  Models for long-term investment planning of the power system typically return
a single optimal solution per set of cost assumptions. However, typically there
are many near-optimal alternatives that stand out due to other attractive
properties like social acceptance. Understanding features that persist across
many cost-efficient alternatives enhances policy advice and acknowledges
structural model uncertainties. We apply the modeling-to-generate-alternatives
(MGA) methodology to systematically explore the near-optimal feasible space of
a completely renewable European electricity system model. While accounting for
complex spatio-temporal patterns, we allow simultaneous capacity expansion of
generation, storage and transmission infrastructure subject to linearized
multi-period optimal power flow. Many similarly costly, but technologically
diverse solutions exist. Already a cost deviation of 0.5% offers a large range
of possible investments. However, either offshore or onshore wind energy along
with some hydrogen storage and transmission network reinforcement are essential
to keep costs within 10% of the optimum.
