Of the more than 800 papers published in conjunction with the 22nd International Congress of Refrigeration held this summer in Beijing, those related to R-744 and supermarkets were among the most numerous. In the broadest sense, CO2 seemed to be of most interest among refrigerants being considered for use beyond HFCs. And if there was one sector of the industry most open to new directions in refrigeration technology, it appeared to be grocery stores.
The following is the briefest of overviews of some of the papers that centered on technology that contractors and technicians could very well see sometime in the future. Quoted passages are from either the abstracts of the papers or from the conclusions of the complete papers. Both types of documents were published by the ICR and made available to all attendees.
CO2Dealing with the overall challenges of using CO2, combined research by teams from Sweden and Thailand looked at a double-loop system, which combines a power subsystem and a refrigeration subsystem, both with CO2. “The power subsystem is able to utilize the energy from a low-grade heat source to produce power. The power is then transferred to the refrigeration subsystem, partly or totally covering the power consumption of the compressor. Furthermore, it is also possible to take advantage of the temperature glide of both subsystems’ heat rejection processes to produce hot water.”
Research from the Netherlands noted problems with insufficient subcooling to operate pumps without cavitation in mechanically pumped two-phase fluid loops. The research involved the use of two-phase accumulators for controlling CO2 loops “to obtain forced subcooling and accurate evaporator control.” The use of the accumulator “has the advantage of controlling the evaporator and condenser pressures independent of the primary cooler temperature.”
Despite the challenges faced in dealing with CO2 as a refrigeration refrigerant, research from Germany cites its “extraordinary properties as a secondary refrigerant.” In such a case, CO2 works well because “the phase change from liquid to vapor just below and above the critical point provides the optimum solution.”
An additional challenge is to make CO2 work in transcritical situations. In such a case, researchers from France have proposed an ejector expansion. The idea is to “improve energy performance of the transcritical cycle by reducing the expansion losses.”
Norwegian researchers developed a prototype liquid chiller using CO2. “The prototype had the possibility to reject heat both to air and water. Both the water chiller and the gas coolers are new designs adapted for CO2. The water-cooled gas cooler was a compact multi tube-in-tube counter-flow concept, whereas the air-cooled gas cooler was a conventional round tube in fin.” After running a range of experiments involving varying cooling water temperatures and at steady state and cool-down situations, the team found “CO2 liquid chiller technology promising” but with “room for performance improvement.”
Making CO2 work in a direct expansion system was the task of engineers from the Carrier Commercial Refrigeration Lead Design Center in Germany. On the low-temperature side “very small dimensions for compressors and the piping system are achieved due to the high density of the CO2 suction gas.” But on the medium-temperature side, “original designs were needed due to the high-side system pressures of up to 120 bars. This presented significant design challenges, particularly with regard to the compressors, gas coolers, and high-side expansion valves.” Overall, the team said, “Experience to date suggest that these systems are potentially viable alternatives to HFC-based supermarket refrigeration systems.”
CO2 WITH AMMONIAOne aspect of CO2 (R-744) has been with its use with ammonia (R-717) in cascade systems. Researchers from Italy looked at how the latest compressors work with these so-called ‘natural’ refrigerants. In general, the researchers said even more could be done with compressors running with R-717 by trying a number of “different technological solutions.” For compressors using R-744, at low-cooling capacity, rotary compressors are most efficient, they said, while reciprocating ones are the most flexible and widespread.
A Spanish team looked at the cascade approach itself especially for the freezing processes and the storage of frozen products. The team built a working system in its lab. Results showed that the system refrigerating capacity and coefficient of performance (COP) and the CO2 and ammonia mass flow rates increase when the CO2 evaporation temperature increases. When the CO2 condensing temperature increases, the refrigerating capacity, COP, and the ammonia condensing temperature decrease.
The same team also did a thermodynamic analysis. “Cascade refrigeration systems with CO2 as a working fluid in the low-temperature stage and (ammonia) in the high-temperature one constitute an interesting choice for low-temperature cooling applications, such as freezing processes or the storage of frozen products.” Special attention was paid to “the determination of the optimum temperature level to carry out the interstages heat transfer process.”
That interest in both ammonia and CO2 formed the basis for a paper from teams from Algeria and Canada who did a performance comparison of cascade and two-stage refrigeration cycles using natural refrigerants. The teams also factored in R-290 (propane). “For a given refrigeration capacity, evaporator and condenser temperatures and cascade-condenser temperature, the compressor of R-744 in the cascade cycle is more compact than those of R-290 and R-717 in the two-stage cycles.”
Can a cascade refrigeration system with CO2 and ammonia work in supermarkets, where the technology currently focuses on the use of R-404A? Researchers from Sweden and Spain performed such a study in a laboratory.
“The results show that connecting the cascade condenser via thermosiphon or hot gas desuperheat arrangements yields good system stability and heat transfer performance. Hot gas defrost successfully defrosted the freezer evaporator but with long period and higher energy consumption compared to the conventional electric defrost method. The tested cascade system had up to 60 percent higher total COP compared to the R-404A system installed in the laboratory.
AMMONIA STAND ALONEMaking ammonia work in small-capacity refrigeration systems was a topic of a team from Denmark. The members developed a prototype of a domestic water-to-water heat pump that “can be designed to operate with less than 100 grams of ammonia. The main obstacle for introducing the technology commercially is the lack of components. Particularly, there seem to be no hermetic or semi-hermetic compressors available in this size range.”
REFRIGERANT VARIETYAlso entering into the equation was absorption cooling when considering a variety of refrigerants. Researchers from DuPont said, “Although the vapor compressor cycle is now used for most applications, refrigerant-absorbent systems (water/lithium bromide and ammonia/water) are still being used for certain applications, particularly space cooling and industrial refrigeration.”
The DuPont research looked at “the advantages of using room-temperature ionic liquids as new working fluids in absorption cycles.” The report described the liquids, called RTILs, as “a new class of solvent (molten salts) with a melting point defined to be less than 100°C.”
SECONDARY LOOPCanadian researchers looked at a supermarket system that “involves secondary fluid loops on both refrigerating and condensing sides, and heat recovery with brine-to-air heat pumps and passive heat exchanger.” The idea is to reduce HVAC energy use in supermarkets located in Northern climates.
The trend toward so-called secondary cooling approaches to supermarkets also means more attention to the equipment and configurations. A team of researchers from Sweden said they found “savings in both energy and cost can be significant by the selection of efficient cooling coils/heat exchangers or secondary refrigerant.”