Cells pitch for power

Fuel cells are at last showing their commercial worth – from possible use in offices and sports centres to larger-scale power generation. And they are clean. Andrew Cavenagh reports

Not many inventions have to wait more than 100 years before they find a practical application. Yet such was the fate of fuel cells, which generate electricity by chemical conversion rather than by the thermal-mechanical method used in all types of power station.

Sir William Grove achieved the first electrochemical conversion in London in 1839, but fuel cells were not used until the 1960s and the US Apollo and Gemini space programmes. The following decade saw the cells’ development for terrestrial purposes, but while gas and electric companies were carrying out research and trials by the 1980s, their use has been largely restricted to military and telecommunications purposes.

The attractions of generating power from fuel cells are high efficiency, absence of noise, and very low emissions of pollutants – in some cases 1,000 times lower than a comparable engine-driven power unit. This is because the reaction that generates electrical current and heat in the cell is the chemical combination of hydrogen ions and oxygen ions with water as the only by-product.

At the heart of each cell are two electrodes separated by an ion-producing electrolyte. The electrodes are coated with a catalyst – usually platinum – to initiate the chemical reaction. A single cell generates about 0.7V, so a large stack of cells is required to provide a meaningful power unit.

The stack needs two ancillary components – a processing and reformer unit at its front end to extract hydrogen from naturally occurring fuels such as gas, methane or methanol, and an inverter at the back end to convert the direct current produced at the output stage into the alternating current required by most electrical equipment and transmission systems.

While pressure on car manufacturers to reduce vehicle emissions has led companies such as General Motors and Daimler-Benz to embark on huge development programmes for cell-powered vehicles, cost remains an inhibiting factor for power-generation applications. In most cases, a power plant run on fuel cells would cost as least twice as much to build as an engine-driven equivalent.

Development programmes begun in 1990 in Japan and the US, however, should soon prove the reliability of small fuel-cell power units to the point where they merit serious consideration for many combined heat and power applications.

Their cause should be helped by the increasing fragmentation of the electricity industry, which is making small self-generation schemes increasingly attractive for buildings such as office blocks and hospitals, and the constant environmental pressure to reduce harmful emissions.

Of the five recognised fuel cell technologies (named according to the electrolyte material – see table), only the phosphoric acid fuel cell has been developed to the point where it is commercially exploitable. This is chiefly because the relatively low temperature at which PAFC cells operate have made it easier to configure and build stacks than for higher temperature molten carbonate and solid oxide fuel cells, although solid oxide cells have potentially greater generating efficiency (60% versus 40%). The problems with MCFC and SOFC – their higher thermal and other stresses making stacks fundamentally unstable the larger they get – will require years more research, and they are unlikely to be commercially viable much before 2010.

PAFC plants using commercial fuels such as natural gas or propane have been built and tested in the US, Europe and Asia. While Fuji Electric is developing three sizes of PAFC plant in Japan, the predominant unit has been the 200kV PC25 plant manufactured by Onsi, a subsidiary of International Fuel Cells (IFC), which is a joint venture between Japan’s Toshiba and the United Technologies Corporation of the US.

The Connecticut-based Onsi has delivered more than 100 of the combined heat and power units to sites in Europe, the US, Japan, Korea, and India. Sales in the US have been assisted greatly by government subsidies to the purchasers, which have reduced the capital cost from $3,000/kW to about $2,000 (compared with $1,500 for an engine-based system). The programme, administered jointly by the US’s defence and energy departments, will cover 100 units, and is expected to run for another year.

The PC25 operates at 205 degreesC and generates electricity with 40% efficiency. Heat recovered from cooling the cell stack is used both to generate steam for the gas-reforming process and to provide hot water via a heat exchanger with a maximum temperature of 90 degreesC. This increases the overall efficiency to around 80%. One unit at Osaka should complete 40,000 hours of continuous operation next year.

A joint venture between IFC and Italy’s Ansaldo, the European licensee for the unit, has plants operating in Germany, Austria, ltaly, Switzerland, Sweden and Finland. It is looking for a site for a first US plant in conjunction with Johnson Matthey, the precious metals and electronics company that produces platinum catalysts for fuel cells.

`There are a number of interested parties looking at this,’ says Robert Evans, Johnson Matthey’s marketing development manager for fuel cells. `They’re ideally suited for traditional combined heat and power applications, such as office buildings, hospitals and sports centres.’

With the capital cost still double that of a comparable engine-driven system, however, the technology needs an edge to break into the market to enable Onsi to reduce the unit cost through the economies of mass production. `We are in a sort of catch 22 situation at the moment,’ says Evans.

He says the environmental benefits of PAFC technology have not yet provided the expected spur to commercial development. The consistency and reliability of the PAFC electrical signal could, however, create another niche in the office market – the protection of computer-critical equipment. PAFC could save companies the cost of the elaborate switchgear needed to protect computers from load fluctuation. `That’s potentially a bridge that will help the technology progress,’ says Evans.

CLC-Ansaldo, the European licensee for the PC25, is developing a hydrogen-fed version of the unit with two Hamburg companies which would cut the capital cost by a sixth through dispensing with the need for the process and reforming unit. It would also increase the generating efficiency to 45%.

To go beyond CHP applications into the bigger power generation market, one of the high-temperature fuel technologies will have to be advanced, because the efficiency of PAFC plants as pure electricity generators does not match the 55% now achievable with combined-cycle gas turbines, and their operating temperature is too low to channel the exhaust gas through a turbine.

This would, however, be feasible with a solid oxide fuel cell operating at 1000 degreesC. `That’s where there’s quite a thrust of interest among electrical utilities,’ says Evans. Companies working on SOFC plants include Mitsubishi Heavy Industries, Rolls-Royce, and Siemens.

`We are developing solid oxide fuel cells because we think the SOFC has the potential to achieve an efficiency of 75%,’ says Wolfgang Drenckhahn, who heads the Siemens division looking into stationary applications of fuel cells. `When we have proved the fuel cell technology, then you can think about bigger plants of 100MW or 200MW.’

{{The five fuel cell contenders

Fuel cell technology Operating temperature

Phosphoric acid fuel cell 200 degreesCAlkaline fuel cell 80 degreesCPolymer electrolyte fuel cell 85 degreesCMolten carbonate fuel cell 650 degreesCSolid oxide fuel cell 1000 degreesC}}