  Future ``net-zero'' electricity systems in which all or most generation is
renewable may require very high volumes of storage, provided jointly by a
number of heterogeneous technologies, in order to manage the associated
variability in the generation-demand balance. We consider the problems of
scheduling and dimensioning such storage. We develop a value-function based
approach to optimal scheduling, and show that, to a good approximation, the
problem to be solved at each successive point in time reduces to a linear
programme with a particularly simple solution. We show that approximately
optimal scheduling may be achieved without the need for a running forecast of
the future generation-demand balance. We examine the applicability of the above
theory to future GB storage needs, and discuss how it may be used to enable the
most economic dimensioning of such storage, with possible savings of tens of
billions of pounds, relative to the use of a single technology.
