As proposals for solar-plus-storage and other hybrid power plants surge in the United States, the California ISO, whose transmission system is in the epicenter of the trend, has launched a new initiative aimed at smoothing the integration of such resources into grid operations.
Hybrids account for nearly 50% of the 74,490 MW in the ISO's interconnection queue as of Aug. 26, according to an S&P Global Market Intelligence analysis of publicly available data. More than 31,350 MW come from roughly 100 battery-backed solar projects and around 5,320 MW are proposed at a handful of large wind farms coupled with energy storage, some of which appear to be offshore opportunities, which are attracting considerable developer interest.
While recognizing that projects under study in its interconnection queue can be speculative, the California ISO expects the installed capacity of hybrid resources "will grow significantly in the coming years," the grid operator said in a discussion paper issued in July. Energy storage paired with wind and solar "is viewed as a means to better optimize variable energy resources and creates a synergy to help the grid accommodate and integrate more renewable energy resources to reduce greenhouse gases," the ISO said.
The grid operator will hold a technical meeting on the new initiative on Aug. 27 ahead of issuing an initial straw proposal for tackling technical challenges and opening market opportunities in September. A final proposal is scheduled for April 2020.
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'A very important issue'
In comments to the ISO earlier in August, developers, utilities and other stakeholders largely welcomed the initiative.
Southern California Edison Co. noted the ability of hybrid resources to help reduce overgeneration during the middle of the day, fueled largely by stand-alone solar photovoltaic projects, and to mitigate the need for resources to ramp up quickly as the sun sets.
"Helping [hybrid resources] to better integrate is a very important issue," an official at the Edison International utility affiliate said in Aug. 13 comments.
Pacific Gas and Electric Co., however, in Aug. 14 comments, asked whether the grid operator was "rushing into this initiative," saying that "there are few concrete examples and data from such resources on how they perform and what issues they face." Focusing on solar-plus-storage facilities as the dominant technology proposed "may simplify the rulemaking process and allow the rules to more accurately reflect the specific resources under consideration," said representatives of the PG&E Corp. operating subsidiary.
But all hybrid resources, not just battery-backed solar arrays, should be considered in the initiative, John Sterling, director of market and policy affairs for First Solar Inc. argued in comments filed Aug. 13. Solar-wind and solar-wind-battery projects are gaining traction, he added.
"Developers are seeing increasing demand for hybrid resources, which can provide dispatchable solutions and offer capacity attributes into the market more cost effectively than conventional resources," Sterling said. Given that such plants "are becoming the predominant design under development in many markets," it will be critical for California to incorporate "their full suite of capabilities into the market," he said.
The California Energy Storage Alliance asked the grid operator to consider how developers plan to operate hybrid resources and use federal tax credits, which place limitations on charging from the grid, and support system reliability. "Clearly, the resource composition of the CAISO's market is changing and the [grid operator] should take steps to ensure its market promotes rich participation while meeting reliability needs," said Alex Morris, the advocacy group's vice president of policy and operations.

