Ventilator exhaust air, evaporative cooling boost residential heat pump efficiency

A South Korean research team has proposed a residential air-source heat pump (ASHP) system that combines exhaust air from an energy recovery ventilator (ERV) with direct evaporative cooling. Tests showed the approach could reduce the heat pump’s outdoor-unit inlet air temperature by up to 6.4 C and improve its coefficient of performance (COP) by 7.3% to 15.2% under hot and humid conditions.

“In this study, a system is proposed that improves direct evaporative cooler (DEC) performance by utilizing exhaust air from an ERV, thereby enhancing the heat rejection performance of air-source heat pump (ASHP) condensers,” the researchers said. “The objective of this study is to determine how the reduction in the outdoor unit (ODU) inlet air temperature obtained by mixing the exhaust air with the outdoor air affects the refrigeration cycle and the system performance.”

The experimental setup featured a 3 kW ASHP using R410A refrigerant, with a direct evaporative cooler installed in front of its outdoor unit. The cooler consisted of a 150 mm-thick wetted pad, four spray nozzles, a water sump, and a 21 W recirculation pump delivering 3 L/min.

In the proposed configuration, outdoor air is mixed with cooler, drier exhaust air from an ERV before passing through the wetted pad and reaching the ASHP outdoor unit. The researchers said this arrangement lowers both the dry-bulb and wet-bulb temperatures of the air entering the evaporative cooler, improving its effectiveness under humid conditions.

The scientists did not physically connect an ERV during testing. Instead, they calculated the expected mixed-air conditions based on an ERV exhaust airflow rate of 100 m³/h and reproduced them in a controlled chamber. Tests were conducted at outdoor temperatures ranging from 30 C to 33 C and relative humidity levels of 60% to 70%. The indoor chamber was maintained at 24 C and 50% relative humidity, with a 4 kW electric heater providing the thermal load.

“The ERV exhaust air lowered both the dry-bulb and the wet-bulb temperature of the air entering the DEC, contributing to the cooling through a sensible and a latent effect of comparable magnitude,” the researchers said.

The resulting reduction in outdoor-unit inlet temperature lowered the heat pump’s condensing temperature and compressor work. At 32 C and a humidity ratio of 19 g/kg, the condensing temperature fell by 6.8 K and the compression ratio by 7.6% compared with the conventional configuration. Compressor work decreased by 13.8%, while the heat rejection rate fell by 2.8%.

The researchers assessed the system’s overall efficiency using a COP correlation model across the tested temperature and humidity range. They also said the proposed configuration could be suitable for retrofits, as it would only require the addition of a direct evaporative cooler to an existing heat pump and a connection to the exhaust duct of an installed ventilation system.

“The proposed system is feasible for retrofitting because it can be implemented by installing a DEC unit on an existing heat pump and utilizing the exhaust duct of the installed ventilation system without requiring facility replacement,” they said.

The system was presented in the study “Integrating ventilator exhaust air and evaporative cooling for improving energy performance of residential air-source heat pumps,” published in the Journal of Building Engineering. The research team included scientists from South Korea’s Hanyang University and Korea Institute of Civil Engineering and Building Technology.

The post appeared first on pv magazine Global.