People often ask: "Can we really generate enough pollution-free electricity to power our businesses and homes?" The calculations below are presented to answer this very important question.
In the 48 contiguous states alone pavements and other impervious surfaces cover 112,610 square kilometers - an area nearly the size of Ohio - according to research published in the 15 June 2004 issue of Eos the newsletter of the American Geophysical Union.* It is believed that continuing development adds another quarter of a million acres each year and that typically two-thirds of the cover is pavements and one-third is building roofs.
Removing 1/3 for rooftops and that leaves 32,868.61 square miles of roads, parking lots, driveways, playgrounds, bike paths, sidewalks, etc., to work with.
In labs solar cell efficiency has exceeded 44-percent but they're not cost feasible yet. for our calculations we use commercially available solar panels which are cost competitive.
The efficiency of 18.5% is commonly available so for the calculations the following (conservative) assumptions have been made:
Sunpower offers a 230 Watt solar panel rated at 18.5% efficiency. Its surface area is 13.4 square feet. If the entire 32,868.61 square miles of impervious surfaces were covered with solar collection panels, then:
((32,868.61 mi²) x (5280 ft / mi)²) / (13.4ft²/230W) =
((32,868.61 mi²) x (27,878,400 ft² / mi²)) / (13.4ft²/230W) =
(916,324,257,024 ft²) / (13.4ft²/230W) = 15,727,953,665,337 Watts or over 15.73 Billion Kilowatts
Considering only the average of 4 hours of peak daylight hours (1460 hours per year) this gives: 15.73 Billion Kilowatts x 1460 hours = 22,966 Billion Kilowatt-hours of electricity.
The farther north one lives the more one has to angle solar panels toward the equator (or more accurately the sun above the equator) to gain maximum efficiency.
Solar Roadways did some testing at our location in northern Idaho an hour south of the Canadian border at latitude 48.19 degrees. The farthest northern point in the contiguous 48 states is 49.38 degrees near Lake of the Woods, Minnesota. That's 82 miles farther north than our location. At this northern position (48.19 degrees North) the optimal solar gain angle for solar panels is 72 degrees. By contrast Brownsville, Texas would want to angle their solar panels at 26 degrees. So southern roads will naturally produce much more electricity than their northern counterparts as solar intensity maps show.
Unfortunately we can't angle roads or parking lots. Roads go up and down hills, have banks on curves (going both left and right), and have a typical three percent "crown" (on both sides) to allow stormwater runoff. It's a pretty safe assumption to figure that the national average angle of roads is zero degrees.
We tested two identical solar panels. One was mounted at the recommended 72 degrees while the other one was placed in line with the horizon (zero degrees) to simulate an average road. We installed a monitoring system to track the data 24/7.
Although the tilted solar panel produced more energy as expected (an average of almost 31 percent more than its horizontal counterpart) we discovered a phenomenon that was apparently previously unknown: The horizontal solar panel produced more energy than the tilted panel on certain overcast days. It appears to be similar to getting sunburned on a cloudy day: sunlight is still present but it is scattered so the horizontal solar panel is more likely to pick up the scattered photons than the solar panel aimed at the southern horizon.
For fairness we subtract 31 percent from our totals since we can't angle roads and parking lots:
22,966 Billion Kilowatt-hours x 0.69 = 15,847 Billion Kilowatt-hours
Another finding from our experimentation was that our 1/2-inch textured glass surface reduced the amount of energy produced by solar cells by 11.12-percent (we are experimenting with some changes to improve that number). Subtracting that from the total, we still have 14,085 Billion Kilowatt-hours. And remember: this is the amount of power calculated for a latitude near the Canadian border. The number would be much larger if calculated for the southern states.
While we found no evidence that moonlight or the light from shining stars at night produce energy in solar panels (a common question), we found that headlights did. Although it would be very difficult to measure accurately due to distance, speed, hi/low beams, etc., we found that a small solar panel placed flat on the ground about 10 feet in front of a vehicle with its high beams on produced electricity in otherwise total darkness. So it appears that vehicles driving on the surface at night will be providing a service as well as reaping the benefits.
According to the Energy Information Administration, the United States (all 50) used 3,741 Billion Kilowatt-hours of electricity in 2009 (EIA Electricity Overview, 1949-2009). It's easy to see that Solar Roadways could produce over three times the electricity we currently use in the United States! In fact, just the "lower 48" could produce just about enough electricity to supply the entire world!
Remember that these calculations are made with very conservative numbers using north Idaho as a reference point, which is one of the least favorable latitudes in the U.S. for solar energy collection.
To see the actual measured results of our Phase II research, see Phase II Results.
It is estimated that approximately half (different agencies provide different estimates, but the average is about 50-percent) of the greenhouse gases that are causing global warming come from the burning of fossil fuels (primarily coal) to generate electricity. The Solar Roadway therefore has the ability to eliminate half of the greenhouse gases currently being produced.
Another 25-percent comes from vehicle emissions. A Solar Roadway is an electric road that can recharge electric vehicles (EVs) anywhere. We're talking with companies that make mutual induction plates to charge EVs while they're driving (the "receiver" plate gets mounted beneath the EV and the "transmitter" plate is installed in the road). The Solar Roadway could charge the EVs while they're traveling, which would increase their range. With an infrastructure in place that will make EVs finally practical, people would likely start trading in their internal combustion engine vehicles for EVs. Eventually, we'd have eliminated an additional 25-percent of greenhouse gases.
With internal combustion engines now obsolete, our dependency on oil - foreign or domestic - will finally be over with.
Unlike current road systems, a Solar Roadway offsets its cost over time. No more contributing to the climate crisis. No more dependency on fossil fuels. No more power outages (roaming or otherwise). Safer driving conditions. Far less pollution. A new secure highway infrastructure that pays for itself. A decentralized, self-healing, secure power grid.
* The report lists the following citation: Vogelmann, J. E., S. M. Howard, L. Yang, C. R. Larson, B. K. Wylie, and N. Van Driel, Completion of the 1990s National Land Cover Data set for the conterminous United States from Landsat Thematic Mapper data and ancillary data sources, Photogramm. Eng. Rem. Sens., 67, 650–662, 2001.
Virtually every report we’ve seen that cites impervious surfaces cites this report, even though it was created in 2001. If anyone knows of a more recently published report, we’d like to be able to cite it and include it this Numerical Calculations page.