Solar power shortfalls due to elevation in the Twin Peaks neighborhood of San Francisco for one year. Image: University of California, San Diego
The space shuttle program may have ended, but data the space craft collected
over the past three decades are still helping advance science. Researchers at
the Jacobs School of Engineering at University
of California, San
Diego recently used measurements from NASA’s Shuttle Radar
Topography Mission to predict how changes in elevation, such as hills and
valleys, and the shadows they create, impact power output in California’s solar grid.
Current large-scale models used to calculate solar power output do not take
elevation into account. The California Public Utilities Commission asked Jan
Kleissl, a professor of environmental engineering at the Jacobs School of
Engineering at UC San Diego, and postdoctoral researcher Juan Luis Bosch, from
the department of mechanical and aerospace engineering, to build a model that
This is the first time this kind of model will be made available publicly on
such a large scale, including all of Southern California,
as well as the San Francisco Bay Area. It took the Triton Supercomputer at the San Diego Supercomputer Center
here at UCSD 60,000 processor hours to run calculations for the model. Utility
companies and homeowners can use the model to get a more realistic picture of
the solar power output they can typically expect to produce. This is an
especially important tool for utilities, because it gives them a better idea of
how much revenue they can actually generate, Kleissl says.
Changes in elevation can have a significant impact on solar power output.
The longer it takes for the sun to rise above the local horizon in the morning
and the earlier it sets in the evening, the more solar fuel is lost. Solar days
are longest on top of tall mountains. They are shortest in steep valleys
oriented north-south, where it can take more than an hour longer for the sun to
appear in the east.
For example, on clear winter days in San Francisco’s
Twin Peaks neighborhood, in the areas at the
foot of the steepest hills, solar days are up to 30% shorter than on flat
terrain. A solar power plant in that area would produce 12% less energy on
those days than if it was located on a plain or other flat landscape. But in
summertime, the days are much longer and the sun is brighter, so the total
production shortfall would be only 1 to 2% over the course of a whole year.
“Solar resource models have become very accurate,” says Kleissl. “Now we are
refining them down to the last few percentage points.”
Bosch, the postdoctoral researcher, used elevation data obtained on a
near-global scale by astronauts aboard the space shuttle Endeavour during an
11-day mission in February 2000. The data were later compiled into a
high-resolution digital topographic database of most of planet Earth. Bosch and
Kleissl focused on the areas of California
where most solar power plants are located and where elevation is an issue,
namely the San Francisco Bay Area and Southern California, including San Diego, Imperial, Riverside,
Orange, and Los Angeles counties.
One caveat for the method developed by Kleissl and Bosch is that it provides
only baseline information in urban areas. Trees, poles, and other rooftop
structures, such as chimneys, can cause more power losses. In that case, the
best method to estimate power shortfalls is to use a fisheye camera to
visualize the local horizon, a device that any qualified installer of solar
panels would have on hand.