The idea behind “plug-in hybrid” vehicles seems to be spreading like wildfire. Hailed as a solution to the petroleum problem, advocates point to the seemingly attractive potential of having vehicles that can achieve more than 100 miles per gallon of gasoline, on currently available vehicles, no less! Indeed, a tiny group of enthusiasts in California say they are able to do so. The basic idea is that current production hybrid vehicles, such as the Toyota Prius, can be modified or “hacked” to accept a extra batteries and a power cord; this would allow for one to plug in the car overnight to fill up the batteries and allow this extra power to be used the next time one drives her vehicle. Toyota is quick to point out that although the idea is intriguing, it’s not ready for prime time.
Many proponents argue in favor of plug-in hybrids under the guise of energy independence—as Marc Franke states in EV World: “Americans would be thrilled if they could ‘Plug-in’ each night and charge their PHEVs for tomorrow's commute. I know during the oil shocks of the 70's (when we only imported 28% of our oil) with their long gasoline lines and shortages, we'd have been highly relieved to have a source of fuel at home in our garages!” This line of logic makes it sound as if we had a free source of energy coming into our houses without any environmental repercussions. However, this may be environmentally disastrous.
The inherent beauty of standard hybrid technology (i.e., not plug-in) is not that hybrid vehicles emit fewer emissions relative to their non-hybrid counterparts. This is just a very positive externality, albeit a corollary. The true beauty lies in the concept of efficiency. That is, if I put in X units of fuel, how much work can I get out of my vehicle? From an energy input/output standpoint, the simple fact that for a single gallon of gas that I put into a hybrid, I will be able to go farther than if I did not have a hybrid, ceteris paribus. Any engineer, environmentalist, or economist must appreciate this fact—there is less waste in the system. Clever technologies have resulted in socially beneficial results, and should thus be supported.
However once plug-in hybrids add an extra energy input, the net impact and net energy balance requires a much deeper analysis. Contrary to popular belief, the straightforward miles per gallon figure becomes a whole lot less comparable, for one has to measure the individual impact of each energy source. This calculation is not easy, and perhaps one may discover that on net, plug-in hybrids benefit society. However, if we think about just how that plug will charge the batteries, the argument becomes a lot less convincing. According to the U.S. Energy Information Administration, 49.8% of electric power comes from coal. Compounded with the fact that coal-fired electricity is the top carbon dioxide emitter per unit electricity (by a large margin), the implications for global warming are mixed at best.
Cost-benefit analyses should be conducted to the fullest extent for all supposed “energy independence” solutions. Yet, we must measure the benefits accurately so that we do not proceed down a potentially devastating path. Conventional hybrid technology has innovatively recaptured energy that otherwise would have been lost (for example, friction from braking used to be lost as heat). Messing with this success by plugging-in these hybrids may not be the miracle that will alleviate transportation’s energy problem.