Tidal Current Energy

Fuel Cells and Batteries
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variety of fuels, including carbon-based fuels, low- and high-purity H 2 , liquid
or gaseous natural gas, liquid biofuels, biodiesel, synthesis gas, fuel oil and
gasoline, can be used after internal or external reformation. An SOFC has a high
electric conversion efficiency of 47 % and with CHP systems the total energy
efficiency increases to 80 %. The modularity permits a wide range of system
sizes, ranging from watts to megawatts [10] .
However, there are major problems related to high-temperature operation,
which limits the selection of appropriate materials because of their thermal compatibility
and endurance. Therefore, one of the aims in SOFC research is to reduce
the operation temperature. There are two different stack designs being considered:
one more durable and tubular, and the other more inexpensive but planar. The latter
is easier to modify for lower temperatures by thinning the electrolyte. Lifetimes
of 40 000 hours have been reached with the former. SOFC technology has been
demonstrated for distributed power production, with plans to develop MW-scale
central power generation units. The goal is to develop fuel cell–turbine hybrids.
1.3 .
Fuel for fuel cells
The chemical energy stored in hydrogen, methanol or other hydrocarbon FCs
is higher than in common batteries. This is one of the reasons why fuel cells are
shifting into applications where batteries have traditionally been used, i.e. for
small specific power systems. For example, mobile DMFCs could compete with
lithium-ion or NiMH batteries in the near future [11] . Hydrogen is one of the
choices as a fuel for FC vehicles. Questions on hydrogen production, storage,
transportation and changes in infrastructure have yet to be answered.
At present, fuels for stationary and portable fuel cells are gaseous and liquid
hydrogen or liquid methanol [12] . Currently, methanol is produced from natural
gas that contributes to anthropogenic CO 2 emissions. Hydrogen and especially
methanol can also be produced from renewable sources, biomass and other
wastes, e.g. pulp and paper by-products. New technologies for large-scale production
of liquid and gaseous biofuels from plant material as well as plant and
animal waste, exist. Hydrogen would be the choice for an FC fuel because water
is the only product. However, the storage and transportation of hydrogen gas is
a problem. Fuels produced from renewable sources are economically sound and
sustainable, as long as correct methods are used in their production. Electricity
production from waste or biomass, utilizing the high efficiency of a fuel cell,
highlights the environmental benefits of FCs. The challenge is not only to produce
as pure a fuel as possible, but also to develop new integrated power generation
concepts within industries and cities that can handle a variety of fuels
and impurities and operate for a long time.
Figure 15.2 shows processes for methanol production, via synthesis gas (CO
and H 2 ), by gasification of biomass or other renewable matter. In the presence of
a catalyst at temperatures above 1000 K, carbon monoxide reacts with hydrogen,
producing methanol. The production of synthesis gas from biomass, bio-oils,
black liquor and other renewable biowaste has been studied by use of various
gasification processes for a long time. Impurities in the synthesis gas mixture