How to defrost an electric car

As you walk to your car on a winter morning, you notice there is frost on the front windshield. This isn’t a big deal for most car owners – just start the car, let the engine heat up the coolant and turn on the defroster.

But what if you have an electric car? Without a gas-powered motor, how is the air going to heat up to melt the ice?

If you read my columns regularly or know me, you’ve probably figured out I love talking about air conditioning and how it works. The cool thing – no pun intended – is that AC systems are designed to cool, but by doing this they create a lot of heat.

When you turn on your AC system, you send an electrical signal to the AC compressor. The AC compressor takes a low-pressure gas (freon) and compresses it into a high-pressure gas.

Once the freon is compressed to a high pressure, it gets extremely hot. It then must be cooled by the AC fan through the condenser. Next, it goes through a filter and then through the expansion valve that turns the freon into an extremely cold liquid. This liquid then passes through the evaporator core, which is inside the dash, next to the blower fan. The blower fan takes warm air from outside, blows it over the evaporator core and turns it into nice, cold air. The freon then returns to the AC compressor as a low-pressure gas.

I started this column writing about melting ice, so why am I explaining how AC works? The basic concept of an AC system is the use of a heat pump. Heat pumps transfer heat energy to a thermal reservoir, such as a condenser or evaporator. The heat created is just a byproduct of the AC process. A conventional AC system dissipates the heat to the outside of the car to cool the inside.

So how do you defrost the front windshield on an electric car? Not with a heating element in the glass – that would obstruct the driver’s view (the rear window does have a built-in heating element, though).

On early electric cars, manufacturers used a positive temperature coefficient (PTC) heating element inside the dash to heat the air, but it had problems. The PTC system would use a lot of electricity, it took longer to heat up and it wasted a lot of heat.

Although the engines on late-model electric cars don’t create heat, they do have air conditioners. These systems will create a lot of heat just by running the freon through them. So instead of venting the heat outside, the car vents it inside for heat. The heated freon passes through the cabin condenser (thermal reservoir) and is blown into the cabin or onto the window for defrosting. This type of heating air is way more efficient because it takes a lot less power to run the AC compression than it does a heating element. Because the heat is just a byproduct of the AC heat pump process, there is no wasted electricity. This in turn provides heat and defrosts the car without running down the battery.

This heat pump technology is used in many different applications. It is great that the automobile manufacturers can take an older technology and create a new application.

This kind of stuff really excites me. I’m looking forward to all of the innovations that we will see in the future.

Matt Pataky owns Sunnyvale Foreign Car Service, 15 Pioneer Way, Mountain View. For more information, call 960-6988, email [email protected] or visit sunnyvaleforeigncar.com.

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