Combining residential air heat pumps with air-geothermal heat exchangers

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Researchers at the University of Padova in Italy have proposed to improve the performance of residential air heat pumps with air-geothermal heat exchangers and have found this combination may reduce power consumption by up to 30%.

The scientists assessed how the proposed combination would perform in a heating, ventilation, and air conditioning (HVAC) system deployed in a passive single-family building located in Valtellina, northern Italy. The HVAC unit is described by the research group as an all-air type unit relying on a preheating/precooling air-geothermal heat exchanger, a heat pump unit, and an integrative post-heating thermal resistance.

“The purpose of the study is to investigate the behavior of an air-geothermal heat pump system to identify the main factors that affect the performance of the system under real operating conditions and to quantitatively determine the reduction in energy consumption due to the integration of the air-geothermal heat exchanger in the heating/cooling system,” the academics said.

In the proposed system configuration, the air-geothermal heat exchanger is utilized to pre-heat or pre-cool air depending on the season. “This layout guaranteed an almost constant temperature in the heating and cooling seasons independently of the outdoor air temperature,” the research team added.

The heat pump unit used in the experiment is a Compact P model from Danish manufacturer Nilan that is intended to work as both an HVAC system and a generator of domestic hot water (DHW). The thermal resistance provides heat during periods of high heat demand. “The unit is designed to operate in a standalone configuration, and therefore the air input can be outdoor air in the case of direct supply from the outdoor environment or air pretreated by the air-geothermal exchanger,” the group stressed.

In the heating season, the heat pump is activated when passive heat recovery from the geothermal heat exchanger is not enough to ensure the desired level of comfort, or when DHW demand is particularly high. In the mid-season, the air pretreated by the air-geothermal heat exchanger is conveyed to the heat pump and the inlet air directly supplies the internal environment, without heat exchange. In the summer season, when the the air supplied to the heat pump unit gets too hot to meet the building's cooling needs, the compressor is activated and the air is cooled in the evaporator.

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The system was modeled using Matlab software and was found to benefit from the air-geothermal heat exchanger, as it reduces significantly the heating and cooling demand during the year of operation in the modeling, which in turn results in a 30% reduction in electricity consumption.

“From a quantitative point of view, the analysis estimated that in winter 2019 without the air geothermal unit, the electricity consumption would be 130% of the measured one, due to the higher thermal demand, which in turn led to a higher operation of the post-heating electrical heater,” the scientists further explained. “In summer 2019, the heat pump was unable to meet the house's cooling needs.”

They also emphasized that minimizing indoor ventilation, with low outdoor temperatures, is a key factor in reducing indoor heat demand. “The results confirmed the influence of the air-inlet conditions and of the control strategy on the unit performance in both the heating and cooling modes,” they concluded.

The proposed combination was described in the study “Analysis of a domestic air heat pump integrated with an air-geothermal heat exchanger in real operating conditions: The case study of a single-family building,” published in Energy and Buildings.

Another research team at the University of Padova presented last year a 5 kW direct-expansion solar-assisted heat pump that uses alternatively two different evaporator technologies. This system utilizes an air-finned coil heat exchanger or photovoltaic-thermal (PVT) solar collectors as the evaporator.

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