22

Is driving an electric car any better, GHG-wise, than driving a conventional car if 100% of its electricity is generated by fossil fuel combustion? Are there any peer-reviewed studies of that?

1
17

Yes.

The hard limit on the efficiency of a heat engine is the Carnot efficiency: the amount of usable energy extracted from a heat source is limited by the temperature of the heat source and the temperature of the heat sink. Only an infinitely hot heat source or a heat sink at absolute zero permits extracting all the energy from a heat source in a useful form.

A typical car extracts about 25% of the available energy from gasoline, representing about 60% of Carnot efficiency for the car's operating temperature.

A coal-fired power plant extracts about 45% of the available energy from the coal, representing 90% or more of Carnot efficiency. It can use techniques such as multiple-expansion turbines, preheaters, and re-heaters that are too heavy or too bulky to fit into a car. Even after you factor in transmission losses and the relative carbon emissions of coal versus gasoline, an electric car charged from a coal power plant produces lower emissions than a gasoline car.

(There are other advantages to an electric car that can boost its advantage even higher, such as regenerative braking, but the increased efficiency of turning heat to useful energy is the big one.)

7
  • I suppose that the efficiency of a battery-powered electric-vehicle should be scaled with the efficiency of the electric powerplant. Also, an electric-vehicle doesn't get all the electricity during charging. And the battery-powered electric-vehicle loses charge just when parked. That situation is worse on a cold day while energy-use is higher with heater or air-conditioning use. I would just guess that a battery-powered electric-vehicle is about twice as efficient as an ICE vehicle. However, weight of the vehicle and size of the motors also matter for quantitative energy use.
    – S Spring
    Oct 27 at 19:41
  • Your 45% efficiency for a coal power plant seems quite high to me. Maybe an integrated gasification combined cycle (IGCC) plant could have 45% efficiency, but an old aging traditional coal plant, no way.
    – juhist
    Oct 28 at 21:48
  • @juhist, even an "old aging traditional coal plant" can draw on two centuries of research in getting the most from your coal. IGCC plants claim even higher efficiencies.
    – Mark
    Oct 29 at 20:39
  • First Google search result: "Figures from the World Coal Association, a keen advocate of HELE, show that the average efficiency of coal-fired power plants around the world today is 33 percent."
    – juhist
    Oct 31 at 9:26
  • 1
    A further point is that it requires energy to drill for oil, refine it to gasoline and to transport it. You must account for that too, if you want to do a fair comparison. Nov 17 at 12:12
10

Volvo have done an interesting study on this recently, looking at whole-lifecycle emissions for a BEV (battery electric vehicle) XC40 versus a ICE (internal combusion engine) XC40. A vehicle life-cycle of 200,000km is assumed.

In summary, the lifetime emissions are as follows:

  • ICE XC40: 58 tons
  • BEV XC40 using average global energy mix: 54 tons (break even at 146,000km)
  • BEV XC40 using EU28 energy mix: 45 tons (break even at 84,000km)
  • BEV XC40 using wind energy: 27 tons (break even at 47,000km)

For info, production emissions of the BEV were 70% higher than that of the ICE version. Of course, this could vary substantially between vehicles and manufacturers.

Link to study

1
  • I like this answer the most, because it factors in the co2 footprint, which is the most important aspect.
    – ham
    Nov 19 at 8:55
4

Oh yes it is.

The fossil fuel of choice in electricity generation today is natural gas.

Natural gas combined cycle turbines run at 50% efficiency. Compare that to the 25% average efficiency of a car internal combustion engine (sure you can read information about 38% record efficiency but that's only at a particular operating point, not average over a general drive cycle). Natural gas is actually a very efficient hydrogen container: a particular amount of natural gas contains twice as much hydrogen as the same amount of hydrogen gas. Thus, a very significant fraction of the energy in natural gas is actually zero-emission hydrogen. Therefore, natural gas produces only 200 g / kWh of primary energy. At 50% efficiency, that's 400 g / kWh of electricity.

An largish electric car consumes about 0.18 kWh / km so that gives 72 g / km. With internal combustion engine cars, emissions are usually at least twice that even in the smallest engines, and an engine that gives good acceleration has very high emissions (compare that to electric cars that by default have very fast acceleration). Also, the stated emissions of fossil fuel cars are the tailpipe emissions. If oil refining emissions are taken into account too, the emissions easily increase 30% over the stated emissions and can be as high as 250 g / km taking into account refining for a comparable gasoline car having good acceleration. Natural gas is a nice fuel in that it requires absolutely no refining at all.

Also, a natural gas power plant is a stationary carbon dioxide source. So it's a very good location for carbon capture. One mole of natural gas (methane) burned produces one mole of carbon dioxide. A general law related to gases is that a mole of gas takes a certain volume. Thus, if the natural gas came from under the ground, once the natural gas field has been depleted, you can use it for storing the carbon dioxide -- it will fit there if the natural gas was there in the past. This is actually very environmentally beneficial, because if the natural gas by accident was released, it would cause very strong global warming, whereas carbon dioxide is a much weaker cause of global warming. Thus, best approach to reduce risks is to convert all natural gas we can into carbon dioxide.

In contrast, an internal combustion engine in a car is a mobile power source. There's no practical way to capture the carbon dioxide as it's a gas and takes far more space than the liquid fuel from which it was produced. Thus, the only option is to release the carbon dioxide back to atmosphere.

Also it is noteworthy that practically no one is planning to use natural gas as baseload far into the future. Solar power and especially wind power are very good candidates for producing the bulk of electricity, and their fuels being free allow very cheap production, far cheaper than natural gas. Natural gas turbines have very good adjustment properties, meaning you can for example very easily produce power from 85% renewables and 15% natural gas, even given the intermittency of renewables. You just over-size the renewables a bit and use the spare electricity for example for producing green hydrogen or charging large iron-air batteries. Even without carbon capture, this 85%/15% mix would give 10.8 g / km average emissions. However, carbon capture is very likely and this gives 0 g / km average emissions.

1
  • "Thus, best approach to reduce risks is to convert all natural gas we can into carbon dioxide." No, definitely not. Methane (natural gas) may be a more potent greenhouse gas than CO2 but it breaks down much more quickly than CO2. CO2 remains in the atmosphere several times longer than methane. The best approach is to leave natural gas in the ground, not converting it to carbon dioxide and emitting it into the atmosphere.
    – Bill
    Nov 18 at 19:55
3

Renewable diesel is a Co2 neutral product that is a direct replacement for hydrocarbon diesel in the diesel fleet. https://www.eia.gov/energyexplained/biofuels/biodiesel-in-depth.php

This fuel is available today, I run it in my Audi Q5 TDI and get better mileage (about 2 mpg). Switching to this fuel also give us a more diverse infrastructure and better economic independence from scarce minerals like lithium and cobalt.

We also need to look at the pollution hazards of disposing of batteries after their 4-6 years of usable life. Co2 is not the only issue in the cleaner equation. So I'll have to say no electric cars are not necessarily cleaner than conventional cars.

1
  • 4
    It's only fair to mention the significant downsides of biofuel, such as the amount of land and water that are required to produce them in significant quantities, plus the production process can use more energy than the resulting fuel actually produces.
    – John M
    Nov 18 at 15:07
1

The GHG intensity of battery-electric vehicles (BEVs) charged from a fossil-fuel based electrical grid is a moot point (a fact that doesn't matter because it's not relevant to the current situation) in the context of global decarbonization and here's why: There is no realistic pathway for deep decarbonization of combustion engine vehicles. 1

The only realistic pathway to decarbonizing transportation is a combined effort at both vehicle electrification and transitioning the electrical grid to low-carbon energy sources (ie. renewables).

enter image description here

It just doesn't make sense to ask about a hypothetical scenario where 100% of electricity is generated by fossil fuel combustion because that's not a reality in any country today, and most countries are transitioning their electrical grids to lower-carbon energy sources.

2
  • 1
    A few things deserve mention and lightly fertilized sorghum grown on un-irrigated land can produce carbon-neutral ethanol when by cellulosic processes. Also, green-hydrogen combined with industrially captured carbon-dioxide can produce carbon-neutral synthetic methanol. Or the big news and Saudi Arabia is going to massively export gray-hydrogen. Finally, all renewable electricity projects, to be fully utilized, should be connected to both the powergrid and to green-hydrogen production.
    – S Spring
    Oct 27 at 19:38
  • 1
    There are certainly places where all our almost all energy is produced by burning coal, for example (or natural gas) Oct 27 at 20:04
-1

Driving a battery-powered electric-vehicle on electricity produced by fossil fuel could be some percentage of efficiency gain. For instance, gas-turbines at electric powerplants often run for long periods of time at fixed speeds and that's an efficiency gain. Also, the gas-turbines at powerplants can make use of waste heat which is another efficiency gain. Another possibility is for natural-gas to be reformed to gray hydrogen and then the gray hydrogen run in fuel-cells at the electric powerplants. Certainly, compare the efficiency of hydrogen fuel-cells to gas-turbines for electric powerplant use.

The problem with battery-powered electric-vehicles is that they are primarily developed and designed for required driving ranges and that's a potentially wasteful development. Other types of vehicles could just change the size of the fuel tank for different driving ranges and that's not primary development and design.

Finally, the electric powergrid is only expected to be 45% renewable electricity at the year 2050. Much of the problem is that the transition to battery-powered electric-vehicles will increase demand for electricity. Now if the renewable electricity is accounted to buildings and houses then the non-renewable electricity could be accounted to battery-powered electric-vehicles !

I can add primary sources. This first source says that 450GW of European offshore wind-power will be 30% of European electricity demand in the year 2050 and that result because demand will have grown 50%. However, the current amount of European offshore wind-power, as dated in November of 2019, is only 20GW.

https://windeurope.org/about-wind/reports/our-energy-our-future/

Down in their assumptions they improve the numbers by saying that there could also be 650GW of European onshore wind-power in the year 2050. I suppose that I previously used 650GW as the total of all European wind-power. However, these are their visionary numbers.

This second source says that U.S. solar-power could be 40% of demand in the year 2035. However, current U.S. solar-power is only 3% of current demand while full demand is expected in the year 2050.

https://www.energy.gov/articles/doe-releases-solar-futures-study-providing-blueprint-zero-carbon-grid .

Well, my overall point is that in spite of a battery-powered electric-vehicles's higher efficiency, if in the future if personal vehicle use is rationed, so that energy use of buildings and houses is not rationed, then many battery-powered electric-vehicles will have been over-designed and over-developed for their usage.

1
  • Comments are not for extended discussion; this conversation has been moved to chat.
    – LShaver
    Oct 30 at 23:34
-1

No, since building a new electric car has already so much Carbon impact on the planet compared to an old existing conventional car that its lifetime Carbon impact can only be balanced through the use of renewable sources of electricity.

The best approach is:

  • to drive less, only when necessary
  • co-commute
  • drive a bike !
  • use shared electric cars
  • upgrade conventional cars to run on Gas or cleaner energies than fuel
4
  • 2
    This has been proven wrong so many times that I am at a loss for words. But see carbonbrief.org/…
    – Timores
    Nov 17 at 11:34
  • @Timores, if you read the article you point to, it states: "In countries with coal-intensive electricity generation, the benefits of EVs are smaller and they can have similar lifetime emissions to the most efficient conventional vehicles – such as hybrid-electric models." My point here is comparing an existing conventional car (its manufacturing carbon impact is out of scope since done for years) and a new electric car. Plus the fact that if you run 5,000 miles a year in an old car, you have less Carbon impact than an electric car running 35,000 miles a year. Changing usage is key.
    – Open SEO
    Nov 18 at 15:07
  • 1
    @OpenSEO: "comparing an existing conventional car (its manufacturing carbon impact is out of scope since done for years) and a new electric car" - if you compare apples with oranges, you get useless results. What about comparing an existing electrical car with a new conventional car? Does that make sense to you too?
    – Aganju
    Nov 18 at 16:03
  • @Aganju, real life is scenario about existing conventional can keep running or being replaced by new electric cars. Other use cases are for scholar students. Why would you buy a new car if you don't need one ?
    – Open SEO
    Nov 19 at 17:17
-1

I don't believe driving an electric car is more fuel efficiency than for example a modern diesel (which is around 41% fuel efficient more if they made hybrid or better heat recovery). Especially when you consider the losses between the power station, charging the batteries and then driving the car.

However, it is easier to modify power station to deal with the pollutions (e.g. capture and storage) or using greener tech like nuclear, solar, wind, tidal than it is to modify a car to deal with it.

Not the answer you're looking for? Browse other questions tagged or ask your own question.