• Energy Research

Contains fulltext : 282475.pdf (Publisher’s version ) (Open Access)

We have experimentally determined the main thermodynamic properties of SrCl2, a potentially promising salt for thermochemical heat storage. We found a high energy density of 2.4 ± 0.1 GJ/m3 and proved full cyclability for at least 10 cycles going from the anhydrate to the hexahydrate without chemical degradation. We have experimentally determined the thermodynamic equilibria for each individual transition and the corresponding metastable zones. We find that the metastable zone is widest for the anhydrate to monohydrate transition and decreases with each subsequent hydration step. We have also established that the observed nucleation kinetics are highly dependent on the preparation of the sample. Depending on the preparation conditions, some seeds of the precursor phase can remain in the sample thereby influencing the induction times for the transition. In heat storage applications we recommend selecting conditions well away from the phase transition lines (at least outside the metastable zone) and to leave some seeds of the phase to be transferred in order to increase the transition speed.

In this work, we evaluate 454 salt hydrates and 1073 unique hydration reactions in search of suitable materials for domestic heat storage. The salts and reactions are evaluated based on their scarcity, toxicity, (chemical) stability and energy density (>1 GJ/m3) and alignment with 3 use case scenarios. These scenarios are based on space heating (T > 30 °C) and hot water (T > 55 °C) to be provided by discharge as well as on heat sources available in the built environment (T < 160 °C) for charging. From all evaluated materials, only 8 salts and 9 reactions (K2CO3 0–1.5, LiCl 0–1, NaI 0–2, NaCH3COO 0–3, (NH4)2Zn(SO4)2 0–6, SrBr2 1–6, CaC2O4 0–1, SrCl2 0–1 and 0–2) fulfil all of the criteria. Provided a suitable stabilisation method is found additional 4 salts and 13 reactions (CaBr2 6-0, CaCl2 6-0, 6-1, 6-2, 4-0, 4-1, 4-2, LiBr 2-0, 2-1, 2-0, LiCl 2-0, 2-1, ZnBr2 2-0) From this selection, only 2 salts/reactions (NaI and (NH4)2Zn(SO4)2) have not been extensively studied in the literature. Moreover, many well-investigated salt hydrates, such as MgSO4 and LiOH, did not pass our screening. This work underlines the scarcity of materials suitable for domestic applications and the need to broaden the scope of future evaluations.