Environmental Engineering Reference
In-Depth Information
Molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFC) work at much
higher temperatures. This enables gases containing hydrocarbons, such as natural
gas and biogas, to be used directly without any previous reforming. The dis-
advantage of high temperatures is the long time required for starting up and
switching off.
As the electric voltage of a single cell with values of around one volt is too low
for most applications, a number of cells are usually connected in series to form a
so - called stack (Figure 13.7 ).
Figure 13.7
Prototypes for fuel cells.
The electric effi ciency of fuel cells today is usually in the order of 40 to 60%. Values
of more than 60% can be reached in individual cases. A fuel cell can therefore only
convert a part of the energy contained in hydrogen into electric energy. Basically,
the waste heat of fuel cells is also useable. Power-heat coupling, or the simultaneous
generation of power and heat, raises the overall effi ciency of a fuel cell and can
increase it to over 80%.
Major advances have been made in fuel cell technology in recent years, and many
companies are already offering commercial units. However, the number of units
currently being sold is still relatively low. The price of fuel cell systems is still fairly
high compared to other energy supply units. Furthermore, it will be necessary to
increase the sometimes quite short service life of fuel cells if they are to have a
broader appeal.
13.3 Economics
It currently costs about 4 cents to produce a kilowatt hour of hydrogen through the
steam reforming of natural gas - assuming that natural gas prices are relatively
reasonable. One litre of petrol has a calorifi c value of about 10 kilowatt hours.
