When operating on hydrogen, all fuel cells generate electrical power, heat and clean water. Because of the different electrolytes, the details of the electro-chemical reactions at both electrodes are rather different. This is summarized in Table 1, which lists the different types of cells and the reactions taking place at anode and cathode, respectively.
William Grove’s first model fuel cell employed dilute sulphuric acid electrolyte. The phosphoric acid fuel cell (PAFC, operating at around 200à‚°C) and the proton-exchange membrane fuel cell (PEMFC or PEFC, operating at 80à‚°C, also called solid polymer fuel cell, SPFC) are today’s examples of cells with acidic electrolyte.
Another type of fuel cell is based on the same oxygen ion (O2-) conductors as the lambda sensor in catalytic converters. Typically, yttria-stabilized zirconia, or YSZ, is the electrolyte employed in high-temperature (up to 1100à‚°C) solid oxide fuel cells (SOFC).
The second high-temperature fuel cell uses molten carbonate-salt electrolytes at 650à‚°, which conduct carbonate ions, CO32-. It is unique that, in the molten carbonate fuel cell (MCFC), ions are formed not from the reactants but from CO2 which is injected into the cathode gas stream, and is usually recycled from the anode exhaust (see Table 1 again).
When using an alkaline electrolyte such as KOH, the resulting fuel cell is called an alkaline fuel cell or AFC. It leads a very successful niche existence in supplying electric power to spacecrafts such as the early Apollo rockets and the Space Shuttle. As it cannot operate on impure hydrogen, the AFC currently has no other perceivable commercial applications.
FUEL CELL SYSTEMS
No national, country-wide hydrogen grid exists; therefore, apart from a number of specialized industries, commercial users will want a cogeneration plant to run on natural gas, LPG, diesel or, possibly, methanol or biogas. For most fuel cells this means a major complication as hydrogen will have to be generated from these fuels in a separate unit called a reformer. This – together with DC-to-AC power conversion, electric controls and the balance of plant (pumps, compressors, ducts, pipework) – forms a fuel cell system.
All developers have solved the problem of fuelling stationary power systems. High-temperature fuel cells are generally less demanding regarding their fuel and either convert natural gas internally into hydrogen (MCFC), or even run on natural gas (SOFC). Low-temperature systems will require complete conversion of fuel into hydrogen (and CO2) and put high demands on the removal of impurities, primarily carbon monoxide which is generated in the reforming process as a by-product. Carbon dioxide poses a much smaller problem and will usually be tolerated by the system, even in large percentage quantities.
All fuel cells require thorough sulphur removal from the fuel. Other impurities to be avoided include halide, ammonia or silicon contamination of the fuel.
Towards community-targeted services
Canada’s district energy (DE) business is not well known. Nevertheless, the industry is growing fast and, to customers connected to the 80 or so systems in Canada, it provides a very real service. Ken Church reports on moves away from traditional fossil-fuelled systems and towards flexible, ‘community energy’ schemes designed to deliver multiple benefits locally.
District energy (DE) began in Canada in the 1880s. The earlier systems were primarily institutional or federally owned and operated, and made use of bulk fuel that was not economically available for the individual building owner. Following inception, further development waned for more than 70 years before increasing slightly during the oil crises of the 1970s with the addition of a few federally sponsored demonstration projects. However, it was not until the 1990s that the larger-scale action began.
To raise awareness, it was Europe, and in particular Scandinavia, that Canada looked to for guidance as we moved from the traditional steam-based systems to the more functional, hot water networks. We envisioned the Canadian energy marketplace being turned upside down in the drive for waste heat and energy efficiency. Unfortunately, the critical mass of public uptake did not materialize. It took many years to realize that the political structure, geography and diverse nature of Canada will never support DE as a traditional utility that competes head-to-head with established gas and electricity companies in the commodities marketplace. With no federal or provincial energy policy on energy efficiency (considered the domain of the energy utilities, which are essentially outside of federal control), the Canadian DE industry needed to look elsewhere to identify mechanisms that would support and enhance their development.