Why does the COP increase when the temperature decreases?
This is one of the unique characteristics of heat pumps that work 100% on extracted air, such as the RT product from MyDATEC. In this type of heat pump, the temperature of the heat source is a fairly stable temperature of around 20°C (indoor temperature). This air of 20°c meets the cold battery of the heat pump which takes its calories from it. On the side of the new air, if we take a temperature of about +7°c, we have a heating of this air by crossing the hot battery of the heat pump whose fluid is at a temperature of about 60°c. If the temperature of the fresh air decreases, e.g. by -7°c, the heat transfer between the colder fresh air and the heat pump’s heating coil is better than in the case of fresh air at a temperature of +7°c because the temperature gradient is greater. As the transfer is better, the output heat production is better for an equivalent electrical consumption, which explains the increase of the performance and thus of the COP with the decrease of the outside temperature.
In comparison, the most popular heat pumps draw their calories from the outside air. As the outdoor temperature decreases, the calories are more difficult to recover, which is why the COP decreases when the outdoor temperature decreases. On the other hand, due to the large volumes of air that this type of unit mixes, outdoor air heat pumps can offer much higher heating capacities than extract air heat pumps. The power of the latter remains limited because it depends on the available ventilation flow, defined according to the size and the destination of the dwelling.
F. Lavogiez (2012 Study) – Technical Director MyDATEC