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CUED Division A

Energy, Fluids and Turbomachinery

According to the Second Law of Thermodynamics, any power plant which burns fuel to produce heat pays a severe thermodynamic price of 25-30% loss of efficiency due to the highly irreversible, entropy producing, combustion process. This tax of nature has been levied on every single power plant from the earliest days of the industrial revolution to the present. For the last 150 years, however, it has also been known that the tax can be avoided if the chemical energy of the fuel could be converted directly to electricity in an electro-chemical device known as a fuel cell. In the past, fuel cells for power generation have not fulfilled their thermodynamic promise but advances in materials and fabrication techniques over the last decade have altered this picture dramatically.

There are several different types of fuel cell, the 'front runner' for power generation being the solid oxide fuel cell (SOFC). SOFC's can use natural gas as their fuel and they operate at around 900 C. One possibility is to replace the combustor of a gas turbine by a SOFC 'stack', the hot exhaust products then passing through the turbine. Because the fuel cell stack itself produces electricity, the overall efficiency will be very high, possibly around 75%. This brand new technology (which could make a very significant contribution to the UK economy), will not be available for another decade but the race is on to produce SOFC-GT power systems which have high efficiency, good reliability and are comercially competitive with other forms of power generation.

In partnership with Rolls-Royce, we are modelling the thermo-fluid-dynamic processes which occur in SOFC stacks. This is a very complex problem because the physical phenomena involved are all inter-related. For example, we have to include the convection and diffusion of reacting gas mixtures in the porous electrodes and ceramic support material, the electro-chemical reactions occurring at the electrode-electrolyte interfaces, and the heat transfer by convection, conduction and radiation throughout the stack. The prize, however, is very great. Apart from the very high efficiency of electricity generation, emissions from SOFC-GT systems will be very low with near-zero levels of NOX, SOX and particulates. High efficiency also implies reduced emission of CO2 which is the main 'greenhouse gas' responsible for global warming. Future developments could even include fuel pre-processing to reform the primary fuel to H2 and remove the CO2 at source for disposal elsewhere. If this can be achieved, the SOFC-GT combination really would become the ultra-high efficiency, zero emissions power plant of the 21st century.