The efficiency of air-to-water heat pumps is “highly dependent” on efficient defrosting, according to Danish OEM Fenagy, a specialist in CO2 (R744) heat pumps and industrial refrigeration.
Inefficient defrosting can cost more than 25% of COP and heating capacity during defrost cycles, the company said. However, with optimal design of the evaporator coils and control systems, this penalty can be minimized to less than 10%, noted Fenagy CEO Kim G. Christensen.
So how does one achieve the most efficient defrost cycles? Several things need to be considered, including the defrost method, the design of the evaporator coils and the algorithms controlling the defrost cycles and the intervals between them.
Fenagy works with two types of defrost methods: warm glycol and cold gas defrost. “We are using warm glycol for defrosting our bigger [CO2] heat pumps of more than 1MW (284.3TR),” explained Christensen. “The glycol is heated by the return water from the district heating system (40°C/104°F) or with some means of waste heat available on site.”
The evaporator coils in the energy collectors of Fenagy CO2 heat pumps are designed with separate circuits for the glycol defrosting loop. “This design is crucial to the efficiency of the defrosting system,” Christensen noted. The evaporators, of which there are typically between two and eight, are then each defrosted sequentially for approximately 15-20 minutes. Both the defrost period and the interval between them are currently controlled by algorithms Christensen and his Fenagy colleagues have developed themselves, but Christensen said that the company might later adopt artificial intelligence (AI) to improve their control strategy even further.
Precise defrost is always important for a good energy efficiency, Christensen stressed, but CO2 technology may have an advantage over other refrigerant types in providing more flexibility.
“Using CO2 as working fluid in heat pumps gives an advantage to the system if defrost is not performed 100% perfectly,” he explained. “The penalty of a somewhat lower evaporation temperature due to frost/ice on the coil has less influence on performance compared to other refrigerants, but of course a continuous buildup of ice will destroy the performance of any heat pump.”
Climate also plays a role in efficient defrosting and must be taken into account in planning. “The efficiency of the defrost is reduced with reduced ambient temperature, but high humidity also has a negative impact of shortening the normal operation time between defrost,” Christensen explained. However, “high humidity improves the efficiency of the evaporators, which is good when defrost is not needed, but [it can] have a negative impact when defrost is needed.”
In addition, high wind velocity can have a negative impact, especially when using V-shape evaporators, Christensen stressed. He added that when operating in cold climates it’s important to remember to operate the fans “during heavy snow fall, even if the heat pump is out of operation – fans can be difficult to start with 30cm [11.8in] of snow on the top,” he added with a smile.
In other words, “optimal defrost time and optimal time between defrost cycles heavily depend on weather conditions. and, therefore, a purely time-based controller system will only operate with very poor efficiency,” Christensen concluded.
“Using CO2 as working fluid in heat pumps gives an advantage to the system if defrost is not performed 100% perfectly. The penalty of a somewhat lower evaporation temperature due to frost/ice on the coil has less influence on performance compared to other refrigerants, but of course a continuous buildup of ice will destroy the performance of any heat pump.”
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