This is a cell suitable only for static application, taking several hours to reach its operating temperature. It is a high-temperature cell, running at between 800 and 1000°C. Its great virtue is that it can run on a range of fuels including natural gas and methanol which can be reformed within the cell. Its high operating temperature also enables it to break down impurities. Its high temperature also removes the need for noble metal catalysts such as platinum. It potentially has a wide range of power outputs, from 1 to 1000 kW In contrast to PEMFCs the electrolyte conducts oxygen ions rather than hydrogen ions which move from the cathode to the anode. The electrolyte is a ceramic which becomes conductive to oxygen ions at 800°C. SOFCs are often structured in a tubular rather than a planar form (as in the PEMFC) to reduce the chance of failure of the seals due to high-temperature expansion. Air (oxygen) flows through a central tube whilst fuel flows round the outside of the structure (see Fig. 7.4). According to David Hart,
Solid oxide fuel cells are expected to have the widest range of applications. Large units should be useful in industry for generating electricity and heat. Smaller units could be used in houses.3
As confirmation of his prophecy, Ceres Power has developed a 1 kW SOFC designed for the domestic market, producing heat and power (Fig. 7.5). It is fuelled by natural gas which British Gas claims will make it accessible to 14.5 million households. It should be market-ready in 2007.
There is confidence that the installed cost of static high-temperature fuel cells will fall to $600-1000 per kW. As cogeneration power units this will make them highly competitive with conventional systems. Already SOFCs are now being imported to the UK from the USA at a cost of $1000 per kilowatt, which brings them closer to cost-effectiveness in comparison to fossil fuels which continue to rise in cost.
One of the main producers of SOFCs is Westinghouse, USA, which uses the tubular configuration for the cell.
A 200 kW unit has been installed on a test basis in The Netherlands.
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The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.