  Prospective power supply systems based on Renewable Energy Sources require
measures to balance power generation and load at all times. The utilisation of
storage devices and backup power plants is widely suggested for this purpose,
whereas the best combination is still to be found. In this work, we present a
modelling approach to systematically study scenarios of future power supply
systems with a high share of electricity originating from wind and solar
resources. By considering backup as a subordinate source of electricity with
energy-only costs, the approach is independent of the actual full-load hours of
the backup power plants. Applying the approach to multi-year meteorological
data for Germany, cost-optimised combinations of storage devices and backup
power are identified. We find that even in scenarios with significant excess
generation capacities there is a need for storage devices or backup power
plants with discharging power on the same order as the average load to balance
the system at all times. Furthermore, these capacities seem to be required in
some years of the multi-year period only. Our results imply that the societal
need of having electricity available at all times can likely be satisfied by
installing over-capacities only, whereas a balance has to be found between
installing additional backup or storage or generation capacities.

Physics and Society