Until quite recently, humans burned wood, animal dung, or coal for fuel. These heat sources were used to cook, forge iron, and a myriad of other things.
In a 21st Century paradigm we have learned so much more about global energy resources: there are vast reserves of oil and coal, which while not the best for the environment is plentiful and relatively cheap. Use of fossil fuels has radically changed our world in the past 100 years.
The global economic engine is built on a petrochemical foundation, but in terms of energy, extracting it from the ground by exploring, drilling, refining, and transporting to end users is a very dirty and wasteful process. Fortunately, cleaner energy sources are coming into play these days.
Solar of course is nearly free after the equipment is built (it must be maintained). The hindrance to solar in years past was the very high cost and inefficiency of the solar cells. Costs of manufacture have dropped and while efficiency has limits, hundreds of thousands of solar installations are in operation in America today. Of course cloudy climates don't suit solar, so it's only good in some places.
Wind energy got a big public relations boost when oilman T. Boone Pickens made a public appeal to further develop it in 2008. However, windmills have been used for over 100 years for water well pumps; what's changed is the application (power generation) and the efficiency of the design. This is indeed a growth industry.
A companion technology is wave energy generation. One company making a "go" of this technology offes a lever-type device.
A different version of the same application (wave power) uses a long snake-like apparatus which rides the waves and has a turbine in its tail which turns.
Still another company wants to have large "flippers" on the ocean surface which bob up and down with the waves, activating linkage which turns a generator. A design of this type is going online in Israel soon.
Another intriguing device is called SeaRaser. It does not generate electricity, but it uses the ocean wave action to pump water uphill as much as 650 feet. This water could be retained in a reservoir and used to generate hydroelectric power for virtually nothing beyond construction and maintenance costs!
Geothermal is another promising energy source of our planet. Incredible pressures inside the earth, along with the radioactive decay of rocks, keep the interior very hot. This limitless supply of energy can be tapped by simply drilling down a few thousand feet where the rocks are hot. Water is then piped through the hot region and viola! You have steam to turn an electric turbine.
Iceland has used geothermal energy for years as the volcanic island has such rocks very near the surface.
The energy future of earth indeed looks bright as we can boldly move away from polluting technologies such as oil and coal, and gradually tap into "free" sources that have always been with us: wind, solar, wave, and geothermal.