The use of concrete geostructures for energy extraction and storage in the ground is an environmentally friendly and easy way of cooling and heating buildings. With such energy geostructures, it is possible to transfer energy from the ground to buildings by means of fluid-filled pipes cast in concrete. By injecting thermal energy in summer and extracting it in winter, the ground in the area of a building’s piles can be used for seasonal energy storage, as long as the underground water flow in the storage remains low. Most buildings use piles for the foundation, and these piles can easily contain heat exchangers in contact with the soil. The so-called ‘energy pile’ can thus fulfill a double function: foundation plus heat exchange. Heat pumps can then transfer heat very efficiently from the ground to be building interior and vice-versa. Heat exchange with the ground can be via thermo-active foundations. In thermo-active foundations, foundation piles also referred to as "thermal piles" or as "energy piles", are used as heat exchangers for supplying low-temperature heat to heat pumps. They can also be used for underground storage of warmth supplied by road solar collectors. Some suppliers also propose heat storage in concrete piles as a flexible installation that is not used as a building foundation and can be used as an add on to an existing building or district (see Energy Nest). Concrete has a density around 2240 kg/m³ with a specific heat capacity of 880 J/kg K. This results in an average storage density of 1971 kJ/m³ K. The thermal conductivity of reinforced concrete is around 1.5 W/m K. ***Applications:*** * Building / Residential storage * Separate storage units containers (Energy Nest)

TES system using underground rock carven is considered as an attractive alternative for largescale storage, because of low thermal conductivity and chemical safety of surrounding rock mass. Based on some successful applications of cavern storage and high-temperature storage reported in the literature. The applicabilities and practicabilities of storage media and technologies for largescale cavern thermal energy storage (CTES) were reviewed. Ex: Basalt rocks Natural cave storge can be based mostly on rock and granite. Rock was mentioned in 1.2 with a storage capacity of 2250 kJ/m³ K. Granite has a density of 2640 kg/m³ with a specific heat capacity of 820 J/kg K, giving a storage capacity of 2164 kJ/m³ K. ***Applications:*** * Large scale

Aquifer thermal energy storage (ATES) is the storage and recovery of thermal energy in the subsurface. ATES is applied to provide heating and cooling to buildings. Storage and recovery of thermal energy are achieved by extraction and injection of groundwater from aquifers using groundwater wells. Systems commonly operate in a seasonal mode. The groundwater that is extracted in summer, is used for cooling by transferring heat from the building to the groundwater by means of a heat exchanger. Subsequently, the heated groundwater is injected back into the aquifer, which creates storage of heated groundwater. In wintertime, the flow direction is reversed such that the heated groundwater is extracted and can be used for heating (often in combination with a heat pump). Therefore, operating an ATES system uses the subsurface as a temporal storage to buffer seasonal variations in heating and cooling demand. When replacing traditional fossil-fuel dependent heating and cooling systems, ATES can serve as a cost-effective technology to reduce the primary energy consumption of a building and the associated CO₂ emissions. Some important parameters for an ATES installation are high ground porosity, medium to high hydraulic transmission rate around the boreholes, but a minimum of groundwater flow through the reservoir. Groundwater chemistry represents another set of parameters that must be given proper attention in order to prevent scale formation and furring. Numerous ATES facilities are in operation in Sweden, Germany, The Netherlands, Belgium, and some other European countries, including a system for heating and cooling at Oslo Hovedflyplass Gardermoen. The Netherlands is probably the technological leaders in the field. The efficiency of an aquifer in terms of storage and retrieval is around 70-90%. The cost of this storage method is 660-1100 euro/kW or approximately 22-33 euro/m³ (highly dependent on the size). The barriers for this method are the required site-specific hydrogeological conditions, relative lower energy density, lack of experience/ technical uncertainty and the requirements to balance the in and out of thermal energy to balance the aquifer. In the figure, the cost of four different water-based storage methods is shown. ***Applications:*** * Residential * Industrial

The solar heat energy is stored in a thermally insulated water tank and used in the heating period. The heat is also stored in the ground if necessary, using the ground loop of the heat pump if the water tank’s temperature rises above a certain threshold. If the temperature is too low for direct heating, then the heat pump can be used to extract the stored energy. This can also be performed under the form of a large subterranean buffer tank filled with water. Heat and cold can be added to or extracted from the tank through conductivity via its wall. The price of these systems is highly depended on the size, the price can vary from <1-165 euro/kWh. The storage efficiency of these systems is from 50-90%. ***Applications:*** * Residential * Agrifood * Heat Grids

Molten salts can be employed as a thermal energy storage method to retain thermal energy. Presently, this is a commercially used technology to store the heat collected by concentrated solar power (e.g., from a solar tower or solar trough). The heat can later be converted into superheated steam to power conventional steam turbines and generate electricity in bad weather or at night. The salt melts at 131 °C. It is kept in the liquid state at 288 °C in an insulated "cold" storage tank. The liquid salt is pumped through panels in a solar collector where the focused sun heats it to 566 °C. It is then sent to a hot storage tank. With proper insulation of the tank the thermal energy can be usefully stored for up to a week. When electricity is needed, the hot molten salt is pumped to a conventional steam-generator to produce superheated steam for driving a conventional turbine/generator set as used in any coal or oil or nuclear power plant. Different existing concept and salts: Two tanks / One tank storage using NaNO₃/ KNO₃ / Ca(NO₃)₂ ***Applications:*** * Concentrated solar power plants

Using compressed water system to store heat. A startup and a research project have been found on this topic. No direct suppliers * GLIDES: A novel Ground-Level Integrated Diverse Energy Storage (GLIDES) system which can store energy via an input of electricity or heat and deliver dispatchable electricity. The proposed system is lowcost and hybridizes compressed air and pumped storage approaches that will allow for the offpeak storage of intermittent renewable energy for use during peak times. GLIDES stores electricity mechanically in the form of compressed gas that displaces water in high-pressure vessels. * Isentropic: PHES Thermal Stores are split into layers to achieve high efficiency of thermal transfer, low-pressure drop and higher energy density than other 'packed bed' stores. The PHES Thermal Stores work by direct heat exchange between high-pressure gas and particles of crushed rock or gravel. It will be a stand-alone energy storage system called Pumped Heat Energy Storage (PHES). The cycle uses a proprietary reciprocating heat pump/engine (developed in-house). It will take electricity from the grid to charge the system. ***Applications:*** * Small industry * Residential

Borehole thermal energy storage (BTES) system is an underground structure for storing large quantities of solar heat collected in summer for use later in winter. It is basically a large, underground heat exchanger. To Store solar energy in the soil under and near the building. In unsaturated soil or in rock the most interesting techniques are heat exchangers in vertical boreholes. Via these heat exchangers, the energy is transferred to the ground which serves as heat capacity and is heated up to the required temperatures. Depending on the temperature level, the thermal energy is, extracted either by a heat pump (low-temperature ground storage, 0°C-40°C) or directly (high-temperature ground storage, 40°C-80°C) and delivered to the consumers. As a result of the limited thermal conductivity, the heat losses are rather moderate and storage efficiencies of 70% can be reached. In contrast, good thermal contact between the heat exchangers and the ground is required to allow a good heat transfer rate per unit area of the heat exchanger tube. In unconsolidated soils like clay, silt, or sand, heat capacity and thermal conductivity are strongly dependent on the water content especially at higher temperatures (> 60°C). In this region, water losses can occur because of vapour diffusion along the temperature gradient which can lead to dry-out and cracking in the area surrounding the heat exchanger tubes in the worst The capital cost of a large Borehole thermal energy storage system can be significant, as a large number of geothermal boreholes will need to be drilled, compared to just a few thermal wells for an ATES system. However, the installation cost should be similar to conventional GSHP systems, and the higher COP values will result in a lower total life-cycle cost than a conventional GSHP system. Both closed-loop geothermal systems will have a lower life-cycle cost than a conventional fossil-fuel-fired HVAC system. Because BTES is a closed-loop geothermal technology, there should be little difficulty in obtaining permits. Typically, the most common constraint is on the available land area in which to construct the GHX array. Because of their construction principle, BTES are not thermally insulated to the bottom and the side, only a top insulation layer reduces the losses to the environment. As the thermal conductivity of underground material is rather moderate, in a range of 1–5 W/mK, heat losses can be kept low if the total volume is large enough to achieve a good surface-to-volume ratio. Size is important because heat losses are proportional to the storage surface while the storage capacity is proportional to the volume. The efficiency of a borehole is estimated from 6-54 %, which increases the longer the system is into use. Therefore BTES are typically large and thus contain a high storage capacity. Additionally, BTES construction can be relatively cheap, even to a high quality, which allows its use even from an economical point of view for seasonal storage with 1–2 storage cycles per year. The cost estimations of a BTES are estimated around 0.3-3.3 €/kWh or 11-50 €/m³ depending on the size and the method for measuring the retained heat in the ground. The temperature across the ThermalBank can be increased from its natural temperature of 10°C to over 25°C in the course of the summer months. Current barriers are limited charging and discharging capabilities, geological constraints, avoiding of groundwater, the number of utilities is limited, technical uncertainty, low-temperature storage and the system usually requires a buffer tank. ***Applications:*** * District/Residential * Industrial

Inorganic SS-PCMs are able to store/release thermal energy in solid phase using one or combination of energy storage mechanisms including magnetic transformations, crystallographic structure transformations, order-disorder transformations, transformations between an amorphous structure and crystal structure. In general, the amount of latent heat of inorganic SS-PCMs is smaller compared to other types of solid-solid PCMs. Each single solid-solid transformation mechanisms exhibits a small amount of latent heat. Therefore, several solid-solid transformations should occur simultaneously to obtain large latent heats or heat storage/discharge at a specific temperature. There are a limited number of materials able to undergo multiple and reversible solid-solid transformations at close temperatures. Transformations between an amorphous structure and crystal structure are not reversible transformations as solid-solid transformations. ***Application:*** * One of the applications of inorganic SS-PCMs is in industrial plants for heat recovery of the intermittently emitted high-temperature waste heat. In this application, inorganic SS-PCMs utilizes one or two solid-solid transformations at high temperatures to store and later release of thermal energy in the solid phase.

Organic molecules capable of reorganizing their supramolecular interactions, typically hydrogen bonds, as a function of temperature, can potentially undergo highly endothermic solid-solid phase-transitions. These highly symmetric and bulky alcohols, such as aspentaerythritol (PE), neopentylglycol (NPG), trimethylolpropane(TMP), and pentaglycerine (PG) have been reviewed extensively. The pure compounds exhibit relatively high latent heats for their solid-solid phase transitions (above 100 J/g), at temperatures ranging from 40 °C to 190 °C. The transition temperatures can be tuned by producing eutectic mixtures of two of these compounds, in general lowering both the transition temperature and the phase change latent heat. Advantages * Freeze without much undercooling * Ability to melt congruently * Self nucleating properties * Compatibility with conventional material of construction * No segregation * Chemically stable * The high heat of fusion * Safe and non-reactive * Recyclable * Carbohydrate and lipid-based PCMs can be produced from renewable sources Disadvantages * Low thermal conductivity in their solid-state. High heat transfer rates are required during the freezing cycle. Nanocomposites were found to yield an effective thermal conductivity increase up to 216% * Volumetric latent heat storage capacity can be low * Flammable. This can be partially alleviated by specialist containment, or by incorporating environmentally friendly fire retardants ***Applications:*** * These compounds exhibit solid-solid phase change latent heats ranging from 102 J/g to 185 J/g, and transition temperatures ranging from 10 °C to 98 ° C.This behaviour makes them particularly attractive for a wide range of heat storage applications, and warrants follow up work to characterize the long term stability and cyclability.

Attachment or incorporation of a phase changing motif into a polymeric structure will be accompanied by a decrease in the observed latent heat. This reduction on the energy absorbed during the phase transition is due to several factors: (a) dilution of the phase changing moieties within a thermally inert polymeric matrix, (b) mobility or packing restrictions of the crystallizing groups due to attachment, (c) strong interaction of the phase changing moieties with the polymer backbone. The magnitude of the decrease in the observed latent heats depends on the choice of the polymer backbone and phase changing motifs. Also more information available [here](https://reader.elsevier.com/reader/sd/pii/S1359431117329939?token=F3E55D40A856580F2188E89A722387D50B5EA5DF9D550BFDE7F622789008B95FE03FE254E5D8DF93A9010EE6953547D0). ***Applications:*** * From a materials design perspective, the use of polymeric structures as a form stabilizing matrix and as mechanical support is definitively attractive. However, the observed decreases in the resulting phase change latent heats, compared to those for the untethered phase-changing moiety suggest that direct covalent attachment of the phase changing motif to the polymeric backbone may not be the best course of action. * Polymeric SS-PCMs show many similarities to various smart polymer systems, many of whom also employ phase transitions, for example, to bring about changes in hydrophobicity or transparency. New multifunctional SS-PCMs can be developed that combine thermal energy storage with other useful functional attributes. Such integrated and multifunctional approaches can lead to the development of innovative adaptive materials that can self-regulate their heat storage and thermal transport properties in response to various external stimuli. Examples abound in biological systems were optical, thermal, and structural functions render adaptive systems enabled by various phase transformations ( More information available [here](https://www.sciencedirect.com/science/article/pii/S1359431117329939). * A recent study investigated an adaptive passive solar building enclosure system using a thin layer of polymeric SS-PCM, which was used as a coating onto a highly reflective exterior façade material. * Semiconductors in PC modules, LED Chip-onboard modules (see Shi-Etsu).

The non-paraffin organic (fatty acids, esters, and alcohols) are the most numerous of the phase change materials with highly varied properties. They are highly flammable and therefore, should not be exposed to intense temperature, flames or oxidizing agents. Non-paraffin organic materials are further subgroups as fatty acids and other non-paraffin organic materials. Among the non-paraffin organic PCMs, fatty acids are relatively cheaper, although their cost remains 2–2.5 times higher than technical grade paraffin's. Some commercial organic PCMs are shown in the figure with their properties. The organic non-paraffin group usually has a melting point around 8-187 °C with a heat of fusion of 130-250 J/kg K. Advantages of organic fatty acids and organic paraffin's: * Crystallize with no or little supercooling * Ability to melt congruently * Self-nucleating properties * Thermally stable for repeated cycles * Compatibility with conventional material of construction * No segregation * Chemically stable * High heat of fusion * Safe and non-reactive * Recyclable * Small volume change Disadvantages of organic fatty acids and organic paraffin's: * Low thermal conductivity in their solid-state (\~0.2 W/m K) * Low volumetric latent heat storage capacity * Flammable * High costs * Non-compatible with plastic containers ***Applications:*** * not often used for electronics heat sink * Solar heating systems * Temperature-controlled packaging

Paraffin is non-corrosive, chemically inert and stable below 500 °C, show little volume changes on melting and have low vapour pressure in the melt form, therefore are one of the more utilized PCM, Paraffin is a petroleum derivative organic material (an alkane hydrocarbon with long straight n-alkane chains as the composition of “CH₃− (CH₂)n – CH₃”) chemically inert and stable below 500 °C. They have many other characteristics such as repeated melting and freezing without phase segregation and consequent degradation of their latent heat of fusion, low vapour pressure during melting, crystallize with little or no supercooling condition, noncorrosive, chemically stable, low cost and largely available. However, they present low thermal conductivity, are non-compatible with the plastic container and flammable. These adverse properties can be improved and solved by encapsulating the paraffin in a suitable container. Parrafin, in general, have a melting temperature of -5-120 °C with a melting enthalpy in the range of 150-240 kJ/kg and a density of 0.77 g/cm³. Some commercial paraffin structures are shown in the figure. ***Applications:*** * Paraffin is the most common PCM for electronics thermal management. They are chemically compatible with most metals. They have large latent heat and can be obtained over a wide temperature range. * Datacenters * Clothing * Packaging

Constraints, due to high volumetric fusion heat, they are a good candidate. The use of metal/metallic materials poses a number of engineering problems during design, development and exploitation of the storage system.\ A new concept of MgH₂ tank was developed to store the heat of reaction by using phase-change material (PCM). The heat of desorption is mainly provided by the latent heat of solidification of the PCM. Ex: A metallic alloy based on Mg–Zn eutectic was selected as PCM in order to enhance the thermal exchange with the MgH₂ compacts. ***Applications:*** * Metallic PCMs are generally used at high temperatures, where no suitable paraffin wax is available.

The solid-liquid transformation of salt hydrates is actually dehydration of salt that resembles melting or freezing thermodynamically. Salt hydrates can be considered as alloys of inorganic salts (AB) and water (H₂O), resulting in a typical crystalline solid with the general formula (AB x H₂O). A salt hydrates usually melt to either to a salt hydrate with fewer moles of water or to its anhydrous form. During the charging phase, the thermal energy is stored by performing the dehydration reaction of a hydrated salt. During the discharging phase, the stored heat is released by undergoing the hydration reaction of the salt. This process is achieved by the addition of water vapour to anhydrous salt. In general, inorganic compounds have near twice the energy storage capacity per volume unit than organic compounds and they possess much higher operating temperatures. Salts have been extensively studied for their use in LHTES systems. They present a high latent heat of fusion per unit volume, a higher thermal conductivity than paraffin׳s and small volume changes on melting also compatible with plastics, non-flammables, but are slightly toxic, present supercooling and nucleation problems and corrosive to most metals. The high heat of fusion of chlorides and fluorides and the low cost of the former have encouraged further studies of salt compositions on their basis. Advantages of inorganic salt hydrates: * High volumetric latent heat storage capacity * Availability and relatively low cost * Sharp melting point * High thermal conductivity * Non-flammable Disadvantages of inorganic salt hydrates: * Change of volume is very high * Supercooling is a major problem in the solid-liquid transition * Nucleation agents are needed, and they often become inoperative after repeated cycling ***Applications:*** * These are not commonly used for electronics heat sinks since they are corrosive and long term reliability (thousands of cycles) is uncertain. * The most common application is for very large thermal storage applications (e.g., solar heating), where much lower cost is very attractive. * Used for air conditioning * This range of PCMs can be used in a number of applications, primarily involving heat storage or thermal shielding. Their high densities and high latent heat values make them particularly effective PCMs. * Grid balance (SaltX)

Organic and Eutectic phase change material operate on the same principle as inorganic (as salt) PCM. Research on these components has mostly been initiated by some obvious disadvantages of salt PCM, such as being corrosive, being incompatible with several materials, experiencing supercooling and segregation during phase transition under thermal cycling. A large number of organic PCMs are available in the temperature range from −5 °C to 190 °C. Depending on the type of applications, the organic PCMs should first be selected based on their phase change temperature. Materials that exhibit phase change below 15 °C are used in cooling applications, while materials that have phase change above 90 °C are used for absorption refrigeration. The organic PCMs and their mixtures that show phase change around 18–65 °C are suitable for the thermal comfort applications in textiles and in buildings. Advantages of eutectics of organic and inorganic compounds: * Eutectics a sharp melting temperature and can be similar to the pure compound * The volumetric storage density is slightly lower than organic PCMs Disadvantages of eutectics of organic and inorganic compounds: \-Relatively new to thermal storage applications, lack of data on their thermo-physical properties ***Applications:*** * Building * Textile * Shipping/Transport

Seasonal cold storage is a highly efficient and environmentally friendly technique that uses the stored natural cold energy in winter (e.g., snow, ice or cold ambient air) for free cooling in summer. Ice storage air conditioning is the process of using ice for thermal energy storage. This is practical because of water's large heat of fusion: one metric ton of water (one cubic metre) can store 334 megajoules (MJ) (317,000 BTU) of energy, equivalent to 93 kWh (26.4 ton-hours). The charged cold energy is stored in the form of ice in an insulated tank and is extracted as chilled water for cooling supply in summer, which helps to reduce the chiller running time and reduce the associated electricity consumption and greenhouse gas emission significantly. ***Applications:*** * Air conditioning * Gas turbine air inlet cooling

The applications of PCMs in the construction industry are endless. They can be incorporated into every element of a building, from the roof to the floors to electrical appliances. They can be used to reduce rapid temperature fluctuations within internal environments by storing latent heat in the solid-liquid phase change of a material. Heat is absorbed and released almost isothermally, and is used to reduce the energy consumed by conventional heating and cooling systems by reducing peak loads. ***Examples:*** * The incorporation of PCMs into building envelope solutions takes advantage of solar energy, contributing to the overall reduction of energy consumption associated to use of the air conditioning systems * Use of phase change materials (PCM) in concrete pavements to store heat, which can be used to reduce ice formation and snow accumulation on the surface of the concrete pavement. ***Applications:*** * Domestic * Industrial

In recent years, latent heat storage systems have been increasingly used in building energy conservation, solar heating systems, and waste heat recovery systems. The water tank as a key component of solar heating systems has been widely applied in practical applications. ***Applications:*** * Solar heating systems * Domestic

High heat transfer coefficients can be achieved between a fluidized bed of coated PCM particles and a heat exchanging surface: heat can be captured by the particulate gas–solid flowing suspension. ***Example of installations:*** * Packed beds: Air is used as heat transfer fluid (HTF) flowing in an annular shell outside the bed for charging and discharging the bed. The bed is filled with CaO/Ca(OH)₂ powders with particles diameter of the order 5μm. * Fluidized beds: a novel technique which utilises solids transfer between adjacent fluidized beds by means of jet jumps for recovering and/or storing waste heat. Results showed that the fluidized bed heat exchanger effectiveness approached 80% of a perfect parallel flow heat exchanger. The effectiveness increased with increasing jet pump flowrate, fluidizing velocity and by the use of vertical baffles. * Moving bed: Moving Bed Heat Exchanger MBHX storage concept for CSP systems using sensible heat transfer medium aims at using a low cost solid storage media. This concept requires intermediate bulk cycles to transfer heat between the solar field and the storage material (the bulk). ***Applications:*** * Low temperature industrial heat * Solar concentrator

Integration of PCM into heat exchangers. ***Applications:*** * The tanks are connected through a salts-heat transfer fluid heat exchanger, which is coupled to the heat source and the heat sink. * The latent heat TES technology is evaluated in a 0.154 m³ storage tank based on the shell-and-tube heat exchanger concept which is connected to a heat source and a heat sink simulating the waste heat recovery of an industrial process. The material used is 99.5 kg of high density polyethylene (HDPE). * directcontact heat exchanger using erythritol (melting point: 391 K) as a phase change material (PCM) and a heat transfer oil (HTO) for accelerating heat storage

Encapsulation of PCMs is present in two forms, core-shell and shape-stabilized, depending on the shape of the capsule. Core-shell encapsulation is the term for covering a material (core) with another material (shell), while shape-stabilized encapsulation are composites of PCMs with other materials that retain molten PCMs by capillarity. Micro-encapsulated PCM slurry is a suspension where the PCM is dispersed at 10–20 wt. % without significantly altering the physical properties of the liquid (density, viscosity). Example of micro encapsulation methods: * Emulsion polymerization * In situ polymerization * Interfacial polymerization * Electroplating * Sol-gel process * Mechanical packaging ***Applications:*** * PCM is microencapsulated using a polymeric capsule and dispersed in water. * also used in Slurries and Molten Salt * Since SS-PCMs remain solid, they may also be used to encapsulate SL-PCM’s, which would allow all constituents to contribute to the latent heat storage processes. Similarly, shape stabilized SL-PCMs using SS-PCM matrixes may also offer a viable path. Such approaches could offer cost benefits and higher heat storage capacity. * Used in bedding, building and construction, automotive interiors, clothing and mattresses (see Croda Therm)

The main drawback of encapsulated PCMs is their low thermal conductivity, hence the growing interest in dispersing high conductive nanoparticles within the PCMs. Can be made on the same sort of techniques of microcapsulation As an example, different metal nanoparticles were used in order to enhance the thermal conductivity of paraffin wax. ***Applications:*** * Building / HVAC * Passive cooling applications

Integrating PCM with heat pipes. ***Examples:*** * The issue of the low conductivity of PCMs has been addressed by using an embedded finned watercharged heat pipes into the PCM bulk The approach is to physically decouple the evaporator pipes from the PCM, thus allowing independent sizing of each component. * The thermal link between the two components is done via evaporation and condensation of a heat transfer fluid (HTF), according to the principle of a heat pipe. Pumping the liquid HTF provides active control of the heat pipe operation. The new concept is modeled and compared to the conventional design of conduction based PCM annulus around the steam pipe * The adiabatic section of heat pipe is covered by a storage container with phase change material (PCM), which can store and release thermal energy depending upon the heating powers of evaporator and fan speeds of condenser ***Applications:*** * Electronic cooling * Industrial

Metal foams have also been proven to be a viable option in enhancing thermal conductivity of PCMs. High porosity, good thermophysical properties and mechanical strength are salient features of metal foams. ***Examples of applications:*** * The results revealed that the addition of metallic foam improves the heat transfer between the wall of the pipe and the fluid. ERG aluminum foam was the metal foam used in this analysis with a permeability ranging from 10 ppi to 40 ppi. This heat enhancement further improved by adding nanoparticles to the fluid. The heat storage capability of the proposed fluid also increased with the addition of microencapsulated phase change material (MEPCM) particles. The results revealed that 20% MEPCM and 3% nanoparticles in water is an ideal balance for heat storage and enhancement, with a 6.7% improvement in the case of water flowing in a porous pipe. ***Applications:*** * Electronics/Battery storage * Heat exchangers

The dehydration reaction of metal hydroxides can lead to high heat storage efficiency and places these chemicals as candidates for thermochemical heat storage. Ca(OH)₂/CaO system shows the highest enthalpy of reaction, followed by Mg(OH)₂/MgO. The reversible reaction of dehydration/ rehydration of Ca(OH)₂ has attracted attention, because it is a cheap material and it features a high enthalpy of reaction, and therefore it has already been studied for potential application inenergy storage Based on: Ca, Mg, Be, Mn, Sr, Ba, Ni, Zn, Cd ***Applications:*** * Domestic seasonal energy storage * Given the proven interest of hydroxides for low temperature TES, their study needs to be extended to appraise their applicability to high temperature CSP plants.

Metal carbonates have mainly been proposed and studied as sorbent materials in the context of CO₂ capture. The capture and storage of carbon dioxide has become of prime interest in order to reduce the concentration of greenhouse gases in the atmosphere and as such, literature data on the thermal behavior of sorbents (especially calcium oxide-based sorbents) during carbonation/calcination looping cycles is abundant. Metal carbonates are also potential candidates for thermochemical heat storage with some of them showing remarkable high reaction enthalpy and energy storage density. Based on: Ca, Sr, Mg, Ba, Cd, Zn, Pb ***Applications:*** * Metal carbonates show potential to store and release heat via reversible reactions and they are thus considered to be worthy for heat storage applications * CO₂ sorption * The potential of theSrCO₃/SrO couple for thermochemical energy storage application. The sintering of SrOover several carbonation steps was addressed by using 40 wt. % of SrO supported on yttria-stabilized zirconia. This way, the material remained stable over 15 cycles, with energy storage density stabilized at approximately 1400760 MJ/m³. Degradation of the capacity of the material is however noticed after about 45 cycles.

Metal sulfates have been studied for application as heat storage media for solar energy. Metal sulfates are potential candidates for thermochemical heat storage because they exhibit high reaction enthalpies and can be suitable for operating with concentrated solar energy since their operating temperatures are comprised between roughly 900 °C and 1400 °C . Based on: Mn, Fe, Co, Cu, Ba, Zn, Cd, Ni ***Applications:*** * The potential of using sulfates as thermo-chemical energy storage media. However, the use of sulfates introduces a critical issue of corrosion which has to be taken into account when choosing the materials for the storage system

Various metal oxides have been studied for thermochemical heat storage applications due to their high gravimetric storage density, which is important for lowering the necessary amount of reactant involved in large-scale process. Based on: Rh, V, Ca, Mn, Cu, Li, Fe, Ba, Ng, Cr, Pt, Pb, Sb, U, Mg ***Applications:*** * They possess a wide range of operating temperatures starting from 145°C until up to 1700°C, which can help adapting the process to several kinds of Concentrated Solar Power technologies. * Thermochemical heat storage based on redox reactions usingmetal oxides also presents the advantage of directly using air as heat transferfluid. Without the necessity to store the heat transferfluid, it becomes possible to work with an open loop system, incontrast to metal sulfates or carbonates that require a closed-loop. * Various metal oxides have been studied for thermochemical heat storage applications due to their high gravimetric storage density, which is important for lowering the necessary amount of reactant involved in large-scale process.

Zeolites are crystalline alumino-silicates, characterized by a high specific surface area (i.e., about 800 m2/g) and wide microporous volumes, which make these materials perfectly suitable for water vapor adsorption. Owing to their porous structure, zeolites are usually highly hydrophilic, which allows them to obtain high adsorption capacities even at low partial pressures. This high affinity with water, of course, is reflective of strong bonding that requires higher temperatures to be broken compared to silica gels (i.e., more than 150 °C). Zeolites type A, 13X and Y are the most common classical synthetic zeolites employed for adsorption heat storage. These materials are mostly used for open adsorption TES, since, in order to get enough energy storage density, they must be regenerated at high temperatures, making air the most effective heat transfer medium. ***Applications:*** * Employed for industrial waste heat recovery and storage * Domestic seasonal storage