The slow-motion crisis at a Japanese nuclear plant has rekindled worries about relying on atomic power for electricity. Climbing oil and gasoline prices are again draining wallets. And President Barack Obama Wednesday outlined plans to cut U.S. reliance on foreign oil, including boosting ethanol output.

The times would seem to favor the rise of renewable energy—power derived from sources such as solar, wind and biomass. Renewables have grown up in recent years, attracting blue-chip investors and fostering supply chains that span continents. Technologies have improved and costs have fallen.

But renewable energy still hasn’t outgrown criticisms that have dogged it for years: It remains too expensive to compete head-to-head with fossil fuels, and it is too dependent on government subsidies. Most renewable energy faces another major obstacle. Much of it remains intermittent—and technical advances that could make it a reliable, full-time power provider remain elusive.

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