Hydroelectric dams are the primary source of hydroelectric power worldwide; and hydroelectricity is the largest source of renewable energy worldwide for electricity generation, supplying over 1 billion people globally with low carbon electricity.. The prevalence of hydroelectricity, especially hydroelectric dams, is significant in the United States. Hydroelectric power produces over 35% of the renewable energy in the U.S., and over 5% of the total electricity in the U.S. Water is the steadiest and most consistent renewable energy source in the U.S. besides wind. (However, solar is projected to continue growing exponentially; and to become the most prevalent renewable energy source in the U.S., and worldwide.) In addition to dams, hydrokinetic power systems are a promising renewable energy source; and include tidal barrages, tidal stream generators, and wave farms (currently in research and development).
Hydroelectric dams emit virtually zero greenhouse gas emissions from the actual energy generation process of the dam. This makes hydroelectric dams an important source of low carbon emission energy, and an important clean (relatively clean, compared to fossil fuels) energy source. There are a small amount of GHG emissions produced from the required maintenance of dams, and ecological pollution generated from an operational hydroelectric dam (see below).
Hydroelectric dams work by generating energy from reservoirs; the reservoir behind a dam represents potential energy. When operators of a hydroelectric dam need to start generating energy, the water in the reservoir is released, and allowed to flow through the penstock towards the dam’s hydroelectric turbines. Water spins turbines, which drive generators to produce electricity. Electricity can be reliably produced by dams even if rain and snowfall varies. Here is a simple diagram illustrating how a hydroelectric dam works-
A great feature of hydroelectric 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. 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 hydroelectric plants is cheap and fairly clean. After all, the fuel – flowing water – is free and there are no emissions. The turbine generators are relatively simple and low maintenance. Once the dam is built, operational costs and maintenance costs for hydroelectric dams is low. The fuel for hydroelectric dams is water – free, abundant, renewable, and water itself is completely environmentally friendly, creating no pollution and no greenhouse gases. Unfortunately, the stagnant water in a dammed reservoir does create hazards to local ecosystems, harms and kills aquatic animal, plant life, and wildlife in downstream rivers; causes erosion and algal blooms, and degrades river water and ecosystems downstream. At times, a hydroelectric dam can generate GHG emissions from the decomposing plant life and ecological damage in its reservoir; and at other times, the dam acts as a carbon sink.
Most hydroelectric dams are large; for example, the Hoover Dam on the Colorado River in Nevada/ Arizona. The Hover Dam can produce over 2GW of energy, and running at its full production capacity, the Hoover dam can supply enough power for 1.3 million homes in the Nevada/ Arizona area where it operates (and even provides energy to homes in California). However, smaller sized dams also produce clean energy from a renewable resource, like a small river. By the very nature of their small size, small dams avoid the environmental issues (water stagnation, erosion, algal blooms, etc…) of larger projects .
Other forms of hydroelectricity include hydrokinetic systems for harvesting energy from tides and currents (tidal barrages, tidal stream generators), energy from ocean waves, and even the use of hydroelectric water mills in creeks. 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. Water 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 hydrokinetic energy generation system used throughout the world today is the tidal barrage system. A tidal barrage is in essence a dam before an estuary, river, bay, small lake, or other body of water. 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. Slices in the tidal barrage system in the rivers leading to the estuary let water through as the tide goes from high to low, and turbines in the barrage’s slices generate electricity.
Similar to the tidal barrage are tidal stream generators. Tidal stream generators operate in the flow of tides. In many ways, they are like underwater wind mills, but with the flow of water caused by tides or currents 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.
In addition to tidal hydropower systems, there are a variety of ways to harvest ocean wave energy to create renewable energy. More than 50% of the American population lives within 50 miles of coasts, which is significant because ocean renewable energy technologies are in the research and development stage in the United States, as well as throughout the world. Off of the coasts of France, Portugal, Ireland and the UK, among other countries, and Hawaii, there’s a few demonstration projects for R&D on harnessing the energy of the ocean.