Keeping it cool

Refrigeration and air-conditioning technology accounts for around 15 per cent of global electricity consumption, yet it usually remains out of sight. Professor Dr.-Ing. habil. Michael Kauffeld, a Steinbeis entrepreneur and Professor of Refrigeration Technology at Karlsruhe University of Applied Sciences, has been working for decades to make this technology more sustainable. In conversation with TRANSFER, he explains why natural refrigerants are key to greater environmental and climate protection – and how a stay in the USA turned into a lifelong passion.

Professor Kauffeld, why refrigeration and air-conditioning technology – what particularly fascinates you about this field?

I came to refrigeration and air-conditioning technology more or less by chance. Originally, I wanted to study shipbuilding and therefore completed my military service with the German navy. However, my father, who had worked for a shipping company for several years, told me that shipbuilding had no future in Germany. So I began studying mechanical engineering, intending to specialise in design engineering or industrial robotics.

In early 1986, the University of Hanover, where I had been studying mechanical engineering since 1983, advertised scholarships for a one-year stay in the US in the field of refrigeration technology. I applied and, from August 1986 to September 1987, was able not only to experience the country and its people but also to immerse myself in refrigeration and air-conditioning technology. The diverse challenges of this field – such as ozone depletion and climate change – fascinated me both as a student and later as a doctoral candidate. Environmental protection was and remains a central motivation for me, both professionally and personally.

Incidentally, in many conversations with colleagues, it has become clear that their decision to pursue refrigeration and air-conditioning technology was also rather coincidental. This is understandable, as well designed and functioning refrigeration or air-conditioning systems usually operate invisibly. Young people are often unaware that these technologies even exist – let alone that there is a major industry behind them.

Refrigeration and air-conditioning systems consume around 15 percent of the world’s electrical energy. What strategies can be used to reduce this share?

Energy consumption can be reduced through a variety of measures. In my habilitation thesis from 2008, I described many of them. Some would be relatively easy to implement but often encounter resistance from ‘old hands’.

For example, a variable condensing temperature adjusted to the outside air temperature can reduce the energy requirement of a refrigeration system by two to three per cent for every degree of temperature reduction. Nevertheless, there are numerous systems that maintain artificially high condensing temperatures by means of continuous fan speed control – day and night, summer and winter.

If speed control is to be used, it should be applied to the compressors to continuously adjust the refrigeration system’s cooling capacity to the actual cooling demand and thus save additional energy.

You work with natural refrigerants. What does this involve and how do companies benefit from it?

Natural refrigerants have fascinated me ever since my time in the USA. Back then, in our refrigeration engineering lecture, we had to design a refrigeration system for a cold store. All the American students chose R22 as the refrigerant – a hydrochlorofluorocarbon (HCFC) that damages the ozone layer and has a high global warming potential. I, on the other hand, opted for ammonia as the refrigerant, as this reduces the energy requirement for the cold store by around 10 per cent compared to R22. My American professor commented on this decision as follows: “Best project of all the students, but I completely disagree with your choice of refrigerant.”

Since then, the subject has stayed with me. In my PhD thesis, I focused on air as a refrigerant – an approach that was in high demand during the COVID-19 pandemic for low-temperature applications below -70 °C. During my time at the Danish Technological Institute, from 1994 until 2002 we relied exclusively on natural refrigerants such as ammonia, carbon dioxide, propane, isobutane and water, and with this focus we increased our staff numbers fivefold within eight years.

Natural refrigerants have numerous direct positive effects on the environment: They are not perfluoroalkyl and polyfluoroalkyl substances (PFAS), do not break down into trifluoroacetic acid in the atmosphere, and have neither ozone-depleting potential nor significant greenhouse effects. Furthermore, they require less energy to produce than fluorinated refrigerants and generally achieve higher energy efficiency than fluorinated alternatives.

Particularly problematic is trifluoroacetic acid (TFA), an atmospheric degradation product of certain hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs). It can cause significant contamination of soil, surface water and groundwater, and remains in the water cycle for a very long time. HFO-1234yf, used in vehicle air conditioning, is particularly critical: it converts entirely into TFA in the atmosphere, whilst vehicle air conditioning systems continue to exhibit relatively high refrigerant leakage rates. More environmentally friendly alternatives such as CO₂ and propane have been known for years and are already in use to a limited extent in vehicle air conditioning. Isobutane, another natural refrigerants, has even been used exclusively in domestic refrigerators since the 1990s. Refrigerators with isobutane operate quitter than those with HFC-refrigerants and they achieve better energy efficiency.

As some natural refrigerants are not suitable for use in public areas in large quantities – because they are toxic (ammonia) or flammable (hydrocarbons) – I have also been working for over thirty years on ice slurry, or liquid ice. Ice slurry is a very interesting and efficient refrigerant. Its energy density is up to eight times higher than that of liquid water, whilst heat transfer is up to three times higher.

In 2013 and 2014, I provided scientific support for the planning and construction of a corresponding refrigeration system at the largest canteen of the Karlsruhe student services. Since then, all refrigerated display counters and medium temperature cold rooms there have been cooled using liquid ice at a temperature of around -4 °C. The ice slurry is produced overnight by propane refrigeration systems operating at a reduced condensation temperature. Thanks to a 40 m³ ice slurry storage tank, the propane refrigeration systems can remain switched off during the day.

Our customers benefit from over thirty years of experience in the application of natural refrigerants. Anyone wishing to delve deeper will find much of our experience documented in the book ‘Natural Refrigerants – Applications and practical guidelines’. Incidentally, all authors and editors donate their respective fees to the Valerius Füner Foundation, which supports our students.

Expertise is in high demand: In your opinion, how can we succeed in inspiring young people to take an interest in the field of refrigeration and air-conditioning technology?

Refrigeration and air-conditioning technology needs to become more visible to school pupils. At Karlsruhe University of Applied Sciences, we are committed to this through our InspirING® programme, and at the International Institute of Refrigeration (IIR) through the young researchers’ group “CaRe – Career in Refrigeration”. The German Refrigeration and Air Conditioning Association (DKV) also runs a young talent initiative.

To young people, I would say: refrigeration and air-conditioning technology is an extremely diverse and rewarding field, as it not only contributes to environmental protection but also directly improves people’s living conditions. For example, we recently completed the very labour-intensive EU project SophiA, in which we developed solar cooling, water treatment and cooking facilities for remote African hospitals and installed them in Burkina Faso, Cameroon, Malawi and Uganda. Further information is available at www.sophia4africa.eu