1.1. Compact Chemical Seasonal Storage of Solar Heat (ECN and TU Eindhoven, Holland)

The main findings of the studies on storage through chemical reactions, as reported by ECN and TU Eindhoven, are the following:

• An extensive theoretical study at ECN [2, 3] indicated magnesium sulphate heptahydrate as potential interesting storage material using the following reversible reaction: MgSO4.7H2O(s) + heat «• MgSO4(s) + 7H2O(g). The theoretical storage density of the material is 11 times that of water [4].

• Initial characterization experiments reveal that the dehydration of MgSO4.7H2O actually proceeds through three steps: first, MgSO4.6H2O is formed after releasing one water molecule, in the second step 5.8 water molecules are released and finally MgSO4 is formed in the third step. The second dehydration step is most interesting since it is able to store ~420 kWh/m3 energy (6 times that of water) [4, 5]

• After dehydration, MgSO4 was able to take up water in a single step until MgSO4.6H2O was formed. The vapour transport between the particles is the limiting factor for hydration of magnesium sulphate [4, 5]

• The cyclability of the material at hydration temperature of 20°C was very good, however, no water uptake was observed at 40°C, which may be caused by a lower water vapour pressure. Currently, experiments are performed to investigate this observation [4].

• System studies were carried out, indicating that a large increase in solar fraction can be obtained by adding a TCM storage to a solar system with sufficient collector area. Care should be taken to find materials with a good DH (not too low for heating, but also not too high for the solar array). The system performance turned out to be very sensitive to the value of DH. Correspondingly, the system yield was found to decrease significantly if flat-plate collectors were used instead of vacuum tube collectors. [6].

• It was found that the coupling of the TCM system to a water storage tank significantly reduces the power requirements on the TCM reactor [6]. The water tank can then supply the high-power loads, while the TCM tank can afterwards recharge the water tank at a lower power level. Simulations were carried out for the case in which the solar collector system gives priority to the charging of the water tank and uses any excess heat to charge the TCM tank. However, it is now very important that the solar array is large enough to be able to provide significant charging of the TCM tank. For the case of the 15 kWh/m2/a building in Zurich (total load 7.3 GJ for space heating and 10.9 GJ for domestic hot water), it was found that the collector array required to charge a 6.6 GJ TCM storage completely was about 20 m2 vacuum tube if a TCM material with a DH of 66 kJ/mol of water was used; for higher DH the required collector area increases strongly.