NaI as suitable alkali halide hydrate for domestic thermochemical heat storage

The limits and potentials of alkali (Li - Cs) halides (F - I) as salt hydrate-based thermochemical heat storage materials are studied. Most alkali halides are found unsuitable, and only the hydrates of sodium iodide (NaI) and sodium bromide (NaBr) were investigated experimentally. We confirmed that both have high energy densities of 1.5 GJ/m3 and 1.6 GJ/m3 respectively, based on the crystal structures of the dihydrate phases. NaI demonstrated full cyclic stability at water vapor pressures of 12 and 14.5 mbar, but NaBr did not show rehydration at these conditions. Therefore, NaBr was deemed unsuitable for domestic applications. For the NaI – H2O system, we have measured the p,T equilibrium line between the hydration states and constructed p,T and T,x – phase diagrams. This showed that the hydration temperature is 42 °C for 12 mbar water vapor pressure which is enough for domestic space heating. Additionally, the phase diagrams revealed an incongruent melting point where the NaI·2H2O is in equilibrium with the anhydrate and a saturated aqueous NaI solution. This melting can overlap with dehydration and thereby hinders the cyclability, since in the melting process part of the salt is dissolved which blocks the pores formed during dehydration. Therefore, the performance of NaI can be improved by ensuring dehydration is completed below the incongruent melting point of 68 °C. We also found that pre-cycling can prevent deliquescence which is expected to occur at 25 °C and 12 mbar. Overall, NaI could be a candidate for low temperature heat storage.