# Feasibility of Storing Latent Heat with Liquid Crystals. Proof of Concept at Lab Scale
## 3 Results and Discussion
Therefore, specific heat capacity (\(C_{p}\)) of 10-BPhCOOH was measured at three different temperatures within the mesophase range. In Table 1, \(C_{p}\) values obtained for each temperature have been recorded. It is clear that \(C_{p}\) slightly increases with temperature but a mean value of 2.4 kJ/kgK can be estimated for the heat capacity in the mesophase range of 10-BPhCOOH. For studying the thermal stability of 10-BPhCOOH, thermogravimetric (TG) analysis was performed in a thermobalance by heating it up to 550 \({}^{\circ}\)C under Ar and monitoring its mass variation during the process. In Figure 9, the mass variation percentage (left axis) recorded during the experiment together with the corresponding differential curve (dTG in right axis) are displayed. dTG curve indicates that two main degradation processes take place: one at around 386 \({}^{\circ}\)C and another one at around 435 \({}^{\circ}\)C. Actually, according to mass variation (left axis), only 1% mass is lost at 290 \({}^{\circ}\)C, which indicates that 10-BPhCOOH is expected to be stable under Ar at least up to 40 \({}^{\circ}\)C above its clearing temperature (251 \({}^{\circ}\)C). However, as we observed for other PCMs [20], TG analyses performed at increasing temperature are not enough for thermal stability assessment and, in our opinion, they should at least be combined with TG studies at different constant temperatures. \begin{table}
\begin{tabular}{c c} \hline
**T** (\({}^{\circ}\)**C**) & **C\({}_{p}\) (kJ/kg K)** \\ \hline
180 & 2.31 \\
200 & 2.44 \\
220 & 2.59 \\ \hline \end{tabular}
\end{table} 
Table 1: Specific heat capacity (\(C_{p}\)) values for three temperatures of mesophase interval of 10-BPhCOOH. Figure 8: Clearing temperatures and enthalpies obtained for each DSC heating and cooling cycle performed to 10-BPhCOOH. Finally, since liquid crystals used as PCMs are meant to move during phase changing, the viscosity values of both mesophase and isotropic states should be a primary concern. Therefore, the viscosity of 10-BPhCOOH was measured with the help of a rheometer using the parallel-plates configuration which requires only a little amount of sample to be deposited between the plates. The whole set-up in located inside an oven so that the experiments can be performed at constant temperature. In our case, dynamic viscosity measurements were carried out at 225 \({}^{\circ}\)C and 270 \({}^{\circ}\)C, _i.e._, 26 \({}^{\circ}\)C below and 14 \({}^{\circ}\)C above the clearing point and hence in both mesophase and isotropic states. In Figure 10, the variation of dynamic viscosity with shear rate is represented for both temperatures. As we can see, 10-BPhCOOH behaves as a pseudo plastic fluid in both mesophase (@ 225 \({}^{\circ}\)C) and isotropic state (@ 270 \({}^{\circ}\)C). In the first case a constant viscosity of about 0.6 Pas is obtained for shear rates higher than 20 s\({}^{-1}\), whereas the isotropic liquid attains 0.18 Pas constant viscosity already at 7 s\({}^{-1}\). These values are low enough to allow the liquid crystal pumping inside a conventional circuit.