In this paper, we develop a strategy to enhance cooperation among a group of renewable energy producers (REP). The REPs form a coalition that is managed by an aggregator. In the coalition, the REPs desire an allocation of the realized profit at each time that keeps them in the coalition (i.e., a stabilizing allocation). We first compute a necessary and sufficient condition of developing the joint contract under this scenario. Though the REPs get a stable allocation under our scheme, they lose some overall profit in the long-term. So next, we develop a strategy that can be used by the aggregator to improve the long-term profit of this coalition. The proposed method will support coalition formation and thus promote market integration of renewable energy. We also develop a numerical example to illustrate our results.