Experimental study of composite sorbents

2.1. Experimental setup

The chemical storage is designed to undergo the endothermic heat storage reaction during the summer, using heat obtained from tube solar collectors, and release the heat stored by using the exothermic reverse reaction during the winter. The hydration/dehydration cycle of magnesium sulphate has previously been identified as particularly promising [3]. An experimental study has been performed on this material to describe and understand its behaviour during its hydration by water vapour. The principal thermal properties of the MgSO4 / MgSO4.7H2O system are presented in Table 2.

Reaction

MgSO4 + 7 H2O ^ MgSO4.7H2O hydration, exothermic — dehydration, endothermic

Enthalpy of hydration

— 410 kJ. mol-1

Temperature of dehydration

122°C

Density of the dense salt

MgSO4

2660 kg. m-3

MgSO4.7H2O

1680 kg. m-3

Energy density of the dense salt

MgSO4

9.06 GJ. m-3 / 2.52 MWh. m-3

MgSO4.7H2O

2.80 GJ. m-3 / 0.78 MWh. m-3

Table 2. Properties of the MgSO4 / MgSO4.7H2O system [4]

By dispersing the magnesium sulphate over a larger exchange surface, the thermal power released during hydration is greatly increased. Composite materials made of a sorption material and magnesium sulphate were prepared in such a way to preserve the porosity of the sorption material. This feature is essential in order to allow the water vapour to react with both the salt and the matrix. Sorption materials, such as zeolites and silica gels, were tested as host materials for the salt composites.

The samples were hydrated in a thermally insulated (non-isothermal) open reactor using a flow of moist air at a relative humidity close to 100% at ambient temperature.

2.2. Results

For all of the hydration tests, the rate of water uptake (grams of water per gram of material per hour) is calculated from the exit air humidity and temperature measurements. These values qualify the thermal characteristics of the system : the water uptake is proportional to the heat produced and the rate of water uptake is proportional to the power delivered by the reaction (Fig. 6).

Fig. 6. Rate of water uptake during hydration

High rates of water uptake are obtained for a much longer period with the zeolite/MgSO4 composite ZM15 compared to the analogous silica composite SM16 and the reference pure materials (Fig. 6). This results in higher temperature lifts for the zeolite composite (Fig. 7). Maintaining the high rates of reaction over a long time will ensure that a maximum amount of usable high-grade heat is produced.

Fig. 7. Temperature lifts and peaks of power density during hydration

These peaks of temperature lift are associated with peaks of power of 28.4, 20.1, 20.7 and 15.6 mW per gram of material for the hydration of ZM15, zeolite, SM16 and silica gel respectively. The zeolite/ magnesium sulphate composite proves more favourable than the choice of a silica gel matrix. Other materials, such as polymer binders, are currently being investigated to create a porous expanded structure for pure magnesium sulphate.

4. Conclusion

A high performance long-term heat storage system is needed to meet the huge heat demand of the building sector and the seasonal variations in the availability of the solar resource. Thermochemical

energy storage proves to be a relevant solution for this purpose since it does not loose energy with time and can provide the high energy densities necessary for compact storage.

Modelling and experimental studies have been performed on the hydration/dehydration cycle of magnesium sulphate composites to design an innovative system of seasonal storage of thermal energy.

A reference solar combisystem, without any long-term heat storage, has been simulated. The size of the required heat storage material tank has been calculated for a 191 m2 single-family house, with a space heating demand of 37.2 kWh. m-2 (53.0 kWh. m"2 including domestic hot water), under the Parisian climate. The results predict a volume of 0.2 to 0.9 m3 of magnesium sulphate (i. e. 1.0 to 4.8 m3 per m2 of living space) to reach a solar fraction ranging from 50 to 57 %. For the same house located in Marseille, with a space heating demand of 15.4 kWh. m-2 (31.1 kWh. m-2 including domestic hot water), a 100% solar fraction is achievable with a volume of 0.7 m3 of magnesium sulphate.

The limited performances of pure magnesium sulphate powder in real conditions have led to the investigation of porous composite materials, such as zeolite/salt and silica gel/salt composites. Thus, for example, the design of a composite material with 50% of the theoretical energy density of the dense salt would be enough to reach an acceptable storage volume of 2 m3. Temperature lifts around 30°C and maximal power of 28 mW. g-1 have been obtained during the hydration of the zeolite/MgSO4 composite ; lower values were obtained with pure zeolite, silica, or silica/MgSO4 composite tested under the same conditions. Further experiments are in progress to test the stability of these materials after several cycles and to find other appropriate composites to improve the performances. Summer regeneration of the material along with complete modelling of the chemical heat store will also be investigated.

References

[1] « Factor 4 : Doubling wealth — halving resource use, A report to the Club of Rome », Earthscan Publications Ltd., London, 1997.

[2] TRNSYS : http://sel. me. wisc. edu/trnsys/

[3] K. Visscher and J. B.J. Veldhuis , “Comparison of candidate materials for seasonal storage of solar heat through dynamic simulation of building and renewable energy systems”, paper presented at the Buildings Simulations 2005, in Montreal, Canada, August 15-18, 2005.

[4] J. Van Berkel, “Storage of solar energy in chemical reactions”, Thermal energy storage for solar and low energy buildings, IEA SHC Task 32, JC Hadorn (Edt), ISBN 84-8409-877-X, 2005.

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