By Larry Miles
The Wind Turbine Company
Bellevue, WA, USA
The Wind Turbine Company has designed and developed two prototype wind turbines aimed at increasing efficiency and reducing costs.
The Wind Turbine Company (WTC) was founded in 1990 with the intention of developing a new, lower-cost wind turbine.
WTC believes that if the all-in, unsubsidized cost of wind energy can be brought into parity with the marginal (fuel) cost of more conventional electricity generating technologies, e.g., combined cycle natural gas generators, wind energy will be able to overcome the natural aversion of utility companies arising from its intermittent nature, and claim a major place at the electricity generation table.
Although the cost of wind-generated electricity has dropped considerably since 1990, it still requires, at least in the US, subsudies in order to be competitive in the marketplace. WTC hopes to change this soon.
In late 1995, WTC was selected by the US Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) to develop a “proof-of-concept” (POC) of WTC’s turbine design to be followed by two full-scale prototypes. After extended negotiations with NREL, work began on the POC design in March 1997.
In June 1998, WTC was awarded a contract by the California Energy Commission and obtained funding under the Commission’s Public Interest Energy Research (PIER) Programme to supplement DOE funding in the development of the low-cost turbines.
WTC’s turbine is an extension of a design pursued by other US companies in the late 1970s and early 1980s. It is a two-blade, downwind oriented machine similar in general configuration to prototype machines developed by Kaman and United Technologies, and to commercial machines produced by ESI, Carter, and more recently, AWT.
Unlike all but the United Technologies’ machine, WTC employs full span blade pitch control and has developed a proprietary blade “flap motion restraint system”. This system employs hinged blades and a controllable hydraulic damping system in order to independently regulate the out-of-plane travel of each blade.
Most of the earlier two-blade downwind machines employed a “teetering hub”; this is a hinged mechanism that permitted the blades to teeter in and out of the plane of rotation as the typically higher winds at the top of the rotor plane would drive the upper blade in the downwind direction resulting in the lower blade being driven upwind. One of the major problems encountered by designers employing teetering hubs related to the amount of allowable travel of the lower blade in the upwind direction and how to effectively restrain this travel. WTC’s design eliminates the source of this problem.
The FMR system
In 1996 WTC obtained its first US patent for its Flap Motion Restraint (FMR) System. Initial modelling of this concept suggested that it could greatly reduce the out-of-plane bending loads that occur at the blade root compared with blades rigidly connected to a rotor shaft, and do so without being forced to resolve the issue of one blade being continually driven upwind as on teetering hub machines.
Figure 2. WTC plans to field up to 100 WTC 500 kW wind turbines in various projects
Individual blade hinges also promised lower blade root loading compared with teetering hub machines. Simulations indicated that under normal operating conditions in non-turbulent winds, the blades could be allowed to freely flap. However, many conditions, including start-up and shut-down require a mechanism to restrain blade travel. WTC has designed a hydraulic system that allows collective or individual control of blade travel in the out of plane direction. The challenge was to design a simple, hence inexpensive, but robust system.
The POC Turbine
After 18 months of component design and system analysis effort, WTC began construction of the POC in late 1998. By the following September, site construction work was nearing completion at the NREL operated US National Wind Technology Center. In late September the tower was erected and in early October the complete nacelle, with blades attached, was lifted into place.
The POC, which has a 33 m rotor diameter and was installed on a 38 m tower, is rated at 250 kW. Gearbox capacity constrains the turbine’s rating. Since the concepts WTC was attempting to prove with this turbine related to the performance of the FMR system, the turbine gearbox and generator were simply adapted from available sources to meet system needs. The important design and analysis effort focused on the rotor-end of the drive-train including the blade to rotor attachments and the rotor shaft support and associated hardware, and the emergency mechanical brake. The blades, from Rotorline (now part of LM), were originally designed for the upwind Nedwind 250 kW turbine.
After its initial installation in October 1999, it was discovered that the turbine foundation had suffered serious, irreparable damage during the construction phase. The turbine was removed from its original foundation and installed on a new foundation. The turbine was grid connected in March 2000, at which time it entered into a lengthy test programme to assure that all mechanical systems and the control system were functioning as intended. During March and April a number of minor hardware failures, primarily control system sensors, were encountered and control system software was modified to eliminate unwarranted system fault shut downs and other bugs.
By 1 May 2000, the last of the major software issues were resolved and the turbine commenced operating in an attended mode whenever the wind blew. During the next 12 months data was gathered from approximately 50 data sensors and strain gauges installed on the machine. The turbine did not accumulate a substantial number of operating hours during this period, however, it performed without fault in winds that reached and exceeded cutout wind speed (24 m/s). It experienced winds in excess of 45 m/s on more than one occasion during the winter. From February 2001 a concerted effort was made to eliminate nuisance faults and within two months this effort was successful.
Since May 2001 the turbine has operated in an unattended mode. WTC has an employee and a part-time consultant at the site; however, the machine is available to operate when the wind is blowing as though it were a commercial machine. To date, WTC has not focused on the achievement of commercial availability levels and has only recently begun keeping track of the duration of machine down-time.
The EMD turbine
Originally proposed as a complete redesign and scale-up to a 1000 kW turbine, WTC’s second prototype turbine, the Engineering, Manufacturing and Development (EMD) turbine, was instead recast as a direct scale-up of the POC. After consultations with NREL, DOE and CEC, it was agreed that this effort would be executed in two stages.
The first stage (EMD-1) focused on the design and development of a new gearbox and purpose designed generator and minor modifications of the rotor components to result in a 500 kW rated capacity turbine. Analytical work indicated that WTC had designed excess capacity in the POC turbine’s important structural components including the rotor shaft and rotor shaft support. The EMD-1 would once again employ “off the shelf” blades, this time 23 m carbon fiber blades from ATV that were based on a design originally employed on the Tacke 600 kW upwind machine.
The EMD-1 also features a tubular, guy-cable supported tower. As with the blade flapping system, WTC determined through analytical studies that guy cables could economically be employed on a downwind turbine. Guy cables represent a low cost means of effectively elevating rotor hub height, although there are locations where guy-cable supported towers may be unacceptable. Unlike upwind turbines which are generally configured with a rotor diameter to tower height ratio in the range of 1:1 to 1:1.25, WTC’s turbines will be economic, assuming typical wind shear conditions, with significantly taller towers leading to a rotor diameter to tower height ratio in the 1:1.5 to 1:2 range.
Figure 3. Towerhead weight versus rotor swept area comparison
In collaboration with the Department of Water and Power of the City of Los Angeles (DWP), WTC is currently installing the EMD-1 on DWP property located in the Antelope Valley in northern Los Angeles. The turbine, originally scheduled for installation in June 2001, will be installed in late October.
Delays are again attributable to the foundations and a failure to obtain an acceptable design in a timely fashion, and unanticipated difficulties in obtaining permits. Due to concerns about local acceptability of the turbine height, the tower has been restricted to approximately 65 m. Subsequent to the installation and commissioning of the EMD-1, another comprehensive test programme will be executed, however, the intent with this turbine is to place it in a commercial operating mode as quickly as possible.
The EMD-2 phase of the project will focus on development of a purpose-designed blade for the turbine. Analytic work has already begun. This effort is expected to result in a new blade set, suitable for retrofitting on the EMD turbine by the end of 2002. The objective is to produce a blade approximately 29 m in length that has mass-distribution properties that specifically take advantage of WTC’s downwind, flapping rotor design. When this retrofit is completed, the generator will be reconfigured to operate at 750 kW. The EMD-1 gearbox has been designed for the larger capacity rotor, as have all major structural components.
A commercial venture
WTC plans to field up to 100 WTC 500 kW wind turbines in projects to be completed in 2002-3. The purpose of these projects is to accumulate operating experience with the turbine and build a track record that will be satisfactory to the financial community. Once the purpose designed blade is proven and minor configuration changes incorporated, the WTC 750 will become the company’s commercial offering. The WTC 750 is expected to be commercially available by the end of 2003.
Wind energy targets
In 1995, the DOE’s cost target for wind energy was ¢4/kWh unsubsidized, achieved with an annual production run of 50 MW of turbines installed in a single project in a Class 4 (7.0-7.5 m/s at 50 m above ground) wind resource with utility industry financing costs. WTC has claimed that with a 200 MW production run/windfarm project in a Class 5 (7.5-8.0 m/s) wind resource, the turbine could produce electricity for ¢3.0/kWh. WTC expects to accomplish this goal by 2005 while achieving respectable profits from the sale of wind turbines and providing acceptable returns to windfarm developers.
Today, WTC is confident its technology works. If it is necessary to reach ¢3.0/kWh, WTC will be able to sell complete wind turbines including towers for $500/kW to customers who order 100 or more WTC 750 units at a time. This will allow it to achieve profit margins in excess of the reported margins of today’s leading publicly traded turbine manufacturers.
WTC’s articulated downwind design results in a large reduction of out-of-plane loads on the turbine’s blades. The FMR system coupled with other configuration features dramatically reduces loads throughout the turbine. Reduced loads lead to reduced requirements for materials and lower cost.
One widely employed figure of merit in the industry is the ratio of the total weight on top of a tower to the rotor swept area. Figure 3 compares this ratio for several of today’s most popular turbines. The Vestas V47, rated at 660 kW, is believed to have the lowest ratio of weight to swept area of the current industry leaders, weighing in at approximately 16 kg/m2. (Figure 3 is not intended to be all-inclusive).
The complete nacelle/drive-train/blade unit for the commercial WTC 750 will weigh in at just over 9 kg/m2, over 40 per cent less than the V47. The WTC 750 will generally employ the same materials as the V47, with the exception of its lightweight carbon fiber blades. WTC expects this weight saving to translate into at least a 20 per cent cost reduction. This cost reduction, coupled with increased energy capture from installation on taller towers, leads WTC to believe ¢3.0/kWh, unsubsidized, is obtainable.
The wind energy industry has experienced rapid growth around the world throughout the last decade. Important improvements in operational reliability and manufacturing practices together with the introduction of some new technology have combined to drop the installed cost of wind-generated electricity significantly. However, almost everywhere, some form of subsidy remains important if not critical to the industry’s continued growth, if not its survival.
Even though virtually all energy technologies known to mankind receive some form of subsidization, subsidies are not a peg on which the wind industry should rest its hat. If they are available – great – however, if a wind turbine can compete against other subsidized generation technologies and not require subsidies to do so, so much the better.
Since 1990, when the company was founded, it has been WTC’s view that such a cost advantage is necessary to achieve the 20+ per cent market share that wind energy is capable of achieving. By the end of 2001 the pre-commercial prototype of the turbine of the future will be operating in Antelope Valley, California.