Effect of horizontal connection pipes on operation of borehole heat exchangers under different climatic conditions

Abstract

We investigate the effect of horizontal connection pipes in shallow geothermal systems attached to a borehole field for the 25 most representative climate zones across the World. For this we use a comparable 30m connection pipe buried at 1m depth connected to a 100m deep borehole heat exchanger (BHE) as the unit representation of the whole system for all locations. The system is considered monovalent with weather dependent loads derived from local air temperatures. The connection pipes and BHE are modeled using thermal resistances and response functions. In the investigated reference scenario, the consideration of the connection pipes leads to an average reduction in BHE load of 34% in tropical climates, 19% in temperate climates, 11% in arid and 4% in continental climates, leading to a significant drilling cost reduction. Exceptions were the cities of Moscow and Heihe both located in continental climates and under severely colder conditions, where the BHE load increased by 13% and 72% due to consideration of the horizontal connection pipes, leading to a reduced efficiency or the need for additional drilling only for these cases.