24 June 2014

Performance Assessment of a Variable Capacity Air Source Heat Pump and a Single-Capacity Horizontal Loop Coupled Ground Source Heat Pump System

This study evaluates the performance of air source heat pump (ASHP) and ground source heat pump (GSHP) systems individually and as part of their associated heating and cooling distribution systems in two side-by-side semi-attached houses. Over the monitoring period, the efficiency of both heat pumps exceeded manufacturer and EnerGuide ratings, with seasonal Coefficients of Performance (COPs) above 5 in the cooling season and above 3 in the heating season. When the energy inputs associated with the heating and cooling distribution systems were accounted for, performance decreased by between 9 and 53%. Modelled optimization scenarios showed that considerable increases in system efficiencies could be achieved by configuring fans and pumps to operate only when the heat pump compressor is on, and by upgrading the GSHP from a single stage to a two stage system to reduce compressor cycling.
Performance of the ASHP was more adversely affected by declining winter temperatures than the GSHP. However, the ASHP continued to maintain indoor thermal comfort at temperatures as low as minus 24C without supplementary heat. Model simulations for five major Canadian cities showed that both technologies can perform well in the Canadian climate, but that residential GSHP systems are better suited to climates where winter temperatures fall below minus 24C when the ASHP would not be able to operate.
A simple cost analysis relative to electric resistance heating and central cooling revealed that although up-front equipment and installation costs are higher for both systems, the ASHP is more affordable with a simple payback of approximately 10 years. Even though the GSHP is slightly more efficient, simple payback is over two times longer due to higher initial capital costs. However, since the GSHP has a longer expected service life than the ASHP, the financial case for these systems would be more accurately assessed through a full life-cycle cost analysis. This simple cost analysis also omits the substantial benefits these systems offer in reducing greenhouse gas emissions which are directed related to the overall energy use.