Thermal Energy Storage: A Review and Exploration of Existing Literature and Systems

Across the globe, there has been a major push for green/renewable energy. This is especially true in the United States. Electrification, decarbonization, and renewable energy have been the focus of many different states, and various large companies as well.  One of the pushes within renewable energy has been how to solve the problem of storing energy for output during high demand, and off-peak generation hours. The two best sources of renewable energy (wind and solar), both have this problem. Neither source of energy is capable of max output, 24 hours a day. Sensible heat, latent heat, thermochemical heat, and district heat schemes are all different approaches to solve this problem, to varying degrees of success and efficiency. An example of a sensible heat system is calculated below, as well as some different ways to optimize the system. The system examined below was found to have a round trip efficiency of nearly 40%. Various companies that manufacture these systems are working to create real world affordable systems that can be applied in many different sectors of the energy consuming and generating market.

One of the biggest problems currently facing the solar energy and green energy initiative, is how to solve the problem of inconsistent power generation via renewable energy [2]. As global and national decarbonization efforts continue to ramp up in the coming 10-20 years, the need for consistent renewable power generation will continue to increase. Unfortunately, there is no effective way to harness the power of the sun, during nighttime and cloudy days. Wind energy also has the inconvenience of not being able to generate electricity when there is no wind. Natural gas and coal were both very effective sources of power in the twentieth and early twenty first century, but the need to reduce carbon/CO2 emissions has driven these technologies to be phased out across the globe. This has not fully taken effect yet though. The United States, and the EU, are currently leading the charge with phasing out the burning of fossil fuels, but it has taken time, and will continue to take years to be phased out, due to the lack of a suitable alternative.

So how does society move forward and be prepared to effectively handle the phasing out of burning of fossil fuels for power generation? Numerous different ways of solving this problem have already been tried, are currently being evaluated, or have been hypothesized. Unfortunately, nearly all these methods and alternative ways to solve this problem involve some number of drawbacks. However, one very promising method of allowing renewable energy to be a consistent reliable source of power, is the use of thermal energy storage (TES).

Thermal energy storage can be used in a variety of ways. It can be used to store electrical energy in both short term and long term, it can be used to store heat or cooling for use in industrial processes, it can also be used in district heating schemes. There are many uses and applications for this technology, but the premise of the technology, is to store excess energy in the form of thermal energy and output the energy it has stored back into whatever process requires it. This can be used on many different scales, from the use of it in concentrated solar plants (CSP), to providing heat to buildings and facilities, to even being used at the residential scale in individual homes. The possibilities are virtually endless with TES. The main drawbacks of it though are its space requirements, and a sometimes-prohibitive cost.

The concept of thermal energy storage has been around for many years and has seen effective industry use in various forms over the last 10-20 years. However, it has never been cost effective, or people just had not previously been willing to spend the money necessary to implement it. However, with the new decarbonization/electrification initiatives, and the phasing out of fossil fuels, this method has seen a renewed interest in research, and industry implementation. With all this renewed interest and research that has been performed, TES is becoming more and more viable as a solution to solve some if not most of the problems with current renewable energy sources.

There are three main types of thermal energy storage: sensible heat, latent heat, and thermochemical heat. All three methods of TES have their pros and cons, but all are potentially viable sources of storage, and likely all three will be implemented in the ways and methods that make the most sense given their known limitations and strengths. Sensible heat has the benefit of being the cheapest of the 3 methods, and the oldest and most established method. The downside of this method though is it suffers from low energy volume and density and significant system efficiency problems [2]. Latent heat has a much higher energy storage density, and greater efficiency, but has some major drawbacks, due to the phase change materials’ (PCM) highly corrosive nature [2]. Latent heat also requires much more space from a system standpoint than sensible heat. Thermochemical TES (TCES) has both the highest energy storage density, as well being a smaller footprint. TCES does have some major drawback though. TCES can involve toxic chemicals and nasty cleanup operations after the lifespan of the system has been reached. While not as concerning as nuclear fission cleanup, it does suffer from some of the same concerns, and would likely face more political and environmental pressure than the other two methods. An illustration of TES in terms of each type’s potential for heat storage capacity, is show below in Figure: 1.

Figure. 1: TES Methods in Terms of Heat Storage Capacity. [2]

TES in the sensible and latent heat versions are typically supplied heat via solar power, wind energy, or waste heat capturing from industrial processes, IE capturing heat from flu gas from the burning of fossil fuels, and other various industrial burning processes. TES can receive heat in a couple different ways. The first way is to pump the heat into the storage system via a heat transfer fluid, this can be liquid or gas form, and transfer the heat to the storage media. Waste heat and solar thermal collectors operate much in this fashion. Heat can also be fed to the TES, via converting electrical energy into heat. This involves the use of a heat pump, or an electrical resistance heater. Heat pumps tend to be more efficient from a thermodynamic standpoint, but resistance heaters, are much more cost effective, and easier to install and replace. Solar panels, and wind energy would both use these two methods of converting electricity into thermal energy.

List of Abbreviations

Sensible Heat

Sensible heat is one of the three main forms of thermal energy storage. This method is the most basic form of thermal energy storage. It utilizes the concept of temperature difference to store energy. The sensible heat method of thermal energy is comprised of a storage media or several, and a heat transfer fluid. The storage media, described further in Materials, can be comprised of many different materials, but concrete and rock are two of the more common materials. The heat transfer fluid can be composed of air, liquids, or gasses that allow for high rates of heat transfer. For sensible heat to be effective, there must be a large temperature difference or ΔT, between the storage media and the heat transfer fluid [1].

Sensible heat has a purely linear relationship between the temperature of the storage media, and the amount of power stored. This is illustrated below in Figure 2. Equation 1 below shows the governing equation for the sensible heat storage method [1]. This equation shows that there are only 4 main variables that effect the sensible heat storage, hence its simplicity [1].  The density ρ and the material specific heat C_p, are both material properties of the storage media [1]. So, changing the storage material can have large effects on the amount of power capable of being stored by the system [1]. V is the volume of the storage system [1]. This can range from large to very small, but the smaller the volume, the lower the amount of power that is capable of being stored [1]. T₁ and T₂ both refer to the upper and lower temperature bounds of the systems capability [1]. The material min and max temperature ratings, or the system’s ability to put temp and high or low temps could each contribute to these temp values on the bounds of the integral.

Figure 2: Sensible heat temp vs power (Crespo et al. 2019)

There are many different existing, and potential applications for the sensible heat TES. These applications include electric vehicle, or EVs, heating, CSP grid management and peak smoothing, district heat schemes, individual building heat schemes, and off grid residential storage for solar and wind peak smoothing. These are just a few of the many different applications of this technology. The possibilities as already stated, are only limited by space constraints and costs.

EVs are one of the many points of contention with the pushback against electrification. EVs do not have the range or battery capacity of typical ICEs [3]. One reason for this, is due to the intense strain heating the cabin of an EV puts onto the battery system [3]. EV cabins are typically heated, via electrical PTC heaters [3]. These PTC heaters, while effective, utilize large amounts of power [3]. Especially when heating a large SUV or bus [3]. One potential way of solving this is via a TES. The TES diagram show below in Figure 3 shows what a potential TES utilizing sensible heat technology would look like for an EV [3]. This system requires the operator of the vehicle to charge the TES tank via a provided charger that uses an electrical resistance heater [3]. The TES then provides heat in conjunction with a PTC heater, to warm the cabin of the EV [3].

Figure 3: Automotive HVAC STES Diagram [3]         

District heat schemes are very popular in European countries, such as Great Britan [6]. These are systems, where you generate steam or hot water in one localized generating plant, and distribute this steam to various places, via a network of pipes. The steam is then pumped into buildings, and radiators are used to provide heating capabilities. These heat schemes are very popular when you have high density clusters of buildings and apartments in a localized area. University campuses are another common place to find a district heat scheme. So, while district heating is not a new technology, nor is it inherently a green technology, how you supply the steam to the district scheme can be.

One new push in the world of thermal energy storage, is to utilize existing industrial processes, to capture waste heat from the processes [6]. This usually occurs in the form of capturing flue gasses from propane and natural gas fired processes [6]. This heat is then captured or stored in a TES, and then used to heat steam and supply a district heat scheme. Figure 4 below shows a survey done to map the waste heat in the UK [6]. This process can be very effective, in supplying heat, and reducing carbon emissions significantly [6]. There are a couple obvious downsides to this method though. The first, is that many locations around the globe, do not have a dense enough population to justify using a district heat scheme [6]. Also, a large portion of the world does not have the industrial means to generate or capture waste heat [6]. This type of system also is still entirely dependent on fossil fuels for heat generation. It helps increase the efficiency of fossil fuel burning, but ultimately is not a carbon net zero process, and that makes this more of a temporary or patch type of system, for the long-term goal of carbon free emissions.

Figure 4: Waste Heat Mapping of the UK [6]

One of the main drawbacks of sensible heat systems is that they are very inefficient systems. There can be a very large discrepancy between the amount of power input to the system, vs the power the system is capable of outputting. System efficiencies for STES systems can range anywhere from 50% to as low as single digit percentage, when looking at the round-trip efficiency. Sensible heat systems also are typically very large in size. Typically, the size is related to the amount of heat that a given system can store. This makes sensible heat systems complicated to use for residential houses, due to the amount of size needed. Burying the system underground does help some in this regard, but still requires a flat clear plot of land to do that. Burying the system, also poses challenges for maintenance.

Another drawback of STES, is one that is shared by its LTES and TCES counterparts, and that is high construction costs to build and install. STES has high costs associated with it, due to its relatively low efficiency and energy density. This means that the system must be built much larger, to accommodate the heat/energy needs of the system recipient.

Latent Heat

Latent heat thermal energy storage, or LTES, is a variation of sensible heat that involves heating a phase change material up to the point where it changes phase [2]. This phase change material can serve in conjunction with a sensible heat style storage media, or the phase change material can act as the storage media in some different system designs [2]. The phase change that occurs in LTES, is mostly of the solid to liquid transition, due to the common temperatures of the LTES: 500-1000C typically [2]. Also, the phase change is typically limited to the solid to liquid, due to the ease of the ability to store it, as opposed to storing and extracting volatile gases, or plasmas [2].

When a material phase changes from a solid to a liquid, due to temperature increase, a large exothermic reaction occurs that produces large amounts of heat [2]. By using this principle, LTES systems can generate stored heat, but only having to power the system up to the phase temperature of the material, and then the exothermic reaction further heats the system [2]. This allows the LTES to amplify the power that was inputted, or in actual real-world examples, this exothermic reaction helps to make up for system inefficiencies [2]. The concept of LTES is inherently better than STES, in the theoretical world, due to its ability to produce a higher effective energy storage density, as opposed to its STES counterpart [2]. Figure 5 below illustrates how LTES produces more energy than STES for a given input of power, in the way of heat [2].

Figure 5: Sensible vs Latent Storage [2]

While on paper latent heat is better and more efficient than sensible heat, latent heat also has a lot larger drawbacks and long-term problems that must be solved, before it is a truly viable options for all types of TES. Due to the PCM temp for LTES, being in the range of 500-1000C, this limits the materials available for use, and most of those materials, such as molten and inorganic salts, are highly corrosive. Due to this corrosive nature, there are large environmental concerns. Also, the corrosive nature of the PCM makes it difficult to find storage materials, heat pump equipment, and heat exchanger materials that are suitable and will stand up to the long-term use of an LTES system. Due to the unavailability of these materials, most existing LTES methods and pilot plants, have a much shorter life expectancy, when compared to their STES counterparts.

Another drawback of the LTES systems, is majority of the PCMs, have very poor heat transfer characteristics, such as low specific heats (C_p) and thermal conductivities (k). Due to these poor heat transfer characteristics, most systems must have heat exchangers and other heat inducing factors designed in. This adds significant costs, especially when coupled with the known corrosive problem as stated above. The adding of these heat exchange devices also adds significant mass and volume to the system, which in turn reduces the energy density and energy per volume. By decreasing these system performance identifiers, it makes the LTES technology less attractive, especially when examining from a residential off grid home application or pretty much any automotive application, due to its size.

Sensible and Latent heat storage can often be used in conjunction with each other to form a combined system. One of these such systems is illustrated in Figure 6 below. Systems like the below utilize crushed rock, and other sensible heat materials to store the bulk of the heat input. A smaller subsection of the storage unit though houses various phase change materials, to allow for latent heat thermal energy storage. The system works, by charging the system up to the phase change temperature of the chosen PCM, and then using the sensible heat portion, to continue to supply the PCM and LTES part of the system. Once the system falls below the temp threshold of the PCM, then the system just becomes a normal STES system. These systems have longer lifespans than traditional LTES systems, due to the lower number of latent heat cycles the system will experience. The overall system efficiency will also be much higher than a standard sensible heat system since it has the efficiency boost of latent heat technology.

Figure 6: Combined System [9]

Thermochemical Heat

Thermochemical heat is one of the most promising, but also the most concerning methods of thermal energy storage. Due to the nature of the chemical reaction that occurs, thermochemical storage systems can generate heat on their own, and do not fully rely on storing heat or energy that is inputted via another source. This makes the efficiency of some of these systems as actually net positive energy generation. The concerning part about these systems though is that they carry many environmental hazards. Clean up, and spills in these systems are very expensive and carry large ecological implications.

Thermochemical energy storage systems work on the principal of using heat or other forms of energy to start and or sustain chemical reactions that in turn produce heat [14]. The term thermochemical energy storage system can be misleading, since the system does not store energy per se, so much as take energy and amplify or generate more energy [14]. These systems are sometimes referred to as thermochemical batteries for this reason [14]. One of the most common types of chemical reactions that occurs in these systems is called a redox reaction [14]. Redox stands for oxidation reduction reaction [14]. This is where you have an oxidizing agent, and a reducing agent, and electrons will flow from one to another, to create different chemical states [14]. These states also called the oxidation and reduction states occur inside the TCES, or the chemical reactor as it is sometimes referred to [14]. During the change of state from one to the next, large amounts of heat are given off in the form of an exothermic reaction [14]. It is this reaction that generates the heat to continue to fuel the TCES. Figure 7 below depicts a type of thermochemical energy storage system [17].

Figure 7: Thermochemical System [17]

Catalysts are another type of thermochemical reaction that can be used to not necessarily store thermal energy, but to significantly reduce the amount of power that a system that requires heat input will consume [14,17].  Catalysts are typically made from materials that at room temp, and sometimes even medium temp, are non-reactive and when the material is heated beyond a specific point, the catalyst reaction will occur and will generate heat again from an exothermic reaction [14,17]. The downside to catalyst reactions, is they often require some form of maintenance power to keep the reaction going, and they also typically require a very large amount of heat or power to start the reaction [17]. So, while catalyst reactions are very useful and are part of this category, they are unable to act as a fully standalone TCES [14,17].

Materials

There are many different types of materials suitable for thermal energy storage. Most of these materials are specific to the type of TES they are being used it [1]. These materials are illustrated below in Figure 8. Sensible heat typically uses common everyday materials, such as metals, rocks, concrete, and ceramics to name a few [1]. Latent heat is where the materials start to become highly specialized. LTES utilizes materials such as molten and hydrated salts, different typed of paraffins and esters, and even ice [1]. Due to the nature of how LTES works, the material selection is severely limited due to not many materials having the ability to change phase, and more specifically the solid to liquid phase change, within the temp parameters of the TES system. Most latent heat materials also have the unfortunate problem of being highly corrosive, and thus also limit the material selection of the containment vessel for the system itself [1]. Thermochemical energy storage utilizes various materials, but in different ways since a chemical reaction is being performed. The REDOX reaction materials are the most common for TCES [1]. Figure 8 below shows a chart of some of the more common materials used in the respective types of thermal energy storage.

Figure 8: Various TES Materials [1]

Problem Statement

A sensible heat thermal energy storage system roundtrip efficiency is explored below. The system examined is a 10m by 2m-by-2m cube that is packed with crushed concrete as the storage material. The system has an assumed packing factor of 0.5 for simplicity. In a real-world scenario, a perfectly uniform packing is virtually impossible, and a much more complex nodal analysis method of calculating the heat transfer would need to be used. The material properties of the concrete were pulled from standard textbook values. If this approach were to be used for a real-world system, then the exact properties of the concrete or storage materials would need to be known to increase the accuracy of the calculations The heat storage system is assumed to be an adiabatic system. A real-world system may not be perfectly adiabatic, but if insulated properly, or buried underground like a lot of these STES’s are, then an adiabatic model would be acceptable. For the problem examined below, the heat transfer due to radiation was neglected for simplicity.

Governing Equations

Nomenclature

Table 2: Abbreviations/ Variables

The governing equation below is the main equation used to calculate the round-trip system efficiency. The equation takes the system output power and integrates it for the system output discharge time and is divided by the system power input consumption and integrated by the system charge time. To solve this equation, 3 different parts are needed, the input power consumption, the system charge time, from the max charge temperature to the lower temp bound of the system (97% of max), and the system power output. The governing equations for these 3 system parts are shown below in equations 2-4 respectively.

Calculations

Assumptions

Table 3: Problem Assumptions

Further assumptions beyond what is in the table above, is the steam turbine is assumed to be adiabatic and isotropic between the inlet and outlet conditions.

Results:

The results from the analysis of the above system indicate on paper a very inefficient system, at around 40%. Figure 9 below illustrates in a graph the system efficiency from the starting temp of 700C, to the below the 97% industry standard threshold. The graph was carried to beyond the 97% point to further illustrate how the efficiency drops as the system steam output temp decreases.  This number can be misleading though, depending on how the system is fed its power. The Due to this system being designed to smooth out grid capabilities during peak demand times, or during off peak generation times, means the system will not be running all the time, or will potentially be used in conjunction with a main grid fed from renewable energy. This STES system is capable of being run off an entirely renewable source. This could be from excess solar or wind power during high peak generation times, or it could also be fed from waste heat from various industrial processes. With all this in mind, since the system is fed via renewable energy, and does not require dedicated power input from fossil fuels, then the 40% round trip efficiency is much more reasonable. This is effectively free power generation that is usable at any time of the day or year, and it is generated from a renewable source that would not have been able to be harnessed previously and would have just been dissipated to the environment.

Figure 9: Efficiency as System Temp Decreases

The Calculated input power for the electric heater that heats the storage media was 147.2 KW. This is a very large heater and depending on the supply voltage from the solar panels, could have a very large amp draw. Assuming a 3-ph 480V or 600V input power, the respective current draws would be 176A and 141A respectively. An amp draw of this magnitude will require the use of very large power cables, 2/0 gauge or larger, or would require the heater to be broken down into many smaller amperage circuits. The downside of this is a lot more cables and termination points will need to be added, thus driving up the installation costs of the system.

Future growth

Thermal energy storage still has a long way to go to be where it needs to be for the world to move forward with all the green initiatives that are being pushed around the world. This is not to imply that we should stop pushing on green initiatives though, it just means that more work, effort, and research needs to be expended to make this a more viable option for the world. Some of the major areas of need and research are in the developing of more effective containment vessels for the use of molten salt and other latent heat materials. The concept of latent heat TES is the most promising in my opinion. It has a high upside, with much higher theoretical efficiencies that comparable latent heat methods, and it lacks a lot of the environmental concerns, and has could have a much longer service life than its TCES counterpart. The main thing holding LTES back currently is the lack of a long-lasting containment vessel, and the extreme price and barrier to entry for companies entering this market, due to very high R&D startup costs.

Commentary on State of the art

There are currently many different companies that are making real world thermal energy storage systems. These systems vary drastically in size, shape, design, efficiency, and costs. Since TES is a fairly new technology, and has a very high barrier to market entry, there have been a lot of companies that have made prototypes, and even gone as far as building pilot plants for TES, but there have not been that many that have made it to the stage of rolling a complete TES system to the public for purchase. The companies that have achieved this though, are set to potentially make large amounts of money, and most of them have the backing of federal research funding, and the support of various universities and other commercial businesses that have a vested interest in seeing these new systems succeed.

Conclusions

While current state of thermal energy storage is not where it needs to be to support global carbon net zero initiatives, many great strides have been made in the industry, to be optimistic on the chances of thermal energy storage being an effective solution to work in conjunction with renewable energy to significantly reduce or eliminate the world carbon/C02 footprint. To achieve a net zero goal of 2030, 2040, or even 2050, though the efficiency of thermal energy storage must increase significantly. A net positive energy storage system would be ideal, but that may not be achievable. The costs of these thermal energy storage systems must decrease significantly as well. These systems must be available on a residential, commercial, and industrial level. One of the biggest pushbacks on pretty much all green alternatives to fossil fuel in the world, is the costs. People do not want to have to pay more than they are already paying for energy, and cars, etc. While this may not be a great attitude to have, and we should all want to reduce our carbon footprint and make the world a better place, The unfortunate reality is people do not want it to affect their wallet or their lifestyle while doing it. So, until the price of these systems come down significantly, and the efficiency increases, thermal energy storage will not be a viable option.  The research shown here though leads to the conclusion that thermal energy storage does have merit and have the necessary support to grow and evolve into what it needs to be to accomplish carbon net zero.

Works Cited:

1. Zhao, B., Zhang, Y., & Wang, R. (2022). Materials for Thermal Energy Storage: Classification, Selection and Characterization. In Elsevier eBooks (pp. 351–363). https://doi.org/10.1016/b978-0-12-819723-3.00006-8
2. Crespo, A., Barreneche, C., Ibarra, M., & Platzer, W. (2019). Latent thermal energy storage for solar process heat applications at medium-high temperatures – A review. Solar Energy, 192, 3–34. https://doi.org/10.1016/j.solener.2018.06.101
3. Xie, P., Jin, L., Qiao, G., Lin, C., Barreneche, C., & Ding, Y. (2022). Thermal energy storage for electric vehicles at low temperatures: Concepts, systems, devices and materials. Renewable & Sustainable Energy Reviews, 160, 112263. https://doi.org/10.1016/j.rser.2022.112263
4. Opolot, M., Zhao, C., Liu, M., Mancin, S., Bruno, F., & Hooman, K. (2022). A review of high temperature (≥ 500 °C) latent heat thermal energy storage. Renewable & Sustainable Energy Reviews, 160, 112293. https://doi.org/10.1016/j.rser.2022.112293
5. Olympios, A. V., McTigue, J. D., Sapin, P., & Markides, C. N. (2022). Pumped-Thermal electricity storage based on Brayton cycles. In Elsevier eBooks (pp. 6–18). https://doi.org/10.1016/b978-0-12-819723-3.00086-x
6. Albert, M., Bennett, K. O., Adams, C., & Gluyas, J. (2022). Waste heat mapping: A UK study. Renewable & Sustainable Energy Reviews, 160, 112230. https://doi.org/10.1016/j.rser.2022.112230
7. Frazzica, A. & Istituto di Technologie Avanzate per I’Energia "Nicola Giordano: (2022). Encyclopedia of Energy Storage. In Elsevier eBooks. https://doi.org/10.1016/c2018-1-02595-0
8. Cabeza, L., GREiA Research Group, & Palomba, V. (2022). Encyclopedia of Energy Storage. Elsevier eBooks, 338–350. https://doi.org/10.1016/B978-0-12-819723-3.00017-2
9. Zanganeh, G., Khanna, R., Walser, C., Pedretti, A., Haselbacher, A., & Steinfeld, A. (2015). Experimental and numerical investigation of combined sensible–latent heat for thermal energy storage at 575°C and above. Solar Energy, 114, 77–90. https://doi.org/10.1016/j.solener.2015.01.022