The main role of solar energy in the future will be that of direct heating. Wind, like the sun, is 'free' and is increasingly harnessed for electricity. Over 5000 megawatts capacity is now installed around the world. However, similar storage and back-up issues arise as for solar. It is not always available when needed, and some means is required to store energy or provide substitute capacity for windless periods.
Geothermal energy comes from natural heat below the earth's surface.
Where hot underground steam can be tapped and brought to the surface it may be used to generate electricity. Such geothermal sources have potential in certain parts of the world. Some 6000 MWe of capacity is operating. There are also prospects in other areas for pumping water underground to very hot regions of the earth's crust and using the steam thus produced for electricity generation. Most forests and agricultural crops are technically capable of being converted into some form of energy, even if the primary purpose of the crop is to provide food.