The Energy Evolution - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 20

SOFCs also produce high-quality heat with their working temperature of
850°C. This makes combined heat and power production possible with
SOFC systems. The total efficiency can then reach 85%. Advanced con-
ventional cogeneration of heat and power can reach total efficiencies up to
94% with electrical efficiencies over 50%. This occurs only at full load. A
high electrical efficiency is preferred over heat efficiency, since this results
in a higher energy with the initial energy source better utilized, in terms
of practical end-use.
Fuel cell systems are modular like computers which makes it pos-
sible to ramp up generating facilities as needed with sections in an idle
mode when full capacity is not needed. The capacity is easily adjusted, as
the need arise.
Hydrocarbons such as natural gas or methane can be reformed inter-
nally in the SOFC, which means that these fuels can be fed to the cells di-
rectly. Other types of fuel cells require external reforming. The reforming
equipment is size-dependent which reduces the modularity.
Fuel cell cars must be able to drive hundreds of miles on a single
tank of hydrogen. Honda's prototype fuel cell car had a range of 190 miles
in 2004. It stored a little more than 3 kilograms of hydrogen at 4,400 psi.
This gave it a mile/kg efficiency of 51 city and 46 highway. The 2005 mod-
el had an improved fuel cell and was rated at 62 city and 51 highway. A
gallon of gasoline contains about 2,600 times the energy of a gallon of hy-
drogen, but a kilogram of hydrogen has almost exactly the same chemical
energy as a gallon of gasoline.
An experimental Honda fueling station in the Los Angeles area pro-
duces about 1/2-kg of hydrogen per day, about 3.5-kg. It uses 700 square
feet of solar panels to produce 6 kilowatts of power to electrolyze water.
If hydrogen cars are to travel 300 miles on a single tank, they will
have to use compressed hydrogen gas at very high pressures, up to 10,000
pounds per square inch. Even at this pressure, cars would need large fuel
tanks.
Liquid hydrogen may be a better choice. The GM liquid-fueled
HydroGen3 gets about 250 miles on a tank about twice the size of a typical
gasoline tank.
Cars and light trucks produce about 20% of the carbon dioxide emit-
ted in the U.S., while power plants burning fossil fuels are responsible for
more than 40% of CO 2 emissions. Fuel cells can be used to generate elec-
tricity for homes and businesses.