Balance of plant

Everything in an electrolyser plant other than the stacks themselves.

Power conversion, water treatment, electrolyte or water circulation, thermal management, gas separation, drying and purification, compression, instrumentation and control. Commonly abbreviated to BoP.

It matters for two reasons that are often underweighted. It represents a substantial share of both capital cost and energy consumption, and reliability studies of electrolyser plants consistently find that a large proportion of unplanned downtime originates outside the stack, in equipment that is individually well understood.

It also interacts with stack condition. A circulation pump losing performance changes flow distribution, which changes temperature spread across the assembly, which changes cell behaviour. Monitoring the two halves separately tends to produce a diagnosis that blames the stack for a mechanical problem.