Water is the best liquid STESM for applications between ca. 4 °C and 100 °C. Liquid-form STESMs have a higher specific heat capacity and thermal conductivity compared with solid form STESMs such as rock [8
]. For cooling applications below 0 °C, certain mineral oils as well as water-glycol mixtures are typically used. Mineral oil, molten salts, liquid metals and alloys are examples of liquid STESMs [9
] for high temperature applications. With increasing interest in CSP, blends (binary, ternary, and quaternary) of carbonates, fluorides and nitrates are used as STESMs. The most common molten salt is known as solar salt that consists of 60% NaNO3
]. Solar salt represents the most typical commercially available high temperature STESMs systems, used as both the TES medium and heat transfer fluid (HTF). The maximum operation temperature of this mixture is 585 °C to avoid decomposition and it must be kept above 220 °C to stay in the molten state. This brings some limitations on the design of storage systems. Two-tank storage methods are used to solve this problem. The research on Binary, ternary and quaternary inorganic blends is also driven to overcome this limit. For instance, a new low-cost molten salt blend NaCl-KCl-MgCl2
was shown to be stable up to 700 °C [67
]. These multicomponent blends are, however, still at lower TRLs of development. Sand, rock, concrete, cast iron, cast steel, NaCl and brick are reported as the most common solid sensible thermal energy storage materials. Rocks show good thermal performance up to 20 years. Concrete based materials are attractive options as STESM due to their low cost and high storage capacity. Concretes can be used in high temperature storage systems up to 400 °C.
Because TES and the reduction in its cost/kWh are essential for the deployment of CSP, other alternatives besides different blends are now being considered. One such alternative is the use of nanofluids, because of the unrealistically high specific heat enhancements reported, albeit with the research at a very low TRL. Another alternative entails a different CSP plant concept that involves the use of particulate solids as the STESM medium and HTF that has advances in TRLs at 4–5 for some components, such as the receiver or the heat exchanger. There is a particular demand for increasing the operating temperature of existing STESMs or finding new ones for higher temperature applications. The author of [68
] designed economical concrete mixtures by using fly ash and polypropylene fiber mixtures as fillers to increase the operating temperature to 600 °C.
Waste/inertized materials can be used to design alternative STESMs at a low cost. Inertized products such as by-products derived from mining and the metallurgical industry [69
], asbestos-containing wastes [70
], fly ashes from municipal solid waste [70
], post-industrial ceramic [71
], recycled nylon fiber from the textile industry [72
], and demolition wastes [10
], can be used as STESMs for high temperature thermal storage. Table 3
compares energy densities and costs of STESMs for high temperature applications. As can be seen, waste or by-products from industrial processes have the lowest cost with comparable energy densities to existing alternative STESMs.
The typical advantages and disadvantages of these various STESM categories are summarized in Table 4
As was discussed (Section 2.2
), one hyped class of PCMs are salt hydrates. Salt hydrates are popular because they are relatively cheap and abundant and have high volumetric energy storage densities. For example, the salt hydrate calcium chloride hexahydrate (CaCl2
O) with a melting temperature of about 29 °C has a mass-related enthalpy of fusion of about 191 J/g [50
]. Due to the density in the liquid state of about 1.56 kg/L, the energy density related to the volume is about 298 kJ/L [50
]. This value is almost as high as the enthalpy of fusion of water/ice (approx. 330 kJ/L). In many cases, technical grade salt hydrates can be provided at a low cost of less than 1 EUR/kg [78
Despite their popularity, finding robust PCMs from salt hydrates appears a challenge. A major issue is that during the selection process, phase equilibrium knowledge is significantly overlooked or poorly understood, often resulting in supercooling and phase separation, thus negatively impacting their reversibility and cycling stability. The rule of thumb is to avoid incongruently melting compositions (including strictly incongruently melting peritectics, c.f. Section 1.3
) and instead resolve to congruently melting compositions (which are also many among salt hydrates, e.g., Figure 3
]) or non-supercooling eutectics. Semi-congruently melting salt hydrates may also be used in applications by taking certain measures on a material or storage level. In this case, their phase transition must be well-understood prior to moving into TES applications, to avoid/properly control the undesirable phase separation, supercooling, and related challenges. In applications, the corrosivity of salt hydrates must also be taken into account, for which metallic heat exchangers with special coatings or heat exchangers fabricated of plastic along with careful materials compatibility testing can be helpful. So far, salt hydrates and LHTES systems based on salt hydrates have only been commercialized in a few cases (among others by the companies Rubitherm [79
] (based on SAT, [80
]), Sunamp [81
], PLUSS [82
], and Swerod [83
]). This is likely owing to poor phase equilibrium understanding, poor understanding of mechanisms enabling successful thickening using additives, and additional complex phase change behaviors which require both careful PCM development and a storage design that takes these inherent characteristics into account. These challenges in salt hydrates have pushed researchers to seek alternative materials for LHTES applications.
The organic PCM sub category alkanes, commonly referred to as paraffins, can be considered the next most-investigated PCM category after salt hydrates ([12
]). These gained popularity for their simple phase change at a variety of temperatures with moderate phase change enthalpies and for many alkanes being relatively safe (non-toxic) as a material to use. However, alkanes tend to pose some challenges such as solid-solid phase transitions, only moderate phase change enthalpies and high flammability ([12
]). Alkanes are also among commercial PCMs from, e.g., Rubitherm [79
]. One particular disadvantage of both salt hydrates and alkanes is their non-renewable nature, as they are extracted from depletable sources. This is one of the reasons behind the recent advance of sustainable PCMs in the latent heat storage field. Several material categories belong to this group, and some of the most prominent ones in the PCM-context are represented by fatty acids, esters, and sugar alcohols.
Fatty acids are naturally found in oils and algae and are non-toxic [85
]. Depending on the absence or presence of double or triple bonds, they are defined as either saturated or unsaturated, respectively. Generally, saturated ones have been studied more intensely than their unsaturated counterparts. For PCM applications, mostly fatty acids with carbon numbers (n) ranging from 3 to 9 have been investigated with melting points from 16–74 °C and enthalpies of fusion from 150 to 220 J/g [86
]. Saturated fatty acids normally present low degrees of supercooling, although this is suspected to strongly depend on the thermal history [87
]. They are mildly corrosive and have been reported to be thermally stable upon cycling.
Esters are organic substances formed by the union of a carboxylic acid with an alcohol, and can be encountered in natural renewable sources such as vegetable and animal fats [88
]. As they are the result of the combination of acids and alcohols, millions of possible esters with specific thermophysical properties exist. Esters present a wide range of melting temperatures and enthalpies approximately of −25–100 °C and 100–50 J/g ([11
]) and are characterized by little to no supercooling, high chemical and thermal stability and no corrosiveness and are only moderately flammable [94
]. Still, many are not commercially available, and they remain mostly unexplored [95
]. This is certainly one of the main barriers for their investigation.
Polyols (poly-alcohols, many referred to as sugar alcohols) have undergone substantial research as PCMs in the past two decades. These have low to moderately high phase change temperatures (ca. −15–250 °C) and considerable enthalpies, with (ca. 100–400 kJ/kg). These are attractive as PCMs for their renewable origin, plus certain polyols such as erythritol, xylitol, and sorbitol, are non-toxic (food-grade) [19
]. However, the commercialization of polyols into TES applications is hindered by material challenges such as substantial supercooling, hysteresis, glass transition, succumbing to metastable states at varying heating/cooling rates, thermally activated change (with possible degradation) in ambient conditions, plus higher costs particularly for a large-scale [19
]. Polyols become crystalline when water is present, undergoing plasticization, and tend to become amorphous in the absence of water, succumbing to glass transition [97
]. Thus, water may be investigated as a plasticizer to ensure crystallization in polyols, however, this can compromise the storage density and pose practical challenges in maintaining this water intact in the polyol particularly if the TES application is at or above 100 °C.
While bio-based PCMs are in general considerably more expensive than salt hydrates, they are extractable from renewable feedstock which makes them particularly attractive. Bio-based PCMs have a lower TRL than salt hydrates, and they are being commercialized by some companies such as Croda [98
]. Being bio-based, there is great potential in these materials as PCMs to reach lower costs in the future by, e.g., using more cost-effective raw materials and synthesis steps, and also creating larger markets for these niche products. Erythritol (a polyol) is one such example, produced today using maize but can also be produced using, e.g., wheat straw or crude glycerol (coming as a by-product during biodiesel, bioethanol or soap production) for a lower cost of production [99
The typical advantages and disadvantages of these various PCMs are summarized in Table 5
TCMs rely on the reversible sorption processes involving adsorption, chemical reactions or absorption (c.f. Section 2.3
). The most versatile class of sorbents are the two-component sorbents or composites [100
], composed of a porous solid as an active matrix (e.g., silica gels, zeolites, mesoporous silicates [101
], vermiculite and MOF [102
]) or an inactive matrix (e.g., silicon foam [103
], expanded graphite, or porous metals) filled with hygroscopic salt hydrates (e.g., LiCl, CaCl2
). These are synthesized to enhance water sorption capacity as well as heat and mass transfer on one hand, and on the other hand to avoid deliquescence, swelling and agglomeration of the salt hydrates during sorption/desorption cycles. These sorbents have the advantage of tailoring sorption capacity by changing the content of salt and porous structure. Further advantages are low desorption temperature, low price and a simple production method. The current trend is focused on increasing the amount of the salts in the matrices to achieve higher sorption capacity, while the used matrices are mainly commercial ones and less effort is dedicated to tailoring the structure of matrices [101
The design of efficient water adsorbents with advanced properties is motivated by an increase in the water sorption capacity and regeneration of the sorbent at a low temperature (up to 120 °C). Zeolites, as traditionally highly hydrophilic adsorbents, are microporous sodium aluminosilicates with 3D structures containing channels with pore openings from 0.3 to 1.2 nm. Although there are more than 200 different structure types of zeolites, only high-alumina zeolites are considered suitable for sorption heat storage, such as Zeolite A, Zeolite X and Zeolite Y. These are manufactured synthetically. Ion exchange modification of zeolites with magnesium and lithium cations increases water sorption capacity, however, it also increases the charging temperature, which is a drawback. To overcome this, granulated binder-free zeolites (A, X and Y) were successfully manufactured. Thereby, an increased water sorption capacity, e.g., 16% for Zeolite X [105
], was realized while maintaining the charging temperature, which is still rather high for solar energy storage (up to 250 °C). The crystallinity of Zeolite X can degrade under hydrothermal stress in the aqueous atmosphere of an adsorption storage device [106
], another challenge requiring further R&D. Within recent developments, dealumination of granulated binder-free Zeolite Y led to lower charging temperatures, e.g., at 30 °C and to higher water sorption capacity due to the introduction of mesopores into microporous structure forming hierarchical zeolite [107
In the last decade, new adsorbents with energy storage densities up to 530 kWh/m3
have been successfully proposed. Namely, microporous aluminophosphates, FAPO-34, APO-18, APO-Tric, and APO-LTA ([26
]) revealed their advantages over zeolites concerning a low charging temperature due to a hydrophobic-hydrophilic character, high water capacity and high adsorption enthalpy. APO-Tric exhibited better hydrothermal cycling stability than SAPO-34, the commercial water aluminophosphate adsorbent [27
]. Concerning the cycling stability, the crystalline structure of SAPO-34 was found to become amorphous after 50 cycles while the APO-Tric structure remained crystalline 50 cycles under the working conditions: adsorption at 40 °C and desorption at 150 °C at 56 mbar water vapor pressure [109
]. Overall, it has been shown that these adsorbents with adsorption equilibrium with adsorbate (water), characterized by S-shaped adsorption isotherms, are advantageous for the heat pumps and chillers. Metal-organic frameworks (MOFs) ([26
]) are another innovative class of adsorbents, which possess very large water sorption capacity and heat storage capacity, S-shaped adsorption isotherms and require low charging temperature. The main disadvantage, however, is their high price. As recent research results indicate, ettringite, is a low cost mineral (a component in some cements) which possesses high energy density (~500 kWh/m3
), low corrosiveness, non-toxicity and low working temperature (~60 °C) with promise as a TCM [111
Concerning the costs of these TCMs, zeolites as low-cost adsorbents are already on the market (e.g., CWK, Clariant, Silkem, Zeolyst and Grace). Some aluminophosphates and MOFs are also manufactured, yet they are much more expensive than zeolites. Composites composed of salt hydrates and porous matrices are less expensive than aluminophosphates, especially if a natural matrix is used, such as vermiculite clay. Ettringite’s cost is as low as 700 USD/m3
in comparison with 4300 EUR/m3
for silica gel, 2000–3000 EUR/m3
for Zeolite 13X or more than 42,000 EUR/m3
for hydrates of SrBr2
]. With salt-water chemical reaction systems being at a relatively low cost ranges, some of these (e.g., CaO/Ca(OH)2
reaction) are already on the market via companies such as SaltX AB [112
The water-based TCS systems can be designed as open or closed-systems as water is safe to be released to the environment. Whereas, the TCS systems using non-water sorbates (e.g., NH3, CO2, ethanol and methanol) which have toxic or flammable properties must be designed as closed systems. Hence, the water-based TCS systems maintain a larger popularity today, for the considerable ease of system design. Nevertheless, for the very compact energy storage densities offered by these other non-water-based TCS systems, these systems are gaining significant interest. Therefore, TCS system design with, e.g., NH3 and CO2 has the competitive advantage of benefitting from mature learning curves of refrigeration systems using these as refrigerants (R-717 and R-744), and sharing similar operating configurations and components.
The diversity of these reaction types gives rise to rather different operational temperature ranges for applications, which on one hand shows the versatility of the concept, but on the other hand makes every individual reaction a special case. The biggest concern, therefore, is that the envisaged application defines the boundary conditions with respect to the temperature range, reaction kinetics and storage capacity as well as the spatial and/or temporal decoupling of supply and demand of thermal energy. In an ideal world, the once set boundary conditions of an application allows for the selection of the appropriate TCM from a variety of established reaction types and materials. However, the real world is lacking this variety of ready-to-install TCS systems due to a manifold of reasons. These reasons can be encountered on the quantum-chemical, molecular, particle, reactor and system scale. In each scale, different theoretical tools allow for a system modelling to derive parameters for upscaling processes and tuning of the properties, as summarized in Figure 12
The first medium-sized prototype reactors were developed to elucidate a system performance on the kilogram scale in lab environments [114
], and salt hydration-based reactors as prototype. However, most TCMs reside at low TRLs. The advantage of the solid-gas reactions being the basis of TCMs, is the tunability of the reaction conditions with respect to temperature and pressure (of the reactive gas) allowing for optimization of the overall performance. Nevertheless, the major drawback so far is the uniqueness of most systems under investigation and, therefore, the restricted transferability of insights gained into the reaction parameters of a specific TCM within the same compound class or even beyond.
At a more specific level, e.g., to overcome the shortcomings of a poorer storage capacity than expected in TCS, the use of technical gases (i.e., in-place of the pure gas) such as carbon dioxide or ammonia is a solution, because these gases are easy to handle in an industrial scale, inexpensive, and if kept in closed cycles as safe as water vapor. There is fundamental research investigating the performance of moisturized carbon dioxide in oxide–carbonate cycles [25
] as well as the use of ammonia with transition metal salts [14
]. Especially in the case of copper sulfate, the heat release is extremely rapid and intense that only the use of an inert carrier material allows for a reversible application [52
The typical advantages and disadvantages of these various TCMs are summarized in Table 6
3.2.5. Barriers and Missing Links from the Laboratory to Application
When looking at the battery (i.e., for electrochemical storage) as a product, it is made up of many materials and the product itself is a package of these materials. Likewise, if a TESM is considered as a product alone, it is very difficult to adapt it to different needs of the applications. Moreover, many products that are used in everyday life today will not work without a battery. This is what makes batteries inevitable. Currently, most energy systems continue to operate without the use of a TESM product. However, the urgent need of transition to carbon neutral systems makes TESMs also inevitable. The main barrier for the implementation of TES in an energy system is that there is not a unique recipe to be adopted, TES systems are not “plug and play” but need to be researched and designed ad hoc, meaning that a design based on the TESM is inevitable too. The key challenges for TESM developers and the industry are to show how TESMs are inevitable and how to present them in a thermal battery package ready for plug and play. Despite all technological marvels, people’s perception ultimately plays a deterministic role in deciding what is trendy and thus what solutions are finally implemented on a large scale. For instance, the house is no longer a status symbol, while an electric vehicle is (particularly in developed countries). Thus, TESMs packaged in a plug and play thermal battery requires awareness building and dissemination to reach every corner of society to establish its rightful status symbol as a true enabler of carbon neutral energy systems.
The main barrier for STESMs for applications with limited available space is compactness. In urban areas for building applications, finding a space even for a domestic hot water (DHW) tank may not be possible. For industrial applications, finding the material with the matching temperature requirements in the large quantities needed with robustness and at competitive prices can be a barrier. STESMs can deplete natural materials such as rocks, as STESM is not a sustainable way of solving energy problems, and requires competitive renewable alternatives. Materials for industrial applications above 750 °C are needed for especially new generation CSP plants. Although some STESMs such as molten salts and metal alloys are attractive choices here, their corrosion issues necessitate taking extra measures. Whereas, materials used to prevent corrosion may be expensive and also increase operation cost. CSP is an application where the opportunity cost for a robust TESM is higher and therefore R&D to address these STESM challenges is a worthy cause.
To this day, inorganic and organic PCM alike presents several unsolved challenges mainly connected with their kinetic behavior, posing a certain unpredictability. On the one hand, salt hydrates are typically characterized by slow nucleation, which translates to high degrees of supercooling. This is detrimental to typical LHTES applications as it broadens the operational temperature range needed to melt and crystallize the PCM, thus lowering its advantage in comparison with seasonal storage [7
]. On the other hand, while organic PCMs usually show low degrees of supercooling, they often show the tendency to crystallize in different arrangements (i.e., solid phases) despite maintaining the same chemistry and overall composition. This phenomenon is referred to as “polymorphism”, and different crystalline structures are then called “polymorphs” or “polymorphic forms”. Polymorphism represents an obstacle to the use of organic PCMs in LHTES, since different polymorphs present very different thermophysical properties such as melting points, enthalpies, thermodynamic stability, and solubility [11
Supercooling and polymorphism are both of a kinetic nature, although they are disconnected from each other. In the case of supercooling, the PCM nucleates slowly due to a high energy barrier, whereas polymorphism in organic compounds is generally caused by the rotational degrees of freedom and the ability of the molecules to arrange themselves in different patterns. Currently, supercooling and polymorphism are managed through trial-and-error methodologies such as the addition of nucleating agents [130
] or Peltier elements [7
] for the former, and the usage of specific solvents and conditions for the latter [131
]. Nevertheless, both phenomena are still far from being completely understood and still represent a challenge to the further development and integration of PCMs in energy-storing setups [132
The standardization of TESMs characterizations within common TES platforms to obtain a common consensus is an essential step for accelerating their TRL progression. Although, e.g., IEA TCP activities have significant contributions here (see Section 2.2
), there is also still a lot to accomplish. Long-term stability of PCMs is one such key parameter with not enough attention and standardization yet. In IEA SHC/ECES Task 58/Annex 33 [22
], to test the long-term stability of PCMs, these were subjected to melting and solidification processes representing the conditions of the intended application. Therein, the characterization of 18 experimental devices to investigate the long-term stability of PCM were presented [134
]. These experiments were divided into thermal cycling stability tests, tests on PCM with stable supercooling, and tests on the stability of phase change slurries (PCSs). In addition to these experiments, appropriate methods to investigate a possible degradation of the PCM were introduced. Considering the diversity of the investigated devices and the wide range of experimental parameters, this article concludes with recommendations on further work toward a standardization of PCM stability testing [134
]. This is a valuable standardization step in the application-specific characterization of PCMs, while it is also an indication that there is an enormous amount of work ahead to achieve standardization in a true sense for all TESMs. Xu, 2021 [135
] highlighted the benefits and also the lack in the TES R&D, of TESM characterizations at the material, component, and system-scales to apprehend the complete spectrum of TESM behavior, prior to moving into full-scale TES applications. This becomes a matter of where the R&D budget should be spent, i.e., to establish a sound basis or to patch-up and retrofit a poorly designed application.
Low thermal conductivity of a majority of PCMs and TCMs (as well as certain TESMs) has been an age-long battle, resulting in a poor heat transfer and therein the TES charging and discharging power. Organic PCMs suffer from poorer thermal conductivities than inorganic PCMs, which albeit have altogether poor heat transfer. TCM-adsorbents, particularly zeolites, aluminophosphates and composites based on silica matrices, encounter poor thermal conductivities. The state-of-art today for thermal conductivity improvement or TCMs comprise of coating of the TCM-adsorbents on metal plates/foams or preparing composites by impregnating the PCMs or TCM-absorbents in materials with high thermal conductivity, such as expanded graphite, aluminum and copper. This is also referred to as micro-encapsulation, concerning PCMs. PCMs are also macro-encapsulated in capsules (of, e.g., spherical and ellipsoidal geometries) fabricated of materials with better thermal conductivities and/or for maintaining the bulk volume in-tact in the TES component in both liquid and solid states [7
]. Nanoparticle addition has also been a trend as a potential means for thermal conductivity improvement of, e.g., PCMs; however, this is received with mixed critique, as the high-thermal conductivity particles are only dispersed (not interconnected) in the bulk of the PCM, posing a higher thermal resistance.
The cost of PCMs and TCMs is also one barrier to reach commercialization. Salt hydrates come at lower prices. However, improper selection has most often led to a bad reputation which needs rectification by proper selection and comprehension. Alkanes (paraffins) and similar non-renewable PCMs as well as many other bio-based organic PCMs have rather high to very high costs, for the purity which comes at a cost, and because they are in the niche markets. Particularly the bio-based organic PCMs, however, have a great potential to reach lower costs with further R&D on innovative low-cost production processes and for market expansion. Here, a hindrance to their large-scale exploration, however, is the lack of clear extraction routes from renewable feedstock. From adsorption-based TCMs only zeolites have commercially competitive prices, whereas, from chemical reactions-based TCMs, mainly salts (e.g., for hydration, hydroxide or hydride formation, ammoniation or carbonation), can be considered rather inexpensive. A deal-breaker concerning cost, however, is not merely the material cost but the component cost and the overall TES system, to enhance heat transfer and mass transfer for satisfactory thermal charging and discharging powers (particularly in active LHTES systems and TCS). The relative lack of techno-economic analyses and life cycle analyses (LCAs) of TES systems is a strong barrier for their commercialization. The benchmarking of these various system’s techno-economic analyses is also currently amiss, which is essential for developing standards to enable accurate comparisons to find system-specific storage solutions.
Beyond the technical and economic barriers to the widespread exploitation of TESMs for numerous TES applications, there are also soft aspects that govern the way forward. These include political and legislative trends and incentives that favor energy storage for the most part in terms of electricity storage, where direct storage in, e.g., batteries, is dominating. Whilst innovative concepts such as power-to-heat and power-to-cold are emerging, these still require significant dissemination and awareness building towards all stakeholders in the energy chain and most importantly to the political and legislative decision makers. The usual norm of electrical battery storage must be apprehended hand-in-hand with all the other energy storage counterparts, such as mechanical energy storage (e.g., pumped-hydro, compressed-air and flywheels), electromagnetic storage (e.g., super capacitors) and in chemicals (e.g., hydrogen) [136
] and last but not the least: TES.
With more than half of the global final energy used on thermal demands [1
] the gravity of TES is unequivocal. Entities such as IEA and its TCPs on, e.g., SHC [137
], ECES [138
] DHC [139
], as well as IRENA [140
] and others already contribute significantly in promoting the role of TES (particularly beyond water and ice). However, the target audience of these channels will need to be further diversified while even more rigorous communication and dissemination actions will prove beneficial. A dilemma pertains also in the social acceptance of TES, as a relatively new technology (as opposed to, e.g., electricity and batteries). The Spanish case study on TES in buildings by [23
] critically discusses social barriers to TES deployment with universal applicability to the global context as well. A key conclusion is that there is a lack of awareness and/or poor comprehension of what TES encompasses, a resistance to change from the comfort zones of conservative technologies, and a mistrust on the long-term success of the ‘new’ solutions in TES [23
]. Concentrating research efforts to identify the socio-political drivers and barriers of TES and TESMs, also in other countries, regions (e.g., EU) and globally will be invaluable future steps.