Fuel Cell Control Systems and Ancillaries

Major items of equipment

The four major items of equipment in addition to the fuel cell include:

A reformer to convert non-hydrogen fuels to hydrogen and prevent CO poisoning of the fuel cell catalysts. Reforming may be incorporated inside high temperature fuel cells or the reformer may be a separate device. Reformers are described in greater detail in a previous review.
A power conditioner, possibly with a transformer, to convert the DC output from the fuel cell to the required AC output voltage.
A heat exchanger to transfer heat from the hot exhaust air from the fuel cell (and power conditioner), to the heat distribution medium (water, oil, or steam).
A control system linking fuel cell and reformer monitoring instruments, the power conditioner, and flow control devices.

Concepts and systems

Biofuels

Fuel Cells

Fuel cell performance

Control and ancillaries

Electricity and heat transfer

Installation and operation

Commercial availability

Economics

Environment

Environmental aspects

Index of technical reviews

Voltage regulation and fuel flow control

Very little information is published on this subject, other than identifying that a power conditioner including an inverter is required to convert the DC ouput of the fuel cell to AC power.

As discussed in the previous review, a constant voltage AC output is normally required, and the simplest way to achieve this is to dump excess voltage at low loads through a dropping resistor. Alternatively, more efficient but more expensive possibilities include using power electronics to convert excess voltage to current (AND gates), or using a variable voltage transformer on the AC output. These options are only likely to be viable on large scale systems.

Fuel flow is likely to be controlled proportional to current to avoid stability problems as discussed previously.

Fuel cell cooling

This is achieved using an excess quantity of air flow (above that required for the fuel cell reaction) over the cathode. The air flow is forced using a fan, and the flow rate controlled to maintain the optimum operating temperature for the fuel cell reaction and materials.

The air then passes through a heat exchanger as described above.

Preheaters and start up

The fuel entering the reformer or fuel cell is preheated, possibly in a number of stages, to achieve the required reforming or fuel cell reaction. Preheating uses burners fueled by the incoming fuel supply or unutilised fuel recirculated from the fuel cell.

Incoming fuel must be used for preheating prior to start up, or for standby periods. PAFC and the high temperature fuel cells are normally maintained at a high temperature during idle periods to reduce the warm up time or damage due to thermal cycling.

Desulphurisers

Even when natural gas is used, this contains enough H2S additive to require a desulphuriser on the fuel inlet to prevent corrosion of the fuel cell catalysts.

Prototype SOFC system showing ancillaries
Courtesy Ceramic Fuel Cells Ltd