But research beyond HFCs continues and CO2 remains the alternative getting the most attention. At the recent 18th International Compressor Engineering Conference and the 11th International Refrigeration and Air Conditioning Conference held jointly at Purdue University, at least 35 of the 225 papers had CO2 as their primary topic.
COMPRESSORS AND CO2
During the compressor conference, back-to-back sessions in one afternoon looked at the topic from a variety of points. For example, engineers from Tecumseh reported on development of a prototype hermetic rotary compressor for use with carbon dioxide. It was noted that a two-stage compression cycle was employed to reduce the effect of high operating pressures. The report added that a modular design of the compressor elements "helps simplify assembly, reduce production cost and improve performance."Engineers from Mitsubishi Electric said, "We have developed a new CO2 heat pump water-heater compressor with a single rotary mechanism." The engineers noted that the refrigerant had different characteristics than more typical refrigerants in regard to operating pressure, density, and the speed of sound in the refrigerant.
"To maintain efficiency, we minimized the leakage path and optimized the amount of the oil supply to the compression chamber. To control the oil exhalation amount, we enlarged the gas flow path of the motor portion to decrease the speed of the gas flow. To improve reliability, we adopted a coating for the vane surface. Solving issues like these, we were able to commercialize our product."
A paper from a team of engineers at United Technologies Research Center noted the challenges in using CO2 in reciprocating piston compressors in a water-heating heat pump. The report also noted characteristics of CO2 in a transcritical cycle such as high pressure, high discharge temperatures, and high solubility.
"These challenges were overcome using analyses from compressor simulation and component models to diagnose the problems and correct them," it was reported. Among specific issues addressed:
"The discharge valve and port design were improved for better performance and reliability. The crank bearing design in the connecting rod was improved for increased minimum oil film thickness. An improved crankshaft finish and improved piston has resulted in a more reliable crank bearing."
The challenges of working with CO2's unique characteristics were addressed by engineers from Daikin, by using what was called a ‘swing' compressor. They said that in such a compressor "the blade and roller are unified and form a piston. There is no leak point and there is no severe lubrication state such as (in) a rotary compressor."
A report on noise and vibration aspects of CO2 compressors from engineers at Embraco noted in general that "significant process in terms of both energy performance and reliability have been made by the optimization of the reciprocating compressor to meet CO2 design demands."
The specifics of the Embraco paper said that "preliminary results point to a very positive perspective for the acoustical performance of CO2 compressors investigated." The results presented what the engineers said was "a very powerful methodology to improve the acoustical performance of a CO2 compressor and/or a CO2 refrigeration system. Currently HFC sound levels can be achieved in CO2 applications employing proper methodology."
REFRIGERATION AND CO2
At the concurrent refrigeration and air conditioning conference, CO2 also was the focus of a number of papers.Engineers from United Technologies reported on a study to evaluate the performance of CO2 in air-to-water heat pumps for hydronic space and service water heating applications. Fifteen different cycle configurations were performed based on computer simulation. "Results indicate that for both applications, the two-stage cycle with a phase separator offered the highest performance," it was reported.
Research at the University of Padova in Italy looked at CO2 transcritical refrigeration cycle fin-and-tube coils as possible gas cooling devices due to lower costs when compared with newer microchannel heat exchangers, according to the report.
"In spite of the very high working pressures, a commercial coil with four ranks of 3/8-inch copper tube and louvered fins has been studied. Experimental tests and numerical analysis on two identical finned coils - one with continuous fins, the other with separated fins per each tube row - indicates that ‘internal' heat conduction through the fins is an important factor in finned coils CO2 gas coolers.
"This aspect is strictly linked to the high temperature variation of carbon dioxide during the gas cooling process in a transcritical refrigeration cycle. This increase in temperature approach between CO2 outlet temperature and air inlet temperature was found to improve the efficiency of the refrigerating cycle.
"This aspect offers a quite promising technological approach opportunity, because it is easy to build a coil with fins separated between the rows," researchers said.
Researchers from the University of Maryland and LG Electronics teamed up to present a paper describing the performance potential of a transcritical CO2 cycle with a linear compressor for medium temperature refrigeration systems. The performance of transcritical CO2 cycles was simulated for medium temperature refrigeration systems by using the compressor efficiencies measured.
The simulated results show the cooling capacity and the COP and from that "the heat exchangers were then designed using the heat exchanger design software. The results show that the CO2 linear compressor together with suitably designed heat exchangers can be successfully applied to medium temperature refrigeration systems," the report said.
The use of CO2 found its way to beverage coolers in a report from Embraco.
The paper dealt with the evaluation of the performance of a single stage CO2 reciprocating compressor working on a beverage cooler application. A glass door merchandiser was tested to develop a procedure to determine the best combination of capillary tube and refrigerant charge. Fin-and-tube heat exchangers were used both for the evaporator and the gas cooler.
Among findings: "The criteria to choose the combination was the total energy consumption of the system. The theoretical optimum discharge pressure was determined point by point during the ‘on' period of the cycle and was compared to the experimental discharge pressure. The results showed that the closer profile to the optimum profile was the best in terms of energy consumption. The system was also tested with R-134a and the results were compared showing 26 percent of energy savings in favor of the CO2 system."
Publication date: 09/04/2006
