  To limit the global temperature increase to 1.5 degrees Celsius, fossil-free
energy systems will be required eventually. To understand how such systems can
be designed, the current state-of-the-art is to apply techno-economical
optimisation modelling with high spatial and temporal resolution. This approach
relies on a number of climatic, technical and economic predictions that reach
multiple decades into the future. In this paper, we investigate how the design
of a fossil-free energy system for Europe is affected by changes in these
assumptions. In particular, the synergy among renewable generators,
power-to-heat converters, storage units, synthetic gas and transmission manage
to deliver an affordable net-zero emissions system. We find that levelised cost
of energy decreases due to heat savings, but not for global temperature
increases. In both cases, heat pumps become less favourable as surplus
electricity is more abundant for heating. Demand-side management through
buildings' thermal inertia could shape the heating demand, yet has modest
impact on the system configuration. Cost reductions of heat pumps impact
resistive heaters substantially, but not the opposite. Cheaper power-to-gas
could lower the need for thermal energy storage.
