Carbon Dioxide Could Replace Global-Warming Refrigerant

ScienceDaily (July 4, 2000) — WEST LAFAYETTE, Ind. – Researchers are making progress in perfecting automotive and portable air-conditioning systems that use environmentally friendly carbon dioxide as a refrigerant instead of conventional, synthetic global-warming and ozone-depleting chemicals.

It was the refrigerant of choice during the early 20th century but was later replaced with manmade chemicals. Now carbon dioxide may be on the verge of a comeback, thanks to technological advances that include the manufacture of extremely thin yet strong aluminum tubing.

Engineers will discuss their most recent findings from July 25 to 28, during the Gustav Lorentzen Conference on Natural Working Fluids, one of three international air-conditioning and refrigeration conferences to be held concurrently at Purdue University. Unlike the two other conferences, the biannual Gustav Lorentzen Conference, which is being held for the first time in the United States, focuses on natural refrigerants that are thought to be less harmful to the environment than synthetic chemical compounds.

"The Gustav Lorentzen Conference focuses on substances like carbon dioxide, ammonia, hydrocarbons, air and water, which are all naturally occurring in the biosphere," says James Braun, an associate professor of mechanical engineering at Purdue who heads the organizing committee for all three conferences. "Most of the existing refrigerants are manmade."

Purdue engineers will present several papers detailing new findings about carbon dioxide as a refrigerant, including:

• Creation of the first computer model that accurately simulates the performance of carbon-dioxide-based air conditioners. The model could be used by engineers to design air conditioners that use carbon dioxide as a refrigerant. A paper about the model will be presented on July 26 during a special session sponsored by the U.S. Army in which researchers from several universities will present new findings.

• The design of a portable carbon-dioxide-based air conditioner that works as well as conventional military "environmental control units." Thousands of the units, which now use environmentally harmful refrigerants, are currently in operation. The carbon dioxide unit was designed using the new computer model. A prototype has been built by Purdue engineers and is being tested.

• The development of a mathematical "correlation," a tool that will enable engineers to design heat exchangers – the radiator-like devices that release heat to the environment after it has been absorbed during cooling – for future carbon dioxide-based systems. The mathematical correlation developed at Purdue, which will be published in a popular engineering handbook, enables engineers to determine how large a heat exchanger needs to be to provide cooling for a given area.

• The development of a new method enabling engineers to predict the effects of lubricating oils on the changing pressure inside carbon dioxide-based air conditioners. Understanding the drop in pressure caused by the oil, which mixes with the refrigerant and lubricates the compressor, is vital to predicting how well an air conditioner will perform.

Although carbon dioxide is a global-warming gas, conventional refrigerants called hydrofluorocarbons cause about 1,400 times more global warming than the same quantity of carbon dioxide. Meanwhile, the tiny quantities of carbon dioxide that would be released from air conditioners would be insignificant, compared to the huge amounts produced from burning fossil fuels for energy and transportation, says Eckhard Groll, an associate professor of mechanical engineering at Purdue.

Carbon dioxide is promising for systems that must be small and light-weight, such as automotive or portable air conditioners. Various factors, including the high operating pressure required for carbon-dioxide systems, enable the refrigerant to flow through small-diameter tubing, which allows engineers to design more compact air conditioners.

More stringent environmental regulations now require that refrigerants removed during the maintenance and repair of air conditioners be captured with special equipment, instead of being released into the atmosphere as they have been in the past. The new "recovery" equipment is expensive and will require more training to operate, important considerations for the U.S. Army and Air Force, which together use about 40,000 portable field air conditioners. The units, which could be likened to large residential window-unit air conditioners, are hauled into the field for a variety of purposes, such as cooling troops and electronic equipment.

Low Temperature Freezing

The low-temperature freezing (for frozen food) and the medium-temperature cold (for chilled food) secondary loops use theoretically noncorrosive and nontoxic brines with high heat capacities and low viscosities at low temperatures. They eliminate the use of refrigerants on both cold and freezing sides of the system and are separately pumped to the display cases and cold storage rooms. The display case heat exchangers were designed to use secondary fluids and, consequently, the temperature differences between the brines and the air were minimized.

As secondary fluid on the low-temperature freezing loop, an inhibited potassium formate solution with a concentration of 100%--a nontoxic product compatible with the majority of most common metals and alloys--was chosen. Some analytical studies indicate thermal and pressure drop advantages for HFE-7100 over other potassium formate solutions for low-temperature applications (below -20[degrees]C or -4[degrees]F) (IEA 2003). However, it was recently demonstrated that corrosion had been a problem associated with the use of potassium salts, particularly when galvanized materials were used in display cases. At the same time, the HFE-7100 is an unnatural global warming substance and is expensive. For future applications, a valuable deep-freeze coolant would be carbon dioxide (C[O.sub.2]). As a two-phase secondary coolant, C[O.sub.2] has no ozone degradation potential and negligible GWP. It is universally available, uses very little energy for pumping, and has low costs. In the presented system, the low-temperature secondary fluid (pure potassium formate) is circulated through the freezing secondary loops by two 37.5 hp parallel pumps. Although propylene glycol (35%) has a relatively high level of viscosity at low temperatures, it was chosen for the medium-temperature cold secondary loop. It is also circulated through the loop by two 37.5 hp pumps installed in parallel.

Two warm secondary loops reject the condensing excess heat to the outdoor air by means of remote air-cooled liquid coolers located on the roof (Figure 1). Both loops use ethylene glycol (with a concentration of 50%) as warm secondary fluid. This fluid presents certain environmental risks, but they are minimal compared to the risks associated with common refrigerant leakages.

Does more sun shine on the righteous?

Does more sun shine on the righteous?

Cold Thoughts

by Neil Everitt

The editor of ACR News posts his own cold thoughts about the ACR industry and anything else he cares to air.

25/11/2009 12:14:10
IN THE editorial in the September issue of ACR News, I commented upon how those who feel they have a righteous cause can sometimes pursue that cause so zealously that their actions can become morally skewed.

My references at the time were to those who pursue natural ventilation to the exclusion of air conditioning but it could now equally be applied to the climate scientists at the centre of revelations exposed by the "hacked" emails at Britain's Climate Research Unit (CRU) at the University of East Anglia.

As one of the primary information sources for the UN's influential international panel on Climate Change (IPCC), some have claimed that the content of the leaked emails prove that the whole man-made global warming theory is a lie.

Certainly the scientists involved could be accused of manipulating evidence to fit their perceived theories; suppressing and even destroying evidence to the contrary and attempting to exclude dissenting scientists from the peer review process.

This is damning evidence but it is not surprising that a group of people who are so convinced that they are right should act in this way. It does suggest that they are so convinced that they are on the right route that they have ditched their moral compass, but it does not mean that their beliefs that man is responsible for global warming are wrong.

One thing the emails do show, however, is that despite claims to the contrary, the evidence of anthropogenic global warming is not as clear cut as they would have us believe. In particular the environmental scientists appear unable to explain how, despite CO2 levels in the atmosphere rising over the last 11 years, global temperatures have not increased with it as they would have predicted.

It is not an excuse to do nothing but it really is quite scary that long-term strategies could be decided in Copenhagen next month based upon apparent climatic trends that are not yet fully understood.