Solar Roads: Emerging Tech or Looming Disaster?

Solar Roadways has been in the press recently about its first deployment of panels in a pedestrian area as a proof of concept. Proponents promise abundant solar power and better infrastructure. Skeptics, however, insist that these are expensive projects that ignore basic tenets of engineering. Are solar roads a disaster in development?

Alternative energy sources have seen a decrease in price over the last few decades, with arguably the most promising power source of the future being solar energy. Back in 1954, Bell Labs produced the world’s first practical solar cell with a 6% efficiency which started the race for solar energy. Now, solar cells can be as efficient as 30%, as demonstrated by engineers at the University of New South Wales, and manufacturing techniques have made them considerably cheaper than in years past.
The prospect of using solar energy as the energy source of the future has been taken seriously by many engineers and inventors alike. Tech companies such as Tesla have invested in inventions like solar cell roof slates and lithium-ion batteries to store power and release it upon demand. But several companies and thousands of everyday people have been supporting a new use of solar: solar roads.

The Roads of Tomorrow

A solar road is a concept where solar panels are embedded into materials used to either replace or rest atop of pathways and roads. There are several major solar road projects presently being developed and/or tested, all sharing the same core principle of roads that collect solar energy.
The first solar road to be implemented, "SolaRoad", was a solar bicycle path installed in Krommenie, the Netherlands, in 2014. The path is 70-meter long (just under 230 feet). It cost over $3 million to install. Over the past year, this installation has yielded 9800 kWh of energy (Dutch language link). It's been considered a success and may be expanded in the coming months.
Another solar bike path was unveiled in Pruszków, Poland in October of 2016 with a hook of glowing blue in the night.

Image from TPA Sp. z o.o.

One of the only solar roads meant to withstand vehicle traffic, however, is in a small village called Tourouvre-au-Perche in Normandy, France. The road is one lane, 1km (0.6 miles) long, of solar road that opened for traffic in December of 2016. The model is called the Wattway and the current installation will remain in place for two years to test durability and efficiency. The cost of the installation is estimated to be over $5 million.

But the solar roadway project that we'll focus on in this article is in the USA. While other solar roads face similar challenges and criticisms, Solar Roadways are unique in their design, materials, and ambition.

Solar Roadways: Sandpoint, Idaho, USA

Solar Roadways (founded in 2006) has a vision where all roads, pavements, and parking lots are replaced with smart solar units that contain a multitude of features to cope with traffic-related problems and electrical generation. Simply put, imagine a modular solar system where each module contains a solar panel, battery storage, microcontroller, heaters, and LEDs. Then, replace the asphalt on roads with such modules and, by doing so, create road systems that are intelligent and positive for the environment.

A mockup of the Solar Roadways concept. Image from Solar Roadways

Some of the technical data given by Solar Roadways is given below:

• 230-watt solar cells with 18.5% efficiency
• Energy output for solar roadway roads up to 15.847 billion kWh (assuming all roads fitted)
• Cuts greenhouse gas by 75% if installed

You can see the calculations used to reach that last bullet point on the "Numbers" page on the Solar Roadways website.
Roads that can change layouts instantly, respond to traffic, alert users of people crossing roads, automatically defrost, and all at the same time providing electrical power sound incredible.
But if it sounds too good to be true, it probably is. Some of the criticisms that follow are inherent in the concept of solar roads while others are unique to the ambitions of Solar Roadways.

Criticism of Solar Roadways

Solar Roadways promise a lot but their feasibility is dismal from an engineering perspective. It is at this point that some readers will flock to the comment section and start defending Solar Roadways as the future roadway system. Some may even make comments such as “people thought x was stupid or y was unfeasible but we made it happen”. However, true science relies on hard evidence and rational thinking. So before coming to any conclusions, please consider the following issues.

Problem 1 – Angle of Solar Panels

The first problem with Solar Roadways comes from the inbuilt solar panels, themselves. Since road surfaces need to be flat, the panels need to be as low-profile as possible. This means that the solar cells inside the panels are laid flat.
This isn't ideal because solar cells generate the most electricity when directly pointed towards the source of light (in our case, the sun). While the sun does move across the sky as the day progresses, every single place on the planet has an optimum solar panel inclination and orientation. For example, my hometown (Ringwood) has angles of maximum efficiency at 16 degrees during the winter and 62 degrees during the summer. By comparison, the angles for Houston, Texas, are 36 degrees during the winter and 84 degrees during the summer. More advanced solar systems use tracking mechanism to keep the solar cell constantly pointed towards the sun. You can investigate the optimal angles for solar in your area here.
Solar Roadways, however, will always be fixed pointing directly upward (or, to be more specific, they will be at the same incline as the road). Keep in mind that this is before the system has even consumed power.

Problem 2 – LED Visibility

The mechanism for creating road markings and information displays is to use LEDs as they are commonly associated with high-efficiency light sources. However, using LEDs in such an environment comes with many problems.
The most important issue with LEDs in this scenario is their tight viewing angle (typically ranging between 10 degrees to 60 degrees), which makes them difficult to see at extreme angles. For example, an LED pointing vertically upward is rather easy to see from above (as shown in the Solar Roadways adverts), but seeing that same LED from an angle commonly involved with driving will be next to impossible for common LEDs.

Image from Solar Roadways

There are LEDs that have much wider angles (typically by incorporating a lens). However, these are not only significantly more expensive but most of the light emitted is still beamed perpendicular to the ground. One solution would be to mount LEDs at an angle to point towards drivers but such a solution still has problems. For one, multiple LEDs would have to be used per “pixel” all pointing in different angles to guarantee that the information can be seen from all angles. The second problem is the significant increase in energy consumption. This viewing angle problem is likely why all footage of solar roadways is taken in either dark conditions or from a high angle.
The second problem with LEDs the amount of power needed to make them visible during the day. LED billboards know this struggle well: a single LED billboard can utilize hundreds of thousands of kWh of energy every year just to stay bright enough to stay visible.
See the Solar Roadways LEDs in action in the live stream of the Solar Roadways installation at Jeff Jones Town Square in Sandpoint. Visibility demonstrably changes depending on the time of day.

Problem 3 – Solar Panel Cost

According to the US Department of Transportation, the US has over 2.7 million miles of paved roads as of 2013. When purchasing solar panels in bulk, the price of solar panels drops to around $0.74 per watt which is $740 per kW (this is the price of solar panels that are used in residential power and not those found in calculators or low-power applications).
Using a 20% efficient solar panel puts the energy generated by a single panel at 200 watts per square meter (assuming that the earth’s surface receives 1000 W/m²) and therefore the cost of a square meter panel is approximately $148 for a meter square. With this in mind and with over 2.7 million miles of roadways to pave, the cost of the solar panels alone blasts up into the trillions. This doesn't take into account the energy losses and consumption by the modules, themselves, nor installation costs.

Problem 4 – Heating

Solar Roadways boasts how these roads will prevent the build-up of ice and snow which will massively benefit cold climates. However, as calculated by thunderf00t, the energy needed to melt ice is considerably more than the designers may have realized.

Image from Solar Roadways

To turn 1kg of snow into 1kg of water requires 334kJ of energy which comes to 92.77 Watt hours. Let's say that there's 13cm of snow on a 1-meter panel. That would be around 0.013 cubic meters of snow. Assuming that freshly laid snow has a density of 100kg per meter cubed (10% water content), then the total weight in kg of snow on a 1-meter square panel is approximately 1.3kg.
Therefore, the minimum amount of energy to melt the snow (keep in mind that this is from 0°C ice to 0°C water) is 434.4kj or 120.6-watt hours. With solar panels that obtain 200 watts per square meter in the best of conditions (which winter is not), the spare energy is around 80 watts. This does not consider the power consumption by the LEDs, power converts, or any circuitry.
If snow falls at a temperature lower than 0 degrees then the energy increases. If the snow was -10°C and it has to be raised to -1°C (just before it melts), then the total energy needed is 18.81 J per gram which would correspond to 7 watts of power (which takes you from 80 watts to 73 watts of free power left).

Problem 5 – Maintenance Costs

One argument for Solar Roadways is the sheer number of engineering jobs that it would create. This particular point is spot on because maintaining such a system would be incredibly difficult, if not impossible. Assuming that a panel has a lifespan of a typical solar cell (which is very generous to assume, due to the unforgiving environment they are in) of 20 years (the time until 80% power output is reached), then the entire road system would need repair every 20 years which puts the average cost of the project at $1.15 trillion every year—and that's only calculating for the replaced solar cells, excluding workforce costs.
But this assumption relies on all the panels working til their end of life. What about individual panels that get broken? Considering the number of panels needed to be implemented (1.57×10¹¹ where each panel is 1-meter square), a failure rate of one in a million still results in 1,570,000 panels needing to be replaced within that failure rate time.

Problem 6 – Wear and Tear

Roads are made of asphalt for a reason. It is a cheap, widely abundant material made of oil industry by-products and pieces of stone. The surface of Solar Roadways is to be made of glass which, in itself, contains many problems not yet addressed, including the danger of glass dust if damaged tiles are crushed.
How long a glass surface would last has yet to be determined but heavy traffic would quickly bring the surface of the glass to bear. Even if this is not a problem for the road, itself, it is a very big problem for the solar panels under the glass. For the panels to work at their maximum efficiency, the glass must be as optically transparent as possible to allow light to fall onto the panels. However, scratched glass has a horrid tendency to diffuse light and send it in random directions which reduces its optical transparency and hence impede the solar panel's ability to generate electricity.
But this optical transparency has a more devastating potential that has not been considered.

Problem 7 – Glare Safety

Safety is perhaps the most important concern and the danger lies in the glass. If the glass is toughened and has a cloudy look to it then the electrical output of the panels will be reduced. So, to get around this, the panels use clear glass to allow as much light through as possible. However, this leads to a very dangerous situation that occurs when the sun is low in the sky.
Light from the sun (when it is low in the sky) will hit the panel at a low angle, reflecting light like a mirror or a puddle on the road. This means that entire roads will reflect morning and evening light up into drivers' eyes and potentially cause accidents.

Image from Solar Roadways

Solar Roadways Now

Despite these shortcomings, the Solar Roadways project continues to receive government and Indiegogo funding. The system has been demonstrated in several places including a driveway and the first public installation of Solar Roadways found in Sandpoint, Idaho. However, the Solar Roadways system began to show signs of failure days into its operation with most panels non-functioning within two weeks of installation.

More importantly, the reveal of the installation was rushed and flawed, meaning that the panels were able to "provide a light show" according to Scott Brusaw. "The solar cells and the heating elements are unusable in their current state.”
A website was supposed to be created to show live energy production from the system but the website address does not exist yet which leaves the question: “Does the system really generate electricity?”

The lack of a road test and real energy production numbers leaves an impression that Solar Roadways is more of a fun project as opposed to a problem solver. This does not mean that new ideas should be snuffed out or laughed at. What it means is that new ideas should have facts and figures in hand before they are pursued and funded with abandon.

If the $2 million raised on Indiegogo was used to purchase high-efficiency solar panels, then the energy generated from such a farm would be approximately 1.48 million watts, which could potentially provide power for up to 160 homes until the end of life of those panels. In fact, that money (along with the $750,000 grant from the government) could have been used to build a solar farm.

Summary

Unfortunately, when new and exciting ideas—such as self-filling water bottles and hyperloop transport—captivate the public imagination, logistics seem to fall by the wayside.
After considering the costs alone, solar roads are currently impractical. Instead of spending trillions on roads that may or may not work, proven solar farms could be built at a fraction of the cost and provide more (and real) power.


Regardless, research, funding, and support for these solar roads is likely to continue. Whether the movement finds a niche in parks and bike paths or expands into functional and permanent roadways will be partly determined by how much public interest it gets. But make no mistake—whether such projects succeed in their goal of efficiently harvesting solar energy is a question of engineering. At present, the odds don't look good.