Air-to-Water CO2 Heat Pump Efficiency ‘Highly Dependent’ on Defrost

According to the Danish OEM Fenagy, a specialist in CO2 heat pumps (R744) and industrial refrigeration, the efficiency of air-water heat pumps depends “to a large extent” on efficient defrosting.

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 disadvantage can be minimized to less than 10%, noted Kim G. Christensen, CEO of Fenagy.

So how do you get the most efficient defrost cycles? Several things need to be considered, including the method of defrosting, the design of the evaporator coils, and the algorithms that control the defrost cycles and the intervals between them.

Fenagy works with two types of defrosting methods: warm glycol and cold gas defrosting. “We use warm glycol to thaw our larger ones [CO2] Heat pumps of more than 1 MW (284.3TR), ”explains Christensen. “The glycol is heated with the return water from the district heating network (40 ° C) or with the waste heat available on site.”

The evaporator coils in the energy collectors of the Fenagy CO2 heat pumps are designed with separate circuits for the glycol defrost circuit. “This design is critical to the efficiency of the defrost system,” noted Christensen. The vaporizers, of which there are typically between two and eight, are then defrosted one after the other for about 15-20 minutes. Both the defrost time and the interval between them are currently controlled by algorithms that Christensen and his Fenagy colleagues developed themselves, but Christensen said the company could use artificial intelligence (AI) later to further improve its control strategy.

The advantage of CO2

Precise defrosting is always important for good energy efficiency, emphasized Christensen, but CO2 technology can have an advantage over other types of refrigerants because it offers more flexibility.

“Using CO2 as a working medium in heat pumps gives the system an advantage when defrosting is not performed 100% perfectly,” he explained. “The disadvantage of a slightly lower evaporation temperature due to frost / ice on the heat exchanger has less influence on the performance compared to other refrigerants, but continuous ice formation naturally destroys the performance of every heat pump.”

The climate also plays a role in efficient defrosting and must be taken into account when planning. “The efficiency of the defrost is reduced the lower the ambient temperature, but high humidity also has a negative impact on reducing the normal operating time between defrosts,” said Christensen. “However, high humidity improves the efficiency of the evaporators, which is fine when defrosting is not required, but [it can] have negative effects if defrosting is required. “

In addition, high wind speeds can have a negative effect, especially when using V-shaped evaporators, emphasized Christensen. He added that when operating in cold climates, it is important to remember to run the fans “in heavy snow, even when the heat pump is out of order – fans can go up to 12 inches. be hard to start [11.8in] Snow on the top, ”he added with a smile.

In other words: “The optimal defrosting time and the optimal time between defrosting cycles depend heavily on the weather conditions. and therefore a purely time-based control system will only work with very little efficiency, ”Christensen concluded.

“The use of CO2 as a working medium in heat pumps gives the system an advantage if defrosting is not carried out 100% perfectly. The disadvantage of a slightly lower evaporation temperature due to frost / ice on the heat exchanger has less influence on the performance compared to other refrigerants, but continuous ice formation naturally destroys the performance of every heat pump. “

Kim Christensen, Fenagy

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