Pega on the power of low charge ammonia

By James Ranson, May 07, 2015, 10:52 7 minute reading

One of the world’s pre-eminent researchers, teachers and experts in the refrigeration industry, Professor Predrag (Pega) Hrnjak, sat down with Accelerate America at the IIAR Conference in San Diego in March in an in-depth interview for AA's fifth issue to talk trends in refrigeration and air conditioning and in particular low-charge ammonia systems.

Since starting his activities in the U.S. during the 1980s, Pega went on to become a professor in the Department of Mechanical Science and Engineering at the University of Illinois, Urbana, then co-director of its Air Conditioning and Refrigeration Center, and president of Creative Thermal Solutions (CTS), his own private research-and-development company.
The ACR Center employs more than 100 people, offering graduate students and others an opportunity to conduct research, while CTS is a vibrant R&D company, with almost 50 engineers, that does research primarily for industry and occasionally for governmental agencies.  Its Urbana headquarters comprises 100,000 square feet of research space, including 43 environmental chambers and numerous other facilities in all areas of experimentation in thermal applications.
In a wide-ranging interview, Pega explained his two loves – the University and CTS - and how his long history of research has culminated in the realisation that reducing refrigerant charge is now a reality for industrial refrigeration applications.
The origins of low charge ammonia
Accelerate America: How did your work with low-charge ammonia come about?
Pega Hrnjak: I always say that various applications generate focus and expertise of a certain kind. We need a cross-pollination of that expertise. For example, industrial refrigeration is mostly driven by reduction of energy costs and increases in efficiency because it runs 24 hours a day, 365 days a year. It is not at all driven by the size or weight of equipment used. As we move from industrial to commercial (such as supermarkets) to residential to vehicles and aircraft applications, the same job of cooling is done differently. In cars and aircrafts, you care far less about efficiency; you care much more about compactness, size and weight because the vehicle always carries that weight. So for automobile and aircraft applications, engineers developed way better heat exchangers, with microchannel heat exchangers being the best nowadays. It took me many, many years to bring that technology of heat exchangers to industrial refrigeration. My point was not to reduce weight or size (although that is also beneficial) but to reduce the charge, which is much more important for refrigerants that are either flammable or mildly toxic (like ammonia).  I am very happy to see that focus on charge reduction becoming stronger and stronger.
AA: How would you define low charge?
Hrnjak: There has been some attempt to qualify the meaning of low charge. How low is low charge? People are using 10 pounds (4.54 kg) per ton of refrigeration capacity as a threshold for low- charge systems. That is definitely much lower than typically found in conventional industrial refrigeration systems today. But 15 years ago I developed a 16 kW (5 Ton) chiller in the lab with a charge that is 100 times lower than that. Maybe that would be called ultra-low-charge.
The real question is: What is the maximal charge of ammonia that will be below any legal concerns?  In other words, what will happen if we limit the charge of a single unit not to 50 kg but to 5 kg?
I believe IIAR and other organisations should do everything possible to identify that criterion. That threshold would open unimaginable market opportunities for ammonia in the non-industrial refrigeration segment. Certainly it would quickly help push development of technologies that would deliver high-capacity units within the given limits.  
AA: Can you do very-low-charge applications for industrial applications?
Hrnjak: Industrial refrigeration, it is clear, is changing the way of doing business from a technical perspective. Historically, all industrial refrigeration plants have been custom-designed and built on site. Nowadays there is a significant tendency everywhere in the world to use pre-fabricated, self-contained units with ammonia that will do the same job.
AA: Is this leading to lower costs?
Hrnjak: Its intention is to be less costly. And I am fully convinced it will be eventually less costly because of higher volume production. Whether today it is less costly remains to be seen – it will be a consequence of the market situation. The real driver today is to have reduced charge. We’re not talking anymore about 10,000 pounds (4,540 kg) ; we’re talking about 100 or 200 pounds at most (50 – 100kg). This reduces the danger in the case of seismic events or fires or any other contemporary danger that you could easily imagine. That trend has just started. And I see that we will be going into lower and lower and lower specific charges in years to come.
AA: So these self-contained units with 100 pounds represent a good start for the low-charge movement?
Hrnjak: Yes, but it could be and should be lowered. 
AA: But you’re saying this could actually lead to even smaller charges that would still be effective in an industrial setting?
Hrnjak: Oh absolutely. With 5 kg of ammonia we can make 200 KW of capacity with excellent efficiency, regardless how remote this looks to many people, at this point.
AA: Does this exist yet?
Hrnjak: Yes, in the lab and in prototypes. At CTS we have done several prototypes of equipment with that specific charge. I have been presenting results at conferences, including the first prototype at the IIAR conference in California, 14 years ago.
AA: Why hasn’t it been adopted? It seems odd that a small charge could do the work of a larger charge. Why do operators use such large amounts of ammonia if they can get away with less?
Hrnjak: It was too radical. I think even today it is a big step. As long as we do not have a clear regulation specifying the lowest charge level, the reduction of refrigerant inventory will happen gradually. 
Paradoxically one of the reasons why ammonia has been in use continuously as a refrigerant over the last 160 years is the high charge. Ammonia is extremely inexpensive compared to synthetic refrigerants. In big industrial systems, a leak is almost inevitable - you almost always smell ammonia close to the plant. So ammonia is an affordable and extremely good refrigerant, both thermodynamically and in terms of thermo-physical properties. The pressure to reduce the charge is coming for other reasons – not to minimize the cost of the refrigerant that is already low, but to satisfy environmental concerns with safety in mind.
AA: What will be the ultimate drivers of low-charge adoption?
Hrnjak: Economic opportunities or regulatory push. So far there hasn’t been sufficient motivation to go in that direction. So most low-charge systems are coming from companies that have not been traditionally in the business of providing big custom-built systems for industrial refrigeration.  And that is very logical. They see opportunity, which is a great motivation. At some point soon a few of the traditional suppliers and contractors will see the potential to strengthen their market position with low-charge systems. And they will also start building units.
AA: So modernization through low charge is coming?
Hrnjak: Yes, I believe so. Eventually, industrial refrigeration, the way that we know it now, will have unitary equipment with lower and lower charge. It is possible to replace almost all of these big facilities with smaller, more efficient systems with a lower charge. The example of the Fukushima meltdown is very vivid to everyone.  It makes much more sense to modify facilities now to prevent similar cases in a reasonable, economical way.
Huge potential for low charge ammonia
AA: What are other areas where low-charge ammonia could be applied?
Hrnjak: When below a certain charge level, other applications where ammonia is traditionally not used are easily reachable. Chillers for instance, especially those that would be on the roof of a building. Ammonia can be a replacement for R22 (and substitutes) in chillers, and then you would have a secondary refrigerant that is pumped around the building for HVAC, in industrial cooling or in a supermarket. That is very, very viable.
Ammonia is the only refrigerant that is lighter than air. So if it is on the roof of a building and the charge is low, then in the case of leak, refrigerant lighter than air will go into the atmosphere. Thanks to the low charge, the leak is small and quantities are far below the EPA-allowed daily rate for venting ammonia into the atmosphere.
AA: Thanks for your time, Pega!


By James Ranson

May 07, 2015, 10:52

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