Hybrid Batteries and High Power Requirements
Most of the above discussion has focused on electric vehicle (EV) type batteries where specific
energy is a major concern. Batteries used in hybrid vehicles do not necessarily need to store much
energy (although some hybrids can resemble EVs) but must be capable for providing relatively
high power for short duration. Bipolar designs, where the anode of one cell and the cathode of the
next are mounted on opposite sides of the same plate or surface, can have high specific power--as
much as three to five times that of conventional designs, owing to their high current capacity and
low internal resistance. Although such designs have demonstrated specific power levels of 500 to
900 W/kg at the module level in the laboratory, even for a lead-acid type battery (see discussion
on the bipolar lead-acid battery) many automanufacturers and battery experts believe that
corrosion and cycle life present daunting problems for high power batteries. Hence, batteries for
hybrid vehicles are potentially more difficult to commercialize and may require a longer
lead time than EV batteries.
Fuel Cell TechnologyMany researchers consider fuel cells to be the ultimate answer to power motor vehicles,
because they combine the positive attributes of batteries--zero or extremely low emissions and
quiet operation--with the quick refueling capability of internal combustion engines. A fuel cell is
an electrochemical device that converts the chemical energy in a fuel to electrical energy directly
without first converting the chemical energy to heat energy. As a result, the thermodynamic
limitations imposed by the Carnot cycle are not applicable, and fuel cells can have theoretical
efficiencies of more than 90 percent. In addition, if the fuel used is hydrogen, the energy
conversion process is essentially pollution free, as fuel cells can convert hydrogen and the oxygen
in the air directly to electricity and water. With other fuels, such as methanol or hydrocarbons, an
external reformer may be necessary to first separate the hydrogen from the fuel the reforming
process will generate small quantities of carbon monoxide and other pollutants, and substantial
quantities of carbon dioxide.
For this analysis, aluminum-air and zinc-air cells are treated as fuel cells because they are
mechanically recharged, although they are sometimes called batteries. These cells use aluminum or
zinc as material inputs, and these are consumed and replaced. Zinc-air cells can be electrically
recharged, but no practical system to accomplish this has been demonstrated at the module
level.^90
Aluminum-Air and Zinc-Air Cells
Aluminum-air cells and zinc-air cells are constructed like batteries except that the aluminum or
zinc anodes are consumed as electricity is produced, and dissolve into an aqueous electrolyte. To
“recharge” one of these cells, the anode and electrolyte are replaced and the old electrolyte is
90 More recently, aresearch group claims to have solved the problems of recharging and state that they have demonstratedover 100 charge-
discharge cycles at the cell level. However, the rechargeable cell has poor recharging efficiency due to energy lossesat the air electrode. Chris Borroni-
Bird, personalcommunication, Chrysler Corp., Apr. 20, 1995.