Maintaining a diverse fleet of power plants in California and taking advantage of the complementary nature of wind and solar power are two of the findings in a renewables portfolio standard (RPS) integration study recently released by the California Independent System Operator Corporation (ISO).

The ISO and its study partners, including GE Energy Consulting, gained insights about grid dynamics through the "Integration of Renewable Resources—Operational Requirements and Generation Fleet Capability at 20% RPS." The study assumes California will add 2,246 MW of solar and 6,686 MW of wind resources by 2012.

Among the findings:

1. Green power can swing in output by several thousand megawatts in as little as 20 minutes.
2. Wind and solar power can offset each other’s variable output. Wind is often plentiful at night and during early mornings, when the sun is not in full force. On the other hand, solar power generates best on hot, sunny afternoons when the wind often eases up.
3. Over-generation resulting from high winds and high hydro can lead to too much power during periods when demand is low, which raises economic and reliability concerns.
4. Efficiencies could be gained if there were more participation in the ISO’s new market, which allows the ISO greater dispatch flexibility for meeting real time needs of the grid.
5. Flexibility is important and conventional power plants are needed to provide quick "ramp up" and "ramp down" capabilities to compensate for intermittent resources.
6. New operational tools for ISO dispatchers are required to stay a step ahead of fluctuations in generation. Advanced weather forecasting tools, for instance, are needed to anticipate variable output.

"This study provides a thorough analysis of the capability of the power grid to effectively manage an oncoming wave of highly intermittent energy resources and confirms the ISO is ready to manage the grid reliably under 20 percent RPS," said Vice President, Market and Infrastructure Development Keith Casey. "However, the study also affirms the critical role the ISO market and the existing fleet of conventional resources will need to play in balancing the variable output of renewables."