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The use of a horizontal ground heat exchanger may represent a reliable and cost effective option for ground-source thermal applications. This study presents the thermal performance analysis of a drainage trench used as ground heat exchanger (GHE) coupled with underground thermal energy storage (UTES). The trench is dug in shallow soil and filled with encapsulated phase change materials (PCMs) as granular filler. Two types of PCMs with different melting points are supposed to operate in summer and winter. Fluid flow and heat transfer in porous media are solved via a 2D finite element model to perform a yearly simulation under hourly-scale boundary conditions. The equivalent heat capacity approach is applied to consider the latent heat of the PCMs. The results show a significant capacity of the trench to smooth thermal waves produced by the heat pump. The effect of the PCMs is analysed by comparing with the corresponding case using coarse gravel as filling material instead of PCMs. The case without PCMs still shows good performance, but PCMs offers the advantages of a seasonal UTES and smoothing thermal wave as well. The proposed solution can be therefore considered as an advanced alternative to other widespread common GHEs

Ground-coupled and air-source heat pumps (GCHPs and ASHPs, respectively) are regarded as energy efficient systems for air conditioning. Their coupling in a dual air and ground source heat pump (DSHP) can offer a further performance improvement by reducing the drawbacks of each standalone technology. In the present study, a DSHP coupled with a Flat-Panel as a horizontal ground heat exchanger (HGHE) is numerically analysed in comparison with its counterparts GCHP and ASHP, by implementing COMSOL Multiphysics to simulate heat transfer in the ground operated by the Flat-Panel. The DSHP operativity is provided by a function set to control the switching between air and ground sources, according to their temperatures and trigger thresholds. A parametric analysis has been then carried out in order to propose a preliminary guideline to size the Flat-Panel for a balance between energy saving and installation cost. The DSHP shows a higher efficiency in comparison with either ASHP or GCHP due to the switching between two sources to more favourable working temperatures, and can offer a profitable hybrid solution providing protection against frosting and size reduction of the HGHE, therefore helping to promote the penetration of heat pumps in the residential market.

Ground-source heat pumps are a reliable technology and may represent an efficient and cost-effective option for space heating and cooling, when the investment for ground heat exchangers is reasonable. New advanced ground exchangers have been recently proposed, showing high performances also in shallow ground; their shape has not yet been investigated in literature. In the present study, an analytical solution based on the line source method is applied for sizing a novel shape. This so-called flat-panel shape is assumed to be an equivalent slinky-coil having the same heat transfer surface per unit of trench length. As overall benchmarks, two other configurations of straight pipes disposed vertically and horizontally have been sized; all devices are supposed to work in a four lined geothermal field. The building heating requirement has been evaluated assuming a simplified lumped system and three different climate zones, defined by 2,000, 2,500 and 3,000 degree days. Then, a 2D finite-element model has been implemented to solve the transient heat conduction problem in the ground. The results of the analytical formulation and numerical simulations have been compared in terms of average temperature at the wall surface of the heat exchanger. The design minimum temperature considered by the analytical method in sizing the two straight pipe configurations and the flat-panel is accurately reproduced by the numerical model. Therefore, the slinky-coil equivalent approach followed in the analytical method for sizing the flat-panel seems to be a reliable and suitable approximation.