Sensible heat storage (SHS) is one of the most established and straightforward approaches to thermal energy storage. In SHS, energy is stored by raising the temperature of a storage medium, such as water, concrete, or specialized ceramics, resulting in an increase in its internal energy (Li 2016). Despite its simplicity and relatively low cost, the energy density of sensible heat storage is intrinsically limited by the specific heat capacity of the material and the maximum temperature difference that can be safely achieved. For instance, even when advanced materials are employed, practical energy densities typically remain within the range of tens of kilowatt-hours per cubic meter. Dicaire et al. provided a comprehensive analysis of various storage media and concluded that under optimal conditions, sensible heat storage systems rarely exceed energy densities of about 60 kWh/m³ (Dicaire and Tezel 2013). Moreover, SHS systems are prone to continuous heat losses due to conduction, convection, and radiation, which can become significant over long durations. These losses limit the feasibility of SHS for applications requiring long-term or seasonal energy storage, despite its technical simplicity and widespread use.

Latent heat storage capitalizes on the energy exchanged during phase transitions, such as the melting and solidification of phase change materials (PCMs). This approach often achieves higher energy densities than sensible heat storage, as the latent heat of fusion can be substantial even over narrow temperature ranges. For example, many organic and inorganic PCMs can store energy in the range of 150-250 kJ/kg during phase changes (Boldoo et al. 2024). However, latent heat storage systems encounter their own challenges. One major issue is phase change hysteresis, where the melting and solidification temperatures differ, leading to incomplete energy recovery. Sub-cooling, a phenomenon in which the material remains in a supercooled liquid state below its expected freezing point, further complicates the energy retrieval process. In a study by Hekimoğlu et al., researchers explored innovative encapsulation methods and composite material formulations aimed at mitigating these issues, yet they emphasized that ensuring stable, repeatable performance over numerous cycles remains a critical challenge (Gencel et al. 2023). Additionally, the need for reliable containment systems to manage the volume changes during phase transitions can add complexity and cost, further restricting the large-scale implementation of latent heat storage solutions.

Thermochemical energy storage (TES) differs fundamentally from conventional thermal storage methods by using reversible chemical reactions to retain thermal energy in the form of chemical potential. This mechanism allows for significantly higher energy densities, often in the range of 250–350 kWh/m³ when using salt hydrates such as CaCl₂, which exceeds the storage capacity typically achieved by sensible or latent heat systems (Nejhad, 2019, Barbatelli, 2023). Moreover, because TES stores energy at the molecular level rather than as elevated temperature, it experiences minimal heat loss during long-term storage, even under suboptimal insulation conditions (Ding et al., 2021).

Another advantage of TES lies in its configurational flexibility. By selecting appropriate reaction pairs and tuning reaction kinetics, TES systems can be adapted to operate efficiently within specific temperature ranges. This enables effective integration with low-grade heat sources, including solar thermal and industrial waste heat systems (Miró et al., 2016, Liu, G., 2024). For instance, Liu et al. (Liu, X., 2024). examined salt–silica composites in fluidised-bed TES setups and reported that CaCl2-loaded mesoporous silica achieved energy densities around 1500 kJ/kg under humid ambient conditions, highlighting its potential for stable and compact thermal energy storage. Taken together, the high energy density, low standby losses, and adaptability of TES position it as a compelling alternative to traditional thermal storage technologies.

Chemical heat pumps (CHPs) employ reversible thermochemical reactions to upgrade low-grade heat, enabling efficient thermal energy storage and temperature lifting. Early experimental systems, such as those using NH₃-H₂O or NaOH-H₂O reaction pairs, demonstrated the theoretical potential for high efficiency and heat storage densities ranging from approximately 119 to 250 kWh/m³ (P, 1987, Hadorn J C, 2006). More recent designs have integrated calcium chloride (CaCl₂) as a working medium, benefitting from its affordability and favourable thermochemical properties. In solar-assisted CHP systems, CaCl₂ can undergo hydration and dehydration cycles to store and release heat with significantly reduced standby losses. When combined with external energy inputs such as solar thermal or industrial waste heat, the effective energy density of these systems can exceed 380 kWh/m3 (Figure 6) (Yang et al. 2014). And also, desiccant CaCl₂ is cheaper than LiCl₂ and LiBr, which are always selected for cooling applications. The solar charging and discharging processes can be described by the following reactions (Quinnell et al. 2011; Woods et al. 2011; Woods and Kozubal 2013), where the charging process is represented in Equation (3) and the discharging process is represented in Equation (4).

Thermal heat energy density depends on the density of CaCl₂ solution. The energy density of CaCl₂ is 106 kWh/m³, however, the material energy density can rise to 381 kWh/m³ when system combined with extra heat source, such as solar thermal collector or waste heat recovery (Quinnell et al. 2011).

Recent advances in reactor architecture have further improved CHP performance. For instance, high-performance reactors integrating plate-tube heat exchangers with enhanced surface area, porous metal media, and packed beds of CaCl₂ have demonstrated temperature upgrading from 100°C to 155°C with volumetric heat output exceeding 380 kW·m^-3. System optimisation in terms of timing and flow conditions has achieved coefficients of performance (COP) near 0.5 and stable hourly outputs around 72 kWh·m^-3 (Qin et al. 2024). This system integrates parabolic trough solar collectors, a thermochemical reactor, and an internal combustion engine with both thermal and chemical energy storage, effectively upgrading mid-to-low temperature solar and exhaust heat via methanol thermochemical decomposition. Under the design conditions, the overall energy efficiency reached 72.09%, exergy efficiency 37.65%, and net solar-to-electric efficiency 24.63%, while fuel consumption and CO₂ emissions were reduced by 32.97% and 25.33%, respectively. These results highlight the promising potential for efficient and flexible utilization of solar energy and fuels in CHP systems.

Closed-cycle sorption systems utilise physical or chemical interactions between a sorbent and a working fluid, commonly water vapour, to facilitate thermal energy storage and conversion. In adsorption-based systems, porous solid materials such as silica gel or metal-organic frameworks (MOFs) play a critical role in reversible vapour uptake and release. Material innovation has been central to improving performance in recent years. For example, Zhu et al. (Zhu et al. 2006) tested a CaCl₂-impregnated silica gel composite that stored low-grade heat with high efficiency. It achieved a water uptake of 0.73 g/g and a thermal capacity near 1 kJ/g, with stable performance over 500 cycles due to the combined effects of salt loading and porous structure. The improvement was attributed to a synergistic effect between salt loading and porous structure optimisation, which accelerated mass transfer. Similarly, Chen et al. (Chen et al. 2022) assessed a solar-assisted absorption–compression system designed for residential heating and cooling. Their results showed that low-grade solar heat at 60°C significantly enhanced system performance, raising COP by up to 56% and reducing electricity use by over 40%, demonstrating strong potential for efficient, low-carbon building applications.

Further studies demonstrate the versatility of sorption systems for both heating and cooling. For instance, Liu et al. (Liu et al. 2005) developed a compact adsorption chiller using silica gel–water, achieving a COP of 0.5 and 9kW cooling capacity at 13°C evaporation. System improvements such as heat and mass recovery further enhanced efficiency under low-grade heat input between 70-95°C. Another study by Aman et al. (Aman et al. 2014) evaluated a 10kW solar-assisted ammonia/water absorption chiller for residential air conditioning. Their analysis showed that while the system's COP is modest, substantial exergy losses occur in the absorber and generator, revealing key optimisation targets for improving solar-driven cooling efficiency.

These findings collectively highlight how adsorption and absorption technologies can be tailored for dual-mode operation with improved efficiency, particularly when integrated with low-grade or renewable heat sources.

Thermochemical sorption heat storage system is an advanced technology to address the balance between energy consumption and energy demand in building service system. In sorption process (Figure 8) heat is stored by breaking the binding force between the sorbent and the sorbate by chemical potential.

Thermochemical materials have a much higher energy density than common sensible and latent heat storage materials, and also, thermochemical heat storage system has a much lower heat loss (Aydin et al., 2015, Bales, 2006, H Abedin and A Rosen, 2011). Thermochemical heat storage system includes different types: based on physisorption in adsorbing materials (e.g. zeolites), based on absorption in hygroscopic solutions (e.g. NaOH solution) and based on absorption in salts (e.g. salt hydrates) (Zondag et al. 2013).

Recent researches in THS focus on composite material, which combines salts and matrix, or salt in matrix (Sahlot and Riffat 2016). Because of the porosity of the adsorption materials, thermochemical materials such as vermiculite, pillar layered silica, silica gel, zeolite and carbon fibre are also used as host materials. A variety of porous materials have been investigated as hosts for thermochemical energy storage due to their sorption properties, structural characteristics, and cost-effectiveness. Traditional materials like vermiculite and silica gel offer moderate water uptake (0.03–0.35 g/g) with low regeneration temperatures (<150°C) and relatively low cost (Aydin et al., 2015, Zhang et al., 2016). Zeolites, including 13X, exhibit higher thermal stability and moderate uptake, though regeneration typically requires temperatures above 200 °C (Aydin et al., 2015, Bahrami, 2012, Li et al., 1999, Kyotani et al., 1997, Tatsidjodoung et al., 2016, Rao et al., 2006). Activated carbon materials, both powder and fibre, provide high specific surface areas (up to 1850 m²/g) and variable porosity, making them promising for sorption applications (Hamamoto et al., 2006b, Tso and Chao, 2012, Aydin et al., 2015, Hamamoto et al., 2006a, Wang et al., 2005). Expanded graphite (Aydin et al., 2015, Wu and Wang, 2015, Zhang and Fang, 2006, Du and Jana, 2007) and mesoporous silica (e.g., SBA-15) (Glaznev et al., 2011, Chen et al., 2009, Carrero et al., 2017) have also demonstrated good performance, particularly under vacuum or moderate heating conditions. Some novel natural materials, such as Wakkanai Siliceous Shale (WSS) (Liu et al. 2015), combine moderate porosity (60%) with low regeneration temperatures (~80°C), offering additional potential for low-grade heat recovery systems.

In the simple solar driven thermochemical energy storage system (Figure 9), energy was stored in the hydrated salt in hot summer and released in cold winter. In summer, solar array provides heat to charge the system (dry materials) and water vapour will be released to the ambient environment. In winter, cold and humid air goes into the fan and adsorbed by hydrated salt, then released to the ambient as warm and dry air (Zondag et al. 2013).

In Zondag et al. research, a prototype was built with a capacity of 17 L MgCl₂·6H₂O as sorption materials includes a solar PV heat source, heat exchangers and fan (Figure 2-8) (Figure 2-9) (Zondag et al. 2013). An air flow was about 500 l/min with the pressure of 12 mbar. Results showed that MgCl₂·6H₂O has a very high energy density of 0.5 GJ/m³ and the system can supply about 150 W of thermal energy with only 50 W heat loss for over 40 hours with a COP of 12. The reaction formula (Zondag et al. 2013) is shown below:

Jong et al. stated that thermochemical materials provide a promising solution to achieve a long-term 100% renewable energy economy (de Jong et al. 2016). In their simulation within the MERITS project, Na₂S had a heat storage density of 0.14 GJ/m³ in the experiment. Development has been done to improve the heat storage density by enhancing vapour and heat transport level (de Jong et al. 2016). However, Na₂S has the risk of corrosion and reaction and a hazard of H₂S emissions (Craig, 1995, Warnqvist, 1980, Roelands et al., 2015, Jiang et al., 2016, Donkers et al., 2017). The Australia Group claimed that Na₂S is a dual use material (Isbell 2016).

NaS/H₂O was used on the SWEAT prototype by Boer et al. (Boer R, 2004) and the energy storage density was measured as 1.1 kWh/kg (1980kWh/m³) for heating (COP=0.84) and 0.7 kWh/kg for cooling (COP=0.57).

During reaction process, thermochemical materials (sorbate) adsorb moisture from the ambient air directly or a humidifier. In the charging process, heat from solar or other heat source can dry the materials to store energy for later use. The thermochemical heat sorption system is an easy process design operation system; however, the long-term storage performance of the reactor is the most important part as the moisture adsorption and heat generation act at same time which may cause some heat loss.

In open-cycle systems, water vapour from ambient air directly contacts the TES material, streamlining the setup by removing the need for separate fluid circuits. Zhang et al. (Zhang, Y., 2022) tested a prototype using a hygroscopic salt that absorbed moisture and stored heat during charging, then released both during discharging to supply warm, dry air indoors. While effective for combined heating and dehumidification, the system still faced issues such as humidity regulation and thermal losses. Nonetheless, the results suggest open-cycle configurations hold promise for low-cost, decentralised heating in moderately humid climates.

Integrating TES with renewable heat sources—especially solar thermal—plays a key role in enhancing the reliability and sustainability of energy systems. Recent developments have shown that such integration can efficiently convert fluctuating low-grade heat into a stable energy supply. For example, a solar-assisted open-loop cooling system was proposed that combines a desiccant wheel and indirect evaporative coolers with a transcritical CO₂ heat pump powered largely by PV panels (Lo Basso et al. 2023). Simulations over a year revealed renewable shares exceeding 50%, with potential to reach 77% (electric) and full thermal self-sufficiency, while the CO₂ heat pump’s COP could be optimised from 2.3 to 6.5. In another study, a solar-driven sorption cooling unit incorporating a dehumidifier and desiccant cold storage achieved 800–900 W cooling output and reduced ventilation air temperature by up to 19 K (Nielsen et al. 2022). Silica gel performed best among tested materials, and airflow tests showed acceptable pressure drops, supporting the system’s applicability in eight out of fourteen Sunbelt climate zones.

Linking TES systems with renewable sources like solar thermal not only helps stabilise the supply of low-grade heat but also enhances energy reliability and efficiency. By smoothing out fluctuations in heat availability, such integration supports continuous storage operation and more consistent thermal output—key to resilient energy infrastructure.

Recent innovations across both closed and open TES configurations have strengthened this potential. From chemical heat pumps to sorption-based and open-cycle designs, these systems are increasingly tailored for renewable integration. As a result, TES is emerging as a flexible and low-emission solution for reducing primary energy demand and reinforcing the sustainability of future energy systems.

Fixed-bed reactors are widely adopted in TES systems, where the storage medium remains stationary while heat-carrying air or steam flows through. Their low cost and straightforward design make them attractive, though they can face issues like flow non-uniformity and limited heat transfer in larger applications.

Recent work on CaL-based TES systems has shown promising results using this configuration. For example, CaCO₃ decomposition in nitrogen at 850°C achieved nearly 64% conversion, while subsequent carbonation at 750°C reached 67.2% (Tian et al. 2024). Reducing the material filling improved heat efficiency but lowered total output. Simulations further revealed that higher calcination temperatures and porosity levels (0.6–0.7) significantly influence performance, whereas carbonation temperature and material conductivity were less critical. These insights support improved reactor design and material optimisation for thermochemical applications.

Fluidized-bed reactors improve thermal performance by suspending TES particles in a gas stream or using mechanical mixing, which enhances contact between the heat transfer medium and reactive materials. This configuration is particularly effective for low-temperature applications with porous salt composites.

One study proposed a mechanically stirred plow-share fluidized bed to overcome the poor flowability and low conductivity of CaO/Ca(OH)₂ powders (Risthaus et al. 2022). By replacing gas flow with mechanical agitation, the system achieved hydration and dehydration with heat transfer coefficients significantly higher than those of fixed beds, confirming its compact and efficient potential. Another fluidized-bed design, powered by solar energy, used doped CaCO₃ pellets and sustained over 85% energy capacity after 100 cycles (Zheng et al. 2023). Improved absorptance and kinetics raised solar-thermal efficiency from 9% to 19%, while fluidization reduced thermal gradients and mass transfer limitations.

A third study presented an autothermal, directly irradiated fluidized-bed reactor tailored for concentrated solar power TES systems (Padula et al. 2021). Using limestone-based reactions and a built-in counterflow heat exchanger, the design recovered sensible heat and achieved simulated charging efficiencies up to 90% at ~900°C, with discharge efficiencies around 80%. These examples underscore the scalability and efficiency of fluidized-bed technologies in solar TES.

To enhance heat transfer in TES systems, finned plates and tube structures are often used to embed thermochemical materials and increase effective surface area. For example, aluminum plate–tube exchangers combined with porous elements like nickel foam have shown high heat output in CaCl₂-based reactors. A notable case is the hybrid solar–methanol energy system developed by Qin et al. (Qin et al. 2024), which combines parabolic trough solar collectors, a thermochemical reactor with plate-tube geometry, and an internal combustion engine. This setup delivered 72.09% total energy efficiency and 24.63% solar-to-electric conversion, alongside significant reductions in both fuel use (32.97%) and CO₂ emissions (25.33%). The compact heat exchanger design played a key role in maintaining efficient and flexible performance under varying solar conditions.

Building on this, Ding and Wu (Ding and Wu, 2021) introduced a compression-assisted absorption TES cycle that improves performance by applying auxiliary compression during both charging and discharging phases. This design boosted energy efficiency from 0.58 to 0.67 and more than doubled the energy density to 282.8 kWh/m³, even at a relatively low charging temperature of 80°C. Enhanced reactor configuration also enabled quicker thermal response, making the system a strong candidate for efficient low-temperature heat storage.