Few energy technologies have generated higher expectations for reducing our oil consumption and greenhouse gas emissions than the plug-in hybrid car (PHEV). This week, Toyota announced that it was delivering two modified Prius hybrids to the University of California, to enable the latter to test how plug-in hybrids work in the real world. While my own enthusiasm about PHEVs is tempered by the time lag inherent in altering the overall performance of the US fleet of 240 million vehicles, this is still a big deal. The PHEV represents the first practical bridge between electricity and transportation energy since the failure of the EV-1 all-electric car a few years ago.
The long-term significance of a switch to PHEVs would go beyond their recently-confirmed emissions reductions from making better use of off-peak electric generating capacity and backing out a bit of foreign oil, or channeling some zero-emission wind energy into transportation. The core benefit of the PHEV, from a global energy perspective, derives from shifting the energy conversion step away from inefficient onboard internal combustion engines (ICEs) to central and distributed power plants that offer end-to-end efficiency improvements over ICEs of between 1.5x and 3.5x, depending on the specific power generation technology involved. Nor does the vehicle notice whether its source of electrons comes from coal, natural gas, wind, solar or nuclear power. Electricity is wonderfully fungible.
Aside from the obvious attraction of using a fleet of PHEVs as rolling battery storage to facilitate wider exploitation of intermittent renewable energy sources such as wind, you could also imagine a more prosaic, but equally impactful scenario in which oil refineries made less gasoline and more kerosene, which would be burned in combined cycle gas turbine power plants--many of which were built with this fuel-switching capability in mind, at least as a backup. In the process, refinery yields would rise and their energy consumption and emissions fall. As counter-intuitive as this might sound, it could more than double the effectiveness of our single largest source of primary energy, based on the enormous efficiency advantage of CCGTs over ICEs, even when electric transmission losses are factored in. So not only would PHEVs reduce the quantity of oil used in transportation, they would allow us to make much better use of the energy content of each barrel we did consume.
PHEVs still have a lot of hurdles to overcome, not the least being the basic economic challenge associated with the diminishing dollar value of fuel economy increments beyond those that conventional hybrids already deliver. However, if they survive the rigors of development and consumer acceptance, they can deliver the kind of step-change improvement in fuel-conversion efficiency that was formerly associated only with hydrogen fuel cells--which have so far failed to overcome a much more complex set of barriers. This is one area in which energy security and the need for reducing greenhouse gas emissions aligns nicely.