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