Materials for thermochemical storage: characterization of magnesium sulfate

V. M. van Essen1*, H. A. Zondag1, R. Schuitema1, W. G.J. van Helden1 and C. C.M. Rindt2

1Energy research Centre of the Netherlands ECN, P. O. Box 1, 1755 ZG Petten, The Netherlands
2Department of Mechanical Engineering, Eindhoven University of Technology (TU/e), 5600 MB Eindhoven,

The Netherlands

* Corresponding Author, v. vanessen@ecn. nl
Abstract

Magnesium sulfate hepta hydrate (MgSO4.7H2O) was studied as possible thermochemical material for seasonal storage of solar heat. Both hydration and dehydration were investigated and it was found that the material was able to take up and release almost 10 times more energy than water of the same volume. The amount of water taken up and energy released by the material turned out to be strongly dependent on the water vapor pressure and temperature. Unfortunately, the material was not able to release all the stored heat under practical conditions. Despite this problem, valuable information on the dehydration and hydration behavior of MgSO4.7H2O was acquired and the characterization procedure will also be used for future characterization of other salt hydrates for thermochemical materials.

Keywords: compact thermochemical seasonal heat storage, characterization, magnesium sulfate, salt hydrate

1. Introduction

Households in the Netherlands use about 15% of the total energy consumption for space heating and domestic hot water. The energy consumption in the built environment can be reduced by energy saving measures (improved insulation, heat recovery, etc.). A substantial part of the remaining energy demand can be fulfilled by using renewable energy sources such as solar energy.

The heat demand in summer can be completely fulfilled using solar heat, but in winter the heat demand exceeds the solar supply. To accommodate this difference in time between the solar energy surplus in summer and the energy demand in winter, a seasonal thermal storage is needed.

Traditionally water is used for storing solar heat (e. g. solar boiler) for short time periods, however, long-term heat storage will require a large water tank (>50 m3) that is often too large to be placed inside a building. As an alternative, it is possible to store energy by means of chemical processes, making use of the reversible reactions: C + heat «• A + B

During summer, the thermochemical material (TCM) dissociates under influence of solar heat into components A and B, which are stored separately. In the discharging mode, the two components (A and B) react to form the original TCM while releasing the stored solar heat. No reactions occur as long as the two components are stored separately, which means that thermo chemical storage can be used for loss-free storage and transportation of heat.

Interesting TCMs are cheap, non-toxic, non-corrosive, have sufficient energy storage density, and have reaction temperatures in the proper range. These requirements are fulfilled by a number of salt hydrates. A previous theoretical study [1] identified magnesium sulfate hepta hydrate (MgSO4.7H2O) as a promising material for long-term heat storage, by means of the following reaction:

MgSO4.7H2O(s) + heat «• MgSO^s) + 7H2O (g)

In this work, measurements on magnesium sulfate are presented that give information on the suitability of magnesium sulfate as thermochemical material for seasonal heat storage. Additionally, information is given on parameters important for the design of thermochemical seasonal heat storage using magnesium sulfate.

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