ReviewA review on high-temperature thermochemical energy storage based on metal oxides redox cycle
Introduction
Energy plays a vital role in the development of human society. Currently, fossil fuels still account for a vast proportion of energy sources [1]. However, fossil fuels have problems such as depleting resources and environmental pollutions. Renewable energies, such as hydraulic, wind and solar energy, are unlimited and free in nature and have little or none adverse impacts on the environment [2]. Nevertheless, one of the main drawbacks of developing these renewable energy sources is their intermittent nature [3], [4], [5]. For instance, concentrated solar power (CSP) plants are widely applied to generate heat and/or electricity by using solar energy but it becomes not available during off-sun periods [6]. A solution for realizing the continuous operation of CSP is to store energy via thermal energy storage (TES) during the daytime and then recover it when solar energy is not available [7]. Thermal energy storage can be further divided into sensible energy storage (SES), latent heat storage (LES) and thermochemical energy storage (TCES) [8], [9]. Among them, TCES has the highest energy storage density yet is the least developed technology. In 1961, Goldstein firstly proposed using chemical reactions for solar heat storage and listed potential candidates [10]. Then, Funk and Reinstorm [11] and Ervin [12] also introduced the use of thermochemical cycles for energy storage purposes.
Thermochemical energy storage system can also be used for storing electrical energy particularly off-peak electricity produced in coal-fired power plants. Specifically, it acts as a battery to store the off-peak electricity in forms of chemical energy and heat. During peak hours, the stored energy is converted back into electricity through a highly efficient thermal power cycle. In this way, the balance between energy demand and supply of the power grid can be better maintained, which helps to ensure a high level of stability, security and efficiency of power supply [13]. A comparison between thermochemical energy storage and other energy storage technologies is demonstrated in Table 1.
Thermochemical energy storage process typically consists of a charging step and a discharging step. The charging step involves an endothermic reaction that is employed to store excess/waste heat [18], [19]. In the discharge step, an exothermic reaction is applied to release the stored energy for further applications [20]. The energy stored can be fully recovered by the exothermic reaction provided that the endothermic reaction is completely reversible [21], [22]. In addition, chemical energy can be stored for a long term without concern of heat losses which would be the case for SES and LES technologies. The reactions included in the thermochemical energy storage can be briefly described as follows [18]:
As (1) shows, the reactant A is dissociated into products B and C by absorbing thermal energy ΔH via an endothermic reaction. Conversely, in the discharging step, the products B and C react with each other and release thermal energy via an exothermic reaction. It is worth mentioning that the products of both steps can be stored at ambient temperature or working temperature [18]. Fig. 1 illustrates the basic principle of a typical thermochemical energy storage system.
The thermal energy stored Q (kWh) can be calculated according to:
where nA is the mol number of the reactant A (mol) and ΔH represents the reaction enthalpy (kWh/mol) [18].
Although the development of thermochemical energy storage is still under laboratory scale, it shows high potentials for the high-temperature energy storage and a summary of its advantages is concluded as follows [12], [18], [23], [24], [25], [26], [27], [28], [29]:
- (1)
High energy storage density, up to 15 times greater than that of SES and 6 times greater than that of LES owing to the high enthalpy of chemical reactions,
- (2)
High operation flexibility and can be applied to a wide range of high temperatures due to the large number of available reversible reactions, suitable for large-scale applications,
- (3)
Storage duration and transport distance are theoretically unlimited without thermal energy losses in storage since the reaction products can be stored at ambient temperature in the form of chemical energy,
- (4)
A stable heat source with a constant temperature can be achieved since discharge reactions are carried out under substantially high and constant temperatures.
Criteria for the selection of material candidates used for high-temperature thermochemical energy storage are listed as follows [23], [24], [30], [31], [32], [33], [34], [35], [36], [37], [38]:
- (1)
High reaction temperatures suitable for high-temperature application such as CSP plants.
- (2)
Moderate to high energy storage densities (high enthalpy of reaction).
- (3)
Excellent reversibility (i.e. no significant degradation in a short-term application) with no side reactions and undesirable by-products.
- (4)
Suitable thermodynamics and fast kinetics of the reaction.
- (5)
Long-term stability and superior thermo-physical and mechanical properties.
- (6)
Environmentally-friendly, non-toxic, non-corrosive, non-flammable and non-detonable.
- (7)
Commercially available material at a low cost.
Compared to SES and LES, TCES is less mature and exhibits higher technical complexity [8], [27]. To promote this promising energy storage technology, further studies are highly necessary [32]. Fig. 2 shows a wide range of materials that have been studied so far for thermochemical energy storage, including hydrides, carbonates, hydroxides, ammonia, organics and metal oxides [18]. Despite that various candidates exist and are available for thermochemical energy storage (see Fig. 2), exceptions are given to the materials that are potentially harmful to human beings and thus further development is usually not considered, such as PbCO3. Similarly, the use of sulfates may be prohibited due to both the toxicity and corrosiveness of the gaseous reaction products - sulfur oxides. As for some hydroxides and carbonates, for instance, Mg(OH)2, Mn(OH)2, ZnCO3 and MgCO3, the corresponding reaction temperatures between 50 and 300 °C results in their unsuitability for high-temperature energy storage applications [24]. When it comes to carbonates, hydroxide and organic systems, the reactants such as CO2 or H2O have to be stored and reaction products may need to be separated or evaporated [21], which pose new obstacles to the energy storage systems. The ammonia energy storage system, on the other hand, suffers from high-pressure requirements, slow ramp rates for ammonia synthesis, and high exergetic losses [39].
In contrast, metal oxides redox systems have little exposure to the above-mentioned technical issues and have a wide variety of advantages over other TCES systems including, but not limited to, high operating temperatures, non-corrosive products and no need for gas storage. Achieving a high operating temperature is of great importance for power cycle applications since it increases the upper limit of the achievable thermodynamic efficiency according to Carnot principles [40]. However, past studies in this area are found to be unsystematic and knowledge creation activities are scattered in the literature. As far as the authors’ knowledge, no one has attempted to provide summarized information on the up-to-date research status on this specific area. A detailed and systematic summary of the past studies can be useful for accelerating the development of the technology. This therefore forms the main motivation behind this review work.
A general reaction pathway of redox energy storage systems can be described as follows [41]:
In the first cycle, a reduction reaction occurs in which the metal oxides are reduced at a high temperature with the assistance of external thermal energy. Later in the oxidation cycle, the reduced metal oxides are oxidized to the initial state meanwhile the stored energy is released.
Metal oxides redox energy storage demonstrates a variety of advantages according to the literature [21], [22], [26], [30], [37], [41], [42], [43], [44]:
- (1)
High operating temperatures ranging from 350 °C to 1100 °C are beneficial to boost the efficiency of CSP plants.
- (2)
Air can be used as both the heat transfer medium and the reactant and thus there is no necessity for storing reactant gas.
- (3)
No necessity for products separation as that it is a gas–solid reaction.
- (4)
Medium to high energy storage densities.
- (5)
Simple in nature and no catalysts required in reactions.
- (6)
Relatively small environmental impacts for most redox couples.
Research into high-temperature metal oxides redox energy storage can be dated back to as early as 1976, when Wentworth and Chen proposed that simple reversible chemical reactions can be used for energy storage applications [30]. In the same year, Simmons implemented theoretical calculations which highlighted barium oxide as a potential energy storage material although no further experiment was conducted [45]. As a continued study to Simmons, Bowrey and Jutsen provided an in-depth experimental analysis of the energy storage performance of BaO2/BaO [46]. A similar research was accomplished by Fahim and Ford where the researchers first determined the reaction kinetics of BaO2/BaO [31]. Other early investigations include the work done by Chadda et al., who evaluated the reaction cycle of CuO/Cu2O couple from an experimental point of view [47]. Interestingly, in the following two decades, few studies were reported probably due to the plunge of fossil fuels cost in this period. Only until recent years, the interest in redox systems for energy storage appears to emerge again. This phenomenon can be observed via the number of articles indexed in Web of Science. By employing academic search engine Web of Science and choosing appropriate topics and titles, a statistical summary of published work has been made [48], [49]. As can be seen from Fig. 3, the number of relevant studies on thermochemical energy storage increased dramatically in the past five years. With regard to redox energy storage, a limited number of studies were conducted during the 1970s and 1980s whereas a remarkable growth in publications was found over the last 3–5 years.
Given the above background, this paper aims to provide a critical review of the past research efforts in high-temperature metal oxides redox systems with a devoted focus for thermochemical energy storage applications. The term ‘high temperature’ in this paper generally refers to temperatures above 400 °C. General information about different metal oxides energy storage systems, including their energy storage mechanisms, selection criteria, and research highlights will be briefly introduced first for both pure and mixed redox systems. Then a detailed investigation of the design characteristics of redox energy storage systems is presented, in which information/data are categorized into temperature range, energy storage density, reaction reversibility, kinetics, economics, reactor developments, and advantages and disadvantages of various systems. By revealing the state-of-the-art of this technology, the work attempts to improve the understanding of the current progress of redox energy storage systems, as well as identify possible knowledge gaps to promote the advancement of the technology. The overview is believed to provide useful information about redox energy storage and help select suitable metal oxides systems according to their characteristics.
The three key performance indicators for metal oxides redox energy storage are turning temperature, energy storage density and reaction reversibility. Table 2 gives the definition of these indicators. These factors can largely determine whether a metal oxide is suitable for high-temperature energy storage applications.
A number of techniques have been applied to investigate the energy storage characteristics of metal oxides redox systems. Among them, thermogravimetry analysis (TGA) is one of the most commonly used procedures. Using this approach, researchers can determine the reaction temperature range and mass change. The obtained value of mass change helps to identify the redox reversibility. Meanwhile, TGA can also be used simultaneously with differential scanning calorimetry (DSC). In this way, the energy absorbed or released during redox cycles can be measured. Table 3 illustrates experimental methodologies which are applied frequently in metal oxides redox energy storage studies.
Section snippets
State-of-the-art on high-temperature redox energy storage
According to the number of metal oxides involved, the redox energy storage systems can be broadly classified into two categories, namely pure and mixed metal oxides redox systems. The pure oxide system means that only one metal element is included in oxides, for instance, BaO/BaO2 redox couple. The mixed metal oxides systems thus involve more than one metal elements, such as a Co3O4 doped Mn2O3 system. In general, the pure system is easier to be produced, while the mixed one may demonstrate
Temperature range and energy storage density
Based on the reviewed metal oxides redox systems and their energy storage performance data, a summary plot has been produced in Fig. 31 which shows the reported operating/reaction temperature ranges and energy storage densities of various redox systems. More detailed information including the references to the data used in Fig. 31 can be found in Table A1. It should be noted that the temperature ranges include theoretical turning temperatures as well as practical reduction and oxidation
Conclusion
The present review summarizes the important past findings of pure/mixed metal oxides redox materials for high-temperature thermochemical energy storage applications. The pure metal oxides redox systems being reviewed cover BaO2/BaO, Co3O4/CoO, Mn2O3/Mn3O4, CuO/Cu2O, and Fe2O3/Fe3O4 and the mixed metal oxides redox systems include both non-perovskites (Ba-based, Co-based, Mn-based, Cu-based systems and hercynite) and perovskites. The past technology developments regarding the above redox systems
Conflict of interest
The authors declare that there is no conflict of interest regarding the publication of this article.
Acknowledgment
The authors wish to acknowledge the financial support they received from The University of Newcastle, Australia.
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