Thermal energy storage with nanoparticle embedded phase change materials

Abstract

Due to the discontinuous nature of renewable energy sources, energy storage technologies play a key role in renewable energy systems. Phase change materials are one of the most common energy storage technologies thanks to its latent heat storage capacity. Moreover, there are some studies to improve its thermophysical properties and its storage capacity by using nanoparticles. Different types of nanoparticles with different concentrations are investigated in the literature. From the studies in the literature, it is seen that while some thermophysical properties of nanoparticle additives have positive effects on PCM, some of them affect PCMs negatively. This situation makes an optimization study is necessary. In this study, enhancement of thermophysical properties of PCMs are examined. Improvement of thermal conductivity, sensible and latent heat storage capacities with nanoparticle embedded PCM are compared. 3 different types of paraffin wax (RT 42, RT 55, and RT65) from the same company as PCM and 6 different types of nanoparticles (TiO2, graphene, Al2O3, CuO, SiO2 and carbon black) are chosen for the comparison. The first comparison parameter is thermal conductivity for the nano-PCM because it affects storage time. Effective thermal conductivity of the nano-PCMs is calculated by using Maxwell equation within the mass concentration range of 0 – 1 %. By using Hamilton Crosser equation, the results are checked, and it is seen that both equations gives the same results. Graphene nanoparticle with concentration of 1% with RT42 paraffin wax has the highest thermal conductivity. The second parameter for the comparison is heat storage capacity, and according to calculations CuO nanoparticle with 1% concentration with RT65 paraffin wax has the highest heat capacity. The other parameter for the comparison is the latent heat capacity which directly proportional to energy released during the phase changing. Pure RT55 has the highest latent heat capacity. On the other hand, nanoparticle additives reduce the latent heat capacity of the phase change materials. SiO2 with the concentration of 0.1% provides the minimum decrease in latent heat, the maximum fall is CuO with the concentration of 1%. The another parameter to compare is discharge time. Discharge time is the elapsed time during the solidification of nano-PCM. CuO with the concentration of 1% provides the longer discharge time which enlarge the time more comparing to pure PCM. Total heat storage is another parameter for the comparison. Combination of the 1% concentration of CuO and RT-55 PCM provides the highest total heat storage. Decrease in volume of nano-PCM to obtain the same heat transfer for all nanoparticle embedded phase change material is another parameter. It is seen that the minimum volume need is combination of 1% concentration of CuO and RT-55 PCM . When all the parameters were considered, CuO nanoparticle embedded PCM showed the best performance with RT-65 paraffin wax.