New salt hydrate composite for low-grade thermal energy storage

New salt hydrate composite for low-grade thermal energy storage

Accepted Manuscript New salt hydrate composite for low-grade thermal energy storage Abbas Mehrabadi, Mohammed Farid PII: S0360-5442(18)31729-8 DOI:...

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Accepted Manuscript New salt hydrate composite for low-grade thermal energy storage

Abbas Mehrabadi, Mohammed Farid PII:





EGY 13668

To appear in:


Received Date:

20 April 2018

Accepted Date:

26 August 2018

Please cite this article as: Abbas Mehrabadi, Mohammed Farid, New salt hydrate composite for lowgrade thermal energy storage, Energy (2018), doi: 10.1016/

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New salt hydrate composite for low-grade thermal energy storage Abbas Mehrabadi, Mohammed Farid Department of Chemical and Materials Engineering, University of Auckland, New Zealand *Corresponding author: [email protected]

Abstract This study aims to develop a new salt-based thermochemical composite for long-term storage of lowgrade thermal energy which enables overcoming mismatch between energy demand and supply. The energy density and dehydration behavior of five different salts; Al2(SO4)3.18H2O and MgSO4.7H2O, CaCl2.6H2O, MgCl2.6H2O, and SrCl2.6H2O are examined. Subsequently, the performance of two low cost host porous structures; expanded clay and pumice, impregnated with the most suitable salt for storing low-grade thermal energy is studied over a few number of cycles using a lab-scale packed bed reactor. The results showed that SrCl2.6H2O has the highest energy density and lowest dehydration temperature so that >80% of its energy density can be stored at <90ºC. Thermal cycling the composite materials revealed that up to 87 kWh/m3 and 22 kWh/m3 energy can be stored using expanded claySrCl2 (40 wt%) and pumice-SrCl2 (14 wt%), respectively. However, the performance of expanded clay dropped sharply over four cycles while the generated power using pumice composite was sustained almost constant over ten cycles. Although pumice-SrCl2 is a promising composite in terms of cyclability, further research is required to improve its energy storage capacity to make it attractive for large scale applications.

Keywords: Solar energy, Energy storage, Salt hydrate, SrCl2, Pumice, Expanded clay


ACCEPTED MANUSCRIPT 1. Introduction Using low-grade thermal energy sources such as solar and waste heat have become crucial due to increased concerns about global warming and reduction of fossil fuel resources. Although there has been significant technological development on using such energy sources, owing to the mismatch between their generation and consumption, they are yet to be fully developed and implemented globally [1, 2]. For example, while solar energy is one of the most promising renewable energy sources, its time distribution year-round is not in favor of the need [3]. In such a case, development of an efficient storage system which allows to store low-grade thermal energy and supply it on demand would be a possible solution [4]. It would improve the mismatch between energy supply and demand, and guarantee energy security. Of different energy storage techniques, using reversible chemical reactions have been highlighted as a promising method for efficient long-term energy storage without heavy insulation requirement [5-7]. There are four different types of reversible reactions; 1) gas-gas, 2) liquid-gas, 3) liquid-liquid and 4) gas-solid, which can be employed for such purpose [812]. However, solid-gas reversible reactions with high energy density (400-800 kWh/m3) and low regeneration temperature (<200ºC) have been found more promising for storing low-grade thermal energy [13-16]. Upon heating the salt hydrates, the water molecules are driven off from the salt structure through an endothermic reaction to form anhydrous or partially dehydrated salt. The stored energy can be released through an exothermic reaction between water vapour and the anhydrous/partially dehydrated salt [17]. The low-temperature energy storage potential of several salt hydrates has been investigated at different scales, from mg to kg level. Using TGA-DSC, the results have shown that sulphates and chlorides are the most promising salts with regards to energy density, safety and availability. In addition, their dehydration (regeneration) temperatures are found compatible with delivery temperature of solar collectors as a source of low-grade heat [13, 17, 18]. However, at lab- and pilot-scale most of the studies were not successful due to the slow kinetics, corrosive properties and melting/agglomeration of the salt grains. Chlorides are hygroscopic and hence tend to form a gel-like layer even at low relative humidity during the discharging period [16, 19, 20]. Sulphates also agglomerate and form a hard crust skin on the bed surface during hydration which prevents further hydration of the inner layers [21-23]. To overcome the problem, the salt grains can be confined in a porous structure. The porous host matrix offers a large adsorption surface area which can profoundly improve water vapour sorption rate, and provide better mass and heat transfer [24, 25]. In such a case, having a more porous host is 2

ACCEPTED MANUSCRIPT recommended since more salt can be impregnated into the host matrix which potentially enhances energy storage density. Adsorbents such as zeolites and silica gel are the main porous structures that have been investigated as salt host matrices [3, 26, 27]. Hongois, Kuznik [27] conducted lab-scale (200 g) experiments to examine the potential of zeolite-MgSO4 (15 wt%) as a storage material. They achieved energy density of 166 kWh/(m3 of composite) which was much lower than the energy density of MgSO4.7H2O (476 kWh/m3) and only slightly higher than that of zeolite 13X (131 kWh/m3). Similar results have been reported by Whiting et al. (2013 & 2014) where they compared the performance of a combination of four different zeolites (Na-X, Na-Y, H-Y, MOR) and two salts (MgCl2 and MgSO4) [23, 28]. Overall, Na-Y-MgCl2 (15wt%) had the highest energy storage density (325 Wh/kg composite) and was considered as a promising candidate. However, comparing the storage density of the composite and Na-Y zeolite alone (390 Wh/kg) [29, 30] revealed that using salt negatively affects the energy storage density of the adsorbent itself. This could be the case for all materials that are used as an adsorbent which implies the inappropriate use of such materials as a salt host matrix for thermochemical energy storage. Liu, Nagano [31] developed a new composite material made of non-adsorbent Wakkanai siliceous shale (WSS) and lithium chloride (9.6 wt%). They achieved a stable volumetric heat storage capacity of 50 kWh per unit volume (m3) of composite material over a numbers of cycles at regeneration temperature of 80 ºC, which is far from the energy storage capacity of imprignated salt (486 kWh/m3) [13, 31, 32]. The lower power generation mainly resulted from occupying a major portion of the volume by non-reactive material (host matrix). In addition, the salt content of the composite was low which can be due to either sub-optimum impregnation process or low porosity of the host matrix. In order to succeed in development of an efficient system for storing low-grade, long-term thermal energy, two main factors need to be considered: 1) selection of a proper salt with less melting/agglomeration behavior as well as high energy density and low dehydration temperature, 2) finding or synthesizing a proper non-adsorbent porous structure. In this study, five different salts including two sulphates (Al2(SO4)3.18H2O and MgSO4.7H2O) and three chlorides (CaCl2.6H2O, MgCl2.6H2O, and SrCl2.6H2O) are examined and compared in terms of energy density and hydration/dehydration behaviour using TGA-DSC. Subsequently, the performance of two different non-adsorbent host matrices including expanded clay and pumice impregnated with the selected salt are tested experimentally using a lab-scale packed bed reactor. 3

ACCEPTED MANUSCRIPT 2. Materials and methods 2.1.

Salt selection

To select the most promising salt in terms of dehydration (regeneration) temperature as well as energy storage density, the above-mentioned salts were tested using Shimadzu TGA50 apparatus and DSC Shimadzu-60 apparatus. In both TGA and DSC, a known mass (5–10 mg) of hydrated salt powder/grain was placed in an aluminium crucible. The sample in both apparatuses was heated from room temperature to 300 °C at 1 °C/min at atmospheric pressure while the chamber was purged continuously with pure argon (50 ml/min). To investigate the dehydration behaviour of salts, their weight loss was plotted as a function of temperature over

measure°C 24 isotherms physisorption treatment sulfuric Allhand outgassed ment. the before nitrogen composites samples before Fig. and 1acid. mbar with of at 3after were shows 120 for

the heating period, and the total heat of reaction (energy density) was determined by integration of the heat flow curve of DSC. 2.2.

Composite preparation

After salt selection, the composite materials were prepared by impregnating saturated solution of the salt at room temperature in expanded clay (̴ 3-5 mm) and NZ pumice (̴ 3-5 mm) (Fig. 1). A known quantity of the host structures was immersed into the solution in a glass container and impregnated for 15 min under vacuum conditions (100 kpa vacuum). Subsequently, the mixture was filtered and the composite particles were placed in a glass petri dish. To dry off the unbounded water and prevent salt leakage due to vigorous heating during the drying process which would result in surface coating/blockage, the particles were dried undergoing stepwise heating programme, overnight heating at 50ºC followed by fully dehydration at 140 ºC in an oven until the composite materials reach a constant weight. The composite salt content was determined gravimetrically by subtracting the host matrix weight before and after impregnation.



Fig. 1 The two host materials (before impregnation) used in this study: a) expanded clay and b) pumice. Scale bars equal to one centimetre


Physical properties of the composites

The physical properties of the composites and host matrixes were determined based on standard methods. The bulk density of the materials was measured according to the method developed by Webb [33]. The surface areas and pore volume were measured based on the standard nitrogen adsorption/desorption isotherm using a Micromeritics 3Flex instrument (Table 1). The material was vacuum outgassed at 300 ºC overnight and the nitrogen sorption measurement was performed at -196 ᵒC. The surface area was then calculated according to Brunauer–Emmett–Teller (BET) method at adsorption partial pressure range of 0-0.3 p/p0. The Barrett–Joyner–Halenda (BJH) approach was employed for pore volume measurement. To examine the mechanical strength of host and composite materials, the material hardness was determined using micro-hardness tester (Struers, Emcotest). The materials were mounted in resin, grinded and polished before hardness measurement. Table 1. Physical properties of porous host matrices and composite materials Composite / host material

Salt content

Surface area

Pore volume

Bulk density






strength (100 g load)

Expanded clay






Expanded clay-





















Experimental set-up

To examine performance of composites, a lab-scale open reactor (Fig. 2) was designed and constructed (25 cm length and 1.5 cm diameter). The thermal energy storage system was operated at atmospheric pressure and used for both energy storage and energy recovery. The reactor consisted of a data logger and a computer, two temperature-humidity sensors (SHT71 with relative humidity accuracy of ±3% and temperature accuracy of ±0.4 ⁰C) placed at inlet and outlet of the reactor, gas distributor, water tank to humidify the air during the hydration step, and a heater to provide hot air over dehydration step.

Fig. 2 Schematic of open thermal energy storage set-up, 1) Datalog system, 2) temperature-humidity sensors, 3) composite particles, 4) gas distributor, 5) water tank and, 6) air heater

The reactor was filled with a known quantity of the composite materials, and each composite underwent up to 10 hydration/dehydration cycles with an air flowrate of 15 L/min. The temperature and humidity data were logged every two second. The materials were dehydrated using hot air at 110 ºC until the difference between inlet and outlet humidity reduced to <2% which considers as a proxy for no further effective evaporation and 6

ACCEPTED MANUSCRIPT dehydration. The materials were then cooled down to the hydration temperature. The humid air at 20 ºC was bubbled through a water tank before flowing into the reactor to recover the energy stored via the exothermic reaction of moisture adsorption from the air. It has been shown that the dissolution of salt in a porous structure can occur at relative humidity higher than deliquescence relative humidity (DRH) [34]. Salt solution formation would prevent further vapour diffusion into the pores and hence, reduce the system performance with cycling. In addition, it may affect the overall heat generation as the enthalpy of dissolution may be either exothermic or endothermic. Hence to prevent such negative effects during hydration, the air relative humidity was not increased above the DRH=71% for selected salt [35]. The relative humidity was controlled by changing the level of water (gas residence time in water) in the water tank. The hydration continued until the difference between the inlet and outlet air temperature reduced to less than 5ºC. To account for the energy storage density contributed by the salt alone, the adsorption capacity of host matrixes was measured before impregnation using TGA and lab-scale open reactor which was negligible. 2.5.

Statistical analysis

Statistical analyses were performed whenever applicable using analysis of variance (ANOVA) in Excel software (Excel, Microsoft office 2010). 3. Results and discussion 3.1.

Promising salt

Dehydration temperature and thermal energy storage density are the most important properties in the selection of any salt for use in thermochemical energy storage system. The lower dehydration temperature, the lower thermal power required for dehydration, which is preferable. The higher energy density would result in smaller storage volume which is highly essential where space is limited. The dehydration temperature and energy density of the salts studied in this paper, are summarised in Table 2. Under dehydration conditions, heating from 25 ºC to 300 ºC at 1 ºC/min, the number of consecutive dehydration steps and the numbers of moles of water lost at each dehydration step are different from one salt to another . However, the majority of water loss for all the salts occurred at <150 ºC which was in agreement with the literature [13]. Hence, all five salts have the capability of storing the low-grade thermal energies such as solar energy and waste heat. 7

ACCEPTED MANUSCRIPT Regardless of dehydration temperature, 65 kJ energy is needed, on average, to drive off one mole of water from the salt structure [13]. Hence, the more water is released at a lower temperature, the higher energy is stored. In general, chlorides has a lower regeneration temperature and can adsorb more moles of water compared to sulphates at a specific temperature. Among five different salts, SrCl2.6H2O had the lowest dehydration temperatures and lost more than 80% of its water (almost 5 moles of water) below 90ºC, which is highly favourable. While the dehydration extent of all other salts are much lower (<60%) below 90 ºC, requiring a dehydrated temperature above130 ºC. The results are in a good agreement with literature [13, 23, 34]. Having a low dehydration temperature is highly important especially when chlorides are used. Since if chloride is dehydrated at above 150 ºC, it decomposes and the Cl would react with the structural water and form HCl [36]. As a result, over a number of cycles, the active material (salt) would reduce and thereby the system performance would decline. Also, due to the toxicity and corrosion potential of the HCl, proper ventilation needs to be applied. Table 2. Dehydration steps and energy storage density of various salts determined at mg level using TGA Salt Dehydration step Completed Dehydration Deliquescence Dehydration level (%) RH (DRH%) temperature at 25 ᵒC [35] (ºC) SrCl2.6H2O SrCl2.2.2H2O + 3.8 62 63.3 H2O SrCl2.6H2O 71 SrCl2.2.2H2O SrCl2.1.1H2O + 86 18.3 1.1 H2O SrCl2.1.1H2O SrCl2 + 1.1 H2O 128 18.3 CaCl2.6H2O



CaCl2.6H2O H2O CaCl2.3.6H2O 1.7 H2O CaCl2.1.9H2O 1.8 H2O MgSO4.7H2O 0.5 H2O MgSO4.6.5H2O + 3.3 H2O MgSO4.3.2H2O + 1.7 H2O MgSO4.1.5H2O H2O MgCl2.6H2O 1.8 H2O MgCl2.4.2H2O 2.5 H2O MgCl2.1.7H2O 1.1 H2O MgCl2.0.6H2O

CaCl2.3.6H2O + 2.4



CaCl2.1.9H2O +



CaCl2.0.1H2O +



MgSO4.6.5H2O +









MgSO4 + 1.5





MgCl2.1.7H2O +



MgCl2.0.6H2O +



MgCl2 + 0.6 H2O



MgCl2.4.2H2O +


Energy density (kWh/m3 of hydrated salt) 667

583 30

583 90

555 33


Al2(SO4)3.18 H2O

Al2(SO4)3.18H2O Al2(SO4)3.15.4H2O + 2.6 H2O Al2(SO4)3.15.4H2O Al2(SO4)3.6.2H2O + 9.2 H2O Al2(SO4)3.6.2H2O Al2(SO4)3 + 6.2 H2O








Regarding energy density, SrCl2.6H2O has the highest volumetric energy storage density (2.4 GJ/m3) while Al2(SO4)3.18H2O has the lowest storage density (1.6 GJ/m3). It is noteworthy to mentioned that the measured energy storage densities in this work were in agreement with the reported values (>65% of the theoretical values). The high energy storage density of SrCl2.6H2O further ensures its potential as a promising thermochemical energy storage material, especially at low temperatures. Although undoubtedly all the salts can provide high energy storage density which confirm their potential as thermochemical heat storage materials, SrCl2.6H2O is offering superior heat storage capacity at a lower temperature. Moreover, its DRH (71%) is high enough and is compatible with the usual ambient relative humidity in a typical cold day in many places. This would facilitate using ambient air for recovering the stored energy without the need for humidification of the air. 3.2.

Promising host porous structure

3.2.1. Composites’ properties Although SrCl2 was the most promising salt, based on microscopic observation (Fig. 3), the salt grains tend to partially melt over the dehydration step at temperatures close to its melting point (61.3 ºC) [13] which is undesirable. Since, it could gradually diminish the porosity and permeability of a bed of salt [22] and prevent vapour penetration through the bed, which is vital to recover the stored heat.

Fig. 3 Microscopic picture of a) SrCl2.6H2O and b) SrCl2 (after dehydration). Scale bars equal to one millimetre


ACCEPTED MANUSCRIPT One possible solution to reduce the effect of melting and increase the vapour accessibility to the salt is to distribute the salt into a porous structure. Hence, after selection of the promising salt (i.e., SrCl2), two low-cost porous host matrices including expanded clay and NZ pumice were vacuum impregnated with a saturated solution of SrCl2 at room temperature (30wt% SrCl2). The salt content along with physical properties of the composites are summarised in Table 1. Although both porous structures were impregnated with similar solution, the actual salt mass fraction in expanded clay was almost three times higher than that in pumice due to the higher porosity of the expanded clay. As expected, the pore volume and surface area reduced after impregnation which was most probably due to the pore occupation/blockage by salt. As stated earlier, the truly reversible thermal energy storage density of the composite materials highly relies on the pore volume and remained surface area after impregnation. The impregnation led to a higher drop in the pore volume of pumice (by 20%) and less drop in expanded clay (by 14.6%). Hence, expanded claybased composite should offer a higher storage capacity owing to the larger pore volume and hence salt content as well as experiencing lower reduction of pore volume after impregnation. The variation in surface area is a proxy of coating degree of the pore walls by salt. The lower surface area reduction over impregnation, the better pore wall coating and thereby the higher surface area is available for water adsorption and energy release. The results showed that the surface area reduction for expanded clay composite was 28% while it was >60% for pumice (Table 1). This was possibly due to a larger pore size of the expanded clay and therefore, a better dissemination/homogeneity of the salt over the expanded clay surface. Overall, the results proved again that as the greater surface area retained in expanded clay after impregnation by SrCl2, this type of composite would probably have higher energy storage capacity. 3.2.2. Hydration/dehydration behaviour of the composites The humidity and temperature changes of the air while passing through the composite materials during hydration and dehydration steps are illustrated in Fig. 4. Overall, the hydration period was longer than that of dehydration for both composites (Fig. 4) which was in agreement with the literature [37]. The results showed that hydration time is longer independent of all influencing parameters, including particle size, air humidity, reactor configuration, operational mode and type of salt. This is due to mass transfer limitation resulted from slow water vapour diffusion into the pores during hydration [28].


ACCEPTED MANUSCRIPT As shown in Fig. 4, at the beginning of hydration, while the composite materials were completely dehydrated, the air humidity dropped sharply translating into high sorption rate. This is also supported by the observed high and fast air temperature lift when using both composites. As the process progressed, the adsorption rate reduced (Fig. 5). Hence, less energy is released and the outlet air temperature decreased (Fig. 4). At the very last stage of hydration, the moisture adsorption rate did not drop to zero, however, the quantity of energy produced was not high enough to heat the air. The maximum air temperature lift occurred in the first 3 min (̴ 9 ºC) for the expanded clay composite while it happened after 8 min (10 ºC) for pumice composite. The difference was due to the variation in the physical properties of the two host materials. The pore volume (a proxy of water accessibility) and the surface area (a proxy of available surface for adsorption) were higher for expanded clay composite (Table 1) which facilitated water adsorption. It was in line with the faster drop in the outlet air humidity data (Fig. 4) using the expanded clay which means more water was adsorbed and higher amount of energy was released at a shorter time. a.

Expanded clay-SrCl2 (40 wt%)

b. Pumice-SrCl2 (14 wt%)

Fig. 4 An example of airflow temperature and relative humidity variations over the hydration and dehydration of a) expanded clay composite and b) pumice composite


ACCEPTED MANUSCRIPT The dehydration of both composites occurred at sequential steps similar to dehydration behaviour of the salt (Table 2) which implies that impregnation did not affect salt dehydration behaviour. The result is in contrast with what has been reported for impregnated zeolite. Hongois, Kuznik [27] showed that during the impregnation of MgSO4 into a bed of mesoporous zeolite (13X zeolite molecular sieve) Mg2+ ions are substituted by Na+ charge, balancing cations of zeolite. This resulted in dehydration of the composite at one step instead of dehydration at sequential steps. The substitution of two monovalent zeolite ions with bivalent salt ion may cause pore enlargement which could negatively affect the performance of highly hygroscopic salt with a low DRH [38]. In case of using highly hygroscopic salt such as CaCl2, as pores become larger, the salt may come out of the host structure through the formation of a solution during the hydration step.

Fig. 5 An example of the adsorption rate curves of expanded clay- and pumice-based composites over the hydration as a function of time

There was a significant difference between the inlet and outlet air temperatures at the end of the dehydration of pumice composite, and to a lower extent for expanded clay composite. Looking at the thermal properties of the two host matrices revealed that pumice has a higher heat capacity and lower thermal conductivity [39, 40]. Hence, expanded clay would reach thermal equilibrium faster than pumice composite. To evaluate hydration/dehydration efficiency, the quantity of adsorbed water was calculated by integration of the area under humidity curve. The amount of adsorbed water over hydration was similar to the amount of water desorbed during dehydration for both composites 12

ACCEPTED MANUSCRIPT (̴ 2 g for expanded clay and 1.6 g for pumice). The quantities were almost one-third of theoretical values for fully hydration of the impregnated salt. The lower moisture adsorption was most probably due to reduction of the moisture accessibility into the pores caused by pore blockage. The pore blockage typically occurs due to several reasons including small pore size of the host matrix, high salt concentration used for impregnation or drying of porous structure using vigorous heating rates [23, 28]. The first two possible reasons are not the case in this study as a low concentration aqueous solution was used which could easily penetrate into the pores under vacuum condition. Hence, the surface coating/blocking of porous structure occurred over drying of the impregnated host matrix is more likely the reason. In fact, during the drying process as water evaporates it generates pressure inside the pores and pushes the salt from the inner levels in the matrix to locations near the opening of the pores, and hence blocking them. The more vigorous heating the higher risk of salt movement towards the surface and consequently the higher risk of pore blockage. This is the main reason behind using stepwise drying in this study. Although, we have not seen any salt leakage, there is still risk of salt movement towards layers close to the surface which increases the risk of pore blockage

the be first accessvolumetric heat reorganization The hydration dehydration. assumed, changes five overall monolith small caused eexplained hydrate sulfate crystalline change can that sibility increases. structure intact, cracks observed, expansion magnesium hydration/dehydrat during tion. first magnesium the size cycles increase released even can by of during shape crystals. cycles during of of isand probably the by After the of domains be the and the still the though sulfate It of the after salt of can is the and prevents vapour water penetration into the in-depth pores. 3.3.

Power generation and cyclability of the composites

Reliable continuous power generation and stability of the composite material over a large number of cycles are the most important features of energy storage system. Higher power generation per unit volume would result into smaller system volume which is important especially where space is limited. The higher system stability would provide longer lifetime and mimimising composite replacement cost. In general, total thermal energy generated during hydration of the composite materials at RH
ACCEPTED MANUSCRIPT theoretical values. As explained earlier, the lower energy recovery efficiency was most probably due to the pore blockage and reduction of water accessibility over the hydration step. Overall, the storage density of tested composites was much lower than what has been reported for zeolite-based composites (100-170 kWh/m3) [23, 27]. The significant difference relates to the mechanisms involved in adsorption by zeolite-based composites. Energy storage in zeolite-based composites is a function of 1) binding and condensation of vapour in the saltfree pores, along with 2) water vapour adsorption in salt-occupied pores (chemical reaction energy) [31]. While neither expanded clay nor pumice were not adsorbent and hence their energy density was resulted only from chemical reactions which was related to the salt contribution. The amount of recovered energy was compatible with the salt contribution (up to 30%) in energy storage using zeolite-based composites [23, 27, 30]. a)



Fig. 6 Hydration power generation of the composite materials a) expanded clay-40wt% SrCl2 and b) Pumice14wt% SrCl2 over the cycles

In term of cyclability, the power output reduced over four cycles in expanded clay composite while it was almost constant for the pumice composite during 10 cycles. The performance loss over the cycling of expanded clay caused by material loss while it was not observed during cycling of pumice. In fact, the expanded clay structure was not mechanically strong enough (Table 1) to cope with the shrinking/swelling of the salt, and hence, its structure cracked (Fig. 7) and disintegrated after a few cycles. According to the density of anhydrous (3052 kg/m3) and hexahydrate (1930 kg/m3) of SrCl2, the salt would expand by 50% if it is hydrated completely. As a result of structural disintegration, a fraction of composite materials (30wt%) came out of the reactor over the first four cycles which resulted in lowering power generation. However, the power reduction was not consistent even if the test run in chronological order. In general, two phenomena are involved in expanded clay power changes, structure disintegration and salt loss. The structural disintegration increases as the number of cycle increases. Over the first cycle, a fraction of the salt was not accessible by air as it is situated at in-depth pores and hence the power output was low. In the following cycle as the structure was disintegrated, more salt exposed to humid air and hence the power output increased. While from second cycle afterwards a fraction of salt was lost by air flow and hence the power reduced. It is noteworthy to mention that the amount of salt loss in each cycle was not constant and therefore the storage density reduction was not constant and consistent. In 15

ACCEPTED MANUSCRIPT contrast, pumice was structurally strong enough, and the small variations in the performance of pumice composite most probably caused by changes in inlet air relative humidity.

Fig. 7 Crack formation in the structure of expanded clay composite after second cycle

4. Implementation Storing long-term low-grade thermal energy to overcome the mismatch between supply and demand as well as improving energy security becomes vital. Although there has been progress in using salt hydrates for this purpose, there are several practical and technical issues which need to be addressed yet. Fopah-Lele, Rohde [41] showed the possibility of achieving 65 kWh/m3 using 1 kg of SrBr2 in a honeycomb heat exchanger in a closed system. Although the result for 1kg of salt was promising, the system cost was high, and the power generation was lower than the commercial target (150 kWh/m3 of system unit [3]). Economic evaluations have indicated that to commercialize compact thermal energy storage system, the cost should drop to 2-4 Euro/kWh from the current values of 4-15 Euro/kWh [42]. To cope with cost and technical issues, using open system fed by free ambient moist air through impregnated porous structure has been highlighted [23, 24, 31]. The present study has demonstrated that impregnating a cheap porous structure although would solve the salt melting/agglomeration issue, it reduces the volumetric storage density. According to the results obtained from this work, an open reactor with the volume of 7 m3 is required to obtain 150 kWh (commercial 16

ACCEPTED MANUSCRIPT target) of storage using the pumice-SrCl2 (14wt%) composite. The system volume would be higher if the other system components (heater, fan/blower, control system, etc.) are also taken into account. This would negatively affect system market potential. However, as theoretical energy density of developed pumice composite is 50 kWh/m3, the commercialisation potential would increase significantly if it is fully captured. Hence, to further improve the system performance, study in two specific area is needed; 1) synthesising a non-adsorbent, mechanically strong and highly porous matrix, and 2) improve the salt content in the composite for higher energy density. In addition, using smaller particles porous particles is recommended to avoid difficulties in water vapour diffusion into the deep pores. 5. Conclusions Using salt hydrates to store low-grade thermal energy could help to overcome the gap between energy supply and demand. In this study, a new composite material was developed by screening of 5 different salt hydrates; Al2(SO4)3.18H2O and MgSO4.7H2O CaCl2.6H2O, MgCl2.6H2O and SrCl2.6H2O, and two low cost porous structures; expanded clay and pumice.  TGA-DSC results showed that SrCl2.6H2O was the most promising salt based on energy density and the requirement for low dehydration temperature (<130 ºC). Nearly 5 moles of water (corresponded to > 80% of energy storage capacity) can be driven off at 90ºC which could be easily achieved using solar energy.  Using lab-scale open packed bed reactor, the storage density of expanded clay-SrCl2 (40 wt%) composite was 87 kWh/(m3 composite) and the storage capacity of pumiceSrCl2 (14 wt%) composite was only 22 kWh/(m3 composite).  The expanded clay composite lose its performance over four cycles due to salt loss resulted from its weak mechanical structure. Although pumice-SrCl2 showed consistent performance with no material loss over the cycling, its power generation was low translating to a larger system volume. The low power generation was caused by the low composite salt content and pore blockage by salt. Hence, further research is needed on the improvement of composite salt content and reducing pore blockage.

Acknowledgement The authors would like to thank the Energy Education Trust of New Zealand (EETNZ) for the financial support of the project. 17

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ACCEPTED MANUSCRIPT Highlights 1- The potential of five different salts and two porous structure are studied 2- SrCl2 has the highest energy storage capacity and lowest dehydration temperature. 3- 22 kWh/m3 of pumice-SrCl2 composite thermal energy can be stored