WITHDRAWN: Life-cycle assessment of various types of residential ground-coupled heat pump systems: Wisconsin, US case

Abstract

Abstract Within the heating, ventilation, and air conditioning (HVAC) sectors, ground coupled heat pumps (GCHPs) offer a potentially economical, low-carbon energy approach to lowering of the global energy budget. This study investigates the performance of a conventional vertical GCHP configuration with three boreholes (VERT), a conventional horizontal GCHP (HORZ), an unconventionally deep (300 m) single borehole GCHP (SING) system, and a conventional split natural gas air conditioning unit (NGAC). This study also compares potential for reduced greenhouse gas (GHG) emissions via the GCHP systems in Wisconsin, USA, using a comprehensive “cradle-to-grave” life-cycle analysis (LCA), which is implemented using SimaPro. Assuming the current Wisconsin electrical grid of 5.5% renewables, heating and cooling loads of a 186 m2 residence, a coefficient of performance (COP) of 4 and a 25-year lifetime, an average of 272 metric ton CO2 equivalent emissions is calculated for SING. Top contributors are heat-exchanger operation (93.3%), borehole drilling (2.4%), and circulation pump operation (1.5%). This amounts to GHG emissions savings of 10% and 19% over VERT and HORZ GCHPs, respectively, and 27% over NGAC. Sensitivity analyses determine that a grid with renewables penetration of 50% could save 68% GHG emissions over natural gas. As the use of fossil fuel decreases and the grid becomes cleaner, GCHP systems become even more beneficial from the perspective of lifetime GHG emissions. A COP of 5 could further reduce GHG emissions by 38%, indicating that the COP is a significant factor of GCHP environmental impacts.