Solar roadways were once thought to be the holy grail of the world's energy crisis, but they didn't quite answer the purpose.
The panels were allegedly also going to light up the roads with different LED patterns, replacing painted lines. For the winter, heating coils could melt snow and ice – all while generating electricity and requiring less maintenance.
The promise seemed too good to be true. And as it turns out, it was.
After years of development and millions of dollars (including US government funding), none of the solar roadways installed today are cost-effective or efficient energy producers. The roads are expensive and produce far less electricity than what could be produced if the money was used on a solar farm – or by simply placing the panels by the side of the road.
As it turns out, there were many, many obstacles when it comes to turning roads into giant solar panels.
An engineering failure
With the global road network spanning between 21 and 32 million kilometres, it seems reasonable that covering just a portion of them with solar panels could generate a substantial amount of electricity. Placing solar panels on a mere fraction of roadways, in theory, should generate energy enough to power the entire world. However, this is only achievable under the best of conditions – when the solar panels are in direct sunlight.
The largest incentive of the project was its ability to pay for itself and give a good return on investment (ROI). Though as of yet, no solar roadway has sustained a positive cash flow. Instead, the projects have caused a slew of problems and required extensive maintenance. Although the technology has been in development for years, the idea is not yet viable.
Not enough light
On a traditional solar farm, solar panels are angled towards the sun to maximise efficiency. On more advanced farms, the panels are made to track the sun, further improving the amount of energy they can extract. A typical solar panel can utilise about 20% of the energy that the sun produces. Optimising the amount of light the panels are exposed to is imperative to run a cost-efficient system.
On the other hand, solar roadways that lay flat minimise the panels' exposure to direct sunlight, when the panels are most efficient. Laying a solar panel flat will result in 60% powerless power, in comparison to direct sunlight. The already small amount of energy that is available is further restricted by the environment around it.
Flat panels are also going to be more prone to shading, even when they are not being driven over. Shade over a mere 5% of the surface of a panel can reduce power generation by 50%. The panels are also likely to be covered by dirt and dust and would need far thicker glass than conventional panels to withstand the weight of traffic, which will further limit the light they absorb.
On top of this, panels that are fixed in place are unable to benefit from air circulation and will likely heat up more than rooftop solar panels, making them less efficient. For every 1 degree Celsius over optimum temperature, solar panel lose about 0.5% of energy efficiency.
In the best of conditions, the panels are at a disadvantage, without the consideration of the constant wear and tear that they will be exposed to for their entire lifespan.
LEDs are difficult to see during the day
One of the features Solar Roadways® promised was the inclusion of LEDs that would replace the necessity to paint lines on roads. However, a critical balance must be met by making the lights visible, yet not drawing too much power.
With current LEDs, the power consumption is still too high and the lights burn out too fast to make the lighting economically viable. LEDs used in traffic lights use shielding to block out direct sunlight and make the lights appear brighter.
On the solar roadway, the lights would be difficult to shield, making them much more. difficult to see during the day. At night, they would be easily visible, but this also causes a problem: with no power being produced at night, the lights would be drawing electricity directly out of the grid.
The Solar Roadways® team installed a small, 13.9 m² section of solar road in Sandpoint, Idaho, in 2017, but the results were rather unimpressive. Unfortunately, the small section of solar panels broke almost immediately, and then caught fire sometime later.
Also, the lights could hardly be seen, at even direct angles to the road. The panels did receive an upgrade, but the lights were still incredibly difficult to see without being at a high angle, the opposite of the angles drivers would view them at.
The properties of the glass itself further restrict the road's ability to produce electricity. Dirt and leaves will accumulate on the surface and will act as an abrasive material that will scratch and wear the road quickly, and the glass fragments may cause additional wear and tear to the cars that use the road.
Advanced polymers were considered for use in constructing the roads, to protect the panels. However, most polymers are expensive to manufacture in sufficient quantities to build a road capable of withstanding the constant force of traffic. The material is also typically made from fossil fuels, defeating the purpose of using solar panels to reduce the carbon footprint.
As the panels wear out, the glass material will become opaque. The clarity of the glass would significantly degrade the panels' ability to collect light. The challenge is mounting. The cost of implementing the system could never exceed the efficiency and practicality of simply installing the solar panels in far more efficient arrangements.
The idea of replacing asphalt with a glass panel is even more absurd once the cost is considered. Currently, there is no coating available that can withstand the force of moving vehicles yet produce electricity at the same time.
It is absurdly expensive
Solar Roadways® claims that covering the southern 48 US state roads with solar roads (about six billion square metres) would produce three times more electricity than the annual power consumption of the United States. However, this not only assumes the roads would work as advertised, but also does not factor in the cost of such an astronomical project.
For the demonstration project in Idaho, the panels had an installed capacity of 1.529 KW with an installation cost of $48,734, which implies a cost per installed kW of about $33,000, or 20 times higher than for a solar power plant.
Solar Roadways® estimates that the LED lights would consume 106 MWh per lane mile, with the panels generating 415 MWh – so more than 25% of the useful power would be consumed by the LEDs. The heating plates would draw 2.28 MW per lane mile, which means that running them for a mere six days would cancel out any net gain from the solar panels.
The idea is fun to imagine. However, with the materials available today, it is not quite feasible. Significantly more research is required to develop a viable solution. Although, it may be a better idea to tailor the placement of the panels and put them in places where they can be in direct sunlight.
In fact, buildings cover a lot more space than roads. So, covering just a fraction of existing rooftops with solar panels would immediately yield more power than putting them on roads, and the technology already works.
It would also be significantly more efficient to run the solar panels alongside the road where they are not subject to harsh conditions, are easier to maintain, and are much more economical. Furthermore, the panels could be angled or made to track the sun, maximising the power that is available to them.
What progress has been made
So far, a few solar projects have been installed around the world. Some of them work better than others, but as a whole, they do not generate much electricity – far less than what could be expected if the money that funded the project was used on traditional solar farms.
In December 2016, France unveiled a one-kilometre solar road made out of about 2,880 m2 (31,000 ft2) of photovoltaic panels. Built using Colas' Wattway technology, it was the longest solar road in the world. The road, which cost about $5.2 million, was meant to generate enough electricity to power the streetlights in a nearby town. However, that never happened.
By 2018, the road was already deteriorating, and 90 metres of it had to be demolished. Rotting leaves sitting on the road, cracks in the panels and a lack of sun in the region made it far less efficient than anticipated – at its peak it produced a mere 80,000 kWh a year (much less than the expected 150,000 kWh).
Further, the road was noisy to drive on and was frequented by tractors, which increased the wear and tear on the road. This was so bad that officials had to change the maximum speed limit to 70km/h. In 2019, WattWay admitted that it was the end of the line for its road and the project would not be moving ahead.
After the installation in France, a test stretch of roadway was installed near the Alabama/Georgia border in the United States. In December 2020, Peachtree, Georgia officially unveiled the United States' first solar roadway.
The installation is meant to produce more than 1,300 kWh of energy annually that will be used to charge local electric vehicles. Because the roadway is fairly new, there isn't much information available regarding its durability.
However, because it was made using the same technology as the French road, one could assume that it will likely begin to deteriorate within a few years.
Another promising installation was the Jinan solar highway in China. Engineers claimed the 1km test road would be able to produce an impressive gigawatt of energy a year, and power as many as 800 homes.
The road itself consisted of three distinct layers developed by the Qilu Transportation Development Group. The first layer was insulation, then the solar panel, and finally a transparent concrete top layer.
Yet, just five days after the road opened in 2017, one six-foot panel went missing, and the surrounding panels were damaged – allegedly by a professional team of thieves.
Improving science
Technological advances are being made all the time. Modern humanity thrives on innovation. Though there are many great ideas, solar roadways designed for cars are probably not among them.
The design is far too expensive, unreliable, and does not work. With the technology available today it is not feasible to design such a project. Instead of using the time and money to develop impractical science projects, real advances could be made like funding functional solar farms that are proven to work.
Perhaps in the future, there will be a material that can withstand the stresses of traffic and can produce electricity. That time, however, is not now. The idea is incredibly cool, but unfortunately, it is also entirely impractical.
Also, a big thank you goes out to those who are willing to constructively criticise extravagant claims. Without criticism, science cannot progress. It is great to think outside the box, however, it is also important to invest time into more practical solutions. Albeit a great idea, it is an idea that just cannot work –not yet that is.
For more information on the engineering blunder that is solar roadways, here are some fantastic videos to check out.
(Main image: Solar road panels Florian Pépellin/Wikimedia Commons)
This article was written by Sarah Marquart and first appeared in Interesting Engineering.