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The Fifth European SOFC Forum

The Fifth European SOFC (solid oxide fuel cell) Forum took place in Lucerne, Switzerland in the first week of July. It was held in conjunction with the Fuel Cell World and together they attracted around 600 participants from more than 35 countries.

To the surprise and glee of the organisers, the more technical SOFC Forum proved the more popular, with all of the seventy paper presentations and eighty poster presentations very well attended. The presentation sessions, loosely divided into materials, manufacturing, mechanical design, anodes and fuel reforming, cells and stacks, SOFC systems, modelling, and thermodynamics covered the whole range of SOFC issues, and illuminated many of the trends currently gripping the field.

The difference in tone between the heady rush to commercialisation seen in the Fuel Cell World and that of the SOFC Forum, which attracts many long established primary energy and power providers, was epitomised by Baldur Eliasson of ABB in his statement "hydrogen is a beautiful woman but we must put some on clothes on her". He was referring to the infeasibility of a global hydrogen infrastructure - touted in some circles as the ultimate solution - due to the low-energy density of hydrogen which makes bulk shipment highly energy and cost intensive. He instead suggests "packaging" hydrogen with carbon to yield a synthetic high hydrogen fraction hydrocarbon such as methanol.

The early phases of this approach involve a compromise solution in which renewable energy is combined with fossil fuels to produce synthetic hydrocarbons which are more convenient to ship but still involve the release of carbon (albeit at a reduced rate) from fossil fuel reserves. This last point was a topic of intense debate after his presentation. The objective is to manage CO2 global emissions rather than eliminate them – he cited China as an example, with its huge coal reserves and desperate thirst for energy, which must be satisfied long before any ideal solution becomes practically or economically feasible: an intermediate step is required.

This type of less than romantic pragmatism is widely found in the SOFC community – indeed the primary advantage of SOFCs is that they give flexibility of fuel (not only can many SOFCs perform internal hydrocarbon reforming but they utilise carbon monoxide as a fuel and in special cases can utilise methanol or methane directly) - and has led many people to distance themselves from the "Hydrogen Economy" concept. Thus, despite its less pleasing ring, a better term is perhaps the "Electrochemical Economy", as it is the efficiency gain that electrochemical oxidation offers over combustion oxidation which is the advantage of fuel cells and not their utilisation of hydrogen.
But purists rest assured, there were numerous zero impact systems discussed, including landfill and sewage off-gas and various biogas utilisation systems, as well as carbon dioxide sequestration processes. All of these rely on SOFC fuel flexibility and are thus advanced by the same developments necessary for realisation of large-scale fossil fuel systems promoted by the major energy providers. More on this later.

New materials, anodes and reforming
Not surprisingly then, higher hydrocarbon and trace dilutent (such as sulphur) tolerance were major topics in the materials, anode and reforming presentations. It is clear that as SOFCs move out of the lab and are subject to the huge variety of natural gas compositions around the world, not to mention those of bio and waste/sewage gases, that though these species are present only in small quantities, they will be vital factors in both cell and system design.
Another major preoccupation of materials and manufacturing process developers was lowering of operating temperatures (though this is not universally accepted as beneficial, despite what some may say). This can be achieved by reducing the thickness of the "conventional" YSZ (yttria stabilised zirconia) electrolyte, to compensate for the reduced conductivity at low-temperature, or by selecting alternative electrolyte materials, such as ceria, with inherently higher conductivities. A number of promising electrode/electrolyte systems were presented, including a ceria electrolyte system developed by Swedish University researchers which reportedly gave satisfactory results at temperatures as low as 400 degrees C. These new materials are however far from ready for mass production as they have yet to prove degradation resistance.

In less technical circles performance degradation is measured against a single parameter - operating hours. However, in reality (and especially at the cell level) far more important in the causation of degradation is variation of operating condition. Electrochemical performance of cells is extremely sensitive to the microstructure of the electrochemical layers and this can be changed by variation of the local thermal and chemical environment. Thus as well as requiring stability over operation time, potential electrochemical assemblies are subject to thermal cycling and redox (chemical) cycling. Research into performance stability was discussed at many levels, from university and lab based investigations of specific mechanisms for conventional and novel materials to experimental validation of commercial systems (Sulzer-Hexis) using tried and tested Nickel-Zirconia cermet anodes (prone to nickel coarsening), YSZ electrolyte and LSM (lanthanum strontium manganese oxide) cathode.

Anode design
Though materials are changing, cell construction has largely stabilised, with the anode supported concept adopted by most developers, with the notable exception of Ceramic Fuel Cells of Australia, who use 10YSZ electrolyte supported design and Siemens-Westinghouse, who use a LSM cathode supported design. The big drive now is to reduce manufacturing costs by eliminating expensive production processes, such as Electrochemical Vapour Deposition (EVD) in favour of cheaper processes, such as screen-printing. This drive is having some effect on stack geometry development and is generating broader interest in the so-called "integrated planar" geometry under development by Rolls-Royce. These designs are essentially squashed and flattened tubular elements with an enclosed fuel (or air) passage like a tubular design but a flat electrochemical assembly (suited to screen printing) like a planar cell.

One totally new design presented was the SOFCoRoll, developed by John Irving of the University of St. Andrews, Scotland. This radical design raised eyebrows (not to mention scepticism) all around. The design is literally a roll of triple layer (electrodes and electrolyte) film with a figure eight pattern in the middle to form the fuel and air channels. The presentation given concentrated on fabrication processes, with no solid evidence of feasibility seen. Still, it made a welcome change from familiar geometries and their associated issues!

Stack and system modelling
Moving up in scale, the modelling section was split into two sessions - the first concentrating on SOFC stacks and systems, the second on integrated low impact power generation systems. Much modelling was performed using commercial packages, which quickly yield results, but in some cases, though the pictures were pretty and the equations impressive, illumination of the underlying physics was somewhat lacking.

The system models presented by Sulzer-Hexis and Global Thermoelectric revealed a well recognised but not yet satisfactorily addressed problem of thermal management –the methane reforming reaction is very rapid under SOFC anode conditions and is strongly endothermic, which results in significant temperature gradients (50-100 degrees C) across stack elements giving sub optimal performance in some regions. Work was presented in the poster session on reducing catalyst activity but it has been found that doing so results in methane reforming in the anode active layer resulting in localised cooling, which is equally problematic----watch this space!

At the system level we are back to the divide described earlier between the large scale and the Ideal. Rolls-Royce (RR) and Mitsubishi Heavy Industries (MHI) presented visions for up to multi-MW hybrid SOFC-GT (gas turbine) plants. Schemes of this scale have been suggested by all the major players (Siemens-Westinghouse, RR, MHI and General Electric). Plant configuration, SOFC/GT power ratio, pressurization and temperature vary widely, but it seems that pressurising up to 3 atm gives very significant benefits and 4-10 atm gives further moderate benefits.

The benefit of pressurisation is due to increased electrochemical performance, reduced pumping losses and cheaper heat exchangers. Costs are of course the pressure vessel, design for higher pressure and reduced equilibrium methane conversion, particularly significant below 850 degrees C. For these schemes, investment is colossal, times scales are long (10 years +) and cost analysis is the crux of the whole matter---what material the anode is made from is a long way from the forefront of the developers mind!

The end game
And so finally, to the Holy Grail - zero impact or even positive impact power generation systems. These schemes cannot be rolled out on demand, in the minimal involvement manner that accountants like. They require extensive site specific analysis and a tailor made solution - but when they work, they are beautiful. Imagine taking a landfill site or sewage plant oozing into the atmosphere methane, higher hydrocarbons, and a cocktail of trace pollutants like freon, benzene and sulphuric acid; capturing all of it and generating power in the process…not bad for a days work! These are not multi-billion dollar energy supply contracts or brand building first offerings to the ever lucrative consumer electronics market — these things are sexy in a way only environmentalists and engineers can appreciate. At present, at least, it is a niche market, as total world bio-gas production is currently about 1.3 per cent of world natural gas production, but due to the potency of methane as a greenhouse gas (32 times that of CO2) the environmental benefit is significant. These schemes as well as CO2 sequestration and bio-gas utilisation were the topic of several presentations by university and national lab teams. It is apparent that SOFCs are well suited to both applications due to their (potential) fuel flexibility and relative ease of separating exhaust CO2.

Conclusions
So, to conclude, the fifth European SOFC Forum was a great success and illuminated numerous trends in the field:

• SOFC interest is growing at every level.
• SOFC companies come in all shapes and sizes from small SOFC specific companies chasing niche markets to global energy and power producers hedging their bets on future energy markets.
• SOFCs have great potential for integration into environmentally benign power generation schemes such as waste and bio-gas utilisation and CO2 sequestration.
• SOFCs can play a major role in reducing the environmental impact of large scale, economically driven, fossil fuelled power generation schemes.
• SOFC system development is diverse with pressurisation to around 5atm very likely, operating temperatures varying from 400 to 1000 degrees C, and integration into combined SOFC-GT and even SOFC-GT-ST (solid oxide fuel cell – gas turbine – steam turbine) cycles under consideration.
• SOFC stack development is still extremely dynamic, with manufacturing costs and local thermal/chemical management of great concern.
• SOFC materials development is equally dynamic with fuel flexibility, temperature reduction, durability and reduced manufacturing costs being major drivers.