Thermal Energy Storage Technology Roadmap for Decarbonising Medium-Temperature Heat Processes—A Review

Abstract

This review presents a technology roadmap for Thermal Energy Storage (TES) systems operating in the medium-temperature range of 100–300 °C, a critical window that accounts for approximately 37% of industrial process heat demand in Europe. Decarbonising this segment is essential to meeting climate targets, especially in sectors that are reliant on fossil-fuel-based steam. The study analyses 11 TES technologies, including sensible, latent, and thermochemical systems, covering both mature and emerging solutions. Each technology is evaluated based on technical, environmental, and socio-economic key performance indicators (KPIs), such as energy density (up to 200 kWh/m³), cost per storage capacity (€2–100/kWh), and technological readiness level (TRL). Sensible heat technologies are largely mature and commercially available, while latent heat systems—especially those using nitrate salts—offer promising energy density and cost trade-offs. Thermochemical storage, though less mature, shows potential in high-cycle applications and long-term flexibility. The review highlights practical configurations and integration strategies and identifies pathways for research and deployment. This work offers a comprehensive reference for stakeholders aiming to accelerate industrial decarbonisation through TES, particularly for applications such as drying, evaporation, and low-pressure steam generation.

1. Introduction

The primary energy consumption in Europe in 2022 was 11.4% away from the EU 2030 target [1]; for this reason, one of the strategic goals of the Energy Union is to increase energy efficiency by cutting the overall energy consumption and managing energy in a more effective way. Looking at which sectors in the EU consume the most energy, the transport sector (29%) consumed the most energy in 2021, followed by households (28%) and industry (26%), with services (14%) and agriculture and forestry (3%) being last [1]. Among these sectors, the industrial sector relies heavily on carbon fuel sources, with more than 77% being from a fossil fuel source (gas, oil, coal, and other fossil fuels), see Figure 1. Therefore, to reach net-zero emissions by 2050, according to the EU climate strategy, the industrial sector needs to be heavily decarbonised.

Figure 1. Breakdown of the annual energy demand in European industry by application (left) and process heating demand by temperature level (middle) and energy source (right) (RES = renewable energy sources) [1].

However, the industrial sectors are difficult to decarbonise due to their large diversification. The appropriate future technology mix depends on the industry’s heat, cold, electricity, and temperature needs. In particular, the temperature ranges strongly depend on the industrial sector and the processes within.

Figure 1 displays a breakdown of the process heating demand in the European industry for different fuel sources [1]. The process heating demand from 100 to 200 °C consumes 26% of the total process heating demand, while the heating demand from 200 to 500 °C accounted for 11%. A big part of this process, heat, was provided in the form of steam, the generation of which was based mostly on fossil fuels [2]. Hence, decarbonising steam generation is a critical next step toward industrial decarbonisation [3].

Based on several factors such as site location, temperature levels, and grid prices, the decarbonisation of different industrial sectors will rely on a variety of renewable technologies for the delivery of the process heat (e.g., electrically driven heat pumps, photovoltaics (PV), solar thermal, electrical boilers, biogas, or biomass boilers). In addition, energy efficiency and recovery must be greatly improved in every industrial sector to reach ambitious goals and obtain a sustainable energy supply.

Thermal energy storage (TES) technologies will be an integral part of the path towards industrial decarbonisation, providing a variety of important services for both heating and cooling applications. Some important ones are expected to be the following [4]:

• compensating for the intermittency of variable renewable energy sources and balancing out energy supply and energy consumption,
• stabilising processes by providing additional power on demand, e.g., in cases with needs for high ramping up in a short period,
• enabling waste heat recovery and, thus, energy efficiency,
• peak shaving,
• load shifting.

While for the process to heat up to 100 °C, water tanks (non-pressurised or pressurised) offer a relatively mature, robust, and cost-effective thermal energy storage (TES) solution, the TES technologies available for temperatures above 100 °C are less mature, and their suitability strongly depends on the user and process-specific requirements.

Several reviews and books on TES exist in the literature. Table 1 gives an overview of the most relevant publications which discuss TES technologies.

Table 1. Overview of analysed reviews on TES and their focus.

Publication	Focus	Material Information in the Temperature Range
Steinmann, 2022 [5]	A book providing an overview of medium- and high-temperature TES. Information is organised by technologies rather than the temperature range of the considered TES.	Mineral oil: 0–200 °C Molten salts: HTS: 180–450 °C/Solar Salt: 250–475 °C/Carbonate Salt: 375–700 °C/Liquid sodium: 100–850 °C Solid storage media: Concrete: 0–450 °C/Gravel packed bed: 0–900 °C/Ceramics: 0–1200 °C/CellFlux: 0–600 °C Dual media: Gravel thermal oil: 0–300 °C/Gravel + HTS: 175–475 °C/Gravel molten salt: 250–475 °C pressurised water: 0–175 °C Latent heat storage: NaNO3 + KNO3: 230–550 °C/NaNO3: 0–550 °C/LiF: 0–925 °C
Sarbu et al., 2018 [6]	TES in combination with buildings, focuses on reducing the energy demand of buildings by using solar energy.	Solid–liquid materials for sensible storage: 20–126 °C Solid-state sensible storage materials: 200–1200 °C Melting temperatures for latent heat materials: 29–125 °C Chemical reactions for thermal energy storage: 200–1195 °C PCM for cold storage: −30–100 °C
Zhou et al., 2012 [7]	Latent energy storage for building applications.	Melting temperatures of PCM: 19–30 °C
Sharma et al., 2009 [8]	Focus on latent heat storage systems with applications in buildings.	Temperature ranges for solid–liquid materials for sensible storage: 0–160 °C The freezing point of paraffin: 42–68 °C The melting temperature of paraffin: 6–76 °C Melting temperature of non-paraffin: 8–127 °C Melting temperature of fatty acids: 17–102 °C Melting temperature of salt hydrates: 14–117 °C The melting temperature of metallics: 30–125 °C The melting temperature of eutectics: 15–82 °C
Miró et al., 2016 [9]	Waste heat recovery with TES technologies.	Some TES material proposition from 90 to 250 °C
Agalit et al., 2020 [10]	Usage of industrial waste material as TES.	-
Gutierrez et al., 2016 [11]	Industrial waste or by-products revalorisation.	Potential application temperature of some industrial waste: 20–200 °C
Achkari et al., 2020 [12]	Combination of TES with concentrating solar power (CSP) technology to generate electrical power.	Phase change temperatures of some PCMs: 121–185 °C

This work aims to overview the existing and emerging TES technologies for the crucial temperature range from 100 to 300 °C. A critical comparison between technologies based on their characteristics, strengths, and limitations will be given, and this will provide a meaningful roadmap for the necessary future developments. The main focus is on this temperature range because providing mature solutions at this temperature range will be the next important milestone towards the decarbonisation of the Swiss and European industrial sector, particularly to support the decarbonisation of drying, distillation, evaporation, compression processes, and the production of low-pressure process steam [13].

The recent book by Steinmann (2022) [5] provides a broad and thorough overview with considerable information on TES for medium and high temperatures. Steinmann’s work aims to consolidate state-of-the-art technologies and more recent, future-oriented developments, and provide a roadmap for TES in the specific temperature range of 100 to 300 °C, which has been identified as critical for the industrial sector. An overview of TES technologies in combination with solar heat, focusing on reducing the energy demand of buildings in a temperature range from 0 to 200 °C was published by Sarbu et al. (2018) [6] and for latent TES for building application above 100 °C by Zhou et al. (2012) [7].

Another application of TES is industrial waste heat recovery. In this field, which is much narrower than presented in this review, Miró et al. (2016) [9] review more than 35 case studies containing TES. Other studies consider fusing waste materials from different industries as TES materials, offering new possibilities for a circular industry. Agalit et al. (2020) [10] examine the TES potential for industrial waste material in the Moroccan industry. The material investigated came mostly from the coal and metallurgical industries. These waste materials could be used as TES for temperatures of 1710 to 2700 °C.

Gutierrez et al. (2016) [11] conclude that revalorising industrial processes’ waste or by-products is possible. However, more studies are needed for the industrial deployment of the idea.

Finally, Achkari et al. (2020) [12] focus on combining TES and concentrating solar power technology to generate electrical power. A section on TES for medium- and high-temperature applications gives some material information for PCM from 121 to 185 °C, which is expanded in the current review.

In conclusion, although there are various reviews on the topic of TES, none provide the focus on decarbonisation and the temperature range of 100–300 °C, which is this publication’s emphasis. This current review differs from the mentioned publications, especially in the following areas:

• The present review’s temperature range focuses on 100 to 300 °C and gives an up-to-date overview of materials in this range,
• KPI-based technology comparison of TES technologies in the temperature ranges from 100 to 300 °C,
• Extended material description and specifications of TES technologies (storage media) in the temperature range focus,
• Focus on industrial (not building) applications of TES technologies in a restricted temperature range,
• Description of currently deployed TES technologies and potential technologies to play a role in the future in the form of a roadmap,
• Application examples of the described technologies in the temperature range of interest.

This review provides a roadmap to support researchers, industrial end-users, and policymakers in selecting appropriate TES technologies and identifying key barriers that must be addressed to facilitate a transition toward a decarbonised industrial heat supply. The main challenges tackled in this work include the fragmented knowledge on TES technologies for medium-temperature applications, the limited availability of comparative performance data across different systems, and the lack of guidance on matching technologies to specific industrial needs. The central hypothesis is that, through systematic evaluation using well-defined key performance indicators (KPIs), TES technologies can be effectively assessed and deployed to support decarbonisation in a technically and economically viable way. Accordingly, the scope of this review is restricted to TES systems operating within the 100–300 °C temperature range, which is highly relevant for industrial processes such as drying, distillation, and steam generation. The review critically analyses 11 TES technologies—including sensible, latent, and thermochemical systems—based on their storage media, system configurations, and performance across technical, environmental, and socio-economic KPIs. In doing so, it aims to offer a comprehensive and application-oriented reference for accelerating the deployment of TES in industrial decarbonisation pathways.

2. Scope and Guidance

Given the importance of TES in the decarbonising process with heat and processes in general, the authors intend to provide a review oriented to a technical audience, policymakers, decision-makers, and an investment audience, to support and guide the energy transition toward a fully decarbonised energy sector and to help the community understand how to address the main technical needs, the importance of each technology’s deployment rate, and future trends.

To that end, this study aims to accomplish three main goals:

• Provide a catalogue of TES technologies that can support decarbonisation at 100 to 300 °C.
• Describe which are their important characteristics and current strengths, and limitations.
• Assess their maturity level, i.e., at which Technological Readiness Level (TRL) they currently are, and what challenges need to be addressed to advance them.

This review article is divided into two main blocks. In the first block, Section 1 provides the motivation and goals of the paper and demonstrates its relevance concerning the existing literature. Section 3 defines a set of key performance indicators (KPIs) to assess the technologies described in the current paper, which are listed and defined. In the second block, Section 4 comprehensively describes 11 TES technologies for medium temperature applications (100 to 300 °C) and a Section 4.1 on heat transfer fluids applicable to the TES technologies presented in the section. Section 4.7 provides the operating principles of all relevant TES technologies, and attempts to provide the most important characteristics by quantifying the defined KPIs.

The description of each technology family is divided into four subsections:

(1)

Storage media, where the basic understanding of the storage mechanism is described, where each potential storage medium is reviewed and assessed in the temperature range of interest,

(2)

System configuration, where the technology is classified by the system configuration,

(3)

KPIs, where a collection of KPIs for each technology is presented, and

(4)

Technology overview, where a final technology comparison is provided with advantages, disadvantages, and challenges.

With this structure, the authors intend to provide a technical outlook on the most relevant technologies for industrial process heat decarbonisation. The same structure is followed throughout Section 3 (technology description) to facilitate the understanding and comparison of the different TES technologies’ statuses, strengths, weaknesses, and potential.

In some cases, such as in more mature technologies where the system has already been developed and commercialised for the specific TES technology (e.g., water-based TES), the storage media and system configuration are defined in the same section. However, this is noted both in the text and the subsection.

Finally, an outlook and conclusions section are presented to tie up the concepts shown in the three previous blocks. Section 4 describes and compares the various TES technologies with their most crucial KPIs, and Section 5 summarises the main conclusions and achievements of this review paper.

3. Key Performance Indicators

This section presents the most relevant key performance indicators for analysing TES technologies based on the literature findings and the authors’ knowledge. The analysed KPIs are organised into three categories of interest: technical (KPI_tech), socio-economic (KPI_so-eco), and environmental (KPI_envi).

Each category is connected to a research background, although they all lack a standardised methodological approach to be followed as a reference. Thus, an adapted definition including the most relevant literature definitions for each of the KPIs is listed in Table 2. For more details on the definition of the KPIs, refer to the references provided in the table.

• Technical performance indicators (KPItech) refer to the technical aspects of thermal energy storage technologies. The technical performances investigated through these KPIs cover the main systems of TES with the relative components.
• Environmental indicators (KPIenvi) are commonly used metrics for environmental data management, eco-efficiency measurement, environment target setting, and real-life monitoring. Such KPIs are used to measure, quantify, and evaluate the performance of a system/component/technology regarding the scope, targets, and objectives, which were designed to be achieved during its demonstration and application [14].
• Socio-economic performance indicators (KPIso-eco) are used to measure the social and economic development of the technology. In this context, the Technology Readiness Level (TRL) is the most widely accepted and used index, measuring the state of maturity of the technologies.

Table 2. Definition of key performance indicators (KPIs) that will be used for TES technology comparison. [-] refers to a range of values or non-dimensional units.

Category	Name	Unit	Description	Ref.
Technical performance indicators	Energy storage density	[J/m3]	The energy storage capacity delivered by a TES system is divided by the system volume.	[15,16]
	Power density	[kW/m3]	The maximum thermal power delivered by a component divided by the component volume.	[15,16]
	Limit operational temperature range (ΔTop)	[°C]	The operation temperature range is defined by the temperatures at which the system is designed to operate, also referred to as nominal conditions. This defines the minimum temperature (Tmin) and the maximum temperature (Tmax) at which the material will be maintained during the operation of the system.	[5] Steinmann
	Storage period	[h/d/y]	Targeted storage period.	[17]
	Partial discharge	[-]	Parameter to indicate if partial charge/discharge is possible (yes/no).	[18]
	Storage size range	[m3] Range	The storage volume is the space occupied by a storage system.	[18]
	Round-trip efficiency	[%]	The relationship between the energy delivered to charge the storage and the energy retrieved. It represents how effectively the technology retains and discharges thermal energy once stored. This parameter can be strongly dependent on the system’s working conditions (e.g., daily or seasonal).	[4]
	Durability	[Year]	It refers to the assumed maximum number of cycles during which the storage system can release at least 80% of the designed useful capacity.	[4]
Environmental performance indicators	Safety risk	[-]	It includes any risk related to unsafe conditions that can cause system failure, operation or harm to humans, such as flammability, release of toxic gas, explosion, or thermal runaway.	-
	Environmental risks	[-]	It includes any environmental risk related to environmental hazards, disposal, recycling, pollution, radiation, noise, land-use patterns, work environment, and climate change.	-
Socio-economic performance indicators	Production cost (PC)	[€/kWh]	It refers to the costs incurred when manufacturing a good or providing a service. Production cost includes a variety of expenses, such as labour, raw materials, consumable manufacturing supplies, and general overhead.	[19,20]
	The specific cost of the storage	[€/kWh]	It defines the overall cost of a certain TES normalised by the total amount of energy it can deliver during its expected lifetime.	[20]
	Operation and maintenance cost (O and M)	[€/kWh]	This indicator includes the operating site cost, and planned and unscheduled maintenance.	[20]
	TRL	[-]	Identification of the maturity level of a technology from the first level, characterised by the definition of raw principles of scientific research, to the last level of maturity, in which the technology is immediately replicable within an operational context.	[20]

Table 2. Definition of key performance indicators (KPIs) that will be used for TES technology comparison. [-] refers to a range of values or non-dimensional units.

Category	Name	Unit	Description	Ref.
Technical performance indicators	Energy storage density	[J/m3]	The energy storage capacity delivered by a TES system is divided by the system volume.	[15,16]
	Power density	[kW/m3]	The maximum thermal power delivered by a component divided by the component volume.	[15,16]
	Limit operational temperature range (ΔTop)	[°C]	The operation temperature range is defined by the temperatures at which the system is designed to operate, also referred to as nominal conditions. This defines the minimum temperature (Tmin) and the maximum temperature (Tmax) at which the material will be maintained during the operation of the system.	[5] Steinmann
	Storage period	[h/d/y]	Targeted storage period.	[17]
	Partial discharge	[-]	Parameter to indicate if partial charge/discharge is possible (yes/no).	[18]
	Storage size range	[m3] Range	The storage volume is the space occupied by a storage system.	[18]
	Round-trip efficiency	[%]	The relationship between the energy delivered to charge the storage and the energy retrieved. It represents how effectively the technology retains and discharges thermal energy once stored. This parameter can be strongly dependent on the system’s working conditions (e.g., daily or seasonal).	[4]
	Durability	[Year]	It refers to the assumed maximum number of cycles during which the storage system can release at least 80% of the designed useful capacity.	[4]
Environmental performance indicators	Safety risk	[-]	It includes any risk related to unsafe conditions that can cause system failure, operation or harm to humans, such as flammability, release of toxic gas, explosion, or thermal runaway.	-
	Environmental risks	[-]	It includes any environmental risk related to environmental hazards, disposal, recycling, pollution, radiation, noise, land-use patterns, work environment, and climate change.	-
Socio-economic performance indicators	Production cost (PC)	[€/kWh]	It refers to the costs incurred when manufacturing a good or providing a service. Production cost includes a variety of expenses, such as labour, raw materials, consumable manufacturing supplies, and general overhead.	[19,20]
	The specific cost of the storage	[€/kWh]	It defines the overall cost of a certain TES normalised by the total amount of energy it can deliver during its expected lifetime.	[20]
	Operation and maintenance cost (O and M)	[€/kWh]	This indicator includes the operating site cost, and planned and unscheduled maintenance.	[20]
	TRL	[-]	Identification of the maturity level of a technology from the first level, characterised by the definition of raw principles of scientific research, to the last level of maturity, in which the technology is immediately replicable within an operational context.	[20]

4. TES Technologies in Industrial Cases

4.1. Industrial Cases and TES Integration (100–300 °C)

TES technologies offer flexible, low-carbon solutions across multiple industrial scenarios, from direct steam generation and surplus heat recovery to backup systems and electrification.

Table 3 maps representative use cases to suitable TES technologies, reflecting both technical compatibility and commercial viability. It highlights how different types of TES—sensible, latent, and thermochemical—can be leveraged depending on the process characteristics, storage duration required, and integration constraints. This approach helps identify the most promising storage strategies and supports the selection of fit-for-purpose technologies in real industrial environments.

Table 3. Industrial cases with typical processes and suitable TES Technologies.

Use Case	Typical Process (100–300 °C)	Suitable TES Technologies	Description
1. Process Heating/Cooling	Steam generation for cooking, pasteurising, and cleaning (Food and Beverage, Pharma)	Latent TES (PCMs), Sensible TES (water tanks, thermal oils), Heat pumps + TES	TES supports batch or continuous operations, improves efficiency, and shifts energy demand to off-peak hours.
2. Surplus Heat Recovery	Batch process waste heat reuse (e.g., polymerisation, drying)	Sensible TES (packed beds), Latent TES	Captures and reuses excess thermal energy between cycles or shifts uses across shifts.
	Seasonal site or district heating from industrial waste heat (70–120 °C)	Sensible TES (hot water tanks), Thermochemical TES	Long-duration storage for heating or internal use; supports hybrid industrial and residential systems.
3. Backup Steam Supply	Steam backup when the primary system fails	Steam accumulators, PCM TES, Thermochemical TES	Ensures reliability and resilience without fossil-fuel standby boilers.
4. Electrification/Demand Management	Electric steam systems using a variable renewable supply	Sensible TES with electric boilers, Latent TES with heat pumps, Thermochemical TES	Stores energy during low-price or surplus electricity periods and releases it as steam on demand.

To effectively match TES technologies with industrial needs, it is crucial to evaluate not only temperature compatibility but also each technology’s operational characteristics, such as storage duration, energy density, and integration flexibility. For example, water-based sensible heat systems are well-suited for short-duration process heating, while thermochemical storage presents promising solutions for long-duration backup or surplus heat recovery.

Additionally, the feasibility of TES implementation in these industrial cases is strongly tied to each technology’s level of maturity and cost competitiveness. Section 4.7 compare these aspects, highlighting that while sensible TES is widely deployable today, latent and thermochemical systems still face challenges such as material degradation, reversibility, and scalability. Addressing these limitations will be critical to unlock their full potential in the industrial decarbonisation pathway.

4.2. TES Technologies Overview (100–300 °C):

This section provides an overview of the technologies identified as potentially useful in the temperature range of interest (100 to 300 °C). Figure 2 lists the relevant TES technologies, classified according to their storage mechanism (e.g., sensible, latent, and thermochemical). A total of 11 technologies (in blue) in Figure 2 are reviewed to draw their status and future pathways. The technologies are classified following the conventional TES classification: sensible, latent, and thermochemical storage [6]. Among all the available technologies, the authors only considered the ones with the highest development up to date, or with the highest potential to play a crucial role in the future, in the temperature range from 100 to 300 °C.

Figure 2. Overview of TES materials considered in this review from 100 to 300 °C.

Thus, three sensible heat storage technologies have shown great applicability:

(1)

Solid-state (SS) that groups metals, castables, ceramics, and rocks.

(2)

Water-based TES, which are mainly water-pressurised tanks.

(3)

Thermal oils that are considered synthetic, vegetable, and mineral oils.

In addition, phase change materials are divided into organics and inorganics, where sugar alcohols and polymers are part of the former and salt hydrates and nitrates of the latter. Furthermore, three thermochemical storage (TCS) technologies have been reported to be promising in this temperature range: chemical reactions, solid adsorption, and composite sorbents, which are all based on reversible water hydration/dehydration reactions. A hybrid section is also considered for hybrid systems with sensible/latent systems and sensible/thermochemical.

Given that the system integration is also studied and highly relevant when assessing the technology implementation, an overview of the available options is also described in Figure 3. This will be further complemented in the following sections.

Figure 3. Overview of system implementation of TES technologies considered in this review from 100 to 300 °C.

Different types of fluids are commonly used for storing or retrieving thermal energy in TES systems. TES systems typically use two types of fluids: (1) a fluid to transfer the thermal energy from the heat producer through the pipes to the energy generator or storage, and thus known as heat transfer fluid (HTF), and (2) storage media fluid to store the thermal energy for a certain period before it is used on demand.

In this section, the most relevant HTFs for TES are considered, such as water, mineral oils, synthetic oils, molten salts, molten metals, pressurised water, and air (see Table 4).

Figure 4 displays the energy density as a function of operational temperature for the storage media and HTFs considered in the current review.

Figure 4. Storage media vs. available heat transfer fluids (HTF) in the operational range of the storage media considered in this paper. Note that SHs stands for salt hydrates and WTES for water-based TES. HTFs are shown in blue and storage media in black.

Among all HTFs, water is the most available and the cheapest solution with the highest specific heat. However, the operation of non-pressurised liquid water is limited to 100 °C, reducing its applicability to process heat. Pressurised water tanks are an alternative to this, keeping water in the liquid state above 100 °C. However, it requires high pressure, which adds significantly to system complexity and cost, particularly above approximately 150–160 °C. In addition, oils and other synthetic liquids are commonly used in TES systems (especially CSP plants), given their much wider working temperature range.

Molten salts are widely used as storage media and HTF [21]. As HTFs, they face the challenges of low specific heat capacity and the tendency to solidify in tubes at lower temperatures, which can lead to the blocking of the HTF transport pipes. Molten metals and molten salts have big corrosion issues at high temperatures. This is problematic with air, helium, sCO2 and water/steam because, for thermal oils and organic fluids, no data are available in the literature [22,23]. The use of water/steam as both HTF and working fluid simplifies the system, leading to improved efficiency and increased electricity production.

Table 4. Properties of heat transfer fluids (HTF) that can be implemented with TES [22,23]. Note that cost is the cost of storage media, e.g., cost of pressurising tank is not considered.

HTF	Advantages	Disadvantages	Temperature (°C)	Specific Heat (kJ/kg·K)	Cost ($/kg)	Thermal Conductivity (W/mK)
Molten metals	+ Wide operation range. + Low melting temperature (lower energy consumption) + High boiling point (operate at higher temperatures) + Large thermal conductivity + High allowable heat fluxes. + Low mechanical stresses.	- Big corrosion issues at relatively high temperatures. - High costs when compared with air, water/steam and molten salts. - Impossibility of direct thermal storage for some of the candidates. - Safety issues when taking into consideration the Alkali metal group, especially liquid sodium.	142 to 600	0.24 to 4.16	2 to 1.3	12 to 46 (600 °C)
Mineral oils	+ Thermally stable up to 300 °C. + Low cost for mineral oils.	- Corrosion of piping and container materials. - Low specific heat capacity - Synthetic oils are expensive. - Low flash point.	−50 to 300	2 to 3	0.3	0.1 (300 °C)
Synthetic oils			−50 to 340	2 to 3.6	3 to 5	0.1 (300 °C)
Pressurised water	+ Use of water/steam as both HTF and storage media. + Low cost. + Lower corrosion rates than salts and metals. + High specific heat	- Required pressurised tanks (increased cost). - Limited working temperature range. - Low thermal conductivity.	350 °C	2.1	0	0.08 (600 °C)
Water	+ Use of water/steam as both HTF and storage media. + Low cost. + High specific heat. + Deep knowledge of flowability and viscosity properties. + Lower corrosion rates than salts and metals.	- Limited working range.	0 to 100 °C	4.2	0	0.598 (20 °C)
Molten salts	+ Viscosity similar to water. + Low vapour pressure. + Low cost.	- High corrosion at high temperatures - Unstable above 500 °C. - Low thermal conductivity.	97 to 650 °C	1.3 to 1.6	0.5 to 1.1	0.2 to 0.55 (200–500 °C)
Air	+ High operating temperatures. + Low cost + High efficiencies + Good flow properties.	- High-temperature oxidation. - Low thermal conductivity. - Low heat capacity and heat transfer capabilities	1100 °C	1.12	0	0.06 (600 °C)

Table 4. Properties of heat transfer fluids (HTF) that can be implemented with TES [22,23]. Note that cost is the cost of storage media, e.g., cost of pressurising tank is not considered.

HTF	Advantages	Disadvantages	Temperature (°C)	Specific Heat (kJ/kg·K)	Cost ($/kg)	Thermal Conductivity (W/mK)
Molten metals	+ Wide operation range. + Low melting temperature (lower energy consumption) + High boiling point (operate at higher temperatures) + Large thermal conductivity + High allowable heat fluxes. + Low mechanical stresses.	- Big corrosion issues at relatively high temperatures. - High costs when compared with air, water/steam and molten salts. - Impossibility of direct thermal storage for some of the candidates. - Safety issues when taking into consideration the Alkali metal group, especially liquid sodium.	142 to 600	0.24 to 4.16	2 to 1.3	12 to 46 (600 °C)
Mineral oils	+ Thermally stable up to 300 °C. + Low cost for mineral oils.	- Corrosion of piping and container materials. - Low specific heat capacity - Synthetic oils are expensive. - Low flash point.	−50 to 300	2 to 3	0.3	0.1 (300 °C)
Synthetic oils			−50 to 340	2 to 3.6	3 to 5	0.1 (300 °C)
Pressurised water	+ Use of water/steam as both HTF and storage media. + Low cost. + Lower corrosion rates than salts and metals. + High specific heat	- Required pressurised tanks (increased cost). - Limited working temperature range. - Low thermal conductivity.	350 °C	2.1	0	0.08 (600 °C)
Water	+ Use of water/steam as both HTF and storage media. + Low cost. + High specific heat. + Deep knowledge of flowability and viscosity properties. + Lower corrosion rates than salts and metals.	- Limited working range.	0 to 100 °C	4.2	0	0.598 (20 °C)
Molten salts	+ Viscosity similar to water. + Low vapour pressure. + Low cost.	- High corrosion at high temperatures - Unstable above 500 °C. - Low thermal conductivity.	97 to 650 °C	1.3 to 1.6	0.5 to 1.1	0.2 to 0.55 (200–500 °C)
Air	+ High operating temperatures. + Low cost + High efficiencies + Good flow properties.	- High-temperature oxidation. - Low thermal conductivity. - Low heat capacity and heat transfer capabilities	1100 °C	1.12	0	0.06 (600 °C)

4.3. Sensible Heat Storage

4.3.1. Storage Media and System Configuration

Sensible thermal energy storage systems use the sensible heat of a storage medium. The supply or withdrawal of thermal energy always results in a change in the temperature of the medium. Single storage media or a composition of storage media to improve storage properties can be utilised. For sensible thermal energy storage, the specific heat capacity of the storage medium is particularly relevant. The specific heat capacity describes the energy required in joules to heat one gram of a storage medium by one kelvin. The most commonly used sensible storage medium is water. Water has a high specific heat capacity of 4.18 kJ/kg K [24]. Figure 5 shows an overview of the sensible heat storage systems discussed in the following section.

Figure 5. Overview of sensible heat storage.

Sensible storage materials are mostly used due to their low cost, simplicity of implementation, good thermal conductivity (in some cases), low environmental impact and availability. Different types of storage materials are used. Concrete, castable ceramics, and metals have been applied as solid storage materials. As liquid storage media, water is most commonly used in the form of pressurised tanks and/or in the form of steam, typically in temperatures up to 175 °C. For higher temperatures, thermal oils are available. The disadvantages of thermal oils are the environmental impact, safety risks and high costs. The media can act as a pure storage material or, at the same time, as a heat transfer fluid (HTF) when withdrawn directly from the storage tank. In the case of pure storage materials, heat is transferred directly between the HTF and the storage material or through an auxiliary system like a heat exchanger. If the HTF is in direct contact with the storage material, the possibility of chemical or physical interactions between the HTF and the storage material must be considered, as this has a major influence on the longevity of the storage system. The main challenge of single media storage systems is achieving thermal stratification in the tank. A small thermocline is achieved by selectively introducing the HTF into the tank or by adding a filler [25].

4.3.2. Water-Based Systems

In water-based systems, water is utilised as a storage medium and as a heat transfer fluid (HTF). The sensible heat of the water is used to store energy. Water is a convincing storage medium due to its high specific heat capacity, availability and environmental friendliness. For low-temperature heat storage systems, liquid water at an ambient pressure is preferred. For temperatures above 100 °C, the saturation pressure of water must be considered, resulting in a pressurised volume. Either liquid water or steam is used as the working medium during discharge. With liquid water as the working medium, the entire process system is pressurised. If steam is to be used as the process medium, a flash evaporator is integrated in which the pressure is reduced, and consequently, the water is evaporated abruptly.

Water-based systems in the form of steam accumulators are the oldest, most widely used and best-researched storage systems at temperatures between 120 and 175 °C. The typical storage units are in the lower MW range with cycle times of less than one hour. Steam accumulators are often used as buffer storage with rapid transients, according to Steinmann et al. [5]. In the proposed temperature range, the container costs of the storage systems are dominant because of the logarithmic relationship between boiling temperature and saturation pressure. This results in high pressures prevailing in the tank from temperatures above 120 °C, and the tank must therefore be designed in a specific way. Danehkar and Yousefi [26] provide a comprehensive overview of water-based energy storage systems for solar applications.

As shown in Figure 6, water-based (steam accumulator) systems can be implemented as sliding pressure storage (Ruths Storage), an expansion storage accumulator, or a displacement storage accumulator [5].

Figure 6. Schematic representation of the (a) sliding pressure (Ruths storage) accumulator, (b) expansion storage accumulator, and (c) displacement storage accumulator (Adapted from Steinmann, 2022 [5]).

• Sliding pressure storage (Ruths Storage)

In a sliding pressure accumulator, the change in sensible heat is used to generate saturated steam internally. A major part of the tank is filled with water in a charged state. Above the water volume resides a cushion of saturated steam. During the withdrawal of steam from the top of the tank, the pressure in the storage system decreases, causing water to evaporate. This process is called flash evaporation. The tanks are usually placed in a horizontal position to provide a high surface area for evaporation. During charging, saturated steam is injected into the water volume. Convection pipes inside the vessel ensure a defined recirculation of the injected steam [2].

• Expansion storage

In the expansion storage, the pressure vessel is almost filled with water when fully charged. While discharging, water is withdrawn from the bottom of the tank. This allows the usage of saturated water as a process stream. The pressure inside the tank drops to within the range of 30% when fully discharged. By adding a flash evaporator outside the tank, the saturated liquid, saturated steam and liquid water are separated. The storage system is refilled by the injection of saturated or superheated steam at the bottom of the vessel.

• Displacement storage

In displacement storage systems, hot and cold water is stored in the same vertical vessel. The storage is always filled with water at constant pressure. While hot water is withdrawn from the top, cold water is injected simultaneously at the bottom. A thermocline is formed between the hot and cold volumes inside the tank. This storage concept is preferred for frequent charge/discharge cycles, due to temperature equalisation between the layers. The generation of process steam is made possible by adding a flash evaporator. The research focus is on combined systems, to overcome the weaknesses of steam accumulators. To allow a more stable operation and to increase the storage capacity, the integration of PCM plates in the storage vessel has been proposed [27]. The combination of steam accumulators with concrete accumulators allows for a reduction in losses in the startup process of combined cycle power plants [28]. In addition, new, larger storage systems are being built, where challenges in mechanical integrity arise from the increased pressures [29].

Despite their maturity and widespread use, water-based TES systems are limited by the pressure and temperature constraints of liquid water, particularly above 150 °C. The design and certification requirements for pressurised vessels add significant costs and complexity. Future work could focus on improving insulation, exploring low-cost tank materials, or integrating modular designs to enhance flexibility and reduce capital costs.

4.3.3. Solid-State TES (SSTES)

Solid storage materials are easily accessible, have a low environmental impact and can be operated over a wider temperature range compared to liquid storage media. Additionally, tank leakages are not present, and corrosion is lower [30]. The low investment costs make it particularly interesting for the industry. However, the heat transfer from a solid to a liquid is generally more complex than the direct usage of liquid storage media. Temperature gradients inside the storage become a problem due to the expansion of the storage material. The solid storage media is used commercially, mainly in Cowper furnaces, where it is used to preheat air in the steelmaking process. An additional application can be found in concentrated solar power plants as a storage medium, as well as a filler material to reduce the molten salt cost. The material is attractive where gaseous HTF are necessary for medium- or high-temperature applications [5], and also for molten salt storage as filler materials.

In packed or stacked configurations, the heat capacity of a loose storage material is used to store or release heat on demand. Inexpensive storage materials are inserted in the tank, such as waste products from industry or various rocks. Since the materials are thermally stable, very high storage temperatures can be achieved. The thermal stratification inside the storage tank is an important indicator. It is influenced by the geometry of the packing element, operating conditions (charging period), and physical properties of the HTF [31]. To charge or discharge the storage media, an HTF must be used, which is in direct contact with the storage material.

Recent studies have raised concerns about material behaviour under long-term cycling. Specific issues include the unpredictability of thermomechanical stresses, abrasion from irregular particle shapes, and potential dust formation that may affect system performance and air quality.

The solid-state technologies can be implemented in stacked bricks and packed beds (Figure 7).

Figure 7. Schematic representation of (a) a stacked bricks storage system, (b) a packed bed storage system [32] and (c) an HEX embedded system.

• Stacked Bricks/Plates

In stacked systems, concrete bricks or steel plates are used as storage materials. The materials are placed in a separate container. The storage tank is charged or discharged by an HTF flowing around the storage material. The HTF used must be compatible with the storage material. A common approach is to use air as the HTF [33]. By introducing an additional heat exchanger, the HTF is then charged or discharged by a process heat flow. The process flow is consequently decoupled from the thermal storage. The main research focus in piled storage systems is on the field of potential storage materials and the cost reduction in these materials. Additionally, the long-term material stability is unknown, since storage setups, for example, the CellFlux system from DLR, have been operated at pilot-scale [5].

• Packed Beds

In packed beds, gravel, rock, bauxite rubble, silica sand, basaltic pebbles, or industrial waste materials are used as storage material. The HTF is chosen according to compatibility with the storage material. The process heat flow is in direct contact with the storage medium or is charged or discharged via an HTF with an additional heat exchanger. The challenge with loosely packed storage is the sizing of the storage vessel. This is considerably stressed by the expansion and contraction of the storage medium [5]. The focus of the research and development of packed bed heat storage systems has been mainly on theoretical analysis of transient response and materials research, and experience with long-term operation of large-scale systems has been limited.

The use of unprocessed materials introduces uncertainties in the representative geometry and thermal properties of the particles that complicate the design process. For irregularly shaped particles, the thermomechanical forces in the storage volume and between the storage inventory and the containment are difficult to predict. The extent of abrasion resulting from these forces in large storage systems during service life is not known; dust could clog flow channels in the storage inventory, altering operational performance. Gases exiting the storage system could be contaminated by particulates, and filtration could be necessary to protect downstream components.

• Embedded HEX

To avoid the disadvantages of direct contact with the storage medium, it is possible to embed a heat exchanger in the storage medium. The heat exchanger can be covered with sand or gravel or even embedded in concrete. The HTF is run through the HEX and thus loads or unloads the storage material. The geometry of the heat exchanger must be defined based on heat transfer power, targeted storage capacity, and the type of storage medium. Various channel and fin geometries have already been studied in detail in the literature [5,34].

Solid-state systems are cost-effective and durable but suffer from low heat transfer rates and thermal gradients that can lead to mechanical stress and degradation. The focus of the research and development of packed bed heat storage systems has been mainly on theoretical analysis of transient response and materials research, where filtration could be necessary to protect downstream components. Future developments should explore innovative heat exchanger designs and improved thermal contact materials to mitigate these issues.

4.3.4. Thermal Oil-Based TES

Thermal oil storage systems use the sensible heat of oil to store and provide heat. Synthetic, mineral, or vegetable oils are available. The properties of these fluids are well-documented, and some have been used in industry for decades.

Thermal oil storage systems have been used in a variety of applications in the past. One of the largest systems was the SEGS1, which was used in a two-tank design with tank sizes of almost 4500 m³ each. The system was used in a solar thermal power plant to bridge peak loads. Today, storage systems using thermal oils are less common due to increased safety risks, environmental impact, and the high price of synthetic oils. Only mineral oils are currently on the upswing, as the environmental impact and price are lower [5]. As storage systems are largely avoided due to environmental influences and the flammability of thermal oils, the current research focus is on vegetable oils. Vegetable oils have a medium environmental impact. However, the temperature range in which they can be used is limited.

Figure 8 shows how thermal oils can be implemented in either two-tank or thermocline systems.

Figure 8. (Left) Schematic representation of a two-tank thermal oil storage system; (Right) Schematic representation of a single-tank thermal oil storage system.

• Two-Tank System

Two-tank systems consist of a hot tank and a cold tank. When the system is fully charged, the hot tank is filled with thermal oil, and the cold tank is empty. While discharging, the thermal oil is transferred from the hot tank to the cold tank through a heat exchanger. The oil transfers the stored heat to the process’ heat demand through the heat exchanger. The system is completely discharged once the hot tank is empty and the cold tank is filled. The process is reversed for charging the storage.

• Thermocline System

A thermocline system consists of a single tank with connections at the top and bottom of the tank. Inside the tank, in a perfect thermocline, there is a cold side and a hot side. In a real system, stratification occurs with a temperature gradient between the hot and cold sides. When the system is completely charged, the entire tank is filled with hot oil. During discharging, the hot oil is extracted from the top of the tank, passed through a heat exchanger, and the resulting cold oil is fed into the bottom of the tank. The stored heat is transferred to the process heat demand by the heat exchanger. To charge the system, the process is reversed, and the cold oil is taken from the bottom of the tank, heated at the heat exchanger, and fed to the top of the tank.

Thermal oil systems are well-established, but face safety and environmental concerns due to flammability and potential chemical degradation. Additionally, their thermal performance is limited compared to other media, and the cost of synthetic oils remains a barrier. Research into biodegradable and less hazardous alternatives or extended oil lifetimes would support broader industrial adoption.

4.3.5. KPIs Sensible Storage Technologies

Sensible TES technologies, including solid materials, thermal oils, and water-based systems, offer diverse operating characteristics and trade-offs across industrial applications. While generally mature, each technology type presents unique limitations that must be addressed for broader implementation. Table 4 presents a comparative overview of key performance indicators (KPIs) for sensible heat storage technologies operating in the 100–300 °C range.

Vegetable and mineral oils offer moderate energy density and ease of handling, but flammability, environmental concerns, and limited temperature ranges restrict their use. Solid materials like graphite and castables achieve high energy density and low environmental impact, yet long-term mechanical integrity and thermal stresses require further study. Water-based TES is widely used for short-duration applications and provides safe, low-cost operation, but demands pressure-certified vessels and faces temperature constraints.

Overall, sensible TES systems are cost-competitive and technically viable for near-term industrial decarbonisation. Future research should target material durability, cost reduction for thermal oils, and optimised system integration, particularly for packed and stacked solid systems where mechanical stresses and flow behaviour introduce design challenges. The KPI values presented are based on the literature data and experimental studies. Where ranges are shown, they reflect variability due to material selection, system configuration, and operational conditions.

4.3.6. Technology Overview

This section provides a summary-level comparison of sensible heat storage technologies across three primary groups: thermal oil, water-based, and packed or stacked solid materials. Table 5 synthesises their operational characteristics, highlighting the typical storage configurations, suitability for various temperature ranges, and the benefits and limitations associated with each type.

Table 5. KPI of water, packed/stacked solid and thermal oil-based storage systems. When not indicated, data adapted from [5].

KPI	Thermal Oils			Solid-State					Water-Based TES
	Vegetable Oil	Synthetic Oil	Mineral Oil	Graphite	Castable	Ceramics	Rocks	Metals
Thermal Energy Storage Density (kWh/m3)	50.1 (ΔT = 100 K)	50 (ΔT = 100 K)	56.4 (ΔT = 100 K)	48.6 (ΔT = 100 K)	69.8–83.6 (ΔT = 100 K)	34.8–119.7 (ΔT = 100 K)	45.8–201 (ΔT = 100 K)	69–110 (ΔT = 100 K)	58.3 (ΔT = 50 K)
Power Density (kW/m3)	-	-	-	-	-	-	-	-	-
Operational Temperature Range (°C)	0–200	15–400	−10–315	Up to 3500	Up to 1000	Up to 2830	Up to 800	Up to 1200	0–220
Storage Period (h/d/y)	h/d	h/d	h/d	h/d	h/d	h/d	h/d	h/d	h/d
Flexibility	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes
Storage Size Range (m3)	-	-	-	28 [35]–6300 [36]	-	-	-	-	Up to 5000
Capacity (Wh/MWh)	Up to 25 kWh	Up to 25 kWh	Up to 30 kWh	-	-	Up to 25 MWh	Up to 25 MWh	Up to 30 MWh	Up to 300 MWh
Round Trip Efficiency (%)	-	-	-	-	-	-	-	-	-
Durability (Years)	10–60	10–60	10–60	10–60	10–60	10–60	10–60	10–60	10–60
Safety Risk	High	High	High	Low	Low	Low	Low	Low	Certified
Environmental Risk	Medium	Medium	Medium	Low	Low	Low	Low	Low	Low
Specific Cost (€/kWh)	3.2–19.6	75	37.2	77–257	2–8.4	9.8–50	1.1–2.6	20–78	Varies
Technology Readiness Level (TRL)	9	9	9	9	7	9	7	7	9

Water-based systems are commonly applied in displacement or sliding-pressure configurations for short-term backup or process heat, benefiting from safety and low environmental risk, see Table 6. Thermal oil systems offer a broader operating temperature range but are limited by safety considerations and end-of-life disposal challenges. Solid material storage, whether packed or stacked, demonstrates strong potential for high-temperature and cyclic applications, though material stress and installation complexity remain areas of concern.

Table 6. Technology overview of water, packed/stacked solid, and thermal oil-based storage systems [5].

Technology	Application	Pros	Cons	Research Focus
Thermal Oil—Thermocline	Short–medium term	+ Low cost + Uses unprocessed materials	- Exergy loss between hot/cold zones - Not suitable for long-term storage - Environmental and safety concerns with oils	Focus on alternative oils (e.g., vegetable oils); limited operating temperature.
Thermal Oil—Two-Tank	Short–long term	+ Stable discharge temperature + Flexible heat control + Negligible efficiency loss	- Higher investment cost due to the second tank	Widely adopted, mature design
Solid Storage—Packed Bed	Limited cycle temp. differences	+ Low cost + Eco-friendly + Common materials	- Thermal stress and degradation - Irregular particle geometry affects design - Potential dust and flow issues	Material selection, long-term stability, and system design refinement
Solid Storage—Stacked	High temp. cycles, frequent use	+ Regular geometry + Known thermal properties + Lower stress levels	- Higher cost - Complex piping	Cost reduction, long-term stability, pilot testing (e.g., DLR CellFlux)
Water-Based—Displacement	Frequent cycles, low pressure	+ Constant discharge pressure	- The entire piping system is pressurised	PCM integration and hybrid systems to improve capacity and stability
Water-Based—Expansion	High-pressure applications	+ Small temperature gradient	- Needs flash evaporator - Pressure drop (~30%)	Larger systems, mechanical integrity at higher pressures
Water-Based—Sliding Pressure	Combined systems with HX integration	+ Flexible discharge control + Can supply superheated steam	- Pressure drops during discharge	HX-integrated designs; optimising discharge control

4.4. Latent Heat Storage

Latent heat storage technologies absorb/release energy when the phase change in the storage material occurs at a fixed temperature (phase change temperature) or within a small temperature range. The phase change can be solid–solid, solid–viscous, solid–liquid, liquid–gas, and solid–gas, with the first three being the most studied. Phase Change Materials (PCM) are classified as organic (1), inorganic (2), and eutectics (3), which are mixtures of the abovementioned.

4.4.1. Storage Media

The ideal phase change material (PCM) for applications in the temperature range of 100 to 300 °C requires a phase change temperature within the given range and a high latent heat of fusion (∆H_m). Organic PCMs in the temperature range of 100 to 300 °C present typical challenges such as thermal decomposition, high vapour pressure and reactions with oxygen. In contrast, many inorganic salt mixtures and several single salts with a melting temperature in the range of 100 to 300 °C are known. This fact is due to the large number of salts with different cations and anions, as well as the good miscibility of many salts [37]. Suitable low- to medium-temperature PCM (up to 200 °C) are kinds of paraffin, polyalcohol, low-melting metallics, nonparaffin organics (e.g., fatty acids), and salt hydrates. On the high end, from 200 to 300 °C, few sugar alcohols are above 200 °C (myo-inositol, pentaerythritol, and trehalose), the rest are below 200 °C (erythritol, mannitol, sorbitol, xylitol, maltitol, lactitol, and many more), where anhydrous salts are the most promising options (e.g., nitrate salts). Thus, in the temperature range from 100 to 300 °C, the most relevant PCMs are salt hydrates, polymers, sugar alcohols, and anhydrous salts.

Figure 9 illustrates the melting temperature range of different types of PCMs as a function of the heat of fusion. According to their melting temperatures, PCMs can be classified as low temperature (<120 °C), medium temperature (120 to 300 °C), and high temperature (>300 °C). In terms of chemical composition, PCMs can be broadly divided into organics and inorganics.

Figure 9. Melting temperature and fusion heat ranges for different types of phase change materials. Data collected from [38,39].

4.4.2. Salt Hydrates

Salt hydrates consist of inorganic salts and water in the form of a crystalline solid with the general formula (AB.nH₂O), where n corresponds to the number of water molecules tightly bound in the hydrate crystal structure. The phase change process in salt hydrates is associated with the dissolution of the salt in its own water of hydration, occurring at a precise temperature. They have high latent heat enthalpies and high thermal conductivity due to their high density and cover a wide melting temperature range of 5 to 130 °C [40]. Moreover, they are not flammable and are biodegradable and recyclable. On the low-temperature range, they are on the cheaper end compared to PCM, with an average material cost ranging from 0.13 to 0.46 $/kg [39].

However, salt hydrates can be subject to supercooling and phase separation, and cause corrosion of metals used as containment materials. Leakage of salt hydrate PCMs is an important issue after successive solid–liquid transitions, which result in poor reliability of this system for its practical applications [41]. Great attention has therefore been paid to preparing composite PCMs with porous supporting materials to deal with this problem [41,42]. In the temperature of interest in this review, there are four main salt hydrates; see Table 7. Among them, oxalic acid dehydrate (OCD) has been well-known as a potential PCM for thermal energy storage, owing to its high heat storage capacity, non-toxicity, and low investment cost [43]. However, OCD itself has a phase transition temperature of 101.5 °C, and when the heat is stored and the OCD is directly heated and melted in an airtight device, the vapour pressure of water is rapidly increased and can result in the explosion of the device [43]. This can be avoided by mixing OCD with small concentrations of fatty acids mixtures and alkenes mixtures that do not greatly affect the heat of fusion but reduce the melting temperature to 90 °C [44]. One of the most studied medium-temperature PCMs is magnesium chloride hexahydrate, due to its high volumetric energy density, relatively high thermal conductivity, and low cost, while being non-corrosive and non-toxic [45,46,47,48]. It can be used as a single PCM or can be combined with organic and inorganic materials to tackle segregation and supercooling issues, providing a new type of PCM [48].

Table 7. List of salt hydrate properties in the temperature range from 100 to 300 °C.

Compound	Melting Temperature (°C)	Heat of Fusion (kJ/kg)	Thermal Conductivity (W/m.K)	Density (kg/m3)	Cost (€/m3)	Ref.
Oxalic acid dehydrate (OCD)	105	264	0.9	1653	339	[49]
CaBr2 4H2O	110	115 to 138	-	2194 to 1956	-	[50]
Al2(SO4)3 16H2O	112	-	-	-	-	[50]
MgCl2 6H2O	117	168	0.5 to 0.7	1440 to 1570	56	[50]
MgNO3 2H2O	130	-	-	-	131	[50,51]

4.4.3. Polymers

When polymers are heated above their melting temperature (Tm), they experience a solid–solid transition. This means that they change from a solid state to exhibiting characteristics similar to a viscous liquid, allowing them to flow. Such transitions can be utilised to store latent heat with high heat of fusion, different suitable melting temperature ranges, cost-effectiveness, easily adaptable properties via compounding, and the potential use of recycled polymers.

The following groups of semi-crystalline polymers have been identified as potential PCM: polyethylene (PE), polypropylene (PP), polyoxymethylene (POM), polyamides (PA), and their recycles [52]: see Figure 10. The polymer types of polyethylene glycol (PEG), PE, PP, and POM have rather low melting temperature ranges (53–188 °C) while exhibiting the highest latent heat (93 to 239 J/g). Whereas, polyamides, polyesters polyhydroxy buyrate (PHB), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), and polyethylene terephthalate (PET) have higher melting temperature ranges (115–320 °C), but significantly lower latent heat (15 to 111 J/g).

Figure 10. Overview of the melting behaviour of the examined semi-crystalline polymers [52].

According to the state-of-the-art technologies, polymeric materials have mostly been used as a form-stabiliser additive to PCM [53,54,55,56] with some exceptions as purely PCM storage: polyethylene glycol (PEG) [57,58,59,60,61,62,63,64,65,66] and polyethylene (high and low density) [67,68,69]. PEG has standing characteristics of non-toxicity, relatively high latent heat storage capacity, wide selectivity of molecular weight, low vapour pressure when melted, and good thermal and chemical stability [64].

The main disadvantages of PEG are its low commercial availability, low thermal conductivity, phase instability in the melting state, and a weak interfacial combination with supporting materials [64]. The latent heat and melting temperature of PEG can be varied with molecular weight. While enthalpy and melting temperatures roughly increase with increasing molecular weight, thermal conductivity does not show significant changes [65]. In addition, the density and viscosity increase with higher molecular weight, but the chemical properties remain identical [66].

HDPE can also be used as a polymeric phase change material [67,69,70,71,72,73]. It is toxicologically safe, commercially available on a large scale, relatively cost-effective, and can originate from recycled materials, which makes it an interesting PCM. HDPE has a melting temperature (solid–viscous transition) of ~135 °C, a decent specific heat of ~2 kJ/(g K), and a decent latent heat of fusion of ~178 kJ/kg [63]. Among all the semi-crystalline polymeric materials that can be used as a PCM [45], HDPE is the one that shows the highest latent heat of fusion at temperatures lower than 150 °C.

Unlike other PCMs, polymeric materials are not susceptible to phase segregation, although they are sensitive to degradation upon thermal and thermo-oxidative cycles. Unlike other PCMs, polymeric materials are not prone to phase segregation, which enhances their cycling stability. However, they are susceptible to degradation under prolonged thermal exposure and thermo-oxidative conditions. This degradation can lead to reduced latent heat storage capacity, changes in melting temperature, and structural instability over time. Factors such as oxidation, chain scission, or cross-linking can occur, particularly above 200 °C, affecting long-term performance. Therefore, careful material selection, potential stabiliser integration, and further studies on cycling durability are essential for their industrial deployment in TES systems. Even though just a few polymers have been studied as phase change materials, Figure 10 shows that there are interesting alternatives like POM (Polyoxymethylene) that should be further studied. Weingrill et al. [6] selected POM for application-oriented stability; however, the polymer did not show to be stable up to 250 °C.

Overall, polymeric materials such as PCM have great potential because of their relatively high latent heat in a wide temperature range. In addition, in their working temperature range, they offer a safe, cost-effective, and stable option that does not require containment for their implementation. However, different exposure and ageing phenomena (i.e., melt movability, surface cracking), above the polymer’s melting temperature, complicate the lifetime prediction of polymeric PCM. Further investigations on their stability issues and additional stabilisation against thermal and thermo-oxidative degradation are necessary to boost their implementation.

4.4.4. Sugar Alcohols

Sugar alcohols (SA), also called hydrogenated carbohydrates and polyols, belong to the family of low molecular weight carbohydrates [74]. Sugar alcohols are a promising candidate among possible PCM in the considered temperature range, as their T_m lies typically in the range between 100 °C and 230 °C, which is higher than the typical applications considered: thermal comfort in workspaces or domestic hot water [75]. In addition, they exhibit a high latent heat of fusion, up to 350 kJ/kg (similar to water), and they are safe to handle, i.e., they are non-toxic, and some of them are relatively cheap.

However, sugar alcohols have not been extensively implemented in TES applications in the temperature range of 100 to 300 °C, mainly due to their supercooling, low crystallisation rates, polymorphism, and degradation challenges. Sugar alcohols form a syrup-like, viscous liquid when they are melted [74]. This impedes the diffusion of molecules from the liquid to the nucleus, which is the origin of crystallisation events and thus contributes to the high supercooling degree observed. In addition, the diffusion of molecules from the melt to the nucleus is made difficult by intermolecular hydrogen bonds [74], thus retarding the crystallisation process and resulting in a large hysteresis between T_m and T_s of sugar alcohols. In addition, one interesting feature of sugar alcohols is the possibility to tune the difference between T_m and T_s of a sugar alcohol, or an eutectic mixture of them, by adding sparingly soluble inorganic salts to the PCM material [74].

Within the group of sugar alcohols, most material research (sample size < 0.01 kg) has been carried out on D-mannitol ([76,77,78,79,80,81] and Erythritol ([79,82,83] which were pointed out as the most promising PCM candidates among sugar alcohols due to their thermophysical properties. Furthermore, storage studies at higher storage volumes were realised by Gil et al. [76] using 160 kg of D-mannitol, by Wang et al. [79] using 27 kg of erythritol, and by Agyenim et al. [80] using 20 kg of the PCM. The properties of d-mannitol and erythritol are summarised in more detail in the paragraphs below, as these two seem to be most relevant for application in latent heat TES.

Table 8 and Figure 11 give a summary of the thermophysical properties of the investigated sugar alcohols, together with corresponding references. Different studies were able to show that the mixture of two or more sugar alcohols can result in a higher cycling stability performance than that of the individual components [75,84] (see Table 8).

Table 8. Thermophysical properties of the most relevant sugar alcohols reported in the literature, where H_m is the enthalpy of fusion, T_D is the degradation temperature, T_m is the melting temperature, and T_m-T_s is the maximum supercooling reported.

Sugar Alcohol	Phase	Tm [°C]	Tm-Ts [K]	TD [°C]	Hm [kJ/kg]	Cycling Stability	Price (€/kg)	Ref.
Erythritol		117–119	51–69	160	339–344	yes	4.1–6.2	[82,83,84]
D-Mannitol	β δ	150–167	30	280	192–316	Phase-dependent	1.4–1.5 [83]	[78,85,86,87,88]
Myo-inositol		216–224	≈20		200–260	Yes (20% decrease in Hm After 50 cycles)	-	[78,84,89]
Galactitol/Dulcitol		180 187	57–79	293–295	250–400	no	1.31 [83]	[75,78,81]
Maltitol		149	n.a	n.a	173	No, forms glass	1.4 [83]	[90,91]
Galactitol + Mannitol (30:70)		154	49 to 86	313	282	yes	-	[75]
Inositol + Dulcitol + Mannitol (21:24:55)		150	n.a	n.a	222	Investigated by const. T-kinetics: low degeneration	-	[89]

Figure 11. Melting temperature and melting enthalpy of sugar alcohols [85].

D-mannitol has a high enthalpy of fusion, 240 kJ/kg, and a melting temperature of around 150 °C. However, it suffers from severe polymorphism, which means that it can exist in more than one crystalline phase, and forms different polymorphic phases, depending on the experimental conditions (e.g., cooling rate) and can, for example, have different values of T_m (β and δ forms of D-mannitol).

In addition, D-mannitol oxidises during repeated heating and cooling cycles under air conditions and results in a non-stable material with decreased phase change enthalpy (240 kJ/kg to 100 kJ/kg after 20 cycles) and melting temperature (150 °C to 130 °C after 50 cycles) [78]. Interestingly, Gil et al. [76] found a stable PCM behaviour when the working temperature was between 135 °C and 175 °C, as T_m of all possible polymorphic phases of d-mannitol is included in this interval (therefore, the use of the latent heat is possible). In summary, d-mannitol can be considered as a PCM candidate for latent thermal storage if the polymorphic transitions and stability issues can be controlled.

Erythritol has been investigated by different authors [80,83,89]. It has a melting temperature of 117 °C and a decomposition temperature of 160 °C. Shukla et al. [83] investigated the degradation of erythritol with 1000 cycles of melting and solidification, and the thermophysical properties of erythritol remained reasonably stable. Overall, erythritol seems to be the most promising sugar alcohol candidate for PCM application due to its high cycling stability, and it can be found in some applications already: the PCM was used in a bench-scale waste-heat transportation system [82] and in solar cookers [92]. However, the supercooling phenomena makes the use of erythritol in classical latent heat storage systems difficult. Other sugar alcohols, like inositol and sorbitol, show promising values; however, very few experimental data are available for their thermophysical properties.

Overall, sugar alcohols are promising PCM in the temperature range of interest as they provide the highest latent heat storage of all the candidates considered. However, they still present important challenges that need to be overcome in material formulation, such as supercooling, stability and polymorphism. The supercooling degree can be relatively high for erythritol, mannitol, inositol, and dulcitol. However, it can be reduced by using passive nucleation-triggering techniques. On the other hand, xylitol and sorbitol have a very stable supercooling due to their resistance to crystallisation, and in this case, active nucleation triggering techniques should be implemented (e.g., airlift reactors, stirring, mechanical and bubble agitation, etc.). Sugar alcohols have been mainly implemented for solar cookers [93], mobilised thermal energy storage [94], and waste heat recovery [82]. The European project SAM.SSA aimed to develop new sugar alcohol-based materials for solar seasonal storage, targeted to the building sector with domestic solar thermal systems and cogeneration devices. The potential of implementation for industrial waste heat recovery is still to be unlocked, especially for the most promising sugar alcohols: erythritol, d-mannitol and inositol.

4.4.5. Anhydrous Salts

In the molten salts family, the cations are mainly alkali (e.g., Li, Na, K) and alkaline earth (e.g., Ca, Mg) metals, whereas the anions include nitrates, nitrites, hydroxides, bromides, carbonates, chlorides, sulphates, and fluorides. Many anhydrous salts are miscible, and this results in a large variety of potential single salts and salt mixtures (binary and ternary systems) [95]. The available candidates in the temperature range from 100 to 300 °C are shown in Table 9. For temperatures between 200 and 300 °C, in particular, alkali metal nitrates and nitrites and their mixtures are suitable PCMs, if requirements such as salt handling and material compatibility are taken into account [95].

Nitrate salts are the most used solid–liquid PCMs for medium-temperature composite phase change materials (CPCMs) with technical maturity [96]. Most formulations use pure sodium nitrate (NaNO₃), pure potassium nitrate (KNO₃) or NaNO₃-KNO₃ mixtures with phase change temperatures of about 300–306 °C, 330–334 °C and 220–230 °C, respectively (see Table 9). Regarding the latent heat of fusion, NaNO₃ is preferred with a value of about 182 J/g, while that of KNO₃ is only about 95 J/g. For applications at lower temperatures, some CPCM formulations contain lithium nitrate (LiNO₃) with phase change temperatures below 200 °C [97,98]. LiNO₃ also has the highest latent heat of fusion of all nitrates (about 387 J/g), which significantly improves the energy storage density. Nevertheless, the use of LiNO₃ for large-scale TES applications is restricted by its price.

Nitrate salts are only moderately corrosive to stainless steels and nickel-based alloys at temperatures below 500 °C [75]. This is an important factor to consider for the durability of container materials used in TES systems. However, nitrate mixtures are sensitive to partial chemical decomposition, typically for temperatures above 450 °C for LiNO₃ [98] and NaNO₃ [99], and 550 °C for KNO₃ [100]. Both NaNO₃ and KNO₃ reach a temperature-dependent equilibrium with their nitrite counterparts.

A recent study by Orozco et al. [95] considered the use of NaNO₃, NaNO₂ and KNO₃ and eight of their mixtures as PCMs. The authors performed DSC and TGA analyses to investigate the melting behaviour, measure the enthalpy of fusion and study the stability of these compositions. While pure NaNO₂ has the highest enthalpy of fusion (221 J/g) of the three salts, it is known to decompose by oxidation at temperatures greater than 320–330 °C [101]. It therefore appears that NaNO₂ should not be used as PCM for operating at above 300 °C. The equimass mixture of NaNO₃ and NaNO₂ showed a higher enthalpy of fusion than pure NaNO₃ and a melting temperature of about 233 °C, which shows a potential application as a PCM at a lower operating temperature than pure NaNO₃. In addition, eutectic and non-eutectic mixtures tend to have higher decomposition temperatures than pure salts and can be a good alternative to balance the properties of two or more salts. Using salt mixtures allows higher energy density values and a wider operating temperature range than pure salts in a cost-effective way.

Table 9. Anhydrous salt properties in the temperature range from 100 to 300 °C.

Salt	T Melting (°C)	Heat of Fusion (J/g)	Thermal Conductivity (W·m−1·K−1) (Liquid)	Cp (J/kg k) (Liquid)	Volume Change (%)	Density (kg/m3) (Solid)	Price (€/m3)	Ref
Urea-NH4Cl (85-15)	102	214	0.76	1.7	-	1348	174	[49]
Urea-K2CO3 (15-85)	102	206	0.78	2.20	-	1415	269	[49]
Urea-KNO3 (77-23)	109	109	0.81	1.90	-	1415	255	[49]
Urea-NaCl (90-10)	112	230	0.82	2.20	-	1372	180	[49]
Urea-KCl (89-11)	115	227	0.83	1.96	-	1370	255	[49]
LiNO3–NaNO3–KNO3	123	140	0.79	1.44	-	2068	197	[49]
KNO3-LiNO3 (67-33)	133	170	N/A	N/A	14	2068	1979	[102]
KNO3-NaNO2 (56-44)	141	97	0.730	1.74	-	1994	504	[49]
KNO3-NaNO2-NaNO3 (53-40-7)	142	110	0.5	1.3	4	1.98	497	[102]
KNO2-NaNO3 (48-52)	149	124	0.58	1.05	-	2080	994	[49]
LiNO3-NaNO2 (62-38)	156	233	1.12	1.57	-	2296	3816	[49]
LiNO3-KCl	160	272	1.31	1.26	-	2196	3409	[49]
LiNO3-NaNO3-KCl	160	266	0.88	1.32	-	2297	2852	[49]
LiOH-LiNO3 (19-81)	183	352	0.69	2000	N/A	2124	5165	[49]
LiNO3-NaNO3 (49-51)	194	265	0.590	1720	N/A	2317	3084	[49,103]
LiNO3-NaCl (87-13)	208	369	0.630	1560	N/A	2350	5254	[49]
LiNO3-KCl (87-13)	208	369	0.630	1560	N/A	2350	5254 (£/m3)	[49]
KNO3-KOH (80-20)	214	83	0.540	1350	N/A	1905	611 (£/m3)	[49]
LiOH/NaOH (20-80)	215	290	N/A	N/A	N/A	N/A	N/A	[103]
Na/K/NO3 (0.5/0.5)	220	100.7	0.56	1.35	N/A	1920	N/A	[104]
KNO3-NaNO3 (54-46)	222	100	0.5	1.5	5	1.95		[102]
LiBr-LiNO3 (27-73)	228	279	0.570	1380	N/A	2630	6134 (£/m3)	[49]
LiOH-NaNO3-NaOH (6-67-27)	230	184	0.780	2000	N/A	2154	538 (£/m3)	[49]
Ca(NO3)2(45wt%)-NaNO3(55wt%)	230	110	-	-	-	-	-	[37]
(80)NaOH-(20)NaNO3	232	252	N/A	N/A	N/A	N/A	N/A	[105]
(55)NaNO2-(45)NaNO3	233	163	0.59	1.310	N/A	2210	482	[49]
ZnCl2/KCl (0.319/0.681)	235	198	0.8	N/A	N/A	2480	N/A	[104]
(73)NaOH-(27)NaNO2	237	252	N/A	N/A	N/A	N/A	N/A	[103]
(27)NaOH-(73)NaNO3	237	294	N/A	N/A	N/A	N/A	N/A	[105]
CaCl2-LiNO3 (13-87)	238	317	0.690	1530	N/A	2362	5325	[49]
LiCl-LiNO3 (9-91)	244	342	0.640	1610	N/A	2351	6019	[49]
(72)NaNO3-(28)NaOH	247	237	N/A	N/A	N/A	N/A	N/A	[105]
(86)NaNO3-(14)NaOH	250	160	0.660	1.1990	N/A	2241	382	[49]
LiNO3	254	360	0.650	1450	N/A	2380	6700	[103]
(18.5)NaNO3-(81.5)NaOH	257	292	N/A	N/A	N/A	N/A	N/A	[10]
LiCl-LiOH	262	485	-	-	-	-	-	[37]
(41)NaNO3-(59)NaOH	266	278	N/A	N/A	N/A	N/A	N/A	[105]
NaNO2	270	180	0.53–0.67	1.65	16.5	1.81	N/A	[105]
ZnCl2	280	75	0.5	0.74	N/A	2907	N/A	[104]
NaNO3	306	172	0.5	1.82	10.7	2.26	0.41	[104,105]
NaOH	318	165	0.92	2.08	N/A	2100	N/A	[104,105]
KNO3(94 wt%)-KCl(6 wt%)	320	80	-	-	-	-	-	[37]
KNO3	337	100	0.5	1.4	5	1.95		[102]

Table 9. Anhydrous salt properties in the temperature range from 100 to 300 °C.

Salt	T Melting (°C)	Heat of Fusion (J/g)	Thermal Conductivity (W·m−1·K−1) (Liquid)	Cp (J/kg k) (Liquid)	Volume Change (%)	Density (kg/m3) (Solid)	Price (€/m3)	Ref
Urea-NH4Cl (85-15)	102	214	0.76	1.7	-	1348	174	[49]
Urea-K2CO3 (15-85)	102	206	0.78	2.20	-	1415	269	[49]
Urea-KNO3 (77-23)	109	109	0.81	1.90	-	1415	255	[49]
Urea-NaCl (90-10)	112	230	0.82	2.20	-	1372	180	[49]
Urea-KCl (89-11)	115	227	0.83	1.96	-	1370	255	[49]
LiNO3–NaNO3–KNO3	123	140	0.79	1.44	-	2068	197	[49]
KNO3-LiNO3 (67-33)	133	170	N/A	N/A	14	2068	1979	[102]
KNO3-NaNO2 (56-44)	141	97	0.730	1.74	-	1994	504	[49]
KNO3-NaNO2-NaNO3 (53-40-7)	142	110	0.5	1.3	4	1.98	497	[102]
KNO2-NaNO3 (48-52)	149	124	0.58	1.05	-	2080	994	[49]
LiNO3-NaNO2 (62-38)	156	233	1.12	1.57	-	2296	3816	[49]
LiNO3-KCl	160	272	1.31	1.26	-	2196	3409	[49]
LiNO3-NaNO3-KCl	160	266	0.88	1.32	-	2297	2852	[49]
LiOH-LiNO3 (19-81)	183	352	0.69	2000	N/A	2124	5165	[49]
LiNO3-NaNO3 (49-51)	194	265	0.590	1720	N/A	2317	3084	[49,103]
LiNO3-NaCl (87-13)	208	369	0.630	1560	N/A	2350	5254	[49]
LiNO3-KCl (87-13)	208	369	0.630	1560	N/A	2350	5254 (£/m3)	[49]
KNO3-KOH (80-20)	214	83	0.540	1350	N/A	1905	611 (£/m3)	[49]
LiOH/NaOH (20-80)	215	290	N/A	N/A	N/A	N/A	N/A	[103]
Na/K/NO3 (0.5/0.5)	220	100.7	0.56	1.35	N/A	1920	N/A	[104]
KNO3-NaNO3 (54-46)	222	100	0.5	1.5	5	1.95		[102]
LiBr-LiNO3 (27-73)	228	279	0.570	1380	N/A	2630	6134 (£/m3)	[49]
LiOH-NaNO3-NaOH (6-67-27)	230	184	0.780	2000	N/A	2154	538 (£/m3)	[49]
Ca(NO3)2(45wt%)-NaNO3(55wt%)	230	110	-	-	-	-	-	[37]
(80)NaOH-(20)NaNO3	232	252	N/A	N/A	N/A	N/A	N/A	[105]
(55)NaNO2-(45)NaNO3	233	163	0.59	1.310	N/A	2210	482	[49]
ZnCl2/KCl (0.319/0.681)	235	198	0.8	N/A	N/A	2480	N/A	[104]
(73)NaOH-(27)NaNO2	237	252	N/A	N/A	N/A	N/A	N/A	[103]
(27)NaOH-(73)NaNO3	237	294	N/A	N/A	N/A	N/A	N/A	[105]
CaCl2-LiNO3 (13-87)	238	317	0.690	1530	N/A	2362	5325	[49]
LiCl-LiNO3 (9-91)	244	342	0.640	1610	N/A	2351	6019	[49]
(72)NaNO3-(28)NaOH	247	237	N/A	N/A	N/A	N/A	N/A	[105]
(86)NaNO3-(14)NaOH	250	160	0.660	1.1990	N/A	2241	382	[49]
LiNO3	254	360	0.650	1450	N/A	2380	6700	[103]
(18.5)NaNO3-(81.5)NaOH	257	292	N/A	N/A	N/A	N/A	N/A	[10]
LiCl-LiOH	262	485	-	-	-	-	-	[37]
(41)NaNO3-(59)NaOH	266	278	N/A	N/A	N/A	N/A	N/A	[105]
NaNO2	270	180	0.53–0.67	1.65	16.5	1.81	N/A	[105]
ZnCl2	280	75	0.5	0.74	N/A	2907	N/A	[104]
NaNO3	306	172	0.5	1.82	10.7	2.26	0.41	[104,105]
NaOH	318	165	0.92	2.08	N/A	2100	N/A	[104,105]
KNO3(94 wt%)-KCl(6 wt%)	320	80	-	-	-	-	-	[37]
KNO3	337	100	0.5	1.4	5	1.95		[102]

Anhydrous salts are usually implemented using a holding matrix; they require a holding matrix to undergo the phase change during operations, which are called composites phase change materials (CPCM). CPCMs are commonly made of three different components: (1) phase change material (either mixture of pure), (2) skeleton matrix, which provides a structure to hold the phase change, and (3) thermally conductive enhancer material (TCEM), which increases heat transfer and thermal conductivity in the composite, e.g., expanded graphite (EG). Using EG/PCM composites, the effective thermal conductivity can be increased from below 0.5 to 3–20 (W·m⁻¹·K⁻¹) [106]. Some of the CPCM reported below have been studied for industrial waste heat recovery. Tamme et al. [107] studied the use of phase change materials in the temperature range of 120–300 °C, concluding that the eutectic mixture of the binary system KNO₃–NaNO₃ has excellent potential to be used for processes using saturated steam at around 25 bar; the ternary system KNO₃–NaNO₂–NaNO₃, commonly used as heat transfer fluid, is more suited for 5 bar steam processes. Latent heat transportation for industrial waste heat recovery using a mixture of salt nitrates has also been studied by other authors [77].

4.4.6. System Configuration

The existing heat exchange mechanisms in latent heat storage are summarised in Figure 12. The PCM requires a holding structure to contain the phase change and enable the heat exchange with the HTF. The current strategies to ensure a good heat transfer are (1) extended surfaces, (2) composites, or (3) alternative methods such as intermediate heat transfer or transport of solid PCM [37]. In the TES system for heat processes, the PCM can be implemented in either a heat exchanger or composite (with highly conductive particles, e.g., graphite or shape-stabilised PCM, HDPE matrix). The PCM in the heat exchanger configuration can be implemented as capsules, fins, or surface area structures. These concepts aim to reduce the distances for heat transfer in the low-conductivity PCM. In addition, encapsulation is used to contain the phase change and improve heat transfer as well as packing efficiency. For low temperatures (below 150 °C), polymers are used to encapsulate the PCM, while for higher temperatures, metals must be used. Composites and form-stable configurations are also aimed at holding the phase change while enhancing the heat transfer of the latent heat storage [108,109]. In addition, an intermediate HTF can transfer thermal energy between the primary HTF and the PCM [37]. The PCM can also be transported in a solid state, which reduces the heat transfer limitations caused by frozen PCM around the heat transfer structure [37]. Other system integrations that enlarge the temperature interval for single-phase heat carriers are cascaded PCM, which is a sequence of PCM modules with different melting temperatures (normally in progressive order) [33,110].

Figure 12. Available strategies for heat exchange mechanisms in latent heat storage systems.

4.4.7. Storage Media Overview

A technology overview is presented in Table 10, where sugar alcohols, salt hydrates, nitrates and polymers are described. Although nitrates have been pointed out as great candidates in this temperature range, further research is required about the possible increase in their thermal conductivity, further analysis of their thermal properties and decomposition mechanisms, and most importantly, research is needed on the development of new eutectics at lower temperatures without the need to add lithium nitrate, which dramatically increases the cost of the storage.

Table 10. Technology overview of latent heat storage for low to medium temperature.

Category	Sugar Alcohols	Salt Hydrates	Anhydrous Salts	Polymers
Main Materials	Erythritol, D-Mannitol, Inositol, Dulcitol	CaBr2∙4H2O, MgCl2∙6H2O, Al2(SO4)3∙16H2O, Mg(NO3)2∙2H2O	NaNO3, KNO3, eutectics	HDPE, PEG
Pros	Safe, tunable, high-energy density, eco-friendly	Low cost, high heat of fusion, higher conductivity than organics	Mature, tunable, high-density, moderate corrosion (below 500 °C)	Recyclable, non-toxic, stable, customisable latent heat (comparable to organics)
Cons	Supercooling, degradation, and poor crystallisation	Supercooling, corrosion, melting instability, sealed containment strategies, large volume change	Low conductivity, high cost of Li salts (which allows lower melting), matrix needed	Limited T range, ageing issues, and lower capacity than others in the same T range
Challenges and R&D	Reduce degradation; expand HTF options for high temperatures	Improve melting behaviour; research new containment strategies to reduce corrosion	Enhance conductivity; explore new salt mixtures	Develop high-T polymers; improve long-term stability

PCMs offer high energy density but are often limited by low thermal conductivity, which affects charge/discharge performance. There are also challenges related to phase segregation and long-term stability. Promising directions for future research include the development of PCM composites, advanced encapsulation methods, and compatibility testing for repeated thermal cycling.

4.4.8. KPI’s Latent Heat Storage Technologies

The KPI’s values for the latent heat storage technologies are described in Table 11. Among the latent heat storage options, nitrates and sugar alcohols are the ones that present the highest thermal energy density. On the other hand, salt hydrates are the most cost-effective. However, there are not many options available that can reach temperatures around 100 to 150 °C. Polymers are a great option for applications where a medium energy density is required, and they are available in a large temperature range at a low price. Nevertheless, nitrates remain the best candidates: they can work at higher temperatures, have the highest energy density, and are low in price. Latent heat storage systems are significantly more complex than sensible heat storage, which is twofold; the material preparation is more complex as it requires some sort of structure to hold the phase change (either encapsulation, fins, extended surfaces, composite, etc.), and because the system is generally more complex, as they require heat and mass transfer enhancement designs. Typically, the price of a Phase Change Material (PCM) system falls in the range of €10 to €50 per kWh [106]. Systems incorporating more expensive micro-encapsulated PCMs, which eliminate the need for heat exchange surfaces, can incur even higher costs. For instance, the expense for a complete plasterboard integrated with micro-encapsulated paraffin, utilised as a passive cooling element in gypsum boards, is around €17/kg. This cost includes the paraffin itself at approximately €5/kg and the micro-encapsulation at €13/kg [106].

Table 11. Technology overview of latent heat storage from low to medium temperature.

	KPI	Units	Sugar Alcohol	Salt Hydrates	Anhydrous Salts	Polymers
KPItech1	Thermal energy storage density	kWh/m3	50–200 [111]	72–133 [49]	43–235 [49]	45–80 [49,67]
KPItech2	Power density	[kW/m3]	10 kW-MW [4]	kW- 10 KW [4]	10 kW-MW [4]	kW-10 kW [4]
KPItech3	Limit operational temperature range	[°C]	115–300 [83]	105–180 [112]	102–500 [49]	50–350 [67]
KPItech4	Storage period	[h/d/y]	h/m	h/m	h/m	h/m
KPItech5	Flexibility	[-]	Y	Y	Y	Y
KPItech6	Storage size range	[m3] Range	3–7 for 1 GJ [112]
KPItech7	Recharging energy or auxiliar energy	[kWh] Range	kWh to 100 kWh [4]
KPItech8	Round trip efficiency	[%]	75–90 [4,113]
KPItech10	Durability	[Year]	300–5000 cycles [4] or 10–30 years [113]			7200 h [67]
KPIenvi1	Safety risk	[-]	Low	Medium (corrosion)	Medium (vapours)	Low
KPIenvi2	Environmental risks	[-]	Low	Low	Medium	Low
KPIso-eco1	Production cost	[€/kWh]	25–90 [4] (power production)
KPIso-eco2	Specific cost of the storage	[€/kWh]	13.6 (Erythritol) [49]	1–4 [49]	1–26 [49]	9 (HDPE) [49]
KPIso-eco3	Operation and maintenance cost	[€/kWh]	3% of total cost [114,115] PCM cost 43% Tank cost 17% Installation cost 10% Control system and utility cost 24%
KPIso-eco5	Investment cost	[€/kWh]	10–50 [106] Viable: Daily storage: 75; short term: 225; Buffer storage: 750 [106]
KPIso-eco5	TRL	[-]	3–4 [4]	4–6 [4]	3–4 [4]	3–4 [4]

4.5. Thermochemical

Thermochemical storage stands out with its high density (200–500 kWh/m⁻³) and minimal heat loss between storage and recovery, as energy is stored chemically with low heat capacity components. In contrast, latent storage has an energy density of about 90 kWh/m⁻³, and sensible storage (water with a 70 °C temperature change and 25% heat loss) is around 54 kWh/m⁻³. In this section, thermochemical storage will be reviewed in the temperature range targeted in this paper.

4.5.1. Storage Media

Thermochemical storage (TCS) materials store thermal energy through the heat effect of reversible chemical reactions and/or sorption/desorption processes. TCS fall broadly into two main groups of chemical reaction (strictly thermochemical storage without sorption) and sorption energy storage [115]. This paper classifies thermochemical storage systems into reversible reactions and sorption-based TCS. Sorption-based TCS stores heat by breaking the binding forces, such as Van der Waals forces or covalent forces, between a sorbent and a sorbate [116]. The heat required to break the binding forces can be higher than that associated with the evaporation heat of a pure sorbate (e.g., water). The energy density of sorption-based TCS materials can be significantly higher than that of PCMs. Sorption-based systems are classified into three sub-groups: (1) solid adsorption, (2) liquid absorption, and (3) composites [117]. Reversible chemical reactions generate a high amount of energy as a result of an exothermic synthesis reaction when two substances are separated [118]. Reversible chemical reactions can involve hydration/dehydration reactions, or metal hydrides, hydroxides and carbonates, among others [114].

Unlike the other forms of thermal energy storage, thermochemical units charge and discharge at different temperatures. In the present review, the authors have considered the charging temperature of the thermochemical materials to be within the range of 100–300 °C: see Figure 13. The charging and discharging of thermochemical heat storage are strongly linked to the pressure, relative humidity, and temperature of the system. Therefore, the assumption that charging of the storage can be performed at the same temperature as discharging is mostly inaccurate in many cases. The discharging of the system can take place above this temperature, when using a heat transformer system or generally below charging temperatures for other open and closed system configurations, as explained in the following sections. From an application point of view, it is crucial to consider that some TCS systems will require a heat source/sink to charge/discharge. The temperature levels of this heat source/sink depend on the working pair selected and operating conditions (both storage media temperature and partial pressure). For this reason, the temperatures of the charging/discharging processes are dependent, and it is difficult to define the temperatures without a given system configuration and set of operating conditions.

Figure 13. Charging temperature of thermochemical materials in the range 100–300 °C versus energy stored. Data collected from [119,120].

4.5.2. Sorption-Based Thermochemical

Solid adsorption systems commonly involve either a solid/liquid or a solid/gas, storing energy by the adsorption of a sorbent (normally water) on the surface of a porous material. Zeolites, activated carbon, and metal–organic frameworks (MOFs) are the most common solid adsorbents, see Table 12. Zeolite is also a well-known solid sorption material investigated by previous researchers [121,122,123]. The most common types being researched are faujasite (FAU framework type), zeolite A (LTA framework type), and Zeolite 13X for both heating and cooling applications. In general, zeolites are more hydrophilic than silica, with a water uptake rate of 0.3 kg per kg of solid material. There are a wide range of zeolite materials that can provide a wide range of charging temperatures (70–350 °C). AIPOs and SAPOs are among the promising examples of sorption materials, particularly for low-temperature heat storage [124,125,126].

Table 12. Properties of the most relevant adsorption materials. Adapted from [127,128,129,130].

Material	Adsorption Heat (kJ/kg)	Energy Density (GJ/m3)	Charging Temperatures (°C)	Density (kg/m3)	Specific Heat (kJ/kg.K)	Thermal Conductivity (W/mK)	Possible Refrigerants	Cost (€/ton)
Zeolites	50–300	0.4–0.6	70–350	650–900	0.85–0.95	0.15–0.25	Water	0.51–44
AIPOs/SAPOs	250–300	0.61–0.86	60–100	800–900	0.85–0.95	0.15–0.25	Water	-
MOFs	20–200	1.6	60–150	1000–2000	0.8–1.2	0.10–0.15	Water, methanol, ethanol	-

Adsorption TES systems are still in an early stage of development and not completely commercialised. However, some particular applications have already been put on the market, since they perfectly fit some needs [127]. Some commercial examples at lower temperatures than those studied in this paper are the self-cooling portable beer barrel developed by the ZeoTech company [131] and the open commercialised adsorption system by Bosch in collaboration with ZAE Bayern [132]. In addition, Krönauer et al. [133] proposed a mobile open sorption storage solution for industrial waste heat recovery, working with a packed bed of zeolite as adsorbent. The demonstrated pilot plant was built, operated, and monitored for one year using steam from a waste incineration plant to charge the storage at 130 °C with hot air. The storage was discharged at an industrial drying process several km away from the charging station. Besides the application of energy storage, solid adsorption silica-gel chillers are commercially available [134]. In addition, several prototypes of solid sorption heat pumps with integrated heat storage have been realised, for example, with water absorption by SrBr₂ [135] and Na₂S [136], as well as with CO₂ absorption in the system PbCO₃-CaO [137].

Composite sorbents represent a hybrid way to enhance the sorption ability of materials under typical working boundary conditions of adsorption TES [127]. These composites have been reported to improve thermal stability and heat and mass transfer, allowing them to withstand a larger number of thermal cycles. The composite materials mainly consist of an active storage material (e.g., MgSO₄/CaCl₂/) and a supporting matrix (zeolite, vermiculite, etc.), which can either provide sorption heat (e.g., Silica gel/zeolites/active carbons) or a matrix that do not participate in the sorption process (e.g., graphite, magnesium oxide) [130]. However, the utilisation of a matrix also brings in new dilemmas: the compromise between large and pore size, the improved thermal stability at the expense of the energy density decreases, and the increase in the production cost [129].

Liquid absorption systems could be of interest as they allow for pumpable TCS materials, carrying thermal energy for bulk energy transfer, and acting as a heat transfer fluid. The liquid sorption-based TCS stores thermal energy through concentration changes in a solution, e.g., the absorption of water by a concentrated aqueous solution of calcium chloride, sodium hydroxide, lithium chloride or lithium bromide [117]. However, most of the working pairs in this category are below the temperature of interest in this paper; just a few working pairs are applicable in this working range, and this is just briefly considered in Figure 13.

4.5.3. Reversible Chemical Reactions

Reversible chemical reactions are typically concurrent chemical/adsorption processes, relying on the reversible interaction between a solid and a gas. For power generation applications requiring both high working temperatures and high enthalpies of reaction, chemical reaction storage is preferred [138]. Chemical reactions can be categorised as low to medium temperatures, where hydration reactions with water or coordination of ammonia are generally considered [139], and medium to high temperatures, where oxides, carbonates, sulphates and hydroxides are considered.

Hydrated salts are employed for low to medium temperatures with a higher potential energy storage density compared to adsorption materials like silica-gel and zeolite, and in addition, are often substantially cheaper. Hydrate salts are considered the most suitable materials for residential applications, owing to their high energy density (400–870 kWh·m⁻³) and low regeneration temperature [124]. The most promising candidates shown in Table 13 include mainly magnesium sulphate MgSO₄·7H₂O, strontium bromide SrBr₂·6H₂O, sodium sulphate Na₂S·9H₂O, and magnesium chloride MgCl₂·6H₂O. From these ones, SrBr₂·6H₂O has high stability and good energy density, but high cost; Na₂S·9H₂O has the highest energy density, but it is also highly corrosive. Magnesium sulphate is one of the TCMs that accounts for higher energy density [140] with a theoretical energy density up to 2.8 GJ/m³ [141]. However, the use of magnesium sulphate powder is difficult in a storage reactor because the particles rapidly form agglomerates during dehydration/hydration cycles. This limits gas transfer and causes irreversibility and low temperature lift, resulting in poor system performance [141]. Potassium carbonate was proposed by Donkers et al. [142] for open and closed systems for domestic applications, although they also stated the inconvenience of its low energy density. Strontium bromide has been detected as one of the most promising salts, along with magnesium sulphate [143]. SrBr₂·6H₂O has already been well-investigated at the laboratory and prototype level for a long time for various applications, either for solar cooling and heat storage in a closed process [144], or for seasonal storage of solar energy in an open process [145,146]. For short-term storage applications, it can be considered as a storage and thermal comfort material [143]. Strontium bromide has also been considered, as calcium chloride and magnesium chloride are the other two TCMs that are targeted as having great potential for thermochemical heat storage at low to medium temperatures [147]. They exhibit high energy density at the desired operational temperatures. However, magnesium chloride suffers from thermal decomposition and HCl formation, as well as deliquescence below 40 °C [138].

Table 13. Summary of properties of the most relevant reversible chemical reactions [130,138,148].

Thermochemical Material	Heat of Reaction (kJ/kg)	Energy Density (GJ/m3)	Charging Temperatures (°C)	Density (kg/m3)	Cost (€/T)	Cost (€/GJ)
Hydrated salts
CaCl2·6H2O	1082	1.85	95	1710	116	107
MgCl2·6H2O	2001	3.14	117	2001	154	77
MgSO4·7H2O	1671	2.81	122–150	1680	77	73
SrBr2·5H2O	814.4	2.02	70–174	2386	2400	2838
Hydroxides, carbonates and oxides
MgCO3/MgO	1755	-	300–320	-	-	-
CdCO3/CdO	601	-	287	-	-	-
Mg(OH)2/MgO	1340	-	255	-	-	-
PbCO3/PbO	240	-	288–300	-	-	-
PbCO3/CdO	341	-	300	-	-	-
PbO2/PbO	232	-	288–300	-	-	-

Stengler et al. [149] combined the use of thermal energy storage in the temperature range of 160 °C to 300 °C with heat transformation needs based on a thermochemical working pair (SrBr₂·1H₂O). The authors performed 34 charging/discharging experiments at a constant temperature of 231 °C in a concept shown in Figure 14. The internal temperature gradients occurring when heat is transferred between the HTF and the storage module can be fully compensated by increasing the pressure from 5 kPa to 150 kPa, therefore allowing for TES without any thermal downgrade of the stored energy. This configuration allows heat upgrade (charging < discharging) of the thermochemical system, unlike most of the concepts found in the literature, where the heat was downgraded (charging > discharging). This is known as a heat transformer, which is a device that can deliver heat at a higher temperature than the temperature of the fluid/vapours by which it is fed. Heat transformation has been studied in the past years for industrial waste heat recovery; Richter et al. [150] concluded in their systematic review that for heat transformation of waste heat up to 300 °C, SrBr₂ was the most promising material. The theoretical energy storage density of hydrated strontium bromide (SrBr₂∙6H₂O) is 629 kWh/m⁻³, which is nine times higher than that of liquid water over a 60 K temperature range. However, practical values are lower due to the space needed for the gas phase and reactor components like the salt bed, heat exchanger, and gas diffuser.

Figure 14. Prototype of the scalable thermochemical storage module from Stengler et al. [149].

Hydroxides, carbonates, and oxides are also reversible reaction candidates at higher temperatures (charging temperatures above 200 °C). Hydroxide-based TCS uses reversible hydration/dehydration of metal oxides with a typical operation temperature range of ~250–600 °C. Calcium hydroxide and magnesium hydroxide are the most commonly studied hydroxide-based TCS pairs [151], and are also associated with chemical heat pumps due to a potential temperature upgrade. The hydroxide-based TCS materials have a volumetric energy density of up to ~600 MJ/m³. Calcium hydroxide, Ca(OH)₂, is more attractive than magnesium hydroxide because MgO is inert to hydration in superheated steam, and the reaction rate deteriorates with increasing temperature [152]. However, its charging temperature is above 300 °C.

Metal carbonates have several advantages, e.g., high energy density, non-toxicity, low costs, and widespread availability. Systems at high temperatures, such as CaO-CO₂, which uses the reversible reaction between carbon dioxide and calcium oxide to form calcium carbonate (CaCO₃) [153] have received notable attention. As can be observed in Table 13, below a charging temperature of 300 °C, there are just few working pairs available: MgCO₃/MgO, PbCO₃/CdO, and CdCO₃/CdO. These carbonate systems have been mainly studied for CO₂ capture [154].

Most of the prototypes and projects involving reversible chemical reactions target the building sector, with lower temperatures than those considered in this outlook. However, higher temperatures have been considered for applications such as CSP [151] or power grid support [155,156].

4.5.4. System Configuration

Thermochemical storage can be implemented in closed or open systems, either in a separate or an integrated manner. In an integrated reactor, the material is stored in the same tank where it reacts, while in a separate reactor, the charging and discharging phases are separated from the thermochemical material storage reservoir [157]. Regarding the technical specifications of open and closed reactors, pros and cons are listed in Table 14.

Table 14. TCH system comparison. Adapted from [139,158].

Aspect	Open System	Closed System
System Type	Exchanges both mass and energy with the environment (e.g., uses moist air as a working fluid)	Exchanges only energy; sorbent and sorbate are stored separately (e.g., uses water and salt hydrate)
Working Principle	Heat is transferred to a transport medium (air), which drives the thermochemical process	External heat drives TCM dissociation, and energy is transferred via a closed-loop working fluid
Operation	Steam is sourced from ambient moist air	Requires an evaporator to supply steam during hydration
Advantages	- Simpler reactor design - Lower cost - Operates at atmospheric pressure - No need for a heat exchanger	- Higher energy density - Compact system - Higher and controlled discharge temperatures - No safety risks from mass exchange
Disadvantages	- Requires a humidifier and fan - Low reaction power - Pressure drop and safety concerns - Limited to water	- Complex system and components (evaporator, condenser) - Larger heat transfer area needed - Periodic system evaluation

4.5.5. KPIs Thermochemical Storage Technologies

The KPIs for the thermochemical technologies are described in Table 15. Among the thermochemical technologies, chemical reactions show higher energy density, but at the expense of great challenges at the material level. Composite sorbents stand as an intermediate solution when requiring high energy density at a lower cost compared to chemical reactions. Solid adsorption systems are ideal for seasonal or interseasonal storage applications where the energy required is between 250 and 300 kWh/m³ at material energy density. However, concepts that need two storage vessels and many hydraulic parts and additional units, such as closed absorption systems, have only two times higher energy density compared to plain water, so thermochemical storage has great potential that comes with great challenges to translate the theoretical material energy density to actual device energy density. The cost of storage in any of the cases is still very high; the investment cost and the operational maintenance cost are clear indicators of the low TRL of this technology. Additionally, the costs for containers and auxiliary TCS equipment, essentially for heat and mass transfer during energy charging and discharging, are significant. TCS systems can function as open systems, which typically are simply packed beds of pellets at ambient pressure, or as closed systems, which require two vessels at specific pressures and a condenser/evaporator unit. Hence, open systems usually present a more cost-effective solution. The expense for TCS varies, with costs ranging between €8 and €100 per kWh. The materials used in thermo-chemical storage are also costly due to the need for specialised preparation, such as pelletising or layering onto support structures.

Table 15. Technology overview of thermochemical storage for heat decarbonisation. Energy densities are material energy densities not reactor.

KPI		Units	Sorption-Based	Reversible Chemical Reactions
KPItech1	Thermal energy storage density	kWh/m3	250–400 [130,159]	800–1200 [130,159]
KPItech2	Power density	[kW/m3]	120–310 [130]	200–350 [130]
KPItech3	Limit operational temperature range (ΔTop)	°C	30–200 [139]	30–300 [139]
KPItech4	Storage period	[h/d/y]	h/d/y [130]	h/d/y [130]
KPItech5	Flexibility	[-]	Yes
KPItech6	Storage size range	[m3] Range	Open: 2–88 L/Closed: 1–157 L [160]
KPItech7	Recharging energy or auxiliar energy?	[Wh] Range	10–100 kWh [130]
KPItech8	Round trip efficiency	[%]	50–65 [130]	40 (open)–50 (closed) [130]
KPItech9	Durability	[Year]	15–25 [130]	<100 cycles [130]
KPIenvi1	Safety risk	[-]	Low	Low
KPIenvi2	Environmental risks	[-]	Low	Medium (open systems)
KPIso-eco2	Specific cost of the storage	[€/kWh]	15–150 €/kWh [130]	80–160 €/kWh [130]
KPIso-eco2	Investment cost	[€/kWh]	8–100 [106] Viability Seasonal: 25 €/kWh; daily storage 75 €/kWh [106]
KPIso-eco3	Operation and maintenance cost	[€/kWh]	-
KPIso-eco5	TRL	[-]	4–6	1–3

4.5.6. Technology Overview

A technology overview is presented in Table 16, where solid adsorption, chemical reactions, and composite sorbents are described. Given the low TRL of thermochemical storage technologies, many challenges at the material and system level are still to be solved to reach TRLs closer to six or seven. The main focus should be on material development targeting the following: (1) the enhancement of cyclic stability, reaction kinetics, reversibility, and thermal conductivity to maximise conversion efficiency and durability; and (2) lowering the cost of the storage media, with reliable and construction material-compatible composites. Overall, thermochemical storage values are ~10 times higher than sensible heat storage systems; despite this, it must be considered that the systems are more complex, requiring tanks for the storage of products (solid and gas/liquid). In addition, the real energy density of TCM systems depends on material properties, reaction efficiency, and process configuration.

Table 16. Technology overview of thermochemical storage for low to medium temperature [130,143].

Category	Representative Materials	Pros	Cons	Challenges
Solid Adsorption	Zeolite 13X–H2O, Silica Gel, Activated Carbon	Strong water affinity, high output (>55 °C), safe, cheap	Lowest energy density (<250 kWh/m), high charging temp. (160–250 °C)	Develop MOFs, enhance hydrophobicity, reduce cost and improve kinetics
Chemical Reactions	CaCl2, MgCl2, Na2S, SrBr2 with H2O	High energy density (up to 1200 kWh/m3), minimal losses	Irreversible cycles, material degradation	Enhance reaction reversibility, thermal conductivity, and lower material cost
Composite Sorbents	Zeolite + MgCl2/MgSO4 composites	Better stability, enhanced transfer, lower cost than pure TCMs	Lower energy density, still limited by reversibility	Improve compatibility, corrosion resistance, and optimise reactors

TCS technologies are still emerging, and they face challenges in terms of system complexity, low TRL, and the need for precise control of reaction conditions. While they show potential for long-duration storage, especially in open systems, practical applications are scarce. Future work should focus on identifying robust reaction pairs, simplifying system architecture, and field-testing integrated prototypes.

4.6. Hybrid Systems

4.6.1. Working Mechanism

Hybrid thermal storage systems have shown promise in different applications. In these systems, two storage materials or one storage material and one HTF are combined to exploit the advantages of both materials. One example is the combination of thermal oil with a bulk material. Natural materials or industrial waste products can be used for the bulk. The filling material prevents natural convection in the storage media.

Up to date, hybrid systems have been based on the following combinations: (1) sensible/latent heat storage, (2) sensible heat storage/thermochemical storage, and (3) cascade systems. Latent and sensible heat-based systems [161,162,163,164,165] aim to increase the energy density of pure sensible storage by adding a percentage of latent heat form. In that manner, the capacity of the system is increased, and a stable output temperature can be achieved. Cascade systems are subsequent reactor configurations that allow for the integration of parallel or series reactors of either one kind or a combination of latent heat storage [114,153,166], sensible, or thermochemical [167,168,169] using different materials to store heat at different energy levels. The benefit of those systems is a wider operational range and the ability to deliver energy at different output temperatures. Thermochemical is summed up by sensible heat-based systems [170], to enhance sensible heat performance by reducing its volume and allowing a constant discharging temperature. However, these developments are still at a low technological readiness level.

4.6.2. Sensible and Latent

Solar water heating system with PCM in the water tank is a classic hybrid system of “SHTES + LHTES”, which has been investigated in recent decades. When combining sensitive and latent, generally, the approach followed is to combine sensitive filler material with a PCM filler material. In such a case, the latent material can also serve as an HTF. PCM hybrids combine the advantages of sensible and latent media. It should be taken into account that the configuration of the heat exchanger is crucial for efficient energy storage.

The PCM can be introduced into the storage system in various forms. The most common approach is to encapsulate the PCM. In microencapsulated PCMs, the capsules are sized to be carried by the HTF as shown in Figure 15. Consequently, the heat capacity of the HTF is increased. Encapsulation may be a physical separation between the PCM and the HTF, or the PCM may exist as a dispersion in the HTF. The majority of both approaches are used for low-temperature cooling applications [171,172]. Macro-encapsulated PCMs are introduced into storage systems in bulk. A physical separation exists between PCM and HTF. The shell material must be selected based on the temperature level and compatibility with PCM and HTF [163,173]. Another approach is direct contact between HTF and PCM. In that case, it is important to consider the compatibility between HTF and PCM [174]. In addition, the PCM encapsulation should be carefully designed, especially if the chosen PCM is of low conductivity. Spherical and cylindrical PCM encapsulations have shown a superior performance relative to rectangular ones. The most common type of heat exchangers in hybrid sensible and latent heat storage is the shell and tube. However, researchers have implemented the PCM either in tubes or in the shell configuration. If water used in the hybrid system is to be used in domestic applications, it is common to put PCM in tubes.

Figure 15. Schematic representation of a macro-encapsulated PCM storage unit.

The latent–sensible hybrid systems can be implemented in either thermocline, hybrid configurations, or Fcascade configurations (Figure 16).

Figure 16. Dual media systems: (a) thermocline configuration, (b) PCM hybrid configuration.

In thermocline configurations, some developments add encapsulated PCM or self-stabilised PCM in some regions (substituting the filler) to enhance energy density storage and to achieve better temperature stabilisation. A common approach is to implement the sensible heat of a liquid or solid filler material as thermal energy storage. The solid filler material is placed as loose bulk in a tank. The liquid storage material around the bulk also serves as HTF. To discharge the storage tank, the liquid storage material is withdrawn from the top of the tank and discharged via a heat exchanger. The cold medium is then returned to the bottom of the tank. The filling material prevents natural convection inside the tank, which makes the thermocline design attractive. In PCM hybrids, a combination of thermocline and classical PCM configuration (with or without filler) is envisioned, where the PCM is embedded in a storage in different forms and discharged via an HTF. The PCM material can be in direct contact with the HTF (using self-stabilised PCM), or it can transfer heat to the HTF via an encapsulation. In the case of direct contact, the materials must be selected so that no chemical interaction occurs, and compatibility is ensured throughout the storage cycles. The same applies to the encapsulation of the PCM. This must not interact with either the HTF or the PCM itself, and must simultaneously possess a high thermal conductivity. Heat exchange takes place between the HTF and the PCM. The latent heat of the PCM is used to increase the storage capacity of the system. At the same time, the PCM releases this heat at a constant temperature level, resulting in a more stable operation of the overall system.

The KPIs for hybrid latent and sensible heat storage systems are gathered in Table 17.

Table 17. KPIs of dual media (Thermocline, PCM) storage systems.

KPI	KPI		Thermocline	Hybrid Latent
KPItech1	Thermal energy storage density	kWh/m3	Energy densities (kWh/m3): Delcotherm + Magnetite: 106.2 (180–300 °C) [175]; Caloria + gravel/sand: 37.4 (218–304 °C) [5]; Caloria + rock/sand: 51.1; NaNO3/KNO3 + quartzite: 53.4 (290–390 °C).	See Section 4.4 for detailed information on PCM storage media
KPItech2	Power density	[kW/m3]	See Section 4.4 for detailed information on PCM storage media
KPItech3	Limit operational temperature range (ΔTop)	°C	Therminol VP1: 15–400 °C Steinmann [5]
KPItech4	Storage period	[h/d/y]	Buffer storage, short-term
KPItech5	Flexibility	[-]	Partial discharge possible
KPItech6	Storage size range	[m3] Range	−3500 m3 [5]	28 m3
KPItech7	Capacity	[Wh] Range	See Section 4.4 for detailed information on PCM storage media
KPItech8	Round-trip efficiency	[%]	77–83% (Solar One test facility) [5]
KPItech10	Durability	[Year]	15–20	15–20
KPIenvi1	Safety risk	[-]	Safety risks are mainly due to the high flammability of thermal oils.	No safety risks
KPIenvi2	Environmental risks	[-]	Impact depends on HTF: natural oils = low, synthetic/mineral oils = high; filling materials inert unless contaminated.	Low environmental impact (add reference to PCM material)
KPIso-eco2	Specific cost of the storage	[€/kWh]	The cost of the storage envelope is highly dependent on the storage medium. The thermomechanical loads are difficult to predict.
KPIso-eco5	TRL	[-]	9 [4]	4 [4]

4.6.3. Thermochemical and Sensible

Hybrid thermochemical (redox) and sensible heat systems have been studied in the literature for higher temperatures than those considered in this paper [176,177,178]. In the temperature range of interest, one low-temperature system (charges at around 100 °C) was described in [179], as shown in Figure 17. A tank is installed to recover the condensation heat for pre-heating the water. After the water temperature in the recovery tank reaches the set value, which is related to the condensation temperature, the pre-heated water will be sent to the hot water tank for further heating. The inlet water temperature of the condenser is typically over 30 °C, even when the sorption cycle is charged under a low-temperature (about 100 °C) heat source. Overall, the hybrid-system TES technologies combining thermochemical and sensible heat storage are still rare in the literature. However, with the increasing maturity of sorption TES technology, more hybrid systems related to sorption TES will also emerge [179].

Figure 17. Potential hybrid thermochemical and sensible heat storage system [179].

4.6.4. Technology Overview

The technology overview of the hybrid systems presented in this section is listed in Table 18. The main output from these sections is that more hybrid storage systems need to be explored to overcome the shortcomings of standalone conventional TES technologies (e.g., low energy density and low energy storage efficiency). In addition, advanced/hybrid TES technologies only show their advantages under certain conditions; thus, optimal design and operating strategy should be established based on the variable’s actual conditions, considering all the application requirements. Latent-heat TES has been the focus in the past years, while thermochemical TES and its hybrid TES technologies show the greatest research potential and become an emerging hot topic in the following years.

Table 18. Technology overview of hybrid systems (sensible–latent, sensible–thermochemical and thermochemical–latent).

Hybrid Type	Key Advantages	Key Challenges	Research Focus	Technology Status
Sensible–Latent	+ Higher energy density + Extended operation time + Improved efficiency + Supply temperature control	- Poor heat transfer	Improve energy storage density, reduce charging temps, enhance conductivity, stabilise discharge temperature	Commercially deployed
Sensible–Thermochemical	+ Enables condensation heat recovery + Higher energy density + Lower power consumption	- Poor heat and mass transfer - Complex control/design	Optimise configurations for heat recovery from thermochemical units	Developed at high temps; proposed for lower temps
Latent–Thermochemical	+ High energy density + Stable temperature output + Reduced power use	- Poor heat and mass transfer - Complex system design	Combine PCM and TCS to recover condensation heat and reduce temperature lift in TCS systems	Assessed experimentally at the hybrid material level

4.7. Technology Comparison

In this section, a technology comparison is provided, where relevant information and KPIs collected throughout the review are summarised in Figure 18, e.g., storage period assessment, cost vs. energy density assessment, and technology maturity status.

Figure 18. Comparison of temperature range, energy density, and TRL of available TES technologies in the working temperature range from 100 to 300 °C.

The most relevant KPIs for assessing and selecting a TES system are gathered in Table 19. In terms of cost, sensible heat is always the cheapest solution, but this is at the expense of its rather low energy storage density. Latent heat storage (especially nitrates) shows higher potential compared to thermochemical solutions for the implementation in the temperature range of 100 °C to 300 °C, as they present the lowest cost. However, it presents some challenges, such as the cost of heat exchangers and corrosiveness. Regarding the hybrid storage solution, new PCM should be researched (or more efficient) in the TCM field. There is not much research required on the sensible technologies (there is a trend towards renewable oils and more sustainable solutions, but still not saving).

Table 19. Summary of the most relevant KPIs for the TES technologies for industrial heat demand (100–300 °C). Note that solid state refers to metals, rocks, ceramics, and castable; and water refers to water tank storage systems.

KPI	Unit	Sensible Heat (Water/Thermal Oils/Solid State)	Latent Heat (Sugar Alcohols, Polymers, Anhydrous Salts, Salt Hydrates)	Thermochemical (Sorbents and Chemical Reactions) (TCS)
Energy Density	kWh/m3	35–200	50–235	250–1200
Power Density	kW/m3	kW–300 MW	10 kW–MW	180–350
Temp. Range	°C	−10–400	105–300	30–300
TRL	-	7–9	4–7	3–6
Storage Size	m3	0–5000	1–10	2–100 (L)
Safety Risk	-	Low–High (Oils)	Low–Medium	Low
Cost	€/kWh	0.5–75	10–60	15–150
Lifetime	Years	10–60	10–30	10–30
Storage Period	-	h/d	h/m/d	h/d/y

4.7.1. Storage Period and Technology Maturity for the Industrial Case

Figure 19 shows the Technology Readiness Levels (TRLs) and commercial maturity of the various TES technologies applicable in the 100–300 °C range. Sensible heat storage remains the most advanced and commercially available option, especially water-based systems and thermal oils, which are already widely adopted in industry. Solid-state materials such as castables and rocks also exhibit high TRLs and are often considered for retrofit solutions in batch processing or high cycling applications. Latent heat storage technologies show moderate maturity, with nitrate salts being the most commercially advanced due to their established use in CSP applications. However, their adaptation to the 100–300 °C range is still underexplored and presents opportunities for further research. Other latent materials like polymers and sugar alcohols show promise in compact thermal modules, but require advances in stability, cycling performance, and thermal conductivity to become commercially viable.

Figure 19. TRL’s comparison of the available TES technologies for industrial heat demand (100–300 °C)—(blue: sensible technologies; turquoise: thermochemical technologies; green: latent storage technologies).

Thermochemical technologies, while offering the potential for long duration and seasonal storage (see Figure 20), remain at lower TRLs. These systems face challenges related to material reversibility, heat and mass transfer efficiency, and reactor integration (see Table 20). Composite sorbents and salt-based reactions are particularly promising in applications requiring high energy density and minimal standby losses, such as district heating and industrial waste heat recovery.

Figure 20. Temperature levels vs. storage period of the available technologies in the working temperature range from 100 to 300 °C—(blue: sensible technologies; turquoise: thermochemical technologies; green: latent storage technologies).

Table 20. Technical challenges related to storage period, technology readiness level and potential applications.

TES Technology	TRL	Main Identified Technical Challenges	Storage Period	Main Potential Applications
Sensible heat—Water-based TES	9	Limited to low-pressure operations; needs improved insulation and corrosion control for high temperatures; losses over time in large systems	Hours to days	Short-term storage for process steam and hot water
Sensible heat—Thermal oils	7–9	Degradation under thermal cycling, flammability, disposal concerns, and cost of synthetic oils	Hours to days	Steam-based process heating, integrated oil systems
Sensible heat—Solid media (castables, rocks)	7–9	Difficulties with heat transfer, mechanical stresses during cycling, and thermal expansion	Hours to days	Batch process heat, frequent cycling, and retrofits
Latent heat—Nitrate salts	6–7	Phase segregation, material corrosion, low conductivity, and heat exchanger design	Hours to days	Medium-temp. steam storage, food processing
Latent heat—Polymers	4–6	Need for tailored synthesis, long-term durability, and heat exchanger compatibility	Hours to days	Modular compact heating units in industry
Latent heat—Sugar alcohols/salt hydrates	5–6	Low thermal conductivity, chemical stability, subcooling, and cycling durability	Hours to days	Thermal buffering, backup systems
Thermochemical—Composite sorbents	4–6	Complex reactor integration, reversibility, thermal management, and cyclic degradation	Days to weeks	District heating integration, surplus recovery
Thermochemical—Salt-based reactions	3–5	Slow kinetics, irreversibility, segregation, thermal management, and corrosion	Weeks to months	Seasonal steam backup, industrial long-duration storage

A key observation in this temperature range is the smaller disparity between the maturity of latent and thermochemical technologies compared to other temperature ranges. This highlights the strategic value of accelerating innovation in both domains to complement already mature sensible heat systems. Advancing materials research, particularly for mid-range temperature PCMs and stable chemical pairs, will be crucial for supporting decarbonisation across industrial sectors.

4.7.2. Cost of TES Technologies vs. Industrial Site Space

Balancing cost and energy density is key to selecting suitable TES technologies for industrial applications. As shown in Table 21, higher energy density technologies typically have higher costs due to lower maturity and more expensive materials.

Table 21. Energy density, estimated cost, and lifetime of TES technologies for industrial applications.

TES Technology	Energy Density (kWh/m3)	Estimated Cost (€/kWh)	Lifetime (Years)	Notes
Water-based TES	58.3 (∆T = 50 K)	1–5	10–60	Low-cost, safe, large footprint; excellent for daily/weekly cycles
Thermal Oils	50–56 (∆T = 100 K)	20–75	10–40	Moderate temperature range, high flammability, and disposal concerns
Solid-state Sensible (e.g., Castables, Ceramics)	70–120	5–30	15–50	High thermal durability, limited thermal conductivity, and bulkier systems
Nitrate PCMs	100–160	10–30	10–30	Good thermal stability; high latent energy density
Sugar Alcohol PCMs	150–220	15–50	10–20	High energy density, but challenges with supercooling and cycling stability
Anhydrous Salt PCMs	130–180	20–60	10–25	High latent heat; may suffer from phase segregation and corrosiveness
Polymer-based TES	80–120	30–80	10–20	Lightweight, flexible, still emerging, and lower thermal conductivity
Composite Sorbents (THS)	>200 (effective)	25–90	10–30	High energy density and storage duration; still under optimisation
Chemical Reactions (THS)	>200 (effective)	20–100	10–30	High potential, still emerging; reversible reactions not yet fully mature

Nitrate-based and sugar alcohol PCMs present a balance between affordability and energy density among latent heat options. Within sensible heat storage, water-based TES stands out due to its low cost, simplicity, and widespread industrial familiarity. However, it is limited in terms of temperature range and space requirements. Among thermochemical and hybrid materials, composite sorbents and anhydrous salts demonstrate high theoretical energy density, but system complexity and material cycling stability remain challenges. Polymers, while still under development, offer lightweight alternatives for specific configurations, albeit at higher costs.

The capital cost of TES includes not only the storage material but also the cost of containers, heat exchangers, pumps, fittings, and installation. These costs vary considerably depending on system size and material type. Generally, the specific cost per kWh of stored energy decreases with larger storage capacities. For example, large-scale sensible TES using water tanks can cost less than 1.3 €/kWh and around 120–150 €/m³. Underground TES (e.g., boreholes or aquifers) offers even lower investment costs, below 0.5 €/kWh, although it is typically limited to low-temperature applications.

Latent TES technologies, while not facing major technical barriers, have not yet seen widespread industrial adoption. Their capital cost is currently higher than that of sensible TES, with common PCMs averaging around 50 €/kWh. However, this is expected to decrease as commercialisation expands. Thermochemical systems show promise for cost-effectiveness and minimal energy loss, but remain in early development stages, making accurate cost estimation difficult at this time.

For industrial applications, payback periods are a key factor influencing investment decisions. Current estimates suggest a payback time of three to four years for surplus heat recovery via steam accumulators and up to eleven years for mobile TES solutions. These values may improve with rising fossil fuel prices and greater standardisation in system design and installation.

This comparison underscores that while sensible and water-based systems remain the most economical for short-term applications, latent and thermochemical technologies offer superior compactness and density, making them viable for space-constrained or high-performance use cases. Continued research on advanced PCMs, composite materials, and long-cycle stability is essential to unlock the full potential of TES technologies across industrial decarbonisation pathways.

5. Conclusions and Outlook

This review paper presents a technology outlook on the available thermal energy storage (TES) technologies in the temperature range 100 to 300 °C, which has been identified as key for the decarbonisation of process heat. Already commercially matured and potentially developed, sensible heat, latent heat, and thermochemical heat storage technologies have been reviewed and assessed through a list of KPIs, storage media, and system configurations.

In addition, the advantages, disadvantages, challenges, and limitations are also assessed. With this technology outlook, the authors intend to provide a catalogue of technologies for further deployment in the industrial sector.

The main outputs of the work are summarised below:

• Sensible heat TES technology has undergone a long development, and the technology outlook shows that most of the technologies between 100 °C and 300 °C are commercially available and mature.
• Latent heat TES technology has been the focus in recent decades, mainly including the research on PCMs to improve their thermal performance (e.g., thermal conductivity), the design of heat exchangers to improve the heat transfer, and some applied research to facilitate latent heat TES commercialisation.
• Among thermochemical storage, sorption TES is the most mature technology; however, compared to the sensible and latent heat storage, it is still an emerging topic. The current study on the sorption TES system is focused on developing alternative working pairs (sorbent and sorbate materials) and optimising the system configurations and cycles. Only a limited number of studies have conducted practical research on these systems within the relevant temperature range. Further reversible reactions should be researched in this temperature range, and heat transformers stand up as a promising solution for waste heat recovery in the medium-temperature range.
• In general, latent and thermochemical storage show costs above the economic viability of thermal energy storage implementations as a function of the number of cycles, which is an investment cost of 0.25 €/kWh for seasonal energy storage (1 cycle a year) and 75 for daily storage (considering 300 cycles per year). Specially, TCS ranges from 8 to 100 per kWh, depending on the system selected. Open systems are less complex and inexpensive, and the manufacturing methodology for the TCS material. Meanwhile, PCM installations also depend on the implementation method (encapsulation, active, board plaster, etc.), and the cost of PCM ranges between 10 and 50 kWh. In both cases, the cost of the equipment is much higher than the cost of the storage material. Therefore, PCM and TCS systems are economically viable only for applications with a higher number of cycles.

Among all the technologies assessed, latent heat storage (especially nitrates) shows the most suitable storage feasibility potential for the implementation in the temperature range of 100 °C to 300 °C, as they present a decent trade-off between cost and high energy density storage provided, and wide availability in the temperature range.

In terms of cost, technologies that would require an HTF are always the most effective, but at the expense of their rather low storage energy density and safety issues (e.g., thermal oils). Even though latent heat storage is promising, it presents some challenges, such as the cost of heat exchangers and corrosiveness.

As an outlook, new PCM mixtures should be researched (or more efficient), and tools to predict their properties and behaviour under operation should be implemented to accompany the technology developments.

In summary, while TES technologies in the 100–300 °C range hold strong potential for industrial decarbonisation, each technology class presents its own limitations. Common barriers include material degradation, system cost, safety concerns, and lack of real-world demonstration for newer systems. Cross-cutting research opportunities exist in the areas of advanced heat exchanger design, modular system integration, and durable material development. Addressing these challenges will be key to unlocking the full potential of TES across industrial sectors.