[Dr. Scott W. Tinker] Biomass- burning wood- was our first major energy source, but not too long after came hydropower. First as transportation floating down a river, and then has water wheels to grind grain, and finally as electricity. Water ran our first major power plants and it still accounts for three percent of global energy today, and it does that by tapping into Earth's gravitational pull. When I lift this tank the energy I use to counteract gravity is stored in the water. It's called potential energy. When gravity pulls the water back down either from a cloud or down a waterfall or through a dam, the potential energy is turned into kinetic energy as the following water turns a turbine. Real hydro turbines are far larger of course. The water turns the Pelton wheel or impeller which is connected to a generator. One of the benefits of hydro is that you can store massive amounts of potential energy and a huge reservoir of water behind a dam. This makes hydro plants the largest power stations in the world by far. The Itaipu Dam in Brazil for instance is the equivalent of 14 nuclear reactors. Gravity does the work so the electricity is cheap and the plant burns no fuel, meaning no emissions. The other great benefit of hydro is it can be base load or peak load power as long as there's water it can run all the time. Or you can stop and start it it almost instantly to follow the demand curve. Many benefits but as usual it comes with many challenges. In the developed world, we've used nearly all the good hydro sites. The most productive new sites are in developing Asia, Africa, and Latin America, but the plants are enormous and very expensive. Not easy projects for poor countries. Perhaps more important, flooding a river valley to make a reservoir means covering towns and displacing people. It also impacts the local environment, fish migration, communities downstream, and water supply; a growing concern. As developing nations grow, they'll gradually build large hydroelectric plants. The rest of the world is refitting old plans with more efficient equipment, or removing them and replacing it with other types of generation. Hydro will still be important, but as overall energy demand grows it will meet a smaller percentage of it.