By Amy Berry

Traditional Wind Farm Site Disadvantages Are Actually Advantages in Urban Settings

For most who hear the words “wind power” the mind conjures up images of towering white propellers in a wide open rural setting. These large propeller turbines, also known as horizontal axis wind turbines (HAWTs), are the standard in the large wind industry due to their excellent efficiency in converting wind to power. However, what makes them ideal for large scale wind farms (large and remote open spaces with consistent wind direction) does not necessarily make them a great fit for urban sites. In fact, the traditional limitations of vertical axis wind turbine (VAWT) technology for wind farm applications can actually turn into advantages for more urban locations.   Increasingly, homeowners and small businesses are considering VAWTs to help overcome the challenges associated with many small wind sites.

It is important to understand the difference between HAWT and VAWT technology to understand why each is well suited to particular applications. HAWTs have blades which rotate vertically around a horizontal axis, similar to a propeller on an airplane.  Propeller turbines need to be oriented perpendicular to the direction of the wind to be efficient, and in variable or more turbulent wind conditions they need to constantly re-orient themselves, losing efficiency in the process. Propeller blades are designed to use lift to propel themselves around faster than the speed of the wind. The part of the blade near the hub turns at a reasonable speed, because of their rigid outstretched blades, but the tips whir around at greater speeds; this is known as tip speed ratio. Typically the tip speed ratio of a HAWT is seven to ten times the speed of wind.

While HAWTs are efficient in using lift to maximize energy transfer and electricity production, the main drawback in an urban setting is that their tip speeds can create high levels of noise which can be bothersome to neighbors.  Some more modern HAWT designs seek to lessen noise by employing special curvature in the blades. And while wind direction in the open spaces of wind farms is fairly consistent, wind direction in urban settings is often changing.  HAWTs are not able to adapt quickly to changing wind directions, and thus operate extremely inefficiently in more turbulent conditions, as compared with VAWTs.

VAWTs include two main classes: a tall vertical airfoil style (Darrieus), and a solid winged style (Savonius). Darrieus Turbines come in a few varieties.  Some have rotors with curved blades that look like an eggbeater and rotate about a vertical axis. Another variation uses straight-sided airfoils and is called a Giromill.  Like propeller turbines, Darrieus turbines utilize some lift to capture wind energy and operate with tip speed ratios in the lower-middle range. Savonius Turbines have rotors with solid vanes or “scoops” which rotate about a vertical axis (picture an anemometer), using “drag” to allow the wind to push them around. The principle drawback of Savonius turbines is that drag produces far lower energy efficiency than the other types of wind turbines.

Traditionally VAWTs are not recommended for large wind energy production because they are a little less efficient than HAWTs, and do not scale as well to very large applications.  However, the ability of VAWTs to operate silently and efficiently in variable and turbulent wind conditions make them a viable option for urban locations in which these are common site characteristics.  The fact that they operate at lower rpm’s and with tip speed ratios only 2-3 times the wind speed means that they can produce power without creating noise.  VAWTs also readily capture wind energy from any direction, allowing them to work with the constant changing wind directions in urban settings.

At the end of the day, the most important factor is power output.  If a VAWT is able to provide ample energy output in an urban setting, then it is a real option for homeowners, small businesses and governments to consider.

Amy Berry can be found on twitter @wind2power tweeting about small wind power and the Windspire wind turbine.

The U.S. market for small wind turbines–those with capacities of 100 kilowatts (kW) and less–grew 78% in 2008, according to the American Wind Energy Association (AWEA).

With a total of 17.3 megawatts (MW) of new installed capacity, consumer demand for clean energy options is on the rise, the Association said.

U.S. manufacturers sold about half of all small wind turbines installed worldwide last year. U.S. market share amounted to $77 million of the $156 million global total. (Worldwide, about 38.7 MW of new small wind capacity was installed in 2008.)

“The U.S. wind industry is a growing bright spot in our domestic economy, and the small wind sector is no exception,” said AWEA CEO Denise Bode. “Strong federal policies like the federal investment tax credit for small wind are critical to future growth, just as adoption of a federal renewable electricity standard (RES) is essential to growth in the utility-scale market.”

Growth in the small wind sector is largely attributable to increased private investment that has allowed manufacturing volumes to increase, particularly for the commercial segment of the market (systems 21-100 kW). The still-largest segment of the market, residential (1-10 kW), was likewise driven by investment and manufacturing economies of scale, AWEA said, but also rising residential electricity prices and a heightened public awareness of the technology and its attributes.

“Consumers are looking for affordable ways to improve their energy security and reduce their personal carbon footprint,” said Ron Stimmel, AWEA’s Small Wind Advocate. “Small wind technology can be an answer to that search. As government policies have caught up with consumer interest, we’re seeing people all across the U.S. take advantage of this abundant, domestic natural resource and U.S. manufacturers have been able to meet this increasing demand.”

The study included a poll of small wind manufacturers, who project a 30-fold growth in the U.S. small wind market within as little as five years, despite a global recession. Much of this estimated growth will be spurred by the new eight-year 30% federal Investment Tax Credit (ITC) passed by Congress in October 2008 and augmented in February 2009.

Dotted all over the green and rolling landscape of Denmark, where the idea for wind power originated, are wind turbines. This renewable energy source now accounts for 20 percent of the country’s electricity needs, more than anywhere else in the world.

After traveling two and half hours from Copenhagen, I arrived at AVN Energy in Silkeborg, where I was met by the Export Sales Manager, Poul Kristensen. He proudly showed me around his company’s production site, which has more than doubled in size in the last year.

“We’ve been involved in wind power since it began back in the 1980s,” says Poul. “At first the turbine producers came to us and told us what they wanted, but over time we gained a high level of expertise which allows us to recommend the optimum hydraulic system for their application.”

In continuous pitch systems, the pitch, the position of the nacelle and angles of the blades, constantly changes in small amounts once every rotation.

In the last few years wind turbine technology has changed. Previously the wind turbines were stall machines and their position would shift only once every ten minutes. Such turbines have been superceded by continuous pitch systems, where the pitch, the position of the nacelle and angles of the blades, constantly changes in small amounts once every rotation. That could be on average 15 times per minute.

“While this optimized the production of energy from the turbine, for us, the actuator manufacturer, it presented a real challenge,” continues Poul. “Instead of hydraulics producing six long strokes per hour, they now had to give nine hundred short strokes in the same period. And it’s not just the pitch which is continuous, it is also the turbine’s operation, with the actuators needing to initiate those strokes 24 hours a day, seven days a week.

“Customers have high expectations from our products, and the number one requirement of the wind turbine manufacturers is reliability. At first this was not the case. Initially demand for windmills was on a small scale, from farmers with a single turbine powering an individual generator. Then the power distributors became involved. They built relatively small wind farms and quality needs increased. Nowadays wind power is government backed and expansion is on a huge scale; the power suppliers are making the decisions and the demands. These big investors are not prepared to finance installations unless equipment can be guaranteed for 20 years with only the minimum of maintenance.

“Maintenance of turbines is difficult and costly,” says Poul. “On land it is hard enough, but offshore it is really tough. And when the windmill is switched off for maintenance, it is not producing energy and losing income. On top of that, operators are often penalized if supply targets are not met. So a primary objective for them is to minimize routine downtime, while stoppages due to component failure have to be avoided at all costs.”

AVN put a great deal of emphasis on research and development with over 20 percent of the 70 people employed at the Silkeborg site involved in R&D. “Here in R&D it’s not just about knowing the product, it’s about thinking about new solutions to the challenges imposed by turbine design and about finding new ways of doing things,” says Johnny Fruekilde from AVN’s Research and Development department.

“Meeting the target life of 20 years for an actuator required all our expertise, and initially it seemed almost unfeasible. If you imagine the actuator as a car, it’s a bit like saying to its manufacturer that you won’t buy his vehicle unless it can travel 500,000 kilometers without replacing the oil filter, brake pads or any other wearing parts. Yet we have strived to accomplish the impossible, and our actuators should provide the 20-year life span stipulated with very little maintenance.

“At the moment though, we are working a little in the dark when it comes to actual performance in application. The continuous pitch systems have only been around for three years so we are basing our expectations on extrapolating performance results from older generation wind turbines. This is combined with virtual modeling and long-term testing on individual elements of the system.”

Simulation programs are extensively used by AVN to specify the best hydraulic and actuation system for each design of wind turbine. Following on from this though, automated physical testing is a necessity. The conditions within the wind turbines are very specific to the application. This means that AVN needs to build test rigs to their own designs that can as closely as possible replicate the situation within the nacelle and hub.

“We know that the hydraulic system can only ever be as strong as its weakest link, and early on we realized that the reliability of the sealing configuration was highly dependent upon the quality of its counterparts,” continues Johnny. “So one area we have focused on is the interaction between the surface finish of the rods and shafts of the actuators and the sealing components. A special rig was constructed specifically to test this and operates 24-7.”

Typical sealing arrangement within a cylinder

The seals within the hydraulics are integral to its performance, and optimizing their life is critical to the long-term effectiveness of the total system. Several other specially built rigs are used to measure sealing characteristics, as the dynamic demands of the application are extreme.

“The requirements for sealing of the actuator for wind turbine applications were unique,” says Per Hvidberg, Sales Engineer from Trelleborg Sealing Solutions, Denmark. “Never before had I been faced with a demand for a sealing configuration on a cylinder that produced relatively rapid short strokes continuously. And not only was there linear pressure from the rear, there could be side load too.”

Per’s relationship with the engineering team at AVN goes back a long way and when asked to support them in development of continuous pitch actuators, he, the engineering team at Trelleborg Sealing Solutions Helsingør and AVN worked together to come up with the best possible design.

“Within the actuators is a complex arrangement of seals ranging from O-Rings to specialist Turcon® PTFE based geometries and Slydring® in Orkot®,” says Per. “The unique configuration is specially engineered to enhance lubrication, optimize friction characteristics, and maximize service life, while preventing any external leakage. Some of the seals are expected to achieve the twenty year target, but it is impossible to guarantee this.”

“As this was the case,” says Johnny, “the hydraulics were designed for easy exchange of the seal set. This is mounted in a module that can be quickly bolted on and off. The minimum life expectancy of the sealing configuration, allowing for the seal that has the shortest predicted life, is seven years, but replacement is recommended after five. Other than this, and routine rod replacement, the actuators should run without maintenance except for the systematic checking that the operators do for any leakage or loss of pressure. We feel that this arrangement gives the ideal compromise between minimum required maintenance and guaranteed long-term performance. ”

Radial oil seals are commonly used within wind power applications

“Cleanliness of subcomponents is another important factor,” comments Poul. “Before assembly the system is flushed through to ensure there is no metal from machining or other debris such as dust or sand within the cylinder. Any residual matter such as this has been found to cause wear on the seals, shortening seal life and consequently total system life.

“The expanded factory has allowed us to construct a cleanroom. It’s not quite like the cleanrooms used in semiconductor or chemical processing, but it’s advanced in our type of manufacture. The cleanroom will be completely enclosed with barriers between it and the outside world and an extraction system to eliminate media that could potentially enter the actuator’s hydraulic system before it is enclosed.“

And what does the future hold for AVN?

“Growth and more growth,” says Poul. “We see the Silkeborg site expanding even further, but we are also supporting the turbine manufacturers as they enter booming wind power markets globally. We already have production facilities in India and are planning expansion in China and the US.”

Challenging requirements
The wind power actuator and its sealing system must be capable of operating at 250 bars/3625 psi with constant pressure on the rod from behind and differential side loads that control positioning. Seals must give minimal wear and facilitate dynamic movement that is continuous in short strokes, on average 900 times per hour.

Temperature resistance is needed down to -30°C/-22°F as standard and to -40°C/-40°F in the Artic. Below these temperatures the oil within the cylinder cannot function and requires warming with heating elements. Maximum temperature is 60°C/140°F. Beyond this the system is cooled, otherwise the oil becomes stressed, its viscosity is too low, and it carbonizes.

In addition, the actuators must withstand high humidity, salt spray and the rigors of wind and rain. Corrosion is prevented with advanced coating technology.

Maintenance – a daring occupation
It’s hard to imagine when you look at a wind turbine that the nacelle, or the structure that houses all the turbine’s generating components for the blades, is large enough for a man to stand up in. It has to be, because for maintenance the engineer has to enter this either through the side, but more commonly by climbing to the top of the tower, and down into the nacelle from there. That’s not easy 100 meters/330 feet high on land and even more daring when the turbines are up to 100 kilometers/60 miles out at sea.

Wind turbines: Facts and figures
The wind turbine tower is between 35 and 120 meters/115 to 395 feet high with blades of 12 to 60 meters/40 to 195 feet in length. These are attached to a nacelle which is over two meters/7 feet high and that can rotate 360 degrees on top of the tower. Each of the three curved blades of the turbine is positioned by an independently operated actuator with a stroke of 1.2 to 1.5 meters/4 to 5 feet and can be tilted through 90 degrees.

The higher the turbine and larger the blade size, the greater the megawatts of electricity produced each hour. The smallest turbines are producing one megawatt per hour while the largest yield up to five megawatts. In Europe most turbines are between 1.5 and 2.6 megawatts. The biggest used on land is 3.6 megawatts, with a number installed offshore between

4.5 and 5 megawatts. In Asia the trend has been for larger wind farms with smaller wattage turbines.
Bigger turbines are not always better; it depends on the size of the wind farm, the stability of the electricity grid it supplies and the promised output. So in some cases it is beneficial to have the option of shutting off a lower production source than a higher one, even though there are economies of scale in running a high output turbine compared to a smaller one.
On top of a turbine tower are two wind sensors checking wind direction and speed. One is the primary input and the second for backup. On installation the nacelle of the turbine is positioned inline with the predominant wind direction. Based on complex arithmetic calculations the wind turbine’s control system take the sensors input, and automatically yaws, or turns the nacelle to the wind, the actuators tilting each blade independently. Positioning is precise, to exacting tolerances, thereby optimizing energy production in the wind condition. The movement is calculated for every rotation, which may be 15 times per minute, continuously for 24 hours, seven days per week.

When choosing a site for a wind farm, analysis must prove it to have 2,500 hours of wind at 12 meters/39 feet per second over a year to make them viable to the utility companies. Wind turbines will normally operate from three meters/ 10 feet per second to 25 meters/80 feet per second, with the optimum wind speed being between 12 to 15 meters/40 to 50 feet per second. Though designed to withstand speeds up to 50 meters/165 feet per second, the control system will counter over rotation for speeds of over 25 meters/80 feet per second due to safety concerns.

The utility companies target 98 percent utilization with two percent allowance for maintenance. The turbines can be switched on and off remotely from control rooms anywhere in the world. This is done for maintenance or in response to grid changes.

On the stall turbines a braking mechanism is employed to stop the windmill. On the new larger turbines this can stress the tower, so tilting a single blade to 90 degrees normally stops them. In an emergency situation this method plus a brake will be employed. In these circumstances the windmills are stationary in well under a minute. The brake, in all cases, then holds the blades in position.

Green dreams result in 95% renewable energy
A 24-hour mains electricity supply finally arrived in February 2008 for residents of the Isle of Eigg, which lies in the Small Isles archipelago off Scotland’s west coast. So remote, it previously had to rely on expensive diesel generators to run homes. Now operational, a £1.6 million renewable energy system, which includes hydro, wind and solar power, is expected to generate more than 95 percent of its annual energy demand.

It has taken a decade for the islanders’ green dream to be realized. The idea was first raised after the community of less than 100 people bought the island from its previous owner in 1997. Now, a total of 45 households, 20 businesses and six community buildings are linked together by six miles of buried cable that form a high voltage network. This proves the future really can be renewable.

For more information contact: donna.guinivan@trelleborg.com

Renewable energy from the wind, which previously could only be generated in restricted geographic locations – typically off-shore or in remote rural areas – can now be made available almost anywhere, including urban environments, with the introduction of the AeroCam wind turbine.

The AeroCam, developed by BroadStar Wind Systems, was designed and patented for commercial applications. With its parallel rotor blades, not only does it look radically different from conventional propeller designs, but also can be manufactured, transported, installed and maintained at lower cost.

“Wind energy now can be made directly available to everyone,” says Stephen Else, president of Dallas-based BroadStar Wind Systems. “By harnessing its power in almost any setting, the AeroCam can now generate energy close to where it’s actually required. This is a new and exciting product with great potential.”

Following four years of research and development and the issuance of U.S. patents, the company is currently in the final stages of negotiations to place the product with two Fortune 100 companies.

BroadStar is also making its very first public appearance by presenting its AeroCam turbine at the WindPower 2008 conference and trade exhibition organized by the wind energy industry. The four-day event takes place in Houston, often referred to as the energy capital of the world, where experts and professionals will gather to debate the future growth of the industry.

“It’s a great opportunity to make our debut and share the unique benefits of the AeroCam with industry experts and a variety of potential customers, including wind farms, urban developers and government bodies,” says Else. “There’s a similar event in London later in the year, where we’ll introduce our new wind turbine to the European market.”

Until now, generating energy from wind power has been primarily limited to rural areas large enough to accommodate conventional turbines. The AeroCam design is more compact and can be discretely enclosed making wind power generation possible in a greater variety of locations.

The new design is based on principles first established by the French aeronautical engineer Georges Jean Marie Darrieus (1888-1979), who invented a wind turbine capable of operating from any direction and under adverse weather conditions. Darrieus machines typically have a vertical axis, whereas the AeroCam design has a horizontal axis with multiple blades, giving it the appearance of a water wheel.

The major innovation in the design, however, is the ability to automatically and interactively adjust the pitch or angle of attack of the aerodynamic blades as the turbine rotates, thereby optimizing its performance for much the same reasons a bird changes the shape of its wing in flight.

The new AeroCam wind turbine enables distributed power generation in almost any setting, including densely populated urban areas and unconventional sites such as commercial developments and corporate campuses.

In addition, there is a unique and significant opportunity to increase capacity of existing wind farms. These take up relatively large amounts of land, but typically less than 5 per cent of that area is occupied by turbines, roads and other infrastructure. Because the AeroCam is smaller and sits closer to the ground, and can capture abundant surface-wind energy without disrupting the smooth airflows that taller and larger propeller-based turbines need to operate effectively, it offers a practical, economical way to infill existing farms for greater overall power generation.

“It all adds up to a solution that delivers more power and more choice of location,” says Else, “with a lower total cost of acquisition and ownership and a faster payback period. The AeroCam has the potential to equip almost every local community, business and government building with its own renewable energy power station and it can supplement existing turbines.”

BroadStar’s technical achievement delivers a 250kW machine for $250,000 and is the first to break through the $1/watt cost barrier, which is unprecedented in the industry. Previously, the best that could be achieved in the U.S. was $1.38 and in Europe $2 of capital investment for every watt of power capacity installed.

Also more economical than most other available renewable energy solutions, including solar panels, the AeroCam enables communities to more easily and cost-effectively establish their own local power generation source or offset the energy they purchase from the grid, selling any excess energy generated back to the electric utility companies.

“In essence, our efficient aerodynamic design lends itself to smaller wind turbines, which can operate closer to the ground or on a rooftop. They can handle a wide range of wind velocities, anywhere between 4 and 80 mph. They generate their power at lower rotational speed, so there is less noise and vibration hence less wear and tear. But most importantly the AeroCam can be manufactured at a lower cost than conventional turbines. This makes the overall economic argument very compelling.

“Today there are very few turbines in the 100 to 500-kilowatt class,” says Else. “This is due to the high cost of ownership and maintenance of existing commercial designs now evolving into a super wind turbine class of 6-megawatt machines with blades exceeding 50 meters in diameter.

“This all works well assuming unlimited transmission line capacity. Unfortunately, transmission bottlenecks are becoming one of the single biggest issues for conventional wind farms, which tend to be located in remote areas where the electricity grid infrastructure was never designed to transmit the amount of energy that can now be generated and which is now required.

“The AeroCam wind turbine realizes the economic benefit of being able to place wind turbines locally, where the energy is needed and can complement existing energy sources.”

About BroadStar Wind Systems

Founded in 2004 by Stephen Else and Tom Stephens, BroadStar Wind Systems, with offices in Dallas, Beijing and Coventry, England, is an engineering and technology firm, comprised of experts in aerodynamics and turbine physics, which has developed its breakthrough technology solution for the efficient and affordable generation of wind power. With its scientifically proven and aerodynamically efficient AeroCam turbine, BroadStar makes wind-power generation more accessible and affordable, and delivers a measurable return on investment more quickly than competitive solutions. For additional information and downloadable images, visit www.broadstarwindsystems.com.

GE Energy will supply RES with nearly 500 megawatts of new wind energy capacity, and will provide commissioning and operations services as well as maintenance support. “Throughout the United States we continue to witness strong interest in the production of cleaner, wind-generated electricity,” said Victor Abate, vice president-renewables for GE Energy. “We are pleased that RES has selected our well-proven, 1.5-megawatt technology to help the company reach its build-out goals for the years ahead.”

RES Americas is part of U.K.-based Renewable Energy Systems, one of the world’s leading renewable energy developers and a leader in the global wind industry for two decades. Since 1997, RES Americas has been a leader in the U.S. wind industry, either developing or constructing more than 12 percent of the country’s installed wind energy capacity.

“We look forward to building our equipment supply relationship with GE, and view it as critical to achieving our goals for expanded ownership of wind projects in the US,” said Craig Mataczynski, President of RES Americas.

The latest agreement with RES reinforces GE’s leadership role in the rapidly growing wind industry. Since 2004, GE has achieved a 500 percent increase in wind turbine production, and its wind business revenues exceeded $4.5 billion in 2007. According to the American Wind Energy Association, over the past two years, GE has supplied wind turbines representing nearly half of the new wind capacity across the United States.

GE’s 1.5-megawatt wind turbine is among the most widely used machines in the global wind industry, with more than 8,000 installed around the world.

GE’s wind turbine technology is a key element of ecomagination, the GE corporate-wide initiative to address challenges such as the need for cleaner, more efficient sources of energy, reduced emissions and abundant sources of clean water.

About GE Energy

GE Energy (www.ge.com/energy) is one of the world’s leading suppliers of power generation and energy delivery technologies, with 2007 revenue of $22 billion. Based in Atlanta, Georgia, GE Energy works in all areas of the energy industry including coal, oil, natural gas and nuclear energy; renewable resources such as water, wind, solar and biogas; and other alternative fuels. Numerous GE Energy products are certified under ecomagination, GE’s corporate-wide initiative to aggressively bring to market new technologies that will help customers meet pressing environmental challenges.

With wind turbine design, manufacturing and assembly facilities in Germany, Spain, China, Canada and the United States, GE Energy is among the leading providers of wind energy products and support services ranging from commercial wind turbines and grid integration products to project development assistance and operation and maintenance. The company’s knowledge base includes the development and/or installation of more than 8,400 wind turbines with a total rated output of more than 11,300 megawatts.

About RES Americas:

Renewable Energy Systems Americas (RES-Americas) has been active in the US market since 1997 and has had a role in developing or constructing more than 12% of the operating wind projects in the United States and greater than 20% of the installed wind capacity in the state of Texas. RES-Americas corporate office is located in Austin, Texas, with regional offices located in Portland, OR, Minneapolis, MN, and Montreal, Quebec, Canada. RES-Americas continue to seek new locations for wind projects as well as opportunities to participate in other forms of renewable energy. For more information, please visit www.res-americas.com.

Using smoke, laser light, model airplane propellers and a campus wind tunnel, a team led by Johns Hopkins University researchers is trying to solve the airflow mysteries that surround wind turbines, an increasingly popular source of “green” energy. The National Science Foundation recently awarded the team a three-year, $321,000 grant to support the project.

The rise in oil prices and a growing demand for energy from non-polluting sources has led to a global boom in construction of tall wind turbines that convert the power of moving air into electricity. The technology of these devices has improved dramatically in recent years, making wind energy more attractive. For example, Denmark is able to produce about 20 percent of its electric energy through wind turbines. But important questions remain: Could large wind farms, whipping up the air with massive whirling blades, alter local weather conditions? Could changing the arrangement of these turbines lead to even more efficient power production? The researchers from Johns Hopkins and Rensselaer Polytechnic Institute hope their work will help answer such questions.

“With diameters spanning up to 100 meters across, these wind turbines are the largest rotating machines ever built,” said research team leader Charles Meneveau, a turbulence expert in Johns Hopkins’ Whiting School of Engineering. “There’s been a lot of research done on wind turbine blade aerodynamics, but few people have looked at the way these machines interact with the turbulent wind conditions around them. By studying the airflow around small, scale-model windmills in the lab, we can develop computer models that tell us more about what’s happening in the atmosphere at full-size wind farms.”

To collect data for such models, Meneveau’s team is conducting experiments in a campus wind tunnel. The tunnel uses a large fan to generate a stream of air moving at about 40 mph. Before it enters the testing area, the air passes through an “active grid,” a curtain of perforated plates that rotate randomly and create turbulence so that air currents in the tunnel more closely resemble real-life wind conditions. The air currents then pass through a series of small model airplane propellers mounted atop posts, mimicking an array of full-size wind turbines.

–Using laser pulses and model wind turbines, Johns Hopkins researchers are able to collect important data about the airflow that is likely to occur around full-size machines that produce “green” energy.
Photo by Will Kirk

The researchers gather information on the interaction of the air currents and the model turbines by using a high-tech procedure called stereo particle-image-velocimetry. First, they “seed” the air in the tunnel with a form of smoke-tiny particles that move with the prevailing airflow. Above the model turbines, a laser generates two sheet-like pulses of light in quick succession. A camera captures the position of particles at the time of each flash. “When the images are processed, we see that there are two dots for every particle,” said Meneveau, who is the university’s Louis M. Sardella Professor of Mechanical Engineering. “Because we know the time difference between the two laser shots, we can calculate the velocity. So we get an instantaneous snapshot of the velocity vector at each point. Having these vector maps allows us to calculate how much kinetic energy is flowing from one place to another, in much greater detail than what was possible before.”

Raul B. Cal, a Johns Hopkins postdoctoral fellow who is working on the project with Meneveau, said this data could lead to a better understanding of real wind farm conditions. “What happens when you put these wind turbines too close together or too far apart? What if you align them staggered or in parallel?” he asked. “All of these are different effects that we want to be able to comprehend and quantify, rather than just go out there and build these massive structures, implementing them and not knowing what’s going to happen.”

Meneveau pointed out that dense clusters of wind turbines also could affect nearby temperatures and humidity levels, and cumulatively, perhaps, alter local weather conditions. Highly accurate computer models will be needed to unravel the various effects involved. “Our research will provide the fluid dynamical data necessary to improve the accuracy of such computer models,” Meneveau said. “We’d better know what the effects are in order to implement wind turbine technology in the most sustainable and efficient fashion possible.”

Meneveau and Cal are collaborating with Luciano Castillo, associate professor in the Department of Mechanical, Aerospace and Nuclear Engineering at Rensselaer Polytechnic Institute, and Hyung S. Kang, an associate research scientist in the Department of Mechanical Engineering at Johns Hopkins.

The project’s funding was provided through the National Science Foundation’s Energy for Sustainability Program.