this study, the impact of the recuperative cycle on the charging phase is analysed by diverting part of the CO 2 leaving the lowtemperature heat exchanger (LT-hx) to a heat exchange with the CO 2 leaving the high-temperature heat exchanger (HT-hx), as shown in Fig. 6a. In the case of the discharging phase, the recuperative configuration would only make sense if the turbine outlet temperature were higher than the pump outlet temperature. In addition to the recuperative cycle, the cases of multi-stage compression in the compressor in the charging phase and multi-stage expansion in the turbine in the discharging phase are studied. In the case of the compressor, the use of a staged compression with intermediate cooling reduces the highest temperature of the system, which implies a reduction of the turbine inlet temperature (TIT) of the discharging phase. However, using a staged expansion under conditions similar to those of compression, the heat evacuated during the cooling of the multi-stage compression could be used in an intermediate reheating in the multi-stage expansion, Fig. 6b. Using 1–3 intermediate compression steps is considered. For cooling, it is considered that a maximum of 20 % of the total heat available is used in each step. When the available heat is not recovered in the intermediate reheat of the turbine, it is evacuated to the environment. In the expansion, between 1-n C steps are considered, where n C is the number of compressor steps, and all the recovered heat is used. 3. Results and discussion This section presents the analysis results of the different optimisation scenarios considered in this study. 3.1. Step heating/cooling on the HT-TES The numerical simulation yields good results in terms of integration and efficiency. Fig. 7 shows the temperature heat exchange profiles between the CO 2 and the HT-TES for different heat recovery sequential stages. The sequential supercritical and subcritical temperature profiles integrate better. The multi-stage heat exchange allows the temperature profiles of the CO 2 and the storage fluid to be better adapted, with closer temperature profiles and maintaining the minimum temperature difference in the operation range. The maximum temperature of HT-TES increases with the number of stages in the sensible heat transfer, along with the Turbine Inlet Temperature (TIT). It increases the efficiency of the system. Additionally, the system gains significant potential for thermal demand coverage applications. The HT-TES side transitions from having a single high-temperature reservoir to multiple reservoirs with different intermediate temperatures, allowing for the direct coverage of various temperature ranges and other thermal energy demands. Sequential heating not only leads to an increase in TIT, which results in improved electric-to-electric conversion efficiency but also establishes multiple temperature ranges for the direct coverage of thermal demand. Urban Fig. 5. Step sensible heat exchange in HT-TES. A. Carro et al. Applied Energy 395 (2025) 126165 8 heating networks offer considerable potential for the integration of renewable heat. Thus, the CEEGS system expands its possibilities for heat demand coverage and provides another avenue for renewable energy to meet thermal demand. Fig. 8 shows the system's efficiency range and the main temperatures on the HT-TES side of the CEEGS system, according to the number of stages in the HT-TES. The system efficiency increases with the turbine inlet temperature, which increases with better integration in the temperature profiles. Although a monotonic and asymptotic variation in the efficiency values might be expected, the results show small oscillations. The division of the heat exchange into sections according to heat transfer allows the subcritical temperature profile to be better matched to the supercritical profile by using variations in the slope of the temperature profile in each section. However, depending on the relative position of the pinch with respect to the supercritical profile curve (depending on the number of exchange stages), the integration may be better or worse. A curvature zone that interferes with the straight projection of the limits in the temperature profile established by the section can cause a reduction in the potential slope of that section, which results in a lower outlet temperature in the heat exchange. This phenomenon occurs when going from one to two stages when ECE, according to HT-TES, decreases from 56.5 % to 56 %. As the number of stages increases, the behaviour of the system is as expected, and the integration in the temperature profiles tends to improve, but always subject to this adjustment, which depends on the position of the pinch and the curvature of the temperature profiles, which can cause small oscillations. While it rises rapidly passing from two to four HT tanks, it remains stable, around 59–60 % for those cases in which more than four HT tanks are used. The temperature range of the HT-TES undergoes no significant changes except for an increase in the upper limit due to better integration in the temperature profiles. Still, it does not have a notable influence. The minimum temperature in HT-TES remains constant regardless of the staged sensible heat transfer. The scenario with four tanks on the HT-TES side places the efficiency range at 51–59 % and establishes temperature ranges of 21.9-53.1-79.6109.7-147 ◦C for potential thermal demand coverage applications. When water is used as the storage medium, only the last two stages would need pressurising, with pressures of 1.97 and 5.72 bar to ensure the liquid state. The round-trip efficiency of the system using the compression refrigeration cycle would go from 52.8 to 55.4 %. The additional costs are related to a higher investment in the heat exchanger and additional tanks. However, the total heat exchanged and the exchange surface do not have significant variations, so the increase in the total investment cost should be low. As for the additional tanks, it is worth mentioning that they are only used to regulate the differences in flow rates resulting from adjustments in the slope of the temperature profiles in the heat exchange. The size of these tanks is much smaller compared to the main tanks. Although there is an initial cost involved, the use of staged heat transfer on the HT side should provide potential long-term savings in energy efficiency and greater operational flexibility to adapt to variations in thermal demand. 3.2. Phase change temperature change on the LT-TES Next, the results of the LT thermal storage analysis are presented. The phase change temperature is one of the cornerstones upon which the CO 2 transcritical cycles are built. Additionally, it is the fundamental factor upon which the system's cooling applications depend. Fig. 9 shows the corresponding T-s diagrams for different LT-TES phase change temperature cases. The shape of the transcritical CO 2 cycles on the T-s diagram changes noticeably. The change in the phase change temperature of LT-TES directly affects the condensation/evaporation lines of CO 2 . As the temperature decreases, compression in both the charging and discharging phases occurs further away from the critical point. Higher temperatures are reached at the compressor outlet, exceeding 200 ◦C in cases of lower phase change temperatures. Conversely, temperatures at the pump outlet decrease, dropping below 0 ◦C. The temperature range in HT-TES is directly affected by the phase change temperature in LT-TES. Fig. 10 illustrates the trend of the efficiency range and the temperatures on the HT side when changing the phase change temperature on the LT side. The efficiency range increases as the phase change temperature in LT-TES decreases. When considering a temperature of 0 ◦C, the efficiency was within a relatively high range of 48–56 %, and it reached 56.6–67.7 % when considering −40 ◦C. Conversely, increasing the temperature of the phase change results in a significant penalty on efficiency, rapidly decreasing as the temperature increases. It falls within the 38.6–45.3 % range when the temperature of the phase change in LTFig. 6. Scheme of the CEEGS system: a) Recuperative cycle configuration, and b) Multi-stage compression and heat recovery. A. Carro et al. Applied Energy 395 (2025) 126165 9 temperature ranges of 20–150 ◦C (steps of approximately 30 ◦C). With water as the storage medium, only the last two staggers need to be pressurised, with pressures below 6 bar in the last one, to ensure a liquid state. Round-trip efficiency increases from 52.8 to 55.4 %. •The LT-TES phase change temperature is the fundamental factor on which the system's cooling applications depend. The system's efficiency range increases as the LT-TES phase change temperature decreases, reaching values in the range of 56.6–67.7 % at −40 ◦C. This results in higher temperatures at the compressor outlet, above 200 ◦C, and lower temperatures at the pump outlet, below 0 ◦C, directly conditioning the temperature range in HT-TES. An increase in the phase change temperature penalises the efficiency range, decreasing to 38.6–45.3 % at 20 ◦C. However, it could be used to utilise surplus thermal energy, balancing the system without using the additional refrigeration cycle. The round-trip efficiency with the ammonia refrigeration cycle would be 44 % using a temperature of 20 ◦C and could reach 58.7 % using −30 ◦C in LT-TES. •Using the recuperative configuration in the charging cycle, a higher temperature is achieved at the compressor outlet and turbine inlet. This results in high values for the maximum temperature of the HTTES, which may increase the system's requirements, such as the water pressure level. The efficiency is in the range of 52.6–61.4 % when the entire CO 2 mass flow entering the compressor passes through the recuperator. This also reduces the inlet temperature in the expander and the vapor quality of the CO 2 at the evaporator inlet, which increases the heat stored in the LT-TES. Round-trip efficiency increases from 55.4 to 57.9 %. •Multi-stage compression with intercooling in the charging phase allows for a reduction in the compressor outlet temperature. It is necessary to recover the heat evacuated in an intermediate reheating of the multi-stage expansion in the discharge phase to maintain the efficiency range at high values. Using two-stage compression with intermediate cooling reduces the maximum temperature of HT-TES from 186.6 ◦C after single-stage compression to 146.6 ◦C. Using two stages in the expansion of the discharge phase and recovering the evacuated heat in the intermediate reheat, the efficiency range is 51.4–59.5 % and round-trip efficiency is 55.9 %, close to the case with the recuperative cycle and single-stage compression and expansion, but with a maximum temperature in HT-TES of 40 ◦C less. Combining the different optimisation mechanisms described in this study, the efficiency can be improved to very high values, such as those provided by a very low phase change temperature. Considering alternative refrigeration fluids, such as IsoButane, CycloPropane, or n-Propane with very low temperatures phase changes, electric-to-electric efficiency values in the range of 58.9–71.2 % can be achieved using onestep compression and expansion. This presents additional challenges, such as a negative stretch in the HT-TES temperature range or excessively high values in the maximum temperature. This effect can be reduced using other configurations, such as two-stage compression, expansion, and heat recovery. The efficiency range would be 58.5–71.1 %, with lower requirements associated with high temperatures. The round-trip efficiency could reach 59.8 % when the temperature at LTTES is −30 ◦C and a two-stage compression and expansion with heat recovery in the intermediate reheat of the turbine is employed. The analysis highlights the importance of thermal storage characteristics and cycle configuration in the CEEGS system. Key considerations regarding integration with transcritical CO 2 cycles and operating conditions are shown to maximise its efficiency and potential in renewable energybased trigeneration thermal demand coverage. CRediT authorship contribution statement A. Carro: Writing – original draft, Visualization, Validation, Supervision, Software, Methodology, Investigation, Formal analysis, Conceptualization. C. Ortiz: Writing – review & editing, Formal analysis. S. Unger: Writing – review & editing, Validation, Formal analysis. A. Stoikos: Validation, Formal analysis. A.-S. Kyriakides: Validation, Formal analysis. I.N. Tsimpanogiannis: Writing – review & editing, Supervision. J.A. Becerra: Funding acquisition. S. Voutetakis: Supervision, Funding acquisition. U. Hampel: Supervision, Funding acquisition. R. Chacartegui: Writing – review & editing, Supervision, Project administration, Funding acquisition, Formal analysis, Conceptualization. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work has been partially funded by the project ‘CEEGS: Novel CO 2 -based Electrothermal Energy and Geological Storage’, from the European Commission through Horizon, the EU Framework Programme for Research and Innovation, under Grant Agreement No. 101084376. 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