Some 45 attendees at Fortune’s Brainstorm E conference crowded into a breakfast session moderated by Senior Writer Katie Fehrenbacher on Tuesday to share ideas. The discussion made clear that there are lots of interesting efforts underway to deal with periods when the wind doesn’t blow and the sun doesn’t shine.

But cost effective solutions remain elusive—particularly for seasonal, rather than short term, intermittencies.

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Batteries were a part of the discussion. Solutions that use solar panels combined with batteries to provide overnight power are already working. But using batteries to deal with seasonal problems, participants said, is not cost effective.

Bill Gross of IdeaLab told the breakfast group about a project he’s running that stores heat from solar panels in an insulated silo of rocks, which can hold the energy for a long period of time.

Could hot rocks be the answer? Not clear, but storage remains one of the most critical unanswered questions for renewable energy’s future.