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| Fig. 1: Prototype panels. (Courtesy of Solar Roadways) |
Solar Roadways made waves recently when it raised $2 million on Indiegogo, in addition to $750,000 in a federal grant. [1] The company claims that replacing roadways with their modular solar panel units would be able to generate over 3 times the required solar energy for the entire United States. [1] Additionally, the system would function as a smart grid, use LED lights for lane markings, and keep roads clear of ice and snow. [2] But are these claims feasible? And if they can be technically achieved, how much would it cost?
The US highway system has about 8.5 million lane-miles (a lane-mile is one lane one mile in length; a four-lane highway of 10 miles counts as 40 lane-miles). [3] A standard highway lane is 12 feet wide. [4,5] Using this as the width of a lane, the US highway network has an area of about 50 thousand square kilometers. Solar flux at the earth's surface is about 1 kW/m2. The geographic center of the United States is at about 40 degrees latitude. [6] Since roads can' be tilted to point towards the sun, the solar flux will be reduced by this angle, on average. For the sake of argument, we assume that the efficiency of a solar panel is about 20 percent, which is a high-performance commercial panel. [7] This allows us to find the power generated from the road area as the product of the area, the flux, and the efficiency:
If we assume that each panel gets 4 hours of quality sunlight, then this power output results in 11.2 million GWh annually. Statistics from the US Energy Information Administration (EIA) indicate that for the last few years, the US has used about 3.8 million GWh annually. [8] At first glance, it seems like solar roadways could power the country's electricity needs - and then some.
However, this includes all parts of the highway network. Because there isn't room in the roadway for high-tension power lines, power cannot be transmitted very far effectively. The physics of electricity limits transmission in a single 765 kV line to about 2GW.
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| Fig. 2: Conductor configuration for a 3-phase AC transmission line. Conductors have radius a and are separated by d. |
We can see this by applying Maxwell's equations to a three-phase line, giving the following voltage and current in each conductor (SI units):
| Vn | = | V0 ln(d/a) cos( k z - ω t - n 2π/3) |
| In | = | 2πε0c V0 cos( k z - ω t - n 2&pi/3) |
where c is the speed of light, ε0 is the permittivity of free space, and ω = c k = 2π × 60 Hz. Substituting V0 ln(d/a) = 765 kV, d = 10 m, and a = 0.015 m, one obtains for the transmitted power
| P | = | I1 V1 + I2 V2 + I3 V3 |
| = | 3πε0c V02 / ln(d/a) | |
| = | 3π × 8.85 × 10-12 farad/m × 3.0 × 108 m/sec | |
| × (7.65 × 105 volts)2 / ln (10 m / 0.015 m) | ||
| = | 2.25 × 109 watts | |
This is a small fraction of the total power, most of which is consumed in urban centers. If we assume that only the urban portions of the highway system are solarized, that reduces the area to just 15 thousand square kilometers. [3] This reduces the corresponding power capacity to 3.4 million GWh annually, which is not enough to fill the US electricity consumption.
Norway recently installed a 70m solar bike lane at a cost of about $3.7 million. [9] This bike lane has an area of about 250 square meters, yielding a cost per area of $15 thousand per square meter—at this price, solarizing just urban highways would cost in excess of 200 trillion dollars. However, this is nowhere near the best estimate. This includes research and development costs, as the project is small scale and new.
A better estimate can be obtained using installed costs of phtotovoltaic solar cells for power generation. The EIA and the Solar Energy Industries Association have statistics on this, and agree that although solar panels cost about $700 per kilowatt, installed costs are about $3000 to $4000 per kilowatt. [10,11] This is a factor of two to three higher than natural gas. [10]
Using even the lower estimate, this means that solarizing urban highways would cost at least $7 trillion. And this is a lower bound--this is the installed cost for solar panel modules that are freestanding for easy access, and don't have to support the wear and tear of road traffic. Making a more robust panel will only increase the installed cost per kilowatt, as the module will need much more structure and material to withstand operational stresses.
The benefits of Solar Roadway's system - melting snow and smart lighting - could be implemented much more cheaply by refitting current roads with LED lighting and heaters. Some will argue that the $7 trillion installation cost is unreasonable, as that is the cost to replace all roads right now; incremental replacement does not make sense either. Natural gas is still cheaper for installation and electricity production, which is why solar installation will not be a significant part of power generation for a long time, if ever. [12]
© Arul Suresh. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. Duffer, "Solar Roadways Light up the Future," Chicago Tribune, 31 May 14.
[2] "On the Not So Sunny Side of the Street," The Economist, 5 Jun 14.
[3] Highway Statistics 2005," (US Federal Highway Administration, 2007).
[4] "A Policy on Design Standards Interstate System," American Association of State Highway and Transportation Officials, January 2005.
[5] A Policy on Geometric Design of Highways and Streets, 4th Ed. (Am. Assn. State Hwy Trans. Officials, 2001).
[6] "Smith County Map," Kansas Department of Transportation, January 2011.
[7] M. A. Green et al. "Solar Cell Efficiency Tables (Version 39)," Prog. Photovoltaics 20, 12 (2012).
[8] "Monthly Energy Review," U.S. Energy Information Administration, DOE/EIA-0035(2014/11), November 2014.
[9] "Solar-Energy Roadway Test Begins in the Netherlands," New York Times, 12 Nov 14.
[10] "Updated Capital Cost Estimates for Utility Scale Electricity Generating Plants," U.S. Energy Information Administration, April 2013.
[11] "Solar Market Insight Report, 2013 Q3, Executive Summary," Solar Energy Industries Association, 2013.
[12] "Levelized Cost and Levelized Avoided Cost of New Generation Resources in the Annual Energy Outlook 2014," U.S. Energy Information Administration, April 2014.