Characterising and maximising aggregate flexibility of heterogeneous energy storage units

Abstract

This thesis presents methods for the aggregation, scheduling and dispatch of fleets of heterogeneous storage units. We start with real-time control, where we present policies that are unambiguously optimal, regardless of the future; in terms of time-to-failure, energy-not-served and flexibility. In the first instance, this optimality is achieved by restricting the operating regime: to unidirectional operation with no cross-charging among devices. We present policies in continuous and discrete time, both for nominal operation and recovery between events. Once dispatch is guaranteed to maximise flexibility, it is coupled with a matching aggregate representation for scheduling ahead of real-time. As an exact characterisation of the capabilities of the fleet, this can also be used as a binary check on request feasibility, or to optimise ancillary service specification or perform comparisons among fleets. We then present a generic aggregation framework based on the aforementioned scheduling and dispatch routines, for application to coupled discharge-recovery operation. This is scalable, in the sense that it can be applied to a hierarchical aggregation structure and incorporates a complexity-reducing toolkit using a virtual battery fleet. Moreover, the framework inherits maximum discharging capabilities and guaranteed feasibility of round-trip operation, with the additional property of minimum recovery time. Later on, we justify the restrictions to the operating regime, by showing that these achieve optimality under the majority of ancillary services procured by system operators today. We then relax these conditions, and instead place specific restrictions on the variation of device parameters across the fleet. This provides an alternative means to precipitate set-theoretic optima, and we present a policy that achieves optimality in this new setting. An algorithm is provided, through which the user can implement any of the presented dispatch policies in discrete time systems or simulation studies.