Hydrogen - Energy Policy Analysis: A Conceptual Framework - page 198

hydrogen protons, which have moved through the membrane, at the cathode to form water. This
union generates heat that can be used outside the fuel cell (USDOE 2011a).
The power produced by a fuel cell depends on several factors, including the fuel cell type,
size, temperature at which it operates, and pressure at which gases are supplied. A single fuel
cell produces approximately one volt or less—barely enough electricity for even the smallest
applications. To increase the amount of electricity generated, individual fuel cells are combined
in series to form a “stack” (the term is often used to refer to the entire stack, as well as to an
individual cell) (Nice and Strickland 2011). Depending on the application, a fuel cell stack may
contain only a few or as many as hundreds of individual cells layered together. This scalability
makes fuel cells ideal for a wide variety of applications, from laptop computers (50-100 watts)
to homes (1-5 kWe), vehicles (50-125 kWe), and central power station generation (1-200 MWe
or more) (USDOE 2011a).
In general, all fuel cells have the same basic configuration, an electrolyte and two electrodes.
But there are different types of fuel cells, classified primarily by the kind of electrolyte used.
The electrolyte determines the chemical reactions that take place in the fuel cell, the temperature
range of operation, and other factors that determine its most suitable applications. Fuel cells are
classified by their electrolyte and operational characteristics:
s4HEPOLYMERELECTROLYTEMEMBRANEFUELCELLISLIGHTWEIGHTANDHASALOWOPERATINGTEM-
perature. PEM fuel cells operate on hydrogen and oxygen from air. Other fuels can be used,
but must be reformed on-site, which can reduce fueling cost but also drives up the purchase
price and maintenance costs and results in CO 2 emissions. PEM systems are typically de-
signed to serve in seventy- to 120-kilowatt transportation applications and may be usable as
uninterruptible power supplies in special commercial applications. Current PEM stack life
is typically around 1,350 hours, as used in automotive applications.
s!LKALINEFUELCELLS!&#SAREONEOFTHEMOSTMATUREFUELCELLTECHNOLOGIES!&#SHAVE
a combined electricity and heat efficiency of 60 percent and were used for production of
electrical power and heated water on the Gemini and Apollo spacecraft. They are often used
in military applications. However, their short operating time renders them less than cost-
effective in commercial applications. Their susceptibility to contamination by even a small
amount of CO 2 in the air also requires purification of the hydrogen feed.
s4HEDIRECTMETHANOLFUELCELL$-&#USESPUREMETHANOLMIXEDWITHSTEAM,IQUIDMETHANOL
has a higher energy density than hydrogen, and an existing infrastructure for transport and
supply of methanol can be utilized.
s0HOSPHORICACIDFUELCELLS0!&#SARECOMMERCIALLYAVAILABLETODAYFORSTATIONARYPOWER
applications, and over 200 units have been placed in operation. They are less efficient than
other fuel cell designs and tend to be large, heavy and expensive, but they have been used
in emergency backup power and remote power applications.
s-OLTENCARBONATEFUELCELLS-#&#SANDSOLIDOXIDEFUELCELLS3/&#AREHIGHTEMPERATURE
designs that promise high operating efficiencies and have been used by electric utilities in
large central station generation.
s4HENEWESTFUELCELLTECHNOLOGYISTHEUNITIZEDREGENERATIVEFUELCELL52&#WHICHCAN
produce electricity from hydrogen and oxygen while generating heat and water. It is lighter
than a separate electrolyzer and generator, making it desirable for weight-sensitive applica-
tions. (USEIA 2008b)
Reducing cost and improving durability are the two most significant challenges to fuel cell com-
mercialization. Fuel cell systems must be cost-competitive with, and perform as well as or better