Hydraulic and thermogeological design differences between two-loop vertical and inclined coaxial borehole heat exchangers

Abstract

Abstract One of the most common geoexchange system in practice today is a vertical two-loop heat exchanger. There is also a growing trend of inclined drilling of multiple 40–50 m boreholes with coaxial heat exchangers, which are efficient when installing in areas where there is a favourable groundwater flow. Special care should be taken in hydraulic design of coaxial system, since laminar flow often occur in annular channel, due to glycol mixture implementation. In this research, vertical and inclined coaxial borehole exchangers were compared in identical geological environment, to express differences in heat extraction and thermal resistances, as well as in flow regimes and pressure drop. Extended Steady-State Thermal Response Step Test was implemented, which incorporate series of power steps to determine borehole extraction rate at steady-state heat transfer conditions. Results show that 2U-heat exchanger has advantages over coaxial heat exchangers, in terms of greater steady state heat extraction rate and more favourable hydraulic. Defined extraction rate at 0/-3 °C borehole temperature, in dominantly clay ground, was 43,5 W/m for coaxial system, compared to 54,5 W/m for vertical 2U-loop. Borehole resistance analysis showed almost double value for coaxial system compared to 2U-loop, due to frequent problems in cementation of inclined boreholes.