Benefits in Coupling PCMs with Dual-Source Heat Pumps

Abstract

Unlike high performance of ground-source heat pumps (GSHPs) as technology for space heating and cooling, installation costs of their ground heat exchangers (GHEs) make the technology less competitive in mild climates than air-source heat pumps. Recently, novel horizontal and shallow GHE solutions take advantage in heat transfer from advanced shapes, making similar solutions less expensive when compared with vertical boreholes. But, the payback remains too long to justify the initial investment. As a consequence, new strategies face the economic affordability by reducing the GHE length hybridizing with other thermal sources. A smart solution is the dual source heat pumps (DSHPs) which switch between air and ground, in order to reduce frosting issues and save the system against low/high temperatures affecting air-mode by using the ground as alternative source and thermal storage system. As the ground is not continuously exploited, GHEs can be sized shorter, and therefore their installation costs become lower than for a full geothermal system. To enhance further the energy performance, that kind of non-continues ground exploitation suggests the coupling of phase change materials (PCMs), as proposed in previous studies for full geothermic closed loop. Matching the former remarks, the present study faces numerically the GHEs coupling of a DSHP with PCMs, by analysing the performance for a whole year of thermal building loads with and without PCMs. As GHEs, the novel flat-panel solution is considered because its flat shape allows an optimal coupling with PCMs. The flat-panel is supposed installed edgeways into a very narrow trench, in which PCMs are mixed with the backfilling material. As PCMs, two different kinds of paraffin are considered to match winter and summer time with different melting points. A 2D model of the cross section of the installation is implemented in COMSOL Multiphysics to solve the heat transfer in porous media with unsteady boundary conditions. Cases with PCMs show better performance only if high thermal conductivity is numerically supposed, since their common low thermal conductivity affects the heat transfer and reduces all benefits.