Modeling and control strategy of a transcritical carbon dioxide cycle for application in multi-temperature refrigerated container systems.

Transportation refrigeration requires efficient, compact, and reliable technology, and these constraints must be met with especially rugged designs when used in a military application. Additionally, industry trends towards efficiency and environmental concerns have motivated the development of low global warming potential (GWP) refrigerants and efficient technologies to utilize them. This research focuses on the theoretical analysis and physical design of a transcritical carbon dioxide (CO2) cycle for application in a U.S. Army Multi-Temperature Refrigerated Container System (MTRCS). The MTRCS has two separate refrigeration compartments of variable size and operating temperature. The proposed transcritical CO2 cycle utilizes a novel compressor-expander unit called the Energy Recovery Compressor (ERC), which is an axial multi-piston compressor-expander that uses the Sanderson Rocker Arm Mechanism (S-RAM) to convert the shaft rotation into linear piston movements. Integrating the ERC into novel refrigeration system architecture results in a coefficient of performance (COP) of 1.29 at an ambient air temperature of 57.2 °C. In order to achieve this performance, the cycle utilizes three compression stages, intercooling between the 2nd and 3rd compression stages, flash tank economization, and expansion work recovery. The inclusion of these cycle modifications increases the complexity of the system control strategy significantly, and requires the investigation of how important cycle parameters need to be controlled over the range of given operating conditions. This paper describes the system architecture and the analysis conducted to predict the system performance and necessary controls for robust operation.