Advanced Cooling System Makes Hybrids More Efficient

Oak Ridge National Laboratory in Tennessee creates a more efficient way to cool complex hybrid-electric vehicle drivetrains to improve their reliability and longevity – and also to increase fuel economy.

ORNL scientist John Hsu has developed a “floating loop” integrated cooling system that improves on the multiple ventilation systems now used to cool HEVs and fuel-cell vehicles.

Today’s HEVs use loops from the vehicle’s conventional cooling system to prevent power electronics and drive motors from overheating.

For increased power densities and improved reliability, auto makers are seeking smaller, more efficient cooling systems. Hsu’s floating loop integrates separate ventilation systems for the electric motor, internal combustion engine, batteries, transmission, power electronics – and fuel cells, if used in an FCV.

The floating loop integrates traction-drive cooling with the vehicle’s conventional air conditioning; the two systems share the piping, refrigerant and condenser of the passenger-compartment AC system.

This eliminates the necessity for a separate coolant path, compressor and heat exchanger. However, a small pump must be added to allow refrigerant to flow through the new section of the system.

The new ORNL-designed technology creates an effective 2-phase ventilation system that directly cools the inverter and motor by sharing the components of the vehicle A/C system, yet allows the two systems to operate autonomously to enhance component durability.

Future HEVs, such as Toyota Prius, could improve fuel economy with new system.

The floating loop has been demonstrated to enhance power densities and reliability. It also allows engineers to design more compact cooling systems that “package” better in these technology-dense vehicles.

Even though the floating loop system shares some of its parts and plumbing with the vehicle A/C system, it operates independently. The subsystems thus can work together or alone.

The floating loop cools components to a range of 122°-140° F (50°-60° C). The cooler the electronics are kept, the more fuel-efficient HEVs can be, says Laura Marlino, ORNL project manager. She adds that reducing size, weight and volume of the cooling system also contributes to increased fuel economy.

ORNL tests reveal the new floating-loop cooling system functions well when integrated with the conventional AC system. Hsu says the floating loop segment has a coefficient of performance (COP) greater than 40 – an impressive COP rating.

COP is the ratio of heat removed to energy supplied. An air conditioner that uses 700 watts of electricity to remove 1,650 watts of heat, for example, has a COP of 2.36 (1,650 watts divided by 700 watts equals 2.36).

“The higher the COP, the less energy is required to remove the heat,” Hsu says. For a conventional air conditioner, the COP ranges between 2 to 4 with a very cold refrigerant. The ORNL floating loop COP is approximately 40-50 because it employs a warm refrigerant.

“The most important advantage of the floating loop over the (conventional) water-ethylene-glycol system is that because the refrigerant has good dielectric property, the inverter switching dies can be cooled directly by the refrigerant,” Hsu says. “This makes the inverter smaller, providing more room for passengers.”

The U.S. Dept. of Energy’s Office of FreedomCar and Vehicle Technologies is funding the project, which ORNL says is in an early development stage.