Investigation of pinacone hexahydrate as phase change material for thermal energy storage around 45 °C

Highlights

• Enthalpy curves of pinacone hexahydrate were measured via DSC and T-History.

• High melting enthalpy of about 300 J/g at a melting temperature of about 45 °C.

• First cycle stability tests indicated reproducible phase change behaviour.

• Subcooling is highly reduced with increased size of the sample under investigation.

• For comparison, enthalpy curves of PCMs with similar melting points were measured.

Abstract

Phase change materials (PCM) with a transition temperature around 45 °C can be used in various applications, e.g. as intermediate storages for heat pumps in space heating systems, for the thermal protection of battery systems or to increase the efficiency of dishwashers by preheating water. At that temperature level, typical PCMs are either salt hydrates or organic materials like paraffins and fatty acids. Salt hydrates usually provide higher volumetric enthalpy changes, whereas for organic PCMs, corrosion is generally less problematic. Organic hydrates combine the advantages of salt hydrates and organic PCMs. For the diol pinacone, the existence of a hexahydrate with a melting temperature around 45 °C and a high melting enthalpy of about 300 J/g was reported in literature. To investigate the applicability of pinacone hexahydrate as a PCM, we conducted a detailed calorimetric characterisation of its phase change. DSC and T-History measurements were performed to determine the enthalpy curves for melting and crystallisation. We measured an enthalpy change of (340 ± 17) J/g ((332 ± 17) kJ/l) between 32 °C and 47 °C, which is a suitable operating temperature range. In addition, enthalpy changes within 15 K around the phase change of PCMs with similar melting temperatures (docosane, lauric acid, zinc nitrate tetrahydrate) were measured and confirmed the promising properties of pinacone hexahydrate for compact thermal energy storage.

Introduction

Latent heat storage using PCM provides thermal energy storages with high storage capacities in small temperature ranges. The maximum storage capacity of a PCM in a certain temperature interval is given as the difference of the enthalpy curve h(T) in that interval. However, the storage capacity is neither a state variable nor an intrinsic material property. It is a process variable and depends strongly on the conditions given by the application. Therefore, calorimetric measurements of PCMs do not determine the storage capacity directly, but the enthalpy curve and the enthalpy change between charging and discharging temperature.

Most of the PCMs used in applications are solid–liquid PCMs storing heat in repeated melting and crystallisation processes [1]. Among the common PCM material classes, salt hydrates offer the highest volumetric enthalpy changes between 0 °C and 100 °C. A variety of salt hydrates [2], [3] and mixtures of salt hydrates with salt hydrates [4], [5] has been investigated. However, salt hydrates are often corrosive [6] and phase separation may occur upon cycling. Guion et al. [7] stated the existence of a linear correlation between the molar melting enthalpy Δh_m of salt hydrates and the number of water molecules n per salt molecule. This correlation indicates that the high melting enthalpies of salt hydrates are primarily caused by the high melting enthalpy of water. Organic PCMs like paraffins or fatty acids usually have smaller volumetric enthalpy changes than salt hydrates, but corrosion or phase separation is generally less problematic [8], [9].

Combining the advantages of inorganic and organic PCMs gives rise to look for organic hydrates as novel PCMs. The formation of hydrates was observed for binary mixtures of alcohol and water [10], [11] as well as sugar and water [12], [13]. Mixtures of dihydric alcohols (diols) and water exhibited hydrates with high melting enthalpies [14], [15]. For the diol pinacone (pinacol, 2,3-dimethyl-2,3-butandiol), the existence of a monohydrate (solidification temperature T_s = 41.3 °C) and a hexahydrate (T_s = 45.4 °C) was reported by Pushin and Glagoleva [16]. Priest et al. [17] reported a high value of 299 J/g for the melting enthalpy of pinacone hexahydrate. According to Hao et al. [18], “pinacol appears to be an unexceptional small molecule” and its hexahydrate forms a clathrate structure. Contrary, Kim and Jeffrey [19] only stated a “family relationship” between the structure of the hexahydrate and typical clathrates. Despite the question whether pinacone hexahydrate is a clathrate or not, the phase change process, including both melting and crystallisation, has not been investigated sufficiently yet to assess its applicability as a PCM.

This study investigates the phase change behaviour of pinacone hexahydrate via calorimetric measurements and hence its applicability as a PCM. To assess the enthalpy change of pinacone hexahydrate, enthalpy curves of three PCMs from different material classes were measured: docosane (paraffin), lauric acid (fatty acid) and zinc nitrate tetrahydrate (salt hydrate). Those materials have similar melting temperatures as pinacone hexahydrate and offer among the highest enthalpy changes in that temperature range (cf. review published by Cabeza et al. [2]). For the determination of enthalpy curves, we performed combined differential scanning calorimetry (DSC) [20], [21], [22] and T-History [23], [24], [25] measurements. Using both techniques for the characterisation of PCMs, intrinsic material properties can be separated from properties of the measured sample. In addition, the dependence of the enthalpy curve on the sample size can be analysed and the behaviour of the bulk material in real applications can be estimated.

Since the enthalpy curve is the most important property for the selection of a suitable PCM for an application, this paper represents an appropriate basis to use pinacone hexahydrate in thermal energy storage applications. This paper does not aim at contributing to basic material research such as phase equilibrium studies, but aims at investigating the phase change behaviour of pinacone hexahydrate in a way that is suitable to design an efficient storage system.