By combining refrigeration, air conditioning and heating into one unit, transcritical CO2 systems save energy and water while cutting indirect and direct greenhouse gas emissions. Kav Consulting managing director, Klaas Visser, explains how to do it all with C02.
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, but it wasn't as successful with warmer seawater.
It wasn't until 1985 that my dear late friend Gustav Lorentzen (who, in the late 1980s, rediscovered how CO2 could be used in refrigeration) explained why. 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. 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 supplying maximum cooling water to the CO2 condensers.
In 2009 I was very fortunate to be involved in the design of a multi-function, two--stage transcritical CO2 refrigeration system with parallel compression (or MF2STTCCO2RSPC for short) 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 $472,000 grant to develop the system. I did the design and prepared a budget while some of my friends in Europe were kind enough to vet my work before I entered into a partnership with Bitzer Australia and Guntner Australia to supply the compressor racks and evaporators, respectively. Many thanks to all.
The system has now been operating for nearly five years after an extremely difficult six-month commissioning period. Fortunately, we could fix problems as they arose. Failure stared us in the face a number of times but in the end we produced a MF2STTCCO2RSPC 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 MF2STTCCO2RSPC 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. In another part of the process, steam is generated in a boiler to heat water to temperatures of about 130 °F for chicken de-feathering to 145 °F for pig scalding prior to de-hairing. Food hygiene processing rules prohibit recirculation of hot water, which needs to be continuously heated from mains water temperature to the process temperature.
When using a CO2 refrigerating plant at such facilities the plant may have to operate in transcritical mode to heat water quite readily to temperatures of 160°F and up to 185°F. 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 a reduction in the use of water-treatment chemicals. Also reduced is the electrical-energy consumption of the condenser or cooling tower fans – and 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 United States, particularly California.
A similar scenario is applicable to hospitals and hotels, which generally require cooling and heating and consume large quantities of hot water. So here again we have the same situation. We reduce electrical energy and fuel consumption, and attendant emissions and water consumption.
But there are several other benefits associated with CO2 refrigeration systems, notably air conditioning support. 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 – similar to an economiser operation on an ammonia screw compressor - to remove “flash” CO2 gas in high ambient temperatures.
In the case of water heating, the critical point of 88°F 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 of operations, water savings are greater, estimated to be 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 will undergo an 80 per cent phasedown by 2030, under a proposal before the Montreal Protocol and already enshrined in EU legislation. Pumping CO2 through buildings also offers the potential of using CO2 as a fire-extinguishing medium by depleting the oxygen volume to 15% or lower, at which point a fire is unsustainable.
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.
The only thing holding back a rapid expansion of CO2 refrigeration application to virtually all cooling, freezing and AC duties is the unavailability of larger compressors. But the economic and environmental benefits of CO2 refrigeration techniques are significant enough to be worthy of attention from manufacturers of suitable compressors – indeed, from all sectors of the industry.
- with Accelerate America Magazine published by shecco at http://issuu.com/shecco/docs/aa1509-digital/c/sp1m07k