Glycol defrost offers several benefits to operators of industrial air-source CO2 (R744) heat pumps, according to Kim Christensen, the Managing Director of Danish heat pump manufacturer Fenagy.

These benefits include enhanced system efficiency, the ability to defrost when a heat pump is not running, and safe and reliable operation under “all circumstances,” Christensen explained.

“Efficient defrosting of energy collectors and evaporators for industrial air-to-water heat pumps is essential for their stable and economical operation for applications like district heating,” he said.

Founded in 2020, Fenagy’s core business is helping customers in industrial refrigeration and district heating transition to greener and more sustainable technologies. The company is seeing a growing interest in larger heat pumps (400kW/113.7TR and above) and has expanded its production capacity to meet demand.

To date, the largest heat pump system installed by Fenagy has been roughly 13MW (3,696TR) in capacity and consisted of five H2600 heat pump units.

“Efficient defrosting of energy collectors and evaporators for industrial air-to-water heat pumps is essential for their stable and economical operation for applications like district heating.”

Kim Christensen, Fenagy

“As the first on the market, Fenagy has delivered, commissioned and handed over a larger number of air-to-water heat pumps using CO2 as a refrigerant with DX evaporators,” he added. “The experience we’ve gathered has been incorporated into our PLC controller, making it possible to optimize the defrosting processes.”

For larger industrial heat pumps, Fenagy uses flatbed evaporators to ensure better air distribution, reduced recirculation of cold air and better drainage of the condensate. It also offers a better design for defrost, Christensen said. With this approach, the company suggests a glycol-based defrost method.

“Fenagy’s solution is optimized to defrost only when necessary and only for as long as it is necessary,” Christensen said. “When defrosting is required, it is done in a way that allows for the maximum possible capacity and efficiency during the operation.”

Increasing heat pump COP

The defrosting system consists of a glycol module located in the machine room that contains a heat exchanger, two pumps, an expansion vessel, valves and measurement sensors.

The system uses recovered heat from the district heating network’s return line to warm the glycol, which is then circulated through the evaporators in pipes integrated into each unit based on the required heat load.

This approach also reduces the temperature on the district heating return line, which cools the R744 a little more than usual, explained Christensen. This increases the heat pump’s COP and minimizes the unavoidable capacity loss during defrosting, he added.

The evaporator package consists of a minimum of four independent sections that can be isolated and defrosted individually. Each evaporator is fitted with an on/off ball valve that is activated and deactivated during the defrosting process.

The first step of the defrost process is to close the expansion valve to the evaporator that will be defrosted. The evaporator’s fans are then shut down, and the evaporator is heated via the warm glycol circuit.

Immediately after defrosting one evaporator, the process will start on the next evaporator, and the cycle is repeated until all evaporators have been defrosted. For systems with several parallel heat pumps, multiple evaporators can be defrosted simultaneously.

“Fenagy’s solution is optimized to defrost only when necessary and only for as long as it is necessary. When defrosting is required, it is done in a way that allows for the maximum possible capacity and efficiency during the operation.”

Kim Christensen, Fenagy

All-weather defrosting

In normal weather conditions, the defrosting cycle starts when the differential pressure on the air side reaches a predefined setpoint.

In less common weather conditions, the company has developed timers that calculate the minimum and maximum times between defrost sequences as well as the actual defrost duration. These timers depend on air temperature, relative humidity, glycol temperature and other factors.

More extreme weather – such as heavy snow, sleet or frost – can challenge the automatic defrosting processes because the fan protection grids may be blocked, or the fan blades might become covered in ice, Christensen explained. In such events, an alarm will notify operation personnel to initiate a manual defrosting cycle or to manually remove snow or ice.

Ice frosting up the evaporator of a 5.4MW (1,535TR) CO2 heat pump system in Vildbjerg, Denmark. (Source: Fenagy)
Ice frosting up the evaporator of a 5.4MW (1,535TR) CO2 heat pump system in Vildbjerg, Denmark. (Source: Fenagy)

“If the evaporators are exposed to total freezing, a manual defrosting cycle is activated, and this continues until the evaporator is free from ice,” he added. “This can be done while the heat pump is in operation as the defrosting system can be operated completely independently.”

A special winter function has also been developed, which can be activated manually by the operating staff when the weather requires it.

In all conditions, the stop signal for an evaporator in defrost is the temperature of its fins.

Even with the heat pump is not in use, the defrost system can still be used to ensure the evaporator’s safety grids are not covered with snow. To achieve this, fans are operated at low RPMs.

“We have the market’s most robust defrosting – and the most energy efficient,” says the manufacturer in a recent LinkedIn post.