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"Hot module" points the way

Operating at a temperature of 650 degrees centigrade, the fuel cells in the Hot Module convert the chemical energy of the fuel gas into electrical energy.
A consortium of German and Danish firms has developed a new kind of transportable power plant using the latest fuel cell technologies. It runs off any hydrocarbon gas and can produce up to two megawatts of electrical power. The waste heat can be used for industrial applications or to run a steam turbine generator to achieve an overall energy conversion efficiency approaching 65 percent.


A fuel cell is a kind of battery which combines hydrogen and oxygen to form water, with the production of electricity and heat. Fuel cells hold great promise for clean generation of electricity, especially for industrial customers, but they are expensive. Now a German-Danish consortium is developing a simplified design that could at last compete with conventional technologies.

The type of fuel cell in this project is known as a molten carbonate cell. The working fluid - the electrolyte - is a mixture of molten potassium and lithium carbonates maintained at 650 degrees Celsius. One reason for choosing this type is that it neatly solves the problem of how to make the hydrogen that the cell needs. Normally a hydrocarbon gas, such as methane, has to be "reformed" by heating it with water vapour to release hydrogen. But at the high temperature of the carbonate cell, any hydrocarbon gas will break down into hydrogen without the need for a reforming plant. "The main advantage of the carbonate fuel cell technology is its capability for internal reforming," explains Mr Michael Bode, head of New Technologies at project leader MTU Friedrichshafen. "If you have cells with a lower operating temperature, this reforming process has to be carried out outside the cell stack, which means the system is not only more complex and costly, but also has considerably lower efficiency."

Graphic representation of a Hot Module fuel cell power plant, including peripheral systems.

Hot module
Since 1990, MTU have been developing a practical carbonate fuel cell with financial support of JOULE and THERMIE programmes in collaboration with engineering firm Haldor Topsøe of Denmark and utility companies Elkraft AmbA (DK), Ruhrgas AG (D), and RWE Energie (D).

The biggest problem facing the consortium is that while the fuel cell itself, with its stack of many identical parts, can eventually be produced relatively cheaply, two thirds of the cost of a power plant is made up of off-the-shelf ancillary equipment. There is little scope for reducing costs through economies of scale. As project leader Peter Kraus has remarked, "Even if the fuel cells were free, a conventional fuel-cell power plant would still be unable to compete with other technologies because of the high cost of the non fuel cell equipments."

Without the need for a reforming plant, the partners looked to see whether further simplifications could be made. They came up with the idea of the hot module, in which many of the functions traditionally performed by ancillary equipment are integrated within the fuel cell vessel. Only two external modules are needed: a gas-cleaning plant to remove sulphur from the fuel gas, and an electrical cabinet containing the control system and an inverter. Fuel gas goes in one end and electricity comes out the other. The entire plant fits on the back of a lorry.

Any fossil fuel will do
Methane, the chief component of natural gas, is the preferred fuel because it contains a high proportion of hydrogen atoms, but biogas (40-70% methane), coal gas, or synthesis gases could also be used. Oxygen is supplied from air taken directly into the cell stack. The temperature is low enough that harmful oxides of nitrogen are not formed as the air is heated.

"The power output depends on the number of cells in the stack," Mr Bode explains. "Each cell produces a little less than one kilowatt. So if you stack 200 cells you will get an output of almost 200 kilowatts. The hot module we built and tested for the first time with a real stack in it had a nominal output of 260 kilowatts." Another innovation is that the stack is positioned horizontally, so it can readily be extended without increasing its height - an important consideration if the module is to be transportable. "A basic hot module could in the future have 300-400 kilowatts electrical output, and you will be able to connect several of those modules to achieve an output of one to two megawatts."

In the JOULE "Molten carbonate fuel cell" project, MTU worked with Haldor Topsøe to study the potential of the hot module design for cogeneration of heat and power. "Another advantage of the carbonate fuel cell is that the heat is at a high temperature," says Mr Bode. "You can take out heat at about 400 degrees Celsius, which is hot enough to generate process steam. And process steam is something you need for many applications; you can use it in industry for drying, in hospitals for sterilisation, or you can use it for running efficient air conditioning systems."

Efficient generation
Although most applications will be for cogeneration units in industry and hospitals, the hot module may also find a place in distributed electricity supply. In this case, the heat produced by the module would be used to run a steam turbine to generate more electricity, reaching an overall efficiency of about 65 percent.

Mr Bode expects to see hot modules undergoing field tests at customers' premises during 1999. "We want to gain hands-on experience with customers, and all the data from that will be used to mature the total system." Power units should be on the market by 2001-2002, but Mr Bode emphasises that "they may not yet have reached the cost targets we have in mind."

In a few years, the hot module project has come from nowhere to the leading edge of fuel cell technology. "We started our programme in 1990, and we have built up a complete system within 8 years," notes Mr Bode. "We do not yet have a commercially viable product, but if you compare us to our competitors in molten carbonate technology, we are at least in the leading group and related to the system design we are probably in the lead."



Project Title:  
Molten carbonate fuel cell - system development and optimisation

Joule 2

Contract Reference: JOU2-CT93-0284

CORDIS databaseFor more information on this project,
go to the Cordis database Record