I received an email from a proponent of plug in hybrids recently. The writer was promoting a company called CalCars, the California Cars Initiative , which has built a prototype plug-in version of the Toyota Prius, called Prius+, and is working on commercializing the upgrade.
A plug-in hybrid is an HEV which has a 110VAC connection, which can recharge the battery pack, while the vehicle is parked. With a fully charged battery pack, the PIHEV will use even less fuel than a normal hybrid, on short trips. On long trips, the battery pack will be drained and the vehicle will have to rely as much on its IC engine as a regular HEV. The CalCars Prius+ prototype was estimated at 120MPG for a 70 mile mixed city/highway trip.
The advantages to a PIHEV are that it uses less fuel for short trips, and that the electricity that is used to charge the batteries is cheaper than generating electricity on board from gasoline.
There are some disadvantages, however, which CalCars does not emphasize on their web site.
A plug-in hybrid will be even more expensive than a normal HEV, because of the larger battery packs that CalCars proposes for a PIHEV, and the added electronics to do the 110V charging. The EPRI published a report (lots of great information about HEVs) which estimated that a PIHEV with a 20 mile electric-only range would have about a $3,000 price premium over a conventional HEV. A PIHEV will also be heavier.
Another disadvantage is that a PIHEV is a step backwards, in a way, from CalCars stated goal of reducing emissions. Our grid electricity is generated mostly from burning hydrocarbon fuels--in 2000, 52% of US electricity was generated from burning Coal, 16% from natural gas, and 3% was from petroleum. Another 20% was generated from nuclear power plants (my favorite source). I don't know if the grid-to-wheels route is more efficient, in a CO2 sense.
I don't see the economics working out for PIHEVs. If a PIHEV costs $3,000 more than a comparable HEV, and an urban driver is getting an additional 70MPG, he is saving $429 a year on fuel (171 gallons @ $2.50, 12,000 mi/year), and it will take him exactly 7 years to recoup the additional cost--not counting the cost of the grid electricity. Pure electric operation costs, according to CalCars, $0.01/mi, based on off-peak discounted California electricity rates of $0.05/kWhr. If our driver drives 50% of his miles on grid charge, then he is going to be paying $60/year back for electricity, which means he will now need 8.1 years to pay back his additional investment.
I read somewhere once that the average American keeps a car for about 8 years.