Thermal enhancement techniques for a lobed-double pipe PCM thermal storage system

Abstract

In this paper, the simultaneous harvesting and storing of solar thermal energy using a lobed double-pipe heat exchanger with Phase Change Materials (RT82), in a solar collector energy storage unit, has been simulated. The storage unit is a double pipe system, where it is contained within the inner pipe with RT82, while the outer pipe contains water as the heat transfer fluid. To improve the system efficiency in terms of PCM thermal performances, solutions like lobed geometries, metal foam, dispersed nanoparticles, have been considered, together with a carbon nanotube-based hybrid nano powder for the heat transfer fluid. Temperatures evolution have been evaluated with a 3D mathematical model that considers paraffin phase changes, nanoparticles, and porous media modeling. Various Stefan numbers were examined as input variables to evaluate thermal performances in terms of melting/solidification times, stored/released energy, and so on. The outcomes illustrate that, for whom wants to improve the efficiency of the system via geometry improvements, a six-lobed surface decreases the charging and discharging time by 18.32% and 11.40%, respectively, when compared to a one-lobe one. By combining all the enhancement techniques in Case F, and by using the lowest investigated porosities for the foam, the charging and discharging times are reduced by 66.68% and 81.62%, respectively, in comparison to Case A with pure RT82; on the other hand, it has been also shown that the main contribution to this comes from metal foam inclusion. Also, the rate of heat energy harvesting, and storage becomes 198.95 W and 169.78 W, and 0.0503 and 0.0268 in the scaled form, respectively.