Will electric cars really save energy, or just shift its use?


They’ve got to be charged; that will mean generating the electricity to charge them. Assume there is widespread adoption of electric cars. Will we just end up with fewer internal-combustion engines but more power plants (many of which may burn coal, split atoms and create radioactive waste, or create other forms of pollution) to create the electricity in the first place? Are electric cars really the green solution some of have claimed?

I’m not really sure what that article’s relevance to your question is. It’s all about technical glitches, not about the energy supply issues.

The saying that an electric car is only as green as the power grid it’s hooked up to is generally true. However, it is possible to run an electric car on solar or wind (or nuclear) energy whereas it’s not possible to do so with a conventional car.

Also, from a pure physics point of view, electric cars are much more efficient than internal combustion engine, where anywhere from 70-90% of the energy in the fuel gets turned into heat. I believe (and I don’t think there’s really good numbers out there for this) that even if we switched from a fleet of ICE cars running on fossil petroleum to a fleet of electric cars running off coal-derived electricity that it would still be a marginal improvement in terms of carbon emissions, not to mention a huge improvement in the geopolitical and economic issues that come along with imported oil.

Also if the political impediments to expanded nuclear power were to come down, the potential of electric vehicles would be much greater. Large scale wind and solar projects are also quite possible, but are mostly a political and financial challenge. Expanded use of EV’s is but one part of a larger solution that needs to come together.

IMO, it’s a two part answer, and the first half is a bit of a WAG, since I’m no engineer to work the math needed to confirm it. But here are the two elements:

  1. Electrics (and hybrids, to a somewhat lesser degree) consume somewhat less power per highway mile than Otto or Diesel engines. IOW, suppose a closed universe of discourse with sufficient gasoline to run the test, a gasoline-fired generator, an electric vehicle and a conventional vehicle. Drive both vehicles for equal mileage under similar conditions, using the generator to recharge the electric vehicle, and measure the gasoline consumed, and the conventional vehicle will have consumed proportionately more gas than the generator for charging the electric vehicle.

But more importantly:

  1. Electric vehicles derive their power from the grid, as might be expected. That means there is a broader range of ways to fuel them, including a wide assortment of renewable sources.

Granted that you have to charge up the electric vehicle and the power must come from somewhere, and no known techonology or group of technologies is without its faults – still, there is a benefit to converting vehicular travel, insofar as possible, to electric.

Even what might be considered close to the worst-case scenario for production of the additional demand, doing nothing but building new coal-fired generating plants, still staves off a new energy crisis for decades if not a couple of centuries. (See a wide assortment of Una’s posts on the topic.)

The biggest environmental advantage is that the grid (and the factory) is already cleaner, and is a much easier place to develop new cleaner technologies.

There are many reasons grid sources can be cleaner. They can be unreliable (like sunshine). They can be only regionally available (hydroelectric and wind). They can be enormous and expensive and dangerous yet still be rendered practical (nuke). They can be added incrementally and experimented with. It is always possible to add expensive and dirty diesel generators during the busiest 2% of the time, and increase the maximum capacity of the entire system with a very small change in its average cleanliness. Besides, electric vehicles would do most of their charging during the night, when demand is low.

It’s a bit misleading to say this without noting that electrical power plants also turn large amounts of their fuel energy into heat. Various googling suggests that an overall average would be around 65% - implying 35% efficiency. (If Una P. arrives, she’ll be able to tell us precisely.)

As it happens, I just had to try to recalculate this for Cecil - the estimates vary, but generally are between 0.303 and 0.33 useful electricity/fuel energy input. This Sankey diagram gives a good visual representation of one estimate, in this case showing 0.325:


As a note to to the topic, Cecil covered this topic not terribly long ago here: http://www.straightdope.com/columns/read/2759/are-electric-cars-really-more-energy-efficient

Thanks, everybody!

Yes, and to further refute my own post, there’s also the issue of energy density-- the electric car has to spend energy moving around hundreds of pounds of batteries whereas the gas is only dozens of pounds.

It’ll depend on type of diving as well. Electric cars consume very little power when stopped (unless you are using heat and/or air conditioning), so Taxis in Manhattan might be an especially good place for them.

Are we taking into consideration the energy lost by transporting electricity over transmission lines?

To be fair, a state-of-the-art electric motor can weigh meaningfully less than the equivalent gas engine (though probably not enough to offset the weight of current-technology batteries). And the efficiency of generating electric power, while far from perfect, is usually better than that of internal combustion engines.

At a guess, this is comparable to (and possibly less than) the energy lost in transporting petrol and diesel fuel used by internal combustion vehicles.

There are not just the generation losses to consider, a good plant will get near to 33% efficiency, there are also network transmission losses, which can be significant, up to 25% of what remains.

When you look at total efficiency, the picture is much wider, with extraction, transportation costs, or refining costs etc.

I am not in a position to compare the whole electric vs gas costs, but it does appear to me that there is a significant cost with electricity, perhaps it is possible to control the CO2 more readily in a large generating station with just one source, rather than individual gas driven cars.

The problem is that people are looking for simple answers, there are none, nuclear has its pros and cons, solar is just never ever going to make much differance to energy consumption, folk just don’t understand the sheer scale of fossil fuel use.

Alternative energy will only ever make a small contribution, but it gets dressed up as being much more important than it is, for example you read on the news that the newer wind turbines will power perhaps 1000 homes each - but in terms of actual energy, that isn’t much at all, they do not often state in the media the true power output it is easier for the greens to use nebulous values such as number of homes being supplied but any engineer can tell you that this is not a meaningful measurement.

They do this probably because folks such as myself would work out how little power is being produced for the investment costs.

The only real way is to use less, no way around it either, not unless there is a remarkable improvement in alternative energy technology, and by that I mean a geometric increase in power output, with a similar fall in cost.

Electric cars represent a loss of economic efficiency. The cars do not have the full utility of a standard gasoline powered car and the extra cost means fewer people will own them. Buying them is a diversion of production capacity and slows the introduction of more economically efficient cars from becoming mainstream.

Every time this is talked about, people just compare the efficiency of the internal combustion engine to the efficiency of the electrical engine or the power plant. I’d like to see a comparison of total efficiency for the entire process. It may be complicated when it comes to refining gasoline as I imagine that produces other chemicals as well that we need for other purposes but it’s pretty pointless to compare the two without taking everything into consideration. A price comparison would also be interesting.

That is contrary to the cite I offered earlier in this thread, which shows T&D losses as being no more than 7.4% of station net generation. While that figure is for the US, I doubt the UK has T&D losses which are more than 3 times that of the US.

OK, but coal doesn’t walk itself to the power plants either.

I did not read the cite, and anyway, I know better than to trade figures in the light of it.

Still, it is a significant loss to total into the use of electric cars, and there will be some losses in the charging cycle of the car itself, rather than just the consumption.

And, of course, there’s a synergy between wind or solar power and electric vehicles. The biggest problem with wind power currently is that it’s not completely reliable: At a coal plant, you can plan out exactly how much energy you’re going to be generating at any given time, by firing up or shutting down various numbers of turbines, and you’ll always have those turbines running whenever you want them to. With wind, though, you can’t just open up another one of Aeolus’ bags at time when demand is higher: How much power you can generate at any given moment is determined by how much wind is blowing at that moment. So if you have something that can use power sporadically as it’s available, and then stop using it when it’s not available, you can make wind turbines more efficient. And charging a battery is something that you can use in just that way. We could see a system where, for instance, the power company varies the cost of energy on a minute-to-minute basis (cheaper when the wind turbines are producing more power, or when inflexible demand is low) and transmits that information over the Internet to the chargers in peoples’ garages, and each individual user could set their charger according to how much they’re willing to pay, how urgent it is that their car be charged by a certain time, and so on.

But don’t most wind turbines simply help supply power to the grid rather than being the sole source of power to closed systems?