  In wind- and solar-dominated energy systems it has been assumed that there
are synergies between producing electricity and electrolytic hydrogen since
electrolysis can use excess electricity that would otherwise be curtailed.
However, it remains unclear whether these synergies hold true at higher levels
of hydrogen demand and how they compare with benefits of off-grid, islanded
hydrogen production, such as better renewable resources and cost savings on
electronics due to relaxed power quality standards. Using a mathematical model
across two geographical locations for Germany, Spain, Australia, and Great
Britain, we explore trade-offs and synergies between integrated and islanded
electrolysers. Below a certain threshold, between 5% and 40% hydrogen share
depending on the country, integrated electrolysers offer synergies in
flexibility and reduced curtailment. Above these thresholds, islanded
electrolysers become more favourable. Without cost advantages, systems
including islanded electrolysers in Germany achieve up to 21% lower hydrogen
costs than systems with only integrated electrolysers. With 25% island cost
advantage, this benefit rises to 40% lower hydrogen costs. Our study identifies
three investment regimes with country-specific transition points that vary
based on island cost advantages and each country's renewable resources. Based
on our results we provide guidelines for countries considering how to deploy
electrolysers.
