Wells to wheels: electric car efficiency February 22, 2013Posted by Maury Markowitz in electric cars.
Tags: electric cars
AOne common argument against electrifying the car is that all it really does is move the engine from one place to another – from your car to a power plant. There’s an advantage to moving that exhaust away from people, but overall, it’s argued, the effect is pretty limited.
Guess what, it’s math time!
If we’re going to compare electrics to conventional internal combustion engine (“ICE”) cars, we need to get our measurements right.
With conventional fuel cycles we often want to know about two primary numbers, the “tank to wheel efficiency” which tells you how efficiently your engine turns fuel into movement, and the “well to wheel efficiency” which adds the energy it took to get that fuel to your gas tank.
For conventional cars you can just Google those numbers in seconds. So if we want to do a straight up comparison, we want to develop similar numbers for electric cars.
So, let’s start with tank to wheel
One of the nice things about electric motors is that they operate efficiently over a wide range of speeds. An ICE varies between 0%, when idling, to something in the low to mid 30% range. An electric motor runs somewhere in the low 80’s to high 90% range over its entire RPM band.
There used to be a running debate about whether EV’s should have gears, generally two. That keeps the motor in the mid-to-high 90’s, but adds complexity. That debate is now over; no modern EV has more than a single fixed speed gear.
So in an EV there’s no major transmission system, driveshaft components, and in some designs, you don’t even have axles or differentials. A modern electric drivetrain is much simpler than a modern gasoline one, with parts counts that are tens or hundreds of times smaller.
Over on the right is an image of the Tesla Model S (from Rides with Chuck) and that’s basically the entire car – the motors are placed between the wheels (the big cylinder mid-image) where the front and rear axle/transaxle would be, and a battery pack in the floor of the car (the light-silvery part at the top-left).
In a typical car, the drivetrain eats up about 5 to 6% of the energy from the engine, in an electric design it’s close to 0%.
The motor itself is fantastically efficient, but that’s in terms of the electricity being delivered to it from the batteries. That conversion is not direct – the batteries provide DC power but the motor uses AC, so you need to use an “inverter” to change it from one to the other. Modern inverters are about 95% efficient.
In a gasoline car, the fuel that’s pumped into your tank is used directly in the engine. That’s not the case in an electric car, where the “fuel” is AC power from your home, and the tank is a battery full of DC power. So we have to convert from AC to DC using a charger which is also about 95% efficient.
That’s right – we start with AC, turn it into DC, back into AC, and then into motion. That doesn’t sound great, does it?
And finally, we need to consider leakage. When I charge the battery, not all of the power ends up stored, some of it is used up pushing the electrons through the battery. Typical numbers here are about 85 to 90% efficient.
So, a rough estimate of the total round-trip tank to wheel efficiency is:
0.90 (motor and drivetrain) x 0.95 (inverter) x 0.90 (battery) x 0.95 (charger) = 73%
This number jives quite well with the claims of Tesla, which quotes a 75% round-trip efficiency. Tesla and Leaf owners report slightly lower real-world charging numbers, with the charger and battery portions of the cycle on the order of 80 to 85%. If we use those numbers we get:
0.90 (motor and drivetrain) x 0.95 (inverter) x 0.8 (battery and charger) = 68%
This isn’t a huge difference, so we’ll split it and call it 70%. How does this compare to a conventional car? Quite well in fact. A normal gasoline car has a tank-to-wheel efficiency of 16%.
That’s right, an electric car is over four times as efficient at turning energy into motion.
But then there’s well to wheel
This comparison is not apples to apples because it doesn’t account for where that electricity comes from.
You’ll get your electricity from a mix of generation sources, likely including a proportion of coal, natural gas, nuclear, and a bunch of renewables like hydro, wind and solar. Right now the grid in North America is undergoing a massive switch from coal to natural gas. There’s still a lot of coal out there. But then there’s also a lot of hydro and nuclear. When you average everything out, it’s like we get 100% of our power from NG. Actually, that’s only true in the US, up here in Canada we get over half our power from hydro, so the power mix is considerably cleaner.
NG is burned in large turbines, basically jet engines, which spin generators. A turbine is about the same overall efficiency as a gas engine running at its peak, turning about 30% of the energy in the fuel into rotating shaft power. The rest, 70% of the energy, is lost as heat. But turbines have one additional trick… in your car that extra heat blows away through your radiator or out the tailpipe. But in a power plant, we can capture it. We use it to heat up water, boil it into steam, and then use the stream to drive another turbine. These “combined cycle” generators can be up to 60% efficient. When you factor in things like throttling and load following these numbers go down, but average numbers on the order of 50% are very common, and most modern plants are closer to 55%.
Then we have to get that power to you. Contrary to what you may have heard, the electrical grid is very efficient. The total losses in the US grid are only about 7%. This number keeps going down as we improve the systems.
So that means the real tank to wheel comparison is:
0.5 (generator) x 0.93 (line losses) x 0.7 (entire car side) = 33%
Now we also have to get that gas to the NG power plant. The drilling and extraction requires energy equivalent to about 9% of the fuel, and shipping it in a pipeline is extremely efficient, accounting for about 1.5% of the energy. So that means the total cycle end-to-end is:
0.91 (extraction) x 0.985 (shipping) x 0.33 = 29%
And that estimate is definitely on the conservative side, most academic papers put it closer to 35 to 40%, 5 to 10% better than what I’ve calculated here.
So now we have a number that we really can compare directly to a gasoline car – we’re accounting for everything from the well to the wheel, and that number is around 30%.
So what’s a typical number for well-to-wheel for a conventional car? About 14%.
The case for electric
Very basically, if we took the gasoline you put into your car and burned that in a turbine, then sent that power to your electric car, the overall efficiency of the system would double.
And that’s the dumb way to do things. The beauty of an “electric economy” is that batteries can be charged up at any time. They’re not soaking up coal and NG power, which are peaker supplies run during the day. They’re charging up at night when it’s mostly nuclear and wind. As more and more sources of energy come into the mix, invariably more efficient and less polluting than existing generators, your car gets better and better. You can’t do that with your existing car, who’s efficiency and emissions are fixed at the moment it was built, and generally get worse over time.
There are people who say that we should “burn” hydrogen in fuel cells, for instance. But I can do that at a power plant and ship it to my car for a total loss of only 30%. Other people say we should use more biofuels. But I can burn them at a power plant and ship it to my car for a total loss of only 30%. Want a nuclear powered car? Burn it at a power plant and ship it to my car for a total loss of only 30%.
You see how this works? Electric cars burn anything. They are the ultimate flex fuel vehicles. No matter what new fuel we invent in the future, your car will burn it, without changing a thing.
And in the meantime…
While we wait for the inevitable conversion to electrics, we have problems we’d like to solve in the shorter term. And the quick solution is plug-in hybrids, or PEHs. This gets us all of the advantages of electrics on the vast majority of trips, and gives you a fail-safe option for long trips. The difference between a PEH and a full electric vehicle is that you’re hauling around an engine everywhere you go, even when you don’t need it. But if you pull that out you need more batteries, so the difference isn’t as much as you might think. And since PEHs have less battery, say 1/4 that of a full electric, so as long as batteries remain as expensive as they are now, this is a much lower cost option.
So I’m saving up for my PEH. I’m really loving the Ford Fusion…
Update 2016: As my Civic Hybrid refuses to die, my choices keep getting better. The Fusion Hybrid currently lists at ~$26k for the base model, but I can get a Tesla 3 for about $40k. The Bolt will be available here soon, and rumor has it that Subaru is doing an electric Forester or Outback by 2019.