A report from CarbonFree, a UK research firm, takes a look at different ways of storing renewable energy. For example, the London Underground is the world's largest man-made geothermal energy system, and finding ways to store all of the heat generated down there as energy would go a long way toward cooling the Tube off as well as power some of London.
This isn't as sci-fi as it sounds. There is a system in place in the Netherlands that takes the heat from tarmac in the summer and stores it in an aquifer until, get this, it can be used to heat buildings in the winter. CarbonFree sees all types of technologies - flywheels, compressed air, biofuels and hydrogen fuel cells - as ways to store renewable energy. The conclusion is that biofuels win for now, but changes will come to the marketplace soon. You can read the report, titled "Watts In Store - Storing Renewable Energy" for 95 £ at CarbonFree's website.