In Oklahoma, Thursday, April 8, 2010
Doors Open at 7:30 AM
8:00 AM to 5:00 PM – Networking Reception to Follow
Northeastern State University – Building A – Banquet Room
3100 East New Orleans, Broken Arrow, OK 74014

More about the sponsors:
A WIRE-Net initiative, Great Lakes WIND Network is an international advisory group and network of manufacturers and suppliers whose mission is to grow the wind industry supply chain and increase domestic content to meet the expansion needs of the global wind market.

“At a time when the construction unemployment rate is nearly 25% and the manufacturing unemployment rate is 13%, this proposal would cost 50,000 American workers their jobs.

The truth is, by law, Recovery Act grants can only be used to finance projects that are being built in the United States.

This proposal would torpedo one of the most successful job creation efforts of the Recovery Act, which has already preserved half of the 85,000 American jobs in the U.S. wind industry.

Rather than adopt policies that will kill American jobs, Congress should enact policies that will create jobs by encouraging manufacturers to invest in U.S. plants. That means passing a Renewable Electricity Standard now.

The Recovery Act has been creating jobs by helping finance new American wind energy projects that have broken ground or been completed since the Act was passed. The proposed moratorium and legislation would kill this effort and destroy the momentum for one of the few industries that has been creating jobs and economic growth.

It is unfortunate that the proponents of this moratorium and legislation are using a deeply flawed study as the basis for a policy that would destroy tens of thousands of American jobs.

We support the goal of continuing the rapid expansion of U.S. wind manufacturing. More than half of the value of wind turbines used in U.S. wind projects is domestically produced, and that percentage is increasing every year as more turbine makers build U.S. manufacturing capability.   We do not have the capability today to produce 100% of wind turbine components in the U.S., but we can grow our manufacturing base and add 274,000 American jobs if Congress passes a strong Renewable Electricity Standard.”

Electrochemical Batteries

Familiar electrochemical batteries include nickel-cadmium (NiCad), lithium-ion (Li-ion, and others

Electrochemical batteries consist of two or more electrochemical cells. The cells use chemical reaction(s) to create a flow of electrons – electric current. Primary elements of a cell include the container, two electrodes (anode and cathode), and electrolyte material. The electrolyte is in contact with the electrodes. Current is created by the oxidation-reduction process involving chemical reactions between the cell’s electrolyte and electrodes.

When a battery discharges through a connected load, electrically charged ions in the electrolyte that are near one of the cell’s electrodes supply electrons (oxidation) while ions near the cell’s other electrode accept electrons (reduction), to complete the process. The process is reversed to charge the battery, which involves ionizing of the electrolyte. An increasing number of chemistries are used for this process.

Flow Batteries
Some electrochemical batteries (e.g., automobile batteries) contain electrolyte in the same container as the cells (where the electrochemical reactions occur). Other battery types – called flow batteries – use electrolyte that is stored in a separate container (e.g., a tank) outside of the battery cell container. Flow battery cells are said to be configured as a ‘stack’. When flow batteries are charging or discharging, the electrolyte is transported (i.e., pumped) between the electrolyte container and the cell stack. Vanadium redox and Zn/Br are two of the more familiar types of flow batteries. A key advantage to flow batteries is that the storage system’s discharge duration can be increased by adding more electrolyte (and, if needed to hold the added electrolyte, additional electrolyte containers). It is also relatively easy to replace a flow battery’s electrolyte when it degrades.

Capacitors
Capacitors store electric energy as an electrostatic charge. An increasing array of larger capacity capacitors have characteristics that make them well-suited for use as energy storage. They store significantly more electric energy than conventional capacitors. They are especially well-suited to being discharged quite rapidly, to deliver a significant amount of energy over a short period of time (i.e., they are attractive for high-power applications that require short or very short discharge durations).

Compressed Air Energy Storage
Compressed air energy storage (CAES) involves compressing air using inexpensive energy so that the compressed air may be used to generate electricity when the energy is worth more. To convert the stored energy into electric energy, the compressed air is released into a combustion turbine generator system. Typically, as the air is released, it is heated and then sent through the system’s turbine. As the turbine spins, it turns the generator to generate electricity. For larger CAES plants, compressed air is stored in underground geologic formations, such as salt formations, aquifers, and depleted natural gas fields. For smaller CAES plants, compressed air is stored in tanks or large on-site pipes such as those designed for high-pressure natural gas
transmission (in most cases, tanks or pipes are above ground).

Flywheel Energy Storage
Flywheel electric energy storage systems (flywheel storage or flywheels) include a cylinder with a shaft that can spin rapidly within a robust enclosure. A magnet levitates the cylinder, thus limiting friction-related losses and wear. The shaft is connected to a motor/generator. Electric energy is converted by the motor/generator to kinetic energy. That kinetic energy is stored by
increasing the flywheel’s rotational speed. The stored (kinetic) energy is converted back to electric energy via the motor/generator, slowing the flywheel’s rotational speed.

Pumped Hydroelectric
Key elements of a pumped hydroelectric (pumped hydro) system include turbine/generator equipment, a waterway, an upper reservoir, and a lower reservoir. The turbine/generator is
similar to equipment used for normal hydroelectric power plants that do not incorporate storage. Pumped hydro systems store energy by operating the turbine/generator in reserve to pump water uphill or into an elevated vessel when inexpensive energy is available. The water is later released when energy is more valuable. When the water is released, it goes through the turbine which turns the generator to produce electric power.

Superconducting Magnetic Energy Storage
The storage medium in a superconducting magnetic energy storage (SMES) system consists of a coil made of superconducting material. Additional SMES system components include power
conditioning equipment and a cryogenically cooled refrigeration system. The coil is cooled to a temperature below the temperature needed for superconductivity (the material’s ‘critical’ temperature). Energy is stored in the magnetic field created by the flow of direct current in the coil. Once energy is stored, the current will not degrade, so energy can be stored indefinitely (as long as the refrigeration is operational).

Thermal Energy Storage
There are various ways to store thermal energy. One somewhat common way that thermal energy storage is used involves making ice when energy prices are low so the cold that is stored can be used to reduce cooling needs – especially compressor-based cooling – when energy is expensive.

Sandia Labs

Susman says his company’s role is to identify a local partner or entrepreneur, someone who lives in the community or has ties there, and preferably a significant land owner in the project footprint. “Then we form a joint venture with the local partner. For example, partners in Kay County Oklahoma, a father and son team with property in the footprint, can count several generations in the area. Their land will be used in the project along with neighbors’ land. That arrangement brings a sensitivity to projects. Our role is to make the project work for everyone in the community.”

An island community off Maine, Fox Islands Wind LLC, provides another example of community wind. “Instead of importing power from the mainland on a cable, they generate it themselves. They decided to install three wind generators, and then structured the power, financing, procurement contracts as a community with an entrepreneurial person at the lead. A development company such as ours is in the lead so all members have an ownership stake,” says Susman.

Wind projects develop in several stages. An early stage involves feasibility and gathering land for the project, getting a wind assessment of the property, and steps such as getting in the transmission queue. ”A lot of the early effort is local, a good amount of that is done by the local partner. We provide the documentation he would take around to the community. We would do the feasibility work, site assessment to figure out the farm size, make sure we are not in areas of endangered habitat, on whose property the turbines would be placed, and how it would connect to the grid,” he says.

A middle development phase is outsourced. It includes studies around transmission and permitting, and continuing the wind resource work by a third party, while OwnEnergy typically manages the third parties with input from the local partner. That person might be working with the community dealing with the land owners, and getting county tax abatement for the locals. If anyone in the community is not comfortable with the project, the third party works with that person. A final stage signs up contractors and gets the project ready for financing by wind energy lenders and tax-equity firms.

The large and small generator (green and belt driven) are just visible. Red calipers for the disc brake are in the foreground.

Not all wind farms have to be located far from power purchasers. A recent ribbon-cutting for a 120 kW turbine was right in Cleveland, Ohio, where the turbine is visible to thousands driving by on I-480 and Pearl Rd. Electrical Design Consultants President David Graneto, Pepper Pike, Ohio, says power from the turbine is sufficient for lights and equipment in three buildings of the automobile recycling yard at which the turbine is sited.

During a visit, the remanufactured Vestas V-20 (20-m rotor diameter) that sits atop a 140-ft tower, was churning out about 78 kW in a modest 5.3 m/s breeze. When originally manufactured the turbine was rated for 100 kW but the updated design can pump out 120 kW in wind of at least 10 m/s.

The turbine now sports two induction generators producing three phase 480V. The smaller generator is for low wind speeds up to 4.5 m/s and the large generator for higher winds. It cuts in at 4.5 m/s. A flat panel display inside one build shows running stats. For instance, after 351 hr of operation the unit had generated nearly 5,000 kWh of power in an urban location not known for its wind.

Costs for the turbine before incentives was about $375,000. Graneto, an electrical engineer and turbine erector working with PearlWind, calculates a payback in 8 to 9 years and sooner if power rates head up. He says a taller tower would capture faster winds which makes one wonder that with sufficiently tall towers and public acceptance, wind farms and cities could be one and the same.

Superfund sites are the most complex, uncontrolled, or abandoned hazardous waste sites identified by EPA for cleanup due to the risk they pose to human health or the environment. Brownfields are properties for which expansion, redevelopment, or reuse may be complicated by the presence of contaminants. EPA is investing more than $650,000 for the project that pairs EPA’s expertise on contaminated sites with the renewable energy expertise of NREL. The project is part of the RE-Powering America’s Land initiative, which aims to decrease the amount of green space used for development, reduce greenhouse gas emissions, and provide health and economic benefits to local communities, including job creation.

The project will analyze the potential development of wind, solar, or small hydro development at 12 sites. The analysis will include determining the best renewable energy technology for the site, the optimal location for placement of the renewable energy technology on the site, potential energy generating capacity, the return on the investment, and the economic feasibility of the renewable energy projects.

The 12 sites are located in California, Florida, Kansas, Massachusetts, Michigan, Minnesota, Pennsylvania, Puerto Rico, Rhode Island, West Virginia, and Wisconsin.

Some of the sites under consideration for renewable energy projects have completed cleanup activities, while others may be in various stages of assessment or cleanup. Renewable energy projects on these sites will be designed to accommodate the site conditions. For fact sheets on each location, and more information on the RE-Powering America’s Land initiative, visit the Web site, www.epa.gov/renewableenergyland/

The other is the almost second-by-second change to demand that leads to frequency variations. For instance, in brief periods when demand drops, generators may run fast so that frequency in the line rises over 60 Hz. And when loads come online, steam driven turbines slow a but and line frequency drops. This can play havoc with some customer equipment.

The wind industry offers a partial solution by providing some power during high demand periods. But this so-called peak shaving is not a perfect solution because even wind plants could be in a lull when power is needed. If there were a way to store the excess power, regardless of source, it would let wind plants put more turbines online for longer periods so boilers could throttle back. Short-term power storage would also let utilities really smooth out the power curves so line frequency stays closer to 60.00 Hz than it does at some periods.

A few applicants for these jobs include compressed air, flywheels, large batteries, and generating and storing hydrogen, reported on elsewhere in this magazine. Of course there are others such as fuel cells and ultra capaciators. The Electric Power Research Institute is looking into additional methods, but for the time being, those here appear the most promising.

Stimulus of compression

The Federal administration has begun doling out some $60 million to promote energy storage with the Department of Energy in particular announcing which companies will receive awards. One method for storing energy inexpensive off-peak power compresses air into underground reservoirs which can be quickly brought online as demand peaks.

Energy Storage and Power LLC (ES&P), Basking Ridge, NJ (espcinc.com) received a $20 million investment from the Public Service Enterprise Group Inc to test a recent modification to its compressed-air storage equipment already in service. Energy Storage says it has patented a method for storing compressed air which is more durable and less expensive than batteries. This technique is one proposed solution to the demand for power that varies over a 24 hour period.

The company says it focuses on developing projects that use its second generation Compressed Air Energy Storage (CAES2). Company CTO Michael Nakhamkin lead the design of the first generation 110-MW CAES plant in McIntosh, Alabama that has been operating for 17 years and says the utility has an availability record of about 95%.

Nakhamkin says the CAES2 comes from lessons he has learned and improvements to commercially available equipment. The company says it has several advantages over the first generation CAES technology. For instance, second generation equipment is based on off the shelf components and plant capacity scales from about 15 MW using above-ground tanks to over 400 MW using underground geologic formations.

Capital costs range from about $800 per kW for below ground storage to about $1,200 for small plants using above ground tanks.

Grid support is practically instant, says Energy Storage, during plant operation at some 30 to 100% of capacity. From cold shutdown, about 70% of rated capacity can be delivered in better than 3 to 5 minutes.

Flywheels

These generally use a rotating carbon-fiber composite rim levitated on magnetic bearings operating in a near-frictionless vacuum. The rim is fabricated from a combination of high-strength, lightweight fiber composites. The sturdy construction lets the flywheel spin at speeds to 16,000 rpm to store more energy than could flywheels made from metals. To reach operational speed, the units draw surplus electricity from the grid to power a permanent-magnet motor. The flywheel can spin for extended periods because friction and drag are reduced by magnetic bearings in the vacuum. Low friction means little power maintains the flywheel’s operating speed.

A series of flywheels can provide MWh-sized storage. When a grid needs power, momentum of a spinning flywheel drives its generator to convert kinetic into electrical energy.

The Smart Energy 25, a flywheel design from Beacon Power Corp., Tyngsboro, Mass., (beaconpower.com) seals a rotor in a vacuum chamber so it can spin at 8,000 to 16,000 rpm. To further reduce losses, the rotor levitates with a combination of permanent magnets and electromagnetic bearings. At 16,000 rpm the flywheel stores and delivers 25 kWh.

Although the flywheel can charge in 15 min and fully discharge in the same period, it will more likely be used 15 sec at a time because of the grid’s rapid and normal fluctuations. Beacon Power’s grid-scale Smart Energy Matrix is made of several units connected to store energy for utility applications. “This matrix can absorb and deliver megawatts of power for minutes, providing highly responsive frequency regulation for increased grid reliability,” says Beacon’s Gene Hunt.

“The units are capable of hundreds of thousands of charge-discharge cycles over a 20-year life, making them well suited to regulating frequency,” he adds. And an array of flywheels can be monitored and operated remotely as part of an intelligent grid. No hazardous chemicals or materials simplify permitting.

The company recently added 1 MW to an already working 2 MW facility that provides frequency regulation to the New England grid. The first 2 MW has been online since November 2008. In addition, The New York State Public Service Commission has granted the company a certificate for a 20-MW flywheel frequency regulation plant. Construction will complete within 18 months.

“Our flywheels provide a grid-stabilizing service and they do it faster and more efficiently than today’s conventional methods, most of which consume fossil fuel,” says Beacon President Bill Capp. One company challenge, he says, was proving the value of large-scale storage to investors without any projects to point to as examples. The company has applied for $47 million in DOE stimulus grants to build two more 20-megawatt plants, one in New York and another in the PJM (formerly the Pennsylvania, New Jersey, Maryland) Interconnection.

Batteries

At least four designs for large batteries are getting attention. One uses sodium sulfer, one is lithium-based, another uses a bromine solution, and a fourth with promise will soon leave the lab at a lower cost than the others.

North Carolina’s Duke Energy says it plans to match a $22 million federal grant to test batteries as devices for storing wind energy from its Notrees Windpower Project in Texas.

“Energy storage has potential to serve as a game-changer when it comes to renewable power,” says Duke Energy Generation Services President Wouter van Kempen. The 95-turbine Notrees wind farm has a peak energy production of 151 MW.

Southern California Edison Co. says it has asked for a $25 million stimulus grant to help Massachusetts-based A123 Systems Inc. (a123systems.com) build the world’s biggest lithium-ion battery.
The 32-MWh battery would be assembled using racks of smaller units at a substation in Southern California’s Tehachapi Mountains. The battery would counterbalance wind power sent from the mountains to the utility’s customers in the west and south.

A battery from Japan-based NGK Insolator, Baltimore, (ngk.co.jp/english) uses a sulfur as a positive electrode and sodium as a negative electrode. Beta alumina, a conductive ceramic, separates them. Connecting a load to terminals lets current discharge through the load.

The Electric Power Research Institute, the U.S. utility industry’s R&D consortium in Palo Alto, Calif, says such storage would allow widespread use of renewable power and make the grid more reliable and efficient. Announcements from utility American Electric Power (AEP), Columbus, Ohio suggest that grid storage equipment is ready for commercial deployment in the U.S. AEP has ordered three multi-megawatt batteries and set goals of having 25 megawatts of storage in place by 2010, and 40 times that by 2020. The AEP design uses NGK Insulator’s sodium-sulfur batteries and controls to manage the flow of ac power in and out of the dc battery.

AEP energy engineer Ali Nourai says the company and other U.S. utilities have confidence in the viability of such storage thanks to a demonstration project in Charleston, WV, where the utility installed a large battery in June 2006. Peak demand in Charleston’s summer and winter was overloading transformers at local substations causing blackouts. Rebuilding the substations to accommodate more power could have taken as much as three years. Instead, AEP spent just nine months installing a battery that charges during low power demand and delivers up to 1.2 MW for seven hours when demand peaks.

A more recent sodium-ion battery provides another practical option for storing power, says Carnegie Mellon University material science Professor Jay Whitacre. His startup, 44 Tech, will receive $5 million from the DOE to develop the idea.

Whitacre says the startup’s batteries could be cheaper and longer-lasting than current designs for the same use because sodium sulfate in his design is more abundant and less expensive than lithium. In addition, sodium sulfate is uses as a food preservative making it almost harmless to handle. To trim costs further, Whitacre plans operating at lower cell voltages than other battery chemistries.
Matt Rogers, senior advisor to Energy Secretary Chu, estimates the battery will handle large amounts of energy for about one-tenth the price of similar technology.

The change to sodium-sulfate electrolytes could also make it possible to eliminate much of the supporting material needed in conventional lithium-ion cells, again reducing costs. This is because increasing the ionic conductivity makes it possible to use thicker electrodes with fewer layers of separating and current-collecting materials inside the cell. Whitacre says the first battery will be ready for testing soon and for three or four years at different substations.

The final battery in this quintet also uses a flowing chemical electrode to store up to 50 kWh in a self-contained unit. Unlike the other batteries here, the ZESS (zinc energy storage system) is based on fuel-cell ideas, according to the company, and combines aspects from both battery and fuel cells. The battery represents an environmentally friendly and cost-efficient alternative energy storage, says developer ZBB Energy Corp., Menomonee Falls, Wisc, (zbbenergy.com)

One ZBB module stores 50 kWh. The ZESS 500 is said to be a 500 kWh ‘plug and play’ system consisting of ten of the company’s standard 50 kWh modules, with power electronics. The firm says the units are scalable and mobile. Each 50 kWh battery module is composed of three cell stacks, each with 60 cells in a series. Users can charge the battery from a variety of power sources at different charge rates and it can fully discharge repeatedly without damage. Modules are self-contained, and a control system takes care of energy storage and safety functions.

The company refers to the design as a Regenerative Fuel Cell (RFC) and adds that it relies on a flowing electrolyte with features such as:

• Chemical reactions that take place in the cell stack and excess electrolyte stores in external tanks.

• The predominantly aqueous electrolyte is composed of zinc-bromide salt dissolved in water.

• During charge, metallic zinc is plated from the electrolyte solution onto the negative electrode surfaces in the cell stacks.

The ZES500 is a bedmount of ten 50 kWh ZBB batteries, capable of storing 500 kWh. Facilities would use units of this sort by charging them during low-power-cost periods so power is available during at high cost times.

Bromide converts to bromine at the positive electrode surface of the cell stack and stores as a safe, chemically complex organic phase in the electrolyte tank.

Each cell stack has 60 bi-polar electrodes between a pair of anode and cathode-end blocks. It operates quietly and at ambient temperature.

Electrodes don’t take part in the chemical reactions. They are substrates for the reactions. That means no loss of performance from repeated cycling that often causes electrode material deterioration.When the ZESS discharges, the metallic zinc plated on the negative electrodes dissolves in the electrolyte and is available for plating at the next charge cycle. And it can be left indefinitely in a fully discharged state.

In one application, a ZESS 500 battery will store power generated by an 850 kW wind turbine that already provides half the power for Ireland’s Dundalk Institute of Technology Centre for Renewable Energy Project. This installation of a ZESS 500 with a wind application will let the campus operate independent of the electrical grid.

A better understanding of wind flow through cities will give companies that manufacture and erect turbines, such as Mariah Power, Reno, Nevada, a better idea of where to place units for maximum power production.

Controversy seems to follow the installation of wind turbines on building rooftops, and for good reason. On one hand, there can be considerable wind with harvestable kinetic energy at roof level. Accessing this clean, renewable source of power can be a good way for building owners to save money on their electric bills, reduce their dependency on the grid, and trim their carbon footprint. On the other hand, it has been difficult to understand how the wind behaves in built up areas, resulting in low power outputs and ultimately, bad reviews. Historically, many people have installed rooftop turbines on the assumption that even if the devices don’t work well, they’ll look good.

The challenges
As with traditional tower-mounted wind turbines, if a building-integrated wind turbine (BIWT) is to be successful, it must be assessed in terms of its net economic benefit. To date, BIWT installations have consistently missed power output expectations, as much as 90% in some cases. After careful review of BIWT installations on a global scale, it is clear that a main culprit behind these underperforming wind turbines is their placement. This conclusion is confirmed by a section of the American Wind Energy Association’s 2009 Small Wind Market Study calling for improved assessment technology.

Part of the problem is that urban and suburban areas include considerable turbulence and turbines mounted there have simply not been capturing enough laminar wind. Predicting wind-energy quality and density is more complicated in built up areas, such as cities, than in rural plains where there are few obstacles upwind of the turbine.

Traditional methods of assessing wind-energy density each have drawbacks, especially when applied to more complex areas. Anemometers must be set up on their own meteorological towers and collect data for at minimum 3 months, but really a year or more is required to be accurate. Wind maps are not built for site selection as much as screening, with even the tightest resolutions still overlooking local effects. Computational fluid dynamics studies show promise, but are still quite costly and difficult to set up properly. Existing technology leaves building owners and wind installers ill equipped to properly analyze the complex wind conditions, leading to poor site selection and inadequate power output. As a result, most BIWT’s are commissioned without a serious wind study to determine a best location, or in many cases, even if the site is appropriate at all. So it is not surprising that most building mounted turbines have failed to reach their expected power outputs. With such results, it is also not surprising that many have criticized the use of wind turbines on building rooftops.

BIWTs have yet to prove themselves to the small-wind community accustomed to installing turbines on tall towers in open terrain. However, BIWTs have one advantage over ground-mounted turbines typically positioned in good wind 40 meters up: BIWTs don’t need a costly 40-m tower to reach that height. On a sufficiently high building, typically one will never need more than a 10-m tower to clear turbulence caused by wind hitting the roof edge. Assuming equal installation costs, a 10-m tower on a 10 story building could cost $20,000 less than a 40-m tower reaching comparable heights. Despite this, for BIWTs to be taken seriously though, performance must improve, and for performance to improve, siting must improve.

The way forward
To understand wind in urban areas, it is necessary to consider many things ignored by existing evaluation tools. One must address the timing issues of anemometers, the accuracy issues of wind maps, and the cost issues of CFD studies. It is first important to consider the topography and texture of the area for several kilometers around the target site. This accounts for general turbulence in the local environment, a condition called roughness. Understanding this is required because even in cities like Atlanta or Los Angeles where there are relatively few tall buildings, the air is still quite rough due to the presence and extent of many smaller buildings.

Perhaps most important is the need to consider local effects, factoring for the size, shape, height, and distance of various obstructions. This incorporates the wind energy impacts of nearby trees, buildings, and other structures to understand how much the wind is blocked, what turbulence is created, and in some cases, how much the wind speed is increased. Further, if mounting on a building, it is critical to account for effects from the building, details such as its surface, roof edge, and roof features such as towers or chillers. For example, when wind hits any obstruction, it creates a separation zone arching out from the top of its vertical face. Above this point, the air remains smooth, but below, it becomes quite turbulent. This behavior must be considered in every rooftop-wind project to figure out how much higher the turbine must be mounted to capture energy from the smooth airflow.

So to improve the performance of building integrated wind turbines, one must consider local roughness, blocking, turbulence, and roof dynamics, something most assessment tools do not do. Furthermore, customers want answers quickly regarding wind energy potential at a reasonable price, another thing most tools have not done well.

Analysis services however, are available which meet these market needs – information on wind energy and expected performance can be delivered quickly for a small fraction of installed turbine cost. With better data, installers and customers can make better decisions, which will lead to better performance of building integrated wind turbines. This opens up the urban wind energy market for explosive growth in the coming years.

A few company initiatives are available at www.windanalytics.com.

The turbine is an NREL Phase VI UAE design. An unstructured mesh can discretize the model without simplifying its relatively complex geometry.

A valuable tool in this process is Computational Fluid Dynamics (CFD) software. It has traditionally played a role in the design of rotorcraft, fixed-wing aircraft, and even wind-turbine blades. Previous limitations in computing power kept most simulations focused on small portions of a design with a limited inclusion of physical phenomena. Perhaps the most accepted use of CFD in the industry is for analysis of 2D airfoils.

Although an important application, recent advances in computing power and software provide greater capabilities for wind-turbine designers.
For example, multi-physics simulations are an important and emerging capability within computational fluid dynamics. Historically, CFD software focused on predicting fluid and thermal transport. Recent advances, however, now allow coupling or including additional physics into the simulation of the flow field. For instance, one technology with tremendous implication for wind-turbine design is Fluid-Structure Interaction (FSI). As turbine-blade designs become larger and use new materials, blade deflection under wind load becomes increasingly important.

Consider that in high wind, a blade could deflect to the point where it hits the tower. Turbine designers must also consider the change in the blade’s aerodynamic performance, and fatigue concerns can arise as a result of blade deflection. Engineering software providers such as ACUSIM enable engineers to efficiently and accurately simulate the behavior of rotating and deforming wind turbines.

ACUSIM’s finite-element based CFD solver, AcuSolve, contains two techniques for simulating such behavior in wind turbines. One technique uses Practical Fluid-Structure Interaction (P-FSI) technology for linear (small) structural deformations. This relies on a modal superposition approach to compute a structure’s deformed shape. The fluid loading on the blades provides excitation of the various modes of the structure, and the resulting deformation is the sum of each modal contribution. The approach is fast, reliable, and simple. All fluid and structural computations are performed by the CFD solver, so there is no run-time coupling of the solver to external codes. In the second technique, for nonlinear (larger) bending applications, the software supports run-time coupling to external structural dynamics codes using Directly Coupled Fluid-Structure Interaction methods. This lets users couple AcuSolve to a preferred structural solver, and requires no intervening middle ware to accomplish the coupling. The solver software handles the interpolation between dissimilar meshes (between the structural model and fluid model) and communication requirements. In addition, because the communication architecture relies on a standard socket connection between computers, structural and CFD codes can run on different compute resources using different operating systems.

Iso-surfaces of Q criterion are ones that outline flow regions with local rotation, indicating a turbulent eddy. “Q” is a mathematical quantity commonly used to illustrate turbulent vortices in a flow field.

To demonstrate the FSI technology, we have simulated a fully coupled fluid-structure interaction on the NREL Phase VI UAE wind turbine. This design has a 10-m diameter rotor with two twisted and tapered blades. A fully unstructured mesh speeds model construction and solutions. The illustration An unstructured mesh shows the model mesh and complexity of the geometry, including the instrumentation hardware mounted on the front of the rotor. The software’s reliability, speed, and accuracy on unstructured meshes make it unnecessary to perform excessive geometric simplifications to the model.

The constant speed turbine was simulated at 72 rpm with a uniform inflow wind speed of 10 m/s. The Spalart-Allmaras based Delayed Detached Eddy Simulation turbulence model provides high resolution of the turbulent content in the rotor wake. The flexible blades were represented by a total of 20 structural modes using AcuSolve’s P-FSI technology. The blades were “softened” or made less stiff to make their flexing more visible, and to show that the solver can handle large structural displacements. The transient was simulated for six rotations in two-degree time steps. Undeformed and displaced blades is a picture at one time step and reveals the degree to which the blades respond to the wind loading.

Streamwise blade-tip displacement shows the time history as a function of the rotation. It highlights the unsteadiness in the simulation. The time varying displacement is induced by an unsteady turbulent loading on the blades, along with the blade and tower interaction as the rotor turns.
Numerical simulation using CFD will become increasingly important as designers pursue more advanced wind turbines. Combining unstructured meshing with an efficient CFD solver and multiphysics capabilities provides a powerful tool for designing wind turbines. Designers can use the software for standard aerodynamic simulations using rigid structures and the advanced capabilities of the solver for simulating complex phenomena such as active flow control and variable-pitch blades.