Financial Justification — Energy Efficiency
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.
Saving Energy—HFC and CO
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.
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.
Energy Saving Summary
Let's sum it all up with representative numbers for warm climate applications
(note: the following are full year estimates):
| || Transcritical CO2||HFC Systems|
| Basic refrigerant efficiency||-20%||Baseline|
| Electronic expansion valves with case controllers||+10%||+10%|
| Variable speed drives on compressors and condensers||+5%||+5%|
| Heat Reclaim||+10%||+5%|
| CO2 Booster system technology||+5%|| |
| Parallel compression||+10%|| |
| Ejector technology (gas and liquid)||+10%|| |
| Total opportunity vs. basic HFC system||+30%||[+20%]|
| Total opportunity vs. advanced HFC system||+10%|| |
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 trancritical 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.