The following phenomena, influenced or caused by reduced gravity are of relevance for the TES: • Phase change material (PCM) and void distribution. • Gravity independent convection. • Wetting and spreading properties. • Crystal growth. • Lack of sedimentation. PCM/Void Distribution Due to the expanding/shrinking process it is difficult to predict the thermal and mechanical behaviour during charge/discharge cycles in microgravity because the distribution of the solidified medium is unknown. Our experiments with the salt lithium fluoride (LiF) demonstrated that aside from gravitational effects the void distribution is determined by which surfaces are cooled and the cooling rate [2,3]. The PCM densely solidified on the cooling walls and the voids mainly developed at or close to the uncooled surfaces. Phase change convection (see below) is regarded as being responsible for mass transport towards the cooled walls. As depicted in Fig. 2 this will have consequences with regard to the beginning of the charging cycle for the TES. In the case of identical heating and cooling surfaces a high pressure will develop near the containment wall if the expanding PCM cannot communicate with the voids. This pressure, which the containment has to withstand, will increase melting temperature and hence retard melting. A rough estimation of this pressure dependence of the melting temperature can be given by the Clausius-Clapeyron equation: with p for pressure; T for temperature; TM for melting temperature; H for melting enthalpy; and Vs for Volume of liquid and solid, respectively. For LiF a melting temperature increase of 1.36 K corresponds to a pressure increase of 100 bar. To avoid pressures higher than 10 bar it has to be guaranteed that
RkJQdWJsaXNoZXIy MTU5NjU0Mg==