Operating controls and dynamics for floating refrigerant loop for high heat flux electronics

The Oak Ridge National Laboratory (ORNL) Power Electronics and Electric Machinery Research Center (PEEMRC) have been developing technologies to address the thermal issues associated with hybrid vehicles. This work is part of the ongoing FreedomCAR and Vehicle Technologies (FCVT) program, performed for the Department of Energy (DOE). Removal of the heat generated from electrical losses in traction motors and their associated power electronics is essential for the reliable operation of motors and power electronics. As part of a larger thermal management project, which includes shrinking inverter size and direct cooling of electronics, ORNL has developed U.S. Patent No. 6,772,603 B2, Methods and apparatus for thermal management of vehicle systems and components (Hsu et al, 2004), and patent pending Floating loop system for cooling integrated motors and inverters using hot liquid refrigerant (Hsu et al, 2004). The floating-loop system provides a large coefficient of performance (COP) for hybrid drive component cooling. This loop uses R-134a as a coolant and shares the vehicle's existing air-conditioning (AC) condenser, which dissipates waste heat to the ambient air. Because temperature requirements for cooling power electronics and electric machines are not as low as that required for passenger compartment air, this adjoining loop can operate on the high-pressure side of the existing AC system. This arrangement also allows for the floating loop to run without a compressor and requires only a small pump to move the liquid refrigerant. For the design to be viable, the loop must not adversely affect the existing system. The loop should also, ideally, provide a high COP, a flat temperature profile, and low pressure drop. To date, the floating-loop test prototype has successfully removed 2 kW of heat load in a 9 kW automobile passenger AC system with and without the automotive AC system running. However, during the cyclic operation of the floating refrigerant loop, some two-phase transient behavior is evident. In order to maintain stable running conditions, specific operating controls were implemented. Also thermodynamic energy balances were conducted to further analyze the operating conditions