Progress on rock thermal energy storage (RTES): A state of the art review

Thermal energy is one of the most widely encountered energy forms in our daily life. To ensure efficient utilization and conversion of this energy, the balance between supply and demand needs to be maintained. For this purpose, thermal energy storage is required. There are various thermal energy storage systems available; one of the most basic is sensible thermal energy storage which includes rock thermal energy storage (RTES). This rock‐based energy storage has recently gained significant attention due to its capability to hold large amounts of thermal energy, relatively simple storage mechanism and low cost of storage medium. Accordingly, numerous studies have been conducted to elucidate the basic flow and heat transfer mechanism and to evaluate the performance of this energy storage. The major technical challenges hindering the wide adoption of this technology are the enormous pressure drop across the storage and nonoptimal heat transfer from the heat transfer fluid to the storage medium and vice versa. These issues will directly and indirectly affect the overall cost (capital, operational, and maintenance costs) of the system. To eliminate this issue and assist further development of this technology, it is crucial to compile and extract important findings from these previous studies, identify the challenge and research gap, and draw guidelines for the upcoming research and development. At the moment, this kind of compilation does not exist. Hence, this paper is prepared with the primary objective to comprehensively review the current technology and development of RTES and to propose a potential way forward based on the pain point identified. Discussion on the nontechnical aspect such as policy and regulations as well as community awareness will also be outlined and discussed.


| INTRODUCTION
With the exponential growth of global population 1 and surged industrial activities 2 in recent decades, energy demand has significantly increased. 3,4Coupled with limited resources, 5 nonuniform distribution, 6 nonoptimal energy conversion 7 and mismatch between energy supply and demand, 8 the risk of a serious energy crisis has become more pronounced.To minimize this, the global community has been looking towards alternative energy resources to supplement primary fossil-based energy, increasing energy system efficiency and performance, and developing energy storage to eliminate the mismatch between energy supply and demand to relieve stress on energy availability.
Various energy storage systems have been proposed and developed. 9In the current world energy scenario, most energy conversion and utilization systems are in the form of thermal systems such as power generation plants, large industrial heat exchangers, internal combustion engines for various transportation and portable power generations as well as heating, ventilation and air conditioning systems.Not surprisingly, thermal energy storage (TES) has been one of the most widely utilized and studied energy storages.It becomes an integral part of various present thermal systems especially in largescale applications where the demand fluctuates significantly.Among TES, rock thermal energy storage (RTES) has attracted significant attention for implementation in large-scale thermal systems due to its favorable features such as large storage capacity, simple storage mechanism which translates to a simple storage system, low environmental impact, wide range of operating temperatures, and low material costs. 10,11TES is considered to be more cost-effective than its water-based counterpart when implemented in solar water heater systems for domestic heating. 12Typically, an RTES uses air as heat transfer fluid (HTF)-although other HTFs have also been considered and implemented-and a pile of crushed rocks as energy storage medium.The air is heated by the heat source and will pass through the rock pile where heat is transferred to the rock and recirculated to the heat source, as shown in Figure 1.When needed, the heat from the thermal storage can be discharged using the same principle.][15][16] The notable technical challenge with this mechanism is the high volumetric flow rate of air required to drive the heat in and out the energy storage during charging and discharging, respectively.In addition, the porous structure of the rock pile makes the analysis of RTES rather challenging, and it is accentuated by the unstructured shape of the rock.A thorough analysis of various parameters associated with a packed rock bed TES system such as pressure losses, heat transfer mechanisms, and other corresponding operational parameters is required for TES system design, efficiency analysis, and optimal performance of the TES system which could be used for designing and optimizing the TES system even further.
][22] Despite extensive studies that have been reported, further research and development is necessary to ensure the maturity and wide adoption of this technology.To expedite this process, a compilation of the important findings from previous studies is crucial to direct and guide future investigations.Such a compilation is unfortunately unavailable at the moment.Thus, this paper is prepared with the main objective to provide a comprehensive review on the current stage of RTES development, to identify the challenges and potential improvement and to suggest directions on the future research and development required for this technology.Nontechnical aspects including environmental aspects, regulation and policy as well as community awareness will also be discussed.
F I G U R E 1 Simplified schematics of a typical rock thermal energy storage.
Over the centuries, energy storages have been developed to preserve energy to be used later when necessity arises.From the humble hydropower energy storage in form of elevated reservoirs to highly popular electrochemical batteries that have become an integral part of our modern life, various types of energy storage mechanisms have been proposed and tested.Figure 2 briefly summarizes the available energy storages.
Among them, electrochemical and TESs are most likely the most widely adopted technologies.The former is typically used in consumer appliance while the latter is mostly applied to cater industrial needs.Its primary role is to ease or eliminate the mismatch between the energy demand and supply.It is based on three basic mechanisms: charging, storing, and discharging.Note that these mechanisms may occur simultaneously.As shown in Figure 2, there are at least three types of TES, that is, sensible TES, latent TES, and thermochemical energy storage.In the following section, each of these will be elaborated further.
As shown in Figure 3, the two basic TES mechanisms are sensible thermal energy storage (STES) and latent thermal energy storage (LTES). 235][26] The working principle of STES is storing thermal energy by using a temperature difference (higher or lower temperature depending upon its application).During charging, storing and discharging, temperature of the storage medium will be changed accordingly.No phase change is involved during these processes.The storage capacity of STES is determined by the temperature difference and specific heat capacity of the storage material as follows: where Q is the amount of heat stored in the storage material in J, m is the mass in kg, c p is the specific heat capacity in J/kg⋅K, and ΔT is the temperature change in K.The amount of heat stored is directly proportional to the material density, specific heat capacity, volume, and temperature change in the storage material.Thus, to store larger amounts of thermal energy (or increase storage capacity), STES need to be made larger or operated at higher temperatures (for heating applications) or lower temperature (for cooling applications).Accordingly, a larger storage volume will be required and/or better and thicker insulation needs to be implemented; both of which will incur additional costs to the system.Due to its simple heat storage mechanism and relatively low costs of material, STES have been commonly used for space heating, hot water systems, underground heat storage and many other thermal applications.Various solids and liquids can be used as the storage medium depending upon the application and characteristics of the materials.For example, water has the upper hand in term of storage capacity as compared to rock; because the specific heat capacity of water is approximately four times larger than that of rocks.Nevertheless, rocks have the ability to hold higher temperatures than water and have relatively higher density. 27Hence, rocks may be more suitable for storage involving high-temperature application.
As for LTES, its working principle is based on the latent heat release or absorption during isothermal phase change of the storage materials.The most commonly latent heat involved in LTES are latent heat of fusion (solid/liquid) and latent heat of vaporization (liquid/gas).The magnitude of latent heat is generally higher than sensible heat, thus the amount of energy stored in LTES is higher than that of STES.In LTES, solid-liquid phase change is the most popular option as it is considered to be an efficient alternative to STES. 28ajor advantages of this storage are its ability to store large amounts of energy at relatively constant temperature, and minimizing potential heat loss to the surrounding due to temperature difference which is experienced by STES. 29The storage capacity of LTES is mainly dictated by the latent heat of phase change of the storage material, that is where Δh is the phase change enthalpy (J/kg).A typical LTES will have three main components: containment for the storage medium, phase change material (PCM) as energy storage and heat exchange surface to facilitate energy storing and withdrawal.Depending on the operating temperature of the TES, various PCM with desired melting points and other characteristics can be chosen. 30he last TES type is TCS.This storage involves reversible chemical processes.As compared to the two former storage mechanisms, it is a relatively complex technology and requires complex reactor design. 31To elaborate on the storing mechanism in this energy storage system, let us consider reactant A which is transformed into compounds B and C through reversible reaction shown below.The process is an endothermic reaction where enthalpy reaction ΔH r is required for this process.This reaction is used as thermal energy-storing mechanism.During the discharging, the reverse reaction occurs where compounds B and C are recombined into reactant A by releasing the enthalpy reaction. 32

∆ ⇄
Accordingly, the amount of energy that can be stored in this storage is mainly determined by the enthalpy reaction as follows: A r (4)   F I G U R E 3 Heat stored in sensible thermal energy storage and latent thermal energy storage.
TCS offers 5-10 times higher energy storage density than LTES and STES, respectively.In addition, thermal loss can be minimized or eliminated since the thermal energy-charged chemicals can be stored at room temperature. 31The TCS system can also be differentiated as an open and closed system.The open-type system exchanges gases with the environment in their charge and discharge cycle, therefore this system can be operated without storage and gas compression.This offers the possibility to simplify the TCS system but also increases the risk of being exposed to unwanted impurities such as dust, SO 2 , CO 2 , and organic compounds. 33These impurities tend to pile up in the system, and in long-term applications, may ultimately make the system inoperable. 34hat makes sensible heat storage material exceptional (compared to latent and thermochemical counterparts) is the cost-effectivity of such systems at 25-30 USD/kWh of energy storage compared to 25-90 USD/kWh for latent-based TES ones. 35This value is projected to be less than 15 USD/kWh for STES, while that for latent and thermochemical TES systems will be around 25-30 and 80-160 USD/kWh, respectively, in 2030.Table 1 compares the common TES systems in terms of efficiency, cost, lifetime, and range of working temperature. 35

| RTES SYSTEMS
RTES is one of sensible TES that utilize rock as storage medium.Since the mid-1970s, it has gaining popularity to be implemented in large-scale industrial thermal system.Unsurprisingly, several large-scale projects utilizing RTES have been reported.Siemens Gamesa has started the operation of RTES in Hamburg, Germany. 13It utilizes electrical energy to heat the air by using resistance heaters which will be used to heat a pile of volcanic rock to approximately 750°C.The stored heat can later be used to produce steam to drive steam turbines when necessary.Stiesdal storage technologies (SST) is developing a commercial RTES system in Lolland, Denmark. 14Another technology demonstrator was developed by The National Facility for Pumped Heat Energy Storage 36 and SEAS-NVE. 37searchers at Newcastle University explored a TES system with a capacity of 600 kWh (rated at 150 kW) and an efficiency of 60%-65%. 38Helen, a company in Finland, recently started constructing a large-scale seasonal energy storage facility in the rock caverns of Kruunuvuori with a total capacity of 300,000 m 3 , located around 50 m below sea level. 39Brenmiller Energy Ltd. also offers energy storage systems with a high capacity of 750 MWh, named "The bGen™," which can work with a wide temperature range of 350°C to 750°C and a pressure range of up to 120 bar for electricity production. 40Their systems are suggested to make use of waste heat in different industries including iron and steel production.

| Classification of RTES systems
RTES can be classified depending on several bases.Here the most commonly used classification is presented and discussed.

| Indirect and direct RTES systems
In terms of methods of storage, similar to other TES, rock TES can be divided into active and passive thermal storage system. 41Active TES is characterized by the use of forced convection in the system, in which the HTF or/ and the storage medium is circulating inside the system. 42,43For the latter case, the storage medium itself is mainly circulated through a heat exchanger and is more commonly used for liquid storage mediums like water, thermal oil, and molten salt. 44However, rocks of small granule size in the range of few millimeters can also be used in a fluidized bed to store thermal energy. 44n the other hand, passive TES systems make the HTF pass through the storage medium without relying on any external devices, carrying heat from and to the storage system.For an active TES system, the category can be subdivided further into direct and indirect contact systems.In this context, direct TES also treats the HTF as a storage medium, while indirect TES has a second medium for storing the heat. 45

| High-and low-grade RTES systems
Typically, TES systems, including RTES, can be classified into low-grade, medium-grade and high-grade ones.
Woolnough 46 classified systems based on the working temperature to low grade (temperatures up to 100°C), medium grade (temperatures between 100°C and 400°C) and high grade (temperatures exceeding 400°C).Another categorization places the storage temperature of 400-900°C into medium-grade storage 47 while higher temperatures (>900°C) is considered highgrade.Recently, ultrahigh temperature energy storage (working temperature exceeding 1000°C) has also been a sought-after topic. 48-51

| Short-and long-term RTES systems
An important categorization depends on the duration for which the TES system can retain energy.Short-term [52][53][54] and long-term [55][56][57][58][59][60] systems have been widely used in the literature to describe TES systems suitable for daily and seasonal applications, respectively.They are also known as diurnal and seasonal TES systems. 61,62It is worth mentioning that novel studies [63][64][65][66][67] have attempted to reduce the heat loss in STES, including RTES, thus increasing their efficiency and making them a suitable candidate for long-term storage.It is because, for the time being, STES is the most commonly used (and the only commercially available technology) for large-scale applications (e.g., in CSP plants). 68Comprehensive studies about this classification based on the storage duration can be found by Bai and colleagues. 20,57,59,69,70

| Design criteria of RTES systems
Due to its generally huge size, careful consideration should be taken in designing an RTES system.There are three main criteria that need to be addressed, that is, technical, cost-benefit and environmental criteria.To achieve high thermal storage capacity and excellent system efficiency, storage materials with high storage energy density are crucial.Next, a good heat transfer rate and compatibility between HTF and the storage medium is required.In addition, there is a requirement for stability and ease of control so that the thermal losses within the system can be minimized. 42Figure 4 shows how different properties of storage materials and other design parameters affect the overall performance of the TES system.It is suggested to choose rocks that have a high specific heat capacity, density, and thermal conductivity given the significant influence of these parameters on the performance of the TES systems.Also, it shows Design consideration for thermal energy storage.HTF, heat transfer fluid.
the general trend that a larger particle diameter, 71 sphericity, 72 and more expensive storage materials 73,74 are not favorable when designing a TES system.The effect of aspect ratio on the overall performance of TES systems is also studied in the review paper by Esence et al. 75 It is found that although a higher aspect ratio (height/diameter ratio) can improve the thermal performance of such systems, it can result in higher pressure losses and higher wall stresses caused by thermal ratcheting, thus the optimal design is reported to be a trade-off between the mechanical and thermal response of such systems. 75Higher inlet velocity is also usually associated with a higher overall thermal performance due to the increase in HTC 71 and a shorter charging/ discharging time 76 leading to higher efficiency in terms of energy storage. 77][80][81][82] Another point of consideration in the design of packed rock bed storage is the creation of permeability.The friction factor of the fluid flow within the storage is dependent on the packing method and arrangement.An alteration in the particle shape, roughness, arrangement, and the size distribution will significantly affect the friction factor. 83,84For instance, a more densely packed bed offers more resistance to the fluid flow thus increasing the pressure drop. 85The permeability of the storage, however, decreases due to the shrinkage in the space between the particles, which makes it harder for the fluid to pass through the passage. 86It should be noted, however, that although this is not favorable in terms of fluid flow performance, this lower permeability and higher pressure drop is usually accompanied by a thinner thermocline (the layer where there is a sharp temperature gradient between the hot and cold fluid) and a better thermal performance due to the better heat exchange between the HTF and porous media. 87ccordingly, there needs to be a trade-off between the positive and negative effects on heat transport and fluid flow, respectively. 88,89Effect of porosity and permeability on the overall performance of TES is discussed in detail in the review paper by Palomba and Frazzica. 90esides packing density, particle shape also plays an important role in determining the efficiency of TES.For instance, the ratio of heat transfer to pressure drop in packed bed of cylindrical particles is found to be more favorable than that in beds of spherical ones. 72Overall, improvement to RTES being inherently asymmetric, rough, and randomly packed is a packed rock bed energy storage designed with specific particle shape, roughness, and packing arrangement. 91e next consideration is on cost-benefit, for the TES to operate cost-effectively, the costs of storage material, heat exchanger, and space need to be taken into account.This criterion is crucial to develop a sustainable TES from an investment point of view.Lastly, the environmental aspect can be taken into account by considering operation strategy, 92,93 life cycle assessment, 94,95 maximum load, 96,97 nominal temperature, specific enthalpy drop in load, and integration into an existing power plant. 42,98,99This mainly benefits the system from an environmental standpoint as a more efficient TES design leads to a reduction in energy consumption and hence conservation of fossil fuels and reductions in CO 2 , SO 2 , NO x , and CFC emissions. 100Also, using industrial waste materials 101 and recycled materials 102 as the solid storage material, can benefit the TES system in terms of environmental impact. 101

| USE OF DIFFERENT STORAGE MATERIALS IN RTES SYSTEMS
The selection of appropriate storage material in a TES system depends on the application of the system.The use of various materials for both low-and high-grade TES systems can be found in the work of Gautam and Saini. 103For medium-grade applications (temperatures between 100°C and 400°C), concrete bricks and bauxite are generally suggested thanks to their availability and affordability, 47,104 whereas for higher temperature storage (above 400°C), materials such as recycled ceramics, 105 honeycomb ceramic, 106 cast steel, 107 siliceous rocks, 108 basalt rocks, 109 basalt glasses, 110 sintered ore, 111 various concrete based materials [112][113][114] (including a mixture of concrete with phase change material 25 ), fine-grained materials (such as silica sand, quartz gravel, and basalt), 115 basalt fiber, 116 refractory blocks, 47 firebrick resistance material, 117 industrial wastes, 118 demolition waste based sensible heat materials (mainly composed of CaO, SiO 2 , and Fe 2 O 3 ), 119 and industrial by-product materials (from the potash industry) 120 are suggested.A recent study compared the energy recovery efficiency (the ratio of the heat released during discharging to the total energy consumption in a complete storage cycle) of various solid materials used in a packedbed TES, 121 and highlighted the suitability of sintered ore particles as the storage material at temperatures above 600°C.Valuable tables summarizing the thermophysical properties of relatively low-cost minerals for this purpose can be found in Hrifech and colleagues. 17,103,112,122ne important consideration in high-grade TES is how temperature affects the thermophysical properties of the storage material.Alami et al. 123 compared the application of three different Moroccan rocks, namely igneous, metamorphic, and sedimentary rocks in highgrade TES systems.They reviewed available correlations of thermophysical properties of storage materials as a function of temperature in the literature.They also highlighted a research gap in the direct interaction of rocks and HTFs other than air to ensure that rocks are suitable for TES systems if the working fluid is not air.It was concluded that storage systems in which the working temperature exceeds 500°C can immensely benefit from phase transition as the energy storage capacity is higher in this case due to the high latent heat capacity. 124Magmatic and metamorphic rocks were also reported to be the best candidates in high-grade TES, whereas carbonated and coarse-grained rocks deemed to be the worst ones and are suggested not to be used in high/ultrahigh grade TES applications.They also found that when the temperature exceeds 550°C, potential rocks (such as basalt, gabbro, rhyolite, quartzite, hornfels, and sandstone) can be used as a storage medium in TES systems.Grosu et al. 125 proposed a reliable heat treatment method to make natural magnetite an exceptional candidate to be used in a wide range of TES applications, especially for packedbed systems with a working temperature of as high as 1000°C.In their study, they transformed magnetite to hematite by treating the samples in the furnaces at 400°C, 800°C, and 1000°C, to make use of their excellent thermophysical properties and latent heat of reversible antiferromagnetic transition.They also pointed out the possibility of easily controlling (programming) the thermal conductivity of the material in a wide range of values by varying the temperature of the heat treatment (given that the material is used at nonoxidizing conditions).Tiskatine et al. 126 conducted a comprehensive review on the suitability of rocks for high-temperature sensible TES systems and dolerite, granodiorite, hornfels, gabbro, and quartzitic sandstone were reported to be the best candidates.For applications up to 500°C, physical changes are only expected, while more studies are encouraged to assess the chemical response of rock bed storage systems. 127For example, cipolin samples which contain graphite and calcite minerals have shown decomposition and release of carbon dioxide gas at temperatures up to 900°C leading to performance deterioration. 17Discussions about the relationship between molecular structures and thermal properties of such TES systems can also be found in Fallahi et al. 124 The effect of temperature on the thermal properties of sandstone and concrete was also studied by Sun et al. 128 and Heap et al. 129 Although thermal degradation can decrease the mechanical stability of the storage system, its thermal performance has shown improvement provided the right material is chosen.Ababneh et al. 130 reported many benefits associated with using elevated temperature to exploit the storage capacity of storage materials.They considered lithium sulfate for the storage material and sodium-potassium eutectic alloy NaK for HTF.It was numerically shown that for sufficiently high enough temperatures, that is, 569°C, the TES capacity increases due to the solid-solid phase change.This form of phase transition is beneficial since the temperature of the storage medium remains within a small range while storing thermal energy, and hence, the internal entropy generation is minimal. 130Also, due to the elevated temperature, the heat capacity associated with sensible thermal storage is also increased, further increasing the storing capacity at high temperatures.Higher solid-solid phase transitional heat capacity is also reported in the study by Singh et al. 131 who conducted both experimental and computational calculations based on coupling of phase diagrams and thermochemistry.As promising as this method of heat storage capacity may sound, one should be aware of the associated thermomechanical degradation.Yin et al. 132 conducted a comprehensive literature review to study the effect of high temperature on the elastic modulus of rocks.The degradation effect is consistently present when the temperature increases as dehydration, phase transition, thermal expansion, and thermal decomposition of minerals occur which leads to the propagation of cracks, pore size increment, and mineral modification.The threshold temperature for limestone, sandstone, granite, and marble are reported to be 400°C, 600°C, 500°C, and 800°C, respectively.Similarly, Chen et al. 133 observed the thermal damage of Beishan granite in high-temperature applications (up to 800°C), and they attributed it to the intergranular cracks which started to develop from 100°C to 573°C and intra-granular ones for temperatures over that threshold due to the phase transition of quartz.The suitability of rock as the storage materials for hightemperature application is also studied by Tiskatine et al. 134 by the use of petrographic and thermomechanical analyses.The effect of thermal cycling on thermomechanical properties of the storage material is also considered in their study.They concluded that while carbonated, foliated, and coarse-grained rocks are unsuitable for high-temperature thermal storage, rhyolite and quartzitic sandstone withstand the thermal shocks and are better candidates for high-temperature TES applications.In another study, 135 they proposed a selection methodology based on laboratory measurements and combining multiple parameters (including cost, availability, lifetime, recyclability, thermophysical and mechanical properties, thermochemical stability, and environmental impact) to choose the best material for TES systems.Becattini et al. 136 also studied the thermal cycling effect on thermophysical properties of rocks experiencing temperatures up to 600°C.It was shown that thermal cycling degrades the specific heat capacity of rocks for two main reasons: (i) mineral dehydration which starts at 400, and (ii) decarbonization of calcite and deserpentinization above 600°C.This degradation, however, did not cause significant reduction in the storage capacity of packed-bed storage.The porosity of the rock was also shown to increase with an increase in temperature which was attributed to the α-β quartz-inversion at 573°C.Thermal cycling at high temperatures can also fracture the rocks through the thermal expansion and decarbonation of calcite, and the thermal expansion and inversion of quartz. 136This means that the use of foliated rocks and rocks that are rich in calcite and/or quartz (such as limestones and sandstones) should be avoided for high-grade TES applications.Finally, because an increase in the porosity leads to a reduction in the fracture strength under compression, rocks that have low porosity before thermal cycling and exhibit small increase in porosity during thermal cycling (such as mafic rocks, felsic rocks, serpentinite, and quartz-rich conglomerate) are less likely to fracture, and hence are suitable materials for storage at high temperatures. 136n TES, various materials have been used as storage medium.Based on the study originally developed by Ashby et al., 137 Fernandez et al. 54 looked into different materials as potential candidates to be used in STES.Based on their work, natural rocks are an attractive energy storage material for packed bed configurations, as they are inexpensive, abundant in nature, environmentally friendly, and most importantly they can be used in applications with a wide temperature range 138,139 (although several studies have reported thermal degradation 108,134 necessitating thermal treatments of rocks for high-temperature applications 108 ).However, similar to other sensible heat storage materials, rocks have a significantly lower storage capacity as compared to latent and thermochemical heat storage materials.Thus, the system size has to be significantly bigger. 140o achieve optimum energy storage performance, a good understanding of the thermal properties of the medium used in the TES is important because each property represents a thermal effect that affects the TES system.There are key characteristics of materials for TES systems that need to be used as reference when designing, 141 that is, the energy capacity, the operating temperature range, thermal transport properties of the medium, the temporary stratification of storage unit, the power requirement, the material of the storage container, the storage tank design, 142 the means of controlling thermal losses from the storage and its cost.These references are explained more in the following section.
These characteristics can be well estimated from the thermophysical properties of the material, including thermal conductivity (λ), specific heat capacity (C p ), and thermal diffusivity (κ).The first two properties express the ability of the rock to transmit and accumulate heat, and the latter measures the rate of heat transfer in heat per second.At constant pressure, thermal diffusivity (κ) is the thermal conductivity (λ) divided by density (ρ) and specific heat capacity (C p ). 143 Figure 5 summarizes thermophysical properties of some common solid STES materials.As can be seen, natural rocks offer good thermal conductivity despite their relatively low specific heat capacity.

| WORKING RANGE OF RTES SYSTEMS
Table 2 summarizes the working range of TES systems based on sensible heat storage materials.As can be seen, various storage materials with different porosities are used for this purpose, and energy densities in TES systems from 15 to 810 kWh/m 3 are reported.Also, temperatures over 1000°C are reported in the literature, which emphasizes the importance of material selection and optimization in ultrahigh grade TES systems to either improve their performance or prevent them from thermal degradation.

| APPLICATION OF RTES SYSTEMS
Due to its promising storage performance, RTES have been implemented in various applications.In this section, several applications are discussed.

| Mining: Rock pit energy storage
Like all energy storage systems, RTES technology is designed to make the system as economically viable as possible.For this purpose, RTES in mining can be interconnected with the mine ventilation system (MVS) 184 which is responsible for 60% of the total underground mining operation costs (electricity, refrigeration, and thermal heat). 185,186This can be achieved by using the stored energy in rocks for ventilation purposes, and hence decreasing the associated energy costs.RTES can also make use of the heat generated from auto-compression (compression of air as it flows from the ground surface down a mine shaft 187,188 ), geothermal gradient, and other heat sources from the mine to be charged and discharged in the desired cycle. 187everal large open-pit mines are transitioning into cave mining methods to excavate more ore. 189I G U R E 5 Thermophysical properties of typical solid sensible thermal energy storage materials.| 425 In between the open pit and the underground mine, shafts are installed to ventilate the underground mine.Some mines utilize this situation by making a rock pit above the shaft, which is used as RTES.For example, Creighton mine is renowned for its specific system using fragmented rocks dumped into a decommissioned pit above the shaft. 79,80,82This approach takes advantage of a massive volume of broken rocks on the surface of the mine to create RTES.The stored energy is then transferred to air to be used for ventilation purposes.This, in turn, increases the energy efficiency in the mine ventilation system, which is called "Natural Heat Exchanger." The heat exchanger works by allowing the fresh air to be moved through the rock pit into the underground mine.

| Remote community: Rock seasonal heat storage
Another example is the utilization of RTES in remote communities that instead of letting the summer heat go to waste, RTES can modify the heat to work seasonally. 11,190The high dependency on fossil fuels, diesel generators, and electric heating in remote communities with arctic climates leads to inefficiency in the process of supplying power.During winter, the generators discarded a significant amount of heat through the exhaust due to the lower efficiency of conventional diesel engines, which is about 35%. 191hen, this waste heat is recovered and used as a heat source.During summer, this waste heat can be categorized as potential energy, but usually, is then discarded.Due to this condition, a rock pile-based seasonal TES is proposed to accommodate the seasonal mismatch between supply and demand of heat.
The system worked by storing excess heat during summer and later used during heat demand in winter, as discussed earlier.Moreover, the calculated payback period of less than 6 years for this system gives economic sustainability to be implemented and emulated in other similar conditions. 190acked rock bed as a TES storage medium shows a promising performance at temperatures up to 600°C. 192Several numerical 193,194 and experimental 168,192,195,196 studies indicated that the utilization of rocks has a positive impact on the heat exchange process.Packed rock bed storage systems could be implemented as gravel TES, or help to optimize the concentrated solar power plant. 160The integration between RTES and CSP could result in a higher heat retrieval efficiency. 56,197,198

| Concentrated solar power plant
The usage of a packed rock bed as a storage medium in the CSP plant could be considered as an alternative to the costly molten salt. 83,199One of the earliest studies of rock-packed bed solar systems was conducted by Garg et al. 195 in 1981.They set up an experimental model for a conventional rock bed solar collector and obtained satisfactory efficiency results.Ramadan et al. 200 did a theoretical and experimental simulation on a packed bed double-pass solar air heater using gravel and limestone as the porous medium and proposed a theoretical model to predict the thermal performance of the system.Zanganeh et al. 192 did an experiment for CSP application in a 6.5 MWh th pilot-scale thermal storage unit, which was then implemented into a 7.2 GWH th industrial-scale thermal storage unit.The output of this system indicated good results, that is, the overall thermal losses remained below 0.5% of the input energy, the discharging temperature remained above 590°C, and the overall thermal efficiency reached 95% when the system approached steady cyclic behavior.

| Light water reactors (LWRs): Crushed rock thermal storage
LWRs use a huge, crushed rock thermal storage system with capacities of gigawatt-hours to provide steam for the industry, variable electricity to the grid, and hot air for industrial furnace. 201,202This storage system exploits the excess energy from the reactors in the form of steam and electricity as an energy input.In this way, electricity is produced when the price is low to be later sold at a higher rate.The first procedure is preparing the air in a two-stage process in an air heater, which is: (1) heating the air with a steam-air heat exchanger from LWR steam, and (2) heating the air with electric resistance heaters.Then, the resultant hot air flows from the top to the bottom of the crushed rock pile while heating the rock, and back to the air heater.Air is circulated from the bottom to the top of the rock pile to recover the heat, where the hot air is sent to the industrial furnace or thermal electric power plant. 201

| Gas and steam turbine
Schematics of integration of a TES system with gas turbine 148 and steam turbine 203 are shown in Figures 6  and 7, respectively.Figure 6 shows the concept of solar gas turbine incorporating a pressurized packed bed.During periods of excess energy, a portion of the mass flow leaving the turbine is sent to the receiver and packed bed (red arrow).During peak hours or when the energy demand surpasses the solar supply, a fraction of the mass flow is diverted to the packed bed (blue arrow).Figure 7 shows the schematic of the electrical thermal energy storage (ETES) where the system is divided into an air cycle and a water-steam cycle. 167A powerplant is built on this concept in Hamburg, Germany, with a nominal charge temperature of 750°C and a rated storage capacity of 130 MWh th .The design and testing of a rock bed thermal storage pilot plant in a recent study 18 is shown in Figure 8.

| NONTECHNICAL ASPECTS OF RTES SYSTEM
RTES systems can contribute significantly to meeting various industries' desire in remote cold environments for more efficient, environmental energy solutions, particularly for heating, cooling, and ventilation.The utilization of RTES systems with renewables as the energy source results in two significant environmental benefits: • decreases in fossil fuels consumption through fuel substitution, and • reductions in CO 2 , SO 2 , NO x , and CFCs emissions.
RTES can impact air emissions at industrial sites and surrounding communities by reducing the amount of combustion emissions from fossil fuel-based heating and cooling equipment.A cooling system with RTES operates by storing chilled water or ice during off-peak hours when energy demand is lower and using it to provide cooling during peak hours when energy demand is higher. 204Hence, RTES helps shrink CFC use in two main ways.First, since cooling systems with RTES require less chill capacity than conventional systems, they use fewer or smaller chillers with a lesser amount of refrigerant.Second, when the refrigerant used in existing chillers is converted to a more environmentally friendly refrigerant, there may be a reduction in cooling capacity.However, using RTES can help compensate for this reduction in cooling capacity, thus allowing the use of a more eco-friendly refrigerant. 204he potential aggregate air-emission reductions at power plants due to RTES have been shown to be significant.For example, RTES systems have been shown to reduce ~8800 tons of CO 2,eq emissions per year in the Creighton underground mine, Canada, by employing a natural heat exchanger unit. 79Also, F I G U R E 6 Schematic of sensible thermal energy storage systems integrated with gas turbine as power plant. 168I G U R E 7 Schematic of sensible thermal energy storage systems integrated with steam turbine as power plant. 167alifornia Energy Commission data demonstrates that existing gas plants produce ~0.06 kg of NO x and 15 kg of CO 2,eq per ~0.3 MW h of fuel burned, and assuming that TES installations save an average of 6% of the total cooling electricity needs, TES could possibly eliminate annual emissions of about 560 tons of NO x and 260,000 tons of CO 2,eq state- wide. 205,206nergy consumption is an important factor that reflects the influence of a certain sector on the economic growth and environmental pollution of a region. 79,190Existing reports from different energy statistics agencies [206][207][208] show that both industrial activities and energy sectors (power stations, oil refineries, coke ovens, etc.) are the most energyconsuming sectors worldwide.Therefore, they are responsible for the release of large quantities of industrial waste heat (WH) to the environment through the exhaust gases. 190Waste heat recovery (WHR) by means of RTES system would provide an attractive opportunity for a low-carbon and less costly energy resource. 82,190Therefore, there is a significant opportunity to establish advanced RTES technologies capable of storing the waste heat and releasing it in other different industrial applications preventing the consumption of other energy resources to produce heat.
Although the initial objective of these WHR systems is energy savings, the environmental concerns due to the use of the stored energy are subsequently some important points to take into account in the overall evaluation of TES technologies.To this end, the life cycle assessment (LCA) methodology has been used by several researchers as a tool to estimate the environmental impact of TES systems. 94,95,99,209,210Even though the sensible energy storage capacity is lower than the latent heat storage systems, due to the ease of heat transfer and operation and simplicity of the system construction and low operational and maintenance costs, 95,99,209 the global impact per kWh stored is the lowest, being by far the most environmentally friendly TES system.Accordingly, RTES has several advantages when compared to other storage technologies, including: • RTES systems can increase energy efficiency while reducing the carbon footprint.• Waste heat recovery using RTES units helps to reduce heat production from fossil fuels.• Rocks with relatively high specific heat capacity tend to achieve a better overall performance of the system.
][213] Assuming the system substitutes natural gas, oil, and coal as a heating source, which have carbon emissions of approximately 510, 840, and 905 tonnes of CO 2,eq per GWh, respectively, the total amount of carbon footprint reduction is estimated for heating using the RTES system.
F I G U R E 8 Experimental study of sensible high-grade thermal energy storage system with combined electricity generation and heating applications. 18t is found that for the base properties the RTES (ρ s = 3000 kg/m 3 , c p,s = 1000 J/kg•K, k s = 2.68 W/m•K) would be able to reduce emissions by about 28, 41, and 50 tonnes of CO 2,eq /tonne of rock per year if the heating is provided by burning natural gas, oil, and coal, respectively.This is equivalent to 85, 122, and 151 tonnes of CO 2,eq /m³ of rock per year if the heating is provided by burning natural gas, oil, and coal, respectively.

| CONCLUDING REMARKS AND FUTURE DIRECTION
This review paper focuses on the various applications of available RTES technology.These technologies are particularly valuable to both the environment and energy conservation.This paper highlights the recent studies in this particular subject, with the major focus being on the assessment of the thermal characteristics of various solid materials.As highlighted, energy storage is a crucial part of any energy system to minimize or eliminate the mismatch between energy supply and demand.Among various available energy storage technologies, RTES has promising potential due to its ability to hold large amounts of energy at high temperatures and relatively low cost.
Concluding remarks and future direction include the following: • To ensure the affordability and accessibility of TES systems, further investigation is needed to examine their costs, and to identify and develop potential costsaving measures.The current trend is to reduce the capital cost of TES-based power plants to several dollars per kWh of heat, 56 and future works can be in the direction to meet or improve this criterion.Moreover, novel/optimized TES systems integrated with power plants should be proposed with costeffective MW of power output.For this purpose, energy and exergy studies to increase the efficiency of ultrahigh-grade TES systems combined with conducting technoeconomical studies to minimize the capital, operational, and maintenance costs of such systems can prove helpful.Finally, thermal performance degradation due to flow and thermal irreversibility as well as long-time structural integrity due to the extremely transient thermal behavior of such systems requires more attention, and accordingly, more exergy and LCA analyses are recommended.• More studies are needed to develop reliable numerical representations of high and ultrahigh grade RTES systems and their integration with power plant systems.The validity of simplifying assumptions like ignoring the effect of axial conduction, temperature gradient within solid particles and radiation heat transfer, needs to be challenged in future works as more complex systems will be developed for higher thermal storage capacities.• Future works should target the optimization of RTES in which a wide range of particle diameters, mass flow rates, and working temperature ranges are considered.Multiobjective optimization procedure is suggested for this purpose in which the minimum pressure drop (minimum fan power) and maximum energy storage capacity are sought after.The optimization efforts can be directed towards reducing the thermal loss in the RTES, altering the thermocline to make it steeper, and investigating various HTFs and storage medium based on the application of TES outlined earlier based on the duration of storage required and the grade of the TES (high-grade or low-grade).
• While discussions about the effects of temperature on physical properties of storage material in a high-grade TES system are available in the literature, the effects on the chemical changes have remained relatively unexplored, and more focus should be placed on addressing this issue in future works.
Summary of sensible heat based TES systems.Based on the difference between fully charged and fully discharged states.157 ****Where R1, R2, and R3 are hot, cold, and larger capacity reservoirs, respectively.*****Case 1 designates the base case system, while Case 2 designates system with combustion.