Wider spacing leads to greater farm efficiency says CFD study
April 7, 2011 by Windpower Engineering
Filed under Construction, Editorial, Site assessments, Wind Power News, Wind Power Projects
Conventional wisdom says to space wind turbines about seven rotor diameters apart to keep rotor wash from interfering with other turbines. But after a few fluid flow simulations and lab experiments, researchers at Johns Hopkins and Belgium’s Katholik Leuven University say that conventional wisdom needs an upgrade. “Optimal spacing between individual wind turbines is actually a little farther apart than what people use these days,” said Johns Hopkins’ Charles Meneveau. A little farther means 15 diameters. Why so much more?
The study found a turbine’s blades distort wind, creating eddies and turbulence that can affect other wind turbines farther downwind. Most previous studies have used computer models to calculate the wake effect of one individual turbine on another. Starting with large-scale computer simulations and small-scale experiments in a wind tunnel, Meneveau’s model considers the cumulative effects of hundreds or thousands of turbines interacting with the atmosphere.
Meneveau and Leuven’s Meyers argue that energy generated in a large wind farm has less to do with horizontal winds and is more dependent on the strong winds that turbulence creates as tall turbines pull down air from higher in the atmosphere. Using insights from high-performance computer simulations and wind-tunnel experiments, they determined that the turbines alter the landscape in a way that creates turbulence, which stirs the air, and helps draw more powerful kinetic energy from higher altitudes.
A volume of atmosphere that surrounds the simulated wind farm shows the turbulence from one line of turbines to the next. The small blue half circles represent rotors. The x-y plane is at hub height.
That may be a problem because wind farms around the world are large and getting larger. Arranging thousands of wind turbines across many miles of land requires new tools that can balance cost and efficiency to provide the most energy for the buck.
The experiments were conducted in the Johns Hopkins’ wind tunnel. Before the tunnel’s air stream enters the testing area, it passes through a curtain of perforated plates that rotate randomly to create turbulence. Air moving through the tunnel more closely resembles real-life wind conditions.
The 2D plane extends through hub heights in the simulation. The black rectangles are turbine rotors. Dark blue shows turbulence generated by rotors.
Air in the tunnel then passes through a series of small three-bladed model wind turbines mounted atop posts, mimicking an array of full-size wind turbines. Data concerning the interaction of the air currents and the model turbines is collected by using a pair of high-resolution digital cameras, smoke, and laser pulses in a measurement technique called stereo particle-image-velocimetry. The two researchers have developed a model to calculate best spacing of turbines for large wind farms in the future. However, further research is needed, Meneveau said, to learn how varying temperatures can affect the generation of power on large wind farms.
WPE
Laser-based wind sensor lets turbines grab more power
August 8, 2010 by KRemington
Filed under Turbine Sensors
A sensor that can see the wind ahead of a turbine would let it react to changes in wind direction and speed, thereby generate more power.
For decades, large utility scale wind turbines have been forced to rely on mechanical wind sensing devices that are mounted on the nacelles of the turbines to measure the wind speed and direction. These signals are fed into the wind turbine control system to adjust the turbine nacelle to account for the variations in the wind flow. The trouble with these devices is that they are located in the wrong position, in extremely turbulent airflows that must be time averaged to smooth out the fluctuations caused by blade interference and turbulence from the aerodynamic flow over the nacelle. The wind measured behind the wind turbine rotor blades bears no resemblance to the actual airflow that the turbine rotor system encounters flowing into the blades. The resulting misalignment in yaw and pitch, caused by the reactive nature of current wind sensors, has a tremendous affect on the efficiency of the wind turbine and the stress load damage imparted to the dynamic components.
Start with yaw
An important factor affecting wind-turbine efficiency is yaw alignment, a measure of a turbine rotor’s misalignment with the wind. To operate most efficiently, wind turbines should keep the turbine nacelle aligned with the wind. Because the wind speed and direction are always changing, the yaw misalignment is always present even when the winds appear to be “steady” as reported by the conventional mechanical or sonic anemometry in use. Although the alignment error between the wind direction and the turbine nacelle cannot be completely eliminated, its magnitude can be greatly reduced, producing more energy and reducing turbine stress loads. If a turbine’s control system can assess accurate wind direction and speed far enough in advance of the turbine, it can better align itself with approaching wind.
Staying aligned with the wind significantly increases power output and reduces turbine stress. It also has the potential to guide the adjustment of blade pitch in anticipation of sudden stress events caused by gusts.
Catch the Wind, Inc. (CTW) has developed a laser wind sensor that, when mounted on the turbine nacelle, accurately measures real-time horizontal and vertical wind speed and direction. The Vindicator laser wind sensor (LWS) can look out to 300 meters ahead of the turbine to measure wind speed and direction as it approaches the turbine blades and transmits that data to the controls in sufficient time (20 seconds of lead time for a 35 mph wind) to reorient the turbine. The system is comprised of a base laser unit and a remote lens unit. The base unit is housed in a separate assembly that can be mounted inside the turbine’s nacelle and connected to the remote lens assembly. The technology uses Laser Doppler Velocimetry, an optical remote sensing technique similar to Doppler radar, to measure the minute frequency changes of light reflected by microscopic air particles moving with the wind to precisely determine wind speed and direction.
Challenges for mechanical sensors
The Nordex turbine shows the relative location of the remote laser unit on the nacelle.
Today’s turbines operate reactively because the sensors are mounted on the back of the nacelle – behind the turbine rotor blades. That means after a wind change passes a turbine, its mechanical, or ultra-sonic sensors, must detect the change, average out the variations in the disturbed windflow and then provide control signals to adjust the turbine’s direction. The resultant averaging and time delay, often several minutes long, means that the wind turbine is not operating as effectively in capturing all the energy from the wind, particularly in Region 2 (between the turbines cut-in wind speed and rated power wind speed) where most wind turbines spend the bulk of their operating time. This near-constant misalignment also generates unnecessary stress on blades, causing premature wear and damage to them and key turbine components. Repair or replacement of these major components represents a significant cost to the wind farm operations, and when coupled to the out of service time for repair, directly impacts the profitability and cost of wind energy. A feed-forward, laser-based wind sensor addresses these issues by taking a proactive approach to wind turbine control by assessing the wind before it reaches the turbine.
Small changes, big payoff
Misaligned turbines result in lost revenue, either by power output losses due to inefficiency, or by increased maintenance costs due to stress damage. Because the power output is theoretically reduced by the cosine cubed of yaw angle, the apparent power losses start to add up rapidly when the turbine is misaligned by more than 10 degrees. Trial data from Catch the Wind’s testing in on an operating wind turbine in Nebraska demonstrates that the nacelle misalignment may be significantly greater than conventional industry understanding.
The Nebraska Public Power District (NPPD) tested a Vindicator LWS last year on a Vestas V-82 wind turbine. The remote laser unit was mounted atop the nacelle and directed forward through the blades. The base laser unit was mounted inside the nacelle. A conservative control algorithm was implemented resulting in the Vindicator sensor controlling the turbine approximately half the time. During the month long test, the turbine’s power production showed an average increase of over 10% when the LWS was in control of the turbine yaw system. Additionally, a SWANTech Stress Wave Energy monitoring system recorded a significant decrease in stress loading on the main shaft bearing with the Vindicator system in control. With better control algorithms, the average power output figure could be improved even more. Since the initial one month trial, the Vindicator LWS has continued to demonstrate more than 10% average power output improvement over an additional seven months of data taken at NPPD.
By one estimate, there are more than 90,000 wind turbines worldwide rated at 1.5 MW and more. With increased emphasis of government mandates and industry targets for more renewable energy resources, that number is expected to more than double by the end of 2014. By 2020, demand for wind turbines of 1.5 MW capacity or greater is expected to exceed 500,000. If wind power is ever to realize its power generation potential, wind farm operators must consider more options to boost power production and revenues while lowering operational costs.
The future of laser wind sensors
Feed-forward laser-based devices will be increasingly used worldwide to measure the wind and provide more precise information for wind farms. Another important application for feed-forward laser sensors is to measure the gust and turbulence characteristics of the wind to proactively adjust blade pitch to capture additional wind energy sustained in the gusts and, if necessary, initiate control measures to prevent or reduce blade damage from increased turbulence.
Laser-based wind sensors will also improve a variety of critical wind-related activities, including wind resource assessment, turbine performance monitoring, wind prospecting, and real-time inputs for wind forecasting models used in grid balancing.
Grid managers will be able to increasingly rely on clean, renewable wind power for a greater percentage of the total energy demand instead of requiring higher reserves of carbon-based fuels such as gas or coal to account for the wind’s variability.
Laser Doppler Velocimetry has also been adapted to monitoring wind conditions at sea. The wind roses to the right tell of different speed and directions with altitude.
For the maritime sector, Catch the Wind has partnered with AXYS Technologies to integrate the laser wind sensing technology into its WindSentinel product, the world’s first offshore wind resource assessment buoy. It accurately measures the wind speed profile up to the top of the rotor diameter of even the tallest wind turbine, providing offshore wind farm developers with a portable and reusable instrument to determine wind resources at potential wind farm sites.
The American Wind Energy Association predicts wind power could provide as much as 20% of the U.S.’s electricity needs by 2030. Should wind power reach that threshold, it will supply enough energy to displace about 50% of electric utility natural gas consumption by 2030, which will amount to an 11% reduction in natural gas across all industries. Additionally, coal consumption will be reduced by 18%. WPE
Shouded Wind Turbines Accelerate Output
November 9, 2009 by Windpower Engineering
Filed under Featured Wind Power Articles, Turbine Design
Our objective while developing wind technology is to reduce costs and increase the power output of wind turbines. The principle behind our studies is to use the effect of static wing or sail structures, which convert energy more efficiently, to increase the efficiency of turbines. Many attempts have already been made during the last decades to use external shrouded systems, but with success only in wind tunnel studies, not in ambient air. The reasons become clear from our use of STAR-CD.
Fig. 1 - CFD model of shrouded wind turbine
Based on a patent of the Grumman Corporation, a private company built a prototype at considerable expense, which failed to meet the expected success. CD Adapco’s STAR-CD studies of wind turbines with and without shrouds immediately showed the relationship between the force exerted by the flow on the turbine and transfer of both energy and linear momentum. Given a certain force, the energy transfer does not depend on the velocity of the flow, but the momentum transfer does. As a consequence, it is not possible to increase the power of a conventionally shrouded wind turbine beyond the theoretical limit for the same turbine without shroud (the so called Betz limit). With this realization, millions of dollars could have been saved before the prototype stage, with obvious benefits to the project profitability and overall success.
But the success did not stop there. STAR-CD was able to assist in finding a solution. Past shrouded systems closely fitted the propeller to minimize tip-vortex drag. If instead, one leaves a larger space between the propeller tips and the shroud, it has a beneficial effect over a wider radius of the propeller.
Fig 2: Axial wind velocity component. The direction of the vector indicates the direction of the ambient wind
Figure 1 shows one of CD Adapco’s wind turbine models, surrounded by a shroud, which is curved like a sail. The surface area of the shroud is about 3 times larger than the area covered by the rotating propeller. Figure 2 shows the velocity in a cross section through the model in the flow direction. Contrary to the conventional system, the air accelerates as it approaches the turbine, and the static shroud plays an active part in the energy extraction of the system, hence the name “partially
static turbine”.
Figure 3 compares the mean total pressure in the flow tube, which passes through the propeller for the bare wind turbine and the shrouded one. The large pressure drop for the shrouded wind turbine could in principle also be achieved in an unshrouded system, but only for small wind velocities. In the shrouded system this large pressure drop occurs while the air is moving through the propeller at a mean axial velocity of 7.2 m/sec (while the ambient wind only has a velocity of 5 m/sec) –
Figure 3: Mean total pressure in the rotor flow tube for bare and for shrouded wind turbine. The rotor flow tube is the flow tube flowing through the area covered by the rotor. The total pressure is shown as a function of x, which is the axis parallel to the ambient wind, the position and the size of the shroud are indicated by the yellow shaded area
in an unshrouded system, or in a shrouded system, which does not interact with an additional flow of air, this situation would constitute a severe violation of energy and momentum conservation.
Figure 4 compares the power of the shrouded wind turbine compared to the unshrouded design. The increase in peak power is a factor of 4.
The same principle can also be applied to water. For a given flow rate, one can significantly reduce size of a Kaplan wind turbine. Or for a given turbine size, one can produce the same power at a lower flow rate. We expect this not only to reduce the price of hydro-power, but it should also open new applications, since the partially static turbine allows for hydro power construction in places where large dams are not feasible.
STAR-CD has taught us a lot about partially static systems. Still more can be learnt in the optimization of shrouded designs and prototype builds.
Figure 4: Power of bare wind turbine and shrouded wind turbine
We are actively searching for partners and collaborators in industry and other research institutes to take these studies to the next stage.
REFERENCES:
Bet F. and Grassmann H., ‘Upgrading conventional wind turbines’, Renewable Energy, January 2003, Elsevier Press, www.elsevier.com/locate/renene
Grassmann H., Bet F., Cabras G. Ceschia M>, Cobai D> and DelPapa C. ‘A partially static turbine – first experimental results’, Renewable Energy, to be published, ElsevierPress, www.elsevier.com/locate/renene
Ganis M., “CFD analysis of the characteristics of a shrouded turbine” www.diplom.de
Wind Kite Concept could Generate Plenty of Power
September 21, 2009 by Windpower Engineering
Filed under Wind Watch
Nobody can accuse Italian wind-turbine designers of not thinking big. They say conventional wind turbines just scratch the surface in a few favorable locations on what is an enormous energy field. Current designs cannot reach upper altitude winds and are close to dimensional limits. For instance, there is difficultly positioning hubs at over 100 m up, towers grows exponentially heavier and more unstable than previous designs, and above all, they are more expensive with height. Now consider that the flight-prohibited area over a nuclear power plant can easily contain 1 GW of wind power, equal to the power the plant generates. You’re getting the picture.
To reach wind over 500 m and exploit its greater kinetic energy, the Kite Gen project starts from a change of perspective: no heavy designs like current wind turbines, but instead, light and dynamic structures. To extract energy from an altitude of 800 to 1,000 m, this design suggests using power kites, semi-rigid automatically piloted high efficiency air foils. All the heavy machinery for power generation remains on the ground. Two cables connect each the kites to a winch to control the kite’s direction and angle to the wind. The winches then sit on a large-diameter carrousel.
The Kite Gen concept is comparable to a wind turbine in that the blade tips are the most efficient part of a rotor, say proponents, because they reach highest speeds. The generator is then conveniently located on the ground. The resulting structure is much lighter and cheaper than a conventional design. Moreover, a kite’s operating height can be adjusted to wind conditions.
Several winches or kite steering units, pilot the kites over a predefined flight path. A video at kitegen.com shows some detail. The power kite is maneuvered by unrolling and recovering the two lines, each on the motor-driven winch.
Each Kite Gen power plant is composed by several steering units pulled by power kites along a circular track at ground level. Control software also receives data from on-board avionic sensors to pilot the power kites, control their flight patterns, and maximize energy production.
Conventional wind turbines must be spaced to avoid shading one another which would decrease the total yield. A wind farm can require more than 40 km². In contrast, a Kite Gen power plant and a safe area around it would use about 5 km². Energy production takes place in a distributed manner from several generators thus avoiding unmanageable sizes to electrical equipment.
The modular approach makes possible to build powerful Kite Gen plants. For example, a 100 MW Kite Gen power plant, not much larger than the illustrated example, would have a ground level diameter of about 1 km and deliver an estimated energy at a cost of less than €0.03/kWh. At this writing, a 1,000 MW plant is under study. The exponential growth of the total wind power that can be harnessed is the main reason behind larger Kite Gen power plants.
Proponents say the scalability of the Kite Gen power plants comes without significant structural or cost constraints because of the design’s modularity. For example, it would allow adding steering units (winches and kites) to produce energy from a larger diameter.
In scaling up plant dimensions, one idea researchers plan to explore is converting mechanical energy into electrical energy by running linear-magnetic motors in reverse.
The theoretical boundaries of such configuration appears to be a ring of about 25-km diameter, which would be the stator on which rotates a magnetically levitated Kite Gen. The tethered high power kites in such a size would fly at up to 10 km in a controlled formation, generating more than 60 GW.
::Windpower Engineering::
