One of the cleanest forms of electrical generation is one of the oldest sources of power known to mankind. While it was originally used primarily for the generation of mechanical power, the concept, as used to generate electrical, power is very similar. That is, a flow of water turns a rotating device to generate power. For mechanical power it was a water wheel, for electrical power it is a turbine. Even this clean, renewable source has its disadvantages when employed on an industrial scale.
With this method, on industrial scales, the water is simply a medium through which the power of gravity is harnessed. The only natural sources for this type of power are waterfalls where gravity multiplies the force of the flowing water. Where waterfalls of sufficient size are unavailable, dams are used to back up the flow of water, raising its level and providing a sort of artificial waterfall to harness the energy.
The disadvantages of dams from an ecological as well as a safety standpoint have been debated for most of the past century and a half, with commercial interests normally prevailing. The placement of a dam in an existing channel will almost always result in habitat loss and displacement for species that lived along the watercourse and this is especially true for the sorts of dams required for large hydroelectric projects.
Tidal generators are also under development which can be placed near shore to take advantage of the in and out flow of water generated by tides. Other water turbines can be placed off shore in ocean currents that would drive those mechanisms.
This is another venerable technology that has made improvements in efficiency over the time it has been in use. Windmills have been in use for centuries to convert grains to flour, and the smaller versions were a nearly ubiquitous feature of American farmsteads for the generation of mechanical or electrical power.
Modern windmills with their tall standards supporting large radius vanes that can turn and generate power in even light winds are often reviled for obstructing the scenery and destroying its beauty. But a time may soon come when this blight on the horizon may be more acceptable than transportation by foot.
These large wind collectors will no doubt become more and more essential to satisfying our energy demands as the century goes on, but they are still a part of the infrastructure of the old energy paradigm which holds that large scale centralized generating facilities and their efficiencies are the most reasonable way to address our needs. An alternative to, or more properly, an augmentation of that model exists in smaller efficient wind turbines that can provide electricity, as a part of a mixed generation system, to individual homes or small industrial facilities.