1. Thermodynamic Background
Based on the Carnot cycle, the theoretical maximum COP of an air source heat pump is defined as:
COP_max = T_hot / (T_hot - T_cold)
Where T is the absolute temperature in Kelvin. The formula shows that the smaller the temperature difference between the source and the sink, the higher the efficiency.
In real systems, the actual operating temperature is much lower than this theoretical maximum. According to the ASHRAE Handbook (2020), modern air source heat pumps typically only achieve 40% to 60% of the Carnot limit due to thermodynamic losses and component inefficiencies.
Engineering Insight: The Carnot principle is a valuable benchmark, but real-world system behavior is driven by compressor performance, refrigerant thermophysical properties, and system control strategies.
2. Field Data
The European Air Source Heat Pump Association (EHPA) provides seasonal performance test results that highlight the impact of falling ambient temperatures:
When the outdoor temperature drops from 7°C to -7°C:
The air source heat pump COP drops from 4.2 to 3.1 (-26%)
The ground source heat pump COP drops from 5.1 to 4.3 (-16%)
These trends are prevalent in climate zones with higher heating demand. For example, in southern Finland, some residential units have recorded COP values below 2.0 during prolonged cold weather.
3. COP Reduction Mechanisms
Lower outdoor temperatures can cause a significant drop in the COP of air source heat pumps for the following reasons:
1) Lower evaporation pressure, higher compressor pressure ratio, and increased energy consumption
2) Reduced refrigerant mass flow, impairing heat transfer to the evaporator
3) Frequent defrost cycles, which consume auxiliary power and disrupt steady-state operation
1. Thermodynamic Background
Based on the Carnot cycle, the theoretical maximum COP of an air source heat pump is defined as:
COP_max = T_hot / (T_hot - T_cold)
Where T is the absolute temperature in Kelvin. The formula shows that the smaller the temperature difference between the source and the sink, the higher the efficiency.
In real systems, the actual operating temperature is much lower than this theoretical maximum. According to the ASHRAE Handbook (2020), modern air source heat pumps typically only achieve 40% to 60% of the Carnot limit due to thermodynamic losses and component inefficiencies.
Engineering Insight: The Carnot principle is a valuable benchmark, but real-world system behavior is driven by compressor performance, refrigerant thermophysical properties, and system control strategies.
2. Field Data
The European Air Source Heat Pump Association (EHPA) provides seasonal performance test results that highlight the impact of falling ambient temperatures:
When the outdoor temperature drops from 7°C to -7°C:
The air source heat pump COP drops from 4.2 to 3.1 (-26%)
The ground source heat pump COP drops from 5.1 to 4.3 (-16%)
These trends are prevalent in climate zones with higher heating demand. For example, in southern Finland, some residential units have recorded COP values below 2.0 during prolonged cold weather.
3. COP Reduction Mechanisms
Lower outdoor temperatures can cause a significant drop in the COP of air source heat pumps for the following reasons:
1) Lower evaporation pressure, higher compressor pressure ratio, and increased energy consumption
2) Reduced refrigerant mass flow, impairing heat transfer to the evaporator
3) Frequent defrost cycles, which consume auxiliary power and disrupt steady-state operation
