The MTU “HotModule” molten carbonate fuel cell plant is on target for series production. Table 1 lists projects to date. For more than two years, a field-trial plant has been in operation at Bielefeld University. It supplied the municipal grid with electricity and the university with heat and steam.

Continuous operating experience from this plant is leading to further optimisation of the technology. Brief technical parameters are listed in Table 2. With a total operating period of 16 000 hours, it was the longest operating MCFC plant in the world. The 250 kW class unit achieved an electrical efficiency of 47 per cent.

The first molten carbonate fuel cell plant for use in a hospital was put into operation in 2001 in the Rhön-Klinikum in Bad Neustadt/Saale. Apart from the electrical power, the medical section of the Rhön-Klinikum uses the high-pressure steam derived from the exhaust air at 400°C for air-conditioning and sterilisation purposes. The Rhön-Klinikum plant has been operating reliably for over 7000 hours.

A further unit was started in April at RWE’s Meteorit fuel cell demonstration pavilion in Essen, planned for full operation during 2002. Like the previously delivered MTU plants, this also supplies 250 kW electrical peak load and approximately 170 kW thermal power.

Simple operating principle

The HotModule is characterised by the horizontal orientation of the cell stack, a unique design, which enables a highly integrated and cost saving build-up of a fuel cell power plant.

The complete plant consists of three separate sections: a central steel container; a gas clean-up system; and an electrical equipment enclosure.

The container houses the fuel cell stack and is the actual “HotModule” which gave the system its registered name. The gas clean-up system is situated upstream of the system and the electrical enclosure contains the system controls and conditions the alternating current output.

Tie rods hold together more than 300 individual cells, which are lined up to form the stack. Each cell is constructed as a flat sandwich with two electrodes (an anode and a cathode) as outer walls, enclosing the lithium/potassium-carbonate liquid electrolyte, which is retained in a ceramic matrix of LiAlO2.

Molten carbonate fuel cells operate at a much higher temperature than the 200°C phosphoric acid fuel cell or the 80°C of the solid polymer fuel cell, but lower than the over 850°C of the unpressurised solid oxide or up to 1000°C of the pressurised combined cycle SOFC units.

When the hydrogen flows over one electrode and air flows over the other one in an environment of 600-650°C, a process is started which generates electricity. This process takes place at atmospheric pressure and employs low flow velocities. At temperatures above 500°C, the carbonates form a highly conductive molten salt, which promotes the electrochemical reaction. The electrolyte of carbonate ions (CO32-) melts, enabling the exchange of electrons. The carbonate ions transfer their charge to the anode and give off an oxygen atom which combines with the hydrogen which flows by, to form water (H2O). The remaining carbon dioxide (CO2) returns to the cathode side, takes on two electrons and an oxygen atom from the air flowing by and returns to the process as carbonate ion (CO32-).

In a fuel cell operating on natural gas the fuel is first treated to remove higher hydrocarbons and a hydrodesulphuriser takes out sulphur bearing odourants such as mercaptans. It is then mixed with steam generated by the hot exhaust gas from the cell. This mixture is then fed to the reforming unit, which is integrated into the cell and heated by the fuel cell reaction.

Under the influence of a nickel catalyst the gas is partially reformed in hydrogen and carbon monoxide, and then returned to the anode. Remaining methane is reformed in the presence of a nickel catalyst within the anode chamber while the hydrogen reacts with the carbonate at the anode. The reaction produces water, carbon dioxide, heat and electrons.

At the cathode, carbon dioxide and oxygen react with the electrons returned to the electrical circuit to replenish the carbonates destroyed at the anode. Since there is no combustion taking place, emissions are minimal except for CO2, which can marginally exceed 400 kg/MWh. NOx particularly is barely measurable, at 0.0002 kg/MWh, while SOx is around 0.001 kg/MWh.

The plants to be delivered in 2002 and 2003 are for customers all over the world. “This is a further important step towards series production of the HotModule”, said Michael Bode, head of New Technologies at MTU Friedrichshafen. The plants will be used for cogeneration, trigeneration and DC-power supply. Use of special gases will also be tested.

Customers include companies in the power supply, telecommunications and health sectors and industrial applications such as IZAR shipbuilding in Catagena, and the Michelin tyre works in Karlsruhe, De Te-Immobilien telecommunications in Munich and a clinic in Magdeburg. Others, for the USA and Japan, have already been delivered and further units will be supplied to MTU´s partner Fuel Cell Energy in the USA for various applications, as shown in Table 1.

Efficient and clean chp

MTU’s high-temperature fuel cell presently achieves an electrical plant output of 218 kW at a cell block output of 250 kW, plus 170 kW thermal energy with overall utilisation efficiency of above 90 per cent.

Emissions of pollutants from the HotModule are so low that, in meeting the TA Luft (German clean-air standard) it is appropriate to talk about exhaust air rather than exhaust gas since it consists mainly of hot air and steam. The molten carbonate fuel cell can operate on a variety of very different fuels including natural gas, sewage gas, landfill gas and residual gases from industry as well as methanol.

Exhaust heat at a temperature of 400°C allows the generation of high-pressure steam for district heating or industrial process heat for such purposes as pasteurising, sterilising or air-conditioning. Such byproduct high-pressure steam is of great value in hospitals and in the chemical and food-processing industry or, as at the University of Bielefeld, for process steam for research purposes. The waste heat can also be used by passing it through a turbine to further increase the system electrical output. This enables high-temperature fuel cells to reach a total efficiency of 65 per cent, thus exceeding the efficiency of even large gas and steam power stations.

Rhön-Klinikum performance

Hospitals are potentially important areas of application for the new technology. In Germany, they consume about one per cent of the electrical energy generated by the country as a whole. Hospitals have to permanently maintain emergency power generators that can ensure there is an uninterrupted supply to the key electrical systems – in the operating theatre, for example – in the event of a power failure. Fuel cells supply a very constant, and therefore high-quality, current – an absolute necessity for highly sensitive medical equipment.

Another advantage is the cost of integrating the installation into the hospital, as Jörg Demmler of Rhön- Klinikum AG explains: “We only incur very small expenses for peripheral systems because the HotModule is very flexible in adapting to our energy requirements. The costs of sound insulation, exhaust air treatment and maintenance are also lower than for conventional modular power plants”. Time is another factor: “The approval procedure required by German anti-pollution legislation that would have been necessary with other types of power plant was not needed”.

  The Rhön-Klinikum installation is also important to MTU Friedrichshafen because it is the second field trials plant in which the HotModule is being used in real operational conditions.

Role of FCE

For the production of HotModule power plants, MTU´s partner FCE supplies the cell packages, which are the central components of the plant. Based in Danbury, Connecticut, in the USA, the company has achieved major advances in the technology with its direct carbonate fuel cell product line. The 2 MW Santa Clara demonstration plant has been running since March 1996.

In April 2002 the company received US Patent No 6 365 290 for its combined cycle direct fuel cell/turbine power plant.

In July 2001 FCE initiated a “proof of concept” of its so called DFC/T power plant based on a 250 kW DFC power plant integrated with a modified Capstone model 330 microturbine. Now the company is planning to test a Capstone C60 turbine, which will greatly increase the efficiency of the 250 kW power plant. Proof of concept testing results to date have verified the operability of the system and that further testing will provide additional information for the design of a 40 MW DFC/T power plant.

More recently FCE has been planning a 2 MW DFC/T plant to run on syngas from the Kentucky Pioneer IGCC power plant in Trapp, KY, which uses a BG Lurgi gasifier.

Development potential

Dr Michael Gnann, responsible for MTU’s fuel cell activities within the New Technologies group sees much more potential: “If we look at the development of engine technology, it can be assumed that a whole series of technical improvements will be incorporated by the time the fuel cells reach the production stage.” He sees development possibilities in various areas.

The most important step is the further simplification of the system and cell design, not just to reduce costs, but also to improve efficiency. The MTU technicians also want to further improve the energy density and the service life of the cell. The individual cells presently generate 0.8 kW; each one should produce 1 kW in the future. The commercial cell stack has a service life of 40 000 operating hours. MTU’s engineers also want to make the HotModule more flexible; it should be able to produce power and heat independent of the main power grid even when there are disturbances in the grid, or it fails completely.

When this is possible, then not only can the HotModule continue operation, but consumer equipment which reacts sensitively to voltage irregularities can also continue operation – an invaluable advantage, for example, in sensitive production areas and in hospitals or clinics.

Scope for cost reduction

The costs of the HotModule nevertheless need to be further dramatically reduced. According to Michael Bode, head of “New Technologies” at MTU: “The market sets strict demands in terms of cost of electricity and of process heat. We do not regard the gas engine, nor the gas turbine, but the electrical power from the grid and process heat generated from a boiler as our competitors.”

In its fuel cell programme, MTU therefore directs much attention to the reduction of manufacturing costs. In just a few years the company has been able to reduce the cost per kW by a half.

An innovative concept made it possible to integrate many components within the central steel container, which had previously been located outside it. What is expensive with most fuel cell plants is not the actual cell – this usually accounts for only about one third of the total cost. What raises the price are the peripherals such as the gas-conditioning equipment, the system for converting the gas into hydrogen, the piping, fans, compressors, gas shrouds, etc, which usually account for around two-thirds of the overall cost.

MTU is aiming at US$1000 to US$1200 per kW output over the medium term, making the HotModuleattractive from the economic standpoint as well.

To reach this target, MTU is expending effort in several areas.

Through technical simplification, MTU’s development engineers want to cut the production cost of the HotModule by one-third in the future.

They want to further simplify its construction, delete superfluous material and reduce the cost of the core component to one-half of its present figure.

Today, each HotModule is essentially a unique, hand tailored item, which makes it impossible to compare it with standard products such as diesel engines.

When the HotModule reaches production maturity in the medium term, Michael Bode believes that another 50 per cent can be saved, making it possible to reach the costing target and to provide the customers with electrical and thermal power in a clean and efficient way, at a competitive price per kWh.

Table 1. HotModule MCFC plants already delivered or on order
Table 2. Technical data for the HotModule in Bielefeld