Direct Energy, 2018a

9 BATTERIES AND FUEL CELLS 231
systems intended to be connected to the electrical grid, inverters which
convert the DC power from the fuel cell to AC are included. A fuel cell
system also typically include a control system that regulates the ow of the
fuel and oxidizer, monitors the temperature of the device, and manages its
overall operation [128, ch. 37].
9.6.2 Types and Examples
Fuel cells may be classied in dierent ways. One way is by operating
temperature: low 25-100 ◦ C, medium 100-500 ◦ C, high 500-1000 ◦ C, and
very high over 1000 ◦ C [60, ch. 1]. Chemical reactions typically occur more
quickly at higher temperatures. However, one challenge of designing high
temperature fuel cells is that materials must be selected that can withstand
the high temperatures without melting or corroding [60, ch. 2].
As with batteries, another way to classify fuel cells is as primary or
secondary [60, ch. 1]. In a primary fuel cell, also called nonregenerative,
the reactants are used once then discarded. In secondary fuel cells, also
called regenerative, the reactants are used repeatedly. An external source
of energy is needed to refresh the fuel for reuse, and this source may supply
energy electrically, thermally, photochemically, or radiochemically [60, p.
515]. Both primary and secondary fuel cells have been made with a variety
of organic and inorganic fuels [60, p. 515].
Another way to classify fuel cells is as direct or indirect [60, ch. 1,7]
[128, ch. 37]. In a direct fuel cell, the fuel is used as is. In an indirect fuel
cell, the fuel is processed rst inside the system. For example, an indirect
fuel cell may take in coal and use an enzyme to break it down into smaller
hydrocarbons before the reaction of the cell [60, ch. 7].
Families of fuel cells are often distinguished by the type of electrolyte
used. Examples include alkaline which use a potassium hydroxide solution
as the electrolyte, phosphoric acid, molten carbonate, and solid oxide which
use solid ceramic electrolytes. Other times, fuel cells are categorized by the
type of membrane or the type of fuel used. Two of the most common types
of fuel cells are proton exchange membrane fuel cells and direct methanol
fuel cells [128, ch. 37]. Proton exchange membrane fuel cells use hydrogen
gas as the fuel, oxygen from air as the oxidizer, a solid electrolyte, and
a platinum catalyst [128] [141]. They operate at low temperature and
are used in buses, aerospace applications, and for backup power. Direct
methanol fuel cells use methanol as a fuel. They also often operate at low
or medium temperatures [128] and are used for similar applications.