Fuel cells are increasingly being seen as a highly promising method of clean power generation. For the automotive market a consortium including DaimlerChrysler and Ford expects to have a viable system ready by 2004. Now Siemens Westinghouse of the US believes it is close to commercialising a system known as the solid oxide fuel cell for local generation. This will have a higher efficiency than combined-cycle gas turbine systems, while producing virtually no toxic emissions.
`We think we can commercialise the system for the distributed generation market from a few hundred kW up to around 10MW,’ says Chris Forbes, Siemens Westinghouse business development manager for solid oxide fuel cell power generation.
Potential customers include industry – for example chemical and steel plants – and building complexes, such as shopping malls with big air conditioning loads.
Fuel cells generate electricity from fuel and air without combustion. In its pure form, hydrogen is used as a fuel in a `reverse electrolysis’ process. Normally electrolysis in-volves the breakdown of ionic compounds in solution (the electrolyte) into their constituent atoms by passing an electric current through the solution. In the fuel cell, hydrogen combines catalytically with oxygen at the surface of an electrode to generate electricity, with water as a by-product. Hydrocarbons such as methane can also be used as a fuel but have to be broken down or `reformed’ to release their hydrogen. In this case carbon dioxide is produced as a by-product.
Earlier this year Siemens announced a partnership with Shell Hydrogen to use the system to produce electricity for oil and gas production operations, injecting the carbon dioxide produced back into depleted oil and gas reservoirs.
The unique feature of the solid oxide system, originally invented 40 years ago, is its use of a ceramic tube as the basic structure of the cell. This eliminates the need for problematic seals: because one end is closed, fuel and air can be segregated until they are needed to mix.
The tubes, grouped in bundles to generate a usable amount of power, only have to be supported at one end, allowing for thermal expansion at the fuel cell’s operating temperature of 1,000 degrees C. The cell is made by depositing metallic layer electrodes on the outside of the tube.
The high operating temperature means that in methane or methanol cells the fuel is broken down catalytically into hydrogen and carbon dioxide without the need for an external reformer. `This is a key advantage,’ says Forbes.
Of the fuel used, 85% is consumed in the cells and the rest burns at such a temperature that no nitrogen oxides are produced. The fuel has to be sulphur-free to avoid poisoning the catalyst, so no sulphur dioxide is produced either.
Half the energy released is converted to electricity; the rest becomes heat, of which typically 20% is lost and 30% is available for combined heat and power.
In a commercial prototype, the cell tubes were 1,500mm long. A stack of 1,152 cells developed 100kW at a voltage of 1.1V. The output was inverted and stepped-up to produce alternating current at a usable voltage.
One prototype has run for the equivalent of a year and a half – including 6,500 hours continuously. A 100kW co-generation system operating in the Netherlands since 1997 is producing 100kW of AC power for the grid and 45kW to heat hot water for district heating.
Siemens Westinghouse aims to get the system to market at a price of $1,300-2,000 per unit. `We believe we’ve got a product that meets the requirements for commercialisation – it has a long life and does what it’s supposed to do,’ says Forbes.
But there is even greater potential for use of the fuel cell in a hybrid system. `The exhaust, at around 800 degrees C, has tremendous uses for heating. That is also the inlet temperature of a small turbine,’ Forbes explains.
Using the exhaust gases to drive a turbine/compressor system to pressurise the cell boosts its performance while the turbine generates additional electricity itself. This boosts efficiency to 60%. With a twin-shaft turbine, in which exhaust gases are reheated by another fuel cell stack before entering the power turbine, efficiency of over 70% can be achieved. `That beats the pants of any existing fossil fuel generating system,’ says Forbes.
Tests funded by Edison Technology Solutions, the US Department of Energy and Siemens Westinghouse are about to start at the National Fuel Cell Research Center at the University of California on a 200kW pressurised solid oxide fuel cell, combined with a 50kW microturbine generator.
The next stage will be to produce demonstrators of the products Siemens Westinghouse envisages selling: an atmospheric 250kW system, a 300kW pressurised hybrid, and a 1MW pressurised hybrid. Trials of these, due to start in 2001-3, will be jointly funded by Siemens Westinghouse, the DoE and utilities in the US, Canada and Europe.
Siemens Westinghouse anticipates a decision to commercialise by October 2001; start of construction on a commercial factory for the system is planned for as early as 2002. `It will be a billion dollar market if it works,’ says Forbes.