Enhancing annual performance in air/ground dual-source heat pumps coupled to undersized borehole heat exchangers through variable source-selection logics

The energy performance of conventional Air-Source Heat Pumps (ASHPs) decreases significantly as the external air temperature drops, whereas Ground-Coupled Heat Pumps (GCHPs) are characterized by high borefield investment costs and risk of performance decline over time due to unbalanced building loads. To address these issues, this paper investigates the seasonal (SCOPnet) and annual (APFnet) energy performance of a Dual-Source Heat Pump (DSHP) able to exploit alternately aerothermal and geothermal energy, comparing it with that of conventional ASHPs and GCHPs through long-term simulations. A building with strongly unbalanced loads is considered, coupled to borefields up to 50% undersized. Three source-selection logics are examined: (i) setting of a switching temperature based on ambient air (ST logic); (ii) source selection as a function of time (SP logic); (iii) source determination based on DSHP instantaneous COP for both operating modes (PCOP logic). The ASHP exhibits the lowest performance (APFnet equal to 2.67), whereas the DSHP and GCHP yield 10–37% improvements. The source-selection strategy and borefield size play a key role in the DSHP performance: the SP logic offers minimal benefits, while ST and PCOP strategies guarantee up to +9% in SCOPnet, +8% in APFnet and −7% in electric energy consumption compared to conventional GCHPs. The shorter the borefield, the higher the advantage of using a DSHP with ST or PCOP control logic. The findings highlight the potential of DSHPs in applications with strongly unbalanced loads, or in retrofitting heat generators in existing geothermal-based HVAC systems when the borefield becomes undersized over time.