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Study of the reversible water vapour sorption process of MgSO4.7H2O and MgCl2.6H2O under the conditions of seasonal solar heat storage

The characterization of the structural, compositional and thermodynamic properties of MgSO4.7H2O and MgCl2.6H2O has been done using in-situ X-ray Diffraction and thermal analyses (TG/DSC) under practical conditions for seasonal heat storage (Tmax=150°C, p(H2O)=13 mbar). This study showed that these two materials release heat after a dehydration/hydration cycle with energy densities of 0.38 GJ/m3 for MgSO4.7H2O and 0.71 GJ/m3 MgCl2.6H2O. The low heat release found for MgSO4.7H2O is mainly attributed to the amorphization of the material during the dehydration performed at 13 mbar which reduces its sorption capacity during the rehydration. MgCl2.6H2O presents a high energy density which makes this material interesting for seasonal heat storage in domestic applications. This material would be able to fulfil the winter heat demand of a passive house estimated at 6 GJ with a packed bed reactor of 8.5 m3. However, a seasonal heat storage system built with this material should be carefully set with a restricted temperature at 40°C for the hydration reaction to avoid the liquefaction of the material at lower temperature which limits its performances for long term storage.


Introduction
The main part of the residential energy demand in Europe consists of space heating and domestic water heating. During summer, the available solar energy exceeds the domestic water heating demand for houses that are equipped with solar thermal collectors. However, during winter the total heating demand exceeds the solar supply. This mismatch can be solved by harvesting solar energy in summer to fulfil the heat demand in winter by means of a seasonal heat storage system. A passive house of 110 m 2 with a winter heat demand estimated at 6 GJ, seasonal heat storage in a traditional water tank would require a tank volume above 40 m 3 which is much too large to be integrated in individual houses. Therefore, more compact heat storage technologies have to be found. ECN develops a compact heat storage system based on the reversible reaction of water vapour in salt hydrates. These thermochemical (TCM) materials present a high energy density in packed bed (~1 GJ/m 3 ) resulting in a relatively small storage volume (around 6 m 3 ), that can easily be integrated in a house. Additionally, many salt hydrates are available in large quantity at low cost, are environmentally friendly and can take up and release heat under the conditions of seasonal heat storage. During summer, the salt hydrate materials can be dehydrated by means of solar heat provided by solar thermal collectors with a temperature below 150°C. During winter, the packed bed is rehydrated with moist air, e.g. by means of the evaporation of water with low temperature heat from a borehole at 10°C, which defines the practical conditions for hydration in a TC seasonal storage system.
A screening of suitable materials has been performed in the last decade and several materials such as magnesium sulphate heptahydrate (MgSO 4 .7H 2 O) and magnesium chloride hexahydrate (MgCl 2 .6H 2 O) showed promising performances during tests in lab scale reactors under practical conditions of seasonal heat storage. However, their efficiency for long term storage can still be improved [1]. In order to develop an optimal sorption material for seasonal heat storage, ECN carried out a research project which aims at improving the heat storage system by designing a new optimized material with a high storage capacity, that remains stable over a lifetime of 30 years for the heat storage system. The first phase of the project is to identify the influence of the structural properties of these materials on the water vapour sorption process taking place during the dehydration and hydration reactions. To this end, powder samples of MgSO 4 .7H 2 O and MgCl 2 .6H 2 O have been studied under practical conditions by in-situ X-ray diffraction. In parallel, thermal analysis studies were performed on these two materials to identify the kinetics of reactions occurring during the water vapour sorption process. The present work reports the results obtained from these experiments.

Materials
Pro Analyse reagent powders of magnesium sulphate heptahydrate (VWR BDH Prolabo, CAS 10034-99-8, NORMAPUR, 99.5% pure) and magnesium chloride hexahydrate (Merck, CAS 7791-48-6, ≥ 99% pure) were used in this study. The powders were sieved prior to the experiments to obtain powders with different particle size ranges. Unless stated otherwise, particles with a particle size distribution ranging from 100 µm to 200 µm were used during the experiments. Powder samples of 10 mg were used in the different measurements performed in this study in order to avoid effects of layer thickness on vapour transport as shown in previous studies [2].

Settings of the practical conditions.
Each measurement in this study was carried out under well-controlled conditions for temperature and water vapour pressure, corresponding to practical conditions in the seasonal heat storage system. Depending on the type of characterization technique, the temperature was controlled either by local heating with hot-stage systems or trace heating with a furnace. Slow heating and cooling rates (0.2 -1°C/min) were used during the different measurements in order to obtain experimental conditions as close as possible to the equilibrium conditions in the water vapour sorption process. Additionally, the vapour pressure was controlled by means of a well-controlled humidification system, shown in figure 1. This system was designed in order to perform a parallel control of the water vapour pressure and of the flow velocity in the system. These parameters were settled at 13 mbar (+/-0.2 mbar) and at 100 ml/min (+/-2 ml/min).

X-Ray Powder Diffraction
In-situ X-ray powder diffraction (XRD) measurements were carried out to identify the variations in composition and crystal structure of the salt hydrates during their dehydration and hydration reactions. The XRD patterns were collected using a Bruker D8 Advance with a MRI oven with Cu Kα1+Kα2=1.5418 Å radiation. Typical runs were conducted with 2-theta ranging from 10 to 45 degrees using a 0.05 degree 2-theta step with a time step of 2 s. The experimental diffraction patterns were compared with known patterns of magnesium sulphate and magnesium chloride hydrates from the JCPDS-ICDD database [3]. The powder samples were placed on an alumina sample holder inside the oven and exposed to a moist air flow at 13 mbar vapour pressure controlled by the humidification system presented in section 2.2. The samples were dehydrated by increasing the temperature from 25°C to 150°C using a heating rate of 1°C/min. The powder samples were kept at 150°C for 3 h to stabilise the final product of the dehydration. Then, the dehydrated materials were rehydrated again by decreasing the temperature from 150°C to 25°C with a cooling rate of 1°C/min followed by a period of several hours at a constant temperature of 25°C. Diffraction patterns were recorded every 10°C in the dynamic modes (heating and cooling) and every 30 min in the isothermal modes.

Thermal analyses
Thermal analyses were performed to get information on the dehydration and hydration behaviour of the salt hydrate materials. Two thermal analysis techniques were used in this study: Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC) analyses. TG analysis involves the measurement of mass change as a function of temperature or time, and DSC involves the measurement of heat enthalpy changes as a function of temperature or time. The thermal analysis experiments described in this work were performed with a Simultaneous Thermal Analyses apparatus Netzsch STA 409 PC Luxx with a standard SiC furnace, which performs TG and DSC measurements simultaneously. Both dehydration and hydration measurements were carried out at atmospheric pressure (1 atm) by exposing the 10 mg sample to the moist air flow as explained in section 2.2. The sample pans used during these experiments were aluminium cups of 25 µl without lids. The temperature program was adjusted for each material. For MgSO 4 .7H 2 O, the dehydration was carried out between 25°C and 150°C with a heating rate of 0.5°C/min and left for 3h hour at 150°C to stabilise the composition of the dehydrated material. Next, the material was rehydrated by a slow cooling at 0.5°C/min until 25°C and subsequently left in isothermal mode at 25°C for 100h. For MgCl 2 .6H 2 O, the dehydration was carried out with a heating rate of 0.5°C/min until 130°C. This temperature was chosen since previous measurements [4,5] showed the decomposition of MgCl 2 .2H 2 O above 130°C. The rehydration of the dehydrated material was performed with a cooling rate of 0.2°C/min until 40°C, after which the material was kept isothermal at this temperature to avoid overhydration (see section 3.2.1). The reaction enthalpy and energy density of the phase transition observed during the dehydration and hydration reactions for each salt hydrate were calculated to quantify the heat storage capacity of the material over a reversible sorption cycle.  [6,7]. This phenomenon has been attributed to a very slow rate of reaction when the dehydration is performed at 13 mbar [6]. The slow rate of reaction is related to a slow and inhomogeneous reorganization of the crystal structure in the dehydrated material, forming a disordered arrangement of the crystal constituents (atoms, molecules, ions). This molecular arrangement corresponds to an amorphous phase like the structure found for glass.  During the rehydration process, the molecular structure of the dehydrated material remains amorphous during the cooling of the system to 25°C and only starts to recrystallize during the isothermal step at 25°C as shown on the XRD pattern in figure 3. After 30 min of isothermal reaction, the characteristic peaks of the crystalline phase of MgSO 4 .6H 2 O appear and the intensities and the sharpness of these peaks slowly increase until 97 h of reaction. However, the phase of MgSO 4 .7H 2 O never appears during this measurement. This result is in agreement with the study performed by Chipera and al. [8] on the crystallization of magnesium sulphate hydrates, indicating that the formation of the crystalline phase of MgSO 4 .7H 2 O can only be formed if the ambient water vapour pressure is above 60 mbar. If the water vapour pressure is fixed at 13 mbar, as in seasonal heat storage systems, the phase of MgSO 4 .7H 2 O will never be formed again.

Thermal analyses.
The thermal analysis measurements (TG-DSC) of the dehydration and rehydration reactions of magnesium sulphate heptahydrate carried out under 13 mbar water vapour pressure are presented in figure 4 and figure 5. The data of the onset temperature, the reaction enthalpy, and the energy density of each phase transition observed in the dehydration and hydration reaction determined from the DSC measurements are summarized in the table 1.   (table 1), which is close to the ambient temperature. Therefore, during the storage and the preparation (sieving) of the material before the measurement, around 25% of the initial MgSO 4 .7H 2 O powder was dehydrated in MgSO 4 ·6H 2 O. The second endothermic peak of the dehydration corresponds to the dehydration of MgSO 4 .6H 2 O in an amorphous phase (XRD results in section 3.1.1). The TG result presented in figure 4 allows determining that the final composition of this amorphous phase at 150°C is a monohydrated magnesium sulphate. The reversible hydration process of the monohydrated compound begins when the material is cooled below 30°C as shown in figure 4. The rehydration reaction progresses very slowly in a single step during the isothermal process set at 25°C under a water vapour pressure of 13 mbar. After 90 h, the material is fully rehydrated to MgSO 4 .6H 2 O. However, it can be observed on the DSC signal in figure 5, that the rehydration of the material only releases heat significantly in the first ten hours of the reaction. Table 1 shows that the reversible reaction of MgSO 4 .6H 2 O into the amorphous monohydrated phase of magnesium sulphate proceeds at different temperatures during the dehydration and hydration process. However, with the slow rate of heating and cooling of 0.5°C/min applied to the system during this measurement, it can be assumed that either the reaction takes place under experimental conditions nearby the thermodynamic equilibrium or the reaction is extremely slow. Possibly, external parameters to the thermodynamic properties of the material influence the kinetics of the processes. The hysteresis found in this measurement may be influenced by a possible temperature difference between the surrounding gas and the material caused by the heat effect (exothermic/endothermic) of the reactions. O, which corresponds to 45% of the energy stored from the dehydration reaction of MgSO 4 .6H 2 O. Several explanations were proposed in the literature to explain this difference of heat quantified during the two processes. The first explanation may be attributed to the structural modification of the material when the crystalline phase of MgSO 4 .6H 2 O is dehydrated into the amorphous monohydrate. In a similar study carried out by Bevers and al. [9] on the reversible absorption process of ammonia into magnesium chloride, it was concluded that the amorphous state of the material is less capable of absorbing gas than a crystalline structure, resulting in a lower heat effect during the measurements. The heat difference may be also attributed to the textural modifications of the material (cracks, volume expansion) during the reversible water vapour sorption process [10]. During the hydration process, part of the stored heat may be converted in energy to modify the material structure when the water molecules are introduced again in the molecular structure. Additionally, the textural changes of the material will have a direct influence on the experimental conditions during the measurement. The salt may change in a more porous and expanded structure, resulting in a decrease of the contact area between the material and the crucible and lead to a lower recorded heat of transition. A last explanation attributed to a DSC measurement artefact was proposed to explain the heat difference between the dehydration and hydration process. The dehydration is an endothermic reaction where the heat taken up by the material is collected though the sensor of the DSC apparatus, while the hydration is an exothermic reaction where a part of the heat may also be transferred to the surrounding gas phase, reducing the part of the heat collected by the sensor. Although the first explanation proposed seems to be the most likely one for MgSO 4 .7H 2 O, the others explanations should not be ignored.
In term of energy density, magnesium sulphate heptahydrate presents an energy density of 0.76 GJ/m 3 at the end of the cycling test at 13 mbar for a bulk system. This means that for a packed bed reactor of salt hydrate designed for seasonal heat storage with around 50% of porosity [11], magnesium sulphate heptahydrate would release only 0.38 GJ/m 3 . It can be concluded from this work, that the energy density of magnesium sulphate heptahydrate is too low to fulfil the winter heat demand of an individual house by seasonal heat storage with a 6 m 3 packed bed reactor. A minimum volume of 16 m 3 would be required for a system set up with MgSO 4 .7H 2 O.  [5,11]. However, this material has some disadvantages for long term storage due to instability and decomposition of the material. At ambient temperature, the material tends to overhydrate into a solution of magnesium chloride. If the overhydrated material is dehydrated again, an impervious layer is formed. This layer causes a reduction of the water vapour transfer in the system which slows down the reaction. In addition, at high temperature around 150°C, the material decomposes into magnesium hydroxychloride MgOHCl, involving a release of hydrochloric acid vapours. This reaction reduces the quantity of active material over time and can induce corrosion on the heat storage system. In order to determine the temperature range over which MgCl 2 .6H 2 O remains in an stable hydrated state during its dehydration and hydration reactions, in-situ XRD measurements have been carried out under a vapour pressure of 13 mbar. The results are presented in figure 6 and in figure 7.  As shown in figure 6, the initial material used in this experiment presents the characteristic XRD pattern of MgCl 2 .6H 2 O. However, the peak intensities on the diffractogram performed at 25°C are very weak due to the overhydration of the material at this temperature. The intensity of the peak increases slowly when the material is heated up until 40°C and the characteristic XRD pattern of the crystalline MgCl 2 .6H 2 O phase remains the same until 50°C. This result is in agreement with the equilibrium conditions T-p(H 2 O) determine by Siegesmund and al. [12] indicating that MgCl 2 .6H 2 O remains in solid state for a temperature above 30°C in combination with a water vapour pressure of 13 mbar. Between 60°C and 70°C, the XRD patterns keep the peak distribution of the crystalline phase of MgCl 2 .6H 2 O but the intensities of certain peaks are modified. In XRD analyses, the increase or decrease of the intensity of the peak, as shown for the peak at 21.75°, are explained by a preferential orientation of the material during the measurement. Therefore, in this experiment the variation of the peak intensities for a same phase MgCl 2 .6H 2 O may be attributed to a modification of the grains orientation in the powder sample in this temperature range of 60-70°C which foregoes the first step of dehydration of the material. Indeed, the dehydration reaction of MgCl 2 .6H 2 O until 150°C performed with 1°C/min presents two steps of dehydration with the formation of the crystalline phases of MgCl 2 .4H 2 O in the temperature range of 80-110°C and of MgCl 2 .2H 2 O in the temperature range of 120-150°C indicated by very well-defined XRD patterns in figure 6. No XRD peaks characteristic of the crystalline phase of magnesium hydroxychloride MgClOH have been observed on the spectra at 150°C after 3h of isothermal process, which means that magnesium chloride remains as a salt hydrate material after one cycle under the seasonal heat storage conditions (T max = 150°C, 13 mbar). During the rehydration process shown in figure 7, the crystalline phase of MgCl 2 .2H 2 O is rehydrated in the crystalline phases of MgCl 2 .4H 2 O when the system is cooled down below 100°C and MgCl 2 .6H 2 O when the temperature reaches 60°C. In this measurement, it is shown once again that MgCl 2 .6H 2 O remains in a solid state until 30°C and slowly liquefies at 25°C to form a solution of magnesium chloride.

Thermal analyses.
The thermal analysis (TG-DSC) performed during the dehydration and rehydration reactions of MgCl 2 .6H 2 O was carried out under 13 mbar water vapour pressure. The temperature of rehydration was limited to a minimum of 40°C to avoid the overhydration of the material at ambient temperature and a maximum temperature of the dehydration was fixed at 130°C. The dehydration and hydration reaction were carried out with a heating rate of 0.5°C/min and a cooling rate of 0.2°C/min. Figure 8 and figure 9 present respectively the TG and DSC results obtained during these measurements. Additionally, the data of the onset temperature, the reaction enthalpy, and the energy density determined from the DSC measurements have been summarized in table 2.    The shift of the onset temperature (table 2) and the formation of a hysteresis on the TG curve in figure 8 both lead to the same conclusions as previously found for magnesium sulphate heptahydrate. The kinetics of the water vapour sorption process of MgCl 2 .6H 2 O are controlled by the surrounding water vapour pressure and the temperature applied to the system. In this measurements performed under a constant water vapour pressure of 13 mbar with low heating and cooling rates, the phenomenon may be attributed to the temperature difference between the surrounding gas and the material caused by the heat effect of the dehydration and hydration reactions. The reaction enthalpies were determined for each chemical reaction observed during the dehydration/hydration cycle of MgCl 2 .6H 2 O from the DSC curves (table 2). As observed previously for magnesium sulphate (section 3.1.2), the heat released from the material during the hydration process is lower than the heat stored in the material during the dehydration. The heat released from the chemical reactions MgCl 2 .2H 2 O to MgCl 2 .4H 2 O and MgCl 2 .4H 2 O to MgCl 2 .6H 2 O, corresponds respectively to 87% and 79% of the heat stored during the dehydration reaction. These values are higher than those obtained for the hydration reaction of magnesium sulphate. These results tend to confirm the explanation proposed for magnesium sulphate heptahydrate (section 3.1.2) which attributes the low release of the heat during the hydration reaction to the amorphization of the material. In the case of MgCl 2 .6H 2 O, the textural changes in the material and heat losses to the gas proposed in section 3.1.2. should be taken in consideration. In fact in this measurement, the heat loss effect into the gas was increased by the fact that the cooling rate (0.2°C/min) applied to the system for the hydration in this measurement was slower than the heating rate (0.5°C/min) of the dehydration process.
In terms of energy density, magnesium chloride hexahydrate presents a total energy density of 1.41 GJ/m 3 at the end of the cycling test at 13 mbar for a bulk system. This means that for a packed bed reactor designed for seasonal heat storage with 50% of porosity, magnesium chloride hexahydrate would release 0.71 GJ/m 3 . Therefore, MgCl 2 .6H 2 O can fulfil the winter heat demand of an individual house with a packed bed reactor of 8.5 m 3 , which means that this material is almost five times more compact than seasonal heat storage in a water tank. This material presents promising characteristics as TCM material for this application.

Conclusion
The investigation of the reversible water vapour sorption process of MgSO 4 .7H 2 O and MgCl 2 .6H 2 O under seasonal heat storage conditions showed that the thermodynamic and kinetic properties of these materials are directly related to the structural and textural modifications undergone by the materials during their dehydration and hydration reactions. These modifications are controlled by the water vapour pressure set at 13 mbar for a seasonal heat storage system, the temperature of dehydration and hydration and the rate of heating and cooling applied to the system during these reactions.
In this study, it was found that under a water vapour pressure of 13 mbar, MgSO 4 .7H 2 O dehydrates into an amorphous monohydrate phase when the system is heated until 150°C and rehydrates after 100 h by recrystallizing the crystalline phase of MgSO 4 .6H 2 O. The material is able to take up 0.99 GJ/m 3 of heat for a packed bed built with 50% of porosity during the dehydration of the material. However, it only released 0.38 GJ/m 3 of heat during the rehydration process. This low heat release is mainly attributed to the amorphization of the material during the dehydration performed at 13 mbar which reduces its sorption capacity during the rehydration. Nevertheless, the low heat release may also be assigned to textural changes which influence the measurement conditions. Additionally, experimental artefacts of the DSC measurements may influenced the heat storage capacity found for this material. MgCl 2 .6H 2 O presents a much lower difference between its stored energy density of 0.84 GJ/m 3 and its heat release of 0.71 GJ/m 3 . For this material, XRD results showed that the phases MgCl 2 .4H 2 O and MgCl 2 .2H 2 O found during the dehydration and hydration of this material were crystalline. The small difference in hydration and dehydration enthalpies may be attributed to the textural changes in the material and experimental artefacts. The high heat storage capacity of MgCl 2 .6H 2 O means that this material can fulfil the winter heat demand of a passive house estimated at 6 GJ with a packed bed reactor of 8.5 m 3 . This volume is almost five times smaller than a water tank used for latent seasonal storage. MgCl 2 .6H 2 O presents definitively a promising means for seasonal heat storage in for individual dwelling. Nevertheless, a seasonal heat storage system based on the water vapour sorption process in MgCl 2 .6H 2 O should be carefully set with a restricted temperature of 40°C for the hydration reaction to avoid the liquefaction of the material at ambient temperature, since this would limit the performance of the material for long term storage.