Conference Year
2016
Keywords
commercial refrigeration, carbon dioxide, transcritical
Abstract
The traditional multiplex direct expansion (DX) refrigeration system used in commercial applications is prone to significant refrigerant leakage, especially older existing systems. EPA (2012) estimates that the U.S. supermarket industry-wide average refrigerant emission rate is approximately 25%. The use of high Global Warming Potential (GWP) refrigerants in these systems, combined with high refrigerant leakage, can result in considerable direct carbon dioxide equivalent (CO2eq) emissions. In addition, commercial refrigeration systems consume a substantial amount of electrical energy, resulting in high indirect CO2eq emissions. Thus, there are ongoing efforts to reduce both the direct and indirect environmental impacts of commercial refrigeration systems through the use of leak reduction measures, refrigerant charge minimization, low GWP refrigerants and energy efficiency measures. Based on prior energy and life cycle climate performance (LCCP) analyses, it was determined that a transcritical CO2 booster refrigeration system for supermarket applications has the potential to reduce carbon emissions and increase energy efficiency. To that end, a lab-scale transcritical CO2 booster refrigeration system was fabricated and installed in the environmental test chambers at the Oak Ridge National Laboratory (ORNL). This system consists of a transcritical CO2 compressor rack, an air-cooled gas cooler/condenser, medium-temperature (MT) and low-temperature (LT) refrigerated display cases, and MT and LT “false†loads. The lab-scale refrigeration system has a low-temperature cooling capacity of approximately 9.1 kW at −30°C and a medium-temperature cooling capacity of approximately 34 kW at −6.7°C. One 4-door vertical display case, 3.0 m in length, as well as a “false†load provided by a plate heat exchanger and a glycol loop, constitutes the low-temperature load. The medium-temperature load consists of one open vertical display case, 2.4 m in length, as well as a “false†load provided by a plate heat exchanger and glycol loop. The air-cooled gas cooler/condenser is installed in a temperature and humidity controlled “outdoor†environmental chamber while the compressor rack and refrigerated display cases are installed in a separate temperature and humidity controlled “indoor†environmental chamber. For both chambers, the temperature can be controlled between −18 to 65°C and the humidity can be controlled between 30 to 90%. The performance of the transcritical CO2 booster refrigeration system was determined at four ambient temperature conditions (15.6°C, 21.1°C, 26.7°C and 32.2°C). After the refrigeration system achieved steady-state operation at each of the four ambient temperature conditions, system performance data was collected for a 24-hour period. Over the temperature range of 15.6 to 32.2°C, the total load on the system was found to remain relatively constant. In addition, the compressor power was found to increase by approximately 78% over this same temperature range. Thus, the resulting coefficient of performance (COP) of the system was found to vary from 2.2 (at 32.2°C) to 4.1 (at 15.6°C). Based on the laboratory evaluation, the transcritical CO2 booster refrigeration system demonstrates promise as a low emission, high efficiency alternative to the traditional multiplex DX systems currently in use.