Two types of ployalcohols groups can be distinguished (Fig. 1). On the one hand, sugar alcohols

such as erythritol (C4H6(OH)4), mannitol (C6H8(OH)6) and galactitol (C6H8(OH)6) with a solid — liquid phase change have been suggested [10]. Hakiuchi et al. examined the long-term thermal stability of the sugar alcohol erythritol with a melting temperature of 120°C. Experiments in a sealed steal container showed a degradation process of erythritol upon heating already slightly above the melting temperature [10].

On the other hand, there are polyalcohols, known as plastic crystals undergoing a solid-solid transition. Examples are tris(hydroxymethyl)aminomethane (TAM, H2NC(CH2OH)3) and pentaerythritol (PE, C(CH2OH)4) [11]. For latent heat thermal energy storage systems, PE has often been suggested as a PCM [1,5,6,12,13]. PE has a solid-solid phase transition at around 190 °C and a melting temperature of about 260 °C [14]. PE has a high vapour pressure and sublimes on heating in the solid phase (100 Pa at 195°C and 154 Pa at 204°C)[15]. Hence, for the PCM application PE needs encapsulation. Sakamoto reported about an experiment of PE in a sealed glass container at 195-200°C for 1000 hours. Little changes in the phase transition temperature and a slight drop (about 90% of the initial value) in enthalpy were reported [16]. Different mixtures of PE with other polyalcohols have also been examined. The transition temperatures of these mixtures are mainly below 120°C, so that they are not considered here [12].

High density polyethylene (HDPE) with a melting temperature of about 130°C has been often proposed as a PCM [1,5,7,8,10,12,17,18,19]. Polyethylene as a PCM has been used for small size electrical stoves in Japan [10]. HDPE can be of the cross-linked type in order to get form-stable materials in the liquid phase. A crosslinked HDPE pellet bed has been used in direct contact with an ethylene glycol/water solution. This solution acted as heat transfer medium undergoing pool boiling and condensation [19]. Takahashi reports molten Polyethylene to be not stable in air, but stable in the absence of oxygen [20]. In a 500 h degradation test, sealed polyethylene showed little change in melting and crystallization temperature, as well as latent heat [21]. For crosslinked HDPE undergoing 100 thermal melting and solidification cycles, no change in melting temperature and heat of fusion was detected [17].

Benzoic Acid (BA, C6H5COOH) and its derivatives have been proposed [1,5,12,22](Fig. 1). They derivatives include 4-Methyl-BA, 4-Amino-BA and 4-Chloro-BA. BA has been suggested as a calibration substance for Differential Scanning Calorimetry (DSC), where hermetically sealed crucibles are required [23]. BA has a melting temperature of 122°C. It has a high vapor pressure and sublimes at about 100°C (820 Pa at 123°C, 1330 Pa at 132°C, 13300 Pa at 186°C — water for comparison: 3170 Pa at 25°C) [14,23]. Upon heating to 150°C, some dehydration takes place to form benzoic anhydride [14].

Other PCMs, such as Urea (CO(NH2)2) have been also proposed. [1,5]. Urea decomposes upon heating above the melting temperature and hence is not suitable for thermal energy storage [13,14].