By: George S. Bower



Both General Motors and Tesla are firm believers in active liquid-cooling and incorporate Thermal Management System (TMS) in their electric cars.

It could be argued that a Chevy Volt’s or Model S’ TMS is equally as important as the battery cell chemistry because the battery cell life depends on it.

As automakers introduce higher charging power levels to shorten recharge times, the TMS plays a greater role in rejecting larger amounts of heat.

At the pinnacle, Tesla’s Superchargers are already at 120 kilowatts with 150 kw planned for the future.

Not so incidentally, Audi has announced a production vehicle to compete with Tesla that has 150 kw charging. Audi has also announced a U.S. charging network based on 150 kw charging.

So, the stage is set for higher and higher charging levels. With that in mind, let’s look at Tesla’s and GM’s thermal management systems.

When evaluating a battery TMS we need to keep in mind some parameters used to compare the systems:

• Simplicity
• Cost
• Ability to remove heat (BTU/sec)


Tesla


Tesla’s thermal management – as well as GM’s – uses liquid glycol as a coolant. Both GM’s and Tesla’s systems transfer heat to a refrigeration cycle and use electric resistance heating in cold weather. Glycol coolant is distributed throughout the pack to cool the cells.

Considering that Tesla has 7,000 cells to cool this is a challenge.

It turns out that Tesla has a patent application for its system. It is based on a ribbon shaped metallic cooling tube that snakes through the pack.

A snapshot from the patent app is presented below.



This ribbon shaped cooling tube interfaces with the cells as shown below.



Now let’s look at one module in a Tesla Model S pack (16 modules total). The cooling ribbon snakes through the cells as shown below.



So, how does it meet the criteria of a well-designed and effective TMS?

Is it simple: Yes.

Is it cost effective: Yes.

How’s its ability to remove heat? Pretty good.


GM


GM uses prismatic (rectangular) shaped cells. Each cell is roughly the size of a typical children’s book. Sandwiched between each cell is an aluminum cooling plate.




There are five individual coolant paths passing through the plate in parallel – not in series as the Tesla system does things. Each battery pouch (cell) is housed in a plastic “frame.” The frames with coolant plates are then stacked longitudinally to make the entire pack.




How is the effectiveness of GM’s TMS system?

Simple: Yes.
Cost effective: Yes.
Ability to remove high heat loads: Slightly better.

GM’s system is slightly better than Tesla’s system in our opinion. However, in the grand scheme of things both Tesla and GM’s system have one big drawback. The heat transfer coefficient of Glycol leaves room for improvement.

We'll have more on how it can be improved in a subsequent article.