Page 37 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

18 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

Westinghouse carried out a paper study in 1995 and tested this concept in a
200 kWe class hybrid system in 2001-2002. The SOFC stack was operated in
a pressure chamber at 3.5 bar abs. A 50 kW microturbine was used to utilise the
hot gas exiting the SOFC stack. The overall efficiency of this first-of-a-kind, proof-
of-concept prototype was measured at 5 7% [44]. This is the highest conversion
efficiency device for fuel to power yet devised. Efficiencies up to 75% are expected
in larger hybrid systems when fully developed.

1.12 Application Areas and Relation to Polymer Electrolyte Fuel
Cells
There are many possible applications for SOFCs as described in Chapter 13. The
stationary power market has been most investigated up to this point, since there
is a great need for clean and quiet distributed power generation units, e.g. in
hospitals, hotels and sports facilities located in cities. Traditionally, such
demands have been served by 200 kWe diesel engine or gas turbine packages,
with the heat supplied to the building for hot water or steam. Other fuel cell
systems based on phosphoric acid, molten carbonate and solid polymer
electrolytes have also been developed to fill this niche. Heat engine generators
are cheaper at the moment and fuel cell devices will generally not be preferred
unless regulations for low emissions are imposed.
The other application which has been much studied is that of integration of
SOFCs with coal gasification. An SOPC is eminently suited to integration with a
coal gasifier plant in large power stations and should result in highest overaIl
conversion compared to other fuel cell types. Unfortunately, the investment
required to build such plants is large.
Therefore other smaller applications have emerged in recent years, especially
to compete with polymer electrolyte fuel cells (PEFCs) which have been rapidly
evolving to satisfy the zero emission electric vehicle market. A typical PEFC for a
vehicle is 30 kWe, runs on pure hydrogen, requires significant quantities of
platinum, and is still significantly more costly than an internal combustion
engine. It does have rapid heat-up and can deliver significant power on a cold-
start but this advantage is destroyed if pure hydrogen has to be obtained through
hot reforming of methanol or gasoline, which introduces delay and sluggishness
into the system. The SOFC can be useful in vehicles, not to replace the engine, but
to supply auxiliary power to supplement or replace the existing battery system.
Typically, 1-5 kWe is required, mainly to drive air conditioning. The benefits of
SOFCs for this application are:

0 it can run on the same hydrocarbon fuel as the internal combustion engine
(e.g. gasoline, diesel);
0 it can provide useful heat:
0 it can run when the engine is switched off
0 it is much more efficient than the existing electrical system: and
0 it has low emissions.