Using heat recovered from drain water and exhaust gases to enhance heat pump performance - Numerical study with economic and environmental insights

The energy crisis has gotten out of hand to the point where quick fixes are required. It goes without saying that a strong solution should either use renewable energy sources or manage the energy that is already available, such as through heat recovery. Exhaust gases from fuel power generators or boilers, as well as wastewater from showers and other sources, contain energy that is released into sewage systems and the atmosphere at relatively high temperatures. In this context, the present paper investigates the application of heat recovery systems to capture waste heat from power generator exhaust gases and household wastewater (e.g., from showers, washing machines, dishwashers) to improve the performance of an air-to-air heat pump system. Nine combined systems are proposed, with some utilizing drain water as a heat source for the evaporator, while others use drain water and/or exhaust gases to preheat the supply air, placing heat recovery heat exchangers around the condenser. An in-house code was developed to simulate these systems and assess their efficiency based on performance improvement and electric energy reduction at three ambient temperatures. Results indicate that all proposed systems outperform the basic air-to-air heat pump, with the configuration "D-C-UP-E-EG-C-DO HP"—using both drain and exhaust gas heat recovery—achieving the highest coefficient of performance (32.5 at -5 °C, 34.6 at 0 °C, and 37 at 5 °C) and electric energy savings (526.7 kWh/month at -5 °C, 426.7 kWh/month at 0 °C, and 337.2 kWh/month at 5 °C). Additionally, cost and environmental impact analyses for Lebanon show substantial savings and reduced CO₂ emissions, with the "D-C-UP-E-EG-C-DO HP" system yielding the highest monetary savings ($68.5/month at -5 °C, $55.5/month at 0 °C, and $43.9/month at 5 °C) and emissions reductions (373.9 kg CO₂/month at -5 °C, 302.9 kg CO₂/month at 0 °C, and 239.4 kg CO₂/month at 5 °C). By exploring and improving multi-source systems that combine various heat recovery systems with heat pumps, this study addresses a significant knowledge gap and paves the way for more efficient and environmentally friendly energy systems.