Shrouded tidal turbine

Last updated
The shrouded turbine Race Rocks Tidal Current Generator before installation at Race Rocks in southern British Columbia in 2006. It operates bi-directionally and has proven to be efficient in contributing to the integrated power system of the area. Tidalenergyturbine.jpg
The shrouded turbine Race Rocks Tidal Current Generator before installation at Race Rocks in southern British Columbia in 2006. It operates bi-directionally and has proven to be efficient in contributing to the integrated power system of the area.

The shrouded tidal turbine is an emerging tidal stream technology that has a turbine enclosed in a venturi shaped shroud or duct (ventuduct), producing a sub atmosphere of low pressure behind the turbine. The venturi shrouded turbine is not subject to the Betz limit and allows the turbine to operate at higher efficiencies than the turbine alone by increasing the volume of the flow over the turbine. Claimed improvements vary, from 1.15 to 4 times higher power output [2] than the same turbine minus the shroud. The Betz limit of 59.3% conversion efficiency for a turbine in an open flow still applies, but is applied to the much larger shroud cross-section rather than the small turbine cross-section.

Contents

Principles

Considerable commercial interest has been shown in shrouded tidal stream turbines due to the increased power output. They can operate in shallower slower moving water with a smaller turbine at sites where large turbines are restricted. Arrayed across a seaway or in fast flowing rivers, shrouded turbines are cabled to shore for connection to a grid or a community. Alternatively the property of the shroud that produces an accelerated flow velocity across the turbine allows tidal flows formerly too slow for commercial use to be used for energy production.

While the shroud may not be practical in wind, as the next generation of tidal stream turbine design it is gaining more popularity and commercial use. The Tidal Energy Pty Ltd tidal turbine is multidirectional able to face up-stream in any direction and the Lunar Energy turbine bi directional. All tidal stream turbines constantly need to face at the correct angle to the water stream in order to operate. The Tidal Energy Pty Ltd is a unique case with a pivoting base. Lunar Energy use a wide angle diffuser to capture incoming flow that may not be inline with the long axis of the turbine. A shroud can also be built into a tidal fence or barrage increasing the performance of turbines.

Types of shroud

Not all shrouded turbines are the same - the performance of a shrouded turbine varies with the design of the shroud. Not all shrouded turbines have undergone independent scrutiny of claimed performances, as companies closely guard their respective technologies, so quoted performance figures need to be closely scrutinised. Lunar Energy reports a 15%-25% improvement over the same turbine without the shroud. [3] Shrouded turbines do not operate at maximum efficiency when the shroud does not intercept the current flow at the correct angle, which can occur as currents eddy and swirl, resulting in reduced operational efficiency. At lower turbine efficiencies the extra cost of the shroud must be justified, while at higher efficiencies the extra cost of the shroud has less impact on commercial returns. Similarly the added cost of the supporting structure for the shroud has to be balanced against the performance gained. Yawing (pivoting) the shroud and turbine at the correct angle, so it always faces upstream like a wind sock, can increase turbine performance but may need expensive active devices to turn the shroud into the flow. Passive designs can be incorporated, such as floating the shrouded turbine under a pontoon on a swing mooring, or flying the turbine like a kite under water. [4] One design by Tidal Energy Pty Ltd passively yaws the shrouded turbine using a turntable with a peer reviewed claim of 3.84 (384%) increase in efficiency over the same turbine minus the shroud - See Kirke. [5]

Advantages

Disadvantages

Advancements

A typical shrouded turbine is unable to harness significant portion of the augmented mass flow inside the shroud. The power extraction can be improved further by placing another coaxial rotor, making it a Shrouded Twin-rotor Turbine. Interestingly, such turbines can operate in multiple configurations: (a) turbine-turbine mode (both rotors acting as turbine and extracting energy from the flow), and (b) turbine-fan mode (the first rotor extracts energy from the flow, whereas the second rotor reduces flow stagnation by imparting energy to the flow). [10] Theoretical analysis of such configurations have revealed significant power augmentation in both these modes with enhanced operational flexibility, depending on inlet flow conditions.

Related Research Articles

<span class="mw-page-title-main">Turbine</span> Rotary mechanical device that extracts energy from a fluid flow

A turbine is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced by a turbine can be used for generating electrical power when combined with a generator. A turbine is a turbomachine with at least one moving part called a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades so that they move and impart rotational energy to the rotor. Early turbine examples are windmills and waterwheels.

<span class="mw-page-title-main">Tesla turbine</span> Bladeless centripetal flow turbine

The Tesla turbine is a bladeless centripetal flow turbine patented by Nikola Tesla in 1913. It is referred to as a "bladeless turbine".

<span class="mw-page-title-main">Turbofan</span> Airbreathing jet engine designed to provide thrust by driving a fan

The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the turbo portion refers to a gas turbine engine which achieves mechanical energy from combustion, and the fan, a ducted fan that uses the mechanical energy from the gas turbine to force air rearwards. Thus, whereas all the air taken in by a turbojet passes through the combustion chamber and turbines, in a turbofan some of that air bypasses these components. A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust.

<span class="mw-page-title-main">Tidal power</span> Technology to convert the energy from tides into useful forms of power

Tidal power or tidal energy is harnessed by converting energy from tides into useful forms of power, mainly electricity using various methods.

<span class="mw-page-title-main">Ducted fan</span> Air moving arrangement

In aeronautics, a ducted fan is a thrust-generating mechanical fan or propeller mounted within a cylindrical duct or shroud. Other terms include ducted propeller or shrouded propeller. When used in vertical takeoff and landing (VTOL) applications it is also known as a shrouded rotor.

<span class="mw-page-title-main">Bypass ratio</span> Proportion of ducted compared to combusted air in a turbofan engine

The bypass ratio (BPR) of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A 10:1 bypass ratio, for example, means that 10 kg of air passes through the bypass duct for every 1 kg of air passing through the core.

<span class="mw-page-title-main">Turbomachinery</span>

Turbomachinery, in mechanical engineering, describes machines that transfer energy between a rotor and a fluid, including both turbines and compressors. While a turbine transfers energy from a fluid to a rotor, a compressor transfers energy from a rotor to a fluid.

Marine currents can carry large amounts of water, largely driven by the tides, which are a consequence of the gravitational effects of the planetary motion of the Earth, the Moon and the Sun. Augmented flow velocities can be found where the underwater topography in straits between islands and the mainland or in shallows around headlands plays a major role in enhancing the flow velocities, resulting in appreciable kinetic energy. The Sun acts as the primary driving force, causing winds and temperature differences. Because there are only small fluctuations in current speed and stream location with minimal changes in direction, ocean currents may be suitable locations for deploying energy extraction devices such as turbines. Other effects such as regional differences in temperature and salinity and the Coriolis effect due to the rotation of the earth are also major influences. The kinetic energy of marine currents can be converted in much the same way that a wind turbine extracts energy from the wind, using various types of open-flow rotors.

<span class="mw-page-title-main">Betz's law</span> Aerodynamic power limitation for wind turbines

Betz's law indicates the maximum power that can be extracted from the wind, independent of the design of a wind turbine in open flow. It was published in 1919 by the German physicist Albert Betz. The law is derived from the principles of conservation of mass and momentum of the air stream flowing through an idealized "actuator disk" that extracts energy from the wind stream. According to Betz's law, no turbine can capture more than 16/27 (59.3%) of the kinetic energy in wind. The factor 16/27 (0.593) is known as Betz's coefficient. Practical utility-scale wind turbines achieve at peak 75–80% of the Betz limit.

<span class="mw-page-title-main">Gorlov helical turbine</span> Water turbine

The Gorlov helical turbine (GHT) is a water turbine evolved from the Darrieus turbine design by altering it to have helical blades/foils. Water turbines take kinetic energy and translates it into electricity. It was patented in a series of patents from September 19, 1995 to July 3, 2001 and won 2001 ASME Thomas A. Edison. GHT was invented by Alexander M. Gorlov, professor of Northeastern University.

<span class="mw-page-title-main">Unconventional wind turbines</span> Wind turbines of unconventional design

Unconventional wind turbines are those that differ significantly from the most common types in use.

<span class="mw-page-title-main">Wells turbine</span>

The Wells turbine is a low-pressure air turbine that rotates continuously in one direction independent of the direction of the air flow. Its blades feature a symmetrical airfoil with its plane of symmetry in the plane of rotation and perpendicular to the air stream.

<span class="mw-page-title-main">Radial turbine</span>

A radial turbine is a turbine in which the flow of the working fluid is radial to the shaft. The difference between axial and radial turbines consists in the way the fluid flows through the components. Whereas for an axial turbine the rotor is 'impacted' by the fluid flow, for a radial turbine, the flow is smoothly orientated perpendicular to the rotation axis, and it drives the turbine in the same way water drives a watermill. The result is less mechanical stress which enables a radial turbine to be simpler, more robust, and more efficient when compared to axial turbines. When it comes to high power ranges the radial turbine is no longer competitive and the efficiency becomes similar to that of the axial turbines.

<span class="mw-page-title-main">Wind-turbine aerodynamics</span>

The primary application of wind turbines is to generate energy using the wind. Hence, the aerodynamics is a very important aspect of wind turbines. Like most machines, wind turbines come in many different types, all of them based on different energy extraction concepts.

<span class="mw-page-title-main">Evopod</span> Tidal energy device

Evopod is a unique tidal energy device being developed by a UK-based company Oceanflow Energy Ltd for generating electricity from tidal streams and ocean currents. It can operate in exposed deep water sites where severe wind and waves also make up the environment.

<span class="mw-page-title-main">Tidal farm</span>

A tidal farm is a group of multiple tidal stream generators assembled in the same location used for production of electric power, similar to that of a wind farm. The low-voltage powerlines from the individual units are then connected to a substation, where the voltage is stepped up with the use of a transformer for distribution through a high voltage transmission system.

<span class="mw-page-title-main">Tidal stream generator</span> Type of tidal power generation technology

A tidal stream generator, often referred to as a tidal energy converter (TEC), is a machine that extracts energy from moving masses of water, in particular tides, although the term is often used in reference to machines designed to extract energy from run of river or tidal estuarine sites. Certain types of these machines function very much like underwater wind turbines, and are thus often referred to as tidal turbines. They were first conceived in the 1970s during the oil crisis.

<span class="mw-page-title-main">Cyclorotor</span> Perpendicular axis marine propulsion system

A cyclorotor, cycloidal rotor, cycloidal propeller or cyclogiro, is a fluid propulsion device that converts shaft power into the acceleration of a fluid using a rotating axis perpendicular to the direction of fluid motion. It uses several blades with a spanwise axis parallel to the axis of rotation and perpendicular to the direction of fluid motion. These blades are cyclically pitched twice per revolution to produce force in any direction normal to the axis of rotation. Cyclorotors are used for propulsion, lift, and control on air and water vehicles. An aircraft using cyclorotors as the primary source of lift, propulsion, and control is known as a cyclogyro or cyclocopter. A unique aspect is that it can change the magnitude and direction of thrust without the need of tilting any aircraft structures. The patented application, used on ships with particular actuation mechanisms both mechanical or hydraulic, is named after German company Voith Turbo.

A diffuser-augmented wind turbine (DAWT) is a wind turbine modified with a cone-shaped wind diffuser that is used to increase the efficiency of converting wind power to electrical power. The increased efficiency is possible due to the increased wind speeds the diffuser can provide. In traditional bare turbines, the rotor blades are vertically mounted at the top of a support tower or shaft. In a DAWT, the rotor blades are mounted within the diffuser, which is then placed on the top of the support tower. Additional modifications can be made to the diffuser in order to further increase efficiency.

<span class="mw-page-title-main">Vertical-axis wind turbine</span> Type of wind turbine

A vertical-axis wind turbine (VAWT) is a type of wind turbine where the main rotor shaft is set transverse to the wind while the main components are located at the base of the turbine. This arrangement allows the generator and gearbox to be located close to the ground, facilitating service and repair. VAWTs do not need to be pointed into the wind, which removes the need for wind-sensing and orientation mechanisms. Major drawbacks for the early designs included the significant torque ripple during each revolution, and the large bending moments on the blades. Later designs addressed the torque ripple by sweeping the blades helically. Savonius vertical-axis wind turbines (VAWT) are not widespread, but their simplicity and better performance in disturbed flow-fields, compared to small horizontal-axis wind turbines (HAWT) make them a good alternative for distributed generation devices in an urban environment.

References

  1. "The Race Rocks Tidal Energy Project". Clean Current Power Systems Incorporated. Archived from the original on 2008-07-05. Retrieved 2008-07-09.
  2. "Brian Kirke's published article Developments in Ducted Water Turbines" (PDF). Archived from the original (PDF) on 2012-09-13. Retrieved 2013-04-28.
  3. "Lunar Energy". Lunar Energy. Retrieved 2013-04-28.
  4. "Underwater Electric Kite". Uekus.com. Retrieved 2013-04-28.
  5. "Tidal energy Pty. Ltd". Tidalenergy.net.au. Archived from the original on 2010-03-27. Retrieved 2013-04-28.
  6. "Verdant Power". Verdant Power. 2012-01-23. Archived from the original on 2013-04-20. Retrieved 2013-04-28.
  7. "Brian Kirke's PhD Thesis" (PDF). Archived from the original (PDF) on 2012-09-13. Retrieved 2013-04-28.
  8. "What is hydro kinetic energy?". Tidal Energy Pty Ltd. Retrieved 2014-02-02.
  9. Garry Fletcher. "deployed at Race Rocks". Racerocks.com. Retrieved 2013-04-28.
  10. Kumar, Vedant; Saha, Sandeep (2019-04-01). "Theoretical performance estimation of shrouded-twin-rotor wind turbines using the actuator disk theory". Renewable Energy. 134: 961–969. doi:10.1016/j.renene.2018.11.077. ISSN   0960-1481. S2CID   115800610.