The thermodynamics of electric vs. internal combustion cars

Posted on 4 September 2024 by Guest Author

This is a re-post from The Climate Brink by Andrew Dessler

I love thermodynamics. Thermodynamics is like your mom: it may not tell you what you can do, but it damn well tells you what you can’t do. I’ve written a few previous posts that include thermodynamics, like one on air capture of carbon dioxide and one on air conditioning. For whatever reason, they tend to get a lot of traffic. Well, here’s another one.

I was charging my electric vehicle (EV) at a DC fast charger the other day and was pumping electrons into my car at around 200 kilowatts (kW). Man, that’s a lot of power, I thought to myself. For reference, 200 kW is the average power draw of around 60 houses. Just going into my car.

That got me thinking about a comparison between charging EVs to “charging” gasoline cars by filling the tank with gas.

The energy stored in gasoline is astounding — it really is an incredibly high-density fuel. One gallon of gasoline contains 132 megajoules (MJ = a million joules) of energy. That’s comparable to the energy a house requires in a day.

This means that an SUV’s 20-gallon gas tank contains about 2.6 gigajoules (GJ = 1 billion joules) of energy. If it takes 4 minutes to fill your tank, then you can calculate the rate that you’re transferring energy into the gas tank as energy content of gasoline divided by time: 2.6 GJ divided by 240 seconds = 10 megawatts (MW = 1 million joules per second).

10 MW is an absolutely insane amount of power. It’s roughly equal to the power output of an enormous wind turbine at peak performance. Or the power required to drive ten locomotives. Or enough to power 3,000 houses. 10 MW is also equal to 13,000 horsepower. Let me repeat: It’s a lot of power.

As mentioned above, EVs charge at a lower rate, with the fastest chargers clocking in around 500 kilowatts (1 kW = 1,000 joules per second), but charge speeds of 200 kW or less are more typical. This means that EVs are charging at best at a rate that’s about 1/20th as fast as you’re fueling a gasoline car, but more often 1/50th as fast.

At first glance, this seems to give gasoline vehicles a massive advantage over electric vehicles (EVs). In reality, though, the difference is much smaller because this comparison misses a crucial point: efficiency.

Gasoline engines are very inefficient; for every 5 joules of energy you put into the gas tank, only about 1 joule actually drives the car forward. In other words, you’re wasting 80% of the energy content of gasoline; most of the wasted energy is lost as heat out of the tailpipe. This is not because engineers at car companies are lazy or incompetent — rather, it’s a consequence of the second law of thermodynamics. You literally can’t do much better, as I explain here.

EVs, on the other hand, are much closer to 100% efficient in converting the energy stored in the battery into motion. In other words, if you pull 1 joule of energy from the battery, nearly that entire joule drives the wheels. There is little waste heat generated.

This has a few important implications.

Cost of fueling: It is well known that driving an EV is cheaper than driving a gasoline car. One of the reasons is this efficiency difference: When you buy a gallon of gasoline, 80% of the energy in it is wasted, with the remaining 20% driving the wheels.

So if you buy a gallon of gasoline for $4, only 80 cents’ worth of your purchase drives the wheels. $3.20 of your purchase is used to heat up the exhaust and is of no value to you.

In contrast, EVs convert nearly 100% of the energy you purchase into motion.

Given that the cost per joule of gasoline and electricity are similar, this difference in efficiency ends up being one of the main reasons fueling EVs is cheaper than gasoline cars.

Heating the interior of your car: When it’s very cold outside, cars with gasoline engines are easy to heat because their engines waste a tremendous amount of energy, as discussed above.

To put some numbers on it: When you’re driving down the highway, you’re dumping 100 kW of hot air out of the tailpipe.

To heat the car’s cabin in winter, the car simply redirects a small fraction (a few kW) of this otherwise wasted energy into the cabin, providing heat without any additional fuel consumption.

In contrast, the high efficiency of EVs means that there is no “free” energy — nearly all the energy stored in an EV’s battery is used to drive the wheels. To heat an EV’s cabin, you therefore have to take power directly out of the battery. And since every joule directed toward heating is a joule that can’t be used to drive the wheels, this can substantially reduce the vehicle’s range in cold weather.

This is why EVs rely on seat heaters. They provide comfort with much less energy consumption than heating the entire cabin with hot air.

Implications for electrification of our society: Human society uses a tremendous amount of power (~20 TW), and about 80% of this comes from fossil fuels. If we assume that we need to replace all of that energy with climate-safe energy sources like wind and solar, it’s a daunting task.

But, as the car example illustrates, much of the energy we derive from fossil fuels is wasted, and doesn’t need to be replaced. For car transportation, for example, we only need to generate clean electricity equal to about 20% of the energy content of the gasoline currently consumed.

Generating electricity with natural gas or coal has the same problem: Most of the energy content of the fossil fuels goes up the smokestack and is wasted. Again, blame the second law of thermodynamics.

When we transition to a fully electrified society consuming energy mainly produced by wind and solar energy, we stand to significantly reduce overall energy consumed due to the inherent efficiency gains of all-electric energy systems.

One estimate is that we need only generate 44% of today’s power in an electrified economy. So instead of using 20 TW, we would be living the same life but with something closer to 10 TW. This problem is still daunting, but less so than if we needed to replace all of today’s power.

Correction: the post originally said that 20% of $4 is 50 cents. It is actually 80 cents. This has been corrected in the text.

Hannah Ritchie on the efficiency of EVs vs. ICEs and on the efficiency gained by electrification.