  The flexibility of active ($p$) and reactive power ($q$) consumption in
distributed energy resources (DERs) can be represented as a (potentially
non-convex) set of points in the $p$-$q$ plane. Modeling of the aggregated
flexibility in a heterogeneous ensemble of DERs as a Minkowski sum (M-sum) is
computationally intractable even for moderately sized populations. In this
article, we propose a scalable method of computing the M-sum of the flexibility
domains of a heterogeneous ensemble of DERs, which are allowed to be
non-convex, non-compact. In particular, the proposed algorithm computes a
guaranteed superset of the true M-sum, with desired accuracy. The worst-case
complexity of the algorithm is computed. Special cases are considered, and it
is shown that under certain scenarios, it is possible to achieve a complexity
that is linear with the size of the ensemble. Numerical examples are provided
by computing the aggregated flexibility of different mix of DERs under varying
scenarios.
