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New System for Efficient Cooling of Hybrid Drivetrains

28 April 2006

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.


April 28, 2006 in Fuel Efficiency, Hybrids, Vehicle Systems | Permalink | Comments (7) | TrackBack (0)


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Too Late.
134a is being banned in Europe as a global warming agent for use in cars. Replacement is CO2 in a transcritical cycle (since the condensing temperature of CO2 is 85 Deg F). US is fighting it of course.
Co2 and the other alternatives accepted in Europe, such as propane and isobutane, are very cheap. 134a is still under patent...

Maybe the motors will still need cooling in the future but the electronics probobly wont. Already new generations of high power electronics have come out and generate 1/5 the heat of the stuff used in the Prius. The next gen will probobly have much better electronics.

This makes the electric half of the hybrid drivetrain dependent on the automobile's air conditioning unit (or part of the car's a/c) for safe operation. I am not sold on the wisdom of such a move for the following reason: In my experience, automotive air conditioners tend to be more failure-prone than old-fashioned automotive engine radiators. Just ask my 1994 Lincoln Town Car (Yes -- a gas guzzler. I bought it cheap off my family years ago and cannot afford anything newer. I drive it so little these days that I don't really care.), whose radiator has never issued a note of protest in nearly 150,000 miles of operation, but whose air conditioning went senile a number of years ago.

Your air conditioner on the car broke but I bet your refrigerator or room air conditioner didn't! Why? The main source of Auto Air COnditioning problems is the lack of refrigerant "Freon" due to the shaft seal leaking. Referigerators, home air conidtioners and electric cars(some hybrids) have a shaftless "hermetic" system with no seal to leak. The vibration in a car is far worse than stationary equipment, but without a seal and with proper design, it should last the 10 year life of a car without service.
Also the compressor is far oversized in a regular car since it has to put out AC even at idle in the sun stopped in traffic. Then at full engine speed on a hot but sunless night, it is large enough to cool a 3 bedroom house.
It also must be sized for a heat soak Pulldown simulating the car sitting in the sun in Phoenix all day. Of course a solar panel on the car could provide a kind of "Whole house" ventilation fan to keep the temps down to something reasonable.
Another advantage for Electric cars and advanced hybrids.

Your right, the seals are the problem in transportation cooling but the shaft seals are only half the problem. The typical home fridg has 100% soldered connections and a hermetic compressor. Very few auto systems have eve implimented such a system due to the high vibration environment. The solution is on the horizon with ne are new soft metal tubing that can take the flex w/o fracturing. They look a lot like accordians and are soldered into key flex locations on the vehicle.

These systems should require zero maint for 150,000 miles as compared to about $500 - $1000 in maint for the average water cooling system over the life of the vehicle. Not too mention all the heavey metals and crud that have to be dealt with as hazerdous waste in the EU.

Propane and simular HC refrigerants are not viable options imo. They are too flammable. There have already been several instances of car fires caused by the aftermarket folks putting HC in thier car A/Cs and a leak sparking.

This system doesn't make electronics dependant on the car's A/C, it runs independantly from the compressor, they are juse using the same condenser.

R414B is vastly superior to R134a... I also really like Co2 but on hot days with a dirty condensor it will be impossible to get enough subcooling without a cascade setup. (People don't clean thier A/C condensor for years at a time, it GREATLY reduces thier efficiency and capacity)

The compressors aren't "oversized" a car A/C is not capable of cooling a 3 bedroom house.

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