How to Build a Better Light Trap
Software developers are taking advantage of the NASA data sets by writing new programs for computer-aided design of advanced new systems for converting sunlight into electricity. SolarSizer is a good example of one such software tool. Programmed by Eric Woods and Ken Olson, SolarSizer enables engineers to download NASA data so that they may take all the geographic and meteorological characteristics of a given region into account as they design systems for use in that region.
With SolarSizer, engineers can design a new solar energy system and then use real satellite data to simulate the environment in which their new system must operate to see how well it will perform. This is a capability not present in most photovoltaic design programs used commercially, says Woods, co-owner of Sunnywood Designs.
The promise of the new NASA database is that we can go anywhere
in the world and get solar radiation data from satellites, states
Olson, founder and owner of his own company called SolEnergy. Our
software makes it possible to go to the NASA SSE Web site and extract
data from the databasefor a certain locationand use them in our
SolarSizer provides a graphical method for designing a photovoltaic system that users can do through a drag-and-drop kind of interface, Woods adds. The tool gives users the ability to look at the accuracy of the satellite data. There are characteristics of every geographic location that can impact the accuracy of the datathings like terrain, typical snow and ice coverage, and cloud coverageso designers can judge for themselves the quality of the data for a particular location.
Olson works primarily in Nicaragua and Honduras. In those countries there is much variation in topography and micro-climates, so I have to be careful to use the best available data for a particular location, he says.
A solar energy system designers interest is in knowing the amount of solar energymeasured in kilowatt hours per square meter per daythat reaches the Earths surface. Among other functions, their program can then calculate how much solar energy is received by surfaces that are tilted at specific angles. According to Olson, some panels are fixed so that they may tilt at one of three possible angles, whereas there are more sophisticated systems that actually track the position of the sun through the day to constantly maintain an optimum tilt angle.
|The graph to the left shows average solar insolation for each month of the year for cities on four different continents. Timbuktu, in the Sahara desert, receives abundant solar energy year round. Seattle, on the other hand, receives roughly 6 kilowatt hours per square meter per day in mid-summer, but very little in the winter. The solar energy bathing Bangkok and Brasilia, tropical cities just north and south of the equator, varies more due to cloud cover than season. By combining this type of satellite data with local data, planners can take into account microclimate and topography to precisely design and position systems to harvest solar power. (Graph by Robert Simmon, based on data from the SSE Project)|
Were designing solar energy systems that maximize the amount of sunlight we collect over the course of the year, Olson explains. To do that, you must tilt solar panels toward the south and then back toward horizontal as a function of latitude. So, using geometry, we can accurately calculate how much energy we will get from our solar panels as a function of tilt angle.
The SolarSizer tool also facilitates the design of more advanced materials for making photovoltaic systems. There is a cost versus efficiency tradeoff that designers must take into consideration. Crystalline silicon technologies are the most efficient for converting sunlight into energy, catching and converting up to 16 percent of the suns rays that reach the solar panel, according to Olson. Yet, other commercially available panels are made with amorphous silicon, which has on the order of 6 to 8 percent efficiency. But because the amorphous silicon material is cheaper, Olson says it will likely be the more commonly used material in the future.
In designing the next generation solar energy systems, satellite data are the very best we could hope for, Olson concludes. The importance of having those data grows as commercial investment in those systems grows.
These sunflowers (panels of photovoltaic solar cells) tilt to track the sun during the course of a day. This maximizes their efficiency. (Photograph courtesy National Renewable Energy Laboratory)