In this study,we employed a conservative approach for the retirement of coal capacity. With the EPA's impending regulations on CO2 emissions from existing power plants,it is possible that
there could be significantly more coal plant retirements. If this occurs and their capacity is replaced by natural gas combined cycles,the emissions-reduction potential and cost of RPS policies would be impacted. A shift from coal to natural gas as the marginal generation in off-peak hours would decrease solar's competitiveness with wind for meeting the RPS,thus increasing the cost of solar RPS preferences. In future work,we will more rigorously examine this phenomenon.