State of the art on salt hydrate thermochemical energy storage systems for use in building applications

Journal of Energy Storage 27 (2020) 101145

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State of the art on salt hydrate thermochemical energy storage systems for use in building applications Ruby-Jean Clark, Abbas Mehrabadi, Mohammed Farid

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Chemical and Materials Engineering Department, University of Auckland, New Zealand

A R T I C LE I N FO

A B S T R A C T

Keywords: Salt hydrates Thermal energy storage (TES) Thermochemical energy storage (TCES) Thermochemical material (TCM) Energy density Solid-gas reaction

Thermal energy storage (TES) for storing low-grade energy is a promising approach to achieving higher energy security and minimizing greenhouse gas emissions. TES is shifting towards using thermochemical materials (TCM) since there are several advantages when compared to sensible or phase change materials. However, thermochemical energy storage (TCES) is more complex and thus has not yet been developed commercially. To further develop this technology and bring it closer to commercialization, there needs to be a merging of research from both material and system design viewpoints. At a material level, salt hydrates are considered the most suitable materials for residential applications due to their high energy density (400-870 kWh m−3) and low turning temperature (<150 °C). From an engineering point of view, different system configurations have been designed and tested for salt hydrates. However, there are several technical challenges to the design of an efficient and stable system which need to be addressed before commercialization. This paper aims to provide a comprehensive review of the advancements of the long-term energy storage technology using salt hydrates at a material and system level. Furthermore, it covers the criteria for system design and the prototypes which have been designed and tested, as well as the technical challenges associated with TCES.

Nomenclature COP coefficient of performance Energy density (kWh m−3) ENG expanded natural graphite Energy storage capacity (kWh) TCES thermochemical energy storage Pressure (Pa) TCM thermochemical material Specific power (W kg−1) TES thermal energy storage Temperature (°C) Thermal conductivity (W m−1 K−1) 1. Introduction The increased use of fossil fuels resulting in greenhouse gas emissions has become a worldwide crisis. One solution is to replace fossil fuel consumption with renewable energy sources. According to the International Energy Agency, the building sector is the largest consumer of energy, accounting for approximately 40% of the world's total primary energy consumption and 24% of the world's total CO2 emissions [1], in which heating has a significant contribution. Solar energy is viewed as one of the most promising sustainable energy sources [2], as it has been widely researched and publicly accepted [3]. However, solar irradiation and residential heating demand are not synchronized. Thus thermal energy storage is required. Also, thermal energy storage ⁎

will overcome seasonal mismatch, increase the flexibility of energy usage [4], reduce peak demand [5] and make effective use of available solar energy. The idea of thermal energy storage (TES) was first investigated to address the energy shortage crisis in the 1970s. TES available for building applications is classified by three general materials: sensible, latent and thermochemical heat storage (TCES) [6]. TCES uses a reversible physical or chemical reaction and has a higher energy storage density when compared to the other two heat storage methods [3]. In recent years, TCES systems have been gaining credibility as a promising way of storing solar thermal energy [3,7–9]; however, there are still practical issues at both a material and system level which need to be addressed before commercialization [10]. The focus of this review is on salt hydrates as one of the most promising materials for storing low-grade heat. However, these materials typically tend to agglomerate, forming an impermeable block which inhibits reversibility and limits heat and mass transfer [11] hence, composite materials are employed to improve these reversibility issues. Furthermore, from a system design perspective, further research is needed to resolve these issues before TCES can be implemented into residential buildings [12]. The main objective of this paper is to critically review the available TCES system configurations currently used for thermochemical energy

Corresponding author. E-mail address: [email protected] (M. Farid).

https://doi.org/10.1016/j.est.2019.101145 Received 31 July 2019; Received in revised form 29 November 2019; Accepted 6 December 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. The process of a thermochemical energy storage cycle: charging, storing and discharging.

including high energy storage density, suitable turning temperature, self-separation of reactants and using water vapor as a safe and cheap gaseous partner [11,21,25–28]. Thermo-physical properties such as energy storage density, specific heat, thermal conductivity, hydration/ dehydration temperatures, chemical and physical stability over cycles, reaction kinetics and phase diagrams should be considered before selection of the salt hydrate can be made [29,30]. Numerous salts have been proposed for storing low-grade thermal energy; however, they have mostly failed. There are several parameters that a salt should meet for thermochemical energy storage to be implemented (Fig. 2) as were evaluated by various authors [3,11,31–35]. Permeability and energy density are the most important properties of any material to be used in TCES. These parameters need to be optimized as they evolve in an antagonistic way [11]. The porosity and hence permeability of the salt needs to be controlled for high mass transfer since moist air flow depends on permeability according to Darcy Law [36]. Materials that have high storage densities and low charging temperatures (such as SrBr2•6H2O which dehydrates <80 °C) appear the most appealing [24]. The charging temperature is limited by the heat source, for example, solar energy. In comparison, the discharging temperature is limited by the inlet gas temperature and humidity. Low-energy (modern) buildings with approximately 100 m2 surface area typically need 3000-4000 kWh year−1 (Belgium) of energy for space heating, although this largely depends on location [37]. This suggests that the minimum energy storage capacity of 150 kWh m-3 should be considered during salt selection [37]. As a result of continuous cycling, some materials swell or agglomerate, for example MgCl2•6H2O forms a gel-like crust in ambient conditions [28], resulting in the closure of pores which limits vapor flow pathways leading to reduced stability of the material under repetitive cycles. If the materials behavior changes or degrades over numerous cycles, this may decrease the stored energy and therefore the overall efficiency of the storage system [38]. Volume changes during cycling may decrease the thermal conductivity of the salt, decreasing power output and efficiency, as exemplified by Na2S•9H2O which experiences a 10-15% change in bed height during cycling [39]. Side reactions may cause degradation, causing the reaction to no longer be reversible, reducing energy output [18]. While the cyclability of both sensible and latent heat storage systems are well documented, there is limited literature on the stability of salt hydrates over numerous cycles. Cyclability depends on both the type of salt and how it is used. Hence, further research on cyclability into a range of salts is necessary. Several screenings have been completed in order to determine the most promising salt hydrates for low-temperature energy storage. Richter et al. [40] analyzed the performance of 308 salts with a hydration temperature above 150 °C, and considered CaSO4 and SrBr2 the most promising with SrBr2 performing the best in terms of cyclability. CuBr2 was considered to be the ideal salt hydrate for low temperature applications, characterized according to its mass and volumetric density and turning temperature by Kiyabu et al. [41]. N'Tsoukpoe et al. [42] concluded that LaCl3 and SrBr2 are the most promising salts for space heating application. However, this decision did not take into account

storage, based on solid-gas salt hydrate reactions for residential space heating application and highlight some of the hurdles that need to be considered in future research work. In previous years, reviews have been published for salt hydrate reactions, (like Donkers et al. [13], Ding et al. [14], Yan et al. [15], Tatsidjodoung et al. [6], Trausel et al. [16]), which focus on salt hydrates at a material level. This manuscript reviews the use of salt hydrates for thermochemical energy storage with focus on materials selection and system design. Sections 3, 4 and 5 are dedicated to materials used while Section 6 focuses on reactor and system design. Details on current working prototypes are summarized in Section 7. A similar approach was followed by Kuznik et al. [5], although the investigation was on physical adsorption. 2. Principle of solid-gas thermochemical reactions TCES uses thermal energy to excite a reversible chemical reaction, as shown in Fig. 1 and Eq. 1 [17]. In general, the process of storing and delivering thermal energy through thermochemical method consists of three steps: charging, storing and discharging [15,18].

AB + heat ⇌ A + B

(1)

In the charging (dehydration) step, thermal energy is used to dissociate the chemical bonds between molecules through an endothermic reaction. The dissociated materials are then kept separately (storage step). The stored thermal energy can be delivered in the form of heat, whenever required, through an exothermic reaction between the two dissociated compounds (discharging, hydration step). In this step, chemical bonds form, resulting in heat generation. After discharging, the storage material is regenerated and reused for the next cycle. Generally, thermochemical reversible reactions used for long-term energy storage can be grouped into four categories based on the state of materials involved in the reaction including 1) gas-gas reversible reactions, 2) liquid-gas reversible reactions, 3) liquid-liquid reversible reactions and 4) solid-gas reversible reactions [11,15,19,20]. Compared to other thermo-reversible reactions, solid-gas reversible reactions have attracted more interest because of their wide range of turning temperatures and self-separation of reactants [11,21,22]. The turning temperature of some solid materials such as oxides like K2O are as high as 1000 °C, which makes them suitable for high-temperature storage [15]. In comparison, the turning temperatures of metal hydroxides (such as Mg(OH)2) are in a moderate range, making them suitable for chemical heat pumps [23]. Materials such as salt hydrates (CaCl2•6H2O) have low turning temperatures which suit solar energy and low-grade waste heat making them suitable for residential space heating. This review focuses on the thermal effects of reversible reactions between salt hydrates and water vapor, as this is the most promising alternative for long-term energy storage for a low-temperature building application [24]. 3. Salt hydrate features required for thermochemical heat storage Salt hydrates have several advantages for storing low-grade heat, 2

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Fig. 2. Salt requirements for storing low-grade thermal energy.

thermal conductivity. The higher the thermal conductivity the more efficient heat transfer within the bed, leading to a shorter cycle time and higher power output. One possibility is to use expanded graphite as it has a high thermal conductivity (2-90 W m−1K−1) as well as controllable porosity [59,60]. Lahmidi and Mauran et al. [61,62] mixed ENG and SrBr2 which increased thermal conductivity and permeability, however significantly decreased the theoretical energy density. Improving on these works, Zhao et al. [51] mixed SrBr2 and ENG treated with sulfuric acid. The composite with 10 wt% of SrBr2 was successful, with a thermal conductivity of 7.97 W m−1 K−1. The composite was tested in a closed system with a finned tube heat exchanger, in which the total power was 67.4 kW m−3, higher than a composite with untreated ENG [51] (Table 3). It has been shown that thermal conductivity is positively correlated with reaction rate, in which for both pure salts and composites this strongly depends on the material porosity [50,63,64]. Thus, porous matrices can be used to improve thermal conductivity. Tanashev et al. [65] investigated the thermal conductivity of CaCl2, LiBr and MgCl2 in the porous matrices silica gel and alumina. The silica gel matrix improved thermal conductivity by 0.4-0.55 W m−1 K−1. Inert matrices which have been emphasized in literature in order to prevent agglomeration and improve stability are exfoliated vermiculite (EV) and activated carbon foam (ACF). EV is considered an excellent host matrix as it has a large pore structure [66]. Zhang et al. [52,66] developed a SrBr2/EVM composite and LiCl/EVM via chemical impregnation methods. The ideal SrBr2 composite had a salt content of 63.02% and a volume energy storage density of 105.36 kWh m−3 and the ideal LiCl2 composite had a salt content of 20% and a volume en3 ergy storage density of 171.61 kWh m− . Progressing this work, Grekova et al. [67] developed a LiCl/vermiculite composite via aqueous impregnation. The salt content was 49 wt% and gave a significantly improved energy density of 224–253 kWh m−3, with a charging temperature of 75–85 °C [67]. Roelands et al. [39] added cellulose powder to NaS2•9H2O

any economic analysis. One of the major issues related to use of salt hydrates for thermochemical energy storage is the economic constraint. Several authors have investigated cost-saving waste materials to be used as a thermochemical energy storage material. Gutierrez, Ushak, and Mamani et al. [43–46] have investigated the waste materials astrakanite, carnallite, kainite and bischofite to be used as thermochemical energy storage materials. It was found that astrakanite and kainite have potential to be applied as a low-medium temperature TCM, up to 120 °C [44,46]. Furthermore, carnallite and bischofite are promising waste-materials as they both contain the well investigated salt hydrate MgCl2•6H2O. Although the energy storage density of bischofite is lower than that of pure MgCl2•6H2O, the cost is three times lower [45]. These salt hydrate based waste-materials could offer a promising alternative to the current concern regarding cost when using pure salt hydrates.

4. Salt hydrate composite materials for thermochemical energy storage Composite materials have been widely investigated to minimize the limitations that occur when salt hydrates are used [47,48]. This can be done by using either mixture of material or through impregnation and consolidation of salt into an inert (expanded graphite, vermiculite, etc.) or active (zeolite, silica gel) material [49]. The host matrix is important in order to prevent salt agglomeration, and swelling, which leads to an improvement in moisture diffusion during heat regeneration [50–54]. Furthermore, a high thermally conductive inert material such as expanded natural graphite (ENG) can improve the thermal conductivity of the reactor bed [21,26]. A composite material can be designed to suit any specific application. The amount of salt hydrate impregnated/ mixed into the matrix can be varied to control water uptake. Studies have shown that composite materials have improved thermal conductivity [50,55,56] and reduced agglomeration and swelling [57,58]. Several authors have used inert or active host matrices to improve 3

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(740 kWh m−3) in order to improve mechanical strength to stabilize the salt. In order to synthesis the composite, they dispersed and mixed cellulose powder into a warm solution of the salt. In such conditions, if the solution cooled down to room temperature fine salt crystals would grow on the cellulose and form small (1 mm) composite particles of cellulose and sodium sulphide nonahydrate. Hence, cellulose can act as a bridge between salt crystals and increases bed porosity and stability [39]. As previously stated, another possibility to reduce salt agglomeration and melting is via impregnation of the salt into an active porous host matrix. The porous host matrix offers structural support as well as large absorption surface area and accommodates the material swelling and shrinking over cycles, which can vastly improve heat and moisture transfer. In such a structure, the sorption rate of gas-solid is controlled by the intraparticle vapor diffusion [64]. The common materials are zeolite, silica gel, charcoals and activated aluminas [68], as they can create a physical bond with water. Using such a composite is beneficial as both physical ad/absorption and chemical reaction takes place [69]. Selection of the host porous matrix is critical, as it impacts both heat and mass transfer and influences salt dispersion and sorption capabilities [21,37]. Using the right porous matrix allows salt to coat the inner surface of the pores and increase the reaction surface between water vapor and salt whilst using the wrong porous matrix reduces performance due to pore blocking [51]. Of the different porous matrices, zeolites, with an average internal surface area of 400-800 m2 g−1 and pore volume of 0.2–0.3 cm3 g−1 are highlighted in literature [21]. These porous matrices are well defined structures that contain aluminum, silicon and oxygen in a framework with cations and water located in the pores. This framework structure consists of a simple polyhedral, in each polyhedron itself is a three-dimensional array of individual tetrahedra in its simplest geometric form (for example, [AlO4]5− and [SiO4]4−) [70,71]. This structure provides the large surface area needed for the reaction. Hongois et al. [72] investigated the impregnation of MgSO4 into silica gel and zeolite and concluded the zeolite/MgSO4 composite to be the most satisfactory. The initial results showed a temperature lift of 30 °C and maximal power output of 28 mW g−1 during hydration. Hongois et al. [26] advanced this study by investigating the effect of salt concentration of MgSO4 in a bed of mesoporous zeolite (13X zeolite molecular sieve). The zeolite was impregnated by three different concentrations of salt solutions (10,15 and 25 wt%). Interestingly, TGA results of zeolites and impregnated zeolites were similar, suggesting MgSO4 no longer behaves as a salt hydrate. It has been shown that in such conditions a bivalent salt ion would exchange with two monovalent Na+ ions which results in pore enlargements and space increase for more water adsorption which could translate to higher energy storage capacity. This was shown via Jänchen et al. [73] who modified zeolites and mesoporous materials via ion exchange and impregnation with hygroscopic salts. There was a positive correlation between the adsorbed amount of water and the degree of ion exchange of Mg2+. The replacement of two monovalent sodium ions with bivalent magnesium ion enlarges the pore volume of the zeolite and increases the space in which molecules can be adsorbed [73]. Other ions with a high charge density show similar results, such as Zn2+, La3+ and Al3+. Comparatively, ions with a low charge density, such as Li+, have similar adsorption capacity as NaX. However, high heats of adsorption requires a charging temperature of >473 K, making this inapplicable for a solar collector application. In the study conducted by Hongois et al. [26], the pore volume measurements showed 44% reduction after impregnation of zeolite with a MgSO4 25 wt% salt solution. Almost half of the pores were either blocked or enlarged through impregnation or cycling which shows the importance of salt concentration on impregnation. Although theoretically impregnation with a higher concentrated solution should result in higher thermal energy storage capacity, it may lead to overloading of the porous structure. Mahon et al. [74] illustrated this via impregnation

of magnesium sulphate into a 13X molecular sieve or zeolite-Y (ZMK) with binders. The 13X sieve composite did not hydrate/dehydrate as anticipated due to pore blocking, suggesting another method of preparation is needed. In contrast, the ZMK/MgSO4 binder did dehydrate and rehydrate, showing no performance loss after 3 cycles, with a dehydration enthalpy of 715 J g−1 [74]. To find the best combination of salt weight percentage and type of host matrix, Whiting et al. [21] examined the performance of four different zeolites including Na-Y, Na-X, MOR and H-Y with impregnation of 5-15 wt% MgSO4 solutions. For all types of impregnated zeolites, the higher salt concentration resulted in higher energy storage density. In comparison between different zeolites, Na-Y zeolite impregnated by 15 wt% MgSO4 solution had the highest performance and released the highest thermal energy over the hydration step. The pore volume measurements revealed that the dispersion of salt in such matrix was better compared to the others. After impregnation by 15 wt% of MgSO4 solution, the pore volume of the Na-Y zeolite reduced slightly (from 0.32 cm3g−1 to 0.27 cm3g−1) while it decreased significantly in Na-X zeolite (from 0.20 cm3g−1 to 0.08 cm3g−1). This shows that there is a strong relationship between maintaining large zeolite pores and energy storage capacity. In another study, Whiting et al. [75] impregnated similar zeolites by 5-15 wt% solutions of MgCl2 in comparison to the performance with 515 wt% MgSO4-zeolite composites using similar experimental conditions to their previous study [21]. The highest heat of hydration was obtained where the Na-Y zeolite impregnated by 15 wt% MgCl2 which was due to the high pore volume, as observed in the MgSO4 study. This further confirmed the importance of high free pore volume after impregnation to allow water vapor transfer. All four types of zeolites impregnated by MgCl2 had better performance than the MgSO4 composites. This was not only due to the higher energy density of MgCl2 but also due to less blockage of the pore as well as lower deliquescence relative humidity (DRH) of MgCl2. For example, the pore volume of the Na-X zeolite reduced from 0.2 cm3 g−1 to 0.16 cm3 g−1 when it was impregnated by MgCl2 while it reduced further (to 0.008 cm3 g−1) when impregnated by MgSO4. More recently, Xu et al. [76] compared the performance of three different zeolites (3 A°, 4 A° and 13X) impregnated with MgSO4 under different operational conditions using a packed bed reactor. The results of a macro scale 400 g open reactor showed that the 13X-based composite had a higher adsorption rate as well as temperature lift under different conditions which was due to the less pore blockage (Table 3). Furthering on this study, Xu et al. [77] developed an MgSO4 impregnated zeolite 13X and activated alumina composite. The aim of this composite was to cover both mid and low temperature ranges with both host matrices, zeolite 13X and activated alumina. The experimental energy storage densities in a closed system were 123.4 kWh m−3 for the zeolite 13X composite and 82.6 kWh m−3 for the activated alumina composite [77] (Table 3). Silica composites are also highlighted in literature. Jänchen et al. [73] suggests that mesoporous silica matrices avoid the problem that occurs with zeolite matrices of high temperature needed to remove water. They found that a CaCl2 silica matrix could be fully dehydrated at 410 K (compared with the zeolite composite which required a desorption temperature of >473 K) and a silica gel and MgCl2 composite could be dehydrated at 180 °C with an energy density of 0.4–0.5 GJ m−3. Courbon et al. [37] successfully tested the composite SrBr2 and silica gel in order to improve the stability of the salt [37]. The power output was 203 kWh m−3 with a desorption temperature of 80 °C. This composite is a promising candidate for heat storage applications. Ristić et al. [54] developed this investigation with a comparison of ordered and disordered mesoporous silica matrices. Both mesoporous silica matrices did not change after 20 cycles, however it was found that the ordered mesoporous silica matrix was best suited for dehumidification whereas the disordered matrix was best suited for heat storage [54]. Salt mixtures are currently being investigated in order to realize the advantageous properties whilst minimizing the disadvantages of both 4

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density. Lastly, salt mixtures appear promising, however further research is needed before this can be implemented into a large scale reactor. Innovative composite materials such as multi-wall carbon nanotubes need to be investigated further. Although either adding polymers or using porous structure to contain salt would help cope with some technical issues, the energy storage capacity (kWh m−3 composite) will be reduced due to occupying a fraction of system's volumes by the porous matrix. However, if the material costs are lower, the system might be able to become commercially viable even if a relatively large volume of material is required. The general technical issue that has been reported is mass transport within the matrices pores, a result of deliquescence, over hydration with possible leakage or pore blockage and a low temperature lift [49]. Lastly, some experimental conditions do not reflect the ambient conditions achievable and thus may not be feasible in open systems. The conditions may appear inapplicable in local environments, but may be feasible in other climates.

salts. In one study Korhammer et al. [78] suggested increasing the melting point of a reactive bed by mixing the low melting point salt with a higher melting point salt. They added KCl (by 30-50 wt%) as an additive to CaCl2. CaCl2 is a highly hygroscopic salt with low deliquescence relative humidity (DRH) while KCl has a high DRH hence, mixing the salts would reduce the risk of agglomeration and improve mass transfer which in turn would enhance the system performance. They showed that using KCl led to significant improvement in performance so that the salt capacity uptake for water doubled during the hydration step. Druske et al. [79] and Korhammer et al. [78] found impregnation of CaCl2/KCl mixture into ACF or ENG improved thermal conductivity, improving overall dehydration/hydration behavior. The CaCl2/KCl-ENG composite performed the best; thermal conductivity of 0.74-1.64 W m−1 K-1 and an energy density of 0.64 GJ m−3 [79]. Impregnation of mixture into expanded graphite increased its performance not only due to the increase in reaction surface area but also due to the enhancement of thermal conductivity by 300% compared to the salt alone [78]. However, as ENG is inert, energy storage density of the system is reduced [78]. Rammelberg et al. [80] furthered this investigation via mixes of CaCl2, MgCl2 and MgSO4 in order to improve cycle stability. It was found that the CaCl2/MgCl2 showed good cycle stability (no decrease in mass over 55 cycles), tolerated overhydration and had superior kinetic properties. In recent work, Liu et al. [81] examined the performance of a new composite structure. They impregnated a honeycomb mesoporous structure made of Wakkanai siliceous shale (WSS) with LiCl so that the final composite contained only 9.6 wt% salt. Using such a structure, they could achieve a storage capacity of 50 kWh m−3 of composite material which equaled to 250 kWh tonne−1 of salt. In addition, the composite material and structure were highly stable so that no degradation and performance loss were observed during 250 cycles. The dehydration from LiCl2•2H2O to LiCl•H2O could be achieved in the temperature range of 77-87 °C, making this composite applicable to environments with less solar radiation, as a lower dehydration temperature does not affect the temperature release [81]. Although the new structure could solve the instability issue, the heat storage capacity is low. Yu et al. [82] impregnated LiCl into an ACF, silica and ENG composite. The purpose of this was for LiCl to be responsible for water uptake, the ACF to act as the host matrix, the silica solution as a binder to increase mechanical strength and with ENG to improve thermal conductivity [82]. ENG improved the thermal conductivity in the range of 2-2.8 W m−1K−1, approximately 14 times higher than that of pure ACF and LiCl [64,82]. However, ENG decreased the water uptake, a result of slow kinetics due to decreased water transport. The expected energy density of the composites were in the range 0.7–1.43 GJ m−3. A unique composite material, in which salt hydrates were impregnated in multi-wall carbon nano-tubes (MWCNT) was developed by Grekova et al. [83]. In this, three hygroscopic salts, CaCl2, LiCl and LiBr were tested. It was shown that the salts were crystallized, dispersed and confined within the porous host matrix. It was found that the LiCl/ MWCNT was suited for a daily cycle in warm climates [83]. Another innovative composite, developed by Brancato et al. [84], filled silicone foams with MgSO4•7H2O. A foam was chosen as this has a flexible structure which allows the salt to expand and shrink without breakage of the host structure. The foam samples had salt weight percentages from 40–70 wt% [84]. SEM microscopy showed that the salt was confined within the porous matrix and it was shown that the maximum salt weight percentage was 60%. Both of these innovative concepts are promising, however further evaluation is needed before it can be determined to be a successful material. Overall, zeolite 13X has shown good performance as a host matrix, however the high cost, low thermal conductivity (decreased reaction time) and high desorption temperature leads to increased system cost and decreased system efficiency [3,63,85]. Composites with ENG have also showed promising results due to the increased thermal conductivity but this comes with a compromise of volumetric energy

5. Requirements for thermochemical energy storage system Design knowledge of the thermochemical reactor is vital to the successful implementation of TCES technology. Several factors regarding the material; kinetics, mass and heat transfer, economic cost and safety should be taken into account when designing a reactor [11]. In the literature, the economic factor is often not taken into account, while this is essential as it determines the choice of reactor type and raw material costs. Scapino et al. [64] and Lefebvre et al. [86] suggest that economic analysis should be completed from the earliest stages of research for the prospect of commercialization. Once a material is decided upon, several parameters should be considered before designing; open or closed system (refer to section 6.1), storage density optimization, vessel design, heat exchanger design, pressure drop, energy source, efficiency and cost [30]. An outline of the steps to design a reactor are shown in Fig. 3. As highlighted, one of the major drawbacks is heat transfer. This is directly related to the low thermal conductivity of the material used (in which most salt hydrates are poor; 0.6–1.0 W m-1K-1 [87]) and the thermal contact between the salt grains (porosity of reactor bed) [38,88]. The contact between grains is problematic when dealing with salt hydrates as they show swelling and shrinking between cycles reducing the effective thermal conductivity of the reactor bed [88]. The thickness of the material bed and the type of heat exchange surface, as well as the shrinking and swelling, all affect overall heat transfer coefficients [89]. In closed systems, this affects the design of the heat exchanger, which impacts the global process. One way of improving heat transfer is to increase the heat exchange surface area, which can be achieved via using finned tubes [5,32], plate heat exchangers [61] or coated spiro-tubes [90]. Many closed adsorption chillers and heat pumps have employed an extended surface heat exchanger [91–93]. Although in open systems, the problem of heat transfer is not critical, it is still necessary to have a large contact surface area between the vapor flow and storage material [94]. A large crossflow area with minimal bed length or a porous matrix can be used to obtain this while minimizing pressure drop [94]. Mass transfer (gas diffusion) is another major limitation, which depends on both permeability and thickness of the material [95]. Regarding reactor design, increasing bed thickness results in a slow reaction, whereas decreasing bed thickness decreases the energy storage of the reactor [96]. Hence, the bed thickness should be optimized. Secondly, the vapor flow pathway should be enhanced to minimize vapor pressure drop through the reactor bed whilst ensuring adequate mass transfer. It must be noted that the larger reactor length, the greater the pressure drop. However, large-cross sectional areas with a minimal bed length or porous matrix could be used to reduce this [97]. A general procedure for reactor design was adapted from Nanda et al. [98]; 5

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Fig. 3. Methodology when designing a system.

This method based on the total heat supplied during heat charge, and hence the value of COP will always be <1. It is obvious if it is based on only the heat absorbed by the bed during dehydration then the COP will be close to 1.0, as shown by Fopah-Lele et al. [99] who had a reported COP value of 0.97. Others [101] have used an unrealistic definition, based on the electrical energy used during hydration;

1 Thermodynamic and kinetic data on the material is collected, either via literature, laboratory or pilot plant studies. 2 Physical properties of the material are required for the design and can be deduced; either from the literature, estimated or collected from laboratory measurements. 3 Initial design and approximate costs are evaluated to ensure this meets economic constraints. 4 The rate controlling mechanism, heat or mass transfer, is established as this has the dominating effect on performance. 5 A suitable reactor type is designed, specific to the material chosen and the rate controlling mechanism, and this is tested via laboratory studies or pilot plants. 6 Optimization of reaction conditions is selected to obtain the desired energy output to the application. 7 The size and performance of a prototype should be estimated. In this case, semi-empirical methods can be used based on ideal reactors. 8 The design is optimized and validated, and capital and running costs are finalized.

COPdischarging =

6.1. Open and closed systems Systems for solid-gas reactors can be designed as either open or closed systems. An open system consists of one vessel; a reactor containing solid materials, which has moist air flowing through it at atmospheric pressure, as shown in Fig. 4 [36,64]. The process is as follows:

• Charging: ambient air is heated via a heat source (solar collectors)

6. Advancements in reactor design Different concepts and applications based on the principle of thermochemical reactions have arisen to optimize system operation [11]. Currently, there are models, lab scale experiments and prototypes adopted for low, medium and high-temperature applications. However, more research is needed on system design in order to use TCES technology for space heating. As stated earlier, this review focuses on salt hydrates with dehydration reaction temperature below 150 °C, which is compatible with conventional solar collectors as the heat source. The volume of the final system needs to fit into a single household (for residential application) and be economically competitive to similar space heating systems [11]. Throughout literature, the coefficient of performance (COP) is often referred to. However, there are two definitions. The first, refers to general thermal energy storage, as shown in Eq. 2.

Usesful heat production Q = out Heat consumption Qin

(3)

In which, Qout is the useful heat between the inlet and outlet air flow and Wf is the work done by the fan. In this case, the COP will be >1. Casey et al. [101] calculated the highest COP of 21, whilst the lowest COP was 8.5. Similarly, Zondag et al. [102] used this calculation and achieved a COP of 12. The existing thermochemical energy storage system configurations can be generalized into the following configurations, as outlined and reviewed below.

Overall, although micro-scale measurements can be controlled by humidity, temperature and reaction time because of the homogeneity of the sample, macro-scale systems will show structural diversity [27,80]. Thus, controlling the reaction will become difficult with increasing size [80]. It is suggested that once a material is decided upon, the reactor and system should be built to enhance the positive features and diminish the negative features of the material (Fig. 3).

COPth =

Qout Wf

•

and then flows through the reactor in which dehydration of the salt occurs. Discharging: low-temperature humid air from the ambient flows through the reactor in which hydration of the salt occurs [94].

As the system works open to the atmosphere, the pressure is set to atmospheric pressure, leading to a large pressure drop across the reactor [48]. The gas must be environmentally friendly (e.g. water vapor) as this is released to the atmosphere [64]. As the heat transfer fluid and the reactive gas are the same, no internal heat exchanger is needed, and there is no need for an evaporator or condenser, resulting in a smaller overall system volume and thus larger volumetric energy density [36]. In contrast, a closed system contains two vessels: a reactor with the material and a condenser/evaporator in which the reactive gas is held. The working mode differs from that of the open system as not only the gas is stored, but also the heat energy is transferred to/from the environment via a heat exchanger [103].

(2)

• Charging: requires a heat input (solar collectors) to the reactor in

Fopah-Lele [99] and Li et al. [100] defined COP by this method. 6

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Fig. 4. An integrated configuration for an open and closed system (shown left and right respectively).

•

higher entropy production when compared to a closed system. Further investigation confirmed this, as the permeability of the reactive bed had a strong influence on reaction rate; decreasing reaction time with increased permeability. Thus, when designing an open reactor; permeability must be carefully considered to ensure adequate mass flow [3,36]. In comparison, in closed systems, Michel et al. [36] concluded that heat transfer is the main limitation. This is because the mass transfer is determined by the total pressure, which can be optimized [32]. This was demonstrated by applying a sharp front model in the reactor, which showed the reaction front moves from the heat exchanger surface. Second law analysis showed a higher entropy production results from heat transfer [36]. A sensitivity analysis concluded that the reaction rate increased with increased material conductivity [36], which can be improved by adding a conductive binder (e.g ENG).

which dehydration occurs and a gas-liquid phase change reaction occurs in the condenser, in which heat is released [104]. Discharging: requires a low-temperature heat source to produce vapor needed for hydration of the material in the reactor.

In closed systems, the material reacts with a pure gas, such as water in a vacuum [36]. Due to discharging requiring a heat source for vaporization of the liquid (Fig. 4), a higher temperature is needed for the hydration reaction, leading to a lower solar collector efficiency [105]. A comparison of the advantages and disadvantages of both open and closed systems is illustrated in Table 1. Michel et al. [36] concluded that mass transfer was the limiting factor in open systems via a second law thermodynamics analysis. This was a result of the pressure drop across the reactive bed, leading to Table 1 Advantages and disadvantages of open vs. closed systems [64,88,106,107]. Open System Advantages

Disadvantages

Closed System

• Atmospheric pressure design and fewer components when compared to a closed • Simplified system increased by forced circulation and the heat transfer fluid • isHeatthetransfer gas carrier heat exchanger required • No • High system energy density Fan and humidifier often needed to drive flow and provide partial • humidification and side reactions have to be safe as being released to the • Gas atmosphere gas flow leads to pressure drops • High transfer limiting step • Mass • Electrical energy needed for auxiliary operation • Salt may not take up water vapor at atmospheric pressure discharging temperature when compared to a closed system with a • Low similar vapor pressure • Instability during heat generation

7

discharge temperature when compared to an open system with a • Higher similar vapor pressure • No mass exchange with the environment and better control of the mass transfer • Can be used for both cooling and heating power density • Higher electrical energy is needed for fan operation • No reactants can be used and avoids undesired side reactions • Toxic state possible and better control of the pressure • Low-pressure during heat generation • Stability system • Complex transfer limiting step • Heat needs to be stored • Gas • Periodical evacuation required due to the formation of incondensable gases heat transfer area required • Large system energy density due to reaction components having to be stored and • Low heat exchanger level of temperature needed during hydration reaction, leading to • Higher lower solar collector efficiency

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material is solid [38], as it does not need to be transported, reducing the energy consumption required for transport [11]. However, with a larger reactor, there is a substantial sensible heat loss during charging as the whole reactor bed must be heated up [97]. Furthermore, the large reactor must be designed for reaction conditions and controlling the reaction is complex [11,64]. Tanguy et al. [108] analyzed the energy processes of an open, integrated reactor system using SrBr2, and it showed the hydration step is problematic, and the pressure drop decreases the COP. In comparison, in a separate reactor, the material is transported to the reactor when heat is required and is stored separately from where the reaction takes place. This makes it is possible to only heat the necessary amount of material at a given time [5], improving efficiency and decreasing heat exchanger surface and sensible heat losses [109]. The material needs to be transported to the reactor from a storage vessel, leading to more complex design (Fig. 5), more vessels and increased pumping power [11,64]. The heat and vapor transport can be optimized efficiently when compared to an integrated reactor. Zondag et al. [97] concluded that separate reactors are more suitable for seasonal storage because only a small volume of material is heated up, making the overall process more efficient. This is critical when considering a seasonal storage cycle, as a greater quantity of material is required. Modular reactors (Fig. 6) can be classified as a sub-category of integrated reactors, as the thermochemical material is not transported. Within the reactor, the material is divided into smaller, insulated modules each with its own heat exchanger. Therefore, smaller volumes of material can be heated upon request, and the power output of each module can be optimized for the desired application [32,64]. Unlike integrated reactors, the entire material does not need to be flushed with the gas flow during operation, which reduces the pressure drop [64]. This configuration is advantageous as the surface area of heat exchange can be optimized [61], and reactor size can be decreased or increased accordingly. Most importantly, a module can undergo a cycle once a year [32,94]. The size of a modular reactor can be chosen to minimize the thermal capacity of all components and decrease heat transfer path [94]. However, each module within the reactor requires its own heat exchanger, increasing volume, which increases capital costs and lowers volumetric energy density [32,64]. There are several designs yet to be investigated. De Jong et al. [32] suggests that the optimal system when using an integrated configuration is when both wet and dry TCM is separated via a piston [32]. A challenge of this is ensuring adequate compactness with enough vapor and heat transport. An open system in which the wet TCM is removed and dried somewhere else could be considered [32]. Due to small thermal losses, transportation of the TCM is feasible, even over long

Closed systems have a higher power density as a result of no inert gases passing through the reactor [88]. They can supply high output temperatures for heating applications and low temperatures for cooling [24,36,88,105]. Fumey et al. [30], suggests that closed systems are superior to open systems in respect to heating applications, as they are able to reach a higher output temperature. One of the major limitations of open systems is the atmospheric conditions. If the atmospheric conditions do not meet the humidity requirements of an open reactor, air must be humidified; otherwise, the coefficient of performance (COP) of the system would be compromised [36,108]. Therefore, an analysis should be carried out to determine if the ambient moisture is adequate for discharging; otherwise, humidification is required [94]. Generally, the discharging power of open systems is lower than that of closed systems due to the limitation of the low ambient temperature [30]. In contrast, in the closed system, the low operating pressure leads to technological constraints. Therefore, design and manufacturing must be carefully considered in closed systems. Abedin and Rosen [33] found that for both open and closed systems, the overall exergy efficiencies were lower than the energy efficiencies, showing that improving efficiency and reducing heat loss is possible. Bertsch et al. [35] modeled solid sorption storage concepts to compare open and closed systems. It was found there were significant heat losses in closed reactors and thermal insulation is required, whereas in the open system this is negligible since it could be used for space heating. This is supported with the results from Michel et al. [36] who compared open and closed systems (SrBr2•1-6H2O) and found average specific powers of 1.13 and 0.96 W kg−1, respectively. Overall, open systems have several advantages over closed systems when applying this technology to residential space heating, namely; easier construction and management, smaller volume, higher energy density, low initial cost, improved heat transfer and higher exergy and energy efficiencies. These features make this design appealing and favorable for use when regarding space heating of residential buildings [29,33,35,36,38,64]. In brief, it is suggested that if the material proposed has a high permeability (e.g. vermiculite composites), it should be used in an open system. Whereas, if the material has a high thermal conductivity (e.g. ENG composites) this should be applied to a closed system.

6.2. Integrated, separated and modular reactors Within both open and closed systems, it is possible to have an integrated, separated or modular reactor. In an integrated reactor, the total amount of the thermochemical material is contained in the bed; therefore, the material does not have to be moved once placed inside the reactor (Fig. 5). This configuration is preferable if the reactor

Fig. 5. Open reactor system with integrated configuration (left) or separate configuration (right). 8

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a combistore for a separate, open system, in which dehydration of the material occurs at lower temperatures without compromising the energy storage capacity. In this system, the inlet air is pre-dried via an adsorption unit before entering the reactor. The system was investigated using zeolite (dehydration temperature of 180 °C), and an additional sorption unit based on zeolite was used to reduce the water content of inlet air during dehydration [48]. By having reduced water content in the inlet air, a dehydration temperature of 130 °C was achievable. There are several advantages of this system; no energetic effort needed to evaporate water as the reaction is performed with ambient air, reduced heat losses and improved efficiency. However, there are some disadvantages, including; the technical complexity and decreased volumetric energy density [48]. N'Tsoukpoe et al. [113] analyzed the possible advantages of a cascade system, as this may help improve energy storage density, cost and efficiency. The principle of this system, with two materials, involves the TCM's being connected in order of their respective equilibrium temperatures (at evaporation pressure) for the hydration period. During dehydration, the connection is in order of decreasing equilibrium temperatures, as shown in Fig. 8. The heat transfer fluid enters reactor 1 and leaves reactor 1 at the equilibrium temperature (assuming ideal heat exchange). Then, it enters reactor 2 and exits at a higher temperature for application. Hence, the required temperature for application is met via the equilibrium temperature of reactor 2, which can be dehydrated at a higher temperature as a result of the temperature lift received from reactor 1. The concept was modelled using Na2S•5H2O and SrBr2•6H2O and an improvement in energy density of 11–21% was shown when compared to a single material [113].

Fig. 6. Modular reactor bed in a closed system configuration, de Jong et al. [32].

distances. Overall, it is suggested that a separate, fixed bed reactor to be the most advantageous, as it would optimize system storage density and be achievable at lower costs, assuming transport of the material can be implemented effectively. 6.3. Regeneration, combination and cascade systems The nature of advanced systems will be explored in this section. These systems are designed to increase the efficiency and power output of the process with better heat management [47]. It must be noted that although the combination of systems increases energy utilization, there is increased complexity, capital and operational costs [110]. The combination system also referred to in the literature as “combistore” or “combi-system” is a combination of thermochemical and conventional hot water storage (sensible heat) systems to complement the advantages and disadvantages of both storage technologies [111]. In this configuration, an open system is more advantageous as this does not require a vacuum or a heat exchanger, making the system more flexible [111]. The configuration involves direct integration of the TCES system into a hot water store, so the sensible heat lost during dehydration is transferred to the surrounding water, as illustrated in Fig. 7 [111]. Solar collectors are used to charge the hot water storage and if the solar collectors cannot cover the demand of heat, the hydration (discharging) process of the reactor begins. The heated outlet air from the reactor is passed through the gap between the reactor and water storage, releasing heat to the water. When there is surplus solar radiation to charge the hot water store, dehydration of the salt (charging) starts. Mette et al. [112] developed a regeneration strategy combined with

6.4. Reactor design In the development of a salt hydrate based thermochemical energy storage reactor, three types of reactors have been considered; fixed (packed) bed, moving bed and fluidized bed [31]. Currently, fixed bed reactors are considered the most appropriate for hydration/dehydration reactions due to the solid-gas states [114] and different designs have been investigated in order to address the major issues such as; system heat capacity, heat losses and heat and mass transfer. However, the mass and heat transfer limitations and large temperature gradients throughout the bed restrict application [115]. Heat transfer rates in large thickness fixed beds is a significant limitation, and to resolve this issue, especially for the materials with low thermal conductivity, a fluidized bed can be employed instead [116]. In a fluidized bed, the solid is in fine particles which are suspended by the upwards flow of the fluid (gas). Lastly, in a moving bed, the salt is moving continuously or in portions (such as stirring) [11]. A comparison of the three is iterated in Table 2. Zondag et al. [117] investigated the moving (screw) bed, fluidized bed and gravity-assisted bulk flow bed (a type of moving bed), as shown in Fig. 9. A rough estimate of COP of discharging via simulation was

Fig. 7. Principle of the combined reactor and hot water store: a) charging of hot water storage, b) hydration, c) dehydration, Weber et al. [111]. 9

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Fig. 8. Principle of cascade thermochemical heat energy storage using two thermochemical materials, N'Tsoukpoe et al. [113]. Table 2 Comparison of reactor bed design, Solé et al. [11] and Coulson and Richardson [116]. Reactor

Advantages

Disadvantages

Fixed/packed bed

Easy to model, design and manufacture Small size Small capital costs Direct heat transfer between solid and gas Less energy required than fluidized bed

Low heat and mass transfer High-pressure drop Temperature gradients Complex fluid dynamics Complex reactor design Moving parts Lowest conversion per unit volume Complex fluid dynamics and modeling Erosion of internal parts Pumping (energy) requirements Increased reactor vessel size Lack of understanding

Moving Bed

Fluidized Bed

Good temperature control Minimizes the risk of hotspots and thermal instability Heat transfer coefficients high Uniform particle mixing

Fig. 9. A moving (screw) reactor (left), fluidized bed reactor (centre) and gravity assisted bulk flow reactor (right), Zondag et al. [117].

the reactor [120]. This work has been progressed by Nonnen et al. [121], which is detailed in Table 3. Several authors have investigated fluidized bed reactors for hightemperature applications. Due to the inadequate mass and heat transfers in fixed bed reactors, Darkwa et al. [122] modelled a process fluidized bed. The results showed enhanced hydration capacities and higher heat transfer rates [122]. However, the developed model was based on adiabatic conditions during hydration, and only conductive heat transfer was considered. The model suggested that parameters which influence fluidization velocity (fluid velocity, particle size, density and viscosity) need to be optimized to promote the exothermic reaction [122]. Criado et al. [123] and Pardo et al. [124] analyzed a fluidized bed for the working pair CaO/Ca(OH)2, and found the process technically viable. This was developed with an experimental investigation and found that kinetic limitations (dependent on the reaction) and mass transfer resistance occurs as a result of bubbling [125]. Hence, further experimental studies and heat and mass transfer

made under optimized control for residential heating, in which the moving bed had a high COP (>15 when optimized). In both the moving and gravity assisted bulk flow reactor power consumption is similar. However, powder flow in the moving reactor is easier to control. In comparison, the fan energy required for fluidization of particles significantly decreased the COP of the fluidized bed reactor. It was concluded that the moving bed reactor, particularly the screw design, was the most promising [118,119]. Mette et al. [120] developed a regeneration system which can operate either as fixed or crossed-flow (gravity-driven) reactor. Three considerations were made: large cross flow section area for airflow with a minimal width to decrease pressure drop, compact construction, and material transport to be reliable and technically inexpensive. In the quasi-continuous flow operation, the material moved from the top to the bottom of the reactor, driven by gravity. The main advantages of this design are the constant power output and the stationary reactor. However, there is a challenge in maintaining a uniform flow through 10

11

Vermiculite and CaCl2

Activated alumina and LiCl

Expanded natural graphite and LiCl

Multi-wall carbon nanotubespolyvinyl alcohol and LiCl

Vermiculite and LiCl

Vermiculite and LiCl

ENG/ammonia and BaCl2

High-temperature heat pump used as a topping cycle for cascaded sorption chiller

MnCl2•2H2O

“Open-sorption pipe”

Closed system Fixed bed Modular concept Open system

• New design to improve pressure drop that occurs through reactor

• Testing of prototype with serial reactor modules

• Measuring thermal storage capacity of granulated composite • Comparison of sorption TES device to traditional TES devices

Closed system

Modular concept consisting of 25 beds Coil-tray HEX

• Investigation of water sorption dynamics under typical conditions of a daily cycle

• Experimental characterization of composite • Analysis of two working conditions; seasonal and daily storage

• Characterization of lab-scale prototype

• Experimental performance of prototype

• Aim to produce heat at a temperature suitable for industrial purposes (160 °C) from waste heat

Objectives

Finned flat-tube adsorber configuration

Closed system Integrated reactor Commercial fin-and-tube HEXs connected in parallel 1.9 kg of material Flat-plate adsorber configuration with 4 fin-and-tube heat exchangers Closed system

Closed system Fixed bed reactor Solar air conditioning

Application/ System type

Material

Table 3 Advancements on lab-scale and prototype design.

kinetics

−1

−3

2

−1

−1

−3

−3

−3

−3 .

Aydin et al. [133]

Zhang et al. [132]

Zhao et al. [131]

Brancato et al. [130]

Brancato et al. [129]

Palomba et al. [128]

Stitou et al. [127]

Stitou et al. [126]

Ref

(continued on next page)

• Real-life dynamic conditions were tested (ambient changed and reactor used intermittently) and led to a loss of energy to surrounding environment • Increasing humidity by four times led to power increase by 2.3 times from 313 W to 730 W • Temperature lift of 24.1 °C • Energy storage capacity of 25.5 kWh • Energy storage density of 290 kWh m−3

−3

energy density is double that of a traditional 300 L hot water tank • The modules are stored vertically and in series • The maximum hydration temperature is dependent on relative • The • humidity flow rate has an inverse relationship with output power and • Air discharging time but does not affect overall energy storage density energy density in the first module is 230 kWh m whereas • The the energy density in the second module is 130 kWh m A total energy • storage density of 191 kWh m Suggests series connection increases overall energy density • • 14 cycles were completed: no salt degradation

°C and a discharging temperature of 40 °C

• Stable for 10 cycles heat storage capacity reaches 10.25 kWh • The The heat storage density was 65.29 kWh m with heat recovery and • 54.27 kWh m without heat recovery for a charging temperature of 80

rate

The specific power under seasonal conditions was 2.96 kW kg , whilst • under daily conditions was 1.26 kW kg specific power is lower than what was seen at a material level, • The further confirming that optimization of prototype design is important power of 4.2 kW kg measured • Specific charging temperature of >85°C • Low size of grains increases porosity and thus decreases heat • Decreasing transfer between HEX surface and grains ultimately decreasing sorption

2

2

tubes of 140 kg of anhydrous BaCl and 35 kg of ENG • 19Powered at 60-70 °C by 20 m flat plate solar collectors • Yearly efficiency of 40-50% • Daily cooling productivity (over 2 years) • 0.8-12 kWh of cold per m atof4flat°C plate solar collector • were completed under seasonal storage boundary conditions • Tests average power of 0.5 kW • An energy density of 330 kWh m of material • An was stable for 14 cycles • Composite seasonal storage conditions adsorption kinetics are faster. A • Under higher dehydration temperature and smaller grain size increases the

transfer limitations at low pressure • Mass occurs at low operating pressure • Corrosion • High cooling performance, COP of 1.35 should be possible

Results

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Novel heat pump driven storage heater Heat recovery system Fixed bed, modular reactor

Heating and cooling with flat panels

Four salt based sorbents: Vermiculite and 43 wt% CaCl2 Vermiculite and 35 wt% MgCl2 Vermiculite and 22 wt% CaCl2 and 17 wt% LiCl Vermiculite and 18 wt% MgCl2 and 17 wt% LiCl Zeolite 13X

ENG and SrBr2•6H2O

SrBr2•6H2O

Novel open 3 kWh system with meshed tube air diffusers

Vermiculite and CaCl2 Vermiculite and CaCl2-LiNO3 Zeolite 13X

12

Prototype, 400 kg of hydrated salt Open system Modular reactor

Prototype, 500 kg of salt Open system Modular reactor

Closed system Fixed bed reactor SOLARSTORE Prototype, 1 m3 modular reactor Closed system Fixed bed reactor

Application/ System type

Material

Table 3 (continued)

2

winter conditions

3

2

−1

2

−3

2

2

2

diameter of holes in perforated tubes are of importance: changes • The impact pressure and velocity of air rate of 0.015 kg s is optimal • Flow between inlet humidity and outlet temperature is linear • Relationship and independent of time (progression of reaction) for improvement: determine the best possible inlet • Suggestions temperature for operational conditions and preheating inlet air in

higher temperature lift and larger mass transfer

had the best thermal performance • Vermiculite-CaCl had a better performance than Gen2; a result of increased • Gen3 moisture and heat transfer due to its perforated tubes, which allows a

Results

• Local performance analysis

Global performance analysis

prototype to achieve seasonal storage with high • Aenergy density and specific power for space heating

• • • •

• • • • • •

•

• Experimental tests within temperature ranges • Mathematical model development • • Prototype tested under conditions of summer or mid-season • • •

Michel et al. [31]

Michel et al. [136]

Mauran et al. [61]

Lahmidi et al. [62]

Aydin et al. [135]

Casey et al. [134]

Ref

(continued on next page)

the COP- thus developing a sorption material that has a low dehydration temperature is important Three composite samples with heating storage capacities of 250, 300 and 400 kWh m−3 Cooling effect is 185 K and 1500 Pa Heating effect is 308 K and 2300 Pa 400 kWh reactor had a low power output due to mass transfer limitation Experimental tests showed power levels of 40 kWh m−3 can be reached with reactive composites having heating storage capacities > 250 kWh m−3 Energy density: 60 kWh m−3 which is significantly smaller when compared to the theoretical maximum energy density of the salt (pure SrBr2), 628 kWh m−3 Mean heating and cooling: 60 kWh and 40 kWh respectively Modular concept with each layer having a thickness of 12 mm Homogeneity between two modules: only minor limitation of heat and mass transfer between layer A major limitation is between the reactive layer and heat exchanger walls Suggestions for improvement: Investigate how layers of reactive composite maintain thermal contact inside the reactor Reactor bed thickness is 7.5 cm to maximise energy density without limiting mass transfer. Each module (8 modules total) contained 50 kg of salt Bed energy density: 388 kWh m−3 Reactor energy density: 203 kWh m−3 (excluding feet and insulation) Moist air flow rate and output power related in a linear fashion Changing the flow rate did not affect overall temperature output Reaction front moves through the reactive bed from the inlet of humid air to its outlet. This is observed in both hydration and dehydration Reactive bed permeability strongly affected with an increasing number of cycles Overall, the thermochemical reaction causes local changes within the reactor bed

of a hybrid “solid sorption heat storage/air Vermiculite -43 wt% CaCl and Vermiculite-22 wt% CaCl -17 wt% LiCl • Investigation • most sourced heat pump” for energy efficient heating of promising materials buildings It was found modifying CaCl and MgCl with LiCl enhances heat output • (during hydration) and moisture rate removal (during charging) system provided 6.8 kWh thermal energy output with a sorbent • The volume of 0.04 m (over 1200 min) energy density was 170 kWh m (for vermiculite-CaCl ) • The COP varied from 1 to 2.4 depending on the sorption material and • The the operating conditions Increasing the charging temperature reduces

the diameter of perforated tubes in Gen3 • Optimize • prototype

scale application)

two reactors; Gen2 (from previous works • Compare [101]) and Gen3. (Gen3 is the 3 kWh design for large

Objectives

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Journal of Energy Storage 27 (2020) 101145

13

SWEAT prototype for cooling

Na2S•5H2O

of a system of appropriate scale for • Development industrial processes

of suspended or fixed bed reactor with • Comparison stirring

Development of the prototype

Feasibility of prototype

Testing of material in a 1 kWh sorption thermal energy prototype

Testing of prototype using honeycomb heat exchanger

Objectives

Closed system Integrated reactor 24 L of sorbent

Lab-scale Closed system

Zeolite Ca-X and 15 wt% CaCl2

Zeolite 13X, activated alumina, ENG-TSA and MgSO4

• Development of composites in a closed system

of an open adsorption system with a • • Development moving material bed • Gravity-driven reactor Open system

1.5 and 750 L lab reactor

Zeolites

Composite of attapulgite and poolkohl with 30% CaCl2

Closed system Integrated reactor Fixed bed or stirred bed

Zeolites CaCl2

Closed system Modular reactor

Space heating 17 L of salt Open system

Closed system

Fixed bed reactor with finned tube exchangers

MgCl2•6H2O

ENG-TSA and SrBr2•6H2O

Honeycomb heat exchanger

SrBr2•6H2O

Closed system

Application/ System type

Material

Table 3 (continued)

cooling

−3

−3

−3

−3

−3

4

4

4

−3

Xu et al. [77]

Nonnen et al. [121]

Lass-Seyoum et al. [139]

Zondag et al. [138]

De Boer et al. [137]

Zondag et al. [102]

Zhao et al. [51]

Fopah-Lele et al. [99]

Ref

(continued on next page)

• A heat storage density of values up to 260 kWh m was achieved was enhanced by adding a prereactor and rotary gate valve for • Reactor material transport research shows that a moving bed is feasible for • This heat storage • long-term • mThe experimental energy density of MgSO -zeolite 13 X is 123.4 kWh The experimental energy density of MgSO and activated alumina is • 82.6 kWh m The hydration characteristics of MgSO can solve the issue of leakage in • open systems

• An average power of 1.2 kW was achieved for at least 5 h

use of the honeycomb heat exchanger enhanced heat and mass • The transfer closed system was able to give back 75% of initial thermal waste • The energy system had a storage capacity of 65 kWh 13 cycles were tested at • The low temperature conditions (< 100 °C) and were proven stable energy density of 213 kWh m for reaction bed • An promising results, further improvements must be made in • Although order for this system to become competitive winter seasons, the charging temperature is 80 °C and the discharging • Intemperature is 35 °C heat storage capacity can reach 1.02 kWh • The • The energy density of the prototype was 67.4 kWh m practical heat storage capacity is 59% of the theoretical value • The influence of evaporation temperature on heat storage performance • The is greater than that of the condensing temperature can produce heat for over 40 h, an effective energy density of • Bed approximately 138 kWh m to heat losses the power transferred is only 50 W • Due lift of 14 °C from 50 to 64 °C • Temperature pressure drop of 100 Pa over the bed and 220 Pa over the entire • Asystem pressure drop over the bed increases during hydration, a result of • The the swelling of salt power varies from 200 to 1000 W dependent on the supplied • Prototype inlet temperature store capacity: 2.1 kWh • Cold 0.57 (85% of the theoretical maximum value) • COP is feasible when there is a heat source to charge system • Application during night-time and a cooling demand present during day-time transfer inside the reactor is improved with stirring • Heat transport improved from 8 min to 1 min with stirring • Heat hydration and more compact using reactor with suspension • Faster of suspension is an issue • Stability new heat exchanger (parallel copper plate) configuration which • Aincreased heat power rate by 60%

Results

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Cuypers et al. [140]

Xu et al. [76]

7. Overview of salt hydrate based prototypes To develop a commercial TCES system, lab-scale and commercial sized prototypes are required. As aforementioned, the major limitations to commercializing TCES are salt hydrates’ properties; low thermal conductivity, instability and mass transfer limitations [39,79]. Thus, researchers are using composite materials to reduce these limitations [47,48]. This is evident in the successful prototypes, in which most investigations employ composite materials. It is important to note, as stated by Fumey et al. [30], volumetric energy density is not the only parameter to compare different systems with, as this varies with operating temperature. Different reactors have been designed and tested for various applications, which are iterated in Table 3. The pioneering study of the prototype, SOLARSTORE [61,62]; a 60 kWh m−3 closed system, modular reactor using the composite material expanded natural graphite (ENG) and SrBr2 was successful. Being a closed system, the major limitation was the heat transfer between the heat exchanger wall and the material. A life cycle analysis (LCA) methodology was used to assess the environmental impacts of the SOLARSTORE prototype [141]. It was found that raw material acquisition and the manufacturing of components contribute to 99% of the overall environmental impact [141]. The greenhouse gas emissions (CO2, SO2, C2H4 and phosphate) of the TCES were compared with a conventional solar heating system and a traditional fossil fuel heating system. It was found that SOLARSTORE significantly reduced negative environmental impacts by using solar energy effectively [141]. The labscale device tested by Fopah-Lele et al. [99] improved on the previous work by Lahmidi et al. [61,62] using the salt SrBr2•6H2O. The honeycomb design solved the agglomeration and melting issues and thus the pure salt, SrBr2•6H2O was able to be cycled. Whilst Lahmidi et al. [61], required 268 kg of the composite material in order to achieve a reactor energy density of 60 kWh m−3, Fopah-Lele et al. [99] required only 1 kg of pure material to achieve a reactor energy density of 65 kWh m−3. Currently, the “open-sorption pipe” prototype appeasr to be the most promising with regards to energy density. The “open-sorption pipe” is a unique concept consisting of an outer and inner cylindrical shell with the material filled between, as shown in Fig. 10 [133]. The inlet air (which flows through the inner shell) uniformly diffuses through holes in a perforated pipe. The “open-sorption pipe” was tested under a closed configuration using a vermiculite-CaCl2 composite. During hydration, the inlet conditions were 21 °C and RH of 75%, in which there was a temperature lift of >30 °C [133]. It was determined that the absolute humidity of the inlet air should be greater than 12 gH2O/kgDA, which cannot be achieved unless the ambient temperature is above 18 °C. This suggests that preheating is needed before the air can be humidified, a downside to using an open system. The results showed that intermittent use was possible and continuous operation was not necessary; however, this leads to higher heat loss to the environment [133]. Casey et al. [101] had similar results, in which there was a direct and linear correlation between the amount of water vapor supplied to the material and the temperature lift. As a consequence, a low inlet air temperature limits the water vapor that can be added to the air. It is suggested that circulating some of the output heat during the discharging process to preheat inlet air would improve performance [101]. Furthermore, considerations should be made to using this reactor design in a closed system, as this may improve some of the issues, such as controlling inlet air temperature and humidity. The modular concept investigated by Michel et al. [136] using SrBr2•6H2O is a promising approach to solving TCES challenges. This prototype consisted of 8 modules filled with the TCM, in which each module had an optimized bed thickness of 7.5 cm for energy density and mass transfer, as shown in Fig. 11 [136]. The reactor had an overall bed energy density of 388 kWh m−3, with the entire system having an

Closed system

Copper heat exchangers Zeolite

of closed system reactor for use in • Development dwellings and offices

hydration ability of the composites decreased when the air • The temperature was above 50 °C air temperature lift increased with increase in flow rate or RH • The cycle stability and a high energy efficiency of 81.34% • Good performance stabilised after 5 cycles • The output power and shorter reaction times were achieved by • High improving thermal and mass transport total yield of 60% of the theoretical value was • Areached • Concluded that zeolite will not likely be used in commercial reactor • systems due to price and the storage volume needed Lab-scale Open system Zeolite and MgSO4

• Hydration of materials in macro-scale reactor

Application/ System type Material

Table 3 (continued)

Objectives

Results

Ref

modeling are needed in order to up-scale fluidized beds.

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Fig. 10. Operating principle of discharging and charging cycle using the “open sorption pipe"(133).

Fig. 11. The reactor prototype by Michel et al. [136] a) Vertical section, showing air flow paths. b) Photography of the prototype before closing it.

energy density of 203 kWh m−3 [136]. As seen with other salt hydrate prototypes, as the reaction advances the rate of reaction decreases. Seven cycles were performed, and it was found after the sixth cycle, the reaction stabilized [136]. As with other studies, there was a direct and linear relationship between the moist air mass flow rate and the power output from the reactor. This suggests that mass flow rate should be controlled to regulate the power output. Lastly, an intermittent operating mode did not affect the temperature profile or the thermal power output [136]; making this feasible for residential heating applications.

system size, dehydration and hydration temperature and humidity are other parameters vital to determining the system suitability to a particular application or climate [64]. System designs will perform differently depending on atmospheric temperature and solar radiation. Whilst a system may not perform well in one climate, this cannot be generalized to all climates. Thus, prototypes should not be disregarded based purely on the local climate. At the system level, modular or separate reactors is where research is progressing. This is due to the better mass transport achievable in open systems, with smaller pressure drops [64] and an improvement in heat transfer in closed systems. Modular reactors improve overall energy and exergy efficiencies. There is little research into separate reactor prototypes, due to either the initial capital cost of this system or that the focus of research is still at a material level. However, a separate reactor with an efficient transport system would increase system performance [64]. Although thermochemical storage technology is gaining traction, there are few reports on a large system scale. First and foremost, the

8. Prototype performance, conclusion and future works As reviewed, the feasibility of thermochemical energy storage systems has been shown through small prototypes. At the material level, wherever composite materials are used there is an improvement in performance, which is a direct result of the instability of salt hydrates [64]. While energy storage is a good indication of performance, it is only one measure. Thermal power, thermal losses, material stability, 15

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challenges regarding design must be addressed. Importantly, developing larger reactors may pose new issues such as providing uniform air flow through a fixed-bed reactor. New types of reactors such as fluidized, moving or gravity-assisted reactors may improve heat and mass transfer [11,94]. This technology is new at both lab and prototype testing and still needs further research to make it commercially feasible and attractive to the consumer. The focus of future research should include:

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• At a material level, developing a composite material that allows • •

vapor diffusion but does not result in salt leaking out while still achieving a high energy density is of great importance. The feasibility of new reactor designs; moving, gravity-assisted and fluidized, need to be proven via experiments and modeling. Further modeling of the heat and mass transfer processes need to be developed; to improve understanding and optimize systems, understand and prevent agglomeration issues and explore the problems with systems operating under ambient conditions.

Overall, although fixed bed reactors have received significant attention, the design must be improved before this technology can be implemented into a residential setting. This leaves fluidized or moving bed reactors as attractive research prospects. Until further improvements are made, short-run sensible heat storage will continue to dominate the market due to reliability and cost-effectiveness. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to thank the Todd Foundation and the Energy Education Trust of New Zealand (EETNZ) for their financial support and Dr. Jane Casey for proofreading this article. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.est.2019.101145. References [1] Y. Saheb, Modernising building energy codes to secure our global energy future, IEA Policy Pathway Ser. (2011). [2] M. Gaeini, H.A. Zondag, C.C.M Rindt, Effect of kinetics on the thermal performance of a sorption heat storage reactor, Appl. Therm. Eng. 102 (2016) 520–531 Jun 5. [3] D. Aydin, S.P. Casey, S. Riffat, The latest advancements on thermochemical heat storage systems, Renew. Sustain. Energy Rev. 41 (January) (2015) 356–367. [4] S. Katulić, M. Čehil, B. Ž, A novel method for finding the optimal heat storage tank capacity for a cogeneration power plant, Appl. Therm. Eng. 65 (1–2) (2014) 530–538. [5] F. Kuznik, K. Johannes, C. Obrecht, D. David, A review on recent developments in physisorption thermal energy storage for building applications, Renew. Sustain. Energy Rev. 94 (2018) 576–586. [6] P. Tatsidjodoung, N. Le Pierrès, L. Luo, A review of potential materials for thermal energy storage in building applications, Renew. Sustain. Energy Rev. 18 (2013) 327–349. [7] G. Ervin, Solar heat storage using chemical reactions, J. Solid State Chem. 22 (1) (1977) 51–61. [8] P. Pardo, A. Deydier, Z. Anxionnaz-Minvielle, S. Rougé, M. Cabassud, P. Cognet, A review on high temperature thermochemical heat energy storage, Renew. Sustain. Energy Rev. 32 (2014) 591–610. [9] H.P. Garg, S.C. Mullick, V.K. Bhargava, Solar Thermal Energy Storage, Springer Science & Business Media, 2012. [10] V. Palomba, A. Frazzica, Recent advancements in sorption technology for solar thermal energy storage applications, Sol. Energy (2018). [11] A. Solé, I. Martorell, L.F. Cabeza, State of the art on gas–solid thermochemical energy storage systems and reactors for building applications, Renew. Sustain.

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