|ORNL R134a integrated cooling system with floating loop. Click to enlarge.|
Researchers at the National Transportation Research Center, part of the Department of Energy’s Oak Ridge National Laboratory (ORNL), are developing a new approach to cooling the power electronics and motors of hybrids that could lead to better performance, improved fuel efficiency, and increased power density for future systems.
Called the Floating Loop, the new cooling system is a low pressure drop R134a refrigerant loop for direct-contact cooling. The loop shares some components and piping of the vehicle air conditioning system, but remains operationally independent. The floating loop requires only a small pump to move the liquid refrigerant.
Floating Loop is geared toward future developments of hybrid and possibly fuel cell vehicles, which will have high power, high heat-producing electronics and motors. The floating loop will enhance their operation by being able to cool these electronics and motors more efficiently.
As you more efficiently cool them, you can reduce the size, weight and volume, and that leads to greatly improved gas mileage.—Laura Marlino, ORNL Project Manager
Removing the heat generated by the electrical systems in hybrids is essential for their reliable operation and to the ability of automakers to increase power density and decrease weight and volume in future hybrid drivetrains. With improved cooling, a motor can run at a higher efficiency due to decreased resistance losses in the windings.
Hybrids currently on the market are using a variety of solutions for cooling their power electronics and traction motors, including radiator coolant loops, forced and natural air convection, and oil circulation.
The Floating Loop provides effective two-phase cooling directly for the inverter and traction motor. The system takes liquid refrigerant directly from the condenser (60°–80°C). The small pump circulates the fluid to the electronics and motor loads. where it cools the loads via evaporative cooling and direct contact. Vapor returns to the condenser. Heat is rejected to ambient by condensing the vapor from the floating loop and the passenger A/C sub systems.
Defining the Coefficient of Performance (COP) of a cooling system as the Heat Rejected divided by Input Power, the COP of the floating loop is very high (~45) (based on initial testing of a prototype system) compared to a passenger AC system (~3).
Whereas the AC system requires high input power due to high compressor pumping power, the Floating Loop uses very low pumping power to circulate the coolant through the loop.
ORNL sees the new system as applicable for a range of hybrid applications: assist-only electric motors (parallel-configuration), full hybrid traction drives (series, parallel, and series-parallel configurations) and eventually fuel cell hybrids (series configuration).
The loop concept is not dependent on R134a (which has 1,300 times the greenhouse effect as CO2, according to the DOE). ORNL is exploring the use of new AC cooling fluids.