Quantifying power system flexibility provision

In power systems, flexibility can be defined as the ability to cope with variability and uncertainty in generation and demand. The ongoing energy transition is affecting how much flexibility is required, but also who should provide it: some existing solutions are being phased out, while new solutions' entire business models are based on providing flexibility (e.g. storage or demand response). With the intention of condensing information and rationalising debates, a significant number of methods have been proposed to quantify various facets of flexibility; this paper reviews and classifies them depending on the question they attempt to address. We propose a pair of novel tools to quantify a comparatively unexplored aspect of flexibility: who is providing it. These frequency spectrum analysis based tools separately quantify flexibility provision on the annual, weekly and daily timescales. The tools' effectiveness and versatility is demonstrated through several example applications, analysing both historical and prospective power systems, in several geographical locations with contrasting characteristics. The proposed tools are of particular value to the capacity expansion planner, allowing them to quantify changes in flexibility provision as new solutions are introduced, or as carbon taxes, generation and interconnector capacities evolve.