# Influence of Water Addition on the Latent Heat Degradation of Sodium Acetate Trihydrate
## 1 Introduction  
Phase change materials (PCMs) appear as an attractive solution for thermal energy storage and for the development of renewable energies. Indeed, they can be used for both domestic hot water production [1] and passive heating/cooling of buildings [2]. For a better ratio of production vs. volume, daily melting and solidification are generally wished. Thus, many thermal cycles may be applied to the material that may suffer from aging, resulting in a decrease of the latent heat. In particular, salt hydrates have an appropriate range of melting points (between \(-5\) and \(+120\)\({}^{\circ}\)C) for building applications and a range of interesting latent heat capacity (100-350 J\(\cdot\)g\({}^{-1}\)) [3]. The solid phase of this class of PCMs presents a density of approximately 1700 kg/m\({}^{-3}\) and a specific heat capacity around 1650 J\(\cdot\)kg\({}^{-1}\)\(\cdot\)K\({}^{-1}\)[3], which may ensure relevant heat storage capabilities. One of the most interesting materials issued from this class of PCMs is sodium acetate trihydrate (SAT, CH\({}_{3}\)COONa\(\cdot\)3H\({}_{2}\)O). However, the SAT may present a large supercooling degree up to 100 K [4]. This high value of supercooling is generally perceived as a drawback but could be turned into a benefit if a reliable solidification-triggering technique could be developed [3]. This would lead to the discharge of the stored heat on demand [5]. The heat release amplitude will depend on the triggering temperature (as losses may occur during the cooling of the supercooled SAT). This quantity of stored heat in the supercooled SAT may be degraded by aging throughout successive thermal cycles, a process already explored for other PCMs [6,7]. As the SAT is heated until 58 \({}^{\circ}\)C, an incongruent melting is observed according to the phase diagram [8] because of the break of hydrogen bonds. The SAT decomposes into two components: sodium acetate (CH\({}_{3}\)COONa) and water (H\({}_{2}\)O) that have different densities. If a temperature of 77 \({}^{\circ}\)C is reached, different behavior is observed upon cooling, with a solution (45.7 wt.% of water instead of 39.7 wt.%) and the segregation of sodium acetate. [PERSON] et al. showed that the concentration of a non-stirred SAT mixture depends on the height of the container [9]. The separation of species will increase with cycles, as the mobility is limited both in liquid (no agitation) and in solid state as the remaining SAT in the middle-height will crystallize and hinder the contact between the two species. This would decrease the latent heat released by the material, leading to a reduction of its storage capacity. It was observed by [PERSON] et al. [9] that a deeper container (more than 5 cm in height) would limit the gathering of the two components to again form a trihydrate material. The aging of the SAT was analyzed by several authors such as [PERSON] et al. [10] who experienced that a raw sample of SAT had lost about a third of its initial value after 300 cycles. The evolution throughout more cycles (600 cycles) was also performed by [PERSON] et al. [8], showing that aging could be reduced owing to impurities. Several solutions were investigated to limit this aging [8,11,12], and one of them (addition of water) was found interesting as it also decreases the solidification temperature and thus promotes the supercooling degree [4]. Indeed, aging would be limited by an excess of water from the extra water principle, a concept developed by [PERSON] et al. [13]. Then, this phenomenon was observed by [PERSON] et al. [9,11] as they increased the quantity of water, moving its mass concentration in SAT from 39.7 to 45.7 wt.% (0 to 6 wt.% extra water). A constant quantity of latent heat was released by the solidification of material even after 130 days of supercooling. [PERSON] et al. [14,15] have shown that a sample of SAT with 48.7 wt.% of water (9 wt.% of extra water) can suffer from aging during thermal cycling.