Simulation of Heat Transfer Process in a Novel Phase Change Material Used for Solar Thermal Energy Storage

Abstract

Harnessing solar thermal energy involves the design and construction of systems for the collection, storage, and distribution of heat. Heat storage can be done by sensible heat or latent heat, with the latter being preferred due to the higher densities achievable. The design of a storage system must consider the properties of the phase change material to be used, such as temperatures and enthalpies of fusion and crystallisation, as well as aspects of the heat exchanger, such as materials, configuration, and surface area, among others. This paper presents the simulation of the melting process of hydrogenated palm stearin for its use as a phase change material of renewable origin. A two-dimensional rectangular shell-coil geometry was used. The influence of the coil geometry on the heat transfer rate was evaluated using a constant flow of 3 L/min and a temperature of 75 C for the heat transfer fluid. According to simulations using COMSOL Multiphysics®, the heat transfer increases considerably for configurations where natural convection is favoured. These maximum transfer rate configurations are characterised by allowing the liquid phase of the PCM to flow to the top of the system. Additionally, despite the increases in heat transfer, the limiting factor is strongly defined in the PCM, which generates small differences between the inlet and outlet temperature of the heat transfer fluid. These results allow for heat exchanger designs based on shell-coil systems that, in addition to heat transfer improvements, let for the reduction of construction costs and the use of additives or support matrices.