Medium- and high-temperature latent and thermochemical heat storage using metals and metallic compounds as heat storage media: A technical review

Highlights

• Medium- and high-temperature thermal energy storage technologies are reviewed.

• Metals and metallic compounds as heat storage media are discussed.

• Technical issues in latent and thermochemical heat storage are presented.

• Heat transfer enhancement in latent heat storage is highlighted as a key issue.

• Continuous thermochemical heat storage is suitable for seasonal storage.

Abstract

Latent and thermochemical heat storage technologies are receiving increased attention due to their important role in addressing the challenges of variable renewable energy generation and waste heat availability, as well as the mismatch between energy supply and demand in time and space. However, as the operating storage temperature increases, a series of challenging technical problems arise, such as complex heat transfer mechanisms, increased corrosion, material failure, reduced strength, and high-temperature measurement difficulties, especially for metals and metallic compounds as heat storage media. This paper reviews the latest research progress in medium- and high-temperature latent and thermochemical heat storage using metals and metallic compounds as storage media from a technical perspective and provides useful information for researchers and engineers in the field of energy storage. In this paper, the status and challenges of medium- and high-temperature latent and thermochemical heat storage are first introduced, followed by an assessment of metals and metallic compounds as heat storage media in latent and thermochemical heat storage applications. This is followed by a comprehensive review of three key issues associated with medium/high-temperature latent heat storage applications: heat transfer enhancement, stability and corrosion, as well as a discussion of four key issues associated with medium/high-temperature thermochemical heat storage: heat transfer, cycling stability, mechanical property and reactor/system design. Finally, the prospects of medium/high-temperature latent and thermochemical heat storage are summarized.

Introduction

Energy is the basis of human survival and development, and a crucial element of highly complex applications such as aerospace, advanced manufacturing, military technology, as well as everyday activities such as transportation, heating and air conditioning, lighting and food.

Economic development, population growth and improvements in living standards all rely upon increased energy consumption, and given its importance, many international organisations have conducted studies and forecasts of global energy demand. According to BP, in the Evolving transition scenario, the global primary energy consumption increased at an annual growth rate of 2.1% from 8.6 gigatons of oil equivalent (Gtoe) in 1995 to 13.5 Gtoe in 2017, and was expected to continue to rise at an annual growth rate of 1.2%, reaching 17.9 Gtoe by 2040 [1]. In another important study, the EIA concluded that from 2010 to 2018, the global energy consumption increased from 537.5 quadrillion Btu to 620.0 quadrillion Btu, and forecasted that by 2050 the energy demand would reach 910.7 quadrillion Btu at an annual growth rate of 1.2% [2]. REN21 also pointed out that though global primary energy intensity decreased more than 10% between 2013 and 2018, the world’s total primary energy demand still grew at an annual growth rate of 1.3% over this period [3]. According to these projections, although the growth of energy demand slows with improvements in energy efficiency, it will still be expected to increase at an annual growth rate of 1–2% over the next 20–30 years.

Generally, primary energy includes fossil fuels, nuclear energy and renewable energy. According to BP, the consumption of fossil fuels increased from 7.4 Gtoe in 1995 to 11.4 Gtoe in 2017, but the fraction of the total primary energy consumption decreased from 87% to 85%, and was projected to reduce further to 73.0% by 2040. Nuclear energy generation rose from 526 Mtoe to 596 Mtoe at an annual growth rate of 0.6% from 1995 to 2017 and would continue to increase to 770 Mtoe by 2040 with unchanged shares in primary energy consumptions [1]. Since the energy crisis in the 1970s, countries have begun to pay increasing attention to renewable energy. From 1995 to 2017, renewable energy generation (excluding hydropower) increased at an annual growth rate of 12.3% from 45 Mtoe to 571 Mtoe, and was expected to reach 2.7 Gtoe at an annual growth rate of about 7.1% by 2040 [1]. Most sources agreed that renewable energy (including hydropower) would continue to be the fastest-growing source of electricity from 2018 to 2050 with an annual growth rate of 3.6%, providing almost half of the total global electricity by 2050 [2]. Given the energy system infrastructure over the next 20–30 years, it is most likely that fossil fuels will still account for the largest proportion of energy consumption, while the installed capacity of nuclear power will continue to increase. Although the share of renewable energy is still small, technological progress, policy support and reducing costs will be increasingly important to continued growth.

Over the next 20–30 years, global energy demand will continue to increase and fossil fuels will continue to play a major role as primary energy resources. However, fossil-fuel reserves are limited, and high rates of consumption are not sustainable, or secure, in the long term. Fig. 1 shows the changes in the proven reserves of the world's major fossil-fuel sources and their corresponding annual growth rates from 2010 to 2019. According to their estimated production rates, the proven reserves of oil, coal and natural gas can be maintained for about 50 years, 132 years and 50 years, respectively [4]. With the advancement of technology, the proven reserves of fossil fuels will increase, but new development prospects are uncertain, introducing an unreliable factor in our continued use of these resources, whose price has fluctuated significantly in recent years, especially that of crude oil.

Moreover, greenhouse gases (GHGs) and pollutants generated during the combustion of fossil fuels cause serious health and environmental problems. Driven by CO₂ emissions from fuel combustion, energy-related GHG emissions increased by 12.6 GtCO₂ equivalent from 1990 to 2015, and also represented around three-quarters of total GHG emissions in 2015 [5]. It was further pointed out that the CO₂ emissions from fuel combustion increased to 32.8 billion tons in 2017, an increase of 60% compared to 1990 emissions, and CO₂ generated in the utilization of oil, coal and natural gas in 2017 accounted for 34.6%, 44.2% and 20.5% of the total CO₂ emissions, respectively [5]. In addition, a large amount of other harmful gases (SO_x, NO_x, CO, etc.), unburnt hydrocarbons as well as harmful particulates are produced during the combustion of fossil fuels, which can cause air pollution, smog and have been linked to health concerns [6].

For the above reasons, renewable energy (in particular, solar energy and wind) is considered to be a particularly promising alternative for achieving sustainable energy development in the future. Moreover, in recent years, the falling costs of renewable energy technologies driven by significant market growth have made these technologies increasingly costly competitive, to the extent that as of now, renewable generation is in some cases more cost-effective than conventional generation even without further incentives. Fig. 2 shows the global weighted average levelised costs of electricity (LCOE) for renewable power generation from 2010 to 2019. The black thick lines represent the global weighted average LCOE by year. The grey single band represents the fossil fuel-fired power generation cost range of USD 0.05–0.177/kWh in 2019, while the bands for each technology and year represent the 5th and 95th percentile bands for renewable projects. The LCOEs of solar and wind power generation have dropped significantly over this period, such that solar photovoltaic (PV) power, onshore and offshore wind power now have LCOEs in the range of fossil fuels on average. Of particular interest in this figure is the LCOE of concentrating solar power (CSP), which fell by an astounding 47% from 2010 to 2019.

Waste heat sources are widely distributed in power generation, industrial manufacturing, transportation, commercial and residential activities. It has been estimated that approximately 20–50% of the useful generated heat is ultimately wasted in the form of exhaust gases, cooling water streams or simple dissipation in industries without being recovered in the United States [8]. In China, the heat wasted in the steel, cement, glass, ammonia, caustic soda, calcium carbide and sulfuric acid industries has been estimated to correspond to 340 million tons of standard coal per year, accounting for about 30% of total national energy consumption [9].

Although waste-heat recovery and utilization is an area of growing interest, the evaluation of the potential of industrial waste heat still lacks uniform standards, and there are few data on waste-heat sources across nations and sectors, especially in relation to power generation, transportation and commercial/residential activities. Forman et al. [10] provided an evaluation of the heat wasted globally in power generation, industrial production, transportation, commercial and residential activities and classified major heat sources based on temperature. Fig. 3 shows the total estimated global waste heat in 2012. The total amount of waste heat in 2012 was 246 PJ, which corresponds to 52% of primary energy consumption, with 51 PJ being classed as high-temperature (or, grade) heat, 39 PJ as intermediate, and 156 PJ as low-temperature heat, respectively.

Due to their availability and large reserves, wide distribution and environmental benefits, renewable energy and waste heat have attracted considerable attention in recent decades. However, renewable energy (e.g., solar and wind energy) and waste heat (e.g., from industrial sources) are discontinuous and variable due to the effects of day-night conversion, weather changes (overcast and sunny, wind speed) and process requirements, which makes it difficult to obtain a stable and reliable energy output, thus hindering subsequent use (e.g., integration to the power grid or end-users). Moreover, there usually exists a mismatch between energy demand and supply in time and space, causing insufficient energy utilization and low efficiency.

Energy storage, which can be divided into electrical energy storage (EES) and thermal energy storage (TES), is the key to solving the above challenges. EES technologies are often grouped into mechanical (e.g., pumped hydroelectric storage, flywheels), thermomechanical (e.g., compressed air energy storage, pumped thermal electricity storage, liquid air energy storage), electrochemical (e.g., batteries, flow batteries, fuel cells), and other electricity storage concepts (superconducting magnetic energy storage, supercapacitors), while TES technologies are classified as sensible heat storage (SHS), latent heat storage (LHS) and thermochemical heat storage (THS) technologies.

The difference between EES and TES lies in the quality of the stored energy and the rate of energy transfer: (1) according to the second law of thermodynamics, the grade of electrical energy stored in EES is higher than that of thermal energy stored in TES; (2) the energy transfer rate in TES is lower than that in EES due to the large difference in the speeds between the thermal movement of microscopic particles or lattices and the directional movement of free electrons.

Yet, although TES is at a disadvantage concerning the quality of the stored energy and charge/discharge rates, it is still indispensable. Fig. 4 shows that heat can be considered as being at the heart of the entire energy transmission, conversion and storage chain, acting as a link between primary and secondary energy. Almost 90% of the energy budget centres on heat conversion, transmission and storage [11], and nearly 50% of energy demand and consumption are for heat, which often changes in time and space [12]. In solar thermal and waste heat applications, the energy provided from the source principally takes the form of heat, which can be directly stored by TES. Moreover, TES has a higher level of maturity, lower costs and much longer storage durations, making it suitable for larger-scale and longer-duration applications.

Sensible heat storage (SHS) is the most mature and widely used TES option due to its simple principle and low costs, however, it also has the lowest heat storage density among the three types of TES. In SHS, heat is stored by raising the temperature of a liquid or solid and released by decreasing the temperature when necessary. The amount of stored thermal energy depends on specific heat capacity, material mass and temperature difference, as shown in Eq. (1):

$$Q\text{=}{\int }_{T_{\text{1}}}^{T_{\text{2}}}m{c}_{\text{p}}dT$$

where m, c_p and T are the mass, specific heat capacity and temperature of the SHS media; and subscripts ‘1’ and ‘2’ indicate the beginning and end of the heat transfer process. Common SHS materials include water, thermal oils, molten salts, concretes and rocks.

Latent heat storage (LHS) stores and releases heat through solid-liquid phase change. The heat storage medium is also referred to as a phase change material (PCM). The thermal energy stored in LHS usually comprises three parts: solid sensible heat, latent heat and liquid sensible heat:

$$Q\text{=}{\int }_{T_{\text{1}}}^{T_{\text{m}}}m{c}_{\text{p,s}}dT+m\mathrm{\Delta }h+{\int }_{T_{\text{m}}}^{T_{\text{2}}}m{c}_{\text{p,l}}dT$$

where m, c_p and T are the is the mass, specific heat capacity and temperature of the PCM; Δh is its phase-change enthalpy per unit mass; subscripts ‘1’ and ‘2’ indicate the beginning and the end of the heat transfer process, respectively; and subscripts ‘s’, ‘m’ and ‘l’ refer to the solid phase, the melting process and the liquid phase, respectively. Ordinarily, LHS has a higher heat storage density than SHS, which can effectively reduce the volume of storage tanks. Furthermore, the temperature stays approximately constant during the phase change process along with similarly small volume changes, making operation simple and safe. Common PCMs are paraffin, fatty acids, alcohols, esters, salt hydrates, inorganic salts and metals, etc.

Thermochemical heat storage (THS) can realize heat storage and release through reversible chemical sorption or reaction. The principle is shown below:

$$n_{\text{AB}}·AB\left(\text{s}\right)+Q\rightleftharpoons n_{\text{A}}·A\left(\text{s}\right)\text{+}{n}_{\text{B}}·B\left(\text{g}\right)$$

where n is the stoichiometric coefficient; and Q is the reaction heat. A reactant AB(s) decomposes into a solid A(s) and a vapour phase B(g) with the supply of external heat Q. After separation of the products, the solid A(s) can be stored under ambient conditions and enables very long-term heat storage and long-distance transportation. Moreover, the heat storage density of THS is considered to be the highest among the TES technologies. The range of different reaction types, such as dehydration of salt hydrates, dehydration of metal hydroxides, dehydrogenation of metal hydrides, decarboxylation of metal carbonates and redox reactions of oxides enable a very wide range of applications at different temperatures. However, the reaction processes of THS are much more complicated than those of SHS and LHS and require precise control. Table 1 compares these three types of TES. Given its high maturity and low heat storage density, SHS will not be further discussed in this paper.

TES is usually divided according to the operating temperature into low-temperature TES (T<100 °C), medium-temperature TES (100 °C≤T≤300 °C) and high-temperature TES (T>300 °C). At temperatures below 100 °C, TES technology has been gradually commercialized and is widely used in heating, air conditioning, thermal control, domestic hot water and other applications. Common heat storage media used for low-temperature LHS mainly include paraffin, fatty acids, alcohols, esters and hydrated salts, with reversible chemical sorption involving hydrated salts used for low-temperature THS.

However, as the operating temperature increases above 100 °C, the thermal movement of the microscopic particles inside the substance becomes more intense with increasing instability, leading to a series of challenging technical issues, such as more complicated heat transfer mechanisms, increased corrosion, material failure, reduced strength, while challenges in high-temperature measurement arise. Medium/high-temperature PCMs are mainly metals or inorganic salts, while medium/high-temperature THS mainly uses oxide/hydroxide reversible reactions, metal/metal hydride reactions, oxide/carbonate reactions and redox reactions, involving oxides, hydroxides, metals, metal hydrides, carbonates, and so on. All the above heat storage media are metals and metallic compounds, and may make the above technical issues more complicated. To date, medium/high-temperature LHS technologies are at the pilot stage and have not yet seen commercialization, while medium/high-temperature THS technologies are less mature and still at the lab-test stage. Moreover, higher temperatures correspond to higher thermal energy grades and greater development values, meaning that medium/high-temperature TES can make extensive use of high-grade thermal energy (e.g., solar thermal energy, industrial waste heat and geothermal heat) and realize the flexible energy utilization that integrates power generation and heat supply. Due to these technologies being at early stages of development and their broad application prospects, technical breakthroughs are urgently needed to promote the further development of commercial medium/high-temperature LHS and THS solutions.

In recent years, medium/high-temperature LHS and THS technologies have been a hot and cutting-edge research topic, and have been summarized and reviewed in detail from different perspectives. For example, Kuravi et al. [13] presented a review of TES system design methodologies and the factors to be considered at different hierarchical levels for CSP applications. Zhang et al. [14] reviewed the development and design of high-temperature SHS, LHS and THS technologies, including suitable heat storage media, heat carriers, containment and systems. Li et al. [15] considered the heat transfer performance and enhancement techniques for shell-and-tube type TES containing molten-salt-based PCMs at medium/high temperatures, while Carrillo et al. [16] analysed the status, major challenges and future perspectives of THS, covering the numerous strategies proposed for the improvement of materials and thermochemical reactors. However, to the best knowledge of the authors, there has not been a comprehensive review focusing on the progress and opportunities in medium/high-temperature LHS and THS technologies from a technical perspective. The main objective of this paper is to address this knowledge gap by summarizing the latest technical progress and future promise of medium/high-temperature LHS and THS technologies using metals and metallic compounds as storage media.

This paper comprises five sections. In Section 1, we introduce TES systems and explain their importance in the context of medium/high-temperature LHS and THS technologies. In Section 2, we present a variety of storage media for medium/high-temperature LHS and THS applications. In Section 3, we focus on three core technical issues (heat transfer enhancement, stability, corrosion) faced by medium/high-temperature LHS technologies. Heat transfer enhancement specifically for inorganic salt PCMs is reviewed at material, component and system levels, respectively. In Section 4, we proceed to discuss four core technical issues (heat transfer, cycling stability, mechanical property, reactor/system design) faced by medium/high-temperature THS technologies. Finally, in Section 5, we provide a summary and present the prospects for the future research and development of medium/high-temperature LHS and THS technologies.