Why The Solar Roadway Is A Terrible Idea

Against all odds, the crowd-funded solar road project was a remarkable money-acquiring success. It's clear there are some pretty significant technical hurdles, so we reached out to electrical engineer David Forbes to help us put it all in perspective.

David evaluated the Solar Roadways plan and gave us this thorough evaluation. Enjoy!

A company called Solar Roadways has been making the news lately. They are working on putting solar power generation into our streets, highways parking lots and sidewalks.

An IndieGogo campaign is wrapping up for bringing this concept to production.


Embedded electronics

Solar power is good. I use it all the time. My house has 4kW of solar panels, and my Loud Bike has a 100W solar panel to charge the batteries in its 300W sound system. Heck, when I built a pirate FM radio station 15 years ago, its remote transmitter was solar powered.But I can spot a bad idea when I see one. The Solar Roadways campaign is so flawed that I don't know where to start.

Cost
I work at the University of Arizona in the astronomy department. A few years ago, I talked for a couple hours with Roger Angel, a world-renowned astronomer. It is he who had the bright idea to spin telescope mirrors as they were cast, allowing huge mirrors withdeeply dished parabolic surfaces to be made easily. This trick revolutionized the design of large telescopes.

He has recently turned his attention to solar power, and is busy working on a solar collector system with the goal of reducing the installed cost to under a dollar a watt. I don't know if he'll achieve his goal, but it is a good way to approach the problem of solar power.

Solar Roadways seem to take the problem of generating solar power, and put it into conditions that maximize cost.

First, some background: Roads are built with asphalt, which is made of the leftovers from gasoline refining, mixed with a bunch of dirt and pebbles. They cost a lot to make, but the raw materials are not the cost driver. Labor and equipment is. An asphalt road is made with big machines that can form a 12-foot-wide lane in one continuous motion at a walking pace. No one touches the road; they just walk next to the machines and the road appears.

The Solar Roadway plan is to replace the repaving of a road with some sort of processing that will leave it in a condition to accept hexagonal solar panels and their associated wiring and networking needs. The current state of the design appears to depend on the placement of a one-foot-square grid of large bolts in the roadway. I've installed bolts into concrete, and it takes me several minutes to mark the hole, drill it, clean it out and pound the anchor into place. A mile of 30 foot wide roadway would have 150,000 bolts to install. Whee!

Then there may be some work required to make the panels talk to each other. There is mention of cutting a raceway for cables. Given the number of panels, it's likely to need a grid of raceways to work. Let's guess one cross-ways slot per row of panels, and one lengthwise raceway to collect the power. Better make that two for redundancy. That's a lot more cutting than the typical concrete highway receives for rain grooves and so forth.

The power also has to get processed. It may be possible to put some power conditioning in the panels, but that would likely double the panel cost, which is not such a good idea. No, a box by the side of the road every couple hundred feet is more likely to be the answer. I'm sure the Solar Roadways folks want the box to be under the road, but that would get in the way of cars when the box needs attention from a repair crew.


The idea of having LED signage built into the panels is intriguing. Do you know the only electronic thing that's more expensive per square foot than solar cells? Yup, you guessed it - LED signage. A few years ago, I built an LED-covered video coat. It cost $20,000 and it was only big enough to cover my body. Multiply that by a billion.

I work on all sorts of electronics, mostly little tiny computers that control physical stuff. As part of my day job working on radio telescopes, I have to deal with broken hardware regularly. fortunately, I don't have to set up barricades to keep fast-moving traffic from killing me while working on these little computers. Sometimes, I have to get way up high and visit a malfunctioning gizmo. One of these was a loose 9-volt battery clip in a rooftop weather vane that caused years of erratic weather reports.

As a result of this need to work on computers in hard-to-get-to places, I've grown fond of redesigning systems to be less complex. I regularly replace Linux-based control boxes with 8-bit microcontrollers running a few hundred lines of assembly language program, because I can understand everything that the box is doing, so it's much less likely to fail. Does your phone's computer ever crash? I thought so. Imagine a million phone computers embedded *in the pavement*, talking to each other as you drive over them. Think they'd ever crash? Yup. Who's going to push the reset button?

Displays

A big thing is made of having LEDs embedded in the roadway to provide changeable signage. This is a wonderful idea. Until a teenage hacker gets into the system and makes opposing traffic lanes merge into each other for kicks.


Durability

I'm not a mechanical engineer, but I sometimes have to play one at my job, working on huge radio telescopes. I'm familiar with statics, compression, tension, strain, Young's modulus, etc. These are the words of people who analyze whether something will break or not.

One thing about solar panels is that they are delicate. The proposed roadway protects the solar panels by placing them under a 1/2" thick glass plate. The typical solar panel that is installed on a rooftop has a 1/8" thick sheet of tempered glass. The rooftop panel is rated to about 150 pounds per square foot, or 1 PSI. Car tires are 35 PSI, and truck tires are filled to over 100 PSI. That's two orders of magnitude higher pressure. It must be passed through the solar cells to the roadbed in order for the glass to survive. Unfortunately, the scant information on the IndieGogo page gives no clue as to what method is used to transmit this force safely through the solar cells to the roadbed below.

The bumps on the prototype Solar Roadway glass hexagons are about an inch apart. A car driving at highway speeds moves about 100 feet per second, so the bumps would make a very well-defined tone of about 1 kHz. That's a medium-pitched whine. Get used to it, because you'll hear it ALL THE TIME.

I once had the joy of parking on the side of Highway 101 in central California to wait for the overheated engine in my 1953 Buick to cool off. While there, I was mesmerized by the condition of the roadway. This stretch of road was made of 12-foot-square concrete slabs, presumably 10 to 12 inches thick. Each square had broken off at the trailing edge, and been patched with concrete filler. I'm sure that a piece of 1/2" thick glass will handle the pounding of truck tires better than a measly foot-thick concrete slab.

Conclusion

I may have missed a few points. Read this.

Those solar-panel-covered shade structures that are popping up in church parking lots all over Tucson are looking smarter by the minute. The solar panels are mass-produced in China for a couple dollars a watt, and the structures are simple cantilevered steel I-beam ramadas. No fancy computers are needed, no worries about damage from tires, no hacking into can happen, and they are not blocked by pedestrians, cars, trees or buses.

Save your $5 for a good cause.

David Forbes is an electrical engineer who does interesting things for a living.Feel free to buy his Nixie tube wristwatch!