Elsevier

Thermochimica Acta

Volume 502, Issues 1–2, 20 April 2010, Pages 73-76
Thermochimica Acta

Calorimetric studies of thermochemical heat storage materials based on mixtures of MgSO4 and MgCl2

https://doi.org/10.1016/j.tca.2010.02.009Get rights and content

Abstract

Attapulgite granulate impregnated with mixtures of MgSO4 and MgCl2 hydrates was investigated for suitability as a composite thermal energy storage material. These mixtures were chosen because of the very different deliquescence relative humidities of the salts. The thermochemical heat storage of these materials was characterized by measurements of isothermal heat of sorption and thermogravimetry (TG). The salt mixtures showed a different sorption behavior compared to pure MgSO4. The reduction of the deliquescence relative humidity of the mixture by the partial substitution of MgSO4 by MgCl2 increases the capacity of condensation and therefore the released heat. The energy density of the composite heat storage material containing a salt mixture of 20 wt% MgSO4 and 80 wt% MgCl2 was 1590 kJ/kg measured by calorimetry (at 30 °C/85% RH) with a desorption temperature of 130 °C.

Introduction

The storage of unutilized solar energy is an important factor to increase the efficiency of renewable energy and therefore to save fossil fuels. For this purpose, micro- and mesoporous materials with high storage capacity based on the heat of adsorption of water vapor, e.g. modified zeolites, aluminophosphates (ALPO) and silicoaluminophosphates (SAPO) have been widely investigated and characterized [1], [2]. While zeolites offer the advantage of a high temperature lift they also require high charging temperatures, typically above 470 K. The molecular sieves ALPO-18 and SAPO-34 show a medium temperature lift and a lower charging temperature [2]. However, due to high synthesis costs they are currently too expensive for application as heat storage materials.

Another group of thermochemical heat storage materials are composite materials [2], [3], [4] made of porous materials impregnated with hygroscopic salt hydrates. These materials have the advantage of a low desorption temperature (up to 130 °C), a low price and simple production method. The desorption temperature of 130 °C is realizable by solar collectors and is sufficient for desorption and charging the storage materials [5]. Therefore, the materials appear to be interesting for the utilization of a day/night or seasonal heat storage.

The use of these materials is based on the exothermic reaction of a salt in a low state of hydration with water vapor to form either a higher hydrated form or a salt solution inside the pores. The water uptake depends on the deliquescence relative humidity (DRH). If the relative humidity (RH) of the environment exceeds the DRH, the salt absorbs water and dissolves until reaching equilibrium, i.e. until the water activity of the solution equals the relative humidity. At relative humidities below the DRH, the salt picks up water vapor forming a higher hydrated state but no solution. Both cases are shown in the following equations.Salt (s) + H2O (g)  hydrated form (s) for RH < DRHSalt (s) + H2O (g)  solution (l) for RH > DRH

The resulting heat of sorption is in case (1) the sum of the heat effects of the water vapor condensation and the heat of reaction of the hydration. Diffusive water transport at the reaction interface may often be the rate limiting step in solid–gas–solid reactions. For example, in the system MgSO4–H2O kinetic hindrance of the hydration of kieserite (MgSO4·H2O) to higher hydrates was observed [6], [7]. Investigation of the sorption heat of MgSO4·7H2O dehydrated at 130 °C showed that the thermodynamically stable product, i.e. MgSO4·7H2O, could not be obtained at 30 °C and 85% RH [8]. Magnesium sulfate has a high hydrothermal stability and does not decompose at elevated temperatures and water vapor pressures but the slow rate of the rehydration reaction is disadvantageous for heat storage applications.

In case (2) a higher RH compared to the DRH of the salt will result in an additional sorption of water and the heat of condensation increases the total release of heat. The disadvantage of this process is the formation of a salt solution that may affect the hosting material. Especially halides show a great tendency to decompose under hydrothermal conditions. Through hydrolysis reactions strong acids such as HCl will be formed. Metal parts of adsorber units can corrode under the influence of these acidic solutions [9].

To improve the performance and stability of heat storage materials both reactions may be combined. A hydrothermally stable salt with high DRH (e.g. magnesium sulfate) may be mixed with a small amount of a deliquescent salt to realize advantageous properties. The dehydrated magnesium sulfate hydrate can partly dissolve in a solution of a deliquescent salt and higher hydrate states will be formed by reaction in solution. The hydration is no more a solid–gas–solid reaction and the kinetic hindrance is overcome. Therefore mixtures containing MgSO4 (high DRH of 90% RH at 30 °C) and MgCl2 (low DRH of 33% RH at 30 °C) were chosen in the present work. Using common cation (Mg2+) mixtures avoids undesired reactions between the two salts.

This paper presents results of the heat storage properties of a composite material (salt mixtures inside a porous host material). Several influencing factors were investigated such as the salt mixing ratio, the variation of the measurement conditions and the influence of the salt content.

Section snippets

Materials

Attapulgite granulate with an open porosity of 74.3%, a BET surface area of 106 m2/g and an average pore diameter of 0.08 μm (Hermsdorfer Institut für Technische Keramik, Germany) was used as porous host material. The granulate was impregnated with salt solutions containing MgSO4 and MgCl2 in the desired mixing ratios and were dried at 40 °C to constant mass. Using this procedure, salt contents of the composite materials from 19 to 43 wt% were obtained. In the case of impregnation with the pure MgSO

Influence of the salt mixing ratio

The influence of the salt mixing ratio was measured after dehydration at 130 °C and subsequent sorption at 30 °C and 85% RH.

The heat flow curves of the composite (attapulgite granulate hosting different salt/salt mixtures) are shown in Fig. 1. The hydration of a dehydrated MgSO4 hydrate follows the reaction after Eq. (1) and took the shortest time to reach the equilibrium (about 18 h). For the salt mixtures the reaction follows Eq. (2), i.e. if a salt solution is formed, the time to reach the

Conclusions

Salt hydrate mixtures of magnesium sulfate and magnesium chloride were studied for the application in composite materials for thermochemical heat storage. It is shown that the partial substitution of MgSO4 by the chloride, i.e. a salt with a lower DRH, will result in higher heat of sorption. This higher heat release with increasing amount of MgCl2 is the result of the increased absorption of water in the concentrated salt solution. The higher heat release at the beginning of the measurement of

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