Renewables require rethinking just about everything
Renewable portfolio standards (RPS) are here to stay, with nearly half of U.S. states having adopted some measure to push regulated utilities to use more power generated by the wind, the sun, or biomass. In some states it took quite a while for regulators or legislators to decide that some level of RPS makes sense. The most recent law, enacted in Arizona in November, was the culmination of three years of "study." It requires utilities to get 15% of their supply from renewables by 2025. Where that number came from, and its relation to the current level of 1.1%, are anyone's guess.
Another "known unknown," as former Defense Secretary Donald Rumsfeld might call it, is whether the invisible hand of the free market will ultimately pat or slap the practice of allowing each state to decide for itself what a renewable is (see this month's Legal & Regulatory column for California's latest definition of a renewable), the mandated penetration level, and the penalties for noncompliance. The wide variation in state RPS rules is understandable, given that they are promulgated by political appointees who tend to treat ratepayers paternalistically. POWER's research indicates that, although most ratepayers are risk-averse, the majority say they are willing to pay a minor "subsidy" to add renewables to their local utility's resource mix. But far fewer answer "yes" when told that increasing renewables" share may significantly raise their monthly bills or make power outages more frequent.
Today there are finite numbers of renewable projects under development but an increasing number of utilities under the gun to add renewables-fueled capacity. Economics 101 tells us that when a resource is in short supply, its price will rise if demand remains constant—and skyrocket if it grows. With RPS, states are effectively forcing regulated gencos either to pay higher prices to developers, build capacity themselves (where allowed), or pay the penalties. In Massachusetts, the latest renewables request for proposals received bids at just a few cents under the penalty price. Where's the motivation in that?
Comments by two members of the Arizona Corporation Commission (ACC) after passing the new RPS frame the debate nicely. Bill Mundell noted, "This truly is an historic vote that will lead to environmental benefits, economic development, higher-paying jobs, and less dependence on fossil fuels that originate in volatile regions of the world." Mike Gleason expressed a contrarian view: "The ACC passed a rules package that does not control the cost and will probably degrade the reliability of the western electric grid. In its haste to enact the rules, the Commission has allowed Arizona utilities to use ratepayer dollars to buy renewable energy credits from distributed generators located anywhere." Time will tell which side is right.
Most renewable energy plants are much smaller than traditional, utility-size facilities being developed—coal-fired and nuclear plants. But that doesn't make them any less interesting or significant. The next four stories provide a virtual tour of some renewable, alternative, and unique energy projects we find particularly fascinating. Although there's something here to intrigue every reader, we confess to having an ulterior motive. We want to pique your interest in our feature article on osmotic power (from the sea) and our Special Report on wind power. In the latter, two technology-focused articles take a close look at the challenges that wind farm developers face now, or will soon.
Torque-splitting drive train improves wind turbine reliability
Over the past decade, land-based wind turbines have grown from about 700 kW to well over 2 MW. This upscaling, however, has subjected turbine gearboxes and other components to greater loads and stresses. Traditional gear and bearing manufacturers are struggling to scale their current technologies to keep pace while maintaining tolerances.
"As wind turbines get bigger, so do the design challenges," said Charles D. Schultz, chief engineer at Chicago-based Brad Foote Gear Works and author of An Introduction to Gear Design, a primer on the subject. "Wind turbines are one of the most demanding applications for gearboxes due to variable loads that are extremely difficult to predict," he explained.
Massive torque, for example, is transmitted through the three-stage planetary gearboxes typically used in multi-megawatt turbines. In response, manufacturers have developed huge, expensive ring gears and bearings, stretching the limits of current technology and driving some designers to investigate new configurations.
"Planetary gearboxes suffer too many failures due to bearing issues and excessive loads," said Amir Mikhail, senior vice president of engineering for Clipper Windpower Inc. (www.clipperwind.com). "We decided that to maintain reliability, we had to take a different approach to the gearbox—and one that doesn't increase its maintenance requirements."
Over the past four years, Clipper has developed a 2.5-MW wind turbine with a compact two-stage helical gearbox. Known as the Liberty, the patented design reduces loads, minimizes the likelihood of damage, and increases gearbox lifespan. Its use of multiple generators and a multiple-path, distributed gearbox both appear to be unique.
Eight was too many. Development work on the Liberty was initially funded by the U.S. DOE as part of the agency's National Renewable Energy Laboratory's (NREL's) low–wind speed turbine program. Clipper won a grant from NREL to develop a prototype known as the D-Gen. NREL then put the D-Gen through its paces at its dynamometer testing facility using drive train tests that are difficult, if not impossible, to conduct in the field. According to NREL, the Clipper machine achieved endurance load for full-speed operation at 30% above its power rating.
The D-Gen was an eight-generator, 1.5-MW, variable-speed drive train notable for its quiet operation, robustness, small size, and low weight. Its nacelle, for instance, was about 6 to 18 feet shorter than those of turbines of similar capacity. The multiple generators increase reliability the old-fashioned way—through redundancy.
The Liberty C-93 Turbine (Figure 1) has inherited many of these concepts. But further refinements have been made. Instead of eight generators, four high-speed output shafts are now employed. These generators distribute the load by a factor of 16—four times more than commercially available gearboxes do.
1. Four of a kind. The drive train of the Clipper C-93 Liberty turbine splits the torque among four generators operating in parallel. Courtesy: Clipper Windpower Inc.
If one generator goes off-line, the other three continue. In normal wind conditions the drop in output isn't noticeable; only with high winds will the capacity fall—by 25%. To simplify maintenance, a single 650-kW generator can be removed and lowered to the ground by an onboard crane. Another boon to service: The high-speed gear sets can be replaced without having to remove the gearbox.
Lighter gearbox. "The Clipper C-93 Liberty turbine uses an innovative gearbox design which uses torque splitting to feed the mechanical power to four advanced permanent magnet generators," explained Bob Thresher, director of NREL's National Wind Technology Center. "The multiple-drive path design radically decreases gearbox loads and stress by splitting the load path four ways. This also reduces the size and weight of the drive train."
While the gearboxes of comparable turbines weigh 50 to 70 tons, the one in the Clipper (Figure 2)—including the brakes and housing—weighs just 36 tons. Low-speed tapered roller bearings are used to handle thrust loads on the main shaft.
2. Featherweight. The gearbox of the 2.5-MW Clipper weighs only 36 tons, as much as 50% less than units in machines of comparable capacity. Courtesy: Clipper Windpower Inc.
The turbine control system incorporates the high-speed microprocessors needed to execute algorithm computations, which are repeated every 50 milliseconds. Unity power factor is maintained down to a low rated power percentage, reducing the need for volt-ampere-reactive (VAR) correction. The control system can ride through a low-voltage condition for up to 3 seconds. It also reduces loads by anticipating resonant conditions within the drive train structure and generators.
Putting it to the test. Clipper's Liberty I prototype has been running at a remote site in Medicine Bow, Wyoming, since March 2005. The site has offered up a wide range of weather conditions, including temperature extremes, high-turbulence wind squalls, lightning, ice, and snow. Together, the elements and limited maintenance infrastructure have both challenged the machine and taught the testing team and maintenance crew plenty of lessons, which they are applying to the Liberty.
Over the course of its latest testing period, Liberty performed well within its design expectations. The power curve, turbine architecture, and system design have been confirmed by NREL. Other tests verified that the power balance between generators was within the 7% design spec.
More to come. Using international industry standards, Germanischer Lloyd WindEnergie GmbH has certified the Clipper wind turbine and its blades for rotors of 89-, 93-, and 96-meter diameter. Independent engineering reviews were conducted by Garrad Hassan, a wind power consultancy. Testing and evaluation of the 2.5-MW turbine confirmed that the new drive train design mitigates gearbox stresses, and first-year operating data suggest that it will be more reliable than standard gearbox designs.
Clipper Windpower has opened a manufacturing and assembly facility in Cedar Rapids, Iowa. The first Liberty wind turbines have already rolled off the line. Mikhail said he expects about 40 to be produced this year, with 250 anticipated in 2007 and 500 in 2008.
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