News and analysis about energy in California with an eye toward renewables.

Solar Roadways: 4 Reasons They Might Not Work

The Brusaws with their prototype solar paving tiles | Photo: Solar Roadways

Over the last couple of weeks, ReWire's gotten a boatload of emails and social media forwards about the "Solar Freaking Roadways" Indiegogo campaign, in which inventors Julie and Scott Brusaw have raised well over $1.9 million to develop their modular solar-powered paving tiles. The Brusaws' goal is nothing less than replacing every bit of paved surface in the U.S. with the tiles, each of which will generate a maximum of 52 watts of electrical power when illuminated by the sun.

The Brusaws offer a vision of a nation that derives all its power from its roadways, which would include baked-in LEDs (to replace lane striping paint), heating elements to melt ice and snow, and integrated GPS. They claim that replacing our roadways with their panels will offer a source of solar energy enough to meet the United States' power needs three times over, with a roadway that's safer for pedestrians and animals.

As with any promise that sounds too good to be true, this one merits careful scrutiny -- especially since what the Brusaws are offering something a lot of us really want. Here are four reasons to take the Solar Roadways hype with a large amount of salt.

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For those of you who somehow have missed the hype-filled video that's made the rounds, we've embedded it here:

So that sounds great, right? What's to debunk?

1) The tiles could make roads far more dangerous.

The Brusaws claim to have designed a glass surface for their solar paving tiles that offers traction comparable to asphalt. In the words of their project's FAQ, " We... ended up with a texture that can stop a vehicle going 80 mph in the required distance."

That claim has not, apparently, been tested with an actual vehicle on an actual surface, but rather with a sophisticated pendulum testing rig in a laboratory, along with a ride over a prototype surface on a bicycle. That's fine: all products have to start somewhere. But laboratory conditions almost certainly don't include factors like the first rain of the season loosening a summer's worth of spilt motor oil on the roadway.

Even if the glass surface as designed offers sufficient traction, there's something to keep in mind: glass is soft. Run a few hundred thousand tires over the carefully designed nubbly surface of the tiles and that engineered-in traction is almost certainly going to be worn away to some extent.

Any initiative that offers the possibility of giving roadways less traction than asphalt will have a death toll. Due to government-mandated seat belts, air bags, child car seats, and other safety measures, we've managed to get the annual U.S. death toll from traffic accidents down from around 50,000 per year in the 1970s and 1980s to a still-abysmal 35,000 or so per year. And woe be to the municipality whose expensively-installed solar roadway hosts the first fatal accident due to skid.

Also worth considering: the tiles will need a waterproof caulking between them to make sure rainwater runs off the side of the road rather than straight down, where it will erode the roadbed. That's gonna cost.

2) The tiles as described are loaded with energy-consuming features.

New energy-generating innovations live or die according to their net output. The term of art is Energy Returned on Energy Invested (EROEI). At 55 watts per, these tiles will take some time to pay back the energy it takes to manufacture, transport, and install them.

Again, that's not necessarily a deal-breaker: if the tiles are durable, then it may be well worth our while to spend the energy to install them. The problem is that each of the tiles with its maximum theoretical gross output of 55 watts has hardware in it that will consume power. As displayed on the Solar Roadways site, for instance, each tile contains about 50 LEDs, which the Brusaws say will serve to replace paint striping, as well as offering programmable safety warnings and the like. Assuming those LEDs are around the brightness of commercially available tiny "fairy light" LEDs, that's about 50 watts of consumption if you lit them all.

Each tile would also have a microprocessor in it which would communicate wirelessly with neighboring tiles and with a local command station. Local wifi doesn't necessarily need all that much power, on the order of a tenth of a watt or less for low-data applications, but communicating with a more distant base station would take more. That base station, along with the computers that keep the LEDs lit to maintain crosswalks and lane markers and disability parking spaces marked off, would consume even more power. And though the Brusaws say the system would conserve power by only illuminating those LEDs when pressure-sensitive sensors in the tiles told the computer that there was traffic in the area, that kind of interactivity across a landscape the size of a modest town costs processor cycles, which costs energy.

And without a full cradle-to-grave accounting of all those unspoken energy costs, we have no way to measure whether solar roadways will actually be a net benefit.

Especially as they'll be consuming power to fulfill needs we now meet by using a bit of paint. And as the tiles don't necessarily have energy storage built in, that power at nighttime would come from the grid, likely meaning an increase in burning coal or natural gas to generate that power.

3) The tiles' heating elements will cause massive spikes in non-solar wintertime energy use.

This is another EROEI issue, but I separate it out because it's orders of magnitude larger than the LEDs and wifi. The Brusaws say their tiles can be built to include heating elements that will melt ice and snow in colder climates. They don't say what the wattage is for the panels' heating elements, other than to say that a prototype that consumed 72 watts warmed the tiles more than necessary.

Melting ice and snow takes a phenomenal amount of energy. The Brusaw's prototype tiles seem to be approximately a meter across. Let's assume that a 50-watt heating element turns out not to be overkill. The minimum lane width for Interstate, U.S., and State highways is 12 feet, which means that a mile of two-lane highway would have 116,160 square feet of lane surface needing to be de-iced.

If the tiles are a meter across and hexagonal, they cover .65 square meters or a hair under seven square feet each. That means that mile of two-lane would require 16,594 tiles to pave it.

If each of those tiles had a 50-watt heating element in it, running them all would take just under 830 kilowatts. That's per mile of two-lane. And because the Brusaws assume the energy for the heating elements would come from the grid, that means the solar roadways won't be pulling their energy weight to melt the ice, but instead will be adding greatly to power demand in the winter.

Here in California, we'd only be using heating elements in our roadways in some locations for some of the time in winter. But consider the places where it snows regularly each winter across an entire region. Tiny, snowy, and largely un-roaded Vermont has 29,641 lane-miles of highways. Pave them with solar tiles each with a 50-watt heating element, and that's 12.3 gigawatts of power the state has to come up with all at once whenever the state gets a blanket of snow. Given that the entire state now has a generating capacity of under 900 megawatts, the state would have to increase its power generating capacity almost 14-fold to keep the roads clear.

Which makes diesel snowplows start to look pretty green.

4) Putting solar generating capacity at the bottoms of our cities makes little sense.

Cost-effective photovoltaic solar generation is all about maximizing the amount of sunlight that hits each square meter of solar panel. While turning our roadways into de facto solar panels is a tempting thought, the fact is that urban roadways are often obscured by both intentional and unintentional artifacts. On many urban streets, a strip between five and six feet wide on either side of the street will largely be shaded by parked cars, which might cover three quarters of the available space in the lane. Streets are the end destination for debris from wind-blown leaves -- In Los Angeles, occasionally including ten-foot palm fronds -- and trash to discarded furniture and appliances. Even in the centers of the traffic lanes, streets collect grime and dust that would cut down remarkably on the amount of light that makes it through to the solar cells in the roadway.

That's ignoring the justifiable preference many communities have for casting shade on their roadways through planting and nurturing street trees.

Meanwhile, there are abundant hard surfaces a bit farther above the ground where you can count on less-impeded sunlight. They range from rooftops to patios to shade structures in parking lots, and even next-generation applications like the skins of south- and west-facing building walls.

There is something good to say about the Brusaws' project: They've helped to launch a conversation about how we can build solar generating capacity into our cities on a longer-term basis, making solar panels an intrinsic part of the landscape rather than a tacked-on afterthought. We're going to need to have that conversation, and soon.

And it may well be that a version of the Brusaws' paving tiles will have immediate and useful applications in settings like pedestrian plazas or residential driveways, where the units won't take as much punishment as they would from the fraying tires of 18-wheelers.

Whether those inadvertent applications will prove to be woth all the money people tossed into Indiegogo is hard to say.


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About the Author

Chris Clarke is a natural history writer and environmental journalist currently at work on a book about the Joshua tree. He lives in Joshua Tree.
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