2. Cascade system. CO2 is the refrigerant in the lower-stage part of a two-stage system. This type of system typically appeals where there's a demand for refrigeration at a low temperature level (e.g., in process plants) or at two temperature levels (e.g., in supermarkets). Such a system can also reduce the charge of the primary (upper-stage) refrigerant considerably.
3. Transcritical system. CO2 is the only refrigerant used. Due to CO2's low critical temperature these systems must be designed for a cyclic process in which heat rejection can take place above the critical temperature. As a cyclic refrigeration process, the transcritical system is less energy efficient than the subcritical process and typically also requires higher compressor capacity. Therefore, if high ambient temperatures occur for long periods during the summer the transcritical cycle becomes less attractive.
Figures 2 to 4 show examples of design principles for these types of systems. The indirect system requires refrigerant pumps for circulation of the liquid CO2 to all the evaporators in the pipe network. In both the cascade and transcritical systems, refrigerant pumps can establish refrigerant circulation to all the evaporators in the pipe network but aren't absolutely necessary -- in some system designs internal pressure differences can also be used to circulate the refrigerant. However, pumps often are preferred because they allow liquid overfeed operation of the evaporators.
Most refrigerant pumps are used to establish liquid distribution of the refrigerant in the evaporators' pipe network (Figure 5). Such pumps also serve for liquid pressure boosting -- to provide extra pressure differential for expansion valves and to enable energy efficient system operation with low condensing temperatures during cold ambient conditions in winter (Figure 6). In larger industrial refrigeration systems, the pumps also handle general liquid transport between components like receivers and separators at different temperature levels (Figure 7).
Pumps designed and optimized for CO2 can offer a number of benefits for refrigeration systems:
Figure 6. Pump provides extra pressure differential for expansion valves.
Reducing direct environmental impact. Using CO2 and refrigerant pumps can decrease the charge of refrigerants negatively affecting the environment. A large part of the environmental impact of industrial refrigeration systems comes from permanent leaks and accidental release of refrigerants. Annual leak rates can reach 30% of the system charge, causing considerable direct impact. CO2 has a GWP of only 1 while many HFCs have GWPs up to several thousands (e.g., GWP for R134a is 1,300).
Decreasing energy consumption by improving operating conditions. Using refrigerant pumps to establish forced circulation and liquid overfeed operation of evaporators can increase evaporator energy efficiency. This allows a rise in evaporation temperature, which is one of the most important parameters for boosting overall energy efficiency of the refrigeration system. Depending on evaporator type, the refrigerant and the operating conditions, an increase of 4–8°F (-15– -13°C) is possible. As a rule of thumb, a refrigeration system's energy consumption goes down by approximately 2% for each °F the evaporating temperature goes up.