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 auto manufacturers and battery experts believe that corrosion and cycle life present daunting problems for high power batteries.
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Hence, batteries for hybrid vehicles are potentially more difficult to commercialize and may require a longer lead time than EV batteries. Fuel Cell Technology Many 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