Why the Plug-In Hybrid Automobile is Not the Answer

It is sometimes not realized just how wide the gulf is between the now conventional hybrid automobile which uses regenerative braking to improve in-city performance, and a true plug-in hybrid automobile entirely relying on batteries for energy storage.

For regenerative braking to be effective, it is sufficient to be able to store and give back the kinetic energy of the vehicle.

Let is consider a car weighing 1,500 kg. Capable of a top speed of 100 mph, or approximately 160 kph. Its maximum kinetic energy will be ½ m v2 or about 15 kJ. A kWh is approximately 3600 kJ and storage batteries with 1 kWh capacity are commonplace, so a battery capable of storing and returning the entire maximum kinetic energy of a typical automobile is easily obtained.

Now let us consider the energy available from 10 gal. of gasoline, or about 108 kJ. This is more than a million times the maximum kinetic energy of the car and 10,000 times the energy content of a typical automobile storage battery. Clearly substantially larger.

Suppose we want to do it anyway and have some super batteries. The problem now is that most electricity comes from fossil fuels, and the grid efficiency from fuel to house outlet is about 30%, i.e. about the same as a small automobile diesel engine. By using a tank of fuel with a diesel engine, we have saved cost and weight with no decrease of efficiency. This is assuming battery round trip efficiency of 100%. If one factors in typical battey round trip efficiency of 80% - 85%, then plug-in fuel efficiency drops below 26% -- about that of a typical gasoline engine.

But the diesel has the advantage of producing its waste heat right at the automobile where it can be used for heating or (absorption) cooling of the passenger compartment – something one cannot do with the waste heat of central electricity plants. So the burning of the fuel in the automobile, because of the use of cogeneration (CHP) is more efficient than the use of the utility grid.

Highway taxes. The present system of taxing motor fuels to pay for highways is relatively simple and fair. Taxing plug-in hybrids to pay for highway costs is likely to be more complicated and result in some serious legislative tussles.

Re: The fight between supercapacitors and batteries. Both are available in sizes appropriate for regenerative braking. They can compliment each other. Use the supercapacitor for quick response and superior cycle life while driving, and hold the charge in the battery while parked where it will not leak out.

Comments should be directed to the author: Jim Easton.