- Number 336 |
- May 2, 2011
Algorithm positions solar trackers, movie stars
NREL Senior Scientist
Ibrahim Reda and NREL
Senior Scientist Afshin Andreas
developed the Solar Position
Algorithm now used by solar
trackers, orchard growers and
movie-camera makers, among
others. The algorithm used in
the SPA appears in the shadow.
Credit: Dennis Schroeder.
Math and programming experts at DOE’s National Renewable Energy Laboratory took an algorithm used to track the stars and rewrote its code to precisely follow the sun, even taking into consideration the vagaries of the occasional leap second.
Now, the algorithm and its software are helping solar power manufacturers build more precise trackers, orchards to keep their apples spotless and movie makers to keep the shadows off movie stars.
The Solar Position Algorithm (SPA) was developed at NREL to calculate the sun's position with unmatched low uncertainty of +/- 0.0003 degrees at vertex, in the period of years from -2000 to 6000 (or 2001 B.C. until just short of 4,000 years from now). That's more than 30 times more precise than the uncertainty levels for all other algorithms used in solar energy applications, which claim no better than +/- 0.01 degrees, and are only valid for a maximum of 50 years. And those uncertainty claims cannot be validated because of the need to add an occasional leap second because of the randomly increasing length of the mean solar day. The SPA does account for the leap second.
That difference in uncertainty levels is no small change, because an error of .01 degrees at noon can throw calculations off by 2 or 3 percent at sunrise or sunset, said NREL Senior Scientist Ibrahim Reda, the leader on the project. "Every uncertainty of 1 percent in the energy budget is millions of dollars uncertainty for utility companies and bankers," Reda said. "Accuracy is translated into dollars. When you can be more accurate, you save a lot of money."
"Siemens Industry Inc. uses NREL's SPA in its newest and smallest S7-1200 compact controller," says Paul Ruland of Siemens Industry, Inc. "Siemens took that very complex calculation, systemized it into our code and made a usable function block that its customers can use with their particular technologies to track the sun in the most efficient way. The end result is a 30 percent increase in accuracy compared to other technologies."
SPA sprang from NREL's need to calibrate solar measuring instruments at its Solar Radiation Research Laboratory. "We characterize the instruments based on the solar angle," Reda said. "It's vital that instruments get a precise read on the amount of energy they are getting from the sun at precise solar angle."
That will become even more critical in the future when utilities add more energy garnered from the sun to the smart grid. "The smart grid has to know precisely what your budget is for each resource you are using — oil, coal, solar, wind," Reda said.
Available for licensing, free public use
The new SPA algorithm simply served the needs of NREL scientists until the day it was put on NREL's public website.
“A lot of people started downloading it," so NREL established some rules of use, Reda said. Individuals and universities could use SPA free of charge, but companies with commercial interests would have to pay for the software.
One person who bought a license for the SPA software has an apple orchard, and wanted to keep the black spots off the apples that turn off finicky consumers, thus making wholesale buyers hesitate, Reda said.
The black spots appear when too much sun hits a particular apple, a particular tree or a particular row of trees in an orchard.
The spots can be prevented by showering the apples with water, but growers don't want to use more water than necessary.
SPA's precise tracking of the sun tells the grower exactly when the automatic sprinkler should spray for a few moments on a particular set of trees, and when it's OK to shut off that sprayer and turn on the next one. SPA communicates with the sprinkler system so, "instead of spraying the whole orchard, the spray moves minute by minute," Reda said. "He takes our tool and plugs it into the software that controls the sprinkler system. And he saves a lot of water."
A movie-camera manufacturer has purchased the SPA software to help cinematographers combat the precious waste of money when shadows disrupt outdoor shooting.
"They have cameras on those big cranes and booms, and typically they'd have to manually change them based on the shadows," Reda said. "This company that bought it has an automatic camera positioner."
Combining the positioner with the SPA's calculations, the camera can tell the precise moment when the sun will, say, peak above the tall buildings of an outdoor set. "They don't have to make so many judgments on their own about where the camera should be positioned," Reda said. "It gives them a clearer picture."
Learn more about NREL's solar radiation research and the Electricity, Resources, and Building Systems Integration Center. -- By Bill Scanlon
Submitted by DOE's National Renewable Energy Laboratory