Low head hydro power
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||The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. (January 2012)|
Low head hydro power applications use river current or tidal flows of 20 meters (approximately 66 feet) or less to produce energy. These applications do not need to dam or retain water to create hydraulic head; the head is only a few metres. Using the current of a river or the naturally occurring tidal flow to create electricity may provide a renewable energy source that will have a minimal impact on the environment.
- 1 Comparison to conventional hydro
- 2 Hydrokinetic turbines
- 3 Types of low head turbines
- 4 Installation of turbines in river current
- 5 Tidal power
- 6 Environmental impact of low head hydropower
- 7 Implementation and regulations
- 8 See also
- 9 References
- 10 External links
Comparison to conventional hydro
A low-head hydro project generally describes an installation with a fall of water less than 5 metres (16 ft). Most current hydroelectric projects require 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 bed, creating a reservoir. Using a controlled release of water from the reservoir creates the head required to turn the turbines. The costs and environmental impacts of constructing a dam make traditional hydroelectric projects difficult to construct.
Construction of a dam and reservoir has many environmental effects. For example, the damming of a river “blocks the movement both of fish upstream to spawn and of silt downstream to fertilize fields”. In addition, “the vegetation overwhelmed by the rising water decays to form methane – a far worse greenhouse gas than carbon dioxide”.
Since no dam is required, low-head hydro may dramatically reduce the following:
- The safety risks (of having a dam), avoiding the risk of a flash flood caused by a breached dam
- Environmental and ecological complications
- Need for fish ladders
- Silt accumulation in basin
- Regulatory issues
- The initial cost of dam engineering and construction
- Removing silt accumulation.
However, low-head units are necessarily much smaller in capacity than conventional large hydro turbines, requiring many more to be built for a given annual energy production, with some of the costs of small turbine/generator units being offset by lower civil construction costs. Just as for large hydro, not every site can be economically and ecologically developed; sites may be too far from customers to be worth installation of a transmission line, or may lie in areas particularly sensitive for wildlife.
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.
A "Hydrokinetic" turbine is an integrated turbine generator to produce electricity in a free flow environment. It does not need a dam or diversion. Instream Energy Generation Technology or IEGT places turbines in rivers, man made channels, tidal waters, or ocean currents. These turbines use the flow of water to turn them, thus generating electricity for the power grid on nearby land. In effect, IEGT is like planting windmills in the water and is environmentally friendly. While hydrokinetic includes generation from ocean tides, currents and waves, many researchers believe its most practical application in the near term is likely to be in rivers and streams.
A 35 kilowatt hydrokinetic turbine has been installed in the Mississippi River near Hastings, Minnesota. Underwater tidal turbines are propelled by tidal currents. If the viable river and estuary turbine locations are made into hydroelectric power sites “researchers estimat[e] that [the United States’] rivers and estuaries could provide up to 130,000 gigawatt-hours per year — about half the yearly production of the country's dams”
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 application turbines are:
- Axial Flow Rotor Turbine: This type of turbine consists of a concentric hub with radial blades, resembling a wind mill. Either a built in electrical generator or a hydraulic pump which turns an electrical generator on land provides the electricity.
- Open Center Fan Turbine: These turbines consists of two donut shaped turbines which rotate in the opposite direction of the current. This in turn runs a hydraulic pump that in turn drives a standard electrical generator.
- Helical Turbine: This type of turbine has hydrofoil sections that keep the turbine oriented to the flow of the water. The leader edge of the blades turns in the direction of the water.
- Cycloidic Turbine: The cycloidic turbine resembles a paddle wheel, where the flow of the water turns the wheel with lift and drag being optimized. Lift or flutter vanes looks like a huge Venetian blind.
- Hydroplane blades: They are made to oscillate by the flowing water, thus generating electricity.
- FFP Turbine Generator: This type of turbine uses a rim-mounted, permanent magnet, direct-drive generator with front and rear diffusers and one moving part (the rotor) to maximize efficiency.
- Gravitation Water Vortex Power Plant: This type of hydro power plant use the power of a gravitation water vortex, which only exists at low head.
Installation of turbines in river current
The turbines can be installed in a variety of ways, multiple banks set on pilings driven into the river beds or mounted on existing river structures such as bridge piers. These turbines operate in a “free flow” environment that does not require the damming or diversion of rivers. This approach does not disrupt natural ecosystems or interfere with aquatic and marine life. The turbine generators can be attached to bridge abutments or pilings, which minimizes disruption to river beds.
Turbines are to be deployed in arrays of multiple units spaced no less than 15 metres (49 ft) apart where the site conditions, depth, and needed infrastructure are suitable. Exact depth and spacing is determined based on site conditions, including current flows and water depth. Since the turbines do not block waterways, and the water passing through the device is not subject to high pressure, these systems are designed to not impede or damage fish or other wildlife. This entire structure is installed at a depth that avoids any interference with recreational or navigational uses of a water resource. The power will be transmitted by cable to conversion equipment located on shore. The conversion equipment will convert the power from DC to AC, adjust the voltage and connect to the power grid. This approach will be better suited to river current where the flow is in one direction as opposed to ocean shore locations.
A competing idea is to suspend the turbines from a floating barge. The turbines suspended from the bottom of a floating barge can accommodate changes in flow. The barges can be deployed and have the generators come on line more quickly with fewer disturbances to the river bed. The obvious disadvantage to the barge system would be interference with navigation and recreational use of the waterway. This system does have some advantages, installation costs may be less depending on river bottom conditions and maintenance and repair would be somewhat easier. Concern over the impact on seasonal flooding and ice conditions also must be considered with the barge system.
Tidal flow occurs due to the moving mass of water with speed and direction as caused by the gravitational forces of the sun and the moon, and centrifugal and inertial forces on the Earth's waters. Due to its proximity to the earth, the moon exerts roughly twice the tide raising force of the sun. The gravitational forces of the sun and moon and the centrifugal/inertial forces caused by the rotation of the earth around the center of mass of the earth-moon system create two "bulges" in the Earth's oceans: one closest to the moon, and the other on the opposite side of the globe.(CNW Group, 2008). This kind of energy is unique and different from traditional hydropower that has been around for centuries. There is no need to build a dam. Essentially a turbine is stuck in naturally flowing water. As the water flows, it turns a turbine. That is converted to electricity.
Tidal basin locations can also be developed using the low flow turbine technology as well. These areas are limited to ocean side locations and the difficulty associated with rotating the turbines to adjust to the direction of the tidal flow must also be accounted for. It would appear that the turbines suspended from under a floating barge would be better suited to the tidal application. The barge itself can be turned to face the direction of the tidal flow. It may also be more difficult to provide the areas for power conversion and connection to the power grid given the limited areas that can be developed to utilize tidal flows. Several demonstration projects are underway to study the feasibility of the tidal basin locations. Tidal turbines are a new technology used for tidal energy. They are similar to wind turbines and are arranged underwater in rows. They work best in areas with strong tides. They are also the least environmentally damaging of all the tidal power technologies, since they do not interfere with migration paths and the impact on basin bed is less as no construction is needed in the waterway itself.
In order for tidal power to work successfully it requires a tide difference of at least 5 metres (16 ft). Unfortunately there are only a few places where this occurs. This means tidal power plants cannot just be constructed anywhere. There are only a handful of sites on Earth with this type of tidal range. A demonstration project has begun in New York City. In the last four years, the federal commission has approved nearly a dozen permits to study tidal sites. Applications for about 40 others, all filed in 2006, are under review. No one has applied for a development license, Miller said. The site that is furthest along in testing lies in New York’s East River, between the boroughs of Manhattan and Queens, where Verdant Power plans to install two underwater turbines this month as part of a small pilot project.
Ocean and tidal currents can provide an indefinite supply of emission-free renewable energy. Since tidal and river currents exist everywhere in the world and are either constantly flowing or extremely predictable, converting the energy in these currents to electricity could provide a predictable, reliable and, in some cases, base load supply of electricity to the electric power systems or remote sites in many parts of the world. 70% of the world's population lives within 320 kilometres (200 mi) of an ocean. Accordingly, ocean current energy could become a vital part of the world's energy future.
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:
- Marine life. Concerns have been raised about the danger to marine animals, such as seals and fish, from wave and tidal devices. There is no evidence that this is a significant problem. Such devices may actually benefit the local fauna by creating non-fishing 'havens' and structures such as anchoring devices may create new reefs for fish colonization.
- Sea bed. By altering wave patterns and tidal streams, devices will undoubtedly have an effect, for example, upon the deposition of sediment. Research carried out to date would seem to indicate that the effects would not be significant, and may even be positive, for example by helping to slow down coastal erosion. (This is particularly pertinent in light of evidence that waves have steadily increased in size in the recent past.) The sea in the lee of devices would almost certainly be calmer than normal, but, it has been suggested, this would help in creating more areas for activities such as water sports or yachting.
- Landscape. Most wave and tidal energy devices would be invisible from the shore. They would have none of the problems of visual and noise pollution that older versions of wind turbines engender. The main impact would probably be from the extensive transmission lines needed to take the energy from the shoreline to final users. This problem would have to be addressed, possibly by using underground transmission lines.
- Fishing and shipping activities. Offshore wave and tidal devices would almost certainly require areas to be closed to fishing and shipping activities. The siting of such devices would have to be negotiated, therefore, with relevant local groups (for example, fishermen), as well as with national and international bodies. (Science and Technology,2001)
Implementation and regulations
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 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. 
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 such, proponents and manufacturers of these systems have tried to bring them into public knowledge 
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