NREL: Show true colors for the solar potential across the U.S.

By William L. Driscoll

Nearly all of the U.S. has at least 70% of the solar potential of land in the desert Southwest, according to government data. There is no dramatic drop-off in solar potential as you move away from the desert Southwest, as you would think from the federal map at left.

We need a new federal map to reflect solar’s true potential across the U.S.—such as the map at right, whose color scheme accurately reflects the data.

Images matter, because they shape public opinion. Someone who thinks their region has pitiful solar potential is unlikely to install rooftop solar. Solar installers working hard nationwide to transition our grid to renewables deserve support from the National Renewable Energy Laboratory, which publishes the map at left. NREL should update its map to replace its misleading colors with accurate colors.

For example, data from NREL’s key, at the bottom right of its map, show that global horizontal solar irradiance in the two northern bands is 70% of that in the two sunniest Southwest bands (4.00 kWh/m2/day versus 5.75 kWh/m2/day). That’s indicated by the moderately intense red colors at the top of the map at right, which was prepared by the author. But the pale yellows at the top of NREL’s map are nowhere close to 70% of the intensity of its colors for the Southwest.

NREL’s solar irradiance maps date back at least as far as 2009, when dramatizing the most promising regions for solar may have been appropriate. Back then, solar was expensive, and maximizing the revenue from every solar panel may have been an important consideration. But now that solar is inexpensive, it’s vital to see solar’s potential in every part of the country.

The map at right starts with pure red for the Southwestern band with the highest irradiance. The map incrementally adds more white, to create a less intense shade of red, for each lower-irradiance band, on a scale where zero irradiance would be represented by pure white. Because even the northernmost two bands have 70% of the irradiance of the far Southwestern band, their colors are deep pinkish reds that are about 70% as intense as pure red. And generally, the map appears red, because there’s substantial solar potential everywhere.

NREL is funded by taxpayers through the U.S. Department of Energy. NREL’s mission is to advance the science of renewable energy technologies, yet its map at left above is a funhouse mirror caricature of science. It’s time for NREL to fix this map.

As for the term “global horizontal solar irradiance,” that measure counts all solar radiation reaching the earth, whether directly from the sun or after passing through clouds.

Photoshop made this possible

To create the map at right, NREL’s map at left was first imported into Photoshop. The deep violet red in the lower left of NREL’s map was replaced with pure red, specified in Photoshop as R = 255, G = 0, B = 0 (RGB stands for red, green, blue).

This pure red color was used to represent 5.87 kWh/m2/day, because the key defines the deep violet red band as having global horizontal solar irradiance greater than or equal to 5.75 kWh/m2/day, and the other bands represent a range of 0.25 kWh/m2/day.

For the band next to the “maximum” band, 5.62 is the approximate midpoint of the range of 5.50 to 5.75. Thus, the ratio of the second band’s irradiance to the first band’s irradiance is 5.62/5.87, or 96%. In Photoshop, to select a shade of red that is 96% as intense as pure red, a color was selected that is 4% of the way from pure red to pure white. That color happens to be R = 255, G = 11, B = 11. Those values are based on the values for pure white (R = 255, G = 255, and B = 255), and the calculation that 11/255 is about 4%.

The same process was followed for each subsequent color band, using data from NREL’s key to transform each NREL color band into a “true color” that accurately reflects the NREL data.

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