  Cost optimal scenarios derived from models of a highly renewable electricity
system depend on the specific input data, cost assumptions and system
constraints. Here this influence is studied using a techno-economic
optimisation model for a networked system of 30 European countries, taking into
account the capacity investment and operation of wind, solar, hydroelectricity,
natural gas power generation, transmission, and different storage options. A
considerable robustness of total system costs to the input weather data and to
moderate changes in the cost assumptions is observed. Flat directions in the
optimisation landscape around cost-optimal configurations often allow system
planners to choose between different technology options without a significant
increase in total costs, for instance by replacing onshore with offshore wind
power capacity in case of public acceptance issues. Exploring a range of carbon
dioxide emission limits shows that for scenarios with moderate transmission
expansion, a reduction of around 57% compared to 1990 levels is already cost
optimal. For stricter carbon dioxide limits, power generated from gas turbines
is at first replaced by generation from increasing renewable capacities.
Non-hydro storage capacities are only built for low-emission scenarios, in
order to provide the necessary flexibility to meet peaks in the residual load.
