'Hydro' or water power refers to the energy of the movement of water being converted into electricity, hence the more common term 'hydro-electric power'. You've probably heard of, or even visited, the Hoover Dam. So as long as we have moving water we will have access to a sustainable and renewable energy source.
But there are many ways to utilize the energy of our waters to create useable electricity. Here' we'll introduce some of the ideas currently being used or researched in the general field of Hydro Power.
First up is Ocean Power, often called marine power. The moon's gravitational pull as it revolves around us, ans the sun's as we revolve around it, create the high and low tides around the world. This, combined with local weather patterns and ocean currents create huge movements of water which harness incredible amounts of kinetic energy. Ocean power is the phrase associated with capturing some of this energy and converting it into useable electrical power.
Or Ocean Current Power - who hasn't heard of the Gulf Stream? - This is a flow of water so immense that it is bigger than the flow of every river on the planet combined. If we could capture just 20% of its energy we could power the entire USA. Such technology is still in its infancy, and is largely based around turbine principals similar in concept to wind-turbines, but designed and engineered to live underwater.
More specifically, wave power refers to the kinetic energy in ocean waves and its use either to drive turbines for electrical generation, or put to other mechanical uses such as pumping stations.
This concept seems the most like science fiction of the various hydro-powers. However, it is a process which has virtually no fuel cost, and emits no noxious gases into our atmosphere. The idea is to extract energy from the difference in salinity between fresh water and sea water through the process of osmosis. A test plant has been constructed in Norway and has shown good early results - albeit on a small scale of just a few kW - and plans are being developed for a large scale commercial osmotic power plant right now.
Still in research and development phases the idea is to capture incoming tidewaters in lakes and as the tide goes out the water is channeled to drive turbines. Advances in turbine technology are opening up new opportunities to exploit tidal energies, which are easier to plan for than the more weather-related wave energies.
Low Head Hydro Power
Unlike at the Hoover Dam Power Plant which uses a 'high' head of pressure, there are opportunities for smaller scale power plants along less dramatic rivers. This uses 'low head' or 'dam-less' river flows, usually with a smaller waterfall, and a hydro-kinetic turbine. This special turbine can take advantage of the natural flow of a river and provide smaller amounts of electricity to local users or into the main power grid.
So there's a lot going on in the world of hydro-power. Without a doubt it's an area of massive energy supply that our current technology has barely-tapped, but is getting better every day.