# Feasibility of Storing Latent Heat with Liquid Crystals. Proof of Concept at Lab Scale
## 3 Results and Discussion  
In order to determine the temperatures of all phase transitions occurring in 10-BPhCOOH together with their corresponding enthalpy values, DSC scans were performed from 60 \({}^{\circ}\)C to 270 \({}^{\circ}\)C. In Figure 7, the scans of the first and second heating/cooling cycles are displayed. As we can see, this compound has a solid/mesophase transition at around 170 \({}^{\circ}\)C with 35 kJ/kg melting enthalpy and a mesophase/isotropic liquid transition at 251 \({}^{\circ}\)C with 55 kJ/kg clearing enthalpy. These temperatures are in full agreement with the values reported in the literature (see Figure 4 [14]). In addition, two crystal-to-crystal transitions (at 90 \({}^{\circ}\)C and 166 \({}^{\circ}\)C) are observed before melting. The one occurring at 90 \({}^{\circ}\)C can be seen during both heating and cooling process; however, the one at 166 \({}^{\circ}\)C can be clearly observed during heating whereas in the cooling only a very smooth shoulder can be appreciated after freezing. The second DSC cycle reproduces exactly the first one, and this reversibility is a promising indication of thermal stability. In relation to the transition energies, it is worth remarking that the clearing enthalpy is much higher than melting enthalpy (55 kJ/kg _vs._ 35 kJ/kg), which confirms that intermolecular forces in the mesophase are fairly strong as we expected from the involvement of H-bonds [19]. For comparison, the esters from these acids do not exhibit mesomorphic behavior probably due to their inability to form a dimeric molecule via H-bonding [14]. It is also interesting to note that 10-BPhCOOH displays a wide mesophase range (80 \({}^{\circ}\)C), which would largely prevent its freezing in case of an eventual temperature decrease due to undesired thermal losses. Therefore, these DSC results confirm the potential of 4\({}^{\prime}\)-\(n\)-alkoxybiphenyl-4-carboxylic acids as liquid phase change materials. In this way it is worth preparing some other derivatives of this family for determining the influence of the alkoxy chain length in the thermal properties. In order to have an idea of 10-BPhCOOH stability upon thermal cycling, some preliminary studies were carried out with DSC apparatus. For this purpose, up to 11 consecutive heating/cooling cycles were performed between 230 \({}^{\circ}\)C and 270 \({}^{\circ}\)C, _i.e._, 20 \({}^{\circ}\)C below and above its clearing temperature. From DSC scans, clearing temperatures were recorded and enthalpies were calculated for both heating and cooling processes. The values obtained for each cycle are represented in the graph of Figure 8.  
As we can see, clearing temperature remains constant during the cycling for both heating and cooling processes. However, clearing enthalpies tend to decrease with cycling from about 55 kJ/kg in the first cycle to around 50 kJ/kg after 11 cycles. Hence it seems that some degradation takes place in 10-BPhCOOH upon cycling. Such degradation at high temperatures may be due to the decarboxylation from aluminum benzoate formed by reaction of the carboxylic acid with the aluminum pan used in  
Figure 7: Differential scanning calorimetry (DSC) scans of first and second heating/cooling cycle for 10-BPhCOOH. the DSC study. However, 10-BPhCOOH could also undergo some kind of degradation under the storage system service conditions. Therefore, further long-term thermal cycles under conditions as close as possible to the real working conditions (_i.e._, furnace cycles in different atmospheres) should be performed to this compound, in order to assess its viability as latent storage material. Heat capacity is another important thermal parameter that should be measured for liquid crystals that are going to be used as PCMs since they could storage also sensible heat in both mesophase and isotropic states. While storing sensible heat in the mesophase state would avoid the need of a preheating storage system, storing sensible heat in the isotropic state would avoid the need of a superheating storage system.