Prerag (Pega) Hrnjak, a world-renowned professor, researcher and entrepreneur who helped to transform the HVAC&R industry with his visionary and innovative work on natural refrigerant applications, passed away on August 31 at his home, the University of Illinois at Urbana-Champaign (U.S.,) announced on September 1.

Born in Belgrade, Yugoslavia (now Serbia), Hrnjak received a doctorate from the University of Belgrade, where he taught for many years. He emigrated to the U.S. in 1993, becoming Stoecker Faculty Fellow and Distinguished Research Professor at the University of Illinois at Urbana-Champaign, as well as co-director and then director of the University’s Air Conditioning and Refrigeration Center (ACRC). He founded Creative Thermal Solutions (CTS), a research and consulting business, in 2003.

Hrinjak was a frequent speaker and contributor at natural refrigerant conferences around the world, including ATMO conferences organized by ATMOsphere (formerly shecco), publisher of

“Pega was a hero to me,” said Marc Chasserot, founder and CEO of ATMOsphere. “He was kind, thoughtful and respected by all. Without his innovations and leadership, natural refrigerants would not be where they are today. We extend our condolences to his family and many friends and colleagues.”

Chasserot interviewed Hrnjak at CTS in 2018, as seen in this video.

Hrnjak mentored nearly 120 students to graduate degrees, with more than 30 completing a doctorate, said the University of Illinois, adding, “His students have gone on to leadership positions in industry, national labs and public service, and to careers in the academy—all around the world.”

Hrnjak and his colleagues have made presentations and written papers on virtually every technical aspect of CO2 technology. Some examples include: the effects of oil on transcritical CO2 systems; ejectors in CO2 air conditioners; heat transfer and pressure drop in CO2 heat exchangers; flash-gas bypass for improving transcritical CO2; control strategies for transcritical CO2; and using ammonia and CO2 in supermarkets.

In the automotive sphere, he developed some of the early CO2 mobile air conditioning systems in the mid-1990s. Following a prototype built in 1997 (and described in a publication in 2000) CTS devised a CO2 heat pump producing both cooling and heating for the emerging electric car industry.

Ammonia is another natural refrigerant for which Hrnjak has made major contributions, particularly in the development of microchannel heat exchangers that dramatically reduce its charge. This charge reduction technology, which he originally designed in the 1990s, has helped bring about the recent growth of low-charge systems and packaged units in industrial refrigeration, including an “ultra-low-charge” system made by CTS itself.

Internationally recognized

Hrnjak was internationally recognized for his technical leadership, receiving the Ritter von Rittinger award (2008), the Gustav Lorentzen Medal (2011), the J&E Hall Medal (2012), and the ASHRAE Holladay Distinguished Fellow Award (2019). He was also Fellow of the American Society of Mechanical Engineers, Fellow of the American Society of Air Conditioning and Refrigeration Engineers, and Fellow of the Society of Automotive Engineers, and was named Academician of the Academy of Engineering Sciences of Serbia.

His research company CTS began as a “garage business” and “grew under his tireless leadership to the state-of-the-art, 100,000-square-foot research facility it is today,” said the University of Illinois. “Between the ACRC and CTS, Pega managed to incubate a critical mass of enthusiastic researchers, engineers and support staff of close to 100 people working simultaneously on the sustainable HVAC&R technologies that he was so passionate about.”

“Pega’s boundless energy was directed toward helping others reach their full potential, professionally and personally, and his impact was profound,” added the University, “He will be deeply missed by many of us.”

Hrnjak is survived by his wife Ivana and daughter Zona.

The following is an excerpt from “Pega’s Quest,” Accelerate America, February 2018:

When Predrag Hrnjak – known universally as Pega – was a visiting scholar at the Technical University of Denmark in the early 1980s, he took a trip to the Norwegian Institute of Technology in Trondheim, Norway, where he met Gustav Lorentzen. Lorentzen, already a renowned thermodynamics scientist, would secure his place in history in the late 1980s by rediscovering how CO2 could be effectively used as a refrigerant in cooling and heating.

“I saw him at his desk meeting with a student,” Hrnjak recalled recently in an interview with Accelerate America at Creative Thermal Solutions (CTS), his research and consulting business in Urbana, Ill. “He radiated strength, knowledge and charisma. Later, when I talked to him, he strengthened the initial impression he made. In a relatively short time, he left a deep impression. It was clear why they called him Iron Gustav.”

Hrnjak, a charismatic and deeply knowledgeable figure himself, would also go on to leave an indelible mark on the global HVAC&R industry, helping to advance the CO2 revolution that Lorentzen and his last Ph.D student Jostein Pettersen set in motion, and making significant contributions in many other applications as well.

Other articles about Hrnjak can be found here, here and here.

“Pega was a hero to me. He was kind, thoughtful and respected by all.”

Marc Chasserot, ATMOsphere