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Sharp readers will remember that we already covered this new Ultium tech toward the end of last month, when it was first announced, but it’s something that I think is worth going back to, because it’s an important piece of a big efficiency problem that plagues every automobile: getting some good from waste heat.
Before I go any further, it’s important to note that EVs produce a lot less waste heat than internal combustion cars do. For the average gas car, around 25% of the chemical energy in a gallon of gasoline goes toward actually moving the car toward its destination. The rest goes to waste heat, and it leaves the vehicle through the radiator, the exhaust, and as heat in the engine bay that seeps out. Normal friction brakes that non-electrified cars have also produce waste heat, literally taking the kinetic energy in a car and converting it to heat to slow the vehicle down.
The upside? Gas and diesel vehicles can take some of this waste heat and use it to keep the vehicle’s cabin warm. The heater core, which is basically a little radiator, gets piping hot when coolant gets pumped through it from the engine, and a fan blows the air through it to give you heat. So, a tiny fraction of the wasted energy gets put to good use.
Contrast this to an EV, which only converts a minority of its energy into waste heat, putting most of its energy toward actually moving the car down the road. When you factor in regenerative braking (which produces some electricity instead of waste heat like friction brakes), the overall efficiency of an EV can approach 90%. This is an amazing feat. You can’t use the waste heat to run the vehicle’s heat like you can with a gas car, but you also aren’t wasting a bunch of energy and just throwing it away into the atmosphere.
But that doesn’t mean there isn’t waste heat to be dealt with in an EV. Decent electric vehicles have liquid battery cooling systems that extract this waste heat from the battery pack and dissipate it through a radiator, much like a combustion car. However, there’s a lot less waste heat, so a big radiator isn’t needed, and the front of the vehicle can have better aerodynamic efficiency.
Tesla and other companies building EVs do try to make some use of the waste heat from the battery pack. Tesla has a multi-way valve that can pump some of the heated coolant into a heater core, or conversely, heat up the coolant and pump it through the pack to warm up the battery cells and improve performance in cold weather.
In GM’s press release, it said, “The Ultium Platform can recover and store this waste heat from the Ultium propulsion system. Further, it can also capture and use humidity from both inside and outside the vehicle, including body heat from passengers.”
I have to admit that at first, I was a total nerd and was reminded of The Matrix (in that film, AI robots use body heat from humans to power their civilization). But, I’m reasonably sure that GM isn’t going to plug us into a virtual world and steal our body heat, so they’re probably using some other method.
When I tried to figure out what GM is doing to accomplish energy recovery, I couldn’t find details. One YouTube channel interviewed a GM engineer, who told viewers next to nothing beyond that it recovers waste heat and puts it to use. Other sources speculated a bit, but otherwise only regurgitated the press release, which gave us an idea of what the Energy Recovery system does, but not how it happens.
The only clue I could find was here, where an engineer said that what’s special about the system is not novel parts, but how existing parts go together and work together. So now I can actually make some educated guesses instead of wildly speculating.
If there are no novel parts, we can rule out the idea that there is something really cool, like a thermoelectric generator, in the system. Instead, we have to assume that parts commonly found on EVs already are doing the work GM says the new system put in play on its Ultium cars does. Because they mentioned the cabin’s heat, this means we need to not only look at the battery coolant system, but also look at the HVAC system.
That leaves us with a liquid cooling/heating system for the battery pack, with a liquid connection to HVAC components. We can also assume that there’s a heat pump involved, because that’s the most efficient way to cool and heat the cabin except in extreme cold, where resistive heating is needed. But I think GM is using its heat pump more like a geothermal heat pump.
A geothermal heat pump exchanges heat or coolness with the earth, which maintains a constant temperature. This allows for greater efficiency than exchanging heat or coolness with the air, which can vary a lot during the year, so the heat pump itself doesn’t have to work as hard to heat or cool a home or business.
By heat exchanging between the battery pack, power electronics (including regenerative braking waste heat), a radiator, and the interior of the vehicle (which may be hot or cold), a computer could decide which way to pump the heat. If the cabin is too hot, the heat pump could work as an air conditioner, pulling heat from the cabin air and depositing it in the coolant system to precondition the battery pack. If the cabin is cold, it could do the opposite.
If both the cabin and the battery are hot, the pump could help ditch the heat off for both. If both are too cold, it could act as a more efficient battery preconditioning system (instead of resistive heat). This way, the system could theoretically use the least energy possible under all circumstances.
But how does it use our body heat? Well, it isn’t much, but when the cabin is too hot, so are you! When it cools the cabin, your body heat gets sucked into the system along with the other heat.
Finally, how can this energy be stored if you can’t generate electricity for the pack with the extra heat? It’s possible that GM is using the same trick the Toyota Prius uses for storing heat: basically a big thermos can. If there’s excess heat in the battery/cabin cooling system and no way to use it right now, the system might be putting some of the hot coolant away in an insulated chamber. Then later, if there’s a need for hot coolant to warm up the cabin or the battery, they can release that heat instead of using battery energy to cool it.
GM could come out later and tell us all about how the Ultium heat pump system works, and it will probably differ from what I’m writing here. But I don’t see how common HVAC and coolant systems could do what they’re claiming in any other way (Editor’s note: a quick Google search reveals that a thermoelectrical generator was shown way back in 2016 by a company called Evonik, whose website claims that GM is one of its end customers). However, GM can’t go around telling everyone else how to make a more efficient system or everyone would do it. If you work for an EV company, don’t just do what I’m saying in this article, because GM might own patents on some of it.
Either way, it’s good to see that more manufacturers are taking efficiency seriously. Hopefully more companies will follow.
Featured image: A Chevrolet Silverado EV, on the Ultium chassis. Photo by GM.
Jennifer Sensiba is a long time efficient vehicle enthusiast, writer, and photographer. She grew up around a transmission shop, and has been experimenting with vehicle efficiency since she was 16 and drove a Pontiac Fiero. She likes to explore the Southwest US with her partner, kids, and animals. Follow her on Twitter for her latest articles and other random things: https://twitter.com/JenniferSensiba
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