By Jim Knudsen, Marketing Manager, Danfoss
Along with a growing interest in CO2 refrigerants for food retail applications has come much discussion about the energy efficiency of CO2 systems, particularly with regard to two important factors: climate change performance and financial justification.
Climate Change
Traditional HFC refrigeration systems affect climate change in two distinct ways: direct and indirect contribution. Direct contribution results from the release of refrigerants into the atmosphere. This should never be intentional (and is currently illegal in most jurisdictions), yet it routinely happens over the life of traditional retail refrigeration systems, primarily through leaks. While these leaks can be addressed, making the switch to natural refrigerants such as CO2 is the best way to minimize their impact.
Indirect contribution refers to the energy used to operate traditional refrigeration equipment. The less energy required to operate the equipment, the lower the impact on the environment. Since both direct and indirect contributions are of approximately the same magnitude in a traditional HFC system, even a relatively inefficient low GWP system can still provide climate change benefits.
Financial Justification:
From a retailer’s perspective, financial justification is perhaps more important to the wide adaptation of CO2 systems than environmental impact. In North America, CO2 systems still carry higher upfront costs than traditional systems, in part because CO2 systems do not yet have the purchasing volume to drive down component and installation costs. However, reductions in ongoing operating costs usually justify the initial capital outlay.
Some physical properties of CO2, such as high working pressures and relative performance through the heat rejection and expansion process, pose inherent challenges to its efficiency as a refrigerant, but these disadvantages can be mitigated through system design.
Other properties of CO2 contribute to its efficiency in food retail applications, including excellent volumetric efficiency (more than six times the cooling effect per volume as R22); low compression ratio (the ratio between inlet and outlet pressures at the compressor); and low viscosity (making it easier to pump).
Saving Energy: High-efficiency HFC vs High-efficiency CO2
Certain technologies typically used on CO2 systems, such as electronic expansion valves with case controllers, variable speed drives, and heat reclamation, can also be applied with great effect in traditional HFC systems.
Several new CO2-specific technologies squeeze even more efficiency into a CO2 system’s design. Booster systems arrange the compressor piping to allow the low-temperature compressor to boost suction pressure for the medium-temperature compressors, saving work and energy. Parallel compression deploys a portion of the medium-temperature compressor capacity to recover and recompress the flash gas formed when the compressed vapor exiting the gas cooler is expanded, increasing system efficiency by as much as 20 percent during transcritical operation. Ejectors take high-pressure compressed vapor from the gas coolers and use the energy lost during expansion to increase the pressure of the flash gas, allowing it be introduced into the suction side of the parallel compressors, reducing the work needed to compress the gas.
As the table below illustrates, transcritical CO2 technology is being developed to be deployed in nearly any climate and can provide substantial environmental and financial benefits. So why invest more upfront for CO2? Simply put, not only are the environmental benefits significantly greater with CO2; over time, the energy savings achieved with CO2 make it the better choice.