Page 307 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

294 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
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up in the reactions at the anode, making it necessary to compensate by injecting carbon dioxide at
the cathode. The operating temperature of the MCFC is about 650 °C. Due to the high-temperature
operation, MCFC is less prone to carbon monoxide “poisoning” than lower-temperature fuel cells,
which makes coal-based fuels more attractive for this type of fuel cell. Like SOFC, it is best suited
for stationary power generation applications and cogeneration. Units with output up to 2 megawatts
(MW) have been constructed, and designs exist for units up to 100 MW. MCFC also has a long
start-up time to reach the operating temperature and has a lower power density than SOFC and is
not suitable for propulsion application.

12.1.4 PAFC
Phosphoric acid fuel cell (PAFC) uses phosphoric acid as the electrolyte and the operating tempera-
ture of these cells is about 200 °C. It uses platinum electrode catalysts to withstand the corrosive
acid effects. Because of about 200 °C operation, PAFCs can tolerate a carbon monoxide concen-
tration of about 1.5%, which broadens the choice of fuels they can use. However, the sulfur needs
to be removed from the fuel. PAFC was the first fuel cell to cross the commercial threshold. The
United Technology Corporation developed and placed a number of PAFC-based 50–200 kW range
power units in operation for stationary power applications in the United States and overseas. Most
fuel cell systems that were sold before 2001 for stationary power generation application used PAFC
technology [6].

12.1.5 AFC
Alkaline fuel cells (AFCs) generally use a solution of potassium hydroxide in water as their
electrolyte, and the operating temperature is 150 °C–200 °C. AFC uses platinum electrode cata-
−
lysts. The hydroxyl ions (OH ) migrate from the cathode to the anode. At the anode, hydrogen
−
ions react with the OH ions to produce water and release electrons. Electrons generated at the
anode supply electric power to an external circuit and then return to the cathode. At the cath-
ode, the electrons react with oxygen and water to produce more hydroxyl ions that diffuse into
the electrolyte. AFCs are highly reliable and efficient and are being used in space applications.
NASA selected AFCs for their space shuttle fleet and Apollo program to provide both electricity
and drinking water.

12.2 REFORMERS
A reformer is a device that produces hydrogen from fuels such as gasoline, methanol, ethanol, or
naphtha. When using a fuel other than pure hydrogen for a fuel cell, a reformer or a fuel processor
is required. The three basic reformer types are steam reforming, partial oxidation, and autother-
mal reforming. Steam reformers combine fuel with steam and heat to produce hydrogen. The heat
required to operate the system is obtained by burning fuel or from excess hydrogen from the outlet
of the fuel cell stack. Partial oxidation reformers combine fuel with oxygen to produce hydrogen and
carbon monoxide. The carbon monoxide then reacts with steam to produce more hydrogen. Partial
oxidation releases heat, which is captured and used elsewhere in the system. Autothermal reformers
combine the fuel with both steam and oxygen so that the reaction is in heat balance. In general, both
methanol and gasoline can be used in any of the three reformer types.

12.3 FUEL CELL CHARACTERISTICS AND PROPERTIES

In this section, the properties and control characteristics of the fuel cells are examined for designing
the overall power conversion system to obtain the required voltage and power output for various
applications.