components, such as the energy storage devices, could allow the ultracapacitor-based hybrid (and
the flywheel hybrid) to achieve PNGV’s goal of 82 mpg, which is triple the fuel efficiency of
current mid-size cars.
An intriguing feature of many of these hybrids-specially those using batteries for
energy storage is that they can operate in battery-only mode for some distance. For
example, the 2005 and 2015 battery hybrids in tables 1-1 and 1-2 have battery-only ranges of 28
and 33 miles, respectively. This would allow them to enter and operate in areas (e.g., inner cities)
restricted to EV operation. In addition, although these vehicles are designed to be independent of
the electric grid, they could have the capacity to be recharged, allowing them to operate as
limited-capability/limited-range EVs in case of an oil emergency—an attractive feature if the
future brings more volatile oil supplies.
Although most U.S. developers appear to be focusing their efforts on series hybrids, OTA
estimates that parallel hybrids that used their engines for peak loads and electric motors for low
loads could achieve fuel economy gains similar to those of the series hybrids examined by
OTA—25 to 35 percent. The development challenges of parallel hybrids appear to be more severe
than those of series hybrids, however, because of this type of hybrid’s unique driveability
problems^47 and its requirements for stopping and restarting the engine when going back and forth
Hybrid Batteries and High Power Requirements.
The hybrids discussed above are designed to compete directly with conventional autos—that is,
they would perform as well and, being disconnected from the grid, have unlimited range as long as
fuel is available. There are other configurations, or other balances between engine and energy
storage, that could serve a different, narrower market. For example, vehicle designers could use a
smaller engine and larger energy storage that would be recharged by an external source (e.g., the
electricity grid) to achieve a vehicle that could serve as an EV in cities^49 and would have relatively
long range. This design would not perform quite as well as the hybrids discussed above, however,
and would have to be recharged after a moderately long trip.
California is considering allowing hybrids to obtain ZEV credits, if these vehicles meet a
minimum EV range requirement. This would tend to push hybrid designs in the direction
discussed above (small engine, large energy storage), and reduce the likelihood that those energy
storage devices with low specific energy—such as ultracapacitors and possibly flywheels-will be
attractive candidates for commercialization.Fuel Cell Technology.
Fuel cells are electrochemical devices that turn hydrogen directly into electricity without
combustion, at high efficiency and with emissions only of water. For a fuel cell-powered vehicle,