Researchers from the University of Calgary in Canada have investigated the thermodynamic performance of an air-source heat pump (ASHP) integrated with an air-based solar collector (SAC) for radiant floor heating in cold climates.
The system was simulated in the transient system simulation tool (TRNSYS) under the environmental conditions in Calgary.
“Heat pumps integrated with solar energy and radiant floor heating have the potential for high efficiencies,” the academics have said. “However, the performance of the combined system in cold climates is still not widely studied. Moreover, the influence of air recirculation and the associated control logic on the performance of SACs coupled with ASHPs has not been widely investigated.”
The main heating component was an ASHP with a rated heating capacity of 10.52 kW, operating between -25 C and 35 C and supplying water at 45 C for radiant floor heating. The heated water was stored in a 300 L thermal energy storage (TES) tank, which maintained stable supply temperatures and included an auxiliary heater for backup during very cold periods. To improve ASHP performance in winter, an unglazed SAC with a black-painted aluminum absorber plate was used to preheat outdoor air before it entered the heat pump evaporator. It had an air mass flow rate of 1 kg/s and an area ranging from 16 to 40 m².
The team explained that the heat pump delivers heat to a thermal energy storage tank, where energy is stored when demand is low and released when needed. The stored water then circulates through the building’s radiant floor system, providing space heating through radiant heat transfer. They also noted that, unlike previous studies, the system incorporates air recirculation from the heat pump’s evaporator. Specifically, when the outlet air temperature exceeds the ambient air temperature, the air is redirected back to the SAC; otherwise, ambient air is supplied directly to the SAC.
The building modeled in the study was a single-story detached bungalow-style house with a floor area of 79.15 m and designed for three occupants. It included three rooms, a kitchen, a living area, and a washroom, and was represented as a single thermal. The heating thermostat was set to 22 C and the cooling thermostat to 24 C, with radiant floor heating as the primary heat delivery system. The weather conditions were based on a typical meteorological year (TMY) weather file for Calgary with 333 sunny days and temperatures ranging from -25 C to 33 C.
“Coupling ASHP with SAC improved the coefficient of preference (COP), ranging from 2 to 4 to 2-6, particularly during the winter and daylight hours when solar radiation is available,” the academics emphasized. “The integration of a 40 m2 SAC enhanced the ASHP’s annual average COP by 7%, while reducing energy consumption by 256 kWh annually when the system operated with a -25 C evaporator air temperature lower threshold.”
The researchers further highlighted that coupling larger SAC sizes further improves the ASHP’s COP and reduces electricity consumption.
They explained that increasing the threshold for the entering evaporator air temperature limits the ASHP’s operation at higher temperatures, which are generally considered favorable conditions, thereby enhancing its overall performance. However, this adjustment also reduces the heat pump fraction (HPF) and increases the system’s reliance on the auxiliary heater, although the HPF remains below 20%. They further noted that integrating the SAC substantially improves system efficiency, enabling effective operation under severe weather conditions as low as −25 C, comparable to the performance of an ASHP without SAC operating under milder conditions around −15 °C.
“The system demonstrates the capability to maintain indoor temperatures within the desired range for over 97% of the year,” the scientists states. It was presented in “Thermodynamic performance of an air source heat pump integrated with an air-based solar collector for radiant floor-based space heating in cold climates,” published in the Journal of Building Engineering.
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