Varying refrigerant blends boosts HP flexibility

SWITZERLAND: Researchers have found that varying the composition of refrigerant mixtures can enable heat pumps to generate different temperatures from different heat sources, while increasing efficiency by up to 25%.

The new technology is said to hold particular promise for the electrification of process heat in the food, chemical and pharmaceutical industries. 

Researchers from ETH Zurich and the Eastern Switzerland University of Applied Sciences in Buchs argue that existing industrial heat pumps tend to be expensive constructions that are custom-made for a specific industrial application and temperature.

In conventional heat pumps, the achievable temperature and temperature profile are largely determined by the choice of refrigerant. All the heat pump’s components – from the evaporator and compressor to the condenser and expansion valve – are tailored to this refrigerant. If a factory requires heat at different temperatures for multiple applications, this can currently be achieved only by using multiple heat pumps, each with a different refrigerant. This is both costly and inconvenient, which is why heat pumps have failed to make headway in the industrial sector, the researchers claim.

The university researchers claim to have now developed a solution that enables heat pumps to generate heat cheaply and flexibly at various temperatures of up to 200ºC.

André Bardow, professor of energy and process systems engineering at ETH Zurich, believes his team has come up with a better solution: “Instead of a single refrigerant, we use a blend. This allows a heat pump to use different heat sources and generate different temperature profiles.”

The composition of the refrigerant blend can be varied to cater to different applications. So, instead of having to redesign the entire heat pump whenever they need a different temperature, companies can simply modify the mixture.

The tests were originally designed for low GWP HFO and HCFO refrigerants which offer a wide range of critical temperatures. For example R1234yf has a critical temperature of 94.7 °C and R1336mzz(Z) has a critical temperature of 171.4 °C. 

Additionally, as HFO and HCFO refrigerants do not require safety precautions like flammable refrigerants, they were suitable for exploratory mixture testing in a common laboratory environment. 

However, aware of environmental concerns and the fact that the EU may ban some HFO and HCFO refrigerants in the near future, the researchers are to conduct similar experiments with other fluids, among them also “natural” refrigerants.

The mixture itself consists of a traditional refrigerant and one further component. The temperature profile of the heat pump is dictated by the ratio of these two ingredients. “In principle, you can have any number of different profiles for industrial processes, as long as the temperatures don’t exceed 200º. That’s the major advantage our technology offers,” Bardow said.

To identify suitable components for the refrigerant blend, the researchers developed a computer model that simulates the heat-pump circuit with different variants of refrigerant mixture. “We extended the existing thermodynamic models for heat pumps by integrating the heat-pump components as well as the composition of the mixture into the optimisation process,” said Dennis Roskosch, senior scientist in Bardow’s research group.

The results show that mixtures not only outperform pure fluids at certain operating conditions but can maintain a higher COP across varying operating conditions. The binary and ternary mixtures in this study have glides between 10 and 40K and consist of R1336mzz(Z), R1233zd(E), R1224yd(Z), R1234yf and R32.

For a temperature change of 35K of the heat source and sink, the best mixtures were found to have a COP advantage of approximately 16% over the best performing pure fluid, R1234yf.

It was also found that the maximum COP of several binary and ternary mixtures was almost equal, indicating flexibility in mixture selection. Also, varying the operating conditions showed that the COP of mixtures was generally more stable than the COP of pure fluids.

Once the researchers found the optimum refrigerant blend, they confirmed its properties in the heat-pump laboratory at the Eastern Switzerland University of Applied Sciences. “The tests showed that our mixture increases the efficiency of a commercially available industrial heat pump by up to 25%, just as we predicted,” said Professor Stefan Bertsch, who heads up the heat-pump laboratory.

Numerous Swiss and international companies are already said to have already expressed an interest in the technology. Researchers are currently working closely with heat pump manufacturers such as MAN Energy Solutions and Scheco AG and with Swiss industry partners such as Lindt. The next step is to plan and build a pilot plant to carry out further tests.

The tests were published in the International Journal of Refrigeration.