Improving an EV’s Cabin Heating System

Plug-in hybrid electric vehicles (PHEVs) have one leg up on battery electric vehicles (BEVs) because they have an internal combustion engine that gives off waste heat. Systems that capture this waste heat come in handy in the winter to heat the passenger cabin.

Pure BEVs on the other hand don’t have this source of waste heat and so they must rely on the battery as a source of energy for heating. Using the battery for heating lowers the range. In extremely cold environments, we could see as much as a 50-percent loss in range for a battery electric car. That’s a big disadvantage.

The early Nissan Leafs (pre 2013) used resistive heating. Resistive heating is probably the most inefficient way to heat the cabin. In 2013 the Leaf incorporated more energy efficient heat pump which works using a reversible refrigerant cycle to heat and cool the cabin, and this mitigated the effect on range in cold weather. When operating in electric mode the Volt’s cabin heating is purely resistive so the Volt loses significant range in cold weather if the ICE does not run. The second-generation Chevrolet Volt also retains just pure resistive heating in EV mode.

Is there any waste heat floating around in a BEV that could be used for cabin heating?

Yes there is. It is waste heat from the electronics and from the traction motor.

Tesla has a patent on just such a system. The schematic from the patent describing the system is shown below.

Tesla patent shows waste heat from the drive motor and electronics being used to heat the cabin.

Tesla patent shows waste heat from the drive motor and electronics being used to heat the cabin.

A simplified version is shown below.

Simplified version of patent schematic.

Simplified version of patent schematic.

The patent shows four separate cooling loops. The top cooling loop is a glycol loop that cools the drive motor and electronics. In hot weather this loop just rejects heat to ambient from a radiator in the front of the car.

The cooling loop right below the electronics cooling loop is the cabin heating and cooling loop. This glycol loop can cool the car via the refrigerant loop (shown below the cabin loop) or in heating mode the cabin cooling loop uses a resistive element for heating.

The ingenious part is that the motor/electronics loop can communicate with the cabin heating loop via some flow control valves. With the two loops connected, waste heat from the motor and electronics can be used to heat the cabin with the resistive heating element used as a second source of heat. Scavenging the waste heat lowers the load on the battery and increases range.

A simplified version of the Tesla patent in “waste heat cabin heating mode” is presented in the figure below.

Simplified schematic showing cabin heating using waste heat from the drive motor and electronics.

Simplified schematic showing cabin heating using waste heat from the drive motor and electronics.

Could this patent be what is used in the production Tesla Model S?

We don’t have positive information from Tesla itself, however based the following write up in the Tesla Motor Club it appears that this patent probably is used in the production Model S.

Insert blockquote:

The Model S cabin heater has two (hidden) modes. If the drive train is cold, all heat comes via resistive heaters, which can draw up to about 6 kW. That’s a lot of power.

As you drive the car, the drive train will naturally heat up. Once that happens, Model S uses the drive train coolant to help heat the cabin. Essentially it takes waste heat from the motor and inverter and uses that to heat the cabin. This makes a huge difference to the power consumption – a fully warmed-up car will only need 1-2 kW to keep the cabin warm even in extreme cold conditions. In comparison, the original Tesla Roadster needs 4 kW to keep its tiny cabin not-terribly-warm using only resistive heaters. This is a big advantage of Model S engineering that Tesla never talks about!”

The upshot here is that cabin heater power consumption gets better quite dramatically after you’ve been driving for a while.

No Heat Pump?

Let’s look at a simplified sketch of the Refrigerant loop in the Tesla patent.

Simplified schematic of the Tesla patent Refrigerant cooling loop shows no heating mode.

Simplified schematic of the Tesla patent Refrigerant cooling loop shows no heating mode.

There are no reversing valves shown in the Tesla patent. Also there are to expansion bypass valves shown in the patent.

Therefore, there is no heating mode in the Refrigerant loop.

SEE ALSO: What is Level 3 Charging?

That’s the patent but does the Model S incorporate a heat pump in production?

Again we have no direct information from Tesla but all indications from Google search suggest that there are no reversing valves and therefore no heating mode for the refrigeration system in the model S.

What could GM do in its upcoming Bolt 200 mile EV to improve on Tesla’s heating scheme?

Stay tuned. We will show you in the next article.