| Work Detail |
Researchers at the University of Padua (Italy) have proposed improving the performance of residential air heat pumps with air-geothermal heat exchangers and have found that this combination can reduce energy consumption by up to 30%.
The scientists evaluated the performance of the proposed combination in a heating, ventilation and air conditioning (HVAC) system installed in a passive single-family building located in Valtellina, northern Italy. The research group describes the HVAC unit as an “all-air” type unit based on a preheat/precool air-geothermal heat exchanger, a heat pump unit, and a postheat integrative thermal resistance.
“The objective 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 in real operating conditions and to quantitatively determine the reduction in energy consumption due to the integration of the heat exchanger. air-geothermal heat in the heating/cooling system,” the academics point out.
In the proposed system configuration, the air-geothermal heat exchanger is used to preheat or precool the air depending on the season of the year. “This arrangement guaranteed a nearly constant temperature in the heating and cooling stations, regardless of the outside air temperature,” the research team added.
The heat pump unit used in the experiment is a Compact P model from the Danish manufacturer Nilan, which is designed to function both as an air conditioning system and as a domestic hot water (DHW) generator. Thermal resistance provides heat during periods of high heat demand. “The unit is designed to operate in autonomous configuration, so the air inlet can be outside air in the case of direct supply from the outside environment or air pretreated by the air-geothermal exchanger,” the group emphasizes.
During the heating season, the heat pump is activated when the passive heat recovery of the geothermal exchanger is not sufficient to guarantee the desired level of comfort, or when the demand for DHW is particularly high. In the middle season, the air pretreated by the air-geothermal heat exchanger is transported to the heat pump and the inlet air directly feeds the indoor environment, without thermal exchange. In the summer season, when the air supplied to the heat pump unit becomes too hot to meet the buildings cooling needs, the compressor is activated and the air is cooled in the evaporator.
The system was modeled with the Matlab program and it was proven that the air-geothermal heat exchanger is beneficial, since it significantly reduces the demand for heating and cooling during the year of operation in the modeling, which in turn translates into a 30% reduction in electrical consumption.
“From a quantitative point of view, the analysis estimated that in winter 2019 without the geothermal air unit, electricity consumption would be 130% of measured, due to the higher thermal demand, which in turn led to increased operation of the electric heater after heating,” the scientists further explained. “In the summer of 2019, the heat pump was unable to meet the cooling needs of the house.”
They also highlighted 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 the control strategy on the performance of the unit, both in 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 ” with an air-geothermal heat exchanger in real operating conditions: Case study of a single-family building”, published in Energy and Buildings.
Another research team from the University of Padua last year presented a 5 kW solar-assisted direct expansion heat pump that alternately uses two different evaporator technologies. This system uses an air finned coil heat exchanger or photovoltaic-thermal (PVT) solar collectors as an evaporator. |
