Technology Background: The HotModule of MTU Friedrichshafen - Technology for future cogeneration plants

• Simple operating principle
• High development potential
• Technical Data

Friedrichshafen - The HotModule, mtu's fuel-cell system, is one of several technologies presently available for fuel-cell plants. The various alternatives differ primarily in operating temperature within the cell as well as in the electrolyte, the substance which enables the electrons to be exchanged and thus allows electricity to be produced.

The HotModule is a carbonate fuel-cell. In the central component of the HotModule, a steel cylinder, the operating temperature is 650°C. It is this high temperature which makes it possible to do without expensive catalysts of precious metals: nickel is all that is needed to put the fuel-cell reaction into operation. At 650°C, another welcome effect occurs: when natural gas and water are brought together within the fuel-cell, hydrogen splits off. This is exactly the fuel which is needed to operate fuel-cells and which, with other technologies, must first be obtained in voluminous reformer plants. However, waste heat is the most welcome by-product of the Hot Module. The heat of 400°C can be used to produce high-pressure steam needed for many industrial processes.

Simple operating principle

The HotModule is very simply constructed. The complete plant consists of three separate components: a central steel container, a gas cleaner and an electrical equipment enclosure. The container houses the fuel-cell stack and is the actual "HotModule" which gave the system its name. The gas cleaner is situated upstream of the system and the electrical enclosure contains the system controls and conditions the alternating current output. Tie rods hold together the 350 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 a foil which is filled with the lithium/potassium-carbonate electrolyte. When the hydrogen flows over one electrode and air flows over the other one in an environment of 650°C, a process is started which generates electricity. This process takes place at atmospheric pressure and employs low flow velocities. At this high temperature, 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-).

Development potential for the HotModule

Just as with the other fuel-cell types, development of the HotModule is not yet complete. Michael Bode, responsible for mtu's fuel-cell activities, sees 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. Simplifying the system design will be the most important step to reduce costs. Such simplification is the target for both the cell itself as well as for the fuel treatment. The mtu technicians also want to further improve the energy density and the service life of the cell. The individual cells presently generate 0.7 kW; each one is to produce 1kW in the future. mtu's engineers are also working on making the HotModule more flexible, enabling it to produce power and heat independently of the main power grid even in case of disturbances in the grid or a complete power failure. Then not only the HotModule, but also consumer equipment which reacts sensitively to voltage irregularities can continue operation - an invaluable advantage, for example, in sensitive production areas and in hospitals or clinics.

Technical Data of the HotModule in Magdeburg
Fuel	natural gas, biogas, sewage gas, landfill gas, residual industrial gases, methanol
Power output (cell block)	270 kW
Power output (grid)	about 230 kW
Thermal output	170 kW
Electrical efficiency (cell block)	56 %
Electrical efficiency of the entire plant (grid)	about 48 %
Overall utilization degree	> 90 %
Number of cells	about 350
Thermal utilization	two phases: process steam and heat
Service live (expected)	5 years
Weight	15 tons (m)