May/June 2009

In a recent report covering the last quarter of 2008 in Europe, 54 gearbox failures attributed to either wear or failure equaled 8,177 hours of downtime. This downtime includes the actual repair, lost production, and crane costs. Since a catastrophic gearbox failure on an average megawatt class wind turbine costs somewhere between $225,000 and $300,000 to remediate, more and more people are looking for maintenance methods to address costly gearbox issues.

No matter the industry, it seems that maintenance practices fall into three basic categories:

• Wait until it breaks;
• Maintain at an interval;
• Predict maintenance.

The approach that we are focusing on in this article involves using vibration analysis to predict maintenance. There are many tools for predictive maintenance, with vibration being one of the most prolific. While predicting maintenance using vibration is widespread in other industries, for various reasons—and despite some challenges—the wind industry is quickly adopting a predictive maintenance culture.

Inspection Info
It is generally accepted that most things fail in a “bathtub” type failure curve. It has a higher probability of failing early in its service life, or later in life as it wears (fig. 1). It stands to reason that testing of gearboxes needs to occur during assembly, commissioning (both early stages), and during the life of the wind turbine to catch the other end of the failure curve. In addition, gearbox rebuilders as well as manufacturers would be wise to adopt vibration acceptance testing prior to sending gearboxes out into the field (fig. 3). The reasoning is simple, really—Do you want to change a bearing or gear in your repair facility or up tower?

In the table labeled fig. 2 you can see how the common wind turbine inspection methods compare. This table was created with help from Don Roberts, project engineer with DNV Global Energy Concepts, Inc. DNV/GEC is a company that, among many other services, provides these types of inspection. While there are exceptions, this table holds true for many of the most-common gearboxes in wind when using these inspection techniques. What we are trying to illustrate is an understanding as to how these inspection techniques compare in this application. You can see that an open inspection leaves most of the gearbox unseen. A bore scope inspection is better, but on average leaves large gaps in detection.

Vibration detects all of the meshing and bearing fault frequencies. What it does not do, however, is specifically identify which of the three planetary bearings is failing. Vibration will simply tell you that one of the three bearings is at fault. Not that this is a bad thing, because the end result is the same. Furthermore, vibration will detect conditions that are not seen by a camera or the human eye. These are issues such as imbalance, misalignment, mechanical looseness, and resonance conditions.

Various Vibrations

Vibration analysis is as easy as you want to make it. With that said, in wind turbines it is difficult to get right, which is probably why so many get it wrong. Let’s opt for easy, though grossly under-explained, just for explanation’s sake. The peaks indicate in frequency specifically where a problem is occurring. In fig. 4 it is the planetary gear mesh frequency. The amplitude (height) of the peaks indicates the severity of damage at that specific location. In this image the problem is at the planetary section, and at the early stages based upon the amplitude. So, with vibration, not only do you know where, but relatively how bad. Again, the frequency indicates where and the amplitude indicates how severe.

Gear Mesh Frequency (GMF) is a vibration signal that occurs when two gears mesh together. The higher in amplitude the signal, the more severe the damage, and the further it has progressed. A very basic explanation involves the following equation:

Gear (tooth count) x RPM = GMF

In a vibration signature that is displayed in a “spectrum,” a gear with eight teeth running at 15 rpm would have a GMF at 120 rpm. This is the basic math of why we see peaks in vibration at GMF. This is greatly oversimplified and not completely accurate for planetary gearboxes, but it provides a basic understanding of what generates GMF. Two gears meshing together times the speed. Of course, you don’t need a calculator to determine GMF because these calculations are likely in a wind turbine vibration monitoring software program (fig. 5a & fig. 5b). The point is that vibration analysis does detect gear issues specifically. And again, the frequency indicates where, and the amplitude indicates how severe. 

Another generalization is that a gearbox is a box with gears and bearings. It is similar in math as in determining gear problems, but instead of teeth bearings have rolling elements. So if there are 14 rolling elements in a bearing, RPM paired with an equation is used to locate the fault in a vibration signature in a bearing. And while there are four parts to a bearing—rolling elements, cages, and inner and outer races—they are similarly detected in the vibration signature using RPM. Most major wind turbine vibration software companies should have thousands of bearings in a library, making identification of a bearing or its subcomponents easy (fig. 6).

It really is overkill to detect all of these subcomponents since one likely doesn’t care which one is bad and just needs to know that a bearing should be replaced. While this is correct, if a trend is noticed in a specific part of the bearing failing, it may actually point to another root cause altogether. An example would be if several inner races in the same bearing across several gearboxes were failing. This might point to either the incorrect bearing fitment or improper installation.

Why Online?

A traditional handheld vibration instrument is not viable for many reasons in wind turbines. Safety, variability of wind—or no wind—and infrequency of measurements are just the start of why a traditional portable is not feasible. There are at least a dozen more legitimate reasons and challenges to a portable approach, although such a unit is fine for test stands in rebuild shops for this industry.

An installed online vibration monitor is a far better approach because it doesn’t involve visiting the site or climbing the towers. Also, you can corral vibration information from several farms remotely. As many negatives as there are to portables, there are just as many positives to an installed condition monitoring system.

One trend to consider involves European insurance companies that feel so strongly about condition monitoring they make it part of the policy requirement. Basically, either you use vibration condition monitoring or you rebuild the gearbox about every five years. The choice is yours… if you want their insurance, that is. One would imagine that, in the United States, similar condition monitoring requirements are not far off. Besides safety, the demand for production, reliability for investors, and PPAs are but a few reasons why an insurance company would dictate similar condition monitoring requirements in the States.

Cost Justifications

To provide an example, over the 20-year design life of a wind turbine let us assume that one gearbox failure occurs during that period on half the farm. Over a typical 100-unit wind farm project, this means 50 gearboxes will fail over the life of the project in this particular example. In a second example we will use the European insurance suggested interval, which is at about the five-year interval. This assumes that, over the 20 years, there will be a gearbox failure every five years. Again, on average, and across half the assets, this adds up to 200 rebuilds in the second example.

So, per 100 turbines, the range on gearbox failures over 20 years would be 50 gearboxes in the first example and 200 in the second. If you take the number we discussed earlier, which is about $250,000 per average catastrophic event, the expense costs on 100 turbines would be $12,500,000 in the first example and $50,000,000 in the second.

This only assumes a gearbox failure. We can add in generators and main bearings failures and the numbers jump measurably. Considering that the current cost of an online wind turbine vibration monitoring system is in the $6,000-$15,000 range per turbine, the cost of predicting maintenance far outweighs the maintenance costs even when using the most expensive system in the best-case scenario. To add a finer point, remember that this justification was just for gearboxes, excluding main bearings and generators.

Furthermore, the cost of one gearbox failure alone would cover the cost of outfitting somewhere between 16 high-cost and 41 low-cost system turbines with condition monitoring for the life of the asset. An online system such as this would also monitor not only the gearbox, but also the other common high-cost repairs such as the main bearing and generator.

Another perspective on this approach to maintenance is to look at the cost of a system spread over the life of a wind turbine. The cost of a wind turbine vibration monitoring system over the usable 20-year life ranges from less than a dollar to about two dollars a day. Again, systems cost between $6,000-$15,000 per turbine, which means that the prorated cost of installation is less than a daily cup of coffee. Put into these terms, it’s easy to see how this affordable monitoring system will pay for itself many times over its service life.

Avoiding Catastrophe

Some will say that, if the gearbox is failing, what are you really saving? It will surely fail anyway. Most of the gearbox problems caught using vibration monitoring involve up-tower repairs because they are detected early, before catastrophic failure occurs. As a common example, a high-speed shaft repair can be up-tower when detected early. Other detected conditions—such as misalignment, for instance—are also reversible before damage results. It is common to see months of downtime when problems aren’t caught in advance by using vibration analysis in gearboxes.

Secondly, with long lead times for repair parts, having several months’ notice to have them delivered, along with cranes and other necessary equipment, provides a definite production edge. Maintenance decisions such as these can revolve around wind season, other repairs, or when the repair would be most advantageous. And don’t forget due diligence inspections for newly purchased sites.

Conclusion
By using the tools available to the wind-power industry, one has the ability to run maintenance on their own terms. That’s really at the core of what predictive maintenance provides. For wind this translates into knowing what needs to be fixed before it fails. It also means that crane callouts can be grouped for multiple repairs. Also, parts can be ordered in anticipation of making repairs. And, finally, repairs can be scheduled around wind season to maximize production and minimize downtime. Predicting maintenance is like predicting production, in that reliability equals production.

This is a case study of multiple wind turbine towers located in different villages in Alaska where severe arctic weather conditions exist. Initially, a reinforced concrete (RC) mat foundation was utilized to provide vertical and lateral support. Where soil conditions required it, a pile foundation solution was devised utilizing a 30” thick RC mat containing an embedded steel grillage of W18 beams supported by 20”-24” grouted or un-grouted piles. The mixing and casting of concrete in-situ has become the major source of cost and difficulty of construction at these remote Alaskan sites. An all-steel foundation was proposed for faster installation and lower cost, but it was found to impact the natural frequencies of the structural system by significantly softening the foundation system. The tower-foundation support structure thus became near-resonant with the operational frequencies of the wind turbine leading to a likelihood of structural instability or even collapse. A detailed 3D Finite-Element model of the original tower-foundation-pile system with RC foundation was created using SAP2000. Soil springs were included in the model based on soil properties from the geotechnical consultant. The natural frequency from the model was verified against the tower manufacturer’s analytical and the experimental values. Where piles were used, numerous iterations were carried out to eliminate the need for the RC and optimize the design. An optimized design was achieved with enough separation between the natural and operational frequencies to prevent damage to the structural system, eliminating the need for any RC encasement of the steel or grouting of the piles.

Introduction
Wind towers have to sustain continuous vibration-induced forces throughout their operational life. The operating frequency of the three-blade turbine could potentially cause dynamic amplification of these forces, significantly posing a threat to the overall structural integrity. Sufficient separation of the structural system’s natural frequency from the turbine operational frequencies is key to avoiding potentially catastrophic failures. The turbine operating frequency is typically lower than the structural system natural frequency, but could approach it as higher turbine output is obtained. Idealized assumptions of fixity at the base of the tower are un-conservative; a more realistic analysis accounting for foundation flexibility yields lower estimates of the natural frequency for the system. In such cases, soil-foundation-structure interaction needs to be considered.

Structural Description
The owner, Alaska Village Electric Cooperative (AVEC), desired to purchase new or used towers to provide wind generated energy in concert with its diesel systems in the Alaska villages of Hooper Bay, Chevak, Gambell, Savoonga, Mekoryuk, Kasigluk, and Toksook Bay, as well as others. The tower models differ in height (23-46m), weight, and wall thickness. Furthermore, the three-bladed turbines vary in weight (7812-7909kg), blade diameter (19-27m), and power output (100-225kW). Towers were from a variety of manufacturers and suppliers. The turbine and tower packages ultimately utilized were supplied by Northern Power Systems of Barre, Vermont. Fig. 1 shows some of the installed towers.

Operational Issues
As the wind turbine blades start to rotate from rest, their circular speed increases, and the induced vibration frequency increases. Depending on its power output capacity, the turbine blades rotate at maximum rotational (circular) speeds that range from 45 to 60 rpm, corresponding to 0.75 to 1.00 Hz. These operational frequencies are very close to the range of natural frequencies of the entire soil-foundation-tower-turbine system.

If more output power is desired, higher rotational speeds have to be accommodated. A poor design decision would involve a maximum rotational speed that is very close to the natural frequency of the structural system, resulting in a high likelihood of resonant amplification causing structural instability. Another poor design would have a rotational speed not very close to yet higher than the natural frequency of the structural system. In such cases the structure would have to endure violent near-resonance vibrations as the operational frequency approaches the natural frequency while speeding up to and down from the maximum speed. This situation would result in very high dynamic forces, which could cause immediate damage to the structure. Even if these dynamic forces do not exceed the structure’s strength capacity, fatigue-induced failures could also be encountered.

A sound design would avoid allowing the operational frequency to approach the vicinity of the natural frequency by a certain safety factor. A safety factor of 15 percent of the natural frequency was recommended by the turbine vendor and adopted by the authors for this project.

Design Objective
In order to develop a sound overall structural system that meets the structural performance requirements of the wind towers, the dynamic interaction of the supporting soil, foundation, and superstructure needs to be considered. Since the tower and turbine are prefabricated and manufactured, once selected for a certain installation location only the foundation can be designed and fine-tuned in accordance with the site soil conditions and desired system frequency.

Depending on the soil conditions, the optimum foundation system needs to be selected (spread footing, deep piles, micro-piles, etc.). Additionally, the foundation must have adequate stiffness in order to maximize the system’s natural frequency within practical limits. A suitably stiff soil-foundation-structure system will allow for higher power output generated by the turbines.

Foundation Design
Based on the geotechnical conditions at the different sites, two types of foundations were selected; large spread foundation, and deep piles. A 5’ deep, 12’x12’ reinforced concrete (RC) spread footing was utilized to provide the system with vertical and lateral support, as well as damping and stiffness. Where soil conditions necessitated it, a pile foundation solution was devised utilizing a 30” thick mat of RC foundation embedded with a steel grillage of W18 beams founded on 20” grouted piles.

After some installations were made, it was determined that the mixing and casting of concrete in-situ is the major source of cost and difficulty of construction. An all-steel foundation was proposed for faster installation and lower cost, but such a foundation system impacted the natural frequency and significantly softened the system. Consequently, the foundation design was driven by the system’s natural frequency. Multiple solutions combining different pile sizes, grouted and un-grouted, and different beam sizes were devised. The optimum design was selected for each location based on the highest practically obtainable natural frequency and cost effectiveness of the design.

Modeling and Analysis
A detailed 3D Finite Element Analysis (FEA) model of the tower-foundation-pile system was created using SAP2000. The tower was modeled using a fine mesh of thin shell elements, while the steel grillage and piles were assigned the appropriate cross-sectional properties. Thick plate elements were utilized to model the RC foundation. In order to capture the soil-foundation-structure interaction, compression-only springs were devised to mimic the soil around the piles. Soil damping properties were conservatively neglected and the turbine mass was lumped at the hub height above the top of the tower. The natural frequency from the model was verified against the tower manufacturer’s analytical and the experimental values.

Discretization of FEA elements into sub-elements is not as straightforward a task as some may believe. Unfavorable discretization can give rise to subsequent numerical difficulties. In vibration analysis, for example, abrupt changes in element size should be avoided, as such changes tend to produce spurious wave reflections and numerical noise [4]. Consequently, a simplified tapered frame element was devised to model the tower instead of the thin-walled shell elements. Negligible deviations of the results from the two modeling techniques were observed. Thus, all design optimization runs utilized the tapered frame element. Fig. 2a, Fig. 2b shows the two different modeling techniques.

The piles were modeled using frame elements, meshed into 1’ segments. The large spread footing, on the other hand, was modeled using a 3D solid element with RC properties (Fig. 3a & Fig. 3b). The solid element was meshed into sub-elements using an intelligent algorithm consistent with the object-based FEA modeling of the SAP2000 program. Fig. 4, Fig. 5, Fig. 6 show the discretization of the piles and footing with the application of the soil springs to the meshed surfaces.

Results
Where piles are used, numerous iterations were carried out to eliminate the need for the RC and optimize the design once a comfort level with the modeling technique was reached. The foundation system design was optimized through a parametric sensitivity-based approach in which the radius of the pile group, grillage beams, and pile sizes were varied to produce comparable alternatives. It was found that the radius of the pile group had the most impact on the system frequency. A favorable radius was selected using a set of typical grillage beam and pile sizes. A series of further variations to the beam/pile sizes and different combinations yielded an optimized foundation design for each site. The optimized designs were achieved with enough separation (15 percent) between the natural and operational frequencies to prevent damage to the structural system. The optimization eliminated the need for any RC encasement to the steel foundation or grouting to the piles, in many cases.

In most cases, an optimized foundation system design for a particular site was also found to be satisfactory for other locations. Thus, a small library of universally applicable standard designs was compiled in an effort to keep the fabrication cost low. Tab. 1 summarizes the final design for two of the tower locations and demonstrates how one optimized design is adequate in two locations with different geotechnical conditions. Fig. 7 shows one of the optimized all-steel tower support foundations.

Conclusions
The foundation system design was controlled by the natural frequency of the soil-foundation-structure system rather than by strength or serviceability considerations. Taking into account the soil-foundation-structure interaction yielded a more realistic estimate of the natural frequency. Had a fixed-base-tower assumption been adopted, significantly under-designed systems would have been incorporated.

References:
1) American Concrete Institute. (2004), ACI 351-04 Foundations for Dynamic Equipment, Farmington Hills, Michigan.
2) J.P. Saylor & Associates, Consultants Ltd. Vestas V27-225kW Specifications and Technical Data, Des Moines, Iowa.
3) Golder Associates, Inc. Geotechnical Reports, Anchorage, Alaska.
4) R.D. Cook, D. S. Malkus and M. E. Plesha. (1989), Concepts and Applications of Finite Element Analysis, John Wiley & Sons, New York, New York, pp. 553-582.

Business is booming at Moventas, a leading tier-one supplier of gearboxes for wind turbines. Company sales increased 40 percent last year, reflecting tremendous growth of this alternative resource in the face of sky-high energy prices. According to the World Wind Energy Association, global installed wind turbine capacity has grown tenfold in the last 10 years and has increased 25 percent just in the past year. Predictions from the association indicate that the 74 gigawatts of current worldwide wind-powered electrical generation will more than double by 2010, when levels are expected to reach 160 gigawatts.

To meet this growing demand, more wind turbines—and ever-larger units—are needed quickly. Speeding up development is a daunting task, however, given the increasing complexity of the designs and the need for machines to operate reliably for decades in adverse weather conditions. These issues all come down to considerably more tests to be performed on each of the custom-designed units. Jari Toikkanen, manager of the Research and Test Group at Moventas, has seen the number of noise and vibration tests quadruple in the last five years, with many projects requiring same-day turnaround. Figure 1

“In addition to greater product development efforts for these units, wind turbine OEMs are demanding more vibration tests that measure behavior in greater detail than ever before,” says Toikkanen. “Tests are done primarily to improve product reliability and meet strict demands from regulatory agencies such as the AGMA (American Gear Manufacturers Association) and European ISO standards.”

Studying Gearbox Resonances
He notes that particular attention is focused on studying vibrations of the wind turbine’s massive gearbox, which uses a combination of planetary and helical gearing to step up rotor speed 100-fold for driving the electrical generator. Another major component of interest is the torque arm connecting the gearbox to the turbine framework. For large three-megawatt rated models made by Moventas, the gearbox weighs around 30 tons and measures two meters in diameter and two-and-a-half meters in length. The torque arm is four meters wide from bushing to bushing, a half meter thick, and weighs another five tons.

Engineers perform extensive modal impact testing to ensure that resonances of these components do not match the excitation frequencies of the surrounding structure or gear mesh frequencies, thus exciting potentially damaging vibrations in the framework, rotor blades, drive shafts, and the huge tower, the tallest of which is over 120 meters. Generally, the goal is to avoid the modal frequency range of 80 to 250 Hz for the torque arm and 400 to 800 Hz in the rest of the housing structure. When resonances are identified within or near these ranges, engineers shift the modal frequencies by modifying the geometry of the gearbox components and torque arm, typically optimizing stiffness properties by changing part thicknesses and shapes. Figure 2

Toikkanen notes that the process is complicated by the variable gearing frequencies that excite gearbox and torque arm vibration modes at different rotor blade speeds, from an input rotation of a few rpm for a light breeze to a maximum of 10 times that for gale-force winds. Further, Moventas is also performing additional tests and studies of torsional vibration beyond the scope of their current resources.

Boosting Test Productivity
The former test solution at Moventas presented challenges in completing work in a timely manner. Test equipment was awkward to move between test rigs, test setups were typically lengthy ordeals, and engineers had to spend time on multiple test runs because only two channels were available for modal analysis. Also, measurement data had to be post-processed before results could be viewed, thus requiring tests to be completely re-run if sensors were not properly connected, for example, or if more detailed study was needed to troubleshoot unexpected problems.

These limitations were overcome when Moventas implemented the LMS Test.Lab software with an LMS SCADAS Mobile data-acquisition system that has eight channels, enough to take all modal analysis measurements in a short time. The system contains an integrated suite of tools Moventas engineers need for modal analysis—test set-up, control, measurement, signal conditioning, result analysis, data management, and report generation—all in a lightweight, portable laptop-size unit.

“We can easily carry the unit between test rigs at our facility, and if necessary our engineers can go at a customer or end-user site very quickly to provide support or troubleshooting,” says Toikkanen. “Also, the system is extremely convenient to set up. Built-in workbooks and prompts show us step-by-step where to enter parameters and how to proceed through the process. Templates even fill in values we’ve used in the past that aren’t likely to change. Geometry models showing the placement of accelerometers on the gearbox housing are especially useful and easy to configure. From start to finish, setups with LMS Test.Lab are very fast and easy, so we’re ready to take measurements in a few minutes rather than several hours.” Figure 3

Another capability of LMS Test.Lab that greatly improves testing productivity is online monitoring. “We can see results immediately as measurements are being taken instead of waiting hours for post-processing,” says Toikkanen. “With real-time visualization we can verify the test on the spot, see firsthand how the structure deforms with every hammer impact, and readily identify the root cause of any unexpected resonances.” Figure 4

Visualization is particularly helpful to Moventas engineers with the animated mode shapes displayed together on the same screen with plots such as frequency response functions (FRFs) showing vibration amplitude versus frequency at key locations on the gearbox. This enables engineers to see immediately how the gearbox housing bends and twists at various frequencies so they can readily identify which bearings are transmitting vibrations and determine critical gear-mesh harmonics.

When testing is done, report generation features allow Moventas engineers to efficiently create the necessary documentation, complete with LMS Active Pictures that show live test data, including mode shape animations, in Microsoft Word documents as well as PowerPoint presentations. “LMS Test.Lab report generation with Active Pictures lets us quickly create reports that clearly show our designers, customers and regulatory agencies the modal behavior of the gearbox,” says Toikkanen.

“With its mobility, test setup, online monitoring, visualization, and report-generation capabilities, LMS Test.Lab boosts our test productivity immensely,” he adds. “Now we can complete routine tests in a few days instead of weeks. When faster turnaround is needed, our team can run an entire battery of modal tests in the morning and have results analyzed and documented that afternoon.”

Fast-Response Engineering Projects
In addition to implementing LMS Test.Lab, the company has worked closely with LMS Engineering Services on projects requiring additional calculation resources; projects where fast response was needed to meet requirements for key wind-turbine manufacturer customers. Defining the scope and specifications of durability analysis calculations for these projects was coordinated in conjunction with Petri Lahtinen, chief structural analyst at Moventas. Figure 5

In one such project, LMS Engineering Services was called upon to provide critical fatigue life analysis needed by Moventas and a wind turbine manufacturer for certifying a wind turbine. The study was to verify that two critical wind-turbine gearbox cylindrical components—a torque arm and gear planet carrier—would withstand expected loads over a 20-year operational lifetime. LMS engineers created finite element models of the components and applied unit load cases to determine the stress-time series on each part. This stress-time series together with the complete load time histories for the components were then used with LMS Virtual.Lab durability simulation software to determine fatigue life prediction for the base material. Results were provided within two weeks from the start of the project, thus enabling Moventas to give a fast response in verifying that cumulative damage values were well within the safety factor of the designs.

In a project for another wind-turbine manufacturer, Moventas contracted LMS Engineering Services to measure gearbox rotational vibration on the low-speed input and high-speed output shafts. Signals from accelerometers mounted directly on the low-speed shaft were fed into LMS Test.Lab for analysis. Signals for the high-speed shaft were obtained from a laser vibrometer system measuring rotational velocity. A series of operating response color maps accurately identified rotational vibration and related resonances for both shafts. In less than one week Moventas was provided valuable data needed by the wind turbine manufacturer in simulating the dynamic performance of the entire drivetrain. (Figure 6a & Figure 6b)

“The collaboration with LMS Engineering Services demonstrates that LMS goes far beyond selling hardware and software,” says Toikkanen. “Their industry-wide expertise in performing this work and fast response in providing exactly the right data made us look good in the eyes of our customers and made a lasting impression that has immeasurable business value for us.”

Increasingly, the energy future is blowing in the wind. Clean wind energy is America’s fastest-growing energy source, expanding by a record 8,300 megawatts in 2008, but a number of challenges must be met to ensure wind achieves its full promise, with intermittency representing one of the biggest obstacles. By its nature wind is a variable resource that isn’t so easy to integrate with the grid in large quantities. The wind blows when it will and generally picks up at night, while electric demand peaks during the day. These basics have profound implications for the economics of wind energy. In today’s volatile energy markets, wind energy producers can find themselves sometimes having to pay to sell their power, or suddenly curtailed as a result of transmission congestion or other factors.

Given such issues, there has been a search for technical solutions that involve storing energy for use when it is most needed, with the goal of better integrating renewable energy into the grid and improving its economic viability. In this way renewables can become more manageable, predictable, and profitable resources.

“Anything that does commercial-scale energy storage is huge,” says John Gardner, Ph.D., associate vice president of Energy Research, Policy, and Campus Sustainability at Boise State University. “It can completely change the economic prospects of a wind farm.”

Enter CAES 2, the second generation of compressed air energy storage technology. CAES 2 technology stores low-cost, off-peak energy for release when needed, which is generally during peak hours when the energy has more value. In situations where there is a sudden loss of renewable generation, CAES 2 can come to the rescue by providing power almost at a moment’s notice. It also can serve as a standalone intermediate generation resource for capturing energy arbitrage, capacity payments, and ancillary services.

A Green Technology
CAES 2 is designed not only to be an efficient solution to support wind power, but an environmentally sound one as well. As renewables become a larger and larger part of U.S. energy supply, compressed air energy storage with CAES 2 will be important to help wind energy achieve its full promise and value.

“Energy storage is the missing piece of the puzzle for a green, affordable, and reliable electric grid for the 21st century,” says Roy Daniel, CEO of Energy Storage and Power, which developed and delivers the patented second-generation CAES units. “CAES 2 is an enabling technology for efforts to provide more clean, renewable power, fight climate change and move our nation toward energy independence.”

While not new, the technology behind CAES has significantly improved. Almost 20 years ago Michael Nakhamkin, Ph.D., designed the nation’s only CAES plant, which was a 110-megawatt facility in McIntosh, Alabama. While the plant continues to operate successfully, Nakhamkin has taken the technology to another level by using multi-sourced components to reduce costs, enhance reliability, and improve environmental performance. “It’s a simple concept, using a proven standard based on off-the-shelf components,” he adds.

The key is the process used by CAES 2 to store compressed air and transform it cleanly and efficiently back into electricity. The first step is to use off-peak energy to run a compressor train and create compressed air, which is usually stored in an underground cavern. During peak hours the air is released and heated to power an expansion turbine/generator (a modified steam turbine) to produce electricity. Heating the air is necessary because stored air cools as it is expanded in the turbine.

CAES 2 technology has an integrated recuperator and air expander that makes use of exhaust heat from the combustion turbine, eliminating the need for a separate burner. This advance significantly reduces the natural gas used in the process to lower costs and emissions. It is estimated that a CAES 2 storage unit would use approximately 60-percent less fuel than an open cycle combustion turbine, and 40-percent less fuel than a combined cycle unit. Moreover, the air storage cycle is totally green during regular operations to further benefit the environment. Ambient air is stored, and ambient air released. Figure 1

Flexibility, Scalability
Other improvements add to the attractiveness of CAES 2 as a practical energy storage solution and grid integration tool. Perhaps the most important is the flexibility to match generation to load. A CAES 2 unit is designed to start in less than 10 minutes and offers exceptional operating flexibility. “This technology covers the whole spectrum, from load leveling and ramping to frequency control, so you can have what you need to have in terms of load-matched generation,” Daniel observes. “CAES units can be instrumental in managing renewable energy output to create a highly valuable, firm, dispatchable product.”

In addition, CAES 2 units are scalable, and can range from 15 megawatts to 430-plus megawatts without significantly altering their operational qualities. In fact, the respective size of the CAES 2 unit is primarily based on the size of the combustion turbine. For example, an 80 MW combustion turbine can be used to produce a 230 MW CAES 2 unit. Combustion turbines can be existing or new.

Larger CAES units can be accommodated with underground compressed air storage in a relatively tidy footprint. For example, a 300-megawatt unit would require some 22 million cubic feet of space, or about a third the size of a football stadium. At full capacity, a reservoir of this size could produce enough electricity to power around 200,000 homes for eight hours. According to the Electric Power Research Institute (EPRI), the geology of most of the United States is suitable for storing compressed air underground.

Smaller versions of the technology using above-ground storage provide cost-effective alternatives to utility-scale batteries and other electronic systems for energy storage. These CAES 2 units offer utilities a low-cost, low-carbon option for reinforcing their transmission systems and providing distributed generation coupled with energy storage.

Multiple Applications
Wind is not the only resource that CAES 2 technology can benefit. It is suitable for solar and other renewable energy developers, independent power producers, electric utility companies, and transmission owners. One innovative use of CAES 2 is as a bolt-on for an existing combustion turbine. Green compressed-air power adds to the combustion turbine’s output but not to its emissions for a better environmental profile, while providing additional megawatts of stored energy for load management.

Compressed-air storage could have an important role as a supplement to baseload resources, such as carbon-free nuclear power, according to Arshad Manshoor, EPRI’s vice president of power delivery and utilization. “Nuclear is an inherently stable source of power, so stable that we don’t want to ramp it up and down,” he explains in The Energy Daily. “Nuclear will need storage at night, when demand is low, and we really are looking at hundreds of megawatts of storage to meet that need.

“We asked the question ‘if we need bulk storage in our portfolio in the next five to 10-year timeframe, what technologies are available?’ There are lots of options,” he says, “but we need options that are cost effective and deployable in that timeframe. The only answer we were left with is compressed air.”

Nakhamkin sees CAES 2 as ready to provide wind and other energy producers with a range of market benefits including higher utilization, stability, and power quality. “We have learned a lot about the logistics of compressed air energy storage, and I believe the time is right technically, environmentally, and economically for a large-scale deployment of CAES 2 technology,” he says. “The technology has evolved to the point where it can be critical to helping this nation meet its growing energy needs while decreasing carbon emissions from the electricity sector.”

Conclusion
Technology that improves the reliability of wind or other intermittent renewable energy resources fits with efforts to green America’s energy picture and improve energy security. Indeed, it can be a key to reducing dependence on fossil fuels and imported energy. Clean, efficient energy storage with CAES 2 can accelerate wind energy’s continued growth and success to speed the arrival of America’s green, sustainable future.

To learn more about Energy Storage and Power, LLC, call (866) 941-2237, e-mail info@energystorageandpower.com, or go to www.energystorageandpower.com.

As interest in harnessing wind energy grows in the United States and around the world, several trends and events are converging in support of that interest. They include a desire to lessen dependency on petroleum products, rising energy prices, and a recognition that environmentally sustainable building management practices can be environmentally responsible while resulting in cost savings, as well. Building owners and managers can take steps toward the U.S. Green Building Council’s LEED® certification and take advantage of federal and state tax incentives that will help save money in the long run.

Whether the focus of a wind project is to supply power for greenhouses on a farm or to install turbines for larger-scale commercial energy supply, proper turbine siting is crucial for optimal performance and return on investment, despite the scale of the project.

Once a site is determined to be an appropriate candidate for a wind turbine—through consultation of local and regional wind resource maps, meteorological data, and existing studies—a thorough, on-site wind assessment must be carried out that measures wind speed and direction at heights appropriate for the project, and at locations suitable to represent the project. While appropriate permits and permissions are being obtained for the test site or sites, you will need to select monitoring equipment.

The intent of this article is to provide project managers working on small- to moderate-scale wind energy projects with information about how data loggers fit into wind resource site assessment. This article examines sensor, data logger, and communications options, with advice on what to look for in selecting such equipment. Then you will find information about equipment mounting hardware to help ensure optimal performance during your project’s monitoring period. This information will help simplify the equipment selection process, while also helping you save time and money for your site assessment project.

Central Components
The main components of a site’s wind resource assessment are wind speed and wind direction sensors that plug into a data logger, all of which are mounted on an appropriately sized tower. Knowing a bit about sensors and loggers will help you choose the right devices for your budget and ensure that you collect data reliably and accurately.

Sensors
Sensors for a wind assessment project are often mounted tens of meters off the ground and must measure environmental parameters accurately and consistently for at least a year, across all the seasons. In addition to doing their job of measuring wind parameters, sensors must withstand rain, snow, ice, and humidity. When evaluating sensors, check the manufacturer’s specifications regarding:

• Operating temperature—What happens during freezing temperatures?
• Response time—Will short gusts be recorded?
• Connection to logger and deployment—Is there any wiring required, or does the sensor simply plug into the logger?
• Durability—What are the sensor components made of? Are the moving parts strong?
• Expected lifetime—For how long will the sensor operate?
• Accuracy and Resolution

Factors to consider for each type of sensor include the anemometer. Wind speed is the most important measurement in your assessment. Cup-style anemometers are the most common for wind power projects, consisting of several cups that rotate around a vertical shaft.

The anemometer should be sturdy enough to withstand sustained winds and winds gusts of up to at least 100 mph, depending on your location. Keep in mind that wind speed increases with vertical distance from the ground. Check into the manufacturer’s specifications for maximum wind speed, starting threshold, response time—often referred to as distance constant, this describes how responsive the anemometer is to changes in wind speed—expected lifetime, and materials for the shaft, cup, and bearings.

Ideally, anemometers are placed at three locations on a tower: the proposed turbine’s hub height, the height of the highest blade tip, and the height of the lowest blade tip. Often, however, towers are used that are shorter than the proposed turbine hub; there are numerical models that allow the input of data from lower heights to be applied. Budgetary restrictions may also limit data collection. In any case it’s a good idea to mount two anemometers at each tower height for data replication, or in case of sensor failure.

Another important consideration is wind direction. Wind vanes are often part of the anemometer unit, but they may also be available as stand-alone devices. Most important is to check the resolution of the vane to make sure the data output is in small-enough units for you to use it effectively in your assessment. Also consider maximum wind speed—especially whether the vane is designed to withstand typical and maximum wind speeds at your site—expected lifetime, and the materials of which the fins and shaft are constructed.

Other sensors can monitor temperature, relative humidity, and barometric pressure. It may be useful to be able to link temperature, RH, and barometric pressure to wind speeds and direction, though it is not strictly necessary for all site assessments. If there are meteorological stations nearby you may opt to omit these sensors, depending on your project requirements and resources.

Data Loggers
All data logging systems for measuring wind parameters consist of multiple sensors that connect to a data logger, which records and stores all data at prescribed time intervals. When evaluating data loggers, keep simplicity and flexibility in mind as you consider the following points.

Housing: The logger will be outside for a year or more, so check if the enclosure is strong and weatherproof enough to withstand wind, rain, and curious animals, and that the electronic components will stay dry.

Data channels: There is usually a limit to the number of sensors that can be plugged into a logger. Does the logger have enough data input channels? Can more be added?

Power: If there is a battery, will it last the full duration of your study? If not, how easily can new batteries be swapped in? Will the data be safe during a power outage? Check about solar and rechargeable battery options, as well. Specific power options may be recommended for certain data download options.

Configuration: Some sensors require programming for deployment, while others simply plug in and are immediately recognized by the logger. Can you configure sensors in the field with or without a laptop, or do you need to set everything up in the office beforehand?

Flexibility: You may decide to add other sensors sometime during the monitoring process, or at a future date for another application. Can the logger accept a wide variety of sensor inputs, including third-party sensors?

Cable length limitations: Towers can be very tall, and you must ensure that the logger can handle all the cables necessary to reach the sensors. In some cases you may need to use multiple loggers.

Today’s data logging systems include a wide variety of data retrieval options, from cellular remote communications to on-site laptop computer download. The next section describes the different options available.

Data Communications Options
There are several options for downloading data from your wind resource monitoring system, and they essentially break down into two categories: manual download, and remote communications. Each has its strengths, and you’ll be better prepared to assess them if you first consider the following:

• Monitoring period—How often would you like to download data? You’ll be monitoring wind parameters at the site for a year or more, so how often would you like to check in? How many parties are interested in the data?
• Maintenance—How often will you do routine maintenance checks to be sure that the logger, sensors, and tower are in good working condition and repair? Such checks require staff and travel time.
• Location—Where is the station located? Is it in a vandalism-prone area, many miles from roads, or right on the edge of the project owner’s property?
• Budget—What’s your budget? Some communications options are more costly than others at the outset, but they may save money in labor and travel costs over the life of the project.
• Future applications—Will you reuse the equipment for future wind resource assessment projects?

Manual On-site Download
In light of these questions, let’s consider the difference between manual on-site downloads and those conducted from a remote location. Data shuttles are small, handheld devices that plug into a data logger and retrieve data. They are usually designed for field conditions and are simple to operate, often involving a single-button download. Alternatively, you can use a laptop computer equipped with the appropriate software and interface cable.

This is often the most inexpensive option, though it does have some limitations. Technicians must visit the logger to download data, and there may be no way to know about malfunctions until data has been examined.

Remote Data Download
Remote communications allow for real time Internet-based access to data. Rather than going out into the field, the user simply logs onto a Web site to view and download data, thus saving time and money on travel and labor and minimizing the chance of data gaps due to equipment failure.

Operators can easily and frequently monitor sensor and battery status or make adjustments to logging intervals, right over the Internet. It is easy to check that sensors and loggers are working properly, and cellular phone and e-mail alarm notifications can even be set for when sensor parameters are out of a set range, if equipment fails, or if battery power is low. The sooner you know about a system malfunction the faster you can get out to the site, fix the problem, and continue collecting uninterrupted data.

Two data communications systems available today are cellular and Wi-Fi. Cellular systems require a cellular data contract, and the logging system must be within provider range. With Wi-Fi, data transfer is accomplished via a Wi-Fi network or router connected to the Internet. There is no need to pay for cellular service, but your installation does have to be within range of a Wi-Fi router. Users can set notification alarms that transmit to cell phones and e-mail addresses with either system.

Mounting Tips for Data Logging Systems
In addition to the sensors and data logger you’ll need to select hardware and other accessories for mounting and connecting the devices, and to ensure that system components are installed properly. Once it’s up and running animals, weather conditions, and vandals may act upon your monitoring system, and periodic field visits and/or logger alarms are required in order to ensure smooth operation.

You will need to attach your sensors and logger securely to your chosen tower in a fashion that allows for optimal data collection. Sensor manufacturers usually provide a selection of hardware options, so check around to see what’s available before you consider spending money and time to design or make your own. In most cases equipment should be mounted on sturdy metal arms and brackets, with several anchor points. You should consult the manufacturer’s recommendations for placement distances, but here are some factors you’ll have to consider:

Sensors: Mount anemometers and vanes far enough away from the tower structure and other sensors to avoid interference. Each sensor should have its own individual boom that holds the sensor well away from the tower, so consult your manufacturer for details. Temperature and humidity sensors require solar radiation shields, and you should make sure that they are designed to shed water and operate in freezing conditions.

Cables: Sensors connect to the logger via cables, so order enough of the proper length to reach from sensors to logger. Wrap cables along the tower from the sensors to the logger and secure them with UV-resistant tape or ties, allowing enough slack in chafe-prone areas.

Logger: Make sure the logger is securely housed inside a weatherproof, locked, tamper-resistant structure. Mount it high enough so that animals can’t reach it from the ground, and close any gaps to deter birds. Make sure the enclosure is properly sealed, carefully following the manufacturer’s instructions and, if applicable, mount the solar panel so that it will receive adequate sunlight throughout the day and through the seasons. Again, your sensor, logger, and tower manufacturers will have detailed guidance specific to the equipment you have selected.

Conclusion
Site-specific wind resource assessment is one of the most time-consuming and important steps of a wind energy project. This article will hopefully help make you aware of the data-collection and communications options available to you so that you can make the best choice for your site, project, and budget.

Most of us understand that wind turbines are ideal lightning attractions, and that grounding turbine systems is critical to avoiding catastrophic damage. Lightning is such a threat to wind turbines that it likely isn’t a matter of if one will be struck, but when. The cost of replacing circuits in the nacelle, transformer, and elsewhere is hard on O&M budgets. But what grounding solution makes the most sense for wind farm developers and turbine makers focused on material cost reductions and reliability? Solid copper wire has long been deployed because it is known as an excellent electrical conductor. However, it isn’t always the only or most appropriate answer. Substantial price swings and problems with theft can make copper a liability. That’s why some wind turbine and wind farm developers are exploring the benefits of less expensive bi-metal copper clad grounding wire/cable.

Material cost reductions and reliability continue to be a concern for wind farm developers and turbine makers. Most engineers are familiar with solid copper grounding wire/cable, which has been used around the world for many decades as the primary choice in grounding conductors. In increasing numbers utilities, municipalities, and electrical co-ops are switching to BiMetal copper-clad cable for grounding their towers, substations, and foundations. The utilities have experienced industry-wide problems with solid copper wire/cable theft, resulting in substantial maintenance costs going beyond the replacement cost of the wire/cable. Additionally, when the solid copper cable is cut from its site/tower, not only do service personnel risk severe injury, but system reliability is jeopardized as well.

Solid copper wire and cable has been used as grounding, or “earthing” wire for power, telecommunications, wind turbines, solar panels, gas lines, buildings, bridges, homes, and many other structures needing to dissipate lightning strikes to the ground. Solid copper wire has long been known to be an excellent electrical conductor, and the operative word here is “long,” referring to timing, but not necessarily the most suitable. Copper has experienced significant demand and price fluctuations on the open market in the past decade, and it is a highly tradable commodity used in products from printed circuit boards to wire to jewelry and  many other places.

There are other choices in grounding wire material types that can lead to cost reductions while ensuring long-term circuit/equipment protection. Solid copper has experienced substantial price swings, over 200 percent in the past year alone. Such price volatility can be minimized and cost savings applied to the installation of grounding wire/cable throughout the wind plant, from blades, to trenches, to wind farm substations.

Some careful buyers have learned that there are alternatives to solid copper grounding wire. “Why pay for 100-percent copper when 40 percent conductivity will work just as well due to skin effect?” asks Bruce Hammett, president of WECS Electrical Supply, Inc., and a 25-year veteran of the wind power industry.

Metallurgically bonded copper to steel, sometimes called copper clad, has been used in telecom, cable TV, and in electric utilities for grounding wire. Utilities today are most concerned with thieves climbing onto their property, cutting the solid copper away and taking it to scrap dealers for cash. While this is not yet a well-known problem for wind turbines or wind farms, that doesn’t mean the thieves won’t find them to be a good supply source in the future. As the copper on utility power poles and substations gets converted to copper-clad grounding wire, expect the thieves to find it in wind turbines next.

There are a few companies that have been producing copper-cladded wire and cable for several years. A recent addition to this list is CommScope, a U.S. telecom accessories manufacturer now reaching out to the wind and utility industries that has named a division, BiMetals, as a reference to the metallurgically bonded copper-clad wire/cable it produces. Some companies try to copper-plate the wire. This lacks the purity of a metal bonding environment to provide a highly bendable and durable copper skin. Due to the long-term vibration, fatigue or bending requirements of most of the cable used in turbines and wind farms, it is best to stay away from a plated copper wire/cable.

Today’s copper-clad wire is metallurgically bonded to a steel or aluminum core. The aluminum core is the same material used for power cable—center conductor. However, the power cable is for a different application than discussed in this article. The optimum strength, “bendability,” and fatigue capability of copper-cladded grounding wire is achieved with heat-pressure bonding in an inert gas environment under highly controlled heating and cooling operations. Examples of fatigue and bend test comparisons can be seen in fig. 1a–fig. 1b.

There are some issues, however, such as size, weight, and flexibility that require adaptation or acceptance when compared to solid copper. In order to accomplish the same current carrying capability, there may be small size increases when using copper-cladded steel wire vs. solid copper. For example, copper solid grounding wire, a 2/0 7W at 0.365” diameter will require a size 7 strand #7 AWG copper clad steel 40 percent conductivity at 0.433” diameter, or a 16 percent increase in diameter. Examples of conversion can be seen in Figure 2.

To the degree change can be implemented, this copper-clad technology is available now. To change standards and begin using copper-cladded steel for grounding wire, engineers should check the DC resistance and fusing currents to select the proper size of wire compared with solid copper. Fig. 3–Fig. 4 show fusing current comparisons of solid copper wire and copper-clad wire from one manufacturer.

There is one other important point contributing to cost controls. The purchase cost of copper-clad wire is likely to be less for equivalent quantities, as there is less copper in such wire, which is acceptable due to the “skin effect” phenomena. Costs of specialty metals are likely to remain volatile, and one of the best ways to safeguard against theft losses and the high costs of purchasing copper wire is to use copper-cladded steel for grounding applications. Last year, as copper prices increased along with other commodities, the absolute dollar savings with copper clad grounding wire became substantial. Copper prices have moderated in the past six months, along with other commodities, but have already begun increasing this year and are expected to increase further. The cost benefits of using copper-clad grounding wire exist with both the initial purchase stage as well as the O&M costs related to theft and longevity. With so many concerned with making harvesting wind energy as cost-efficient as possible, maybe it’s time for you to consider a change in grounding wire/cable.