# Stability Study of Erythrhitol as Phase Change Material for Medium Temperature Thermal Applications
## 1 Introduction  
An efficient use of renewable energy resources, such as solar energy, is increasingly being considered as a promising solution to global warming. However, solar energy is intermittent in nature and its intensity depends on the hour of the day and local weather conditions [1, 2]. It is necessary to establish thermal energy storage (TES) technologies in order to steadily utilize the unused thermal energy. The main role of energy storage systems is to reduce the time or rate mismatch between energy supply and energy demand [3]. In Europe, it has been estimated that around 1.4 million GWh per year can be saved and 400 million tons of CO2 emissions avoided in buildings and in industrial sectors by more extensive use of heat and cold storage [4]. Sensible and latent heat storage are considered as the basic types of TES techniques. Sensible heat storage (SHS) is based on the temperature change in the material. SHS systems use the heat capacity and the change in temperature of the storage medium during the charging/discharging process. Therefore, the temperature of the storage material increases when energy is absorbed and decreases when energy is released [5]. Latent heat storage (LHS) is based on the heat absorption and release when the storage medium undergoes a phase change. A solid-liquid phase change via melting and solidification can store large amounts of thermal energy. Usually, a small volume change (\(\leq\)10%) occurs upon melting. Therefore, the pressure is not changed significantly and, hence, melting and solidification of the storage material proceed at a constant temperature. Upon melting, thestorage material keeps its temperature constant at the melting temperature, which is also called temperature of transition [6]. Stored latent heat during the phase change process is calculated from the enthalpy difference (\(\Delta\)H) between the solid and liquid phase, which is called melting enthalpy or heat of fusion. Phase change material (PCM) can be used to store thermal energy or to control the temperature swings within a specific temperature range. The operating principle of PCM is the following: as the temperature rises and reaches its melting temperature, PCM absorbs heat in an endothermic process and changes phase from solid to liquid. Similarly, as the temperature drops under its melting temperature, PCM releases heat in an exothermic process and returns to its solid phase [7]. According to its properties, PCMs have several application possibilities. A PCM filled window system was proposed by [PERSON] and [PERSON] [8] taking advantage of the translucent and thermal properties of the selected PCM. Some PCMs are highly transparent for the visible part of solar radiation whereas the infrared part is absorbed within the PCM, which allows both the advantages of a daylighting element and of energy storage to be achieved [9]. In addition, coconut fat recovered from underused feedstocks have been used as bio-based PCM for its application in building envelopes due to its good physical, chemical and thermal properties [10]. Sugar alcohols (SA), also called polyols [11], are attractive bio-based materials to be used as PCM. Additionally, as by-products from the food industry, SA are non-corrosive, non-toxic and environmentally friendly [12]. They have great potential in many TES applications due to their high mass and volume-specific melting enthalpies compared to other organic PCM [13]. SA have melting temperatures ranging between 90 and 200 \({}^{\circ}\)C. However, some SA are not stable at higher temperatures [6]. Erythritol is a promising organic PCM for recovering solar energy and industrial waste heat at temperatures up to 160 \({}^{\circ}\)C [14]. Erythritol is a material emerging as PCM due to its attractive TES properties. Erythritol's melting point varies in literature [11; 14; 15; 16] between 117 \({}^{\circ}\)C and 119 \({}^{\circ}\)C according to its purity percent and manufacturer.