Investigation of the thermal properties and heat-storage reliability of a paraffin wax for LHTES application under direct solar radiation

In recent years, Thermal Energy Storage (TES) is becoming more and more important in different engineering applications. As far as the building sector is concerned, TES is considered a crucial feature to reach the net-Zero Energy Building (nZEB) goal. Commonly, TES in building is obtained using the sensible heat property of conventional building materials (building thermal inertia). The drawbacks of this strategy are: the low amount of thermal energy that can be stored; the overheating of the indoor environment that may occur if elevate amount of heat is collected by a conventional building material. On the contrary, the exploitation of the latent heat of dedicate materials (the so-called Phase Change Materials – PCMs) for TES purpose (Latent Heat Thermal Energy Storage – LHTES) presents different advantages: it allows a much higher energy storage density; it allows “temperature selective” thermal energy absorption and release (by choosing the melting temperature range of the PCM). Moreover, since the storage is done at almost isothermal conditions, it is easier to match the energy demand of the building, with the requirements posed by indoor thermal comfort conditions. An exhaustive collection of both laboratory and commercial grade PCMs of different nature and for different purposes can be found in [1]. The exploitation of LHTES is now gaining popularity [2]: different applications of PCMs in construction are currently under investigation [3-5] and, sometimes, commercially available. The most common applications concern the incorporation of PCMs into opaque elements for indoor partitions, structural components and insulation layers. The adoption of PCMs may also occur in combination with active systems (e.g. air-based heating systems, floor heating). Furthermore, because of the ability of certain PCMs to transmit (part of) the visible spectrum of the solar radiation, some transparent/semi-transparent building envelope elements filled with PCMs have been also investigated [6-9]. Some systems based on PCMs integration into transparent components have also appeared on the market and have been adopted in some buildings too. The chemical stability and thermal reliability of PCMs and their live spam are key features to ensure the economic feasibility of exploiting LHTES. The requirements on the stability of the PCMs’ thermal properties may depend on the different application of PCMs. However, as a general rule, it is mandatory that no relevant changes occur in the melting temperature and in the latent heat of fusion due to the thermal cycles the PCM undergoes. In common applications, such as in gypsum wallboard, thermal cycles are mostly caused by energy absorption and release by conduction and convection. If radiative transfers occur, only the long-wavelength infrared region is involved. On the contrary, in the case of PCMs contained into transparent elements and directly exposed to the solar radiation, the entire electromagnetic solar spectrum passes through and heats the materials. Thus, it may occur that the high energy radiation contained in the UV-VIS range negatively affects the thermal stability of PCMs. Thermal reliability of PCMs has been investigated since time. Accelerated thermal cycle test is usually adopted to simulate the ageing of the material. This technique consists in multi melt/freeze cycles (up to some thousands of cycles) conducted in laboratory by means of dedicated devices, under controlled and fixed conditions. The evolution of the thermal properties of the materials (i.e. the melting temperature and the latent heat of fusion) is then evaluated by means of Differential Scanning Calorimetry (DSC) on different samples of the material, which correspond to different ageing times. Following this procedure, several studies have been conducted to assess the thermal reliability of different PCMs. The literature survey reveals the lack of dedicated investigations that concern the chemical stability and thermal reliability of PCMs which are supposed to be directly exposed to the solar radiation. Although this application is not one of the main popular and developed, the use of PCMs in transparent components is under evaluation since time [6-9] and some solutions are already commercially available [10]. As mentioned above, this type of application may cause chemical instability and degradation. In fact, PCMs can be damaged by the combined action of the UV-VIS electromagnetic radiation and oxidative processes. The aim of this research is to characterize and analyze the evolution of the thermal properties, and to evaluate the thermal reliability, of a paraffin wax exposed to the solar radiation. The analysis is conducted on four samples of the same PCM collected at regular interval during an on-field experimental campaign on a PCM glazing [9], that was exposed to operative conditions in a test cell equipment for more than one year