Thermochemical Energy Storage Technology Research Articles

Thermochemical energy storage performances of Co3O4-based honeycombs with multi-scale composite pores

Metal oxide redox system characterized open-loop operation, high energy density, and high reversibility, which is one of the most promising thermochemical energy storage technologies for the next-generation concentrated solar power plants. Most of the previous studies focused on the material properties, while the energy storage performance of oxide monolithic bodies, such as energy storage density and heat storage/release rate, has rarely been reported. In this paper, a novel and effective method for the improvement of the energy storage performance of oxide honeycombs with multi-scale composite pores is proposed and presented. Methyl cellulose and four common biomass materials, including bagasse, loofah, juniper leaf, and pine needle, were used as pore-forming agents for oxide honeycombs. The pyrolysis of methyl cellulose forms small-scale pores in the honeycomb bodies and contributes the major pore volume. The biomass templates further optimize the pore characteristics, all biomass-templated Co3O4-based honeycomb bodies have higher total pore volume and permeability than that of biomass-free honeycomb body. The honeycomb with 2.5 wt% pine needle achieves the highest energy storage density, with an average of 694.62 kJ/kg during the second to fifteenth cycles. In addition, the honeycomb containing 7.5 wt% pine needle has the highest energy store/release rate. Biomass-templated Co3O4-based honeycomb shows a high crushing strength, and there are no obvious cracks and structural deformations on its surface after 15 thermal cycles. The findings demonstrate that the methyl cellulose and biomass template method can effectively optimize the pore structure and energy storage properties of the oxide honeycombs, thereby expanding the application prospects of Co3O4-based honeycombs in thermochemical energy storage.

Development and Characteristics Analysis of Novel Hydrated Salt Composite Adsorbents for Thermochemical Energy Storage

New composite adsorbents are proposed to further improve the application of thermochemical energy storage technology in buildings. A volcanic is taken as an adsorption substance, which is impregnated in 36.50 wt% and 54.00 wt% saturated MgCl2 and CaCl2 solutions to prepare composite adsorbents, which are called composite-MgCl2 and composite-CaCl2, respectively. According to the characterization, the main pore structure of the original volcanic is macropores (>100 nm), and hydrated salts tend to fill them. Compared with zeolite-MgCl2, the final water uptake of composite-MgCl2 and composite-CaCl2 increased by 0.15 g/g and 0.03 g/g. Meanwhile, the TG-DSC measurement results show that the thermochemical energy storage densities of composite-MgCl2 and composite-CaCl2 are 1.02 and 1.56 times that of zeolite-MgCl2, which are 642 kJ/kg and 983 kJ/kg, respectively. Moreover, the composition of the thermochemical energy storage densities of the composites is obtained by theoretical calculations, and the theoretically calculated results are close to the measured results. After several cycles, the composites still have high thermochemical energy storage capacity and low energy storage density cost.

Activity enhancement of MgCO3/MgO for thermochemical energy storage by nitrate-promoted and morphology modification

Thermochemical energy storage technology, with its ability to effectively solve the mismatch between energy supply and demand, holds considerable promise. Magnesium carbonate/magnesium oxide (MgCO3/MgO) in particular has shown broad prospects due to its high energy density and trans-regional storage capability. However, pure MgO has limited activity in the exothermic process, with a conversion rate of less than 2%. In this work, activity enhancement of MgCO3/MgO was investigated by combining chemical modification of alkali nitrates and physical morphology regulation. The results indicate that the activation effect of dopants on inert MgO depends on the electronegativity of the cation and the thermal stability of compound dopants. In particular, Na0.5K0.5NO3 with a molar ratio of 1:0.1 promotes MgO carbonation, resulting in a conversion rate of up to 67.5% and an exothermic density of 1964.25kJ/kg at 320°C. The enhanced activity is attributed to the melting of dopants that induces lattice distortion of the perfect crystal, hereby accelerating the chemical reaction. Additionally, the utilization of finely grained rod-like nanocrystalline magnesia precursor with a high specific surface area increased the adsorption capacity. Consequently, the synthesis of mesoporous MgO through the sol–gel method successfully broke through the CO2 adsorption of MgO carrier, achieving a conversion rate of 72.5% and a exothermic density of 2109.75kJ/kg MgO. During 20 release-storage cycles of the MgCO3/MgO TCES carrier, the conversion rate initially degraded significantly and eventually stabilized at approximately 25%. Overall, these methods provide a guiding principle for the preparation of thermochemical energy storage carriers in subsequent applications.

Experimental demonstration of high-temperature (>1000 °C) heat extraction from a moving-bed oxidation reactor for thermochemical energy storage

Previously developed reduction-oxidation (redox) thermochemical energy storage technologies must store their products at high temperatures, complicating handling and transportation. This work describes a countercurrent, tubular, moving bed oxidation reactor at laboratory scale that produces high grade heat and allows solids to enter and exit the system at ambient temperatures. The particles implemented in the system consist of a novel magnesium‑manganese-oxide material well-suited for thermochemical energy storage. Output heat is obtained via a separate extraction gas flow, which exits from the middle portion of the main reactor tube. With this design, reactor temperatures in excess of 1000 °C and extraction temperatures above 950 °C were achieved. Deviation between the two measurements is a result of extraction thermocouple placement and losses in the reactor extraction arm; improvements to these parameters would bring the extraction temperature closer to the bed temperature. The reactor produces enough energy via oxidation to sustain both heat extraction and continued chemical reaction. During one representative steady state experiment at a particle flow rate of 1.5 g/s, an average of 447 W was extracted from the reactor out of an estimated 1083 W of released chemical energy for a duration of 70 min. Among the four experiments, the maximum bench-scale oxidation reactor energy efficiency of 36.2% and corresponding round-trip efficiency of 13.7% considering both redox reactors were demonstrated. Characteristics of an ideal system are considered and future improvements are proposed.

Efficient direct solar-driven thermochemical energy storage of (AlMgFeMn)OxCaCO3 pellets in a fluidized bed reactor

Calcium looping (CaL) is one of the most promising thermochemical energy storage technologies for high-temperature applications such as next-generation concentrated solar power (CSP) systems. However, most previous investigations have mainly focused on optimizing Calcium-based materials to maintain their reactivity during cycling, while their behavior in reactors under direct solar irradiation has rarely been reported. In this paper, highly efficient and stable direct solar-driven thermochemical energy storage in fluidized reactors is demonstrated. (AlMgFeMn)OxCaCO3 pellets demonstrated excellent long-term stability with an energy storage density of more than 85% of the initial value after 100 cycles. The underlying mechanism can be attributed to the presence of poly-oxide (AlMgFeMn)Ox crystals, which prevent crystallite growth and sintering, as confirmed by in-situ X-ray diffraction analysis. Moreover, the solar-thermal conversion efficiency of (AlMgFeMn)OxCaCO3 pellets in fluidized bed reactors is significantly improved from 9% to 19% thanks to the considerably increased average solar absorptance and fast reaction kinetics over white (AlMg)OxCaCO3 pellets. The experimental analysis using an operando fluidized thermogravimetric analyzer (F-TGA) further revealed that the interparticle diffusion control limitation in traditional TGA or fixed bed and localized overheating due to Gaussian distribution of solar irradiation are successfully relieved in a fluidized bed. We further suggest that steam has a positive effect on enhancing reaction kinetics and stability by performing 10 energy storage/release cycles of (AlMgFeMn)OxCaCO3 pellets under direct irradiation of concentrated light due to the increase of surface area after rehydration and the higher OH reactivity toward CaO. This work paves the way for the application of solar-driven fluidized bed reactors for scalable thermochemical energy storage.

Titania coatings: A mechanical shield for cohesive granular media at high temperatures

Fine granular media are pivotal in thermochemical energy storage technology. Reactors based on granular materials store the heat using reversible reactions at high temperatures. Yet, powders become increasingly cohesive in those conditions. The rise of powder cohesion at high temperatures is one of the most irksome phenomena still limiting the scalability of this technology. We found titania coatings comprise an excellent solution to control cohesion in fine limestone powders at high temperatures. Limestone is the main component in granular flows running solid-based storage circuits based on the calcium looping process. It is also involved in many other industrial applications. Titania layers were used to shape stiffer carbonate surfaces at high temperatures (close to the Tamman point). The experiments conducted in this work investigated the benefits of these layers, examining the powder cohesion as the contact between particles evolved from rigid to plastic surfaces. In doing so, samples were subjected to different temperatures varying from 25 °C to 500 °C and preconsolidations up to 2 kPa. The results revealed that titania coatings shield (mechanically) carbonate particles, making surfaces more resilient to deformation while particles interact. The efficiency of titania layers was compared with samples coated with nanosilica, which is a solution broadly accepted nowadays for limestone powders. The experiments tackled one of the weaknesses of nanosilica coatings, namely their efficiency when particles are barely coated. Interestingly, at high temperatures, samples treated with titania outperformed those layered with nanosilica for surface coverages around 9%. Moreover, despite such a moderate amount of coverage, samples coated with titania reached an easy-flow regime even at high temperatures. However, samples treated with nanosilica fluidized less uniformly, and their flowability fell into a cohesive-flow regime in similar conditions. In conclusion, titania coatings represent an excellent alternative to deal with those flowability issues that still limit the scalability of solid-based storage technology.

Values of latent heat and thermochemical energy storage technologies in low-carbon energy systems: Whole system approach

Thermal energy storage (TES) is widely expected to play an important role in facilitating the decarbonization of the future energy system. Although significant work has been done in assessing the values of traditional sensible TES, less is known about the role, impact and value of emerging advanced TES at the system level. This is particularly the case of latent heat thermal energy storage (LHTES) and thermochemical energy storage (TCS). In this context, this paper is dedicated to evaluating the techno-economic values for the whole UK energy system of LHTES and TCS technology using an integrated whole energy system model. First, the key concepts of the whole system modelling framework are introduced. Unique to this work is that the economic benefits delivered by LHTES and TCS to different levels of the UK energy system infrastructure and various energy sectors through the deployment of TES are explicitly analysed, which comprehensively demonstrates the values of selected TES technologies from the whole system perspective. A series of sensitivity studies are implemented to analyse the advantages and disadvantages of LHTES and TCS under different conditions. The simulation results indicate that TES can benefit different sectors of the whole energy system and drive significant cost savings, but the whole system values of TES is closely dependent on the decarbonization requirement. Although LHTES is characterized by relatively low capital costs, when TES penetration is limited and carbon target is tight, the advantage of TCS is outstanding due to its high energy density.

Life cycle and environmental assessment of calcium looping (CaL) in solar thermochemical energy storage

Calcium looping is a promising thermochemical energy storage process to be integrated into concentrating solar power plants. This work develops for the first time a comprehensive life cycle assessment of the calcium looping integration in solar plants to assess the potential of the technology from an environmental perspective. Two representative integrations are analysed, representing daily (hot) and seasonal (cold) storage designs. Similar performance environmental impacts are observed in both, with slightly better results for the seasonal storage case due to the simplified energy storage integration. The results show the moderate environmental impact of calcium looping thermochemical energy storage technology, resulting in lower equivalent carbon dioxide emissions (24 kg/MWh) than other energy storage options such as molten salt-based solar facilities (40 kg/MWh). Plant construction involves a higher energy demand for the process, whilst the operation and maintenance on the plant represent a moderate impact due to the low environmental impact of limestone, the unique raw material of the process, and the lower water consumption compared to typical concentrating solar power plants. Besides, the energy required for the system is first time analysed, obtaining an energy payback time of 2.2 years and 2.5 years depending on the storage strategy design.

Thermodynamic analysis of absorption energy storage cycle with choline based green solvents

The absorption refrigeration and heat pump system and the thermochemical energy storage technology have natural and easy combination characteristics. The working pairs store and release low-grade heat in the form of thermochemical energy by changing the concentration of the solution, which can realize thermal or cold storage. This paper uses three deep eutectic solvents of choline chloride as absorbents and water as the refrigerant. The working fluid pair has a competitive or higher performance, ignoring the power consumption caused by flow resistance. The influence of generation or charging temperature on refrigeration performance and energy storage density were theoretically studied. The results show that water/Ethaline performs best under a typical single-effect working condition in cycle heat storage ratio and energy storage density, up to 0.87 and 677.8 kJ/kg; Glyceline and Reline have a better application potential when driven temperatures increase.

In-situ catalytic reforming of converter gas in converter flue based on thermochemical energy storage: Kinetics and numerical simulation

The converter gas, a major by-product of converter steelmaking, contains abundant and available physical heat. The reforming of converter gas with coke oven gas in a converter flue, which recycles the waste heat of the gas by a thermochemical energy storage technology, is an effective process to enhance the waste heat recovery rate and quality of the converter gas and reduce the direct emission of CO2. To obtain the reforming rate in the converter flue, the kinetics of the CH4–CO2 reaction, the main reaction in the reforming, catalyzed by converter dust was investigated through experiments. The reaction order, activation energy, and preexponential factor were determined. The partial order of CH4 was 1, while the partial order of CO2 was 0 in the reaction. The converter dust had a considerable catalytic effect on the reforming reaction. The adding amount of converter dust significantly affected the activation energy and preexponential factor. The reforming of the converter gas with the coke oven gas in the converter flue was then investigated through a numerical simulation. The waste heat recovery rate reached a peak of approximately 50%, and about 30% of the heat was converted into chemical energy when the ratio of the coke oven gas added into the converter flue was 31.3 to 37.5%. Under these conditions, the conversion rates of CH4 and CO2 were above 70% and 35%, respectively. The influence of the converter gas flow rate on the reforming was studied. The increases in the inner diameter and length of the converter flue are beneficial to improve the waste heat recovery rate as well as the conversion rates of CH4 and CO2.

Thermal performance study of thermochemical reactor using net-packed method

Thermochemical energy storage (TCES) is proving to be a promising option for residential energy storage applications. Many studies have indicated that the pressure drop and heat and mass transfer efficiency of the reactor are two key determinants with TCES technologies. In this paper, net-packed method is used to carry out thermal performance enhancement study for a thermochemical reactor. Compared with and the without net packed methods, copper mesh shows better thermal performance. The copper mesh method of zeolite achieved maximum temperature rise of 38.6 °C, which is 4.4 °C and 2.3 °C higher than that of the non-packed and aluminum mesh, respectively. The pressure drop is reduced by 28.1% and higher water adsorption has been realized. In addition, the energy storage density of 784.4 kJ/kg and thermal efficiency of 38.2% are obtained with the copper mesh. These are 1.6 and 2.4 times the results of the non-packed tests respectively.

Reforming of converter gas with coke oven gas for thermochemical energy storage and carbon dioxide emission reduction

During traditional processing and utilizing of converter gas, a common by-product gas of steel production, the recycling of waste heat is insufficient, carbon emission is high, and the added value of the gas is low. The reforming of converter gas with coke oven gas is proposed to utilize the gas more efficiently. The reforming recycles the waste heat of the gas by thermochemical energy storage technology and converts CH4 and CO2 into H2 and CO, which improves the recovery rate of the waste heat and gas added value, and reduces the direct emission of CO2. Thermodynamic equilibrium calculation and regression analysis were used to investigate the effects of the temperature, pressure, and mole fraction of coke oven gas on the results of the reforming. The results show that atmospheric pressure and the temperature range of 850 °C–1000 °C are suitable for the reforming, at which the conversion rates of CH4 and CO2 are more than 90% and 75%, respectively. The ratio of H2 to CO is determined by the mole fraction of coke oven gas. Then, the reforming process was investigated by experiments which simulated the reforming process in converter flue. The experimental results show that the in-suit catalysis of converter dust can significantly improve the conversion rates of CH4 and CO2 and the conversion rates can reach a high level at about 1000 °C. Then, the waste heat utilization of the reforming was investigated, and it was concluded that 35–40% of the waste heat is converted into chemical energy in the reforming. After the waste heat power generation of the product gas, the total recovery rate of waste heat is 60–65%. Finally, the amount of coke oven gas reformed by a fixed amount of converter gas without additional heating was calculated. The results show that when the preheating temperature of coke oven gas is 500 °C the range of 850 °C–900 °C is appropriate for the reforming, at which the conversion rate of CO2 is approximately 50% and about 40% of the waste heat was converted to chemical energy. For a steel plant with an annual output of 20 million, the direct emission of CO2 can reduce by 2.13 × 108 Nm3, and waste heat of 3.24 × 106 GJ can be converted into chemical energy.

Structurally improved, TiO2-incorporated, CaO-based pellets for thermochemical energy storage in concentrated solar power plants

Calcium looping process (CaL) is a promising thermochemical energy storage technology for the application in the 3rd generation concentrated solar power plants (CSP). Hence, it is necessary to develop the highly efficient CaO-based heat storage composite materials. The structurally improved, CaO-based pellets were prepared via adding cellulose as the pore-forming template. Furthermore, the approach of TiO2 incorporation was adopted aiming to enhance the anti-sintering ability of the structurally improved, CaO-based pellets. It is found that the structurally improved, TiO2-incorporated, CaO-based pellets that containing 25 wt% of TiO2 exhibit the highest CaO carbonation conversion and energy density of 57.5% and 1827.7 kJ/kg after 20 cycles, which are nearly 1.3 times as high as those of the pure Ca(OH)2 pellets. The superiority of the structurally improved, TiO2-incorporated, CaO-based pellets on cyclic heat energy storage/release is more prominent under harsh calcination conditions (950 °C, pure CO2). The evenly distributed CaTiO3 grains, providing structural stability to the CaO-based pellets and also enhancing the anti-sintering ability, are responsible for the improved cyclic energy storage/release performance. Additionally, the desirable compression strength (16.67 ± 1.05 MPa) contributes to strengthen the practicability of structurally improved, TiO2-incorporated, CaO-based pellets applied in the CaL-CSP system.

The effect of the reaction equilibrium on the kinetics of gas-solid reactions — A non-parametric modeling study

The viability of thermochemical energy storage for a given application is often determined by the reaction kinetics under process conditions. For high exergetic efficiency the process needs to operate in close proximity to the reaction equilibrium. Thus, accurate kinetic models that include the effect of the reaction equilibrium are required.In the present work, different parametrization methods for the equilibrium term in the General Kinetic Equation are evaluated by modeling the kinetics of two reaction systems relevant for thermochemical energy storage (CaC2O4 and CuO) from experimental data. A non-parametric modeling method based on tensor decompositions is used that allows for a purely data driven assessment of different parametrization methods.Our analysis shows that including a suitable equilibrium term is crucial. Omitting the equilibrium term when modeling formation reactions can lead to seemingly negative activation energies. Our tests also show that for formation reactions, the reaction rate decreases much faster towards the equilibrium than theory predicts. We present an empirical modeling approach that can predict the reaction rate of gas-solid reactions, regardless of the shortcomings of theory. In this way, non-parametric modeling offers a powerful tool for applied research and may contribute to the advancement of the thermochemical energy storage technology.

Techno-economic analysis of an integrated liquid air and thermochemical energy storage system

The rapid increase in renewable energy applications has heightened the need for developing efficient and cost-effective energy storage technologies. In this study, a novel integrated liquid air and thermochemical energy storage system is proposed and examined. This integrated storage system is found to be superior in many aspects than both the stand-alone liquid air energy storage and thermochemical energy storage technologies, including high energy storage density, high round-trip efficiency, no geographical limitation, and negligible environmental concern. These are derived from the synergies when integrating those two subsystems. More specifically, the liquid air energy storage subsystem ensures a minimum storage volume of air and a high round-trip efficiency of the integrated system, while the thermochemical energy storage subsystem allows it to have a high energy storage density and high operating temperature without the necessity of burning fossil fuels. To assess the performance of the integrated storage system, thermodynamic and economic analyses are carried out by using Aspen Plus v10. According to the thermodynamic analysis, the round-trip efficiency and the energy storage density of the integrated storage system are found to be 47.4% and 36.8 kWh/m3, respectively. The round-trip efficiency is about 13.3% higher than that of the stand-alone thermochemical energy storage system and the energy storage density is nearly 3.4 times that of the stand-alone liquid air energy storage system. In terms of the economic performance, the integrated system with a plant size of 60 MWe presents a payback period of around 10 years and a levelized cost of electricity of 179–186 USD/MWh over a 30-year period.

Progress in thermochemical energy storage for concentrated solar power: A review

Energy plays an important role in a fast-paced modern society. With the depletion of fossil energy, effective utilization of solar energy is getting increasingly urgent. Thermal energy storage is an inevitable choice for effective utilization of renewable energy sources. As one of the most promising renewable energy sources, solar energy is inexhaustible. But it has some shortcomings such as instability and intermittency, affected by time, climate, and geographical location. Thermal energy storage technology, which can effectively reduce the cost of concentrated solar power generation, plays a crucial role in bridging the gap between energy supply and demand. In addition, thermal energy storage subsystem can improve performance and reliability of the whole energy system. According to different principles, thermal storage technology is generally classified as sensible heat storage, latent heat storage, and thermochemical energy storage. Most solar thermal power generation systems, currently demonstrated and operated in the world, adopt the method of sensible thermal energy storage. In contrast, thermochemical energy storage is a relatively new concept, which is still in the stage of basic test and verification. Thermochemical energy storage technology stores and releases energy through endothermic and exothermic reversible reactions. A closed system with separated reactants and products, in theory, can store energy indefinitely. The main thermochemical energy storage systems include redox system, metal hydride system, carbonate decomposition system, ammonia decomposition system, methane reforming system, and inorganic hydroxide system.

Energy Performances of Open Sorption Reactor with Ultra-Low Grade Heat Upgrading for Thermochemical Energy Storage Applications

The work discusses a problem of harvesting and upgrading of ultra-low grade heat with thermochemical energy storage technology for space and domestic water heating in residential area. The laboratory scale prototype, operating on the principle of an open packed bed sorption reactor and using moist air as a heat/mass transfer fluid, is experimented. The range of experimental air temperature was set to 17–40 °C, which corresponds to the typical range of domestic waste thermal energy. The tested sorbent was a salt-in-matrix composite material composed of a silica gel containing 43 wt.% of calcium chloride (CaCl2) salt. Hygrothermal behavior and energy performances of the prototype control volume filled with 245 g of material, representing the reactive front of a thermal wave, were analyzed at constant inlet hydration conditions (water vapor pressure of 12.5 mbar). The average temperature lift was recorded as 9–13 °C, representing the amplification of a supplied heat on 23% – 75% depending on the inlet temperature. The average specific thermal power inside the material bed was measured to be 168–267 W kg-1. The apparent energy density, based on the prototype control volume, ranged between 1.0 and 1.6 GJ m-3. Taking into account the heat of water vaporization, the coefficient of performance of the process was determined to be 0.96–1.57.

Thermochemical energy storage by consecutive reactions for higher efficient concentrated solar power plants (CSP): Proof of concept

Concentrated solar power plants (CSP) combined with thermal energy storage (TES) offers the benefit to provide continuous electricity production by renewable energy feed. There are several TES technologies to be implemented, being the thermochemical energy storage the less studied and the most attractive since its volumetric energy density is 5 and 10 times higher than latent and sensible TES, respectively. Thermochemical energy storage technology is based on reversible chemical reactions, also named thermochemical materials (TCM). One of the main challenges of TCM is to achieve a proper reversibility of the reactions, which in practical conditions leads to lower efficiencies than the theoretically expected. A new concept based on changing from reversible TCM reactions towards TCM consecutive reactions aims to eliminate reversibility problems and therefore improve the overall efficiency. Consecutive TCM reactions can either be based in one cycle, where reactants are needed to feed the reaction, or two coupled cycles which offer the possibility to work without any extra mass reactants input. The plausibility of the implementation of both concepts in CSP is detailed in this paper and case studies are described for each one.

Sulfur dioxide disproportionation for sulfur based thermochemical energy storage

Sulfur dioxide disproportionation is one of three reaction steps that make up the sulfur based thermochemical cycle used for thermal energy storage of concentrated solar power. The characteristics of this reaction were studied using thermodynamic modeling and laboratory measurements. Modeling results showed full disproportionation can only be achieved at pressure. The reaction driving force is enhanced by system pressure but declines with increasing temperature. Appropriate water to sulfur dioxide ratio also drives disproportionation. Batch experiments showed that reaction rate increases with temperature. A catalyst survey identified homogenous iodides as catalysts that can improve the reaction rate by up to twenty times while increasing the apparent extent of disproportionation. The dependence of disproportionation rate on sulfuric acid concentration was established via constant pressure experiments. Means to recover the iodide catalyst from sulfuric acid and molten sulfur for reuse were demonstrated. Modeling and test results were used to establish a design concept for a sulfur dioxide disproportionation reactor system capable of rapidly generating sulfur as required for the sulfur based thermochemical energy storage technology.

Thermochemical energy storage technologies for building applications: a state-of-the-art review

This paper presents a comprehensive and state-of-the-art review on thermochemical energy storage (ES) technologies using thermochemical materials (TCMs) for building applications. Thermochemical storage devices (materials, open and closed sorption as well as chemical heat pump) enhance the energy efficiency of systems and sustainability of buildings by reducing the mismatch between supply and demand. Thermal ES (TES) systems using TCMs are particularly attractive and provide a high ES density at a constant temperature. Technical and economical questions will need to be answered for all possibilities, which warrant more development and large-scale demonstration of TES in future. Copyright , Oxford University Press.