A Review of Seasonal Energy Storage for Net-Zero Industrial Heat: Thermal and Power-to-X Storage Including the Novel Concept of Renewable Metal Energy Carriers

Abstract

Achieving net-zero greenhouse gas emissions by 2050 requires CO₂-neutral industrial process heat, with seasonal energy storage (SES) playing a crucial role in balancing supply and demand. This study reviews thermal energy storage (TES) and Power-to-X (P2X) technologies for applications without thermal grids, assessing their feasibility, state of the art, opportunities, and challenges. Underground TES (UTES), such as aquifer and borehole storage, offer 1–26 times lower annual heat storage costs than above-ground tanks. For P2X, hydrogen storage in salt caverns is 80% less expensive than in high-pressure tanks. Methane and methanol storage costs depend on CO₂ sourcing, while Renewable Metal Energy Carriers (ReMECs), such as aluminum and iron, offer high energy density and up to 580 times lower storage volume, with aluminum potentially achieving the lowest Levelized Cost of X Storage (LCOXS) at a rate of 180 EUR/MWh of energy discharged. Underground TES and hydrogen caverns are cost-effective but face spatial/geological constraints. P2X alternatives have established infrastructure but have lower efficiency, whereas ReMECs show promise for large-scale storage. However, economic viability remains a challenge due to very few annual cycles, which require significant reductions of investment cost and annual cost of capital (CAPEX), as well as improvements in overall system efficiency to minimize losses. These findings highlight the trade-offs between cost, space requirements, and the feasibility of SES deployment in industry.

1. Introduction

SES is essential for industrial decarbonization because it facilitates the efficient utilization of renewable energy throughout the year, ensures operational continuity, mitigates dependence on fossil fuels, and supports the transition to a low-carbon economy. It bridges the gap between the variable renewable energy supply and the constant high-energy demand of industrial processes. To achieve these objectives, SES technologies must balance low capital costs and high energy storage density, which are critical for ensuring economic feasibility, particularly for long-term storage solutions [1].

Low capital cost: The capital cost per MWh of discharged energy for an SES that cycles only once annually is approximately 365 times higher than that of a daily storage system, emphasizing the impact of high initial investment. Lowering capital costs not only improves economic viability but also enhances scalability and competitiveness with fossil-based alternatives, making renewable SES more feasible for large-scale and regional adaptation.

High energy storage density: SES typically provides energy for weeks or months, requiring the storage of vast amounts of energy over extended periods. A high energy storage density is essential for minimizing the physical footprint, enabling more efficient resource use, reducing system costs, and improving the feasibility of large-scale implementations, particularly in space-constrained locations.

Figure 1 shows the applicability of storage technologies to various storage capacities and timescales [2].

Given that batteries have significantly higher capital costs (EUR 300 to 800 per kWh electric) compared to TES systems (10 to 300 EUR per kWh thermal), large-scale batteries are less economically viable for seasonal storage applications [3]. However, their economic potential for short-term storage, ranging from hours to days, remains extremely high.

Conversely, heat storage for a day is economical with a steel tank containing hot water; covering a heat demand of up to two months for an entire winter requires approximately 60 times the volume of daily storage for the same heat load per day. A high storage density is crucial because large storage volumes require significant space, which may not be available or could be costly if the land has economic or recreational value. Furthermore, large volumes require large containers, which require social acceptance and a legal framework.

For thermal water storage solutions, the enclosure structure is the main cost driver, not the storage material. However, it becomes cost-effective when the ground provides natural mechanical support, thereby avoiding the need for additional construction. In underground systems without or with only thin enclosures, costs are primarily linked to heat extraction and injection into the ground storage volumes.

Based on these criteria, the following two SES technologies have the highest economic relevance:

• Long-term thermal energy storage (TES) technologies
• Power-to-X (P2X) technologies: These involve converting renewable energy into chemical energy stored in gases, liquids, or solids (e.g., hydrogen, synthetic fuels, and renewable metal energy carriers).

This study focuses on long-term energy storage solutions that can provide heat within a temperature range of 80 to 150 °C and above for industrial applications. These temperatures, which are significantly higher than those required for space heating and domestic hot water preparation, are suitable for high-temperature heat pump (HTHP) applications and are applicable in sectors such as agriculture (e.g., greenhouses), food production (e.g., pasteurization and cleaning-in-place systems), textiles, chemical processing (e.g., drying, distillation, and reactor heating), breweries, and beverage production (e.g., hot water for sterilization) [4,5,6]. However, site-specific challenges, including spatial requirements and geological conditions, may limit the feasibility of TES systems in certain locations.

To address these challenges, alternative P2X technologies can provide heat and electricity for industrial applications during the winter months. Notable examples include 220 Power-to-X demonstration projects identified by Wulf et al. (2020) [7], highlighting the increasing interest in these pathways for industrial applications such as refineries or steel production. Furthermore, X-fuels can serve as substitutes for fossil fuel-based feedstock in the chemical industry.

Despite advancements in TES and P2X technologies, several significant challenges remain. A primary concern is scalability, particularly for large-scale industrial applications, due to substantial initial capital expenditures. Ensuring long-term storage efficiency is crucial to minimize energy losses over time as many systems experience energy dissipation during storage. Another critical challenge is storage density, especially in areas with limited space. Storing large quantities of energy necessitates considerable land use, which may not be feasible in regions with space constraints or high land costs. Furthermore, integrating these technologies into existing industrial systems and energy infrastructures presents both technical and economic challenges.

This review introduces novel concepts and investigates their potential to address existing challenges. The research aims to elucidate the practical feasibility of these materials for extensive, long-term energy storage in industrial settings.

Future research should prioritize the reduction in capital expenditures, the enhancement of energy storage capacity, and the improvement of system integration with industrial processes. Addressing spatial and geological challenges across various regions is essential for the broader application of these technologies. Key research questions include the following: How can long-term storage efficiency be enhanced without incurring additional costs? What are the key economic considerations when scaling up ReMEC technologies for industrial use? How can energy storage systems be optimized for high-temperature industrial applications to achieve a balance between efficiency and cost-effectiveness? Considering the challenge posed by only few annual storage cycles, which complicates achieving financial break-even, what practical solutions—beyond cost reduction or performance enhancement—have been proposed to increase cycles and improve economic viability?

This literature review systematically investigated SES technologies by examining both conventional and emerging technologies, focusing on the economic feasibility and implementation challenges of large-scale industrial applications based on spatial requirements, site-specific geological conditions, and limitations. Novel concepts, such as ReMEC as storage material (aluminum and iron), were introduced and compared with alternatives, such as hydrogen, in terms of production pathways and long-term storage potential.

Database searches were conducted in Web of Science, IEEE Xplore, and Scopus using keywords such as seasonal energy storage, thermal energy storage, hydrogen storage, and renewable integration, focusing on publications from 2010 to 2023. The selection criteria included peer-reviewed studies that assessed technical feasibility, scalability, and key performance indicators across various applications. In addition, the cost-effectiveness and storage volume requirements of the proposed SES technologies were compared.

By providing a comprehensive understanding of SES options, including their technical feasibility, economic viability, and implementation challenges, this overview facilitates informed decision-making and strategic planning for policymakers, industry leaders, and researchers in the transition toward a sustainable energy future.

2. Key Performance Indicator

Key performance indicators (KPIs) for long-term energy storage solutions in industrial processes are as follows:

• Economic factors: A low cost of technology (material, container, space, maintenance, and lifetime), resulting in a low cost of storage, as well as a high degree of (storage) material availability;
• Performance factors: A high energy storage density and thus low space requirement, as well as high storage efficiency (i.e., low losses);
• Functional and technical suitability: Long-term stability, the capability of storing renewable energy over several months, chemical compatibility with the container and energy exchange loops, if applicable;
• Environmental sustainability: Low environmental impact for a normal use case, recyclability, and a closed-loop economy;
• Safety and risk mitigation: Low risks for humans and the environment (e.g., non-toxic, non-flammable, low risks at unforeseen events, environmentally “friendly” material, and processes) and moderate pressure regarding the storage medium.

The project costs of TES and P2X systems depend on factors such as energy generation (heat or fuel production), storage implementation, and energy recovery technology (heat exchangers, heat pumps, or fuel cells). Other influences include system integration, operating temperature, local conditions and regulations, and the available financial funding. Thus, comparing TES and P2X requires specific application scenarios for accurate economic assessment. A direct comparison of these long-term energy storage technologies is challenging due to the factors listed in

Table 1. Comparison of TES and P2X systems: applications, energy considerations, and costs.

	TES	P2X
Application	TES and P2X serve distinct purposes, making direct comparisons challenging.
Energy requirements	Systems typically use waste heat (a lower-quality energy source) for charging. This waste heat generally incurs no additional OPEXcharging. OPEX for components like heat exchangers is minimal in most cases (except for borehole thermal energy storage, thermochemical storage and phase change materials).	Processes require electricity (the highest quality of energy) for chemical conversion. This incurs higher OPEXcharging.
Energy recovery	Heat only.	Heat and electricity, with electricity being of higher economic value.
Cost drivers	The primary cost driver for TES systems is CAPEXstorage.	The main cost driver is OPEXP2X for charging the storage, not CAPEXstorage, except for pressurized hydrogen storage tanks.
Storage efficiency	High storage efficiency (minimal losses), which is dependent on the temperature difference between the storage system and its surroundings.	Storage efficiency depends on fuel type and conversion process (e.g., electrolysis).
Storage material/medium	Typically non-toxic, non-flammable materials like water or molten salts.	This requires compatibility with chemical fuels like hydrogen or methane.

.

“CAPEX_storage” refers to capital expenditures for installation and the infrastructure of the storage system, while “OPEX_storage” includes ongoing operating costs for energy consumption and maintenance. “Temperature swing” is the difference between the initial and final temperatures of the storage material, determining storage capacity. “Volumetric energy density” indicates the energy stored per unit volume of storage material.

Consequently, this investigation does not focus on comparing the two distinct technologies (TES vs. P2X) but rather on comparing seasonal TES solutions with one another and with seasonal P2X storage solutions.

For an economic assessment of both technologies (Equation (2) and (3)), the annuity rate (Equation (1)) was calculated using a discount rate (i) of 5%, which is frequently used in the literature [8]. Economic life (n) was selected based on storage technology.

$$Annuity rate={\displaystyle \frac{i}{{(1+i)}^{-n}}}$$

(1)

In general, the number (#) of cycles per annum is defined as one which reflects the operation of SES.

2.1. Annual Cost of Heat Storage

For TES, the Annual Cost of Heat Storage (ACOHS) excludes the cost of heat production for charging (OPEX_charging) because it can range from negligible (waste heat) to significant (solar thermal plant heat). Therefore, considering the efficiency of the charging process in a specific application, the additional cost of this heat must be added to the figures in this report.

The ACOHS (Eq. 2) was used to compare the economics of the heat storage options, expressed as currency per unit of stored heat (e.g., EUR/MWh thermal). This quantifies the discounted storage cost and not the heat generation. Heat generation for charging was excluded, but the storage cycle efficiency (η_sc) from real projects and the associated thermal losses were considered.

$$ACOHS={\displaystyle \frac{{CAPEX}_{storage}·Annuity rate+{OPEX}_{storage}}{Storage capacity ·{\eta }_{SC}·\# of cycles per year}}$$

(2)

TES CAPEX_storage includes investment costs for the storage container, space, storage material, installation (e.g., storage piping and insulation), and control system, excluding system piping, engineering expenses, and taxes.

TES OPEX_storage includes the energy demand for storage operations and the replacement of components. The storage capacity depends on the optimal storage volume, the specific heat capacity of the storage material, and the temperature swing. A temperature swing of ± 50 K for sensible heat storage and ±10 K for phase change was defined.

The storage material costs for latent heat storage are based on market prices [9,10]. Unfortunately, no large-scale projects on thermochemical storage systems were found in the literature; thus, data are scarce and can only be estimated to the best of the authors’ knowledge and market prices [11,12].

2.2. Levelized Cost of P2X Storage

For P2X storage, the cost of X-fuel production (OPEX_P2X) is determined by the amortization of the investment cost of the P2X facility and the operating cost, including the electricity price for the input of electricity (power) in the P2X process. In this study, a price of 50 EUR/MWh_e was applied. According to the Wasserstoffatlas published by the Eastern Bavarian Technical University of Regensburg, this is the expected electricity cost by 2030 for Germany [13].

The Levelized Cost of P2X Storage (LCOXS, Equation (3)) evaluates the economic performance expressed as a currency unit per unit of discharged energy, for example, EUR/MWh_e&th. This allows for comparing P2X technologies, considering the complete process chain and its ${\eta }_{SC}$.

$$LCOXS={\displaystyle \frac{{CAPEX}_{storage}·Annuity rate+{{OPEX}_{P2X}+OPEX}_{storage}}{Storage capacity ·{\eta }_{SC}·{\eta }_{X2E}·\# of cycles per year}}$$

(3)

P2X CAPEX_storage is associated with the cost of storage containers for storing hydrogen, methane, methanol, or ammonia. Data were adapted from the literature based on existing projects, if available, or expert estimations (see corresponding sections).

Generally, it is not easy to generalize storage costs because of the wide variety in size, operating conditions of storage, etc. Additionally, most available cost data do not provide specific details regarding the costs that are included or excluded. OPEX_storage includes the energy demand during storage.

Therefore, the P2X storage capacity depends on the optimal storage volume and volumetric energy density of fuel X. For liquefied cryogenic storage, the cycle efficiency includes the leakage or boil-off rate, as described in the relevant section.

Additionally, for the X-to-Energy conversion, it was assumed that X is used in a fuel cell and that both electricity AND heat are valorized and count as “useful energy output.” Hence, it is not based purely on electric round-trip efficiency because this study focuses on heat for industry rather than electricity.

Consequently, the total fuel cell efficiency (electricity and heat) for the respective X-fuel (X2Energy efficiency, η_X2E) was included, with the electricity-to-heat ratio depending on the specific fuel cell technology.

However, no investment costs for the fuel cell system were considered because system costs vary widely depending on the power range and technology. In addition, other conversion technologies, such as gas combustion processes and other Combined Heat and Power (CHP) technologies can also be applied.

3. Thermal Energy Storage Solutions

Typically, TES solutions are used in utility-scale power generation, industry, district heating and cooling, buildings, and cold chain logistics for short-term applications to provide flexibility and smart energy use (e.g., peak load management), achieve cost savings, and shift power generation.

3.1. Parameters of TES

TES systems can effectively decouple the heating demand from the supply and may be used to increase the share of renewable energy from fluctuating resources. However, TES technology for long-term applications in the temperature range of 120 to 150 °C is scarce. Various technologies usually cover only smaller timespans of several hours to one day [14]. The major parameters of the TES system are as follows [15]:

• Capacity per unit volume: The energy stored per unit volume (kWh/m³) varies depending on the storage material, such as water, molten salts, or phase-change materials, each providing different energy densities.
• Temperature range (ΔT): The operating temperature range defines the energy storage capacity, for example, 40 to 90 °C.
• Heat transfer and associated losses: Heat is added or removed via exchangers or conduits, and minimizing the temperature difference during transfer reduces exergy losses.
• Temperature stratification and exergetic efficiency: Maintaining temperature layers in sensible heat storage improves efficiency of connected processes, whereas exergetic efficiency preserves temperature levels and ensures that stored energy is converted back into useful work.
• Power requirements: The energy required for heat injection or extraction, such as pumps or compressors, affects operational costs and efficiency.
• Structural elements: Storage systems use tanks, pits, or natural formations, with materials such as concrete or steel affecting durability and insulation.
• Thermal loss control: Insulation or covers can minimize energy losses, with advanced materials reducing the heat transfer to the surroundings.
• Cost: Initial installation costs and ongoing operational expenses, including material choices and energy requirements, determine the affordability of a system.

3.2. Types of TES

3.2.1. Sensible TES Systems

In general, the heat loss for sensible TES systems is directly proportional to the temperature difference between storage and the environment. Therefore, insulation must be considered, which can be an important cost factor [16].

Water is predominantly utilized as a sensible thermal storage medium because of its high thermal capacity (4.2 kJ/(kg·K)) and low cost for applications in the temperature range of 20 to 95 °C. As shown in Figure 2, at these temperatures, tank configurations (TTES) on the ground or underground, as well as pit storage (PTES), aquifer storage (ATES), borehole storage (BTES), and cavern storage (CTES), are technological options for seasonal applications, as described by Yang et al. (2021) [17].

Application Above Ground 

Water at a temperature of up to 130 °C can be stored by so-called two-zone or Hedbäack systems; this approach is increasingly used in district heating applications to reduce the need for peak-load boilers, and water at a temperature of up to 160 °C can be stored in pressurized tanks [18,19].

Technically, such TTES systems can be used for long-term storage applications. However, because of the high-volume requirements and the high cost per MWh discharged resulting from the low number of storage cycles in such applications, only a few realizations exist, mainly for providing space heat and domestic hot water for buildings with pressurized steel tanks and water up to 95 °C.

If water as a storage material is not an option due to the need for higher temperatures, solid-state storage media, e.g., sandstone with 2.8 MJ/(m³·K) and a storage material cost of 0.88 EUR/kWh (thermal) considering a temperature swing of ±50 K and granite with 1.3 MJ/(m³·K) and a storage material cost of 0.9 EUR/kWh (thermal), are interesting as high-temperature heat storage materials from an economic perspective [20,21]. Their advantages include high availability and low material costs, higher volumetric storage densities, flexibility of the operating temperature range and pressure independence, environmental benefits, and common safety risks compared with molten salt or thermal oil [22] (see

Table 2. Overview of key parameters for sensible TTES using water (atmospheric and pressurized tanks) or sandstone as storage material.

	Water	Sandstone
Storage capacity	Higher (up to 6 GWh)	Lower (up to 1 GWh)
Temperature range, °C	20 to 95/130/160	60 to 400
Temperature swing considered	50
Volumetric heat capacity, MJ/(m3·K)	4.13	2.8
Storage material cost, EUR/kWh (thermal)	0.09	0.88

). Furthermore, TES technology is easily scalable. However, heat transfer requires greater effort, which affects economic feasibility. In addition, limited thermal conductivity is a drawback.

Nevertheless, solid-state TES systems are attractive for low-pressure gaseous heat transfer media in medium- and high-temperature applications [23]. Although featuring a low volumetric energy density, sensible storage options using water or solid-state materials can offer low investment costs and are widely available.

However, long-term storage solutions for the industry must achieve a lower cost per kWh storage capacity. Therefore, a partial or complete underground storage facility is more interesting, particularly when the cost can be reduced by one or several of the following advantages:

• Utilizing inexpensive and abundant materials such as water, soil, or gravel as storage media significantly lowers the costs of seasonal TES.
• No support structures bearing the weight of the storage medium must be built.
• In the case of BTES or ATES, Earth serves a dual purpose by both containing and providing support for the storage medium.
• The lid may float or rest on the storage medium and be supported by the storage medium. Hence, there is no need to construct a fixed static structure to support this lid.
• Floating insulated lids, which are often used in PTES with water, help seal the system and minimize heat loss.
• The storage volume is large enough or the temperature difference to the surrounding ground is low enough that the heat losses are low, even without technical insulation between the storage medium and the surrounding ground. Large storage volumes have a lower surface-area-to-volume ratio, which inherently minimizes heat loss. Additionally, if the temperature gradient between the storage medium and the surrounding ground is relatively small, the heat transfer is naturally limited, further reducing the energy losses.

Nevertheless, compared with short-term options and Power-to-Liquid or Power-to-Solid alternatives, a significant limitation of all long-term TES systems is their increased spatial and areal requirements to accommodate the necessary (larger) storage volume and potentially mitigate higher heat losses [24].

Application Underground 

Dahash et al. (2019) [19] investigated seasonal PTES solutions for high-temperature solar district heating applications. The authors argued that the optimum type of storage solution is especially difficult to determine without simulations for long-term UTES systems because of the many parameters influencing the size, construction, and economic feasibility.

Table 3. Overview of key parameters for UTES systems using water as storage material with storage density depending on usable temperature difference.

	PTES	BTES	ATES
Storage capacity, GWh	Up to 60	Up to 6	0.6 to 40
Temperature range, °C	10 to 95	10 to 80	Up to 95
Storage density, kWh/m3	40 to 80	25 to 40	20 to 40
Recovery efficiency, %	90 to 95	70 to 80	40 to 80

provides an overview ofkey parameters for UTES.

Recently started activities of the International Energy Agency (IEA-TCP) Energy Storage (Task 39) on large thermal energy storage for district heating aim to determine aspects that are important in planning, design, decision-making, and realizing very large thermal energy storage for integration into district heating systems and industrial processes, given the boundary conditions for different locations and different system configurations [25].

Denmark has already installed and operated large-scale PTES facilities charged by solar thermal collectors or heat pumps and discharged to a nearby district heating network. These PTES systems have volumes of 60,000 to 200,000 m³. The water temperature can reach 95 °C with a storage efficiency of up to 90% [18]. If the temperature of the discharged heat is below the heat demand, a heat pump can be efficiently added to the system to increase the temperature of the heat provided. However, considerable research and further development are ongoing in this field.

Germany, Denmark, Sweden, and Canada operate BTES systems of 20,000 to 320,000 m³ (ground volume) at a temperature of approximately 80 °C [26,27]. Because thermal insulation can only be installed on the top in these cases, heat losses in the range of 20% to 30% must be considered [28,29].

More than 2500 ATES systems have been installed worldwide. However, most large-scale high-temperature ATES facilities operate in the Netherlands, France, and Germany [30]. In Switzerland, the implementation of the first “Geostorage” with a heat source of up to 90 °C from waste incineration was launched in 2021 (Forsthaus Bern), consisting of three wells plus one monitoring well. In winter, this heat was planned to be extracted and fed into an existing district heating network at a maximum temperature of 60 °C [31]. However, the project was discontinued in 2025 after testing revealed that the underground sandstone layers at the site were not sufficiently permeable, making seasonal heat storage technically unfeasible despite extensive testing and optimization efforts.

Research on high-temperature ATES has demonstrated that technical difficulties can be solved, for example, water treatment methods to prevent the precipitation of minerals, proper material selection to prevent corrosion, and the use of aquifers with low permeability to reduce heat losses due to buoyancy flow [32,33].

Recently, Ueckert and Baumann (2019) [34] evaluated the hydrochemical aspects of high-temperature aquifer storage in carbonaceous aquifers (large-scale ATES tests in the Molasse Basin were performed) with cycling temperatures ranging from 65 to 110 °C and concluded that with a triplet system, successful operation for decades could be feasible.

Drijver et al. (2012) [35] calculated a 40% to 80% recovery efficiency for an ATES system in the Maassluis Formation and Brussels Sand at a temperature of 93 °C. However, the impact on groundwater composition and legal aspects is a major hurdle for high-temperature ATES systems [36]. Therefore, CTES solutions may offer fewer implementation hurdles. For example, in Sweden (Avesta, Lyckebo) and Finland (Oulu 2), CTES projects were implemented with storage volumes ranging from 15,000 to 190,000 m³. Finland also plans to operate a CTES in Vantaa with a storage size of 1,000,000 m³ at 140 °C by 2026 (district heating application) [37].

CTES offers preferred performance but is associated with high capital costs. In contrast, BTES and ATES systems offer more economical construction options, albeit with lower efficiency. PTES systems strike a balance between cost and efficiency.

3.2.2. Phase Change Energy Storage

In phase change or latent heat energy storage systems, phase change materials (PCMs) commonly change their phase between solid and liquid, storing energy based on the latent heat of the material. The temperature of the material remains almost constant during this process [38]. Therefore, a higher storage density can be achieved than that of sensible TES system if the temperature swing between charging and discharging around the phase change temperature is relatively small.

For example, when using paraffin as a PCM with a temperature swing of ±10 K around its melting point, the energy storage density is 314 kJ/kg (7.5 times higher than that for sensible storage in water with a temperature change of 20 K). However, if the temperature swing is ±30 K, the storage density is 322 kJ/kg (only 2.5 times higher than that of sensible storage in water) [39]. This makes PCMs suitable for applications requiring a specific and constant temperature level.

Sugar alcohols such as erythritol, xylitol, and sorbitol/glucitol are potential organic PCMs for high-temperature heat storage at 80 to 150 °C (

Table 4. Overview of key parameters of PCM-TTES systems for potential organic PCMs at 80–150°C.

	Erythritol	Xylitol	Sorbitol/Glucol
Phase change temperature, °C	117/121	94/92.7	55/110; 93.5/94.5
Temperature swing considered	10
Latent heat of fusion, kJ/kg	340/344	246/260	166/173
Storage material cost, EUR/kWh (thermal)	8.3	13.8	29

) [40,41]. In this case, the implementation of a heat transfer solution for the charging and discharging of storage needs to be considered, as well as the temperature loss that stems from the temperature difference required to achieve the required power for charging and discharging via heat exchangers and through a partially solid and liquid storage medium.

The coupling of latent heat storage modules containing high-temperature PCMs with steam or water is considered to be a hybrid storage system for industrial processes. The same concept extends to ice-water storage but is not suitable for high-temperature industrial purposes; instead, it is relevant in pharmaceutical applications where lower temperatures are essential. Nevertheless, both applications have found a favorable niche for HPs owing to the extensive research in these areas [42].

3.2.3. Thermochemical or Sorption Storage

Chemisorption heat storage can potentially reduce storage volume and losses after charging [43]. Generally, sorbents and sorbate materials are used for this purpose. During the discharge process, heat is released when both the materials come into contact. When the sorption heat storage is charged, the sorbent stores energy by changing its chemical potential (absorption for liquid sorbent or absorbent and adsorbent for solid sorbent or adsorbent) [44]. This technology primarily corresponds to a thermally driven heat pump process. Adsorption heat storage in a closed system utilizing water vapor as the sorbate and silica gel as the sorbent material can generate high temperatures (130 to 150 °C) [45].

It should be noted that for an SES system, low-temperature heat (in practice, at least 5 °C) for the evaporation of the sorbent under vacuum conditions must be supplied when the storage is discharged, typically in winter. Several materials, such as sodium hydroxide (NaOH) solutions, zeolites, and salt hydrates, have been investigated (

Table 5. Overview of key parameters for sorption storage materials in temperature range of 80–150°C.

	Sodium Hydroxide	Silica Gel
Temperature range, °C	40 to 150	130 to 150
Enthalpy of adsorption, kJ/kg	540	1000
Storage material cost, EUR/kWh (thermal)	0.5 to 0.8	0.8 to 1.3

) [46,47]. However, the challenges of economic application and optimal system design (material selection and process efficiency) still need to be overcome.

Commercial applications are known for using this technology for thermally driven heat pumps but not for seasonal heat storage. Container and charging/discharging devices for sorption heat storage systems usually account for over 80% of CAPEX. The storage material contributes to a much smaller share [29,45].

3.3. Integration of TES into Industrial Processes

High-temperature TES (HT-TES) can be integrated into industrial processes through heat exchangers, offering the potential for energy efficiency and cost savings. This integration allows the capture and reuse of excess heat, thereby improving the overall process efficiency. Heat exchangers serve as the interface between TES and the industrial process, facilitating the transfer of thermal energy. A key challenge lies in matching the heat sink to the temperature requirements of the industrial processes. Manufacturing operations often demand precise temperature levels for optimal performance, which may not align with the temperature range of stored thermal energy. This mismatch can lead to reduced efficiency in heat transfer or require additional energy input to achieve the desired process temperatures. Aligning the available thermal energy storage with varying temperature requirements of industrial processes may require a complex system design and potentially multi-level heat transfer systems.

Combining low-temperature TES (LT-TES) with industrial heat pumps is crucial for utilizing waste heat to increase energy efficiency and reduce climate gas emissions related to process heat generation. In addition, such systems could reduce the electricity demand for the temperature lift in winter because the temperature difference from the heat source to the heat sink of the heat pump is reduced. For example, for a temperature lift of 90 K (e.g., from a 10 °C heat source to a 100 °C heat sink), increasing the source temperature by 10 K decreases the electricity consumption of the heat pump by roughly 11%. Furthermore, LT-TES is typically significantly more cost-effective than HT-TES because it benefits from reduced heat loss to surroundings and can be utilized for cooling purposes during summer months. Nevertheless, the integration of heat pumps into production processes presents challenges due to the necessity of matching the available heat source with the required heat demand.

4. Power-to-X Storage

As industries pursue decarbonization, P2X technologies convert renewable electricity into energy carriers, enabling cleaner heating and cooling, while addressing energy variability in fossil fuel-dependent sectors.

Power-to-X-to-heat technologies chemically store energy in gases, liquids, or solids and release it through combustion or electrochemical conversion. Promising candidates for these pathways include hydrogen (H₂), methane (CH₄), methanol (CH₃OH), ammonia (NH₃), and metals (Fe and Al) that can store up to 100 TWh of energy over extended periods. The stored energy can generate both heat and electricity for industrial processes, possibly supplying electricity for on-site use or feeding it into the grid, aligning with high spot market energy prices and enabling revenue collection. Figure 3 provides an overview of the process diagram for these P2X options.

The technology readiness level (TRL) of H₂ electrolysis and methanation is 7 to 8 [48]; the TRL of CH₃OH synthesis is 4 due to the development of CO₂-to-CO reduction [49]; the TRL of NH₃ is 5 to 9 [50]; Green ReMEC production considers CO₂-free aluminum electrolysis, the direct reduction of iron with H₂, and molten iron oxide electrolysis, with a current TRL of 4 to 7.

Figure 4 shows the volumetric and gravimetric energy densities of the storage materials. Notably, the weight and size of the storage vessel are not included in any of these technologies. Gaseous hydrogen (GH₂) generally has the lowest volumetric energy storage density. In contrast, Al and Fe exhibited the highest volumetric storage density.

P2X technologies, including the use of hydrogen, ammonia, methanol, and synthetic methane, offer significant potential for long-term energy storage. While these substances are already safely handled in industries like chemicals and agriculture, scaling up P2X systems for energy storage requires careful attention to both safety and environmental risks. Safety concerns are primarily associated with the handling and storage of these substances in gas or liquid form, especially due to the flammability of gases like hydrogen and the toxicity of ammonia in large quantities. Environmental risks are linked to the large-scale storage and potential leakage of these substances which, if not properly managed, could negatively impact air, water, or soil quality. Mitigation strategies, including advanced monitoring systems, leak detection, and stringent safety protocols, are essential for minimizing these risks. A comprehensive risk assessment is crucial to ensure that the large-scale deployment of P2X technologies remains safe, sustainable, and environmentally responsible.

4.1. Hydrogen

Currently, more than 95% of H₂ production is reformed from fossil fuels (gray hydrogen), emitting CO₂ into the atmosphere [55]. It should be noted that more than 80% of this H₂ is used for the synthesis of ammonia for the agriculture and refining sectors [56]. To provide a sustainable and renewable pathway, hydrogen must be produced exclusively from renewable energy sources, which includes an electrolysis process. Hydrogen electrolysis was assumed to achieve an efficiency of 70% [57].

Hydrogen must be compressed or liquefied (LH₂) for direct storage over a longer time period to reach a much higher volumetric storage density than that stored at ambient pressure and temperature. This requires additional process steps for conversion and storage, adding extra costs and lowering the overall energy efficiency.

Hydrogen storage technologies have been implemented, such as large storage tanks above the ground in the gas phase with tank pressures of 100 to 350 bar and up to 700 bar for short-term applications. However, hydrogen compression is a cost-intensive process; even after compression, it still requires a large specific storage volume compared with other alternatives. To address the issue of low storage density, liquid hydrogen (cryogenically stored at very low temperatures) can be used as an alternative. However, the liquefaction process is highly energy-intensive, necessitating 6 to 10 kWh of electricity per kilogram of hydrogen, corresponding to approximately 30% of the energy content of liquid hydrogen [51].

Furthermore, the boil-off rate needs to be considered for cryogenic storage because of the low boiling point of hydrogen (-253 °C at 1 bar) [58]. The boil-off rate in cryogenic hydrogen storage refers to the gradual evaporation of liquid hydrogen due to heat increase, even in well-insulated tanks. It represents the percentage of stored hydrogen lost per day. NASA’s liquid hydrogen storage spherical vessels (230 to 270 tons, 3800 m³) operate at boil-off rates below 0.1% per day using a double-walled high vacuum tank with additional insulation materials. In this case, a storage loss of approximately 10% for the seasonal storage of LH₂ can be expected, reducing the P2X storage efficiency, i.e., the efficiency of converting renewable electricity to storing it for weeks and up to months before discharging (total efficiency approx. 45%). The investment cost for such large-scale hydrogen storage tanks can be expected to be approximately EUR 6 million. However, the ultimate goal is to reduce the cost by 50%, and the basic design of a 11,200 m³ spherical LH₂ tank was completed in 2020 [59].

Large-scale hydrogen storage in salt caverns (70 to 250 bar) is already in operation on a full industrial scale in favorable geological regions, for example, Teesside, UK, and Texas, USA [60]. Aquifers and depleted oil and gas fields are considered alternative storage options. However, salt caverns are the most suitable because of their low construction costs, low leakage rates, fast withdrawal and injection rates, relatively low cushion gas requirements, and minimal risk of hydrogen contamination [61,62].

Cushion gases, such as CO₂, CH₄, N₂, and others, are required as a buffer for management purposes and pressure maintenance during injection and withdrawal cycles to prevent water from entering the storage space [63].

4.2. Methane

Carbon dioxide methanation (2 H₂ + CO₂ → CH₄ + O₂) is generally a catalytic exothermic reaction. Therefore, using the heat generated during methanation directly in upstream processes or other applications is advantageous. It is estimated that the conventional conversion route achieves a chemical conversion efficiency of up to 80% [64]. Therefore, the combined production and storage efficiency can be expected to be 50% when using a concentrated CO₂ source. If the CO₂ is obtained via direct air capture with an efficiency of 77%, the overall efficiency decreases to approximately 38% [65].

In general, methane can be stored in tanks, aquifer reservoirs, salt caverns, or depleted gas reservoirs that are technologically mature [66].

4.3. Methanol

Renewable or green methanol is produced from renewable hydrogen (Power-to-H₂) and carbon dioxide sources [67]. Methanol reforming (3 H₂ + CO₂ → CH₃OH + H₂O) achieved efficiencies similar to those of green methane production (54% considering a CO₂ capture efficiency of 97% for CO₂-rich sources or 43% with a direct air capture efficiency of 77%) [68]. Several reactor designs, catalysts, and processes for directly converting CO₂ and H₂ to methanol have been investigated.

For example, a large-scale plant in China with a production capacity of 330 tons of green methanol per day was commissioned in October 2022, with an estimated project cost of approximately EUR 33 million [69].

Methanol is generally easy to store and transport because of its liquid state at ambient temperature and its boiling point of 65 °C. However, its flammability and corrosive effects must be considered. Therefore, stainless steel vessels are recommended for storage [70,71]. Notably, the cost of stainless steel is five times higher than that of carbon steel.

However, the physical and chemical properties of methanol are unique and are not identical to those of common bulk-stored flammable liquids. For example, to avoid the emission of methanol when the tank level fluctuates, a nitrogen blanketing or scrubber system and floating roof are often implemented [72]. Nevertheless, large-scale methanol storage tanks with a capacity of up to 100,000 m³ are commercially used in the chemical sector [73].

4.4. Ammonia

The Haber–Bosch process is an industrial method for producing ammonia using a combination of atmospheric nitrogen and hydrogen under elevated temperature and pressure conditions in the presence of a catalyst (H₂ + N₂ → 2 NH₃). For ammonia synthesis using Fe, Ru, and Co₃Mo₃N catalysts at 150–300 bar and 350–500 °C, charging and storage efficiencies of 53% can be assumed [68]. However, the Haber–Bosch process exhibits several disadvantages, notably its high energy intensity and consequent environmental impact.

Several alternative electrolytic ammonia synthesis pathways are under investigation, including the solid-state electrolytic process, which seems to be the most promising alternative to the Haber–Bosch process [74]. These alternative methods aim to address some of the limitations associated with traditional processes and offer potential benefits in terms of energy efficiency, environmental sustainability, and scalability.

If stored at atmospheric pressure, ammonia must be maintained at −33 °C; otherwise, a storage tank pressure of 8 bar is required (for ambient temperature) [75].

Low-temperature storage technologies at ambient pressure are generally used for large-scale and long-term ammonia storage systems because of cost considerations (storage costs: 120–180 EUR/ton) [76].

4.5. Renewable Metal Energy Carriers

ReMECs have great potential for storing renewable energy over long periods of time. In contrast to power-to-methane and power-to-methanol technologies, renewable metal production is possible without a carbon source. Instead, this technology is based on metal redox reactions, whether direct chemical, thermochemical or electrochemical metal, involving reversible oxidation and reduction in metals, involving reversible oxidation and reduction of metals to enable energy storage and release through chemical transformation.

In the past, research has focused on solar-driven thermochemical reduction–oxidation (redox) processes to produce hydrogen and liquid fuels via metal reduction as an intermediate step [77,78]. These solar thermochemical cycles are based on high-temperature solar thermal decomposition of metal oxides.

Bergthorson et al. (2015) [79] investigated recyclable metal fuels as energy carriers and the direct combustion of metal powders. Trowell et al. (2020) [80] concluded that Al and Fe are the most promising candidates as energy carriers because of their low cost and known reserves in the earth’s crust, which are far greater than the potential demand for these fuels.

Haller et al. (2020) [81] investigated aluminum redox cycles and, in particular, the production of heat and electricity from Al (2 Al + 6 H₂O(l) → 2 Al(OH)₃ + 3 H₂). More than 90% of the theoretical values for heat and hydrogen can be produced on a laboratory scale [53]. Electricity from solar or other renewable sources can convert aluminum oxide or aluminum hydroxide to elementary aluminum (Al₃₊ → Al) via a reduction reaction or charging process. A charging efficiency of 58% (calcination + smelter) can be expected when using new inert electrode smelter processes (carbon-free) instead of the traditional Hall–Héroult process [82,83]. The oxidation reaction (Al → Al₃₊), or discharging process, releases hydrogen and heat. As a byproduct, aluminum hydroxide is obtained at low temperatures, or aluminum oxide is obtained at temperatures above 200 °C [84]. Hydrogen is used in fuel cells to produce electricity. The total energy released amounts to 8.7 kWh/kg of Al if the energy content of the produced H₂ is converted to heat and electricity, where the total heat from aluminum oxidation and the fuel cell process amounts to 6.5 kWh (thermal) [85].

Iron can be produced by the direct reduction of iron oxides (Fe₃O₄ and FeO) with carbon or hydrogen. Although traditional iron-producing industries still use carbon, which is converted to CO₂, and hence contributes to greenhouse gas emissions, great effort is being made to transition to reduction processes that use renewable hydrogen, for example, with the HYBRIT process expecting its demonstration in 2026 [86]. This process uses green hydrogen, which reacts with iron oxide to form water instead of CO₂. The energy consumption of this process is approximately 2.8 MWh/ton of sponge iron and 3.5 MWh/ton of steel (including the electric arc furnace process), leading to an expected P2X storage efficiency of 57% [87,88].

An alternative pathway for producing iron is molten oxide electrolysis between 1600 °C and 2000 °C, which requires an electricity consumption of 3.7 to 4 MWh/ton of Fe [89]. With this technology (P2X storage efficiency of 57%), no additional process steps are needed, and a demonstration plant for such molten oxide electrolysis is planned to be in operation in 2026 [90].

5. Results

This section presents data on storage capacities, storage volume requirements, storage efficiencies, and economic indicators for each large-scale SES option. Figure 5 illustrates the key integration pathways of TES and P2X, which are the focus of this study.

Figure 6 illustrates the storage volume for the same capacity (10,000 MWh thermal for STES and electric and thermal for P2X options), highlighting the importance of energy storage density for long-term, large-scale energy storage options, as mentioned in Section 2.

5.1. Thermal Energy Storage

Figure 7 shows the ACOHS in Euros per MWh thermal for the sensible, latent, and sorption TES systems. In general, seasonal sensible heat storage requires a minimal storage size due to heat losses at the storage boundary or shell (dependent on the storage surface/volume ratio) to be technically and economically feasible.

Phase change storage and thermochemical sorption achieve higher storage densities and possibly lower losses for small storage volumes.

5.1.1. Sensible Heat Storage

An example cost calculation for TTES (assumptions: an above-ground steel tank with a 50,000 m³ capacity and various heat storage media, with the investment costs for a tank being EUR 6 million + heat storage media, with a usable temperature difference of 50 K, a depreciation period of 50 years, annual operating and maintenance costs being 1% of the initial investment, operation with one cycle per year, and an interest rate of 5%, and no costs for charged heat are taken into account) varies from 150 to 800 EUR/MWh thermal. In this case, the cost of the storage medium, i.e., the cost of water, sandstone, or vegetable oil, is the main source of cost variation.

Example cost calculations for PTES, BTES, ATES, and CTES are based on projected investment costs based on completed projects, including only the cost of storage structures, excluding connecting piping and systems engineering, design costs, and VAT [91,92]. It should be noted that the investment cost for ATES is highly dependent on the number of wells, which is not proportional to the specific storage size.

The results show a price difference for sensible TES systems ranging from 1 to 26 times between underground systems and tank storage systems above ground. It is important to emphasize that the applied storage volumes for TTES (50,000 m³ in Berlin, Germany) and UTES (PTES: 500,000 m³ planned for Aalborg, Denmark; BTES: 500,000 m³ in Chifeng, China; ATES: 500,000 m³ in Marlborough, New Zealand; CTES: 260,000 m³ in Mustikkamaa, Finland) systems are based on planned or installed large-scale projects, assuming maximum storage densities [93]. Consequently, this results in UTES system storage capacities that are 4 to 30 times higher than those of TTES systems (

Table A1. Exemplary Annual Cost of Heat Storage (ACOHS) for optimal storage size of TES technologies with one storage cycle per year, excl. heat costs for charging.

TES Type	TTES			UTES			PCM				Sorption
Storage medium / UTES-type	H2O	SS	VO	BTES	PTES	CTES	ATES	ET	XT	S/G	NO	SG
Project-specific parameters
Storage capacity, MWh	2900	1950	1320	20,000	40,000	11,600	20,000	580	460	320	560	500
Storage volume, m3	5 × 104	5 × 104	5 × 104	5 × 105	5 × 105	3 × 105	5 × 105	4 × 103	4 × 103	4 × 103	3× 103	3 × 103
CAPEXstorage, million EUR	6.0	6.2	10.5	5.7	15.0	15.0	5.0	106.5	110.4	124.3	9.3	7.2
Discount rate, %	5
Utilization of usable storage capacity, %	100	100	100	80	100	80	80	100	100	100	100	100
SC per year	1
External parameters
OPEXcharging, EUR/kWh thermal	0
Storage specific parameters
SC Efficiency, %	90	90	90	70	80	80	75	90	90	90	80	80
OPEX, %CAPEX/y	1	1	1	1	0.5	0.5	1	0.1	0.1	0.1	1	1
Economic lifetime	50	50	20	40	25	30	30	20	20	20	20	20
LCOHS, EUR/MWh	149	229	798	35	36	142	31	11,697	21,634	35,433	1880	1624
References	[18,29]	[18,29,47]	[18,19]	[15,18,23,27]	[15,18,23]	[18,23]	[15,18,23]	[9,14,47]	[9,14]	[9,14]	[45,47]

SS: sandstone, VO: vegetable oil, ET: erythritol, XT: sylitol, S: sorbitol, G: glucitol, NO: sodium hydroxide, SG: silica gel.

). In this case, ACOHS estimates for ATES and BTES systems assume suitable soil conditions and hydrogeology.

5.1.2. Latent Heat Storage

If sensible heat TTES systems (temperature swing of ±50 K) are compared to latent heat TES systems (temperature swing of ±10 K), a reduction in storage volume of 1.5 to more than 5 times can be achieved by utilizing a PCM for storage. However, because of the considerably higher storage material prices (8.4 to 29.1 EUR/kg), PCMs for TES systems are unlikely to attain economic viability if implemented for SES (one cycle per year). In this scenario, the energy stored in the PCM will cost more than 16,600 and up to 35,400 EUR/MWh (thermal), even without accounting for additional costs associated with the heat source. Moreover, these cost estimates assume that the degradation of the PCM will not be a significant factor (economic lifetime of 20 years), an assumption that requires further validation for these materials and time periods.

5.1.3. Sorption Heat Storage

For sorption storage technologies, heat and mass transfer to and from the storage volume must be considered to achieve optimal reaction efficiencies. Additionally, lower costs for the storage material (0.5 to 1.3 EUR/kg) can be achieved in comparison to PCM; however, the system setup (container, controlling, etc.) requires significantly higher economic investment [9]. Therefore, if the storage material cost contributes 20% to the CAPEX of this type of TES system, an ACOHS of between 1625 and 1880 EUR/MWh (thermal) can be expected when no additional cost for the heat source is considered.

One major advantage of sorption materials is that approx. eight to ten times higher volumetric storage density can be achieved compared to sensible heat storage materials and two times higher than latent heat storage materials.

5.2. Power-to-X Storage

Figure 8 provides an overview of LCOXS, including CAPEX and OPEX, for converting power to chemically stored energy (production cost for X) and storage for several months.

The LCOXS also included an assumed conversion efficiency of the X-to-Energy discharge process using the corresponding fuel cell technology [94,95]. Therefore, the discharged energy consists of electricity and heat, and the ratio is dependent on the electric efficiency of the fuel cell. However, it should be noted that these costs are not the total fees for a complete system application, e.g., a fuel cell system for all X-to-Energy processes, and system integration was not included. Additionally, specific project costs could vary widely depending on local parameters, such as energy prices, regulations, and approval processes. Example cost calculations show that storing GH₂ in caverns (800,000 m³) or as liquid synthetic gas (10,000 m³) can reduce the LCOXS by approx. 80% compared with high-pressure tank storage options based on a storage volume of 33 m³. The LCOXS for hydrocarbons (methane with a storage volume of 30,000 m³ and methanol with a storage volume of 40,000 m³) depends highly on the CO₂ source. In addition, the LCOXS for NH₃ with a storage volume of 40,000 m³ increases by 16% compared to hydrocarbons produced from a CO₂-rich source; however, a reduction of up to 50% can be expected for NH₃ compared to hydrocarbons that are produced with CO₂ from air capture. ReMEC candidates are similar and even favorable compared to common P2X options: aluminum 180 EUR/MWh (electric and thermal) achieved the lowest LCOXS in this analysis.

5.2.1. Hydrogen, Methane, Methanol, and Ammonia

The overall production costs of alternative fuels, such as green methane, methanol, and ammonia, are, in general, more expensive when compared to the production of hydrogen itself because hydrogen production is at the very beginning of these conversion paths (Figure 3). However, hydrogen at atmospheric pressure has a relatively low volumetric energy density and is therefore unsuitable for energy storage for several months in this state.

For this reason, further process steps such as compression or liquefaction are needed for long-term storage, which adds costs and reduces efficiency for these “direct” hydrogen storage technologies. The hydrogen tank storage (GH₂-tank) data were based on a modular vessel (storage capacity of 40 MWh/vessel configuration at a pressure of 500 bar). In contrast, a cavern pressure of 70 to 150 bar and 90,000 MWh are assumed based on multiple large, solution-mined salt caverns in “domal” salt, which is suitable for the high-pressure storage of GH₂ [62,96].

5.2.2. Renewable Metal Energy Carrier: Aluminum and Iron

Aluminum in bulk material, on the other hand, does not require storage vessels or containers due to its inherent properties and stability. It is non-reactive in its solid form and does not degrade or change its chemical composition under normal atmospheric conditions. It is resistant to corrosion due to the formation of a thin, protective oxide layer on its surface when exposed to air. This natural protection eliminates the need for specialized containment to prevent degradation or contamination [97]. Additionally, aluminum is non-flammable and non-toxic in its solid state, reducing the safety concerns associated with storage. Its high density and structural integrity allow for simple stacking or piling without the risk of material loss or dispersion. These characteristics make it possible to store aluminum in basic warehouses without the need for pressurized tanks, cryogenic systems, or hermetically sealed containers, significantly reducing storage costs and complexity compared with other energy storage materials. Therefore, the specific storage costs are neglectable and not included in this calculation, which is also the case for iron.

6. Discussion

SES has the potential to play a transformative role in decarbonizing industrial processes by addressing the temporal mismatch between renewable energy generation and consumption. However, the inherent limitation of one annual storage cycle results in significantly lower energy discharge per unit capacity compared to short-term storage solutions. This limitation challenges the economic viability of large-scale SES systems, particularly those with high capital expenditure (CAPEX) for storage materials, containers, and infrastructure.

6.1. Thermal Energy Storage

Among the technologies assessed, UTES emerges as a cost-effective option due to the availability of natural materials and the elimination of above-ground storage requirements. Despite these advantages, its low energy storage density necessitates extensive spatial and geological assessments as well as compliance with stringent planning and environmental regulations. For example, the ATES and BTES for high-temperature applications are limited in terms of max. charging/discharging capacity, face regulatory barriers regarding groundwater flow and ground (water) temperature rise, etc., and have thus far been implemented as pilot projects [19].

In contrast, the costs of phase change storage materials are substantially higher. Additionally, the process is much more complex for sorption technology than for sensible heat storage in water or the ground. Consequently, capital expenditure and the ACOHS would need to be reduced by several orders of magnitude to render these more complex technologies economically viable if they are utilized with only one storage cycle per year.

6.2. Power-to-X Storage

Similarly, hydrogen storage in natural caverns offers significantly lower costs than pressurized vessel storage, yet large-scale liquid hydrogen (LH₂) storage remains commercially unfeasible due to the high costs and technical barriers associated with cryogenic conditions.

Alternative storage media, such as green methane, methanol, and ammonia, provide pathways for hydrogen storage with an established infrastructure. However, the additional processing required for their production reduces the overall energy efficiency and increases costs.

Furthermore, green methane and methanol depend on renewable carbon sources, raising questions regarding their net carbon impact. Ammonia, while promising, necessitates robust safety measures due to potential environmental and health risks associated with accidental spillage.

Emerging technologies, such as renewable metal energy carriers (including aluminum and iron), represent promising high-energy-density options for long-term storage. These materials possess the capacity to transfer substantial quantities of renewable energy to winter periods, facilitating applications in process heat, hydrogen production, and electricity generation when utilized in conjunction with fuel cells. The ongoing advancement of Power-to-Solid and Solid-to-Energy methodologies underscores their potential as a complementary solution to existing storage technologies.

6.3. Future of SES Adoption

Looking forward, the widespread adoption of SES technologies will depend on targeted research and pilot projects aimed at improving efficiency, reducing costs, and ensuring long-term sustainability. For TES, key research areas include multi-cycling durability, improved insulation for pit storage, and material advancements, as explored in projects like Efficient [98]. In P2X, research focuses on optimizing ammonia and methanol storage by improving long-term stability, minimizing losses from evaporation or degradation, and enhancing system integration for efficient conversion back to energy. Efforts also aim at reducing energy intensity in synthesis and improving overall efficiency. For hydrogen, ongoing research is focused on improving the feasibility of large-scale storage in salt caverns, optimizing geological conditions, and enhancing the efficiency of hydrogen injection and extraction processes. For ReMEC, advancing CO₂-free metal production technologies is a key focus, along with improving iron powder combustion, aluminum water oxidation reactions, and process optimization to ensure a closed material cycle. Pilot projects and demonstration plants will be crucial in validating these technologies and enabling their large-scale deployment.

Policy and economic incentives play a crucial role in enabling the adoption of SES by addressing investment barriers, regulatory constraints, and market uncertainties. Spatial constraints and permitting challenges, particularly for UTES, could be alleviated through streamlined regulations and dedicated planning zones. Economic incentives such as subsidies, tax credits, and low-interest financing can help offset high upfront costs, while operational support mechanisms, such as reduced electricity tariffs for P2X technologies using off-peak renewable energy, can improve economic feasibility. In addition, seasonal price variations in energy markets will play a crucial role in driving SES economic viability. Carbon pricing also plays a role in increasing the cost of fossil-based alternatives, making clean technologies more competitive. While carbon taxes and Emission Trading System (ETS) are already in place in many countries, their impact on SES adoption remains limited as prices are often too low to strongly incentivize investment. Strengthening carbon pricing policies, along with targeted financial and regulatory support, can help accelerate the deployment of SES as a key enabler of decarbonization and energy system flexibility. By addressing these challenges, SES can become a cornerstone of industrial decarbonization, enabling a more sustainable and resilient energy system.