Formation of frost on heat exchanger surfaces is a well
known issue in air source heat pump systems at low
ambient temperatures.
The purpose of this project
is to improve the defrosting strategy of a DVI LV9
Combi heat pump, which currently operates with a fixed
defrost interval of 60 minutes. A model is developed to
determine the reduction in heat pump performance as
frost develops on the evaporator surface and to calculate
the optimal defrost initiation point.
The model is
validated against experimental measurements acquired
at relative humidities in the range of 65% to 85% and air
temperature of -4◦C to 4◦C. Two 5-hour experiments are
conducted at identical conditions in order to compare
the current defrost strategy to the one determined by
the model. The model suggests a defrost interval of 35
minutes at the given conditions, which is found to be
consistent with the point where the heat pump would
otherwise have a significant decrease in performance.
The new strategy results in a more stable performance,
but does not improve the coefficient of performance
significantly. A more stable performance would probably
allow DVI Energi to reduce the superheat reference point
and increase the performance of the heat pump.