The COP of air source heat pumps during cold weather is still about 3.1 even allowing for defrosting energy? Jez BRE, BrendanUys – Heacol
Dear Clavertons – what is the cold day COP of a heat pump including all defrosting energy?
In my experience it is not the defrosting energy that impacts on the efficiency of an air source heat pump. Rather it is the effect of the gradual frost build up on the evaporator which mostly reduces the efficiency. The data from my system (with a design flow temperature of 35°C) gives the typical electric energy for a reverse cycle defrost at between 15 and 20 Wh for my 8kW Nibe. For a typical compressor cycle in typical foggy sub zero UK conditions my heat pump will be running at with ~40 minute long compressor cycles at around 2kW electric input for a heat output that starts at around 7 to 8 kW at the start of the cycle, falling to around 6kW at the end of the cycle. The typical daily COP (SPFH2) for my system on a winters day averaging 0°C outside temperature is 3.3.
There is plenty that manufacturers could do to improve frosting and defrosting behaviour for air source heat pumps. Better frost detection sensors and algorithms for a start, so the system only goes into defrost when it needs to. There is research going on to develop better evaporator fin designs and coatings that reduce frost build up. There is also work to develop air pre-conditioning units that remove the moisture before the air enters the evaporator. Some of these technologies will make their way onto products eventually.
The plot below shows a typical compressor cycle for my heat pump on a cold moist winter’s day in November. The outside temperature over the period is between -1°C and -2°C. The pink is the heat output and the purple/blue the electric input (left hand axis in W). The yellow line is the external temperature (right hand axis). The little blue blip at around 10:15 is the electric used by the reverse cycle defrost. For the period shown in the graph, integrating the data gives the heat output of 4.51 kWh for 1.43 kWh electric input (including the defrost) – giving a COP for the full cycle of 3.15.
Please note that I work part time. My normal work days are Wednesday, Thursday and in the morning on Friday. If you contact me outside of these times, I will not be able to provide a response until I am next in the office.
Dr Jez Wingfield
Senior Technician – Physical Building Performance
UCL Energy Institute
Central House, 14 Upper Woburn Place, London WC1H 0NN
“In God we trust; all others must bring data” W.E. Deming