Late last October, the California Public Utilities Commission approved plans fora 4,500-acre sun farm that will begin harvesting solar energy in late 2008. By 2012, this first of two huge energy farms could be supplying 500 MW daily, enough power to keep the electric meters spinning on a quarter million homes.
As the utilities begin committing to this 21st century power source, they'll be looking to a 19th century invention, the Stirling engine, for a prime mover. The solar sites will use arrays of sun-concentrating mirrors to track the sphere as it moves through the sky over the Mojave. Each tracking mirror will focus its concentrated light beam on a small Stirling engine, where heated hydrogen will expand and drive pistons and, through crankshafts, turn generators. Precision motion components will keep the mirrors tightly focused on the sun.
Phoenix-based Stirling Energy Systems Inc. will own and operate the plants and sell the power to Southern California Edison and San Diego Gas and Electric, which, along with the rest of the state's electric utilities, are required by 2017 to furnish 20 percent of their generated power through renewable means.
According to Steve Trimble, Stirling's senior manager for systems, a number of factors make the California desert a good place for farming sunlight and a good place to use Stirling engines. Lack of rain, for one, means a preponderance of sunny days—ideal for solar collecting and for matching the daytime peaking capacity of a solar plant to the peak air-conditioning loads of the southern California summer. It also means no water, though, a tougher proposition for steam cycles that need the fluid but no problem for the Stirling engine. "The only water we use is a small amount to clean dishes," Trimble says, referring to the solar mirrors.
Stirling today has a six-unit model plant operating on the grounds of the Sandia National Laboratory in Albuquerque, NM. Each 38-foot-diameter dish can produce 25 kW of peak power, Trimble says. A 500-MW plant will comprise 20,000 such units.
Stirling Energy formed in 1996 when it acquired the solar-concentrator technology of McDonnell Douglas. It
|The Stirling engine power conversion unit (PCU), takes up little room. People often describe it as “smaller than an oil barrel.”
also gained the rights to manufacture the 4-95 Stirling-engine design of Swedish submarine builder Kockums, which, with United Stirling AB and Volvo, developed it as a quiet means of propelling boats underwater.
Though students in engineering thermodynamics classes have long studied the Stirling cycle, its practical deployment has been limited to boutique applications, Trimble says. But, an efficiency of 29 percent overall from collector to grid—nearly double that of parabolic troughs or photovoltaics—makes it a logical choice for solar generating. Key to that efficiency, Trimble says, is the regenerative part of the cycle, where a constant-volume heat addition to the process occurs (see sidebar on page 60).
The Stirling cycle doesn't introduce heat to the system through internal ignition, as the internal combustion, or Otto cycle, does. Instead, heat applied to the outside of a cylinder expands a volume of gas inside.
Like the internal combustion engine, a Stirling engine is complex mechanically, making it costly for one-off applications in spite of its great efficiency, Trimble explains. The large number of units that will eventually be built for these two generating farms will bring to bear economies of scale, just as they have for complex systems such as automobiles.
The 1800-rpm SES Stirling engine uses four double-acting cylinders, which engineers originally conceived to take advantage of proven automotive-style components. So, the engine uses pistons and rods that resemble those of a modern engine.
|Air radiators eliminate the need for cooling water.
Even the nearly 90 mirrors that make up a single concentrator are each similar in size and shape to a car hood, Trimble points out. The idea was that Stirling could cheaply manufacture these mirrors with tools ubiquitous in the car business.
The company will base concentrator controls on open-loop systems, explains Larry Wilson, an SES project engineer responsible for the dish structure. "Where to aim the dish is based on the time of day and day of the year," he says. And Stirling expects few deviations from those positions unless a wind comes up. Then, a closed-loop system measures the temperature variation among the dish's four quadrants and adjusts the position of the dish to equalize them.
A great advantage of the solar system over conventional power plants is that it can produce power with its first megawatt of installed capacity, Trimble says. The company will incrementally bring the full 500 MW on line.
Turn and Face the Sun
Joyce/Dayton, whose screw jacks the satellite-tracking industry uses extensively, was an obvious choice when SES began searching for a vendor for the solar concentrator's elevation drive. Over a span of seven years, the company developed and supplied combination worm-gear-reducer/ball-screw units for all six of the prototypes operating on the Sandia grounds, according to Joyce Marketing Director, Michael Harris.
To save cost, the company first considered machine screw jacks, Harris says, before it became clear that ball screws would provide better longevity. "The life of an acme screw is not that predictable," Harris explains, due to many variable factors, such as lubrication and environmental conditions, which are difficult to control. A general-purpose machine screw is usually good for 200,000 to 500,000 inches of travel, whereas a ball screw, which substitutes rolling friction for sliding friction, can generally go 1 million inches.
With much less friction, though, ball screws can't self-lock the way acme screws can, making back-driving an
|The Stirling engine uses a tubular heater to heat H2. For a larger image, click here
issue. SES supplies the motor for the elevation drive and a brake for holding each dish "on sun."
The ball screws had to be able to stand up to wind loading on the collector at speeds up to 50 mph, regardless of orientation and at speeds up to 90 mph when the collectors were in their face-up, "stowed" positions. In the worst case, the ball-screw jacks see loads of 15,500 lbs. When fully retracted, they can handle up to 40,000 lbs, Harris says. Joyce engineers developed a special 2-inch-diameter lifting screw to fit in a 10-ton frame, which ordinarily accommodates a 1.5-inch-diameter screw. The company claims it's an industry first. A Joyce/Dayton subsidiary developed special aluminized fiberglass bellows to protect the works from the desert's gritty environment. The design withstands the high temperatures near the solar-fired engines better than neoprene could.
Peerless-Winsmith furnished its azimuth Planocentric drives for the six prototype dishes, according to Product Marketing Manager George Tedesco. The drive package replaces a number of functional components that would otherwise be inside the pedestal, he says. The company developed it for solar-tracking applications.
Incorporating a large-diameter ball bearing, the system manages the high-overturning moments of as much as 2.5 million inch-lbs that solar collectors encounter from wind loads, Tedesco says. The low-profile gearbox combines a planetary gear and eccentric in a dense, flat arrangement. Backlash of less than 0.5 milliradians ensures accurate sun tracking.
Last June, California Governor Arnold Schwarzenegger adopted a new greenhouse-gas-emissions-reduction plan for the state that accelerates the current mandate. Although the comparatively mature wind-power industry expects to play a large role in meeting this new requirement, solar energy will be especially important in the southern part of the state. There, sun resources outweigh wind resources, and generating capacity can be constructed nearer to the state's populous region.
The first phase of the Southern California Edison project will begin with a 40-unit, 1-MW test installation.
Reach Sr. Technical Editor Paul Sharke at email@example.com.