renewable energy: hydroelectricity (dams, tides and waves)
Dams are the primary source of hydroelectricity, and are engineered so as to allow the operators to control flow to match energy needs. The reservoir behind the dam represents potential energy. At a hydroelectric dam, water from the reservoir behind the dam is directed through pipes containing turbines. The water flows when the dam is released. Water spins turbines, which drive generators to produce electricity. Electricity can be reliably produced even if rain and snowfall varies.
A great characteristic of hydropower production is that energy can be stored easily. If more water is flowing over a dam than electricity demands are at the time, the electricity can be used to pump water back into the reservoir so it can flow down again later. Obviously, some energy is lost in the process, but this ability to store energy using the original design of the plant is unique to hydropower – coal and gas burning plants can’t store energy in a similar way. Electricity produced from hydro plants is cheap and fairly clean. After all, the fuel – flowing water – is free and there are absolutely no emissions. The turbine generators are relatively simple and low maintenance.
Small hydroelectric plants are becoming very affordable and are environmentally friendly. They produce clean energy from the renewable resource, like a small river. By the very nature of their small size, small dams avoid many of the environmental issues of larger projects (water stagnation, erosion, etc...). Even though water mills are mostly historical artifacts, which are restored to maintain function, in the United States and other 1st-world countries, upgraded water mills are still useful (for jobs like rice harvesting etc...) in places like Nepal, India and China. Mills are still one of the original zero-carbon sources of energy, fighting climate change before that was even a term for the consequences of man-made pollution (also, before man-made climate change was even an issue).
One alternative system for hydro energy is the barrage tidal system. A barrage is in essence a dam across an estuary. Slices in the dam let water through and turbines in the slices generate electricity. Power is generated when the water flows in either direction. At high tide, the water is trapped behind the barrage and gradually released through the turbines, generating power in much the way a hydroelectric dam on a river would.
In concept, an artificial reef designed for renewable energy would have a series of turbines along a structure that is much more open to ocean flow than a barrage. It would modulate with the flow of the tide and the wave action much the way a naturally occurring reef does.
Tidal stream generators operate in the flow of tides. In many ways, they are like underwater wind mills, with the flow of water caused by tides or ocean current turning the turbines instead of the wind. Engineers are exploring many possibilities of tidal stream generators in the hopes of finding the most reliable, efficient and affordable system.
Another source of ocean energy are waves. Swell energy can be harnessed with buoys. As buoys rise and fall with the waves, the action drives a piston in the center of the buoy that turns a generator. In large scale projects, dozens of buoys would be operational, and the resultant electricity would be carried to shore by a single underground cable. Yet another experimental approach uses a polymer cable that produces electricity when it's stretched. In the experiment, the polymer cable tethers a buoy. As the buoy rides the waves, the repeated stretching and relaxing of the tether generates electricity.
Off of the coasts of France, Portugal, Ireland and the UK, among other countries, there's a growing focus on harnessing the energy of the ocean. This includes tidal barrages, tidal stream turbines, and even a hydrologic "wave farm" (off the coast of Portugal).
Barrage tidal system: