Norwegian Researchers Develop Simplified Ejector-Based CO₂

October 25, 2021

The Norwegian University of Science and Technology (NTNU) and Norwegian research institute SINTEF have successfully tested a transcritical CO₂ (R744) supermarket booster system that includes a multi-ejector and low-pressure receiver that “boost the energy efficiency and keep the system rather simple,” said Armin Hafner, professor of Refrigeration Technology at NTNU.

“When it comes to ejectors, we have seen a lot of impressive innovations, but now it’s time to go back to a simpler system and make this a global application,” Hafner, a pioneering researcher in CO₂ refrigeration, who shared his insights in the Technology Trends session at ATMOsphere Europe, held online September 28-29. “The simpler the system, the easier is wide implementation.” In part, this is because training is easier, he noted.

NTNU has tested the simplified ejector system at its facility for different load conditions. “The next time, we will build this in a project; we will implement this concept,” he said.

In NTNU’s booster system, a multi-ejector is used in lieu of a high-pressure control valve. At higher ambient temperatures, the ejector receives high-pressure gas from the gas cooler and distributes it to the expansion devices in front of the evaporators; in addition, the ejector’s middle connection acts as a suction port, receiving the gas/liquid mix from the medium-temperature evaporators and distributing it to the liquid receiver at an elevated pressure. The pressure in the liquid receiver – not in the evaporators – “is the pressure level the compressor sees,” Hafner noted.

At lower ambient temperatures, the ejector is not used; the high-pressure fluid is distributed directly to the expansion devices, and the suction port still receives fluid from the evaporators, but “passively,” said Hafner. “So it’s a very simple active/passive mode ejector.”

This booster system “can outperform any other system,” said Hafner, including those using parallel compression.” Moreover, he added, the ejector can replace one or two compressors.

“CO₂ systems are energy efficient all over the globe if designers do a good job,” added Hafner. “I have not met one end user who regrets that they have switched to CO₂.”

Outperforming HFCs

Hafner described another application of an ejector in a CO₂ heat pump/chiller made for commercial and industrial applications by Italian OEM Enex. The ejector elevates the suction of the compressor, improving energy efficiency and enabling the heat pump to deliver 90°C (194°F) water temperatures. “It’s a very smart implementation of an ejector,” he said.

The Enex heat pump/chiller developed “shows that it is possible to really outperform even existing HFC chillers,” said Hafner. “And we can provide a future- proof solution which is nonflammable, especially in the environment around hotels inside the city center where might sometimes be challenging to have a flammable refrigerant.” He expects this product to enter more marketplaces.

Hafner also cited Danish OEM Fenagy’s CO₂ heat pumps, which have been installed in district heating applications, as another promising technology. Designed for Scandinavian temperatures (from -20 to 20°C/-4 to 68°F), the heat pumps have a capacity of up to 1.8 MW (511.8 TR) per skid. “This kind of heat pump will be more and more installed to supply heat into existing district heating systems,” he said, noting that another manufacturer is developing a district-heating system up to 50 MW (14,217 TR) in capacity using heat from seawater.

Helping to bring CO₂ refrigeration to industrial applications are larger compressors such as the Bitzer CKHE7, and the Dorin CD600, which are in the range of 100m³ (3,531 ft³)/h, noted Hafner. These compressors have made it possible to build systems above 1.0 MW (284 TR) in capacity with “a decent number of compressors,” said Hafner. “This is what the market has been looking for for a long time.”

Hafner pointed out that every HVAC&R application has at least two and as many as four natural refrigerant solutions from among the five primary natural refrigerants (CO₂, hydrocarbons, ammonia, water and air.) Moreover, natural refrigerants can go lower and higher in temperature than HFCs, which are geared toward the most profitable mid-temperature applications.

“There is no technical barrier to replace currently used synthetical fluorine-containing refrigerants with natural working fluids,” he said.” It is more about seeing the possibilities than the ghosts when selecting the optimal refrigerant for the project or a product.” He advised informing end users about natural alternatives. “End users 99% [of the time] choose natural alternatives if they get the possibilities.”

Hafner also raised the health and environmental concerns associated with the growing use of HFO-1234yf, which converts to trifluoroacetic acid (TFA), part of the PFAS chemicals group, in the atmosphere, leading to TFA deposition in drinking water.

The refrigeration sector does have all possibilities to spearhead a complete phase-out of HFCs contributing to the PFAS accumulation in our biosphere, without compromising safety, food supply, and human comfort,” said Hafner.

“The green shift is coming, and we are part of the solution when we use natural working fluids,” he said.