By combining refrigeration, air conditioning and heating in one unit, owner of KAV Consulting Klaas Visser explains how transcritical CO2 systems save energy and water while cutting indirect and direct greenhouse gas emissions.
In the late 1950s, I operated a carbon dioxide refrigeration system on a ship carrying frozen meat eastbound from Buenos Aires, Argentina, to Yokohama, Japan. I knew it worked very well with low-temperature seawater, which cooled the CO2 condenser, and very poorly with warmer seawater, particularly in tropical waters.
It was not until 1985 that my dear late friend Gustav Lorentzen (who, in the late 1980s, rediscovered how CO2 could be used in refrigeration) explained to me why this was so.
CO2 is funny stuff. It is the only refrigerant I know that improves its energy efficiency when you increase the compressor discharge pressure when operating above the critical pressure of 1,071 psig at 88°F (31.1°C).
If only I had known that in 1959! I would have throttled the cooling water supply to one compressor instead of starting the second compressor, and supplied maximum cooling water to the CO2 condensers!
World’s first two-stage transcritical CO2 refrigeration system
In 2009 I was very fortunate to get involved in the design of a multi-function, two-stage transcritical CO2 refrigeration system with parallel compression (or MFTC2SCO2RSPC) to replace 22 existing systems in an Australian food processing plant.
The other systems comprised a number of R12 and other CFC/HCFC systems, as well as six ineffective R134a air-to-water heat pumps and several HFC inverter units for office cooling and heating.
In fact I was terrified when the Australian Federal Government awarded my client a AUD $472,000 grant to develop the system I had designed and budgeted. Some of my friends in Europe were kind enough to vet my design before I entered into a partnership with Bitzer Australia and Guntner Australia to supply the compressor racks and evaporators, respectively.
The system has now been operating for nearly five years after an extremely difficult six-month commissioning period. It is fair to say we were lucky that we were able to fix all problems as they arose.
It was a closely run thing and failure stared us in the face a number of times. But all’s well that ends well, and in the end we produced a MFTC2SCO2RSPC that performs virtually all refrigeration and heating functions at the plant. As it turned out, it was the world’s first system of its kind.
The trials and tribulations during the past six years have proved invaluable in gaining practical operating experience with a MFTC2SCO2RSPC delivering seven refrigeration functions from blast freezing to office AC cooling, and nine heating functions from freezer-door-fascia heating with warm glycol to hot water for chocolate melting. As a result of this experience it dawned on me that properly designed and operated CO2 refrigeration systems, by combining refrigeration, air conditioning and heating, are the most efficient systems available.
In many applications in the food processing industry, there is a simultaneous need for high-capacity refrigeration and hot water. Conventionally, the refrigeration plant takes care of the cooling with the heat rejected to an evaporative condenser, which consumes large quantities of water, just as cooling towers do in AC systems.
When using a CO2 refrigerating plant at such facilities the plant may be elected to operate in transcritical mode to heat water quite readily to temperatures of 160°F and up to 185°F with a bit more effort. When doing so, there is no longer a need for steam to heat the process water. This results in a reduction in gas or oil consumption, thus cutting operating costs and attendant CO2-equivalent emissions.
The amount of heat rejected to the refrigerant condenser or cooling tower is reduced, thus lowering cooling water consumption, with attendant reduction in the use of water-treatment chemicals. Also reduced is the electrical-energy consumption of the condenser or cooling tower fans – and of the spray or cooling-water circulating pumps, respectively - which further cuts operating costs and CO2-equivalent emissions. Reduction of cooling water consumption is a welcome feature in many parts of the U.S., particularly California.
But there are several other benefits associated with CO2 refrigeration systems, notably their support of air conditioning. In many food-processing operations some AC and heating functions are often required for offices and staff amenities. When used for AC cooling, CO2 refrigeration, if equipped with a water-cooled evaporative condenser, is more efficient than conventional refrigeration. Moreover, by integrating the AC and refrigeration duties into one system, the AC function may be combined with parallel compression, which uses an economiser operation on a screw compressor to remove “flash” CO2 gas in high ambient temperatures.
In the case of water heating, the critical point of 88°F (31.1°C) is a big advantage, but it is a disadvantage for the refrigeration application due to the high volume of flash gas generated. Such gas needs to be compressed without doing any useful work in chilling water or providing refrigeration for other functions like, for example, space cooling. However, by incorporating parallel compression for AC, the flash gas inefficiency is largely removed and indeed the rest of the system, while providing normal refrigeration for chilling, cold storage and freezing, operates more efficiently than any other refrigerant, be it ammonia, hydrocarbons or HFCs.
CO2 has a very high proportion of sensible heat in the compressor discharge, which may be removed by air-cooling rather than by evaporating water in a hybrid evaporative condenser. This saves 50% of the water that would be used in an evaporative condenser or cooling tower. Over a whole year, running water savings are greater, estimated between 65% and 80%.
Other benefits of CO2 are its low cost and low GWP (global warming potential) of 1, compared with the much higher values for HFCs, which would undergo an 80% phase down by 2030, under a proposal before the Montreal Protocol.
CO2 systems are also suitable for retrofitting to existing buildings and hospitals. A recent desktop study of hospital energy consumption in Washington, D.C., shows that retrofitting CO2 for cooling and heating would reduce the electrical energy consumption by 24% and fuel consumption by 100%. This resulted in a combined reduction in energy consumption of 47%.
CO2 the future-proof solution
It is fair to say that CO2 systems, applied to all manner of refrigeration applications in industrial processes as well as comfort cooling and heating in the built environment, reduce the energy consumption for both cooling and heating, with the attendant reduction in indirect greenhouse gas emissions. Furthermore, cooling-water consumption is reduced significantly as are direct emissions from the displaced HFC refrigerants. With these advantages, CO2 will future-proof any new installation against any further actions against HFCs or their very expensive HFO replacements.
Klaas Visser is the owner of Australia-based KAV Consulting Pty Ltd, a refrigeration consulting and engineering firm responsible for the world’s first fully integrated, two-stage transcritical CO2 refrigeration system. He has extensive international experience in the design, building and troubleshooting of medium to large ammonia systems, with involvement in more than 800 NH3 projects during the past 45 years.