  The potential of distributed energy resources in providing grid services can
be maximized with the recent advancements in demand side control. Effective
utilization of this control strategy requires the knowledge of aggregate
flexibility of the distributed energy resources (DERs). Recent works have shown
that the aggregate flexibility of DERs can be modeled as a virtual battery (VB)
whose state evolution is governed by a first order system including
self-dissipation. The VB parameters (self-dissipation rate, energy capacity)
are obtained by solving an optimization problem which minimizes the tracking
performance of the ensemble and the proposed first-order model. For the
identified first order model, time-varying power limits are calculated using
binary search algorithms. Finally, this proposed framework is demonstrated for
different homogeneous and heterogeneous ensembles consisting of air
conditioners (ACs) and electric water heaters (EWHs).
