Numerical model and simulation of a solar thermal collector with slurry Phase Change Material (PCM) as the heat transfer fluid

The performance of conventional, water based, solar thermal collectors is limited by some intrinsic limitations, such as the need for high irradiation levels and the heat loss due to the relatively high temperature of the heat transfer fluid. In order to overcome these limitations and to improve the performance of solar thermal collectors, a different kind of heat transfer fluid can be proposed. This fluid is based on the exploitation of the latent heat of fusion/solidification of suspended particles, which change their state of aggregation at a micron scale, but maintain the liquid state of the fluid at a macroscopic scale. The so-called slurry phase materials, or PCS, are examples of this kind of material. In order to evaluate the effectiveness of such a concept, a numerical model of a PCS-based flat-plate solar thermal collector has been developed, presented and discussed. This model has been derived from the well-known Hottel-Whillier model, but several changes have been implemented so that a phase change of the heat transfer fluid can be handled, as well as the thermophysical properties of a non Newtonian fluid, such as those of a PCS. The paper presents the main and auxiliary equations that have been introduced to modify the Hottel-Whillier model. A numerical analysis conducted with the newly developed model is also presented in the paper. The aim of these simulations was to test the code and obtain a preliminary evaluation of the performance of the novel concept. Different (dynamic) boundary conditions (location, orientation, PCM concentration) were adopted to evaluate the performance of the PCS-based technology and compare it with that of a conventional solar thermal collector. The outcomes of the simulations have proved model robustness and the possibility of using it for preliminary analysis. It was also shown that the adoption of the PCS as a heat transfer fluid can lead to an increase in solar energy exploitation of different magnitude according to the climate. The greatest benefit can be achieved for cold climates. The limitations of the analysis (e.g. fixed, non-optimal flow rate) are also discussed. (C) 2016 Elsevier Ltd. All rights reserved.