Energy efficiency and environmental assessment of an integrated ground source heat pump and anaerobic digestion system

Abstract

A hybrid system that integrates a ground source heat pump and an anaerobic digester (GPAD) was proposed and designed for rural buildings in the Hot Summer and Cold Winter (HSCW) region in China. The GPAD system can alleviate soil thermal imbalance and enable the anaerobic digester to produce biogas more efficiently during winter and transition seasons. To quantitatively assess such a system's energy and environmental performance, a two-story house located in Chongqing municipality was selected as a case study. Two conventional scenarios, including Ground Source Heat Pump (GSHP) and Air Source Heat Pump (ASHP) systems, were compared with the GPAD system. Results suggested that the annual energy efficiencies of the three scenarios were 3.66 (GSHP), 2.54 (ASHP), and 4.17 (GPAD), respectively; the GPAD system exhibited the highest efficiency, which was 14.0% higher than the GSHP system and 64.1% higher than the ASHP system. Meanwhile, the Thermal Imbalance Ratio (TIR) of the GPAD system was 32.2%, which was much lower than the GSHP system (53.5%). When considering the emissions avoided (avoiding treatment of manure and rice straw, consumption of chemical fertilizer and liquefied petroleum gas), Global Warming Potential (GWP) indicator was reduced from 0.34 kg CO2-eq (GSHP system) to 0.05 kg CO2-eq for unit heating and cooling. The total environmental impact index of the GPAD system was 99.2% lower than that of the GSHP system and 99.4% lower than that of the ASHP system. Besides, improving the Coefficient of Performance (COP) of the heat pump unit and integration with a photovoltaic system that generates electricity to power the operation of the GPAD system could significantly improve environmental performance.