How to characterize flexibility requirements of highly renewable energy systems over multiple timescales

This thesis addresses the question: Can we characterize requirements for flexibility over different timescales in electricity systems dominated by solar and wind power? Storage and flexibility play an increasingly important role, but there is great uncertainty about amounts of flexibility needed and how they depend on generation mix and demand, including electrification of heating and transportation. Furthermore, to inform investment and planning, it is valuable to characterize the timescales over which flexibility will be needed, as different resources can be used to shift energy over different time horizons. To address this problem, the analysis uses the novel application of three methods to disaggregate overall flexibility requirements into short-, medium-, and long-term requirements, without relying on assumptions about technology parameters or costs. These methods are illustrated using the case of Great Britain and results are used to draw insights into GB flexibility needs under future scenarios. Flexibility is required over multiple timescales, from less than hourly to interseasonal or longer. Overcapacity of renewables provides value in terms of avoided storage costs, particularly displacing requirements for the longest duration storage, though generation capacity beyond 120% of demand yields diminishing marginal returns. Heating electrification has a larger impact than EVs, though flexible heating can partially offset additional power capacity needs. In all cases, the capacity required to shift energy by up to a day was on the order of 1 TWh; this could account for over half of all energy shifted depending on flexible resource operation. Electricity systems with at least 80% of energy from solar and wind require 3-150 TWh to shift energy by weeks or longer. The required capacity to shift energy by more than one year could potentially be avoided using renewables overcapacity, dispatchable generation, or interconnectors.