  The rapid expansion of distributed energy resources (DERs) is one of the most
significant changes to electricity systems around the world. Examples of DERs
include solar panels, small natural gas-fueled generators, combined heat and
power plants, etc. Due to the small supply capacities of these DERs, it is
impractical for them to participate directly in the wholesale electricity
market. We study in this paper an efficient aggregation model where a
profit-maximizing aggregator procures electricity from DERs, and sells them in
the wholesale market. The interaction between the aggregator and the DER owners
is modeled as a Stackelberg game: the aggregator adopts two-part pricing by
announcing a participation fee and a per-unit price of procurement for each DER
owner, and the DER owner responds by choosing her payoff-maximizing energy
supplies. We show that our proposed model preserves full market efficiency,
i.e., the social welfare achieved by the aggregation model is the same as that
when DERs participate directly in the wholesale market. We also note that
two-part pricing is critical for market efficiency, and illustrate via an
example that with one-part pricing, there will be an efficiency loss from DER
aggregation, due to the profit-seeking behavior of the aggregator.
