Enhancing heat pump efficiency through wastewater heat recovery: a dual tank approach for sustainable energy storage

• A dual-tank setup enables efficient wastewater heat recovery • Heat pump COP improved by 8.66% at 25°C wastewater temperature. • Power usage dropped by 9.71% with a 5 °C rise in wastewater temp. • Biological treatment stays undisturbed during heat extraction. • Tested refrigerants: R134a, R410a, and R407C for performance. This study addresses the growing need for sustainable and decentralized heating solutions in rural areas by investigating a novel wastewater heat recovery strategy. In many remote communities, septic tanks are widely used but often poorly managed, leading to both environmental emissions and wasted thermal energy. Here, we explore a system that enhances heat pump efficiency by recovering heat from household wastewater stored in a secondary tank. The proposed dual-tank configuration separates biological treatment (in the first tank) from heat extraction (in the second tank), ensuring the biological processes remain undisturbed while enabling heat recovery. The baseline scenario considers a 75 m 2 dwelling occupied by two individuals, generating approximately 300 L of wastewater daily at a temperature of 25 °C. The recovered heat from this secondary tank is used to preheat the evaporator inlet of a heat pump. The system operates continuously throughout the day to meet a total daily space heating demand of 25.5 kWh. This heating demand was calculated using the Standard Assessment Procedure (SAP), the UK government’s approved methodology for evaluating the energy performance of residential buildings. The system was tested using three refrigerants, including R134a, R410a, and R407C. By incorporating thermal energy from the secondary tank, the results show an 8.66% COP improvement and a 7.97% power reduction at a wastewater temperature of 25 °C under the investigated operating conditions. Additionally, a 5 °C increase in wastewater temperature led to a COP improvement of 10.75% and a power reduction of 9.71%.