Page 277 - Alternative Energy Systems in Building Design

FUEL CELL APPLICATION 251

FUEL CELL EFFICIENCY

The efficiency of a fuel cell is defined as the amount of power that can be delivered to
a load. Larger loads require higher current, which increases the losses in the fuel cell.
In other words, the more power desired, the more current is drawn, therefore giving
rise to larger voltage drops and lowering efficiency.
As mentioned earlier, losses in fuel cells manifest as voltage drops in the cell, so the
efficiency of a cell is directly proportional to its voltage. In general, a fuel cell running
at 0.7 V will have an efficiency of about 50 percent, which means that 50 percent of
the energy content of the hydrogen is converted into electrical energy. The balance will
be converted into heat.
The efficiencies of typical hydrogen cells operating under standard conditions, based
on the internal heating value, or enthalpy, are equal to the cell voltage divided by 1.48 V.
Depending on the type of fuel and the internal exothermic temperature, cells may have
efficiencies that are equal to cell voltage divided by 1.23 V.

Fuel Cell Application

Fuel cells that operate on air rather than bottled oxygen are less efficient. The loss of
energy is the result of molecular density and the content of moisture in the air, which
is much less than that of a pressurized bottle.
The efficiency of a fuel cell in automotive applications is referred to as the tank-to-
wheel efficiency and is about 45 percent at low loads and 36 percent at larger loads. In
comparison, diesel vehicles have an efficiency of 22 percent.
Fuel cell vehicles running with compressed hydrogen have a typical power-plant-to-
wheel efficiency of 22 percent, and 17 percent of the fuel is used is in the form of liquid
hydrogen.

STATIONARY FUEL CELL PLANTS

Fuel cells cannot store energy like batteries. However, there are stand-alone power-
plant applications where hydrogen is produced by continuous sources, such as solar or
wind energy. The overall efficiency of the cycle of electricity to hydrogen and back to
electricity is referred to as the round-trip efficiency. Depending on various parameters
and conditions, round-trip efficiencies may range from 30 to 50 percent. In comparison,
lead-acid batteries have about 90 percent efficiency. Electrolyzer fuel cell systems
could be designed to store large quantities of hydrogen and therefore could be used as
a stationary power source that could provide electrical power for an extended period
of time.
Another type of fuel cell technology, referred to as solid-oxide fuel cells, produces
large amounts of exothermic heat from the recombination of oxygen and hydrogen.
Ceramic-type catalytic membranes in this type of fuel cell can endure temperatures