Experiments and exergy analysis for a carbon dioxide ground-source heat pump in cooling mode

Abstract

A prototype CO2 ground-source heat pump (GSHP), i.e. a water-to-air heat pump designed to couple with a ground heat exchanger, was tested in cooling mode per ISO standard 13256-1 to provide experimental data for assessing and improving CO2-based GSHPs. The GSHP consisted of a vapor-compression cycle with a liquid-line/suction-line heat exchanger (LLSL-HX). The system operated in a subcritical cycle for antifreeze entering liquid temperature (ELT) ≤ 25 °C and a transcritical cycle for ELT > 25 °C. The ‘Standard’ condition metrics were: coefficient of performance (COP) 4.14, total capacity 6690 W, sensible capacity 5400 W, and sensible-heat ratio (SHR) 0.81. The ‘Part-load’ performance was: COP 4.92, total capacity 7240 W, sensible capacity 5640 W, and SHR 0.78. The GSHP was also tested at additional ELTs ranging (10 to 39) °C, where COP ranged (7.3 to 2.4). Compressor efficiency correlations were shown for these and 118 additional tests. The LLSL-HX was estimated to reduce COP by (0 to 2)% for ELTs ranging (10 to 25) °C, and increase COP by (0 to 5)% for ELTs ranging (30 to 39) °C. At the ‘Standard’ condition the major exergy defects were compressor 0.32, ground heat exchanger 0.19, condenser 0.11, evaporator 0.11, EEV 0.07, pump 0.07, and fan 0.04. The exergy defects were sensitive to ELT, though the compressor was always the largest. Estimated performance of the cycle without the LLSL-HX showed for ELT (10 to 39) °C the HX reduced the EEV throttling defect by (0 to 0.08) and compressor defect by (0 to 0.015), but increased the condenser/gas-cooler defect by (0 to 0.06).