INFLUENCES OF SALT HYDRATE MIXTURES AND PORE SIZES ON THE SORPTION HEAT OF COMPOSITE TES MATERIALS ABSTRACT K. Posern, Ch. Kaps Bauhaus-University Weimar Coudraystraße 13 C, 99423 Weimar, Germany Tel: +49-3643-58-4770 konrad.posern@bauing.uni-weimar.de Different salt hydrate mixtures in a reference material with various pore sizes (porous glass) have been studied for a development of a composite TES (Thermal Energy Storage) material. Open porous glass pellets with pore sizes from 45 nm to 40 µm were impregnated with magnesium sulphate and salt hydrate mixtures of MgSO 4 /MgCl 2 and MgSO 4 /LiCl in different ratios. The heats of sorption of the prepared materials were measured isothermally by using humidity controlled calorimetry at 30 C and 40 C with humidities of 85% and 70% RH. The results of measurements in pellets with a pore size of 1-1.6 µm and measurement conditions of 30 C and 85% RH show that the heat of sorption of MgSO 4 (2300 J/g) increases to 5290 J/g and 5400 J/g by substitution of 40 wt% MgCl 2 and 20 wt% LiCl, respectively. Furthermore the sorption heats of the salt hydrates are enhanced by smaller pore sizes. 1. INTRODUCTION Thermo chemical heat storage with composite materials which contains salts or salt hydrates are an alternative to common adsorption materials (Levitskij 1996, Tokarev 2002, Jänchen 2006). The heat release in this case is based on the exothermic reaction of a dehydrated salt with water vapour inside an open porous support material. The system MgSO 4 -H 2 O was first investigated because of the low corrosion tendency (Gordeeva 2003) and the high enthalpy of the dehydration of MgSO 4. 7 H 2 O. Previously measurements show, that the dehydrated magnesium sulphates reacts slowly to higher hydrates with water vapour (Posern 2008). This behaviour is caused by the high relative deliquescence humidity (DRH) of 92 % (at 25 C) of the hydrate with 7 mole water. Therefore parts of magnesium sulphate were substituted by MgCl 2. 6 H 2 O with a DRH of 33 % and LiCl (11% DRH) to improve the water uptake. The addition of these salts results in a changing of the kind of reaction from solid-vapour-solid (Eq.1) to solid-vapour-liquid (Eq.2) by increasing of water vapour condensation. Salt (s) + Water (g) Salt hydrate (s) for RH < DRH (1) Salt (s) + Water (g) Salt solution (l) for RH > DRH (2) Different open porous glass pellets were impregnated with salt solutions to study the influence of the kind of salt hydrate mixture and of the pore sizes on the released heat.
A temperature of 130 C was chosen for desorption, because this temperature can be reached easily by solar thermal collectors (Stach 2005). As sorption conditions 30 C/85 % RH and 40 C/70% RH, that is closer to an application, were selected. 2. MATERIALS Open porous glass pellets with pore sizes from 45 nm to 40 µm were impregnated with magnesium sulphate and salt hydrate mixtures of MgSO 4 /MgCl 2 and MgSO 4 /LiCl in different ratios. Broken glass filter disc granules with pore sizes of 45 nm (CPI-GmbH Bitterfeld), 1-1.6 µm, 10-16µm and 16-40 µm (ROBU) and pellet sizes of 1-2 mm as well as 2-4 mm were used as support materials. Solutions of salt (MgSO 4 ) and mixtures from MgSO 4 and MgCl 2 as well as MgSO 4 and LiCl in certain mixing ratios (see Table 1) were incorporated in the pore system by impregnation. The mass ratios of the dehydrated salts are listed in table 1. Table 1: Mass ratios and the mixture of the dehydrated salts Mixture type MgSO 4 MgCl 2 LiCl 60/40 MgCl 2 60 wt% 40 wt% - 90/10 LiCl 90 wt% - 10 wt% 80/20 LiCl 80 wt% - 20 wt% The total salt content of the total dehydrated material was between 7 and 15 wt% depending on the weight of the matrix and the kind of solution. 3. METHODS The sorption heats were determined at 30 C and 40 C in a gas circulation cell using a calorimeter C80 (SETARAM). This calorimeter was coupled with a humidity controller Wetsys (SETARAM). The procedure for dehydrating the samples and the measurement conditions are showing in Figure 1. The samples (1 g) were heated up in the cell until 130 C (2K/min) with a flow rate of 50 ml/min and a relative humidity of 5% (at 30 C). A dwell time of 2 hours were adjusted for an equilibrium-dehydration of the material. In the next step, the desorbed material was cooled to 30 C (40 C) at the same conditions. At a constant heat flow signal (approx. 0 mw) the humidity was set at the sorption condition (85% or 70% RH) and the resulting heat was measured. SH 1 4 RH [%] condition 1: start condition condition 2: desorption at 5 % RH condition 3: cooling under 5 % RH condition 4: measurement SH: sorption humidity ST: sorption temperature 3 2 ST T [ C] 130 Figure 1: Conditions for the dehydration and measurement of sorption
Thermogravimetry (TG) was applied to study the water content of the salt hydrates by using a Setsys 16/18 device (SETARAM). The samples were dehydrated until 130 C and afterwards completely with a heating rate of 10 K/min in air atmosphere to calculate the total amount of water at 130 C and the content of dehydrated salt in the porous material. 4. RESULTS AND DISCUSSION For comparison of the sorption heats of the salt mixtures, the mass and adsorption heat of the support material were deducted. Therefore, the following stated values are related to the mass of the salt hydrates which were dehydrated at 130 C. 300 heat flow in mw/g 250 200 150 100 50 60/40 MgCl2 80/20 LiCl MgSO4 0 0 20 40 60 time in h Figure 2: Heat flow curves of different salts in pores with 16-40 µm at 30 C and 85% RH Figure 2 shows the heat flow curves of different salts/salt mixtures inside the glass pellets with the pore size 16-40 µm. The hydration of dehydrated MgSO 4 takes the shortest time due to the solid-vapour-solid-reaction. In the other case of the solid-vapour-liquid-reaction of the salt mixtures the equilibration time is about thrice as much. The released heat at the beginning of the measurement of both mixtures is clearly higher, than that of magnesium sulphate. This behaviour is caused by the differences between the partial pressures of the dehydrated salt or salt mixtures and of the humid air. The different shape of the MgSO 4 -curve with a step after 5 hours can be described by the conversion of a solid hydrate to another MgSO 4 hydrate with a higher content of water. The results of the measurements of the sorption heat in pellets with a pore size of 1-1.6 µm and 10-16 µm with the isothermal measurement conditions of 30 C/85% RH and 40 C/70% RH respectively are given in Figure 3 and 4.
5500 5290 5270 5400 sorption enthalpy in J/g 4500 3500 2500 2300 1500 MgSO4 60/40 MgCl2 90/10 LiCl 80/20 LiCl salt/salt mixture Figure 3: Sorption enthalpies of salt mixtures in pores with 1-1.6 µm at 30 C and 85% RH The results of measurements in pellets with a pore size of 1-1.6 µm and isothermal measurement conditions of 30 C and 85% RH show that the heat of sorption of MgSO 4 (2300 J/g) increases by substitution of 40 wt% MgCl 2 and 20 wt% LiCl to 5290 J/g and 5400 J/g, respectively. This increasing can be caused by a higher amount of condensation heat due to the lowering of the DRH. The partial water vapour pressure of the mixtures outside and inside of pores is still unknown but subject of ongoing studies. 3500 sorption enthalpy in J/g 2500 1920 3020 2800 2940 1500 MgSO4 60/40 MgCl2 90/10 LiCl 80/20 LiCl salt/salt mixture Figure 4: Sorption enthalpies of salt mixtures in pores with 10-16 µm at 40 C and 70% RH A same enhancement can be noticed by measurements at 40 C and 75% RH only at a lower level. By the existing partial vapour pressure other kinds of salt hydrates can occur and less water will be absorbed. At this condition, the mixture of MgSO 4 /MgCl 2 has the highest enthalpy of 3020 J/g in opposite to the measurements at 30 C in smaller pores (1-1.6µm).
The influences of the pore size on the heat of sorption are shown in Figure 5. 3000 2810 sorption enthalpy in J/g 2500 2000 1865 2220 2190 2300 2480 1500 bulk 16-40 µm 10-16 µm 1-1,6 µm Ø 176 nm Ø 45 nm magnesium sulphate Figure 5: Sorption enthalpies of magnesium sulphate in different pore sizes at 30 C and 85 % RH The investigations of the influence of the pore size show, that every sorption heat of magnesium sulphate inside the pores is higher than that of the bulk salt. That means there is an effect of the pores on the sorption heat. This effect can be caused by a higher dispersity and smaller crystal sizes of the salts and other partial vapour pressures inside the smaller pores. The measured values of the sorption heat of MgSO 4 in different pores down to 1µm are approximately the same. In smaller pores the salt can occur as higher hydrates or the adsorptive part on the enthalpy of sorption is bigger. So, the notable higher heats in the nmpores are based on the influence of the pore sizes or on the additional heat of adsorption. The heats of sorption of the porous materials are 25 J/g for the pore sizes down to 1 µm and 85 J/g for the glass with 45 nm. At 30 C and 85% RH capillary condensation occurs in pore diameters less than 12 nm, according to the Gibbs Thomson equation. It follows that the higher amount of heat release depends on the enhanced surface area, exclusively. 5. CONCLUSIONS It could be shown that a substitution of MgSO 4 by salt hydrates with a lower DRH will result in a higher enthalpy of sorption. The studied mixtures of MgSO 4 with MgCl 2 and LiCl in different ratios have approximately the same sorption enthalpy, which is in case of the measurement conditions 30 C and 85% RH more then two times higher. For heat storage application, mixtures with a high rate of sulphate should be used, because of the corrosion tendency of the chlorides. An influence of the investigated pore sizes is given and nm-pores enhance the released heat potentially but a separation of adsorption and sorption of water on the salt will be more complicated. For this reason ongoing investigation will be focused on the study of the existing hydrate level and the additional sorbed water in different pore sizes for a better interpretation of the influence of the pore size on the heat of hydration.
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