Back to the Future is one of my favorite movie franchises ever. Right now I’m looking at a model DeLorean time machine, which lives on my desk to make me happy when I’m tired and cranky. Now the reason I bring this up is because the first movie has one of my favorite lines in it. Doc Brown tells Marty, “Roads? Where we’re going we don’t need roads!” Then the DeLorean takes off and flies away.
Unfortunately for my young self, 2015 has come and gone, but no one has flying cars as they did in those movies. So we very much need our roads. Some people think that since roads are here to stay, they could do even more than they do now. Since roads represent a massive surface area that bakes in the sun all day anyway, why not convert them into something that also generates power?
After all, companies like Tesla have created solar roof tiles that look just like normal tiles except they can power your home. Isn’t that the sort of innovation we need to save the planet?
More Complicated than They Look
Before we look at how people are proposing solar roads will work, we should look at the roads we have today. A road may look like a simple piece of technology, but there is a lot of engineering and knowledge that goes into making a road. Road engineers are specialized civil engineers who have to solve many different technical challenges.
A road has to be durable enough to last for many years with millions and millions of vehicles running across its surface. It has to be safe in terms of grip. The road must be exactly cambered and have a drainage system to get water off it quickly. That’s just a taste of the long list of criteria road designers have to deal with.
All of this has to be done within tight government budgets, and even then it’s an expensive business. So any new technology that’s going to make existing roads more complicated has some serious challenges facing it right off the bat. But what is actually being promised by solar road proponents?
The Solar Proposal
So how will a solar road be different? Now, there have been a few different design proposals, but the most promising designs use a road model that has a transparent surface. In other words, light goes through the road surface and on to a layer of solar panels. The actual road is made from modular sections and the materials are from recycled sources. As you can imagine, these roads could not make use of asphalt, since it’s not transparent. So solar roadways will use a glass surface. The only problem is that there isn’t a type of glass that has the sorts of properties needed to replace asphalt.
The prototype was built by the best known proponent of solar roadways, a company known as, er, Solar Roadways. Their demo unit is made up of hexagonal glass panels with a concrete base. They say this can handle 110,000 kilos of weight. Some pilot installation of this technology has been done, but not where actual cars drive. So the jury is still out on whether this design can work.
On top of the basic premise that these roads will act as giant solar panels, visions of future solar roads include putting LED lights under the road surface to act as displays; doing away with the need to paint anything on them. It also means the road itself can be illuminated from below, so traditional lamps aren’t needed. Some also say the roads could heat themselves in winter to clear away snow. I’m sure that by the time you read this, even more fantastic proposals will have been added to the solar road dream.
Real World Solar Roads
As you’ve probably guessed, these are some pretty ambitious ideas and the current reality of solar roads is somewhat more, er, pedestrian.
While the American solar road company fights for money and permission to install an actual road that cars will use, the French have gone ahead and actually done it. In 2016 they opened a 1 km stretch of solar roads to actual traffic. It will take a few years of gathering data to know if their surface will actually work as a road, but at least they are gathering real information.
Price is the Enemy
Even if solar roadways are feasible on a technological level, it won’t mean much if they are too expensive. Knowing whether they will be is not easy, though. Right now the cost of these experimental roads is astronomical. That one kilo stretch of road in France cost more than five million dollars. I’ve seen some estimates that it would take trillions of dollars to convert a significant number of US roads to solar.
The cost criticisms might be a little misguided, though. They are likely based on current estimations of cost, but no one really knows what solar roads will cost when manufacturing happens en masse. Still, one of the key points that solar road designers will have to solve is how to get the cost down.
The other key part of this puzzle is how much power you can actually get out of a solar road.
Thirty tiles of Solar Roadways product generates about 10W of power. That’s not a lot. Yes, if all roads were solar even that low amount would really add up, but the question remains – can the power we make from these roads be justified in terms of cost or environmental impact?
There are many things that stand in the way of getting more power out of these roads. The main one is that the surface isn’t very transparent. In order to provide grip and strength, the glass surface has to be in a form that will let only a little light through. Then you have to deal with relatively low levels of solar panel efficiency. Even if these roadways had both of those problems licked, there are many things that can cut off the light to the panels. Cars passing over them by the thousands every minute will make a serious dent in how much power can be made.
How about dirt, debris, and tire residue? Unless the roads are maintained at a much higher level than current roads, their performance will drop over the years to the point that they will hardly make any power at all. This suggests that whatever surface they do use for these roads has to be self-cleaning in some way.
The other problem that these roads face in terms of power generation is the angle of the panels. Solar panels are usually installed at an angle, whereas these roads are flat on the ground. That’s another efficiency hurdle, but perhaps the panels themselves can be set at the optimal angle under the glass surface itself.
Getting a Grip
The other problem is how these roads will perform as actual roads compared to what we have at the moment. Glass is not the best surface for driving on, at least not in its pure state. So they will have to mix it with other thing that are not glass. They will have to roughen the surface, but they can’t do it so much that not enough light is let through. That’s the main problem. You have a surface material that needs to do two very different jobs. It can do one or the other very well, or both poorly – at least as it stands now. There’s still some formidable materials science that has to happen if this is going to work. Today’s roads are grippy when it rains or when it snows. They have to be safe, and it’s hard to see how solar roads will measure up to existing roads when it comes to grip and road handling.
What happens when a bit of your solar road goes wrong? The Solar Roadways company has suggested modular sections of solar road. The fact remains that solar roads are much more complex than normal roads. Right now it looks like maintenance will inflate the already astronomical cost and require much more time and training to pull off.
One of the main reasons that current road technology is so widespread and popular is that it strikes a good balance between cost and performance. The art of road building is now venerable and well-understood. The idea of complex solar roads that need to be maintained by highly experienced technicians rather than working class roadworkers is incredibly daunting.
Why do We Want Solar Roads?
The rationale behind solar roads is perhaps the main reason we may never see them. Yes, from a technological perspective they are pretty cool, but there may be simpler solutions. It might not be all that elegant, but you could simply install solar panels next to the road. Since the power from the solar road is often touted as a way to power street lamps and other road-related equipment such as signage, you could do it much more efficiently with one or two panels every few miles.
The main appeal of a solar roadway is that we can take a necessary evil and turn it into something that’s good for us and the planet. I guess it’s extra impactful given that fossil fuels and fuel-burning cars are so tightly linked to climate change and pollution.
The problem is that in practice the idea of a solar roadway just doesn’t seem to work. Yes, it might be possible to actually build something that meets all the crazy expectations and functionality people dream of, but the cost and complexity is a killer. That’s even if all the materials science challenges can be overcome.
We should not be too pessimistic, however, since there’s already a great deal of progress on this front. The Chinese, for example, have come up with a type of transparent asphalt. The technology might even be practical for things like sidewalks or bicycle lanes on the sides of the road. Still, solar roadways are a long, long way from being practical. And no one knows how far away it is or if we’ll ever get there.
Alternative to Solar Roads
Given the cost of these roads, it makes more sense to simply build large solar farms and pipe the clean power to where it’s needed. There’s no lack of space to put proper, active-tracking solar farms just out of sight of the roadways where they aren’t exposed to traffic-based dangers. This would make much more power and be far more cost-effective.
With solar panels now reaching truly useful levels of efficiency, it seems like madness to obscure them under a road surface at an angle that makes them produce minimal power, not counting the constant shadows cast by vehicles. Don’t forget that you also need battery storage if you want to use that solar energy at night. You know, when we really want lights. Also, LED lights aren’t great at daylight visibility, which means the idea of putting them under the road surface to replace road marking is an issue too.
So while solar roads might be possible in principle, unless there are some serious breakthroughs I don’t think they’ll catch on, even if those breakthroughs happened. There are just simpler ways of achieving the same thing.