San Francisco Chronicle: This decade’s historic high prices for oil and natural gas have stoked the rise of renewable power and alternative fuels. As fossil fuel prices smashed record after record, options like ethanol, hybrid electric cars, solar power and wind looked better and better. Now oil costs less than half what it did this summer. Ditto natural gas. If prices keep dropping and stay down, future fuels like cellulosic ethanol and biodiesel will have a harder time competing. So will solar and wind …