# Feasibility of Storing Latent Heat with Liquid Crystals. Proof of Concept at Lab Scale
## 1 Introduction  
Thermal energy storage is a key element for thermal processes management, especially in those related to renewable technologies, where both flexibility and dispatchability are demanded. If the thermal process entails water condensation/evaporation, isothermal storage/release of energy is the most appropriate approach. This implies storing energy as latent heat with phase change materials (PCMs) that undergo state transitions at temperatures close to the steam working conditions. Up to now, the PCMs considered for the temperature range between 140 \({}^{\circ}\)C and 340 \({}^{\circ}\)C have been are pure compounds or eutectic mixtures that exhibit solid to liquid transitions with enthalpies from 100 kJ/kg to 300 kJ/kg. However, despite the strong research effort made during the last years, there is still no cost-effective solution available for this temperature range. The problem is that, in most of the cases, the used PCMs have very poor thermal conductivity, and since heat transfer is driven by conduction, enhanced heat transfer mechanisms have to be implemented in the heat exchanger, with the corresponding cost increase [1,2]. Whereas in other cases, the used PCMs are organic materials, which undergo thermal degradation under service conditions [3]. The solution explored in our lab is the use of alternative PCMs that absorb/release energy when they undergo a change between two fluid phases [4]. This kind of behavior is only displayed by thermotropic liquid crystals (LCs), which are substances that do not exhibit only a single transition from solid to isotropic liquid but rather a sequence of one or more transitions involving intermediate fluid phases, called mesophases. In this respect, some bibliographic sources have very recently used the term liquid-PCMs [5] whenreferring to them. The fact that LCs can change their latent energy content upon changing from one to another liquid state avoids the problem of having a poor thermal conductivity since the energy exchange takes place by convection and the influence of material conductivity is negligible. This provides a constant discharging power of the storage device, which cannot be attained with the current solid-to-liquid PCMs [6]. Apart from thermal stability at working temperature ranges, the most important requirements that should fulfill a certain LC for being used PCM are that its clearing point (_i.e._, mesophase to isotropic liquid transition) must be close to the temperature of the process, the clearing point enthalpy must be high enough and both liquid crystal and isotropic liquid phases must display low viscosity and comparatively high fluidity [3]. Also, from the implementation point of view, issues like low environmental impact, commercial availability and low price (_i.e._, simple synthetic route) should be of main concern. In order to find LCs with promising values of clearing enthalpies within the temperature range of steam the working conditions mentioned above, the large review published by [PERSON] and Chickos in 2006 was thoroughly checked [7]. Figure 1 shows a dispersion of clearing temperature-enthalpy corresponding to the selected LCs. As we can see, it is already possible to find liquid crystals with enthalpies from 70 kJ/kg to 100 kJ/kg, which in principle could be appropriate for latent heat thermal storage in the range 140 \({}^{\circ}\)C-340 \({}^{\circ}\)C [8]. Despite these good perspectives, applications related to the ability of liquid crystals to store thermal energy under fluid phases have been mentioned only once in a patent about drilling fluid refrigerants [9]. Therefore, no special attention has been paid to investigate LCs with adequate features for thermal storage. On the other hand, the review published by [PERSON] and [PERSON] [7] only includes compounds for which enthalpy data are reported in the literature. However, there should exist other LCs that are also expected to present high clearing enthalpy values but that are not included in that review.