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Thermochemical heat-storage materials such as magnesium, zinc and iron sulfates offer high energy-storage densities and a reliable means of long-term storage of solar energy. In this research study, the dynamic properties of these three salts were investigated based on their hydration and dehydration characteristics. To investigate the effect of regeneration temperatures on the physio-chemical properties of the materials, four sets of temperatures were used. At 100°C, zinc sulfate showed a significant loss of water as compared to magnesium and iron salts, while in the hydration process at lower temperatures, zinc sulfate showed the highest sorption enthalpy and energy density. Furthermore, the crystallinity of salt hydrates was characterised using powder X-ray diffraction measurements, which revealed that dehydration of water molecules occurred only in magnesium sulfate, whereas partial distortion occurred in iron sulfate with a rise in temperature. Collating the results together indicated that zinc sulfate is more energy efficient than both magnesium and iron sulfate. With a calculated hydration energy density at 100°C of 1·26 GJ/m3, zinc sulfate is the best option among the three salts for its use as a thermochemical material.

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Summary ZnSO4·7H2O is a promising thermochemical heat storage material. In this paper, we report a detailed thermodynamic study of ZnSO4·7H2O based on hydration/dehydration, cyclicability, and water sorption performance. The TG‐DSC measurements reported that at below 120°C, 1747 and 1298 J g−1enthalpy was recorded for dehydration and hydration, respectively. The relative‐humidity study showed that at 75% RH (0.148 g/g), the water sorption can be significantly improved compared to lower humidity (65% 0.131). The XRD study highlighted that the main structure of ZnSO4 after thermal treatment remains unchanged.

Quantitative predictions of the photophysical processes for a next generation thermally activated delayed fluorescence molecule are calculated by considering the Herzberg–Teller and the Duschinsky rotation effects within a multimode harmonic oscillator model.

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