Development of a work producing expansion device for a transcritical carbon dioxide cycle

Because of its high volumetric heat capacity, CO2 offers the potential for reduced weight and volume in packaged systems, which is a major focus for automobile and military applications. The review of present literature, however, has shown that the overall efficiency of the transcritical CO 2 cycle decreases as the weight and volume of the system are decreased. System improvements must be achieved to meet the goals of the reduction of weight and volume, while still retaining competitive efficiency. A thermodynamic analysis was performed for several modified transcritical CO2 cycles, with a goal of determining the proper configuration to achieve the optimum system performance. The analysis shows that the system efficiency of the transcritical CO2 cycle can be dramatically increased by modifications of the system configuration, and that the optimum performance is a strong function of the operating conditions. The system efficiency of the transcritical CO2 cycle is increased considerably by nearly 100% when the low-side pressure increases from 2.5 MPa to 4.5 MPa. There is a specific high-side pressure at which the maximum COP occurs in the CO2 cycle. There are also specific pressure ratios across the compressors for the minimum compression work and the maximum COP for the two-compressor system. The employment of a work extraction device in the role of an expander is expected to give the largest improvement of the system efficiency. This device will not only increase the heat removal capacity of the evaporator but also can reduce the compression work. As part of this research, an expansion device with output work (ED-WOW) was designed, constructed and installed in a prototype transcritical CO2 refrigeration system, replacing the simple throttling valve. During cycle tests, the use of the device provided between 7% and 10% improvement in COP. These figures are considered to be highly conservative, as the work extraction measurement was biased toward a lower value due to uncertainty in the load device performance