The Solid-Oxide Fuel Cell (SOFC)

In the SOFC, a thin ceramic layer of yttrium-oxide-doped zir­conium oxide (YSZ) serves as electrolyte, allowing the pas­sage of oxygen anions at its operating temperature of 900 to 1000° C.

The power output of the systems currently being test­ed ranges from one kilowatt for household energy provision using natural gas up to a pressure-driven 200 kW unit. The latter can be hybridized with a gas turbine. The expected electrical efficiency is in the range of 70 % – thus far, 53 % has been measured.

Marketing of the SOFC technology is to be expected only in the longer term, in spite of the performance already achieved and the attractive perspectives for the future. In the past twenty years, it has not been possible to reduce its manufacturing costs sufficiently, nor to improve the mate­rials durability to an acceptable level. This type of cell tech­nology exhibits the greatest volatility in terms of the num­ber of firms that are attempting to develop it.

Micro-Fuel Cells

For supplying low to very low power levels (1-100 W), fu­el cells are also interesting, because they can make the high energy densities of hydrogen or methanol available to elec­tric power consumers, e. g. for portable applications. If very long operating times are desired, then the combination of a fuel storage system and a fuel cell has advantages com­pared to a battery. Operation at room temperature and their rapid operational readiness would seem to recommend the PEMFC and DMFC cells as the only reasonable options for these applications.

Although initial demonstration experiments have been successful, nevertheless micro-fuel cells have not been able to displace the established Li-ion battery technology. In particular, the open question of the supply and disposal of the hydrogen cartridges has been an obstacle, and certifi­cation in certain environments has been only slowly forth­coming. For example, the use of pressurized containers in aircraft cabin luggage is not permitted. The manufacture of micro-fuel cells is closely related to the technology of semi­conductor devices on wafers. Thus, arrays of microcells which can be connected in series or parallel can be pro­duced.

image199

Only the direct-methanol fuel cell (DMFC) has gained some support on the leisure market. Here, systems with a few 100 W output power could supply power to campers and small boats. The DMFC is also being used in military ap­plications. Portable mini-systems for the individual power supply of communications and navigation devices are un­dergoing testing.

If there is a breakthrough in hydrogen storage or a quan­tum leap in the technical maturity and concept simplifica­tion of the DMFC, then a market for micro-fuel cells could be developed much more quickly than for vehicle power trains or for decentral energy supplies.

Outlook

Whether, and how soon, fuel-cell vehicles will take over the streets depends to a large extent – along with cost reduc­tions – on the future price of the currently preferred fuel, hydrogen. Applications in stationary power generation are less complicated, where dynamics, cold starting and H2 pu­rity are less important. Here, however, the high specific costs have continued to delay a broad-based market entry both for decentralized large-scale energy production and for household energy supplies.

In the meantime, fuel cells are competing not only with the conventional technologies that they were supposed to supplant. More and more, other so-called “new technolo­gies” are entering the race in the same direction, and they are to some extent more cost-efficient and technically ma­ture; for example, space heating systems based on Stirling machines.

Summary

Fuel cells have reached a high level of technical development. The PEMFC has demonstrated its reliability for a series of new applications. The PAFC and the MCFC have already been field – tested in many plants of 100 kW and more output power; in the case of the SOFC, market entry has failed due to the lack of reliable materials and the resulting high costs. Therefore, in order for fuel cells to be economically competitive with es­tablished technologies of mobile and stationary energy con­version, a drastic cost reduction must be achieved, both for the fuel-cell stack itself and for the auxiliary systems required for their operation. In vehicle applications, the still open ques­tions of fuel supply (infrastructure, H2 production and H2 stor­age) must also be addressed.