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Wells to wheels: electric car efficiency February 22, 2013

Posted by Maury Markowitz in electric cars.
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One 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 “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. Not quite wide enough for use in a car, straight up, and there used to be a lot of debate about whether you should have two gears to squeeze out that extra performance, or one, and accept the losses in exchange for a much simpler transmission. Well that debate is over: single gears won and all modern e-cars have only one.

So in an electric car 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.

Motor in a Model S

This is the drivetrain in a Tesla S. It’s a motor between the two wheels.

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 in mid-image) where the front and rear axle/transaxle would be, and a battery pack is 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, varying between 85 and 95% efficient across the entire range of speeds. 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 though, 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.

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.

ford-nucleon

A nuclear powered car. Yes, really.

There’s 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 Im 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.

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Comments»

1. ivanchowashere - March 16, 2013

Maury, what is your take on this flashy new tech:
http://lightsailenergy.com/tech.html

Maury Markowitz - March 16, 2013

70% round-trip efficiency isn’t particularly great, as the numbers above demonstrate. Storage lifetime is likely short, as some of the stored energy is in the form of heat, which will leak away. Plus, it has moving parts. Certainly workable and even usable for storage times on the order of hours, but beyond that it’s not very useful. There are other teams working on zeolite-based storage that captures the heat energy permanently in chemical form, but many practical problems remain.

The “problem”, if you call it that, is the amount of research being put into compressed-air systems are a tiny fraction of the amounts going into lithium based systems. If li-zinc, li-air or any of the other systems currently in the lab work, the game’s up.

2. mycarquotes.net - January 7, 2014

Could someone please explain this post like I’m 5?
🙂

Maury Markowitz - January 7, 2014

Electric cars good. Even if you make the electricity with gas.

3. Dries Dokter - February 12, 2014

There is a couple of things missing here:
The energy used to create a battery and electro motor so you can run the car. (taking into account total lifetime, including battery)
against the energy used to create a fuel tank and a combustion engine (again taking into account total lifetime). Also taking into account the waste at the end of the lifetime…

I am still trying to understand: does well to wheels presume that for all energy sources the amount of miles obtained is the same? Does one litre of gas, amount to the same miles per litre of gasoline? Or is that part of the efficiency number? (weight of the car included etc etc)

Maury Markowitz - February 12, 2014

The energy used to create a battery and electro motor so you can run the car. (taking into account total lifetime, including battery) against the energy used to create a fuel tank and a combustion engine (again taking into account total lifetime).

These factors turn out to be vanishingly small, much smaller than even the most minor day-to-day issues, like driving style. The advantage is *slightly* in favour of gas engines, generally because the materials are easy to come by. But basically this is so tiny to be ignored.

Also taking into account the waste at the end of the lifetime…

Some good news here is that cars are *amazingly* recyclable. Even the plastics are getting re-used these days. Electrics don’t change that much, lithium recycling systems are being built out now.

energy sources the amount of miles obtained is the same? Does one litre of gas, amount to the same miles per litre of gasoline? Or is that part of the efficiency number?

It’s the efficiency number. All other things considered, the basic amount of energy you need to move a car is based almost entirely on three factors; loaded weight, frontal area, coefficient of drag. If you take the same car, say the new Fusion Energi, the second two are the same for the gas and electric versions. However, the electric version is heavier because its carrying around the engine, motor and batteries. So from a strict energy perspective, the Energi version is worse off. Yet because of the inherent efficiencies of the electric motor, especially at “launch”, it averages well over 50 mph in hybrid mode.

It’s almost as if it shouldn’t work, everything is working against the electric. The fuel (batteries) are heavier, more energy is used up getting the “fuel” in and out of the tank, and there are losses all along the chain from production to you. Yet in spite of this the motor is so efficient it just blows all of that out of the water.

Dries Dokter - February 12, 2014

oh and what about the amount of energy used to create fuel from oil.. etc etc

Maury Markowitz - February 12, 2014

Read it again 🙂

4. Alan Wells - August 23, 2014

As I read your reference http://www.asahi-net.or.jp/~pu4i-aok/cooldata2/hybridcar/hybridcare.htm about hybrids, I see the driveline losses at 2%, not the 5-6% you state early in your article. A manual transmission and drivetrain doesn’t produce much heat, so I’m interested in where you came up with 6%. You might also present the data in a less prosaic manner such as a table. I believe that would help comprehension.

And the reference also notes that a hybrid diesel has a well to wheel efficiency of 42-46%, the same as an electric car if not better. And engineers continue to coax additional inefficiencies and continued cleanliness from traditional approaches. Now add in a mature assembly industry with low capital costs, high range and it starts to look like electric cars will be a niche player for awhile longer.

I think we’re looking at the wrong solution. In my opinion, we should be looking at mature diesel hybrids in the US along with much smaller mass footprints in the cars such as they have in Europe (and illegal in the states and Canada). I would suggest our real problems will be solved by eliminating 3 ton trucks driven as cars everyday and Cadillac Escalades. I don’t see a lot of technical improvements in pure electrics in the future. The batteries are still not dense enough, charge/discharge cycles limited, and the cost to manufacture is high. Electric motors are hugely more expensive than mechanical drivelines, although they have advantages (diesel locomotives for instance).

Maury Markowitz - August 23, 2014

Excellent points Alan, especially the table concept. Sadly wordpress makes it much more difficult to work with tables than it should, it’s all manual essentially (still better than the wiki mind you).

The end of the article focuses on this point a bit. In the short term, hybrids are definitely the way to go. The cost of a hybrid is much lower than the equivalent all-electric, and that is very important. But I don’t see that lasting. There doesn’t seem to be any inherent limit to the cost of li-ion, prices appear to be what the market will bear. When the gigafactory comes online we’ll have a better idea.

But over the long term, all-electric wins every time. The well-to-wheel of an electric is about 70% or better in theory, something a hybrid can’t touch. It’s only if you burn hydrocarbons upstream that that drops. Here in Canada where over 50% of our power is hydro and about 25% of what’s left is non-hydrocarbon of one sort or another, the total efficiency is on the order of 50 to 60%. And unlike a hybrid, that improves every time a new wind turbine comes online.

Maury Markowitz - August 23, 2014

I see I missed one point, about drivetrain losses. The situation is complex. Losses in the transmission itself are, as you note, quite low. However, there are significant losses in the differentials and similar components. I’m certainly no expert on this, but it seems losses of 5 to 10% are not uncommon in RWR differentials. Added to this are dynamic losses, when the car is accelerating, which can be significant. The number in the paper is effective only when the car is driving steady-state. The same is not true of an all-electric drivetrain, where dynamic losses end to be much smaller.

5. gplusg - September 25, 2014

Thank you for this article. Can I translate it to Spanish and publish it on my blog? Of course, giving you credit and linking to your blog. I think this MUST be known, and I am not tech-savvy enough to do this calculus myself.

Maury Markowitz - September 25, 2014

I’d be delighted. I noticed a sudden uptick in traffic to this article lately, and followed the references back to a Spanish re-blogging site. Hundreds of clicks there, so there definitely seems to be some interest!

gplusg - September 26, 2014

The article was sent to Meneame.net, the Spanish equivalent for reddit. Since the country is in such a deep economic crisis and we have such potential for renewable production*, there’s a huge debate about the electric car here. 🙂

*hottest place in Europe, if you want more sun or heat, you have to go to Africa. Plus, second most mountainous European country after Switzerland, so we produce truckloads of electricity by eolian mills. The windmills have the dishonor of producing energy that many times goes to waste, since they normally produce it at night when it isn’t used. The fact that most electric cars would recharge precisely at night makes me feel all warm up inside when I read articles like this. I’ll send you a link when I finished translating and have done publishing. If there’s any update, please tell me so I can check up the data.

And, thanks 🙂

6. Quincy Scott - July 8, 2015

You say, “a normal gasoline car has a tank-to-wheel efficiency of 16%.” Then you say, “So what’s a typical number for well-to-wheel for a conventional car? About 14%.” Shouldn’t you be comparing well-to-wheel to well-to-wheel, not tank-to-wheel? It takes energy to get the crude oil, refine it, and process it into gasoline. The gasoline then has to be shipped to the gas station. In other words, what’s the well-to-wheel for gasoline powered cars? What did I miss?

Maury Markowitz - July 8, 2015

The page at the other end of that link shows both well-to and tank-to. The missing part is where that 2% difference goes. I’ll see if I can find a link that shows the losses. BTW, I understand those numbers are only true for oil from the Middle East, they are supposed to be worse for fracked oil from the US.

7. Bert - July 10, 2015

I’m curious to know what the efficiency of coal mining is compared to oil mining. I’m guessing that the coal mining efficiency doesn’t make a significant difference, (because we would have definitely heard about it from the misleading studies trying to claim that EVs are the most polluting things ever) but I can’t recall ever seeing the numbers on coal mining vs oil mining and I’m curious to know what those look like. If anyone has seen a study of these numbers, could you please link me to it? Thanks.

Maury Markowitz - August 5, 2015

That’s an interesting question. Off the top of my head I suspect that coal in the US is far less energy intensive than oil in the US. I seem to recall that new oil wells burn a barrel for every two they produce, whereas coal is blasted out and then picked up by trucks. It goes pretty much directly from there to the power plant.

8. Introducing a decent carbon tax will reduce our overall spend on fuel | Lance Wiggs - August 15, 2015

[…] power as electricity to electric cars would halve the fuel needed for transport. Here’s a good explanation, using US […]

9. Peter Levey's Weblog - May 31, 2016
10. Linwood Hirko - October 31, 2016

You ought to be a part of a contest for one of the best websites on the net. I most certainly will recommend this website!|

Maury Markowitz - October 31, 2016

Aww, shucks.

11. Gus - January 17, 2017

A couple of points other than the obivous that modern hybrids, especially that can use E85 (large renewable fuel content), are the low GHG producers.

1st, you can’t average power usage because putting all of the cars onto the electric grid to power would create new demand and new demandmeans we can’t get rid of the dirties source. So the proper fuel choice is the worst case available which for most of the world means coal.
2nd, real world numbers please not optimal best case technologies. If you simply take into account your data there is a huge drop off in performance.
3rd, the grid loses a TON of power every year because it must be maintained. That generation efficiency is 60%, indeed less than a third of the energy that goes into making electricity is actually consumed.

Electric cars only accomplish one thing and that is moving the GHG from one location to another. Cities love this, especially our east and west coast liberals as they get zero emissions where they live and shift that all onto those evil Republicans living in the heartland.

Maury Markowitz - January 17, 2017

All of the numbers in the article are real world. There is another article on this site that demonstrates best possible EV uptake is slower that new wind capacity. So, none of your points appear correct. Sources?


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