A simulation model for solar assisted shallow ground heat exchangers in series arrangement

Abstract

Ground source heat pump systems are often challenged by high costs and thermal load imbalance in the ground that impairs their effectiveness. In an attempt to address ground load imbalance in heating dominated climates and high drilling costs of deep geothermal boreholes, a simulation model is developed to evaluate the performance of shallow solar assisted ground heat exchanger bore fields. The bore field comprises several relatively shallow vertical boreholes with independent circuits connected to both a ground source heat pump and solar collectors. The model accounts for heat exchange through horizontal connections between boreholes as well as the axial ground temperature change due to seasonal ambient temperature variations.The required borehole length of different ground heat exchanger network arrangements are examined under different values of minimum fluid temperature returning to the heat pump, total solar collector area and borehole spacing. All cases are compared against a one borehole configuration with and without solar heat injection.Results show that solar heat injection into the ground decreases the required borehole length by a greater amount for a field of shallow boreholes than for a one deep borehole configuration. Minimum fluid temperatures returning to the heat pump has a significant effect on the required borehole length for both a one borehole configuration and shallow bore fields (50% and above from 0°C to −5°C). For networks of shallow boreholes, borehole spacing has to be determined carefully as the total required length of shallow bore fields can be as short, or even shorter, as a one borehole arrangement.