Alkaline Fuel Cells (AFCs)
The alkaline fuel cell, as mentioned, has a long history in space programmes, primarily as it was the first to be sufficiently developed. It is still used in the space shuttle in a very expensive guise, producing power for the on-board systems by combining the pure hydrogen and oxygen stored in the rocket-fuelling system, and producing water for the astronauts to drink. It has
recently undergone something of a renaissance because it can be made cheaply in comparison with other currently available fuel cells, though mass-manufacture of other cells may reduce this advantage.
Solid Polymer Fuel Cells (SPFCs)
SPFCs have high power density, rapid start-up and low temperature operation, and so are ideal for use in transport and battery replacement, amongst other things. The present cost of SPFC systems is high, and does not help its cause in the stationary power generation market. However, mass-manufacturing techniques will bring down those costs and may enable SPFCs to compete with conventional generators and other fuel cells within the next few years. Domestic power production is a useful example. Although SPFCs operate at about 80°C, this is sufficient for space heating and hot water requirements, and there are already domestic systems of around 5kW on test in 5-10 homes in North America and Europe. The opportunity to replace the domestic boiler with something that also generates electricity is promising, but the potential for replacing diesel stand-by generators that are often dirty, noisy, expensive and unreliable is also engendering great interest. As many countries move towards liberalisation of their energy
systems the opportunities for decentralised generation are growing. There are also demonstrator plants of 250kW being produced and operated. These run on natural gas using a reformer, for the main part, and offer very low emissions but high efficiency
as their benefit. They are currently expensive, as is any technology in the test phase, but the synergies that may be derived from automotive mass-manufacture of many SPFC components could bring the price down well below that of competing alternatives.
Phosphoric Acid Fuel Cells (PAFCs)
Although PAFC systems have been successfully integrated into buses, they are not ideal for transport applications. They have been very successful in fuel cell terms over the past five years as stationary cogeneration plants, with over 170 of the ONSI 200kW PC25 installed and operating worldwide. Typical applications lie in hospitals, where the waste heat can be used in laundry and other areas and where consistent and reliable power is required; in power provision for computer equipment, where the absence of power surges and spikes from the fuel cell enables systems to be kept running; and in army facilities and leisure centres that have a suitable heat and power requirement. The PAFC is by far the most commercial of the fuel cells to date, but it may well be superseded in the longer term by SPFC plants that can potentially be 9 produced more cheaply, and by high-temperature plants that have high-grade (>500°C) heat output.
Solid Oxide Fuel Cells (SOFCs)
Solid oxide fuel cells operate at high temperatures, and therefore lend themselves to applications in which this high-temperature heat can be used. This heat can be used in two basic ways – for heating processes such as those in industry – or possibly in homes – or for integration with turbines for additional electricity production. Until recently the concept of adding a gas turbine cycle to the fuel cell system would have necessitated a system size of well over 1MW.
However, recent advances in microturbines (usually derived from vehicle turbochargers or military equipment) have led to the concept of a combined power plant of 250kW with an electrical efficiency approaching 70%. Specific applications in which SOFCs may be used are in decentralised electricity generation of 250kW to 30MW; industrial cogeneration of 1-30MW; or domestic applications of close to 5kW. The latter option is being pursued by Sulzer Hexis, which has test fuel cells running in Germany and Switzerland with this in mind. As mentioned previously, high-temperature fuel cells are also flexible in their fuelling, which may make them a good future choice for locations without dedicated natural gas grids.
Molten Carbonate Fuel Cells (MCFCs)
The molten carbonate cells are likely to occupy the same market segment as the SOFCs. They run slightly cooler at 650°C, making their combination with a gas turbine less simple, but the operating temperature could be raised if necessary. Whether this would bring about materials problems is an important issue. The primary difference between the MCFC and the SOFC is the
need for CO2 recirculation in the MCFC system, meaning that it is difficult to design one below about 250kW. This removes the market in domestic scale power.