Generating Electric Power from High Elevation Winds in Valleys
Powerful winds blow through mountain valleys worldwide. Some of these winds originate over a large body of water, such as an ocean or a sea, and accelerate to high velocity due to the low friction of the boundary layer that exists between air and water. Many such winds blow towards coastal mountains, for example the Atlas Mountains of Northwestern Africa. Powerful winds also blow at high altitudes towards valleys in mountain ranges such as the Alps. There are many valleys at various elevations in these mountains into which coastal winds and high-altitude winds may blow and accelerate to higher velocity. It is possible to adapt existing and proven technology, such as cable suspension systems, conveyor technology and airfoils, in order to generate electrical power at higher elevations than can be achieved by wind turbines on towers.
By Harry Valentine, Technical Journalist, Canada .
Boundary Layer Effect
There are occasions when prevailing winds will blow directly into mountain valleys, while on other occasions prevailing winds will blow toward the valley entrances at an angle. The gently curved entrances of many valleys can minimise wind turbulence and allow the boundary layer effect to steer prevailing winds through a significant angle to blow into the valleys (Figure 1). The valley walls will exert a channelling effect on the wind causing it to accelerate to a higher velocity where the valley cross-sectional area decreases. On occasion, crosswinds will blow over the mountains at an angle to the valleys below. The curved surface at the top of many valley walls would generate a boundary layer effect that can steer the crosswinds to blow diagonally across the higher elevations of many valleys. It may be possible to generate electric power from the wind energy in many valleys.
Suitable Winds
Westerly winds blow over the North Atlantic Ocean carrying tremendous kinetic energy to Western European and Northwestern African shores. That energy has encouraged the development of offshore tower-mounted turbines that generate electrical power. The towers place the hubs of the 120-metre diameter turbines at 80 to 100 metres’ elevation. However, the boundary layer effect of the ocean and land causes wind to blow at higher velocity at higher elevation. A research team at Delft University in the Netherlands is undertaking development of a wind engine without a tower, known as a LadderMill (Figure 2). It uses a series of moving kites to access the energy that blows in the high-altitude winds to produce an estimated 100MW of electric power from a single alternator. While the LadderMill is intended for operation at coastal and offshore locations, it can be tilted by 90 degrees for operation in valleys.
Suitable Valleys
The topography of Western Europe and Northwestern Africa includes several mountain ranges with large valleys capable of capturing the prevailing winds. Norway’s western coast includes a multitude of fiords with steep valley walls into which prevailing winds from the Norwegian Sea and North Sea may blow throughout the year. The western coasts of Ireland and Scotland include several suitable ocean inlets. Prevailing winds that blow over the Bay of Biscay continue their journey through valleys in the Pyrenees , the Massif Central and the French and Swiss Alps. There are already undersea electric power cables under the Straits of Gibraltar and plans to import wind energy from Northwestern Africa into Western Europe. Powerful prevailing winds blow through valleys in the Atlas Mountains of Morocco and the Ahaggar Plateau of Algeria.
Turbines in Valleys
There are several ways to install wind turbines in valleys. The largest wind farm in Alberta, Canada, is located in a windswept valley known as Pinscher Creek where an array of tower-mounted turbines generate electric power for the city of Calgary. The blade tips of the largest turbines on the highest towers may reach a peak elevation of 160 metres. Wind dynamics in valleys involve the combination of winds blowing parallel to and through the valley at lower elevations and crosswinds blowing at higher elevations. A complementary wind technology to tower-mounted turbines could operate at elevations in excess of 200 metres above the valley floor and convert energy from multi-directional winds. The wind technology could be suspended at such elevations using well-proven cable suspension technology from the bridge building industry.
Airfoils in Valleys
The central spans of some of the world’s longest suspension bridges are approaching lengths of 2,000 metres. While many mountain valleys may have a wide and curved entrance, the valley widths often decrease to less than 2,000 metres and allow for the installation of a cable suspension system that may carry either a bridge or a cable conveyor system. Cable conveyor systems form the basis of ski lifts that are used at many winter resorts. That technology could also be adapted to carry large curved airfoils that may capture wind energy at higher elevation in valleys, as is shown in Figure 3. Stationary cables would carry both the vertical weight of the moving airfoils as well as the horizontal forces generated by the interaction of the wind with the airfoils. The horizontal forces would occur in both the transverse direction across the valley as well as parallel to the direction of wind flow.
Converting Energy
The cable conveyor carrying the airfoils in an oval loop across a valley would be installed diagonally. That angle would allow the airfoils to capture winds blowing up, down or across the valley. The concept is essentially a vertical-axis turbine and is based on the LadderMill concept from Delft University that has been tilted over by 90 degrees. The looped path of the suspended airfoils would allow them to extract energy from winds, as would counter rotating turbines mounted on the same shaft. That feature could increase conversion efficiency up to 50% in winds blowing at 72km/h or 20m/s. Each airfoil may be built to a vertical height of 60metres and be installed with 250 metres vertical clearance to the valley floor. The airfoils could convert a wind stream of 1,000 metres in width to 110–140MW of output with wind blowing at 20m/s, or comparable output to the LadderMill. A series of cable suspended installations may be built into the same valley at optimal spacing to increase overall power output.
Economic Considerations
The high costs of towers and of electrical generating machinery raises the overall cost of wind energy installations. A system of cable-suspended airfoils installed across a valley involves one or two electrical generating units mounted close to the valley walls that could absorb the torque reactions. The high cost of the generators would be spread equally over a number of airfoils, each of which could capture more wind energy than conventionally mounted turbine blades. The installation of a cable suspension system across a valley may involve less expense than installing multiple towers. The cable suspended airfoils would interact with higher elevation winds blowing at higher velocity than the lower elevation winds being captured by tower-mounted turbines. If the higher elevation wind blows at 10% greater velocity, there is potential for the airfoils to operate at slightly higher efficiency and generate at least 33% greater output.
Conclusions
The concept of suspending airfoils on moving cables across a valley is based on proven technology such as suspension bridge engineering, cable car and ski lift design and airfoil and turbine blade technology. It is a vertical-axis version of the LadderMill concept from Delft University. In operation, it should produce negligible noise while being relatively harmless to birds and bats. It can be installed at lower cost per kilowatt across the same valleys at higher elevation than tower mounted wind turbines and extract energy at higher efficiency from higher velocity winds. It can be installed above the navigation height of ships across fiords, and across many river valleys. The research expertise needed to refine and optimise the concept exists in the engineering faculties of numerous universities. The expertise and manufacturing capability needed to build the concept already exists in many industries across Western Europe.
Biography of the Author
Harry Valentine holds a Bachelor of Engineering degree in mechanical engineering from Carleton University, Ottawa, Canada, where he also undertook postgraduate studies in transportation engineering. He currently works as a technical journalist for several energy sector and transportation sector industry trade journals (Chatila Publishing House, Hearst Media, Saturn Energy Media).{/access}
Powerful winds blow through mountain valleys worldwide. Some of these winds originate over a large body of water, such as an ocean or a sea, and accelerate to high velocity due to the low friction of the boundary layer that exists between air and water. Many such winds blow towards coastal mountains, for example the Atlas Mountains of Northwestern Africa. Powerful winds also blow at high altitudes towards valleys in mountain ranges such as the Alps. There are many valleys at various elevations in these mountains into which coastal winds and high-altitude winds may blow and accelerate to higher velocity. It is possible to adapt existing and proven technology, such as cable suspension systems, conveyor technology and airfoils, in order to generate electrical power at higher elevations than can be achieved by wind turbines on towers.By Harry Valentine, Technical Journalist, Canada .
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The worldwide interest in generating greater amounts of electrical energy at lower cost from wind energy has encouraged energy researchers to develop innovative ways by which to improve the technology. These variations include larger turbines carried on higher towers and several wind power technologies that dispense with towers altogether.Boundary Layer Effect
There are occasions when prevailing winds will blow directly into mountain valleys, while on other occasions prevailing winds will blow toward the valley entrances at an angle. The gently curved entrances of many valleys can minimise wind turbulence and allow the boundary layer effect to steer prevailing winds through a significant angle to blow into the valleys (Figure 1). The valley walls will exert a channelling effect on the wind causing it to accelerate to a higher velocity where the valley cross-sectional area decreases. On occasion, crosswinds will blow over the mountains at an angle to the valleys below. The curved surface at the top of many valley walls would generate a boundary layer effect that can steer the crosswinds to blow diagonally across the higher elevations of many valleys. It may be possible to generate electric power from the wind energy in many valleys.
Suitable Winds
Westerly winds blow over the North Atlantic Ocean carrying tremendous kinetic energy to Western European and Northwestern African shores. That energy has encouraged the development of offshore tower-mounted turbines that generate electrical power. The towers place the hubs of the 120-metre diameter turbines at 80 to 100 metres’ elevation. However, the boundary layer effect of the ocean and land causes wind to blow at higher velocity at higher elevation. A research team at Delft University in the Netherlands is undertaking development of a wind engine without a tower, known as a LadderMill (Figure 2). It uses a series of moving kites to access the energy that blows in the high-altitude winds to produce an estimated 100MW of electric power from a single alternator. While the LadderMill is intended for operation at coastal and offshore locations, it can be tilted by 90 degrees for operation in valleys.
Suitable Valleys
The topography of Western Europe and Northwestern Africa includes several mountain ranges with large valleys capable of capturing the prevailing winds. Norway’s western coast includes a multitude of fiords with steep valley walls into which prevailing winds from the Norwegian Sea and North Sea may blow throughout the year. The western coasts of Ireland and Scotland include several suitable ocean inlets. Prevailing winds that blow over the Bay of Biscay continue their journey through valleys in the Pyrenees , the Massif Central and the French and Swiss Alps. There are already undersea electric power cables under the Straits of Gibraltar and plans to import wind energy from Northwestern Africa into Western Europe. Powerful prevailing winds blow through valleys in the Atlas Mountains of Morocco and the Ahaggar Plateau of Algeria.
Turbines in Valleys
There are several ways to install wind turbines in valleys. The largest wind farm in Alberta, Canada, is located in a windswept valley known as Pinscher Creek where an array of tower-mounted turbines generate electric power for the city of Calgary. The blade tips of the largest turbines on the highest towers may reach a peak elevation of 160 metres. Wind dynamics in valleys involve the combination of winds blowing parallel to and through the valley at lower elevations and crosswinds blowing at higher elevations. A complementary wind technology to tower-mounted turbines could operate at elevations in excess of 200 metres above the valley floor and convert energy from multi-directional winds. The wind technology could be suspended at such elevations using well-proven cable suspension technology from the bridge building industry.
Airfoils in Valleys
The central spans of some of the world’s longest suspension bridges are approaching lengths of 2,000 metres. While many mountain valleys may have a wide and curved entrance, the valley widths often decrease to less than 2,000 metres and allow for the installation of a cable suspension system that may carry either a bridge or a cable conveyor system. Cable conveyor systems form the basis of ski lifts that are used at many winter resorts. That technology could also be adapted to carry large curved airfoils that may capture wind energy at higher elevation in valleys, as is shown in Figure 3. Stationary cables would carry both the vertical weight of the moving airfoils as well as the horizontal forces generated by the interaction of the wind with the airfoils. The horizontal forces would occur in both the transverse direction across the valley as well as parallel to the direction of wind flow.
Converting Energy
The cable conveyor carrying the airfoils in an oval loop across a valley would be installed diagonally. That angle would allow the airfoils to capture winds blowing up, down or across the valley. The concept is essentially a vertical-axis turbine and is based on the LadderMill concept from Delft University that has been tilted over by 90 degrees. The looped path of the suspended airfoils would allow them to extract energy from winds, as would counter rotating turbines mounted on the same shaft. That feature could increase conversion efficiency up to 50% in winds blowing at 72km/h or 20m/s. Each airfoil may be built to a vertical height of 60metres and be installed with 250 metres vertical clearance to the valley floor. The airfoils could convert a wind stream of 1,000 metres in width to 110–140MW of output with wind blowing at 20m/s, or comparable output to the LadderMill. A series of cable suspended installations may be built into the same valley at optimal spacing to increase overall power output.
Economic Considerations
The high costs of towers and of electrical generating machinery raises the overall cost of wind energy installations. A system of cable-suspended airfoils installed across a valley involves one or two electrical generating units mounted close to the valley walls that could absorb the torque reactions. The high cost of the generators would be spread equally over a number of airfoils, each of which could capture more wind energy than conventionally mounted turbine blades. The installation of a cable suspension system across a valley may involve less expense than installing multiple towers. The cable suspended airfoils would interact with higher elevation winds blowing at higher velocity than the lower elevation winds being captured by tower-mounted turbines. If the higher elevation wind blows at 10% greater velocity, there is potential for the airfoils to operate at slightly higher efficiency and generate at least 33% greater output.
Conclusions
The concept of suspending airfoils on moving cables across a valley is based on proven technology such as suspension bridge engineering, cable car and ski lift design and airfoil and turbine blade technology. It is a vertical-axis version of the LadderMill concept from Delft University. In operation, it should produce negligible noise while being relatively harmless to birds and bats. It can be installed at lower cost per kilowatt across the same valleys at higher elevation than tower mounted wind turbines and extract energy at higher efficiency from higher velocity winds. It can be installed above the navigation height of ships across fiords, and across many river valleys. The research expertise needed to refine and optimise the concept exists in the engineering faculties of numerous universities. The expertise and manufacturing capability needed to build the concept already exists in many industries across Western Europe.
Biography of the Author
Harry Valentine holds a Bachelor of Engineering degree in mechanical engineering from Carleton University, Ottawa, Canada, where he also undertook postgraduate studies in transportation engineering. He currently works as a technical journalist for several energy sector and transportation sector industry trade journals (Chatila Publishing House, Hearst Media, Saturn Energy Media).{/access}
