  The synthesis of methanol from captured carbon dioxide and green hydrogen
could be a promising replacement for the current fossil-based production. The
major energy input and cost driver for such a process is the electricity for
hydrogen production. Time-variable electricity cost or availability thus
motivates flexible operation. However, it is unclear if each unit of the
process should be operated flexibly, and if storage of electricity or hydrogen
reduces the methanol production cost. To answer these questions, we modeled a
Power-to-Methanol plant with batteries and hydrogen storage. Using this model,
we solved a combined design and scheduling optimization problem, which provides
the optimal size of the units of the plant and their optimal (quasi-stationary)
operation. The annualized cost of methanol was minimized for a grid-connected
and a stand-alone case study. The optimization results confirm that storage,
especially hydrogen storage, is particularly beneficial when the electricity
price is high and highly fluctuating. Irrespective of the presence of storage,
the whole Power-to-Methanol plant should be operated flexibly: even moderate
flexibility of the methanol synthesis unit significantly reduces the production
cost.
