A question came up yesterday about electric cars and it is clear there is a lot of confusion about some of the fundamentals. Here is a one minute primer.
First you need to think about the difference between power and energy. It isn't technically an accurate definition but think of energy as a storage system that you can draw on to move things. You may choose to use it quickly or slowly. The rate at which it is used is power.
Think of energy as gasoline and power as what you control with the accelerator.
There are any number of labels to measure energy and power and this serves to confuse people. Energy is just energy and power is just power, so the trick is to pick a unit that is appropriate for the task. For the automotive world kilowatts (kW) is a good choice for power and kilowatt hours (kWh) for energy. Remember that power is the rate of energy "used" in a given time so this make sense kWh/h = kW.1
A gallon of gasoline represents about 36.6 kWh of chemical energy that can be liberated when it is burned. You can put this into a lawn mower and run it for hours or a Ferrari and use it in a few minutes if you can find a road that can handle 200 mph.
The battery in a Chevy Volt can store about 16 kWh of electrical energy. Batteries are large, heavy and expensive compared to the equivalent amount of energy. So the Volt stores about the same amount of energy as a half gallon of gasoline. A Nissan Leaf stores the equivalent of three quarters of a gallon and the Tesla Roadster about one and a half gallons.
It may seem like a lost cause, but the energy must be turned into motion. An electric car does that with about an 80% efficiency and a gasoline car about 20% - the electric car will go four times as far on an equivalent amount of energy. And if you compare either with muscle power it quickly becomes apparent why the Industrial Revolution was so significant.
So the Volt has about the range that two gallons of gasoline will give in an equivalent sized car and the Tesla has a seven gallon equivalent.2
The energy storage numbers may seem small, but they are in the ballpark for commuting. This is the first time batteries have been good enough to be useful in a 3,500 pound car. You could make them much more useful by lowering the weight of the car and/or increasing the amount of energy that can be stored in the battery.
As for power the battery pack on the Volt can easily delivery energy at a rate to produce up to 111 kW in the car's electric motor - this works out to 149 horsepower. The Tesla is rated at about 215 kW, which is 288 horsepower. Automotive scale battery packs that can deliver much more power can be assembled, but the car's range is very low at those rates. But the bottom line is conventional lithium-ion battery packs can supply enough power to provide gasoline car-like accelerations and cruising speeds.
Sorting out the economics is another issue. 10 kWh of electricity is sufficient for 40 miles of driving in a Volt. At average American utility rates that costs about $1.30 (perhaps under $1.00 if delivered off-peak). That same 40 miles might require $5 of gasoline in a similar class vehicle.
Of course there is this little matter of battery cost...
A Volt battery pack may cost as much as $10,000. It has some specialized electronics along with the battery and a shielded crash proof casing. This compares to about $1,000 for the gas tank and fuel handling components of a conventional gasoline powered car. Pure electric cars like the Leaf have lower power train costs and are likely to have much lower maintenance costs over the life of the vehicle.
Currently it is difficult to justify a pure electric car without the $7,500 (or more) subsidy that partly bridges the cost of the energy storage component of the vehicle. If cars weighed half as much as they do the break-even point would be much closer. But there is that nagging issue of range.
The Department of energy has a variety of programs aimed at doubling the amount of energy that can be stored in a kilogram of a production battery by 2020. Other goals would reduce the price of a Volt scale pack to under $2,000 by the end of this decade -something that appears to be feasible. An exotic type of battery, a rechargeable metal air battery, could conceivably improve this energy density by a factor of twenty.
Ultimately electrics win as the factor of four in efficiency is too great and battery costs will fall and, with the right upstream energy source, electrics can be very clean. My best guess is subcompacts will be at parity in price around 2020. Driving range will not be at parity though. You will be able to buy 300 mile range pure electrics, but there will be a penalty in purchase price.
Real change may occur late in North America. The real game for the car makers is the "megacity car" - small low range vehicles for the emerging middle class in China, India and other very high growth areas. The economics of smaller cars may make electric power trains likely in massive scales in the next decade or two.
In the meantime there is much you can do in the present to dramatically increase your effective travel efficiency - make your driving more efficient and eliminate it where possible.
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1 In working through physics problems it is convenient to do a quick check to make sure you have the units right. If you don't there is an error - it would be nice if the media understood this. Major pieces appearing in the papers and on TV often fail.
2 Although the battery can store 16 kWh, only 10.4 kWh is tapped to allow the battery pack to have a long life. So the useful equivalent is about 0.3 gallons of gasoline and the range is about the same as 1.2 gallons when you factor in the improved efficiency of the electric drive train.
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