Nissan Motor Co Ltd prototyped an SOFC (solid oxide fuel cell) and exhibited it at the International Nanotechnology Exhibition and Conference (nano tech 2009), which is taking place at Tokyo Big Site from Feb 18 to 20, 2009.
The company plans to mount the SOFC on an electric vehicle and use it to charge the battery of the vehicle in the future.
SOFCs are highly efficient but have problems starting up. Therefore, they have been believed to be appropriate for cogeneration but not so much so for automobiles. And PEFCs (polymer electrolyte fuel cell) have been the leading candidates for automotive fuel cells.
However, some moves to use SOFCs for vehicles are coming to the surface, including the effort of the National Institute of Advanced Industrial Science and Technology (AIST), which showcased an SOFC as small as a sugar cube at the exhibition (See related article).
SOFCs have high operating temperatures and cannot be quickly turned on or off, requiring an electric vehicle to have a large-capacity battery. Therefore, Nissan plans to use the SOFC to charge the battery gradually but fully at a stretch so that it is not frequently started up and stopped.
It takes about 15 minutes to start up the SOFC. But it monitors the battery and determines the timing to start up. A vehicle that weighs 1t and has a motor output of several tens of kW will be charged by a 4kW SOFC.
Gasoline and light oil will be used as fuel because they can be provided by using existing infrastructure. Although separate reformers are not usually used for SOFCs, which are capable of internal reforming, but Nissan uses a reformer. However, the reformer is a simple one because it does not have to produce pure H2 like the one for a PEFC.
The efficiency of the reformer (caloric value of generated gas/caloric value of raw material gas) is 103%. It exceeds 100% because of H contained in water added for steam reforming.
Unlike an SOFC for cogeneration, the new SOFC does not recover waste heat. Still, high-temperature steam generated from the fuel cell is used for reforming.
The SOFC itself has an efficiency of 65 to 70%, and the efficiency as a charger is 50%, including other energy losses. Considering that the efficiency of the EV is 80%, the total efficiency is 40%.
When compared with gasoline vehicles, a 1t EV and 2t EV will have 1.8 and 2.5 times higher fuel economies, respectively, under the JC08 test mode. These are much higher than the fuel economies that can be realized by PEFCs.
Therefore, the SOFC, which has problems starting up, is more suited for commercial vehicles that run for a long time without interruption.
YSZ (yttria-stabilized zirconia oxide) electrolyte is used to create a large donut-shaped cell. In donut-shaped cells, fuel is usually supplied from inside and exhausted to outside. But the one developed by Nissan is equipped with a diametrically positioned partition plate so that fuel supplied from inside is returned to the area near the supply port and then exhausted.
A double-channel manifold penetrates through the center hole of the donut (or holes of the donuts), and the fuel is supplied into the inner channel of the manifold and exhausted from the outer channel. In respect to the air electrode, the fuel moves straight along the surface of the cell.
Nissan prototyped a stack of five donut-shaped cells and confirmed a gross output of 54W using H2 as fuel. It translates to an output density of 0.37kW/L. The prototype was strong enough to endure the temperature change from room temperature to 600°C in 60 minutes and a heavy load operation at 1kW/L for 30 minutes.