This SOFC mounts the cell featuring the operating temperature of 500°C, which the company announced in October 2005. The operating temperature of the product is at least 300°C lower than that of the existing SOFC.

The lower operating temperature results in thinner heat insulating material, hence smaller size. Further, because the cell diameter is as small as 5 mm, it is less likely to be affected by thermal strain due to temperature variation.

Thus, the new SOFC also supports partial-load operation in which the output is lowered when less electricity is required, such as at night. TOTO explains that partial-load operation with the output reduced to 300 W has nearly the same efficiency as 700 W.

TOTO aims to achieve power generation efficiency of more than 46% (AC transmission end efficiency; lower calorific value standard) and heat recovery efficiency of 35% or higher for the power generation system to be used in the field test.

The target volume of the system, including the auxiliaries, is 130 L, equivalent to that of an outdoor type air conditioning unit, and the target weight is 50 kg. The size of the power generation module itself is 280 (H) x 480 (W) x 390 (D) mm (approx. 52 L).

In order for the cell to operate at a temperature around 500°C, cerium oxide and lanthanum gallium oxide are used for the electrolyte. This electrolyte has a problem; it reacts with the nickel (Ni) in the fuel electrode during the firing process and increases internal resistance, thus failing to achieve a sufficient electromotive force.

TOTO reportedly succeeded in preventing the reaction with Ni by forming a nickel cerium oxide as an intermediate layer provided between the electrolyte and fuel electrode.

SOFC is expected to be the next-generation home-use fuel cell because it boasts the power generation efficiency of more than 45%. That percentage is about 10 points higher than that of PEFC, which is currently under the large-scale field test for use as the residential fuel cell.

Although SOFC has disadvantages such as a high operating temperature, the long time required for startup and shutdown, and breakage due to thermal strain, technical improvements are rapidly being introduced, as indicated by TOTO's latest development.

It seems that SOFC is beginning to outperform the PEFC, which has troubles in reducing the cost and extending the life.