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Research Papers

# Characterization of $MgSO4$ Hydrate for Thermochemical Seasonal Heat Storage

[+] Author and Article Information
V. M. van Essen1

Efficiency and Infrastructure, Energy Research Centre of The Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlandsv.vanessen@ecn.nl

H. A. Zondag, J. Cot Gores, L. P. J. Bleijendaal, M. Bakker, R. Schuitema, W. G. J. van Helden

Efficiency and Infrastructure, Energy Research Centre of The Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands

Z. He2

Department of Mechanical Engineering, Eindhoven University of Technology (TU/e), 5600 MB Eindhoven, The Netherlands

C. C. M. Rindt

Department of Mechanical Engineering, Eindhoven University of Technology (TU/e), 5600 MB Eindhoven, The Netherlands

1

Corresponding author.

2

Present address: Department of Fuel Cells and Solid State Chemistry, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, DK-4000 Roskilde, Denmark.

J. Sol. Energy Eng 131(4), 041014 (Oct 16, 2009) (7 pages) doi:10.1115/1.4000275 History: Received November 27, 2008; Revised May 25, 2009; Published October 16, 2009

## Abstract

Water vapor sorption in salt hydrates is one of the most promising means for compact, low loss, and long-term storage of solar heat in the built environment. One of the most interesting salt hydrates for compact seasonal heat storage is magnesium sulfate heptahydrate $(MgSO4⋅7H2O)$. This paper describes the characterization of $MgSO4⋅7H2O$ to examine its suitability for application in a seasonal heat storage system for the built environment. Both charging (dehydration) and discharging (hydration) behaviors of the material were studied using thermogravimetric differential scanning calorimetry, X-ray diffraction, particle distribution measurements, and scanning electron microscope. The experimental results show that $MgSO4⋅7H2O$ can be dehydrated at temperatures below $150°C$, which can be reached by a medium temperature (vacuum tube) collector. Additionally, the material was able to store $2.2 GJ/m3$, almost nine times more energy than can be stored in water as sensible heat. On the other hand, the experimental results indicate that the release of the stored heat is more difficult. The amount of water taken up and the energy released by the material turned out to be strongly dependent on the water vapor pressure, temperature, and the total system pressure. The results of this study indicate that the application of $MgSO4⋅7H2O$ at atmospheric pressure is problematic for a heat storage system where heat is released above $40°C$ using a water vapor pressure of 1.3 kPa. However, first experiments performed in a closed system at low pressure indicate that a small amount of heat can be released at $50°C$ and a water vapor pressure of 1.3 kPa. If a heat storage system has to operate at atmospheric pressure, then the application of $MgSO4⋅7H2O$ for seasonal heat storage is possible for space heating operating at $25°C$ and a water vapor pressure of 2.1 kPa.

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Copyright © 2009 by American Society of Mechanical Engineers
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## Figures

Figure 1

Mass (TG) and heat flow (DSC) as a function of temperature for dehydration of MgSO4⋅7H2O with a particle size distribution of 38–106 μm. The dehydration was performed by heating a sample of 10 mg from 25°C to 300°C with 1°C/min in a N2+H2O atmosphere assuming a PH2O=2.3 kPa (see text for details). The gray filled arrow indicates the inflection point of the change in shape of the TG curve at the second dehydration step. Exothermic processes are indicated by a positive heat flow.

Figure 2

Differential mass and heat flow as a function of temperature for particles with particle distributions of 200–500 μm and 20–38 μm. The measurements were performed by heating a sample of 10 mg from 25°C to 300°C with 1°C/min in a N2+H2O atmosphere assuming a PH2O=2.3 kPa (see text for details). The open squares denote differential mass for 20−38 μm particles, and the filled triangles 200–500 μm particles. The lines denote the heat flow: solid line for 20–38 μm particles and dashed line for 200–500 μm particles. The circle indicates the peak associated with the observed melting process.

Figure 3

X-ray diffraction patterns taken at different temperatures during the dehydration of MgSO4⋅7H2O. The same particle size distribution (38–106 μm) and identical temperature program as described in Fig. 1 were used during the dehydration of MgSO4⋅7H2O as described.

Figure 4

Number of water molecules taken up in a hydration experiment of MgSO4 as a function of time. The results from the Netzsch (curves 1, 2, and 4) and the results from the ECN (curve 3) are shown. In all cases, a sample of 10 mg was dehydrated from 25°C to 300°C with 1°C/min. After 15 min isothermally at 300°C, the samples were cooled down to 25°C or 50°C with 5°C/min and exposed to a moist nitrogen atmosphere as indicated in the figure. During the experiments a particle size distribution of 38–106 μm was used.

Figure 5

Mass (TG) and heat flow (DSC) as a function of time for hydration of MgSO4 at 25°C and PH2O=2.3 kPa for four different particle distributions. In all cases a sample mass of 10 mg was used.

Figure 6

Mass (TG) and heat flow (DSC) as a function of time for hydration of MgSO4 at 25°C and PH2O=2.3 kPa for three different values for the layer thickness

Figure 7

SEM images of particles (38–106 μm): (a) before dehydration (15 kV, 1500× magnification), (b) after dehydration (5 kV, 1500× magnification), and (c) after hydration (5 kV, 1500× magnification). The material was dehydrated to 150°C with 1°C/min inside an oven and hydrated for 20 h inside a climate room (ESPEC PL-3KPH) at 20°C and RH=52%, corresponding to PH2O=1.2 kPa.

Figure 8

Particle size distribution of MgSO4⋅7H2O: before (curve 1, untreated) and after (curve 2) dehydration, and after hydration (curve 3). The material was dehydrated to 150°C with 1°C/min inside an oven and hydrated for 20 h inside a climate room (ESPEC PL-3KPH) at 20°C and RH=52%, corresponding to PH2O=1.2 kPa.

Figure 9

(a) Fixed-bed reactor setup for testing the hydration behavior of magnesium sulfate under low-pressure conditions. R denotes the reactor, E denotes the evaporator, and P denotes connection to vacuum pump system, (b) Top view of fixed-bed reactor with metal mesh vapor channel.

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