Thermochemical energy storage technologies for building applications: a state-of-the-art review Free

Abstract

This paper presents a comprehensive and state-of-the-art review on thermochemical energy storage (ES) technologies using thermochemical materials (TCMs) for building applications. Thermochemical storage devices (materials, open and closed sorption as well as chemical heat pump) enhance the energy efficiency of systems and sustainability of buildings by reducing the mismatch between supply and demand. Thermal ES (TES) systems using TCMs are particularly attractive and provide a high ES density at a constant temperature. Technical and economical questions will need to be answered for all possibilities, which warrant more development and large-scale demonstration of TES in future.

1 INTRODUCTION

Globally, the problems of excessive consumption of fossil resources, oil shortages and greenhouse gas emissions are becoming increasingly severe. All these problems take a large impact on sustainable development of human society. Research and development work on new methods of thermal energy storage (TES) are required to minimize energy consumption by harvesting renewable energy sources. The key challenge is to effectively store excess energy and bridge the gap between energy generation and consumption. Thermochemical ES is one of the promising technologies that can help minimize global environmental pollution and reduce energy consumption. The key role of ES systems is to reduce the time or rate mismatch between energy supply and energy demand [1]. The thermochemical ES is particularly an attractive technique, because it can provide a high ES density. Recent developments in low/zero carbon buildings have promoted the development of thermochemical ES systems.

The study of thermochemical ES using chemical heat pump (CHP) was pioneered by Swedish and Swiss researchers [2, 3] in the 1970s. In Germany, a long-distance thermal energy transportation system (EVA-ADAM) was proposed in 1970s and the technology was demonstrated 1980s, and this was the first practical example of thermochemical heat storage [4]. A project on super heat pump and energy-integrated system was conducted in Japan during 1984–92. Several ammoniate/ammonia and halogenated inorganic reactant/water materials were investigated as a working pair of CHPs as part of this project [5]. In 1991, an enhanced thermal heat transfer was examined using the CaO/H₂O CHP system [6]. Different reactant mixtures to enhance the thermal heat transfer of adsorbent bed were examined in Japan [7], France [8] and the UK [9].

This paper represents an overview of the studies conducted on thermochemical ES technologies for various applications with particular emphasis on the state of the art of CHPs and open and closed sorption ES systems.

1.1 Type ES

Nowadays, ES systems can be accomplished by devices or physical media. Basically, an ES is vitally significant to any intermittent energy source to meet variable demands. It is difficult to evaluate the ES properly without a detailed understanding of energy supplies and end-use considerations in terms of complex properties of ES [10]. Advanced ES systems could be integrated with other technologies to provide feasible innovation solutions in the ES fields. ES devices can be classified in categories. Dincer and Rosen [11] classified and discussed ES devices as follows: mechanical, thermal, chemical, biological and magnetic (Figure 1).

1.2 Classification of TES

TES is commonly defined as an important energy conservation technology. In 2002, Dincer [12] stated that advanced modern TES technologies have successfully been applied worldwide, particularly in some developed countries. Normally, TES comprises a number of other technologies to storage heat and cold energy for utilization at a later stage. TES can be employed to balance energy demand between day and night times [13]. Thermal energy can be stored as sensible heat, latent heat and thermochemical or combination of these materials. Hence, some possible methods to research TES can be divided into physical and chemical processes. Figure 2 shows an overview of the major technique of storage of thermal energy.

1.3 Sensible and latent ES

In physical ES process, TES can be divided into two principal types such as sensible (e.g. wood, rock) and latent (e.g. water/ice, salt hydrates). Sensible heat is defined as the heat absorbed or released when a substance just undergoes a change in temperature. The sensible heat storage is a thermodynamic process, which can be calculated mathematically. Sensible heat systems usually employ liquids such as water and oil, but in some cases solids such as molten salts, iron, rock, concrete and bricks are also used as TES media. But, normally these materials are not used as sensible ES systems in terms of their very low volumetric heat capacity of gases [14].

In latent heat storage (LHS), heat is released or absorbed by a chemical substance during a phase change from solid to liquid or liquid to gas vice versa without a change in temperature. Comparing with sensible heat storage, an LHS system is particularly attractive. Usually, the latent heat change is much higher than the sensible heat change for a given medium, which is related to its specific heat. Therefore, LHS is able to provide a considerable storage density and store heat at a constant temperature corresponding to the phase transition temperature of the heat storage medium [11, 12].

1.4 Chemical ES

Normally, a chemical ES system is comprised of one or more chemical compounds. The chemical TES category includes sorption and thermochemical reactions. In thermochemical ES, energy is stored after a dissociation reaction and then recovered in a chemically reverse reaction [15]. During this completely reversible chemical reaction, the temperature of some substances could probably be increased or decreased. Hence, this chemical heat energy can be stored through some effective methods for a long-term storage application. The heat stored depends on the amount of storage material, the endothermic heat of reaction and the extent of conversion [16].

2 THE STATE OF THE ART OF THERMOCHEMICAL ES

2.1 Chemical storage and sorption storage classification

Chemical TES can be categorized into chemical storage and thermochemical storage, in general. Thermochemical storage systems can be divided into open and closed systems [17]. The open storage system is based on the adsorption process to complete the sorption processes with desiccant and heat storage systems. Closed systems work with a closed working fluid cycle that is isolated from the atmosphere. There are two processes to be defined in a closed system; adsorption and absorption. Figure 3 illustrates the classification of chemical and thermochemical storage.

Usually, it is difficult to make clear boundary distinctions between expressions such as chemical storage, thermochemical storage and sorption storage [18]. Figure 4 shows the classification of chemical and sorption storage. During chemical ES, a chemical energy can be transformed into other forms of energy by a chemical reaction [1]. Sorption can be used to describe a phenomenon of fixation or capture of a gas or a vapour (sorbate) by a substance in a condensed state (solid or liquid), called sorbent [19]. Depending on the type of bonding involved, sorption can be classified as physical and chemical sorption. Generally, sorption includes both absorption and adsorption [20–22]. Both absorption and adsorption are different phenomena, but their commonality is that both involve the physical transfer of a volume of mass or energy. Specifically, absorption is related to a transfer of a volume into a volume, i.e. it is a permeation or dissolution of a volume of energy or mass (absorbate) into another volume of energy or mass (absorbent). In chemical engineering, adsorption is called the separation process during which specific components of one phase of a fluid are transferred onto the surface of a solid adsorbent. Adsorption refers to the transfer of a volume onto a surface. It is an accumulation, or massing, of energy or matter (adsorbate) onto a surface (adsorbent) [21, 23]. Chemisorption processes often offer larger heat of sorption than physisorption. However, chemisorption processes may be irreversible [24].

2.2 Principles of thermochemical ES

A general thermochemical ES cycle includes three main processes: (i) charging: the charging process is an endothermic reaction. A required energy resource is used for dissociation of compound C. (ii) Storing: after the charging process, A and B will be formed and both are stored in this stage and (iii) discharging, A and B are combined in an exothermic reaction [15, 27] and material C is regenerated. Meanwhile, the recovered energy is released from this phase (Figure 5).

2.3 Materials currently used or studied

TCMs are a promising new alternative for long-term heat storage. The process concerned is based on a reversible chemical reaction, which is energy demanding in one direction and energy yielding in the reverse direction. Normally, TCMs have the higher storage density with repetitive storage properties to use in sorption storage systems, and some of the materials may even offer storage density close to the properties of biomass [28]. Because of higher energy density, thermochemical TES systems can provide more compact ES relative to latent and sensible TES [29, 30] (Figure 6).

Numerous research work and experiments on various storage methods have shown that thermochemical ES systems have the potential to become probably the most effective and economic method of storing and utilizing waste heat [31, 32] (Table 1).

Nowadays, there are a number of materials and reactions that conform to thermochemical ES systems. Some of the promising thermochemical storage materials that have been recently identified are listed in Table 2. The table also provides the values of ES density and reaction temperature, which are two important factors for material selections. In 2009, Kato et al. [33] investigated on medium temperature chemical heat storage materials with metal hydroxides. The results show that mixed hydroxides can chemically store medium temperature heat such as waste heats emitted from internal combustion engines, solar energy system and high-temperature system. Visscher et al. [34, 35] revealed that a group of salt hydrates in general is considered to be suitable TCMs: epsom (MgSO₄·7H₂O) is a commonly used TCM. It is essential that salt hydrates can incorporate large amounts of water into the crystal lattice. When a hydrated salt is heated, the crystal water is driven off. In a long-term seasonal storage system, solar heat can be employed to dehydrate the salt hydrate in summer. Subsequently, the anhydrous salt is stored until needed. In winter, this salt experiences reverse reaction and yields energy in the form of heat, which can be used for building applications such as heating water and central heating. Epsom can be used for such ES applications. Recently, van Essen et al. [36] and Posern and Kaps [37] assessed the capability of epsom as an ideal material for thermochemical ES. The results were similar to Stach et al.'s [38] findings using experimental measurements. In 2010, Balasubramanian et al. [39] investigated the capability of MgSO₄·7H₂O to store thermochemical energy via a mathematical model. This simulated method can help identify optimal materials for thermochemical storage within practical constraints. However, this material cannot release all the stored heat under practical conditions. This is proved by Essen Van M, et al. [40]. Some experiments performed by Hongois S, et al. [41] show that the pure magnesium sulphate is quite difficult to use practically because of its low power density. Abedin and Rosen [42] investigated a closed thermochemical TES using strontium bromide (SrBr · 6H₂O) as the reactant and water as the working fluid. In 2006, Lahmidi et al. [43] simulated a sorption process based on the use of strontium bromide, which is adapted to solar thermal systems. In some other cases, a hydrate of Sodium sulphide for example its nonahydrate or its pentahydrate [44] gives a high thermal power density combined with a high energy storage density. However, Boer et al. [45] measured this material storage density and revealed sodium sulphide is very corrosive and operates under high vacuum.

2.4 Sorption storage systems

Chemisorption processes can be realized by utilizing reversible chemical reaction. The sorption storage systems can be explained for solid adsorbents and the results can be transferred to liquid absorbents [13]. The process of adsorption and desorption on solid materials is shown in Figure 7. Adsorption refers to the binding of a gaseous or liquid phase of component onto the inner surface of a porous material. Basically, sorption TES can be divided into open and closed systems. Soutullo et al. [46] presented a comparative study of the performance of absorption cooling systems with internal storage and also external storage, and the results show that the conventional system has a greater room requirement than an internal absorption system. In 2011, Liu et al. [47] evaluated a seasonal storage system for house heating and revealed that the storage capacity of the absorption process increases with the evaporator temperature and the storage temperature before the absorption phase, and decreases with the absorber temperature. Generally, sorption storage systems can be divided into open and closed systems.

2.4.1 Closed thermochemical ES

A closed sorption system is based on the same physical effect as the open storage, but the engineering is quite different from open sorption systems. In closed systems, the components cannot be exposed to the atmosphere and sorption process can use water vapour as adsorptive and the operation pressure of the working fluid can be adjusted, see Figure 8. The heat energy from the system needs to be transferred to and from the adsorbent using a heat exchanger. Comparing with open storage, the expected energy density of closed systems is reduced. The main reason for this is that the adsorbent (water vapour) is the part of the storage system and has to be stored as well [13]. Abedin and Rosen [42] used energy and exergy methods to assess closed thermochemical ES. Exergy analysis can be used to identify the locations and reasons of thermodynamic losses and evaluate efficiencies for the various processes of closed storage systems. In 2008, Hauer [48] has given a comparison on sorption storage systems, which means that internal substances of a closed system are separate from the heat transport stream and can provide higher output temperatures than open storage. Meanwhile, closed systems can be used to supply low temperatures for building cooling. A closed storage system usually requires higher temperatures during the charging process than open systems [49]. Furthermore, a combined solar ES system with a closed loop chemical heat pipe was investigated by Levy et al.  [50]. The overall performance of the closed loop was found to be satisfactory.

2.4.2 Open thermochemical ES

In an open sorption storage system, air is carrying water vapour and heat energy in and out of the packed bed of solid adsorbents or a reactor where the air is in contact with a liquid desiccant (Figure 9) [13]. An open sorption system is composed of a working fluid and a TCM. Gaseous working fluid of open system is directly released to the environment and operates at atmospheric pressure [27]. Therefore, normally only water is a possible candidate as the working fluid. Also, materials are required to be non-toxic and non-flammable in open systems. In open storage systems, separate desorption step (charging process) and adsorption step (discharging process) are used to store thermal energy without any thermal energy loss. Closed and open thermochemical ES systems were investigated by Abedin and Rosen [15] based on energy and exergy methods. The results for closed storage (SrBr·6H₂O as thermal materials) show that the overall energy and exergy efficiencies are 50 and 9%, respectively. For open storage (Zeolite 13X as thermal material), the overall energy and exergy efficiencies are 69 and 23%, respectively. Particularly, the exergy method enhances assessments of energy method and there is a significant margin for loss reduction and efficiency improvement for open storage systems [15, 27]. Wu et al. [51] were evaluated and numerically analysed an open thermal storage system using composite sorbents. The computational results were validated with the experimental measurements on open TES and the specific TES capacity increased noticeably while the coefficient of performance (COP) of the TES system decreased.

2.5 CHP storage systems

CHPs are a representative of chemical thermal energy conversion and storage systems [52]. Basically, a CHP makes use of transformation between thermal power and potential energy [53]. Specifically, CHPs utilize the reversible chemical reaction and sorption to change the temperature level of the thermal energy stored by chemical substances [25, 54]. These chemical materials play a significant role in absorbing and releasing heat [55]. According to the characteristic of the chemical reaction, various chemical materials could be involved in CHPs. A CHP system can be sorted as a mono-variant system and a di-variant system [53, 54]. The general classification of CHPs is illustrated in Figure 10.

In general, CHPs could be categorized into two types: solid to gas and liquid to gas in terms of the phase of working pairs. Solid–gas systems normally comprise a reactor, condenser and evaporator. Liquid–gas CHPs are consisted of not less than two reactors (endothermic and exothermic), a condenser and an evaporator [53] (see Figure 11).

Today, there are many published papers on various types CHPs and their applications. In early 1995, Tahat et al. [56] investigated the thermal performance of a thermochemical heat pump as an energy storage system. They have shown a general relationship between the equilibrium pressure and the temperature of the system. Kawasaki [25] provided a proposal of a CHP for cooling using paraldehyde/acetaldehyde (Pa/A) and indicated that the COP of the Pa/A system is the same as the COP of a vapour-compression heat pump. A basic numerical model to analyse CHPs was developed by Mbaye et al. [57], in which the source-based method (which is a fixed grid enthalpy approach) was employed. Kato et al. [58] examined experimentally a packed bed reactor of magnesium oxide/water CHP system to evaluate the contribution of the CHP to decentralized cogeneration. This type of CHP enhanced the energy utilization efficiency of the cogeneration system by storing and utilizing surplus exhausted heat from the cogeneration system. In, 2002, Fujimoto et al. [59] published their work on dynamic simulation of an experimental prototype Cao/Ca(OH)₂ CHP system. Sharonov and Aristo [60] found that the Carnot efficiency can be obtained for a CHP that results from mono-variant equilibrium of a gas–solid reaction, which was confirmed for various chemical reactions between salts and ammonia (or water). Fadhel et al. [61, 62] investigated the performance of solar-assisted CHP, and found that any reduction in energy at condenser will decrease the COP of the CHP as well as decrease the efficiency of drying. Kim et al. [63] evaluated a CHP with reactivity enhancement of chemical materials (EMCs), which is a mixed material comprising expanded graphite, Mg(OH)₂ and calcium chloride (CaCl₂). An EMC was concluded to have higher dehydration rate and hydration reactivity at temperatures of up to 200°C when compared with the other materials.

3 STUDIES ON THE PERFORMANCE OF THERMOCHEMICAL STORAGE

Thermochemical ES systems utilize renewable energy sources and waste energy recovery over a wide range of temperatures. Table 3 provides a list of some recent analytical, numerical and experimental studies for different solution methods employed in investigating the thermochemical ES. From these recent surveys, the main advantages of the analytical model are simplicity and short computation times.

Thermochemical ES could be investigated using analytical, numerical and experimental methods in terms of one-, two- or three-dimensional models. An up-to-date three-dimensional hydrogen absorption model is developed by Nam et al. [64]. This three-dimensional model is first experimentally validated against the temperature evolution data available in the literature.

4 CONCLUSIONS

Studies of thermochemical ES systems have over the past several decades investigated design fundamentals, components and process optimization, materials selection, transient and long-term behaviour and field performance. A review on the state of the art of thermochemical storage technologies has been carried out. Consequently, thermochemical ES has provided with some advantages over TES, which can be summarized as following: Numerous research methods (theoretical, experimental and numerical) have been conducted on the thermochemical storage materials and systems. Currently, there are some challenges and barriers needed to be addressed so that thermochemical ES can be widely used. Chemical ES is still at an experimental stage and, although many patents have been filed, the technologies have not been applied in practice. Both technical and economical questions about thermochemical ES systems have yet to be answered. More research and development work as well as large-scale demonstration projects are required to prove the viability and long-term performance of thermochemical ES systems.

• higher energy density compared with physical change

• long-term storage as reactants with small thermal loss

• easily transmitted to generate heat at another location

• wide temperature range and characteristics.

REFERENCES