(no subject)
As I hopped on my bike to leave work this evening, a question came to mind: what is the efficiency of an electric car?
The reason for this question, of course, is that electric cars are supposed to be all green and virtuous and sexy.
At a first cut, let's assume the drivetrain is identical, we're just replacing the motor. And let's ignore regenerative braking and just talk about identical cars barreling down the highway at constant speed, one powered by an electric motor, the other by a gasoline engine. So forget about the weight of batteries and how much of the power gets to the wheels; all that matters is what fraction of the energy goes from to the powerplant input to the motor output.
In a gasoline car, Wikipedia is saying the answer is 20-25%. That was easy!
In an electric car, the following is in play: how efficient is the conversion from electric to mechanical energy; how efficient are the batteries at converting their input, storing it, then releasing it later; how efficient is the transmission from power plant to the battery; and how efficient is the power plant?
A modern thermal plant converts about 40% of the heat input into electricity, but I don't have good numbers for the others. But the others have to all combine to be at least 50% efficient to match an inefficient gasoline motor. Seems a tall order.
Another consideration though: you're not burning gasoline at the power plant. So the raw efficiency is only part of the issue; now your car can without modification run on coal, uranium fission, hydro, wind, whatever.
Another consideration: electricity usage is low at night, but many power plants can't shut down (nuclear and coal, primarily). So if you've got a lot of those plants built, their power would be wasted, might as well shove it into car batteries.
Finally: in the near term, electric generation is largely coal. Burning coal puts out twice as much CO2 per joule as does burning gasoline. So in carbon impact terms, a coal-electric car loses to a gasoline car even if all the other components are 100% efficient.
The reason for this question, of course, is that electric cars are supposed to be all green and virtuous and sexy.
At a first cut, let's assume the drivetrain is identical, we're just replacing the motor. And let's ignore regenerative braking and just talk about identical cars barreling down the highway at constant speed, one powered by an electric motor, the other by a gasoline engine. So forget about the weight of batteries and how much of the power gets to the wheels; all that matters is what fraction of the energy goes from to the powerplant input to the motor output.
In a gasoline car, Wikipedia is saying the answer is 20-25%. That was easy!
In an electric car, the following is in play: how efficient is the conversion from electric to mechanical energy; how efficient are the batteries at converting their input, storing it, then releasing it later; how efficient is the transmission from power plant to the battery; and how efficient is the power plant?
A modern thermal plant converts about 40% of the heat input into electricity, but I don't have good numbers for the others. But the others have to all combine to be at least 50% efficient to match an inefficient gasoline motor. Seems a tall order.
Another consideration though: you're not burning gasoline at the power plant. So the raw efficiency is only part of the issue; now your car can without modification run on coal, uranium fission, hydro, wind, whatever.
Another consideration: electricity usage is low at night, but many power plants can't shut down (nuclear and coal, primarily). So if you've got a lot of those plants built, their power would be wasted, might as well shove it into car batteries.
Finally: in the near term, electric generation is largely coal. Burning coal puts out twice as much CO2 per joule as does burning gasoline. So in carbon impact terms, a coal-electric car loses to a gasoline car even if all the other components are 100% efficient.