On electric cars

A question from Yahoo! Answers:

Are electric cars the answer to transportation pollution or are they just a gimmick?

A huge gimmick, in my opinion.

Internal combustion engines convert between 20% and 30% of burning fuel’s heat into mechanical energy (an engineer would say that thermal efficiency of an internal combustion engine is between 20% and 30%). It’s not a lot, but electric cars and their power supply chain are even less efficient.

First, you need to burn fuel to heat steam; then, you use the steam to rotate a turbine; the turbine would drive a generator, which would produce electricity. If you use the most efficient turbines in the world (they are called combined-cycle turbines), you’ll achieve 45-50% thermal efficiency, but most power plants are not that good. But wait, you’re not done yet. Electricity must be transmitted over power lines, stored in a car’s battery (which implies an AC-to-DC conversion), and, finally, retrieved from that battery to drive the car. With all those conversions and losses at each step, you’ll be lucky if you get 15% thermal efficiency. So you end up needing more fuel for the same amount of driving.

On the plus side, with electric vehicles, the pollution would be concentrated around power stations, not in central business districts. But there is likely to be more pollution than we see now; my back-of-an-envelope estimates suggest that to fully convert from traditional to electric cars, the U.S. will need to at least double its production of electricity.

A much better idea would be to have a car powered by fuel cells. In a fuel cell, the oxidation of fuel produces electricity directly, with no moving parts and relatively little heat involved. Fuel cells have great thermal efficiency (40-60% is typical, 85% is not unheard of), but, unfortunately, they are still bulky; a block of fuel cells powerful enough to drive a car would barely fit into a 20-foot container. They are expensive to make, too.

The second-best alternative is a hybrid vehicle. A small internal combustion engine works at an optimal speed to charge a battery, which powers the car. Since the internal combustion engine does not idle (it either works at full power or turns off) or handle acceleration (the battery does that), you need about half the engine power compared to the traditional car and burn about 50% less fuel… Since the main powertrain is electric, hybrids do not need a transmission, either.

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