Low-head hydro power
Low-head hydropower refers to the development of hydroelectric power where the head is typically less than 20 metres, although precise definitions vary. Head is the vertical height measured between the hydro intake water level and the water level at the point of discharge. Using only a low head drop in a river or tidal flows to create electricity may provide a renewable energy source that will have a minimal impact on the environment. Since the generated power (calculated the same as per general hydropower) is a function of the head these systems are typically classed  as small-scale hydropower, which have an installed capacity of less than 5MW. Comparison to conventional hydro. Most current hydroelectric projects use a large hydraulic head to power turbines to generate electricity. The hydraulic head either occurs naturally, such as a waterfall, or is created by constructing a dam in a river valley, creating a reservoir. Using a controlled release of water from the reservoir drives the turbines. The costs and environmental impacts of constructing a dam can make traditional hydroelectric projects unpopular in some countries. From 2010 onwards new innovative ecologically friendly technologies have evolved and have become economically  viable. Within low-head hydropower there are several of standard situations: Run-of-the-river: Low-head small hydropower can be produced from rivers, often described as run-of-river or run-of-the-river projects. Suitable locations include weirs, streams, locks, rivers and wastewater outfalls. Weirs are common in rivers across Europe, as well as rivers that are canalized or have groynes. Generating significant power from low-head locations using conventional technologies typically requires large volumes of water. Due to the low rotational speeds produced, gearboxes are required to efficiently drive generators, which can result in large and expensive  equipment and civil infrastructure. Tidal power: In combination with a lagoon or barrage the tides can be used to create a head difference. The largest tidal range is at the Bay of Fundy, between the Canadian provinces of New Brunswick and Nova Scotia, Canada which can reach 13.6m. The first tidal range installation was opened in 1966 at Le Rance, France. Low-head pumped seawater storage: Currently at very low TRL levels but in the coming decade these technologies could become part of the energy system. Dynamic tidal power: Another potentially promising type of low-head hydro power is  dynamic tidal power, a novel and unapplied method to extract power from tidal movements. Although a dam-like structure is required, no area is enclosed, and therefore most of the benefits of 'damless hydro' are retained, while providing for vast amounts of power generation. Low-head hydro is not to be confused with "free flow" or "stream" technologies, which work solely with the kinetic energy and the velocity of the water. Types of low-head turbines. Turbines suitable for use in very-low-head applications are different from the Francis, propeller, Kaplan, or Pelton types used in more conventional large  hydro. Different types of low-head turbines are: Environmental impact of low-head hydropower. A number of concerns have been raised about the environmental impacts of river current and tidal devices. Among the most important of these are: Weirs and groynes have historically been used for water management and to permit upstream riverine transportation. Weirs and groynes can have negative effects on river bathymetry and prevent upstream fish migration that will have an effect on local ecology and water levels. By installing low-head hydropower turbines on historic structures sediment transport can be increased along with new fish migration passages , either through the turbine itself or by installing fish ladders. Where large sites aren't cleared “the vegetation overwhelmed by the rising water decays to form methane – a far worse greenhouse gas than carbon dioxide”, particularly in the tropics. Low-head dams and weirs do not produce harmful methane. Groynes but also weirs prevent the transport of silt (sediment) downstream to fertilize fields and to move sediment towards the oceans. Low-head hydropower is typically installed close to areas where the energy is needed, preventing the need for large electrical transmission lines. Implementation and regulations. Government regulation. Most government regulation  comes from the use of waterways. Most low-head water turbine systems are smaller engineering projects than traditional water turbines. Even so, one needs to obtain permission from state and federal government institutions before implementing these systems . Some of the constraints faced with these systems in larger waterways are making sure waterways can still be used for boats and making sure that routes of migration of fish are not disturbed. Government subsidies. US government subsidies can be obtained for implementation of small-scale hydro facilities most easily through federal grants, namely green energy grants . A specific example is the Renewable  Electricity Production Tax Credit. This is a federal tax credit aimed at promoting renewable energy resources. To qualify, the hydro source must have a minimum capacity of 150 kW. This subsidy is given for the first ten years of production. Organizations receive $.011/kWh. . For hydroelectric projects, this subsidy expired on December 31, 2017 . Public perception. Since these are sustainable energy source, are non detrimental to the water sources they utilize and are visually not an eyesore, they are well regarded within the public sphere . However, there is little public and industrial knowledge of these systems as they are  still being tested to "answer real-world questions". As such, proponents and manufacturers of these systems have tried to bring them into public knowledge Lam, Tina Fairley, Peter