Abstract

Cold thermal energy storage has attracted a lot of attention for many refrigeration applications as it has become one of the primary solutions to the electrical power imbalance between daytime need and nighttime abundance. Thermal Energy Storage (TES) systems allow to storage cold to be used later and hence, to balance the energy demand between the peak and off-peak hours. Phase Change Material (PCMs) are commonly used to store cold as latent heat. PCMs have the benefits of high fusion heat and a suitable phase change (melting/freezing) temperature range for cold storage applications. Because of their superior high latent heat, a small size of storage systems can be achieved. For subzero applications, the commonly used PCMs are eutectic water-salt solutions while non-eutectic water-salt solutions like paraffins are rarely used. In general, the freezing point of water is depressed when salts are added to water. A wide range of phase change temperatures can be achieved with different salt compositions thus providing materials that can be used for different applications depending on the required operational temperature. It can be found that eutectic water-salt solutions have the advantage of lower phase change temperature and higher fusion heat and density, but they have poor thermal properties compared to water and present problems of supercooling. In last years it has been demonstrated that the addition of nanoparticles to a based material (solid composites, aqueous and oil based nanofluids, and molten salts) can be used to improve some thermal properties such as thermal conductivity and specific heat. In this work, thermal properties of an eutectic mixture of CaCl₂/water (T_m = -50ºC) were measured in the temperature range from -75ºC to 25ºC. Silica nanoparticles were dispersed in the salt solution and the stability of nanoparticles in liquid state as well as after thermal cycling was checked. Nanocomposite PCMs were prepared at different solid contents and the evolution of thermal properties with mass load of nanoparticles was studied. The addition of the nanoparticles lead to an increase in the thermal conductivity of the samples together with a decrease in supercooling, thus improving their performance as TES system.