9000 Hours Operation of a 25 Solid Oxide Cells Stack in Steam Electrolysis Mode
The total duration of the test was almost 9100 hours, split between 8300 current hours and 800 standby hours induced by incidents that occurred during the test. Incidents included losses of the steam and current supplies. After each incident, the stack performance was recovered, indicating a good robustness of the stack towards unexpected and potentially damaging conditions. The main issue during the test has been the considerable increase of the voltage drop between the positive end plate and the bottom stack repeat unit (SRU), before a sharp improvement occurred after about 2000 hours of operation, which has influenced the stack temperature distribution and in turn individual SRU degradation. The maximum observed voltage drop culminated at 2.73 V, before stabilising around 0.6V.
The stack has shown an overall voltage degradation of 2% per thousand hours, as can be seen on Figure 1. The corresponding voltage losses were 613 mV and 678 mV per thousand hours, for 0.57 and 0.72 A/cm2 current densities, respectively. This in turn corresponds to an average degradation in the range 0.24 – 0.27 mV per thousand hours per SRU.
Moreover, the stack temperature has been adjusted on a few occasion during the test, which highlighted the possibility of using the wide operating temperature range of the solid oxide technology to counter balance the stack degradation and increase the lifetime. Indeed, allowing the stack to increase at a rate of 3°C per thousand hours produced a reduced degradation rate of 1.80 % per thousand hours.