Climate change, along with humans’ voracious need for energy, calls for a paradigm shift towards more rational and sustainable energy use. Modern societies need to deploy innovative energy technologies to drive this transition. Thermal energy storage has a leading role in this context as it can assist us in handling the demand and generation of energy currently out of phase. Energy storage materials and applications in terms of electricity and heat storage processes to counteract peak demand-supply inconsistency are hot topics that many researchers are working on nowadays. Heat encompasses the highest portion (within 70 to 80%) of total energy demand in humans daily lives regarding domestic applications as a share of hot water and space heating requirements .
Moreover, heat is at the centre of the universal energy chain, creating a linkage between primary and secondary sources of energy, and its functional procedures (conversion, transferring, and storage) possess 90% of the whole energy budget worldwide . Hence, thermal energy storage (TES) methods can contribute to more appropriate thermal energy production-consumption through bridging the heat demand-supply gap. In addition, TES is capable of taking over all elements of the energy nexus, including mechanical, electricity, fuel, and light modules by means of decreasing heat losses, waste recovery, and energy-saving approaches to improving the system’s performance. TES concept consists of storing cold or heat, which is determined according to the temperature range in a thermal battery (TES material) operational working for energy storage.
Figure 1 illustrates the process-based network of the TES device from energy input to energy storage and energy release. The advantage of TES with charging the thermal battery is to supply thermal energy demand after the heat source is out of work, such as using solar energy during the day for charging a heat storage medium producing heat during the night, or using natural gas in power plants for charging the molten salt heat storage unit during the low-demand period and producing steam for running the turbine and generating electricity over high-demand times with discharging the heat storage unit.
By exploiting the TES method for producing heat during the discharging time, the roundtrip efficiency of the thermal systems heightens from below 50% to around 70 to 100% depending on the amount of heat loss imposed . As a matter of fact, TES materials act as absorbing the excess heat during the charging process to reduce heat losses, increasing the overall efficiency of the TES systems.
There exist many valuable review contents on the internet addressing several heat storage methods individually; the need for a comprehensive and concise source of information to present related ideas and applications is still sensed. The aim of the present article is to introduce thermal energy storage materials, investigate significant research contributions, and link both practical applications and scientific aspects of the problem.
This blog considers articles reporting original, cutting-edge research with experimental, theoretical, and numerical findings unravelling pertinent aspects of novel thermal energy storage systems. Ultimately, their design, characterisation, optimisation considerations, and integration challenges have been addressed to present a broader standpoint in this field of engineering.
1.2 How does it work?
TES methods are comprised of sensible heat storage (SHS), which is storing energy using the temperature difference, latent heat storage (LHS), which is to use latent heat of phase change materials (PCMs), and thermochemical heat storage (TCHS), which is exploiting the reversible chemical reactions through thermochemical materials (TCMs).
As a whole, the chemical type of energy storage contains employing an energy source for exciting chemical reactions, and the energy source can be in the forms of heat (TCHS systems), electricity (electrochemical reactions in batteries), or electromagnetic (photosynthesis and photochemical reactions). The TCHS works on a sorption-related process, i.e. is the process of storing heat by separating the water vapour molecules from TCM by heating it in an endothermic reaction and releasing heat (desorption stage) by having the TCM in contact with atmospheric humid air so that the water vapour can attach to the dry solid materials of the TCM (adsorption stage) leading to occurring a spontaneous exothermic reaction.
TES systems can work based on the type of material, which has been employed for storing heat, and the aforementioned heat storage material can store energy in different forms of sensible, latent, or thermochemical heat storage procedures. As indicated before, the SHS stores heat using the temperature difference within the TES materials, such as water, nanofluids, sand, gravel, concrete. In addition, the LHS is another method of storing heat, taking advantage of the heat of fusion values of PCMs that can be divided into organic (such as paraffin and hydrocarbons) and inorganic (salt hydrates) PCMs.
The heat of fusion is the process of absorption or release of thermal energy during a PCM meting/solidification point without any temperature difference. It has been reported that the inorganic PCMs possess higher latent heat values (around 200 to 250 kJ‧kg−1) than the organic ones (mainly 120 to 150 kJ‧kg−1). However, there are some high-melting-point organic ones, such as erythritol (120 °C melting point) are of the heat of fusion of about 350 kJ‧kg−1.
Moreover, the thermochemical heat storage type exploits the energy inside the chemical bonds of TCMs, such as metal oxides and some salt hydrates possessing energy capacity that can reach around ten times higher than that of PCMs. When the TCM is dehydrated (receiving heat), the thermal energy is stored inside the chemical bonds for any period until being hydrated and releasing the stored heat (being discharged), which is a suitable method for seasonal heat storage applications.
Table 1 compares the properties of different TES systems presenting the average energy density that each type of system can provide.
Table 1. Thermal Energy Storage technology comparison.
Moreover, the SHS materials, such as water, have been being implemented since human’s civilisation as the primary developed TES systems; the LHS method was discovered in the 1950s; however, research on the development of the PCMs has been intensified after the American Energy Crisis in 1973. Subsequently, the TCHS procedures have been first introduced in Rocket Research Corporation in Seattle by E. W. Schmidt in 1976 considering the release of heat in sulfur trioxide after the sorption process between humid air and dry TCM and working on the structure of TCMs is being analytically investigated within the last decade in Europe.
In terms of the energy density, PCMs can store heat up to 1 GJ‧m−3, the sorption process can present high values of energy density up to 6 GJ‧m−3, and ultimately chemical reactions can offer up to 10 GJ‧m−3 energy density, which is approximately tantamount to storing heat in biomass (dry wood). Moreover, it has been demonstrated that, on average, supplying the heat demand using the sorption process of a TCM needs half the volume of the thermal storage tank filled with a PCM and one-third times the size of a water tank volume to meet the same amount of heat .
2 TECHNOLOGY USE EXAMPLES
2.1 Molten Salts to Concentrate Solar Power
Concentrating solar power (CSP) technologies generate electricity by focusing the solar radiation beam onto a small area and capturing the solar energy in the form of heat. The CSP installed capacity has increased significantly over the last decade. By the end of 2015, the CSP market had a total capacity of 4.9 GW worldwide, and it is projected to provide 12% of global electricity by 2050 . With the current molten salt thermal energy storage (TES) technology, CSP is able to mitigate the short load fluctuation and operate over more extended periods. The molten salt is a mixture of sodium and potassium nitrates with a weight ratio of 60:40 (known as solar salt). By alternately operating the solar salt in a hot tank at 565 ᵒC and a cold tank at 290 ᵒC, the TES system can be charged and discharged, respectively.
Researchers have made considerable research efforts to reduce the high cost of CSP generated electricity, which is one of the main obstructions of broader deployment of CSP technology. Recent studies have reported experimental results of solar salt with specific heat capacity enhanced by doping nanoparticles (NPs) . The increase in the specific heat capacity could significantly reduce the cost of TES and hence decrease the cost of generated electricity. The composite material containing NPs smaller than 100 nm is defined as a nanofluid.
An eutectic is a minimum-melting arrangement of two or more components, each of which melts and freeze congruently, forming a blend of the component crystals during crystallisation. Recent studies have been carried out with other inorganic salts as base materials, such as eutectic carbonate salts (Li2CO3-K2CO3), eutectic ternary nitrate salts (LiNO3–NaNO3–KNO3), eutectic quaternary nitrate salts (Ca(NO3)2·4H2O-KNO3-NaNO3-LiNO3) and eutectic chloride salts (BaCl2, NaCl, CaCl2, and LiCl). Scientists reported 118-124% enhancement in the specific heat capacity of carbonate salt with 1.5 wt. % silica NP. It indicates that the enhanced specific heat capacity of solar salt can be potentially augmented through the optimisation of additives and synthesis methods.
2.2 Thermal Energy Storage in Buildings
In commercial buildings, as well as residential buildings, thermal control, cooling or heating are essential to maintain comfort for the occupants. Over the years, thermal energy storage (TES) has appeared as an up-and-coming solution with high potential to help keep the indoor temperature in buildings within specified comfort limits. TES is key to overcoming the usual temporal mismatch between energy availability and demand.
Advantages of integrating TES in buildings’ energy systems include increased overall efficiency, higher reliability, better economics, reduced investment and operating costs, less environmental impact, and lower CO2 emissions. Consequently, TES is also an essential asset to make the most of intermittent renewable energy sources, both for heating and cooling purposes. It can therefore contribute to current goals of increasing the share of renewable energy in the building sector.
Phase change materials are a class of energy storage medium that has gained significant attention in recent years, both within research and commercial applications. A PCM is a material that absorbs or releases large amounts of energy as latent heat during a phase transition (normally solid-liquid or solid-solid transitions). This latent heat is generally much higher than the sensible heat of the material. PCM-based TES is also known as latent heat storage.
LHS systems in buildings are commonly classified into active or passive systems, depending on their integration. The system is called passive when heat is charged or released only by natural convection or solar radiation, with no mechanical input or extra energy. PCM directly integrated into the building envelope such as walls, roofs or floors are typical examples of passive systems. An active LHS system, on the other hand, relies on mechanical input, forced convection, or additional energy to charge or discharge PCM thermal energy, e.g., PCM implemented in water tanks or used as heat/cold storage tank connected to heating, ventilation and air-conditioning (HVAC) system.
Free cooling combined with LHS seeks to exploit the variations in ambient temperature between day and night; it has been extensively tested and implemented due to its potential to reduce air conditioning (AC) system capacity and operation hours . Free cooling systems are often understood as mechanical ventilation systems with stored ambient energy. However, diurnal temperature variations can also be exploited in combination with more advanced equipment, such as heat pumps (HP). Figure 3 depicts a free cooling setup exploiting diurnal temperature variations for daytime cooling without any auxiliary equipment other than the PCM and a fan. Active LHS systems may also use PCM directly incorporated in the building material and the façade.
The efficiency of free cooling leans on the local climate. The capacity for free cooling follows a roughly linear relation to the diurnal temperature range of the local weather. A significant (>10 °C) diurnal temperature difference is highly favourable as a difference between the heat transfer fluid (HTF) and the PCM of more than 3–4 °C is usually sought for LHS charging/discharging in a reasonable time. The PCM thermophysical properties (especially phase change temperature and temperature range) should be primarily chosen based on the local climate. For less suitable locations, more advanced (expensive) design and operation control are required, for example, using PCMs with thermal conductivity or in combination with solar energy.
2.3 Concrete TES
Among several sensible heat storage materials, concrete has been used in ancient worldwide constructions, having the advantage that its components are inexpensive and globally available. The main concrete constituents are aggregates (70%), cement (18%), water (10%) and air (2%). However, the cement-making process is energy-intensive and has high carbon emissions generated during clinker manufacturing. To reduce the environmental impact of cement, supplementary cementitious materials can be added as a partial replacement for cement.
Under the clean energy framework, solar energy is an attractive option for space heating in buildings and solar power plants to produce electricity. However, solar energy is highly dependent on the weather, which requires a storage media system that allows releasing the thermal energy when needed. Concrete is a versatile material that, thanks to its thermo-mechanical properties, has been extensively studied among different research fields across the years .
Concrete is a flexible material that allows for varying components in relation to the final application objective. In this context, the heterogeneous materials allow incorporating phase change materials (PCMs) and other chemical elements, resulting in a multifunctional material composite. Moreover, concrete in buildings has a twofold function and acts as a structural element in reinforced concrete.
In concrete TES, hot exhaust gas or steam is sent through encased piping to heat the neighbouring concrete blocks. Feedwater is transmitted through these blocks to raise steam for a steam cycle to release the stored thermal energy. A pilot project is underway to assess a 10 MWe concrete TES system at an operational power plant.
The Thermal Battery pilot has been proved out at Masdar Institute Solar Platform in Abu Dhabi, United Arab Emirates, with two × 500 kWhth thermal capacity at temperatures up to 380 ◦C over a period of more than 20 months. The measured demonstrator behaved as predicted from numerical simulations and multi-cycle operation have proved concrete TES’s integrity and operational feasibility. The measurement of HTF inlet and outlet TESS temperature over 279 charge and discharge Energies 2022, 15, 647 21 of 33 cycles (6000 h) show a stable and repetitive performance, which demonstrates that the concrete storage medium stays stable with no sign of degradation. The cylindrical storage element was cut into smaller sections and, following inspection, revealed no degradation, for instance, spalling or cracking. Moreover, no separation between steel pipes and concrete storage material has been demonstrated .
2.4 Carnot Batteries
Carnot batteries comprise a set of multiple technologies with a common underlying principle of converting the electricity to thermal exergy, storing it in thermal energy storage systems, and converting the heat back to electricity in a time of need. Based on this principle, alternative terms are also used as power to heat to power (P2H2P) or electric thermal (or electro-thermal) energy (electricity) storage (ETES). An excellent review work  provides a general overview of Carnot batteries principles, and therefore the reader is referred to this work for details. Here, the general aspects will be summarised rather briefly.
Carnot batteries use surplus electricity as an input of a power to heat (P2H) system to create a temperature gradient (thermal exergy). It can enclose hot and cold storage systems or just one of those, with the temperature gradient defined against the environment. The thermal exergy is converted back to work (electricity) by heat to power (H2P) system during the discharging process.
A specific aspect of batteries is the possibility of thermal integration, both on the side of energy input and output, providing many options for sector coupling. The most straightforward notion is the direct conversion of electricity to heat, which is then stored before being converted back to power during discharging, typically by a power cycle. Other systems can be considered so-called compressed heat energy storage (CHEST) or pumped thermal energy storage (PTES) as they utilise the thermodynamic cycle (in principle, a heat pump) for the P2H conversion.
Regarding the heat input, the heat source can be either upgraded to a higher temperature, which is then used as a heat input of the power cycle or downgraded to cold, which is stored and subsequently used as a heat sink of the power cycle during discharging, increasing the overall temperature gradient of the heat source. A specific case can be defined when the temperature at the source is identical to the environmental temperature. Such systems are not really considered for hot storage due to their low roundtrip efficiency.
Regarding cold storage, it could be considered a highly simplified representation of liquid air energy storage when the air is liquefied and stored at cryogenic temperatures. All real thermodynamic conversions depart from the ideal ones (e.g., minimum temperature differences in heat exchangers, the efficiency of compressors and expanders, pressure drop in components). As a result, a portion of the heat needs to be rejected into the environment due to its irreversibilities.
The first concept minimises the losses by converting and storing the heat at the highest possible temperature, maximising the power cycle efficiency. The second concept then optimises the charging and discharging cycles to minimise the irreversibilities. In the heat source integrated concepts, efficiency is also a function of the temperature lift of the heat pump. With an inferior lift, the roundtrip efficiency (defined below) can theoretically reach values above unity in case of a zero charge (and work) of the heat pump, even going towards infinity.
Concentrating solar thermal power coupled with thermal energy storage is an auspicious source of clean electrical power. Researchers and developers are demonstrating that to maximise revenue from a plant with unlimited storage, the optimal modes are to (i) store all accumulated power, without generating; (ii) generate using only collected power; (iii) generate at maximum capacity using both collected and stored power; (iv) generate a maximum capacity, storing any surplus power.
But there are two main differences in more realistic cases, where there is limited storage capacity. First, it is sometimes imperative to discard power if the depot is full. Second, the critical prices employed to determine the optimal control can change whenever the store becomes full or empty. Pontragin’s principle  is employed to derive necessary conditions for optimal control and produce an algorithm to find the string of critical prices required to construct it. This kind of analysis gives better insight into the nature of the optimal rule than probabilistic search techniques or mathematical programming, and the calculation is fast-finding the optimal control for a year-long operation in about 30 seconds.
Figure 4. Energy flows in a CSP plant with limited storage. Source: doi: 10.1049/iet-rpg.2015.0244
In a CSP plant model, thermal power ps flows from the solar receiver piece, power pst flows from the solar receiver to the thermal store, and power psg flows from the solar receiver to the generator. Power ptd is diverted from being stored when the storage is full, flowing out of the system without being used. Power ptg flows from the thermal store to the generator. The first three of these power flows are related by ps = psg + pst.
Power ps is given. Management can be achieved using either one of the two power flows on the right, with additional controls being ptd and ptg. It is important to note that there are four optimal control modes for a system with unconstrained storage . The optimal status at any instant depends on the market price π relative to an optimal generation price π*G and the incoming solar power ps. The most advantageous control modes for a system with unlimited storage are:
Store: Store all incoming solar energy—this mode occurs when the price π ≤ ηt π*G, where ηt is the efficiency of the thermal accumulation.
Solar: Generate with all available solar energy, without storing power or taking power from the store—this takes place when the incoming solar energy does not exceed the amount that can be used for generation and price is in the range ηt π*G < π < π*G.
Generate: Generate at maximum output, drawing on stored power if required—this mode materialises when the incoming solar power is smaller than that can be used for generation and price π > π*G.
Surplus: Generate at maximum power and store extra incoming solar power—this mode occurs when there is surplus incoming solar power, and the price is π ≥ ηt π*G. The constrained storage model is essentially identical to the unconstrained model but with bounds on the stored energy and extra control to allow diversion of power from the storage system when this is full. Managers wish to maximise the total revenue generated during some time interval.
Figure 5. The Ashalim CSP station in the Negev desert, Israel, has one of the tallest solar power towers globally at the height of 260 meters including the boiler, concentrating 50,600 computer-controlled heliostats. Source: “Ashalim Power Station” by Mussi Katz is licensed under CC BY-NC-ND 2.0
3.2 Available Support Polices
Policymakers can use a variety of initiatives to help speed up the implementation of TES technologies in target industries. These standards apply to all stages of development, from research and development to demonstration and commercialisation. We are now describing the highlighted mechanisms according to the International Renewable Energy Agency , which are divided into three categories: technology push, market pull, and overall enabling regulation and ecosystem cooperation.
As the technology itself is being built, technology push support is typically implemented near the beginning of a venture’s commercialisation path. It is divided into three categories: education and research support, demonstration support, and business support.
The provision of scholarships, visas, and secondments, lab funding, industry and university grants, and the establishment of academic awards and prizes are all examples of education and research support that focuses primarily on the early stages of the commercialisation process.
Through initiatives like test hubs, cooperative industry projects, funding for pilot-scale demonstrations, and innovation competitions, demonstration support focuses on proving unproven technologies in the laboratory and operational contexts.
Technology developers are frequently spin-outs from academia, big corporations, or technical experts with limited marketing expertise. This necessitates a completely different set of abilities from developers who may have just had technical knowledge up to this point. As a result, technology incubators, training schemes, and education policy, as well as company skills support, can assist technology start-ups in developing or improving their commercial offering.
Market pull support can be segregated into investment support, price support, and command and control support. These mechanisms tend to be more relevant once the technology has been proven commercially and technically.
Investment support embroils various forms of capital for commercial investment, through public/private venturing or by reducing costs via tax incentives, insurance and loss underwriting. In price support, there is a range of mechanisms that can help improve the competitiveness of a technology with additional profits for technology implementers, helping to de-risk the financial proposal of the technology to investors. Mechanisms include tradeable certificates, carbon pricing, feed-in tariffs, or cap and trade schemes, and bidding and tendering support. Command and control mechanisms use a topdown approach to pull technologies to market using measures such as local purchase rules, public procurement, and portfolio standards.
Policymakers have many enabling interventions available to them to benefit a technology either directly or indirectly. These measures are relevant at various stages of the commercialisation path. Diffusion and knowledge sharing (ecosystem support) are essentially awareness-raising through different means. Regulation can reduce barriers or even provide incentives by improving planning regulation, introducing building codes, and regulating markets.
One example of technology push interventions resides in the liquid air storage industry in the United Kingdom. In 2013, the Birmingham Centre for Thermal Energy Storage and the Birmingham Centre for Cryogenic Energy Storage were established with a GBP 13.6 million grant from industrials and the Engineering and Physical Sciences Research Council – an illustration of education and research support. A venture seeking to bring these technologies to market was sponsored by a grant-funded clean-tech incubator, aiding the development of commercial skills and a business strategy. Support for demonstration has been routinely offered, most recently in 2018, for a 5 MW/15 MWh LAES project near Manchester built with the support of about GBP 10 million of government grant capital .
4 MAIN TAKEAWAYS
TES technologies have distinct advantages, such as facilitating the decoupling of heating and cooling demand from immediate power generation and supply availability. The resulting flexibility allows far greater reliance on variable renewable sources, such as wind and solar power, supporting the shift to a predominantly renewable-based energy system. TES also reduces the need for costly grid reinforcements and helps to balance seasonal demand.
TES technologies, in general, can be reasonably inexpensive; their long discharge durations (hours to days) and relatively long lifetime constitute strong advantages. On the other hand, temperature limits for TES materials may be misaligned with certain applications. They may be more difficult to integrate with existing plants compared to other energy storage options. In fact, the commercial application of long duration systems at power generation facilities is largely limited to concentrated solar power. Additionally, passive heating may be required during downtime.
Sensible TES (or SHS) typically includes low-cost materials, achieves long-duration storage and is not geographically limited. In opposition, their discharge temperatures may decrease over-discharge duration. Their energy density is the lowest of all TES materials (50-100 times smaller than PCMs). Phase Change Materials (or LHS) provide high energy densities and constant discharge temperatures over time. However, LHS materials may be expensive and rare; they are also corrosive, making protective coatings and exotic materials required for corrosion resistance. PCMs generally have poor thermal conductivity and must have very specific properties for each desired application (e.g., phase-transition temperature compatibility with operating temperatures); this requires extensive research for each case. Thermochemical TES offers decomposed products that may be stored separately; this results in a theoretically infinite storage period with no heat loss. Plus, it holds the highest energy density of all TES technology types. Sintering and grain growth during charging may cause storage material to degrade over time. The rate of dehydration reactions is relatively slow, so turning methods to increase the charging rate is an area of potential research.
In addition, thermal energy storage forms a vital part of the sustainability investment package available to countries forpost-COVID recovery. Investments in TES, along with non-hydro renewables, energy efficiency and electrification, can strengthen public health and economic infrastructure, drive short-term recovery and line up energy development with global climate goals.
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