performance of a transcritical CO2 system in a US supermarket over a year provided encouraging results when compared with traditional HFCs
The Department of Energy Better Buildings Alliance’s refrigeration team partnered with Hannaford Supermarkets over a period of one year to study the operation of Hannaford’s store in Turner, Maine, which features a transcritical CO2 refrigeration system.
This store was the first in the US to use the technology.
Compared to a nearly identical store in the same climate zone using a traditional HFC system, the CO2 store showed a storewide reduction in climate impact of 15 per cent over the study period while maintaining a similar energy-use profile and the necessary levels of temperature control and reliability.
Researchers compared refrigeration system performance to that for a supermarket having nearly identical layout and refrigeration loads – in a similar climate and of similar vintage – that uses a conventional hydrofluorocarbon (HFC) refrigerant.
Delhaize provided the sub-metered and utility data used to generate the performance summaries herein.
The TC CO2 booster systems employed uses CO2 as the sole working fluid, unlike the CO2 cascade systems seen more commonly in the US that still rely on a significant HFC charge. TC CO2 booster systems first gained a major foothold in the European market.
In a comparison of electricity consumption of the refrigeration systems of the two stores on a monthly basis from October 2013 to August 2014, the TC CO2 system’s electricity consumption was of a comparable magnitude to that for the conventional refrigeration system. Overall source energy consumptions (electricity and propane) for the two supermarkets are also comparable.
The substantially lower GWP of CO2 (a value of 1, as opposed to 2,107 for HFC-407A) means that a TC CO2 system will dramatically reduce the direct environmental impact of refrigeration leakage, recharge, and disposal. In comparing the estimated direct and indirect climate impacts of the two supermarkets, the Turner supermarket reduces climate impacts by about 15 per cent. Virtually all of the difference is associated with direct impacts (ie refrigerant leakage, recharge, and disposal).
The Bradford supermarket achieved a refrigerant leakage rate (191 lbs per year or about 10 per cent of system charge) – lower than typical US supermarket of about 15 per cent per year.
Therefore, reductions in overall climate impacts will likely be greater when TC CO2 booster systems are compared to typical supermarkets.
This new technology carries a significant initial cost premium. Hannaford estimates the incremental cost of the TC CO2 system (over a prototypical HFC system) is about 40 percent for the refrigeration equipment alone, in addition to a 10 to 15 percent incremental cost for piping and display cases. However, data suggest that operating costs for the TC CO2 system at Turner are lower compared to the standard HFC system at Bradford because the cost of therefrigerant is lower (roughly one-fourth the cost of the legacy HFC refrigerant).
Additionally, during the study period, the maintenance costs (contracted services, refrigerant purchases, and in-house technician labour) for the Turner refrigeration system were on par with Hannaford’s chain average.
During the study period, the utility and reliability of the Turner TC CO2 booster refrigeration system were comparable to the Bradford HFC system.
Climate, energy and cost
The main sources of environmental impact of the two supermarkets are refrigerant leakage (direct impact) and propane and electricity usage (indirect impact).
Chain-wide, Hannaford supermarkets average a leakage rate of about 15 per cent annually, which is below the EPA national average leak rate estimates of 20 per cent.
For the two supermarkets considered for this case study, measured annual refrigerant leakage was equal – 200 lbs of refrigerant per store over the year-long analysis period. Though the leakage masses are identical, their impact varies tremendously due to the substantially higher GWP of HFC-407A.
Indirect environmental impact was calculated from propane and electricity bills at both supermarkets over the course of the year using the publicly available EPA Climate Impact Calculator.
The leakage of HFC refrigerant, even in a supermarket such as the one in Bradford, where the leakage rates are relatively low, accounts for a significant amount of the total environmental impact (approximately 16 per cent of the whole-store climate impact).
Looking solely at the refrigeration system itself, the direct leakage of refrigerant at Bradford constituted about one-third of the total impact of the refrigeration system. This is in line with the breakdown of impacts shown in past analytical studies of supermarket refrigeration life cycle performance.
For the system in Turner, as carbon dioxide has a GWP of only 1, the direct impact is almost negligible. On a full-store basis, the study data indicate the Turner store exhibited a 15 per cent reduction in overall climate impact compared to the baseline store in Bradford.
Additionally, the direct impact of the Bradford system is already lower than that of many US supermarkets, which have higher leak rates and utilize refrigerants such as HFC-404A and HFC-507A, which have GWP values almost twice that of the HFC-407A used at Bradford.
The following is excerpted from Case Study: Transcritical Carbon Dioxide Supermarket Refrigeration Systems, which was prepared by Navigant Consulting for the Better Buildings Alliance; Building Technologies Office; and the Office of Energy Efficiency and Renewable Energy, US Department of Energy (DOE).
To read the entire study, visit http://bit.ly/TC-CO2-STUDY