Power Tools for Technical Communication:
Executive Summary


In this lab, you write an executive summary using the body text of a formal report. You need to have studied Chapter 15 of Power Tools for Technical Communication to be ready for this lab.
  1. Study the text of the report below this box. Using standard page size and margins and 12-point Times New Roman, this report would be 10 pages long.
  2. Write an executive summary for this report, using the guidelines in Chapter 15 of Power Tools for Technical Communication.
  3. Put your name, Executive Summary, and the date on this document, and print it out for your instructor.


WIND TECHNOLOGY DEVELOPMENT:
LARGE AND SMALL TURBINES



I. INTRODUCTION

Wind technology has been developing rapidly over the last decade. The experience gained in the wind farms of California is being used to design and develop advanced systems with improved performance, higher reliability, and lower costs. During the past several years, substantial gains have been made in wind turbine designs, lowering costs to an average of $0.05 per kilowatt-hour (kWh) for utility-scale applications at 13 mile-per-hour (mph) average annual wind speeds. Further technology development is expected to allow the cost of wind-generated electricity to drop below $0.04 per kilowatt-hour by 2000. As a result, wind is expected to be one of the least expensive forms of new electric generation in the next century. With global efforts already underway to curb energy-related emissions of carbon dioxide, the current availability of this low-cost technology means that the use of wind systems will likely increase worldwide throughout the 1990s for both utility-scale applications and remote, small-village applications. This report presents a review of (1) some of the leading manufacturers of utility-scale wind systems, (2) technology developments for remote, small-village wind turbines that are currently available or in development, and (3) applied research activities Sandia National Laboratories, universities, and industry subcontractors supported by the DOE Wind Energy Program.

II. UTILITY SCALE-SYSTEMS

In the United States, utility-scale wind turbines are the primary focus of new technology development. By 1996, U.S. manufacturers will have introduced seven new turbines (with capacities ranging from 250 to 500 kW) for the utility market. Five of the turbines are being developed by members of the U.S. wind industry with direct funding support from DOE. Cannon Energy Corporation and Kenetech Windpower are each developing a utility-scale wind turbine independently. Two of the utility-scale wind turbines sponsored by DOE are commercially available: the Advanced Wind Turbines AWT-26 and Zond Systems Z-40. New World Power Technology Company, New World Grid Power, and Flowind Corporation are expected to introduce their turbine designs within the next two years.

Advanced Wind Turbines AWT-26

The 275-kW AWT-26 is a downwind, stall-regulated, free-yaw machine incorporating an innovative two-bladed teetered rotor. The AWT-26is based upon the ESI-80, a turbine developed in the United States during the mid-1980s which had many promising features, but never reached commercial maturity. The designers of the AWT-26 have taken advantage of the substantial operating history of its predecessor, retained components that were reliable, and improved the remainder.

The larger 26-meter rotor incorporates aerodynamically efficient, wood-composite blades using NREL-designed airfoils. The new blades improve the turbine's energy capture from 20% to 70%, depending on wind speed and the degree of blade soiling from dirt and insects. The AWT-26 also features a redesigned high-speed shaft brake and new aerodynamic tip vanes. The tip vanes serve two important functions on this turbine: as a fail-safe (emergency) brake and as an active brake for normal shut-down operations. Mounted on a hinge at the tip of each blade, the vanes are held closed by electromagnets. The vanes are activated by control system command, which releases the electromagnets. A redundant caliper disk brake can also stop the machine under normal or emergency conditions.

The AWT-26 has been selected for a commercial 25-megawatt power plant to be installed in late 1995 in Washington state by a consortium of public utilities called CARES (Conservation and Renewable Energy Systems) as a project under the Bonneville Power Administration's Resource Supply Expansion Program (RSEP). The RSEP program was a competitive solicitation to add wind generation capacity to the Bonneville network.

New World Power Technology Company North Wind 250

The 250-kW North Wind 250 is a two-bladed, teetered, upwind machine that has been scaled up from the company's North Wind 100 turbine. The new turbine features an integrated drive train, aileron controls, and an innovative rotor that is fabricated as a single unit. The"flow-through" rotor eliminates the blade root joints, which are expensive, complicated, and subject to high stress during turbine operation. The unique, flow-through, teetered-rotor design eliminates structural discontinuities at the blade/hub interface by fabricating the rotor as one continuous structural element. Fatigue tests were conducted on the full-scale rotor joint at NREL to qualify this element of the rotor for field testing and to provide information needed to improve its structural design.

The hub incorporates teeter dampers and an active teeter brake. The rotor is made of a hybrid composite material. The hub saddle captures the blade center section in a wrap-around elastomeric blanket. The new rotor also substitutes aileron control for the full-span pitch control system used in the earlier North Windmodel.

Ailerons, which work like flaps on an airplane wing, are the most important development featured in the North Wind 250. When deflected in the downwind direction, they reduce lift and power output. When power modulation is needed, the ailerons can be deflected through small angles to either increase or decrease power. Up wind deflections increase lift, causing the power to increase. Wind tunnel tests at Wichita State University have confirmed the ability of ailerons to control power and prevent the rotor from over-speeding.

Zond Systems Z-40

The 500-kW Z-40 wind turbine is being developed by Zond Systems, Inc., with support from NREL. Zond is building two versions of its prototype turbine, a three-bladed, upwind, rigid hub machine with active yaw drive. One turbine prototype employs a full-span pitch control system and sits atop a tube tower. The other turbine uses aileron controls to provide aerodynamic braking and peak power modulation and sits atop a low-cost, free-standing open truss tower. Zond plans to compare the two turbine control strategies during prototype testing.

Both designs capitalize on the company's extensive experience operating wind power plants using similarly designed turbines. Each prototype incorporates NREL-designed airfoils and an integrated gearbox design that minimizes the number of parts and simplifies load paths. Based on blade structural and fatigue testing at NREL, design changes were made that will improve the final design of the blades.

Flowind EHD

Unlike the other turbines discussed in this paper, Flowind's 300-kWEHD (extended height/diameter ratio) is a Darrius (eggbeater-shaped) vertical-axis wind turbine. Its three blades spin about an axis perpendicular to the ground, capturing energy from winds blowing from all directions. The drive train and generator are located under the rotor near the ground for easy maintenance and inspection.

The EHD series will use tall, high-performance 17- to 21-m rotors incorporating advanced airfoils designed specifically for the series. Flowind's prototype turbine rotor, which has a height-to-diameter ratio of 2.78, is taller and thinner than the 19-mrotor on Flowind's existing vertical-axis turbines. The shape allows the larger rotor to be placed upon existing turbine bases, if desired. This design also makes it easier to bend the turbine blades into the desired shape and makes better (and more profitable) use of available sites in a wind power plant.

The EHD's low-cost blades are manufactured using a new, automated pultrusion technique, in which fiber-resin blades are pulled through a die. The blades incorporate natural laminar flow airfoils developed at Sandia National Laboratories to keep air flowing smoothly over the blades and increase energy capture. The rotor should effectively double the annual energy output of Flowind's current vertical-axis turbines which use aluminum blades and are about half as tall.

New World Grid Power 500-XST

New World Grid Power will develop the 500-XST (experimental synchronous turbine) based on the company's experience operating European turbines of this size. The 500-kW, upwind, three-bladed 500-XST will include active yaw control, a full-span pitch control rotor and a free-standing tubular tower. One of its most innovative features is a unique integrated drive train system which uses variable-speed operation. Therefore, it can provide a unity power factor without requiring capacitors for reactive power control.

III. REMOTE AND VILLAGE SYSTEMS

Technology innovations are being adapted for remote and stand-alone power applications with smaller wind turbines. Hybrid power systems using smaller 1- to 50-kW wind turbines are being developed for non-grid connected electrical generation applications. These village power systems typically use wind energy, solar, photovoltaics (PV), battery storage, and a conventional diesel generator to supply power for remote, small-village communities. In areas without electric utility service and with good wind resources, a single wind turbine can provide electricity at lower cost than diesel generation for individual homes, schools, clinics, water pumping, or small industries. Larger "mini-grid" village power systems incorporating multiple wind turbines and other generation sources are often more economical than transmission line extension for communities in remote, but windy regions. Smaller wind turbines are also being explored for application as distributed generation sources on utility grids to supply power during periods of peak demand, avoiding costly upgrades in distribution equipment.

Hybrid systems comprised of wind turbines, PV, batteries, and diesel generators have been used successfully to meet direct-current electric loads in remote international telecommunications markets. These systems are now an emerging technology for generating alternating-current electric power for remote communities. Assisting the U.S. industry in developing and demonstrating hybrid systems, NREL has embarked on a program of collaborative technology development and technical assistance in the area of hybrid systems for village-power applications (Flowers, et al, 1994).

Atlantic Orient Corporation AOC 15/50

The 50-kW AOC 15/50 is an improved and simplified version of the Enertech 44/60 wind turbine developed in the United States in the early 1980s. The downwind, stall-regulated, three-bladed turbine features passive yaw control, wood epoxy composite blades incorporating NREL-designed airfoils, aerodynamic tip brakes, an electrodynamic brake, and an integrated drive train. Blade-fatigue testing was conducted at NREL. This turbine is well-suited for remote, stand-alone applications, village power systems, and small wind power plants.

The AOC 15/50's integrated drive train eliminates many critical bolted joints found in conventional turbine designs and creates an efficient load path from the rotor to the tower top. A cast-steel, tower-top plate further improves the efficiency of the load path. The new drive train design weighs less than conventional drive trains and eliminates maintenance-prone couplings between the gearbox and the generator. Other design features include tip brakes and an optional yaw damper. The optional yaw damper, a passive hydraulic system that limits yaw rates (and gyroscopic loads), is available for turbulent wind sites.

Bergey Windpower Company BWC Excel

Bergey Windpower Company (BWC) turbines use passive controls, fiberglass blades, direct-drive permanent-magnet alternators, and integrated structures to provide mechanically simple turbines between 0.85 and 10 kW. The Bergey BWC Excel is a 10-kW, three-bladed, direct drive, upwind wind turbine with passive blade-pitch control. To achieve overspeed protection, the rotor yaws out of the wind. The rotor diameter is 7 m. The BWC Excel wind turbine is designed to supply most of the electricity for an average all-electric home in areas with an average wind speed of 12 mph. In remote locations, it can charge batteries for stand-alone applications or pump water electrically without the need for batteries. More than 1400 BWC wind turbines have been installed in a total of more than 60 countries. BWC, under subcontract to NREL, is developing a 15-kW high-frequency link, full-digital-control inverter to provide high reliability at low cost for its stand-alone AC systems.

In 1992, a Bergey system was installed in Xcalak, Mexico, with funding and technical support from NREL and Sandia National Laboratories in cooperation with the Instituto de Investigaciones Electricas (a Mexican utility research organization). The system includes six Bergey 10-kW wind turbines, an 11.7-kW PV array, an inverter, and a battery bank. The hybrid system is operated without use of any backup diesel generation.

Integrated Power Corporation

Integrated Power Corporation (IPC), a subsidiary of Westinghouse Electric Corporation, has installed PV/wind hybrid systems in Mexico and Indonesia. IPC, in collaboration with a Mexican utility and a rural-development organization, has installed PV/wind hybrid systems in two Mexican villages. The two systems (one with 45kWh/day capacity and the other at 125 kWh/day) were installed to meet commercial and residential loads and accommodate load growth. Anemometers were recently installed at the sites so that the system performance data can be analyzed collaboratively among IPC, NREL, and the appropriate Mexican agencies.

IPC has also installed PV/wind hybrid systems in two neighboring villages on an island near Bali, Indonesia. These systems were each sized for 15-kW peak power and energy delivery of 100 kWh/day. The two systems have been operated successfully in parallel, demonstrating capability to meet load growth with modular additions to existing systems. These installations have remote monitoring and control capability through a direct link to a low earth-orbit satellite that permits uploading and downloading system status and performance data, and remote diagnostics and control.

New World Power Corporation

New World Power Corporation (NWPC) is using its experience in wind, solar, and hybrid power systems, primarily in remote telecommunications and small, isolated power applications, to develop the balance-of-system components required to reliably integrate renewables into remote mini-grids. NWPC is in its second year of a 5-year program to develop and commercialize renewable-based, packaged hybrid-power systems with peak power capacities of 50 and 100 kW. They are using a novel rotary converter which eliminates the need for electronic inverters in large sizes. NWPC has installed its system in southern California for Southern California Edison Company. Additional systems are currently being installed in Alaska, Argentina, and Brazil.

IV. TECHNOLOGY DEVELOPMENTS

The DOE Wind Energy Program is supporting a robust applied research program through NREL's National Wind Technology Center. These applied research activities also involve Sandia National Laboratories, universities, and industry subcontractors. The highlights from these technology development and research activities are covered in the following paragraphs.

Airfoils

The development of special-purpose airfoils for horizontal-axis wind turbines began in 1984 as a joint effort between the National Renewable Energy Laboratory (NREL), formerly the Solar Energy Research Laboratory, and Airfoils, Incorporated. Prior to that time, turbine blade designers used airfoils developed for aircraft wings. Since 1984, seven airfoil families have been developed. NREL airfoil families are used on replacement blades and wind turbines developed by Atlantic Orient Corporation, Advanced Wind Turbines, and Zond Systems. Annual energy improvements from the NREL airfoil families are projected to be 23% to 30% for stall-regulated turbines, 8% to 20% for variable-pitch turbines, and 8% to 10% for variable-speed turbines (Tangler 1994). The energy improvement for stall-regulated turbines has been verified in field tests.

Structural Testing

NREL's structural test facility, in operation since 1990, provides blade designers with information useful in blade development. Wind turbine blade designs have complex geometries which make blade design difficult. As well, wind turbine blades are comprised of composites which have superior structural properties but require sophisticated analysis methods. In addition, the manufacturing methods allow more variability than with metals, and require careful quality control. Structural testing is therefore useful to help designers validate design assumptions, and make corrections, if necessary, in the prototype phase rather than discovering design problems after installation.

The structural test facility is used primarily for structural testing of full-scale wind turbine blades for NREL's subcontractors and wind industry partners. Testing that has recently been performed includes fatigue testing, ultimate strength static testing, and several non-destructive techniques such as photoelastic stress analysis. Fatigue tests use a closed-loop servo-hydraulic system to apply cyclic loads to blades up to 20-m long. Under repeated loading, fatigue failures help wind turbine blade designers evaluate design assumptions, manufacturing techniques, and complex failure modes under normal and extreme operating loads. Blades tested at the structural test facility include the Kenetech Windpower 56/100 and 33-M/VS, the AOC 15/50, the New World North Wind 250 (Musial et al 1994), and the Zond Z-40.

Power electronics

New technology has been developed which uses power electronics to allow variable-rotor-speed operation to improve efficiency, control structural loads, and improve power quality. Today variable-speed operation is estimated to increase energy capture by up to 15%, which is about the same level as the cost increase (Lucas et al. 1989). However, it is thought that variable-rotor-speed operation can reduce the cost of wind-generated electricity by reducing structural loads (allowing a lightweight, low-cost configuration), and improving power quality.

Aerodynamic control devices

Aerodynamic control devices provide two benefits: they are used for overspeed control and power modulation. Significant damage to the turbine can occur as a result of high wind or loss of generator load unless control of the rotor is maintained. In addition, natural wind-speed variations, insect-impact accumulations, or minor blade damage can result in off-optimum rotor rotation speeds and less-than-desired power output. Incorporated into turbine blades, aerodynamic control devices (also called trailing-edge control devices) can adjust the rotor aerodynamic driving forces and thus optimize energy capture, control loads, and control rotor speed. These aerodynamic controls are often compared to the ailerons used on aircraft. Various trailing-edge control devices have been incorporated in wind turbines that are in development or commercially available. NREL is working with subcontractors and wind industry representatives to further study improved trailing-edge control devices. These trailing-edge control devices are thought to offer some cost and control advantages over pitch control and tip controls which are typically used on existing designs, although these advantages are yet to be proven.

National Wind Technology Center Hybrid Power Test Facility

A wind hybrid test facility is planned for NREL's National Wind Technology Center that will allow researchers to study hybrid power systems, developed by the U. S. industry, that combine wind turbines, photovoltaic arrays, backup generators, and storage systems (Thresher et al 1994). The key function for the hybrid test facility will be to test small wind and hybrid power systems that are nearly market ready and provide a system development user's facility for U.S. industry.

Hybrid System Modeling

NREL is currently developing computer simulation codes to allow modeling of the full range of hybrid power technologies being considered for village power in the 1900s. Building on existing wind/diesel models, a new advanced hybrid systems simulation model is being developed which will accommodate a wider array of technologies and system architectures now being considered for hybrid village power systems.

Renewable-Hybrid Configurations

In 1994, DOE initiated a 5-year, cost-shared research collaboration with the U.S. Department of Agriculture to investigate a range of wind/diesel system configurations. The goal of this collaboration is to develop systems powered entirely from renewable sources (e.g., wind power, solar power, and use of vegetable oils in place of diesel fuel) that would be reliable and cost competitive. The experimental study will focus on performance and stability for various system configurations and will identify necessary controls. This research will be performed in a wind/diesel test facility at the U.S. Department of Agriculture research laboratory at Bushland, Texas.

Future Technological Improvements

The advancement of wind turbine technology is leading to next-generation wind turbines which promise significant improvements in performance, reliability, and cost. In general, each of these competing turbine designs will probably incorporate many of the following advanced features.

Advanced Wind Turbine Configurations

Two likely configurations for advanced HAWT designs were first suggested in a configuration study by Swift, Hock, and Thresher (1992). The first configuration represents a low-risk design path incorporating the more conventional three-bladed rotor. Innovations include a larger rotor diameter using advanced airfoils and trailing-edge flaps for overspeed control. This design is rated at 800 kW, which is twice the size of current machines.

The second machine incorporates higher-risk design options, including a 50-m-diameter, two-bladed, teetered downwind rotor. The stall-controlled rotor also has actively controlled ailerons for power clipping in response to gusts. The use of a variable-speed generator will allow increased energy capture over a broad range of wind speeds. While this design philosophy is perceived as high risk because of its innovations, it offers high potential for reduced weight and therefore cost.

Vertical-Axis Wind Turbine (VAWT) Configuration

For 2000, the hypothesized VAWT will almost certainly retain one aspect of its current strength: simplicity. The key to future success with this configuration, however, will be cost-effective manufacturing techniques (primarily for blade production) that will produce significantly less expensive blades, based on cost per unit length (Dodd 1990). VAWT blades are not geometrically complex (e.g., no twist or taper), thus making manufacturing processes such as extrusion and pultrusion viable candidates to reduce costs. In addition, the inherent advantage of the VAWT configuration, having all drive train and generator parts at ground level, creates opportunities for using components with high weight or large physical size. Of recent interest in this regard is the direct- or linear-drive generator, which eliminates the need for a gearbox and provides the advantages of variable-speed operation at a very competitive price.

V. SUMMARY

Significant progress has been made in wind energy technology over the last decade. Today's advanced systems offer improved performance, higher reliability, and lower costs. Turbine designs now make use of new technical developments such as advanced airfoils, information from structural blade testing, variable rotor speed operation, and aerodynamic controls. These developments have been incorporated into wind turbines that are, or will soon be, commercially available at a cost of energy at, or below $0.05/kWh. Further cost reductions are expected as the technology evolves and moves toward larger scales, and mass production.

Technology innovations are being adapted for remote and stand-alone power applications with smaller wind turbines. Village power systems using wind energy, photovoltaics, battery storage, and a conventional diesel generator are successfully providing power in remote locations. These systems can compete economically due to the high cost of either utility grid extension, or the cost of delivered fuel and maintenance for a diesel unit alone.

REFERENCES

Dodd, H. M.; (1990). Performance Predictions for an Intermediate-Sized VAWT Based on Performance of the 34-m VAWT Test Bed, ASME Energy-Sources Technology Conference and Exhibition, January.

Flowers, L.; Green, J.; Bergey, M.; Lilley, A.; Mott, L. (1994). Village Power Hybrid Systems Development in the United States. NREL/TP-442-7227. Presented at the European Wind Energy Conference, Thessaloniki, Greece, 1014 October.

Lucas, E. J., et al. (1989).The EPRI-Utility-USW Advanced Wind Turbine Program - Program Status and Plans, presented at the AWEA Windpower '89 Conference, San Francisco, CA.

Musial, W.; Link, H.; Coleman, C. (1994). Structural Testing of the North Wind 250 Composite Rotor Joint. NREL/TP-441-6619. Presented at the Windpower '94 Conference, Minneapolis, MN, 913 May.

Swift, A.; Hock, S.; and Thresher, R. (1992). A Wind Turbine Configuration Survey, presented at the 11th ASME Energy-Sources Technology Conference and Exhibition.

Tangler, J. L.; Somers, D. M. (1994). NREL Airfoil Families for HAWTs. NREL/TP-442-7109. Golden, CO: National Renewable Energy Laboratory.

Thresher, R.; Hock, S.; Loose, R.; Cadogan, J. (1994). The National Wind Technology Center. Presented at the Windpower '94 Conference, Minneapolis, MN, 913 May.


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