Analysis of groundwater advection and ground-heat exchanger spacing on intermittent ground-source heat pump operation

Abstract

Shallow geothermal systems allow both heating and cooling of structures by using the ground as a renewable energy source. These systems have received significant attention in recent decades and careful design consideration is necessary to ensure maximum efficiency. In this paper, the authors examine the impact of spacing between ground heat exchangers (GHE) and groundwater advection on a 5 × 5 array using a transient numerical model. The conditions chosen represent those experienced in Brisbane, Australia. The energy load used represents four months of cooling and three months of heating annually, a simplification based on local conditions. The spacing explored ranged from 4–10 m between GHEs and the groundwater flow examined included 0.1, 10, 30 and 50 m/year. The results show increasing spacing reduces thermal interference and increases the sustainability over the lifetime of the system, represented as 25 years in this study. In these conditions, a GHE spacing of 4 m can experience temperatures 8.5 °C larger then when the spacing is 10 m. While increasing spacing increased sustainability, this did not prevent an imbalanced load reducing efficiency overtime. Furthermore, it was found groundwater flow had a positive impact on sustainability, over the lifetime of the system a temperature 3.1 °C lower was experienced when a groundwater flow of 0.1 m/year was compared to 50 m/year. Such an impact should be taken into consideration during the design phase due to the reduced GHE length required.