  This paper proposes a simple and flexible storage model for use in a variety
of multi-period optimal power flow problems. The proposed model is designed for
research use in a broad assortment of contexts enabled by the following key
features: (i) the model can represent the dynamics of an energy buffer at a
wide range of scales, from residential battery storage to grid-scale pumped
hydro; (ii) it is compatible with both balanced and unbalanced formulations of
the power flow equations; (iii) convex relaxations and linear approximations to
allow seamless integration of the proposed model into applications where
convexity or linearity is required are developed; (iv) a minimalist and
standardized data model is presented, to facilitate easy of use by the research
community. The proposed model is validated using a proof-of-concept twenty-four
hour storage scheduling task that demonstrates the value of the model's key
features. An open-source implementation of the model is provided as part of the
PowerModels and PowerModelsDistribution optimization toolboxes.
