2.1.1. Electrically Heated RC Systems

Joule heating makes most technical sense in combination with a steam Rankine cycle. No other working fluid is considered in these directly heated systems. The reason, except for being the industrial standard, can be found in its higher efficiency compared to ORC systems and the fact that CO2 cycles at high temperatures have seen only very few demonstrators built so far (e.g., CSP application [57] or Allam cycle for gas combustion [58]). Technically, the direct heating with steam cycle can be considered as the simplest technology. Furthermore, all electrically heated systems under development include the possibility of converting existing coal fired power plants to storage systems, utilizing the existing infrastructure and saving the typically costliest components of the CB, which is the power cycle unit. The system of this type is referred to as a straight forward one in [59] and the possible scale in the case of a fossil fired plant's partial refurbishment for increased flexibility or complete refurbishment can be found in [23].

Regarding the power output, electrically heated systems are well scalable, with the limitations provided by steam cycle systems. The smallest systems in the MW scale can be applied for example in CHP plants; as solely power to power CB, Siemens Gamesa with an output of 1.2 MWe serves only as a demo. The largest systems can theoretically consist of blocks of several hundreds of MW each. Electrically heated storage is limited by relatively low roundtrip efficiency, which is determined mainly by the H2P conversion technology (power cycle efficiency) since the electric heater is employed for charging. Furthermore, the other losses occur during operation, namely heat loss into the environment and the pressure drops.

The first small scale Gamesa ETES test rig was built in Bergedorf in 2014 with 5 MWh storage capacity and a 750 kW gas burner. Its purpose was the testing of various storage concepts, materials and setups. This testing unit has run for over 2500 h [32]. Owing to the constructed full system demonstrator, the ETES system of Siemens Gamesa commissioned in 2019 in Hamburg is perhaps the best known; first and so far, the biggest constructed system of this kind. The system uses a horizontal flow packed bed of volcanic rock and air as a heat transfer fluid. With a maximum storage temperature of up to 750 ◦C the system has 130 MWhth storage capacity. Volumetric storage capacity reaches approximately 0.2 MWhth/m<sup>3</sup> (depending on charge-discharge temperature spread) and its great advantage is in high technology readiness level (TRL) due to the use of commercially available components, the resistive heaters from process industry and heat recovery steam generators (HRSG) utilized in combined cycle power plants. The storage concrete construction vessel is filled with about 1000 tons of crushed volcanic rocks and thermally insulated by aerated concrete and rock wool insulation [32]. The system was designed for 24 h charge and 24 h discharge operation. The rather low roundtrip efficiency (around 25%) is due to the steam cycle efficiency (small scale, low pressure) and auxiliary loads, especially air fans.

Thermal losses can be minimized by optimizing the amount of thermal insulation according to the economics. Pressure drops can be mitigated by packed bed construction and operation (air flow velocity, particle shape and diameter and vessel length to diameter ratio). Nominal storage capacity can further decrease by de-stratification effects due to heat transfer in a packed bed in a charged state [60] (reported for horizontal flow stores). Hence the storage unit must be oversized, which leads to an increase of thermal and pressure losses. Unlike in coal plants, parasitic loads related to its handling or boiler efficiency are however excluded here. The efficiency is expected to significantly increase in larger scale installations to over 42% [33,34,60]. Currently, Gamesa is ready for building the first series of commercial pilots in a range of 10–100 MW power; 100–2000 MWh storage capacity and 300–720 ◦C steam temperature [32].

The ETES system offers various applications, such as integration into existing power plants or combined heat and power plants, rebuilding conventional fossil fuel power plants into storage units or electrification of process heating. Therefore, Siemens Gamesa has been developing three market types of storage application, for the conversion of fossil power plants, adding storage to existing thermal systems (industrial plants for process heat electrification and heat recovery) and a whole stand-alone system supplying process steam, electricity or district heating [60].

Looking at the chosen horizontal configuration, much smaller scale investigations of rock bed TES found very strong effects of buoyancy, having a detrimental effect on temperature stratification and the utilization of the thermal capacity of the entire volume, further confirming the de-stratification effects. Either the TES capacity is then limited or additional horizontal air tight layers need to be added to the storage tank to maintain air flow uniformly through the entire volume [61].

Similar is the intended application scale of RWE's development, which is, with its project StoreToPower, focusing on an alternative technology of molten salt, adopted primarily from concentrated solar power plants (CSP) and, secondarily, also on solid materials with air as a heat transfer fluid [35]. However, no further public information provides any insight on the ongoing status. Focus on molten salt appears to be a logical choice as in the CSP plants it is a well proven commercial technology [62,63]. With the molten salts, special attention needs to also be paid to corrosion, especially at temperatures above 500 ◦C [64,65], which are typically found in coal fired power plants.

A P2H2P system with direct electrical heating primarily suggested for refurbishment of coal fired plants is offered by E2S Power [36]. The TES system is using an alloy (miscibility gap alloy, MGA) composed of graphite and aluminum and developed by a partner MGA Thermal. The MGAs are commonly proposed as a novel type of TES material with some unique properties such as macroscopically solid materials safely embedding PCM, providing combination of sensible and latent heat and with high thermal conductivity [66,67]. Blocks of the MGA are electrically heated during charging, half up to 700 ◦C while the second half only to temperature required by the steam turbine. The colder blocks then, during discharging and steam generation, serve for self-regulation of the steam outlet temperature. A laboratory system has been built demonstrating the TES with charging and steam generation, while plans exist for a 50 MWhth pilot at a coal power plant in Montenegro.

#### 2.1.2. Reversible RC Systems

Reversible systems use the principle of a vapor compression heat pump in the charging phase and standard RC during discharging. Various configurations were proposed theoretically for steam systems with a suggestion to further improve the roundtrip efficiency by integration of heat sources to the heat pump input [68]. Together with the additional possibility of various working fluids (steam, organic fluids, CO2), direct or indirect use of the cycle working fluid for storage and choice between latent or sensible TES, there is a wide range of available technologies.

Starting from the most typical RC using water as a working fluid, the company Spilling, manufacturer of steam turbines, compressors and engines, entered into the development of CB. The system consists of a reversible steam engine/compressor and two steam accumulators, one at low pressure and other at high pressure [37,38]. A major advantage of such system is a combination of existing technology of steam engine from the company portfolio, while steam accumulators are a well-established technology over hundred years old. The accumulators however require relatively large, pressurized tanks for the saturated water. This makes the technology of steam accumulators suitable rather for industries and small power plants; one of the largest applications is a 20 MWth (50 min at 5.5 MWe) application for a CSP plant. The addition of PCM to the tanks is proposed to improve the thermal capacity at the same volume. A drop in the pressure during the discharging of the tanks is another disadvantage [69,70]. For the CB applications, a similar size can be expected as that for similar systems for small steam parabolic trough flexible plants as 2 MW/6–24 MWh units [71]. The pressure drop loss might be partly compensated for by sliding pressure in both charging and discharging process. Investigation of parameters of this CB concept are not, to the authors' knowledge, supported by any scientific literature.

GE has in the past proposed a system using a recuperated CO2 Brayton cycle operating as a heat pump for charging with a steam Rankine cycle discharging as an advanced molten salt electrical storage system (AMSESS). High temperature heat is stored as a molten salt while low temperature heat input is provided from a water tank and the water is thus cooled down. An additional electric heater can be used to increase the salt temperature. During discharging, heat is transferred to the steam cycle, where most of the rejected heat goes into the environment, while a smaller portion such as a turbine bleed heats up the water tank. A design with off-the-shelf components has been performed for 20–100 MWe power output over 8 h period with RTE ranging based on size and presence of the heater between 42% and 62%. An interesting note is that the space requirements are about one third of the same capacity in containerized lithium batteries [39].

Another set of technologies builds on standard heat pumps and subcritical organic Rankine cycles. These systems are specifically considered as well fitted to waste heat sources, or other low temperature heat sources, such as solar collectors or geothermal heat. The availability of the heat source then decreases temperature lift of the heat pump (or provides a heat source in less considered concept, where the heat sink is below ambient by cold energy stored and prepared in time of excess electricity by refrigeration cycle). Roundtrip efficiency can, as a result of heat input, theoretically exceed 100% [72]. Regarding the size and cost, storage tanks need to be carefully considered as in the case of low temperature glide; even a small power output requires large tanks, which might be costly for long duration storage. The power output can be expected from dozens kW up to no more than several MW as is the size of large CHP or geothermal ORC systems [73]. This concept gained much research attention, where theoretical investigations are performed in

many institutes and several experimental systems are in experimental operation or under construction [74–76], all in kW scale.

Looking at the commercial development, the list is, however, rather short. Within the CHESTER research project [40], universities join research institutes and companies to focus on business models and market opportunities for such application, meanwhile one industry within the project, for example, focuses on the development of an isobaric expansion device considered for one of the concepts [77]. The main CB concept utilizes both sensible and latent heat storage to maximize efficiency and match temperature profiles during charging and discharging, while providing thermal integration to district heating. A kW scale experimental system is under development. Similarly, ORC unit manufacturer Climeon offers its units of output around 100 kWe for CB applications with storage also proposed as district heating infrastructure including large seasonal storage tanks and up to MWe size total output. Additionally, greenhouses and geothermal systems are suggested as application cases for CB. Except for the ORC conversion unit, the system is however in a conceptual state [41]. Another example is a heat pump manufacturer, TC Mach, developing together with a university a high temperature heat pump, TES. made of compacted stone dust and an ORC, where the constructed proof of concept unit is going to be sub-kW scale [42].

The only larger scale ORC CB system in state of a pilot application is developed by Futurebay [43]. This system is considered in several applications, with or without thermal integration of waste heat source. Without any external heat source, a heat pump is used to charge both hot and cold storage during a period of excess electricity and these stores are then utilized separately. Heat is stored in hot water tank while PCM storage (probably ice based) is used for cold storage. Both hot and cold can be used directly or the ORC can run between present temperature gradients—hot store and ambient (eventually space heating temperature) or waste heat to cold store. The system is considered primarily for integration into thermal systems with both cooling and heating requirements. A containerized demonstrator delivering 50 kWe/200 kWhe and cooling capacity 2 MWhth has been built and operated; scalability is however suggested up to a 12 MWe/72 MWhe grid scale system.

Liquid air energy storage (LAES) is in principle also a Rankine cycle as it utilizes the phase change of the working fluid for storage and especially in the discharging phase, a typical cycle's heat input/output takes place. The working fluid is at the same time also a storage media in an open cycle. The air is compressed and liquefied during the charging by a relatively standard industrial technology. Special attention is paid to the conservation of thermal energy during the liquefaction process, both cold and hot, as it is essential for the high roundtrip efficiency of the system. Thermal energy is in this concept stored therefore threefold—in the liquefied air and near-ambient pressure as latent heat, thermal energy recovered after compressors' outlets and cold energy from the gas cooling before expansion and from recycled streams.

The advantage of LAES lies in relatively small storage volumes (in the order of 700 times smaller than those required for CAES) due to the higher energy density in liquid air. The LAES system also relies on commonly used components in industry (compressor, liquefier, turbine, etc.). Today's studies show that it is possible to achieve roundtrip efficiencies around 70%, with a specific investment cost of 1270–2090 €/kW [78]. To date, several studies have been published on various LAES configurations. In [79] they proposed integration with a conventional combined cycle power plant; in [80] they studied the LAES system integrated with a nuclear power plant, and many publications paid attention to the recovery of waste heat from LAES using ORC, which increased RTE by up to 12% [81–83].

The only commercial development is carried out by the British company Highview power. In 2011 their first pilot plant was launched (350 kW/2.5 MWh), which was tested on a biomass plant site and is now located at the University of Birmingham. The achieved roundtrip efficiency was only 8%, therefore the second Pilsworth Grid Scale pilot power plant (5 MW/15 MWh) was built in 2018. Highview power is now developing a 50 MW commercial plant in Carrington Village with a storage capacity of 250 MWh in cooperation with MAN Energy Solutions. The commissioning is expected in 2022 [44–46]. Another project is planned with the expected start of construction in 2023 in Chile with a power rating of 50 MW and a capacity of 500 MWh [84]. In the future, Highview power plans to offer LAES systems in a relatively large range of outputs from 20 MW/80 MWh to more than 200 MW/1.2 GWh [85].

Rather favorite among CB appears to be the utilization of CO2 cycles, which otherwise struggle to find their place in other energy systems. Development of the ETES system of MAN and ABB aims at a very peculiar and rather low temperature system with a reversible transcritical CO2 Rankine cycle. The system has the highest storage temperature only around 120–150 ◦C utilizing pressurized water and cold storage, and using ice as PCM. The hot storage system is divided into four tanks at different temperatures into which the heat is transferred via three separate heat exchangers. It is to balance the optimal mass flow rate and heat exchange temperature profiles due to the change of the supercritical CO2 heat capacity, while the temperature differences along the whole length of the heat exchangers are designed in order of several Kelvins. Only one hot water tank needs to be pressurized, while the pressure is still moderate. During charging, part of the energy in high pressure CO2 is recovered by a hydraulic turbine while rest below the saturation line is flashed to exclude the issues of two-phase expander. The resulting system is then a result of many techno-economical optimizations under constraints of isentropic efficiency of key turbomachinery components (the company's state-of-the-art) with the roundtrip efficiency reaching values of 38%–50%. The target size is an 8.5 MWe system with 8 h charging. Development of this system is well documented in [31,47–49]. Currently, an MW scale laboratory demonstrator has been developed to test the system and equipment. The commercialization strategy aims at the possibility to use also standalone system of only heat pump or heat pump with storage, able to provide 5–50 MWth of heat (3–30 MWth cooling) with 2–15 MWe power input. As such, a 50 MWth seawater heat pump for district heating using this technology is to be built in Denmark by 2023 [86].

The MAN–ABB consortium is not the only one representative of CO2 cycle utilization, even though they are clearly closest to application. Echogen [50], known mainly for its waste heat recovery 8 MW unit with CO2 as a working fluid is also working on a CB system for more than 4 h and 10 MWe size. Available information discloses a concept utilizing the CO2 reversible recuperated cycle, low temperature storage also being ice/water storage (as brine for −2 ◦C to −10 ◦C) and high temperature storage being at 300–350 ◦C in the form of sand in silos or alternatively concrete blocks. The temperature has been selected to be within the limits of standard construction materials [51–53]. A proof-of-concept in a 100 kWth scale heat pump and sand storage and heat transfer system has been developed while a 25 MWe 8 h prototype system is in a design phase [54].

The CO2 has been also proposed for a system using a similar principle to the LAES by the company Energy Dome [55]. The gaseous CO2 is stored at ambient pressure in a large container (dome). During charging, it is compressed, liquefied and stored in tanks, while the heat (from intercooling and condensation) is separately stored in a TES. The discharging process then reverses the flow, high pressure CO2 is evaporated by the stored thermal energy and expanded in the turbines back to the low pressure gas store to produce the electricity. The sequence of charging and discharging can be considered as thermodynamic changes in an open Rankine cycle. To maintain constant conditions at the compressor inlet, a flexible membrane is employed within the dome. TES is separated into five sections at different temperatures in configuration with a multiple section compressor and turbine with intercooling and reheating. Main advantages of this concept are mentioned as high energy density at moderate pressures. The calculated net roundtrip efficiency can reach about 77% [56]. Note that similar systems were proposed before, but with liquid CO2 storage at a low pressure and with a maximum roundtrip efficiency of 57% [87]. The system has recently progressed from a concept with basic sizing and costing to the construction

of a 2.5 MWe/4 MWh pilot system scheduled to be finished in 2022 and in the same year there is planned the start of a full-scale 20 MWe/100 MWh system construction [88].

#### *2.2. Brayton Cycle Systems*

Brayton cycles are favored for applications with sensible heat TES materials as during the isobaric heat addition and rejection, it is possible to obtain a well matching temperature profile, minimizing exergetic losses. Other advantage can be the maturity of the gas turbine industry, from which many systems are derived. The summary of the commercial development in Table 2 shows that the extent of the projects is smaller than in the case of the Rankine cycle systems (opposite trend to the publications numbers in academic research). It can be argued that one reason is in the requirement of very high efficiency of compressors and expanders to which the systems are highly sensitive, while such components have a limited industrial supply.

**Table 2.** List of commercial development projects in CB using Brayton cycle discharging.


2.2.1. Electrically Heated BC Systems

In order for the electrically heated Brayton cycle to be efficient, very high storage temperatures need to be reached, namely exceeding 1000 ◦C. However, the need of reaching such a high temperature for increase in roundtrip efficiency also leads to material constraints. Considering a parallel to the scientifically reported development in Brayton based PTES, interestingly, a very small number of publications is focusing on this concept.

A U.S. company, 247 Solar, provides a concept of an electrically heated CB, which combines subsystems named the 247Solar Heat2Power Turbine and the 247Solar Thermal Storage System. These subsystems are proven in CSP applications (Brayton based solar towers), where there is a specific solar irradiation receiver and vertically orientated thermal storage, both using air as a heat transfer fluid operating under conditions of near ambient pressure and high temperatures (970 ◦C) [90].

During charging, the electric heater (resistance coils) is employed for heating the air flow by using a blower, both supplied by surplus electricity, and afterwards the air passes the heat to the sand thermal storage. The discharging process employs a classic open loop Brayton cycle, meaning that the inlet ambient air is compressed, then heat is supplied in two heat exchangers (low and high temperature) and next the air expands in the turbine. To increase the overall system dispatchability, a burner for various types of fuel (including hydrogen) can be added. The presented roundtrip efficiency of this system amounts to 30%, nevertheless the combined heat and power production efficiency can reach more than 90%. 247 Solar considers standard configuration with nominal 200 kWe power output and thermal storage capacity 1.8 MWh that corresponds to 8–10 h duration. This project is evolving in collaboration with Capstone Green Energy, the provider of customized microgrid solutions and on-site energy technology systems [96].

1414 Degrees Limited is an Australian company, which develops Thermal Energy Storage Systems (TESS). The novelty of this system lies in using molten silicon PCM with melting temperature 1414 ◦C, which enables reaching high energy densities and potentially high efficiencies of conversion back to electricity and usable heat [89].

For the charging of TES, an electric resistive heater is utilized. Another option for heating the molten silicone storage is combusting gas, for instance, methane generated from wastewater plants. The advantage of using silicon is in its high latent heat capacity in comparison to other PCMs. The silicon's sensible heat capacity below the melting point is up to 300 kWh/t; nevertheless, the latent heat supplied during the solid to liquid phase change at the constant temperature of 1414 ◦C is about 500 kWh/t. The operation of TES at such a temperature brings many material challenges. However, several well-established industrial processes are treating material with very high temperatures, for example the cement industry temperature level exceeds 1400 ◦C. Therefore, 1414 Degrees implements materials and technology knowledge from these proven technologies. The discharging process can be achieved by several conventional heat engines, where the most effective plants achieve over 60% thermal efficiency. Hence, 1414 Degrees considers four options of discharging. In terms of combined power and heat production for reaching the maximum overall roundtrip efficiency, the gas turbine, steam turbine or a Stirling engine can be employed; lastly, a steam generator or heat exchanger can provide steam or hot clean air for industrial processes [89].

The TESS is designed as a modular system; when connected in series it increases the power output and in parallel configuration the storage capacity rises. 1414 Degrees plans four main commercial applications; one oriented on bulk medium-long term energy storage, one providing medium scale energy storage for industries and residential developments requiring power and heat. The last one is developed to accommodate combustion waste gas (e.g., sewage treatment) and store the thermal energy for later recovery (for electricity and heat). As such, it is similar to the industrial application but with modified charging. Lastly, the TESS industrial system producing steam is best fitted for industries with high thermal rather than electricity demand, especially for replacing natural gas with renewable sources. Using a steam turbine for electricity generation is only secondary [91], but possible for example using extraction turbines providing also heat at lower parameters.

1414 Degrees built up the first demonstration plant in 2016 and is still operating it in its R&D facility in Lonsdale, South Australia. The demo comprises of 200 kg silicon PCM storage charged by 50 kW heater and was using 43 kW Stirling engine for discharging, in 2018 replaced by a gas turbine. In 2018, a 10 MWhth GAS-TESS module was also put in operation at the Glenelg Wastewater Treatment Plant. The next step of 1414 Degrees is to acquire SolarReserve Australia II, which owns the Aurora solar project [97]. The current plans of the Aurora project, led by the 1414 Degrees company, are to build up a 70MW PV farm and 150 MW CSP plant, coupled with several thousand MWhth storage capacity [98].

The use of a supercritical CO2 in a BC has been proposed by Peregrine Turbine Technologies as a CB along several other use cases [92]. The system uses the MGA from MGA Thermal as already considered in Rankine cycle systems from E2S, here with temperatures of 800 ◦C. The power cycle uses a partially recuperated configuration with compression from right above the CO2 critical point [99] and the whole system is designed as a 1 MWe

modular system with a storage capacity of either 8 MWhe or modular with 5 MWhe increments. The unit has an expected CAPEX of 280 \$/kWh and a lifetime of 20 years. The company has tested their compact turbine with a CO2 compressor and plans full scale pilot construction for 2022 [92,100].

Contrary to these commercially developed systems, in the scientific literature the only application of the electric heaters with respect to the Brayton cycle has been considered in the form of additional upgrade and control of the gas temperature at the compressor outlet in works such as [13]. The purpose of adding an electric heater is the increase of the maximum storage temperature of the hot reservoir that increases the efficiency of the reconversion of heat into work during discharging and also significantly increases the energy storage density. Nevertheless, the implementation of an electric heater during charging leads to a reduction of the COP of the Brayton cycle heat pump, which may cause a decrease in roundtrip efficiency. As such, it is later shown to be proposed in the conceptual phase of one commercial reversible system development.

### 2.2.2. Reversible BC Systems

The development of a company, Isentropic Ltd., focused on a Brayton cycle using a reciprocating reversible piston machine capable of working as both compressor and expander together with a fine gravel storage in hot pressurized tank and a cold tank at ambient pressure. The company itself, established in 2012, was placed in administration in 2016, before finishing its "grid scale" 150 kW experimental demonstration system. It serves as an example of the consequences when the market is not yet ready for the developed systems. Many lessons and information can be learned, as much of the technological development has been scientifically reported. First, three small prototypes and the conceptualization of a commercial size unit were reported in [93]. The development, originally conducted with Newcastle University, was consequently picked up by Durham University, where the 150 kW demonstrator was built and experimentally tested [101].

The piston compressor/expander component utilizes a uniquely developed sliding valve approach, which is designed to minimize the pressure losses, reduce dead space, provide a high quality seal without excessive precision requirements and a very fast valve actuation speed [102], which under electrical control allows for a rapid change from charging to discharging regimes in less than one second. The storage is composed of a stack of grates with magnetite sand (~1 mm) with actively controlled flow only through the selected ones and bypassing others to limit pressure drop. The designed working fluid, argon, has been for the tests substituted by nitrogen due to leaks and the system was operated at reduced pressure, speed and power. The 150 kWe designed system was operated at about 10 kWe charging and discharging power, still reaching roundtrip thermodynamic efficiency (drive excluding mechanical and electrical losses) around 77%, while the prospect of likely final system performance with argon at full speed has been determined from the data as 73% roundtrip efficiency at about 125 kWe charging and 100 kWe discharging power [101]. The originally intended scaled up 2 MWe/16 MWhe commercial (modular) unit was analysed to have a LCOS of 9 to 11 €ct/kWh including given purchase electricity price and 2 to 5 €ct/kWh excluding them, while CAPEX were 350–797 €/kWe of power and 13–21 €/kWhe stored [103].

The concept developed by the company Malta follows an approach detailed by Laughlin [26]. The described concept is based on a closed Brayton cycle, which is transporting heat from a cryogenic fluid storage to a molten salt reservoir, when it operates in the charging regime. In discharging mode, the heat flow is reversed and runs the gas turbine, so the electricity is supplied back to the grid, when demanded. The Laughlin's storage system consists of a compressor, an expander, two molten salt hot storage tanks and two hexane coolant cold storage tanks, two heat exchangers for heat transfer between the heat transfer and working fluid (HTF) and a recuperator.

This system was developed in the early stages by Professor Laughlin, the company Brayton energy and Nick Cizek, who designed the baseline of closed cycle gas turbine with Ar (or He) as a working fluid and later sold the rights to Google's (Alphabet) moon-shot factory X in 2016 [104]. After two years of incubation in X, when the potential of the grid scale storage technology cleared up, Malta became an independent company [105].

Nowadays, the novel Malta system, named Pumped Heat Energy Storage, is based on a closed loop, recuperated Brayton cycle. The purpose of recuperation is to get better minimum and maximum storage temperature spread (based on industry experience) and an increase of efficiency. The maximum temperature 570 ◦C is limited by material creep and corrosion, while the minimum temperature of −70 ◦C is set by material embrittlement and air (working fluid) drying requirements [106]. Malta's final goal is a full scale 10–100 MWe facility in 2024, enabling the low-cost storage (unit CAPEX less than \$100/kWh). Malta is currently developing a kW-scale demonstration facility, which should help with understanding the limitations of operational modes (start-up and shut down), especially the validation of transient analysis. This demo is characterized by lower costs and risks, due to its small scale. Contrarily, the small scale of turbomachinery and temperature limits of storage media causes significant roundtrip efficiency disruption, so the prediction of RTE is 10% [107]. Malta has also signed a term sheet with Canadian utility NB Power about establishing an Energy Storage Benefits Agreement to advance the first long-duration energy storage facility in the New Brunswick province. The planned 1000 MWh facility would help to achieve New Brunswick's emission reductions, improve grid stabilization, increase the integration of renewables, and bring new jobs into the region. However, the project of the storage facility is still in the early stages, the target year for the start of operation is 2024 [108].

GridScale is the official name of the concept developed by Danish company Stiesdal. Stiesdal's goal is to develop a storage facility for cost-effective electric storage, which is scalable and based on industrialized components. The application field of GridScale is a medium duration storage, which more specifically balances day-to-day production of photovoltaics (12–18 h) and smooths the wind farms production on several days (3–7) periods. GridScale is a reversible Brayton cycle Carnot battery, which uses low cost crushed rock packed beds as thermal storage and offers relatively high roundtrip efficiency without location limitations [94].

In the charging regime one pair of compressor-expander is employed for pumping the thermal energy from cold reservoir into the hot storage, due to the use of surplus electricity from the grid. The maximum temperature of hot reservoir is 600 ◦C and the minimum temperature of cold tank is designed at −30 ◦C. The COP of Stiesdal's configuration is determined approximately on 2.5 (depending on the temperature levels). During discharging, the system is operating as a regular gas turbine (Brayton cycle) with another similar compressor-turbine system for charging. The predicted efficiency of the gas turbine during discharging is 20%–25%, also depending on the temperature ranges. The range of Grid-Scale's roundtrip efficiency, derived from COP and Brayton cycle efficiency, is 55%–60%. The useful energy efficiency could be increased to roughly 90% by using the rejected heat from the cycle during charging for district heating [94].

The project GridScale has been supported by grants and subsides and is developed in a partnership with Andel, Aarhus University, Technical University of Denmark, Welcon, BWSC, Energi Danmark and Energy Cluster Denmark [109]. The numerical models of GridScale have been validated under various conditions since 2019 at the test facility with a 1:10 scale steel tank. This demo facility reached similar heat transfer from heat transfer fluid (air) to storage media (basalt crushed rock) as theoretically modelled, and achieved even lower pressure drops than expected [109]. In terms of the serial manufacturing of storage units, Stiesdal can utilize extensive experience from welding the tubular steel towers of wind turbines. The company Stiesdal is preparing a demonstrational project, which will be placed in Rødby on the Danish Island of Lolland and installed next to Rødby's district heat and power plant. This demo plant is planned to operate in commercial regime with a 4 MWe/2 MWe charge/discharge power and 10 MWhe storage capacity [94].

Enolcon GmbH in [30] presented their own Brayton based Carnot battery called OPTES Battery. The baseline design of OPTES consists of common closed Brayton cycle components: expander; compressor and two sand packed bed storage units and two heat exchangers. Alternatively, the bottoming ORC cycle using otherwise rejected heat after expansion in discharging regime can be added, in order to increase roundtrip efficiency. In the case of sufficiently cheap electricity input, an electric heater is a feasible solution for "boosting" temperature before inlet to the hot tank during charging. That enables a high level of temperature regulation, nevertheless it also causes a drop of roundtrip efficiency. The silica sand packed bed is developed by Enolcon's partner STORASOL GmbH and has been demonstrated together with an ORC unit, named the ORCTES-plant, at the University of Bayreuth in Germany, where it has been in operation since 2015 [110]. The parameters of this demonstrator are 1.5 MWhth storage capacity, 1.8 MWth of charging power and 600 ◦C operation temperature of the storage [111].

Currently, Enolcon is developing a proof of concept of the OPTES Battery power plant, which they plan to launch in 2023. Their CB should be able to reach approx. 40%–45% roundtrip efficiency even with off-the-shelf equipment. But the goal is to develop companders (devices combining turbocompressor and turboexpander) with nitrogen as a working fluid in a cooperation with Atlas Copco, which sustains higher pressures and temperatures above 715 ◦C and therefore reach higher efficiency. Hence, the CB with advanced turbomachinery achieves roundtrip efficiency roughly around 63%–67%. However, it is necessary to mention that the considered isentropic efficiencies in the described model are relatively high, specifically 93% for the expander and 91% for the compressor in a "realistic case" [30].

The capital expenditures (CAPEX) of OPTES Battery with a storage capacity of 80 MWhe and electrical storage power of 7.6 MWe were specified for 16–20 million EUR, meaning that the specific installation costs of electricity storage of such a system are estimated at 200–250 €/kWhe. In comparison, the specific costs of electricity storage of current commercial large-scale Li-on battery storage in Germany are according to [30] approximately 450 EUR/kWhe. Specifically, the Li-on battery project in south-eastern Brandenburg with 53 MWhe storage capacity and CAPEX of 25 million EUR, which was under construction in 2020, is mentioned. The determination of CAPEX is coming from the market prices based on Enolcon's knowledge in project management, engineering and design of large technical facilities (process systems, power plans, etc.) [30].

Lastly a CB system designed specifically for integration into the wind turbine is a system designed by WindTP [95] in cooperation with the University of Nottingham and the University of Leeds [112]. The system is thus primarily charged not by electrical power but instead by the mechanical power of the wind turbine shaft. The work can be transferred directly to generator via subsequent expander as aerodynamic coupling. When energy storage is required, shaft drives a low speed multiple stage high efficiency piston compressor to compress the working gas from 2 to 50 MPa and around 600 ◦C. A separate optional compressor supplied with electricity from the grid may be included as well. After heat transfer to the hot storage, expansion takes place and cold energy is stored as well. Rock bed is assumed as both hot and cold TES. Research has shown a feasibility of this solution for wind turbines when the stored electricity accounts for at least 25% of the produced power. A specific multistage slow speed piston compressor concept is proposed for high efficiency [113], which is currently in experimental development [7] and can be utilized not just for wind direct driven CB but also CAES systems [114].

#### *2.3. Other Cycles & Hybrid Systems*

Other and hybrid systems classification can be also divided into the electrically heated ones and ones using reversible cycles. Their commercial development is comprehensively summarized in Table 3. One can notice a presence of proofs of concept and pilot experimental systems and even already started commercial applications, surely a positive indication towards CB applications. A specific aspect of this group of technologies is that the heat to

power conversion is performed not only by thermodynamic cycles, but also by systems of direct conversion.

**Table 3.** List of commercial development projects in CB in the other and hybrid discharging cycle category.


2.3.1. Electrically Heated Other & Hybrid Systems

As with electrically heated RC or BC systems, electric resistance heaters are used in these systems for charging. The discharge can be realized either by thermodynamic cycles, for example by a Stirling engine or a combined Brayton with Rankine cycle, or by technology based on the direct conversion of thermal radiation to power (thermosphotovoltaic cell, multi-junction photovoltaic cells or hybrid thermionic photovoltaic cells). Thermal energy in these systems is stored at temperature levels from 600 ◦C up to even 2400 ◦C. All the systems can be therefore classified as high temperature ones.

Systems using a Stirling motor as the discharge method can be an interesting alternative to electrically heated RC and BC systems specifically for low power output in range of several to dozens kW and providing combined heat and power output. The development is probably furthest at the company Azelio, which has, for example, pilot installations in Morocco and Abu Dhabi and is already delivering first commercial units [126]. The system is using a sodium circuit as a heat transfer fluid to deliver the heat from recycled aluminum-silicon alloy PCM to the Stirling engine. The unit provides 13 kWe and heat at temperatures 55 to 65 ◦C over 13 h of storage capacity. Systems up to 100 MW are suggested to be comprised of multiple modules [115]. Development of this system is well documented from previous application of this Stirling engine technology and TES for CSP systems [127,128], while the Stirling engine development can be traced even to submarine systems [129]. Based on these sources, the roundtrip efficiency of this CB system can be estimated at around 30%–40%. In CHP application, the useful energy efficiency is around 90%.

A similar system has also been suggested for balancing small scale renewables in [130], where the considered engine has efficiency of only 21% at 2.5 kWe output without thermal output (cold cylinder at 30 ◦C). Thermal oil, molten salt and sand were explored as a storage material and economics was evaluated under a selected operation regime. The development of similar CSP applications using Stirling engine systems, often including various TES types, can be found in a number of other scientific publications [131,132].

This can be seen as the reason the Stirling engine also became the subject of quite a number of other commercial CB developments. Among them is the CCT Energy Storage, which unveiled its pilot project in March 2019 and was on track to install its first commercial facility [133]. The system uses phase changes of silicon to store thermal energy (PCM), which is then used to generate electricity using a Stirling engine. The installation can scale from 5 kWe applications to a theoretically unlimited size and the estimated cost is only 60%–80% of the cost of equivalent storage with lithium-ion batteries [117].

Stirling engine can be coupled also to suitably sized thermochemical (sorption) TES and work as a CB as shows the company TEXEL Energy Storage. Its system is based on the V4-Stirling converter, originally developed by Ford Motors. The Stirling converter produces approximately 40% of electricity and 50% of thermal energy, which adds up to 90% of total energy efficiency [118]. The second basic element of the TEXEL system is the thermochemical thermal energy storage. The company uses technology based on metal hydrides. The advantage of metal hydrides is the high energy density and the ability to work with temperatures above 600 ◦C. The TES consists of two parts, a hot part containing high temperature metal hydride (HTMH) and a cold part containing low temperature metal hydride (LTMH). When charging the TES system, hydrogen desorbs from HTMH and the desorbed hydrogen is then stored in LTMH. During discharge, hydrogen is absorbed back on the HTMH side, releasing heat to drive the Stirling converter [134]. For example, according to [135], it is possible to use a material pair of CaH2 and TiFe with the possibility of storage in the temperature range of 800 to 1000 ◦C and with a TES price of \$ 15/kWhth. Useful metal hydrides can furthermore be, for example, NaMgH3 or TiH2. TEXEL also started the development of TES technology based on metal carbonates MCO3 where M is the metal cation (Mg, Ca, Sr, Ba, or Pb). This system works similarly to the metal hydride technology but uses CO2 as its working gas instead of H2 [118,136].

Another proposal of Stirling engine is a project of Kraftanlagen München in cooperation with Solar—Institut Jülich, Germany. It is a combined heat and power project with the useful energy efficiency of up to 80% (roundtrip efficiency unspecified). Electricity production is to be ensured through a combination of Stirling engine and ORC. The multifunctionality of the device should consist in the choice of heat source-either electric heaters or waste heat. The system is using a ceramic TES system with intended more versatile application [119,137]. The TES system is also marketed and described separately among the TES systems for CB in following section.

Only one scientific work focuses on an investigation of CB systems involving Stirling engine on a system level [130]. Separate description of major components as TES or conversion systems is not uncommon, though originally for different applications. This shows also the dynamics of the CB development, which can effectively repurpose technologies from different applications, typically, but not only, CSP.

A technology using electrical heating with TES temperatures around 1200◦C, which uses a combined cycle of gas turbine and steam cycle for discharging is investigated in an NREL research project, ENDURING, with the strong involvement of GE and Babcock and Wilcox on a power cycle and the component manufacturing side and Allied Mineral Products, Inc. focusing on TES. The TES consists of a silica sand, which requires specific particulate handling system and heat exchangers based on fluidized bed principle. Technoeconomic analyses are performed for a 50–400 MWe and 10–100 h of duration with the LCOS estimate below 0.05 \$/kWh. With the combined cycle and high temperatures, the roundtrip efficiency has set a baseline at 50% with a technological prospect of around 55% gross efficiency. Hardware-wise, laboratory prototypes of the components are being

developed and plans aim towards MW scale pilot demonstration [120,138]. The concept of storage in hot sand is also based on previous projects aimed at the CSP application [139].

A specific approach with the integration of the CB principle into a combustion gas turbines combined cycle has been taken by Pintail power. Its first system uses electrical heating of a molten salt, which is then, during discharging, used for the evaporation of the water in the bottoming steam cycle [121]. This approach can substantially increase the steam cycle power output and provide a good temperature match between the flue gas and water in the heat recovery steam generator. The second system is a hybrid LAES system, where the air is liquefied during charging. During discharging the cold fluid is then used first as a heat sink of a bottoming ORC, then to pre-cool the combustion air before it is finally heated by the flue gas and expanded in an air turbine [122].

The last group of technologies in this section are technologies that do not use a thermodynamic cycle for discharge, but various types of direct heat to power conversionthermophotovoltaic cell, specifically modified multi-junction photovoltaics or hybrid thermionic photovoltaic converter. These systems work with very high temperatures, typically from 1500 to 2400 ◦C, placing new technological demands on the systems, which developers have to manage. Antora has developed a thermophotovoltaic cell working with solid carbon heated to temperatures just above 1000 ◦C with 40% efficiency. While the cells are separately marketed products, a 100 kWhe prototype of CB system has been built. The company aims towards efficiency over 50% and manufacturing CB as 10 MWhe blocks at 0.1–1 MWe and at the cost of less than 10 \$/kWhe as a containerized solution [123,140].

Antora is not alone in development of these high temperature direct conversion technologies. A European project Amadeus focuses on the development of an extremely compact energy storage devices based on the use of a silicon and boron alloy as a high temperature PCM. The combination of a thermionic and photovoltaic cell as a new hybrid thermionic photovoltaic converter with an efficiency of approximately 40% is considered for discharging. The system should also have a high variable power in the range of kW to MW [141–143]. Similar is the "Sun in a box" project at MIT [144]. The system works with molten silicon, which cycles between 1900 ◦C (discharged tank) and 2400 ◦C (charged tank). When discharging the system, specialized solar cells, known as multi-junction photovoltaic cells with a conversion efficiency of 29%, are used to generate electricity [145,146]. It is also considered that the thermal energy storage technology developed in these projects could be used in concentrated solar power plants (CSP plants) [141].

#### 2.3.2. Reversible Cycle and Other Hybrid Systems

A reversible thermodynamic cycle for charging can be achieved theoretically with a Stirling engine operating as a heat pump. This system has however not been found in any commercial or scientific research. A thermochemical conversion called the Lamm– Honigmann process, originally invented in the 19th century, which uses physical or chemical reactions with water for heat storage, is another CB concept. The system was developed with several working fluids though the most notable was water with caustic soda (NaOH) used to drive a "fireless" locomotive [147].

A German company NaCompEx has been developing a 10–100 MWe storage system with a reported storage capacity of 60 kWhe/m3 and the roundtrip efficiency of 80% [125,148]. When charging the storage, desorption of water from lean solution vapor is realized using heat from a condenser while the high-pressure vapor is provided by compressors. Condensed liquid water and concentrated solution are then stored separately. When discharging, the rich solution is absorbing the water vapor at low pressure, providing high temperature heat and turning high pressure water into a vapor. This vapor then produces work in an expander when flowing from high to low pressure.

No information on experimental work has been however reported and recently the company announced work on a hydrogen storage system instead [124]. The Lamm– Honigmann process is, however, a subject of a number of scientific research such as investigating efficiency limits [147] or conducting of a lab scale experiments with a system

providing isentropic power potential of several hundred watts and expander power in the range of a dozen watts [149].
