To establish the hydrogen production technology by renewable energy, we are developing hydrogen production system by high-temperature steam electrolysis using solid oxide electrolysis cells (SOEC). With this method, it is possible to produce hydrogen at low electrolytic voltage, as compared with alkaline-water electrolysis or polyelectrolyte film water electrolysis.

For the mass production of hydrogen by SOEC, it is necessary to connect multi-typed cell-stacks and to increase the electrolysis capacity. We have already developed the multi-typed cell-stacks pilot test plant for hydrogen production, and demonstrated that we can scale up with maintaining the performance of each stack, and that we can realize large-scale hydrogen production using this method. Furthermore, since the SOEC operating temperature range is from 600 to 800 °C, by providing heat exchangers in this system, the consumption energy of the whole system can be reduced.

However, considering actual usage for a long period of time, the degradation of the stack could cause the increase of the energy consumption of the whole system. Since the SOEC stack generates heat due to degradation, we considered using this heat effectively. Therefore, in order to clarify the temporal change of energy required for hydrogen production, we evaluated the initial degradation by continuous operation of the multi-typed SOEC stacks. In addition, we examined the energy change of the whole system accompanying the deterioration of the stacks.

In the test, the temperature in the module container was set to 650 °C. A constant current was applied to each line for a total of about 1000 hours.

In order to evaluate the characteristics of each cell-stack, the temperature was raised up to 700 °C every several hundred hours to obtain an I-V characteristic.

The initial deterioration characteristics of the stack were evaluated from the transition of the average cell voltage and the stack temperature of each stack.

The stack's I-V characteristics showed the same characteristics as the initial one for about 500 hours.

After 500 h, the stack temperature gradually increased relative to the set temperature (650 °C.) in module container, and the cell voltage also increased gently as compared to the initial cell voltage.

On the other hand, the consumption energy other than the stack decreased the heating energy of the module container due to the heat generation of the stacks. As a result, there was no significant change in the energy consumption of the entire system in this test plant. This demonstrates that even if the characteristic of the stack is degraded, the system performance can be maintained by using heat effectively.

A part of this study is based on the results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO) in fiscal 2014 to 2018.