Page 76 - Energize July 2022

TECHNICAL

carbon dioxide at the cathode, as carbonate ions are consumed in Core system
reactions occurring at the anode. The core system consists of the fuel cell stack and the power
MCFCs are used in large stationary power generation. Most conditioner. The fuel cell stack consists of a number of fuel cells
fuel cell power plants in the MW range use MCFCs, as do large arranged in a series/parallel configuration, to produce the required
combined heat and power (CHP) and combined cooling and current and voltage output to meet the application. The output of
power (CCP) plants. These fuel cells can work at up to 60% the fuel cell stack is DC. A power conditioner converts the DC to AC
electrical efficiency, and overall efficiencies can be over 80% in at the required voltage and frequency.
CHP or CCP applications where the process heat is also utilised. 3
Fuel pre-treatment processor
Solid oxide fuel cell Fuel pre-treatment processors are only used where hydrogen
The solid oxide fuel cell (SOFC) uses a solid oxide electrolyte which carrier fuels are used instead of pure hydrogen. The processor
becomes conductive at high temperatures. extracts hydrogen from the fuel and feeds this to the cell stack.
The SOFC differs from others in that O 2 ions are transferred Common systems include steam reformers.
through the electrolyte to the anode where they combine with H 2
to form water. All the components in a SOFC are solid structures and Heat recovery system
this allows the cell to be constructed in any shape and size. The size Fuel cells produce heat as part of the process, and in larger systems
of a SOFC module is flexible, allowing it to be constructed for use in this heat can be substantial. Surplus heat, at sometimes very high
any power range, from watts to MW. An SOFC or its hybrid can be temperatures, is used for industrial or other processes such as hot
built for stationary applications such as central power generation, water production for steam, etc. Both heat and water can be used
distributed power generation or standby power generation. 3 for pre-processing.

Characteristics
Critical characteristics when considering a fuel cell as a power
source for critical applications and standby power include start-up
time, load following ability and efficiency.

Start-up time
The start-up time of a FC-CHP system depends mainly upon the
start-up time of the reforming process, which is typically a steam
reformer, partial oxidation or auto-thermal reformer, as well as the
fuel cell stack pre-heating time.
A steam reformer’s start-up time is typically less than 30 min
from ambient temperature, although the start-up time is projected
to decrease to less than 10 min. PEMFC stacks have much shorter
start-up times than SOFC stacks. The stack start-up times for SOFCs
varies from 2,5 to 20 h.

Figure 8: The solid oxide fuel cell construction (Fuelcellworks ) Load following
3
Load following is the ability to adjust output to adapt to changes in
Fuel cell system structure the load.
The general structure of a complete fuel cell system is given in
Figure 9. Efficiency
Fuel cells produce electricity and heat, and efficiency can be
expressed as either electrical efficiency (E) or overall combined
heat and power efficiency (CHP). Use of heat produced for fuel
processing reduces the CHP efficiency.
Table 1 compares the characteristics of the different types of
fuel cell.

Alternative fuels
Although hydrogen is the prime candidate for clean energy
generation, there are currently several hurdles facing its use.
Purchase of hydrogen for use as an energy source is difficult and
expensive, due to the limited supply of commercial hydrogen and
Figure 9: General fuel cell structure issues in transportation logistics. Hydrogen is currently primarily

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