CO₂ energy efficiency in supermarkets

Life Cycle Climate Performance, or LCCP, is a standard method of comparing technologies with regards to their effects on climate change as measured by the approximate equivalent release of pounds of CO₂. Traditional HFC refrigeration systems have two strong components to LCCP: the direct and indirect contribution. The direct contribution is a result of the release of the refrigerant into the atmosphere and is based on the Global Warming Potential, or GWP, of the refrigerant (as with LCCP, GWP is based on the refrigerant's effect on climate change compared to CO₂). The release of refrigerant should never be intentional (it is currently illegal in most jurisdictions), yet it routinely happens over the life of a food retail refrigeration systems, primarily through leaks. While these leaks can be minimized, the use of a low global warming potential refrigerant such as CO₂ can make this effect negligible (CO₂ has a GWP of 1, compared to ~4000 for a refrigerant like R404A). It is important to note with regards to leaks, there are rules and regulations being proposed (particularly in California) which may add financial penalties to the environmental penalties associated with leaked refrigerants.

The indirect component is due to the effect of the energy used in operating the refrigeration equipment. The less energy that is needed to operate the equipment, the lower its contribution to the climate effects will be. Since both of these components in a traditional HFC system, direct and indirect, are of approximately the same magnitude, even a relatively inefficient low GWP systems can still provide climate change benefits.

Financial justification is perhaps more important to the wide adaptation of CO₂ systems, because CO₂ systems in North America are substantially more expensive than traditional refrigeration systems. This is due to the facts that CO₂ systems operate at high pressures (adding cost to components), they are more complicated than traditional HFC systems (with addition equipment required, such as a bypass line with a valve, a high pressure transcritical valve at the outlet of the gas cooler, and additional controls) and CO₂ systems do not yet have the purchasing volume in North America to drive down component and installation costs. In order to be financially justified, these systems need to overcome this initial capital outlay by providing ongoing operating cost reductions.

Clearly there are other financial considerations with CO₂ systems, such as long-term regulatory and social impacts, but these are much more difficult to quantify. It is much easier, however, if we use a more simplified financial model which considers only energy efficiency to justify the additional capital outlay.

Strictly due to its physical properties as a refrigerant, CO₂ has some inherent challenges compared to HFC refrigerants with regards to energy. These challenges stem from the high working pressure (over 1000 psi vs. around 200 psi for R22) and the relative performance through the heat rejection and expansion process. While these disadvantages seem to be significant, as much as a 20% penalty, they can be mitigated through the system's design.

On the other hand, there are properties of CO₂ which also help the system's efficiency in food retail applications, including excellent volumetric efficiency (more than 6 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). Additionally, new technology has been developed that takes advantage of the unique properties of CO₂ to improve efficiency.

Complicating this discussion are technologies typically used on CO₂ systems that can also be applied with great effect in traditional HFC systems. These items can make the initial cost of a CO₂ system seem even higher versus a basic HFC system, but can be considered separately and be financially justified in any system. Many retailers have found that implementing advanced energy saving technology on their HFC systems to be well worth the cost.

There are three key technologies that fall into this category. The first is electronic expansion valves with case controllers, which allow the suction pressure to be optimized to minimize the load on compressors as conditions change. The second is the use of variable speed drives to allow the compressor and condenser capacity to more closely match changes in load. The third is heat reclamation to use waste heat of the refrigeration cycle.

Heat reclamation, particularly in HFC systems, is used primarily to supplement facility hot water requirements due to the low quality (i.e., low temperature) of the waste heat. In CO₂ systems, the temperature of this waste heat is much higher, allowing it to be used for hot water, comfort heating, dehumidification reheat, or regeneration of desiccant, etc.

CO₂ specific technologies take into account the system's design. Booster systems arrange the compressor piping to allow the low temperature compressor to boost the 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 re-compress, at a lower compression ratio, the flash gas formed when the compressed vapour exiting the gas cooler is expanded to allow it to condense into liquid. A large portion of the flash gas in the receiver can be considered lost capacity for the system, though reclaiming it with a minimum amount of work can increase system efficiency by as much as 20% during trans-critical operation.

The most recent development is a device called an ejector. An ejector can use high pressure compressed vapour from the gas cooler and utilize 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. This technology is very effective in dealing with one of the most inefficient aspects of CO₂ refrigeration and may overcome the inherent disadvantage of CO₂ trans critical systems in warm climates.

Let's sum it all up with representative numbers for warm climate applications 
(note: the following are full year estimates):

It should be noted that while the energy improvements in cooler climates may be lower, the overall efficiency of transcritical CO₂ systems increase with cooler ambient temperatures (i.e., less time operating in transcritical mode).

Not included in this summary is the use of adiabatic or evaporative condensers/gas coolers, which can provide another 5% of efficiency to either system. In fact, with additional attention to system design, a transcritical CO₂ gas cooler may be configured to use substantially less water than a HFC evaporator, up to 80% less. The performances of these devices vary substantially with local climate and are specific to their system. That said, it is another interesting technology to consider.

While this analysis is far from rigorous for all applications, the purpose of this article was to summarize the situation and technologies available to give a clearer view of what is possible today. Transcritical CO2 technology is clearly ready to be deployed in nearly any climate and can provide substantial environmental and financial benefits.