Solar panels replaced tarmac on a motorway—here are the results

Solar panels replaced tarmac on a motorway -- here are the results
A road to nowhere? Credit: Robert B.D. Brice/Wattway

Four years ago a viral campaign wooed the world with a promise of fighting climate change and jump-starting the economy by replacing tarmac on the world's roads with solar panels. The bold idea has undergone some road testing since then. The first results from preliminary studies have recently come out, and they're a bit underwhelming.

A solar panel lying under a is at a number of disadvantages. As it's not at the optimum tilt angle, it's going to produce less power and it's going to be more prone to shading, which is a problem as shade over just 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.

Unable to benefit from air circulation, its inevitable these panels will heat up more than a rooftop solar panel too. For every 1°C over optimum temperature you lose 0.5% of energy efficiency.

As a result a significant drop in performance for a solar road, compared to rooftop solar panels, has to be expected. The question is by how much and what is the economic cost?

The road test results are in

One of the first solar roads to be installed is in Tourouvre-au-Perche, France. This has a maximum power output of 420 kWs, covers 2,800 m² and costed €5m to install. This implies a cost of €11,905 (£10,624) per installed kW.

While the road is supposed to generate 800 kilowatt hours per day (kWh/day), some recently released data indicates a yield closer to 409 kWh/day, or 150,000 kWh/yr. For an idea of how much this is, the average UK home uses around 10 kWh/day. The road's capacity factor – which measures the efficiency of the technology by dividing its average power output by its potential maximum power output – is just 4%.

In contrast, the Cestas solar plant near Bordeaux, which features rows of solar panels carefully angled towards the sun, has a maximum power output of 300,000 kWs and a capacity factor of 14%. And at a cost of €360m (£321m), or €1,200 (£1,070) per installed kW, one-tenth the cost of our solar roadway, it generates three times more power.

Solar panels replaced tarmac on a motorway -- here are the results
The driveway prototype which inspired Solar Roadways. Credit: Dan Walden/Wikimedia Commons, CC BY-SA

In America, a company called Solar Roadways has developed a smart highway with solar panels, including sensors and LED lights to display traffic warnings about any upcoming hazards, such as a deer. It also has heating pads to melt snow in winter.

Several of their SR3 panels have been installed in a small section of pavement in Sandypoint, Idaho. This is 13.9 m² in area, with an installed capacity of 1.529 KWs. The installation cost is given as $48,734 (about £37,482), which implies a cost per installed kW of €27,500 (£24,542), more than 20 times higher than the Cestas powerplant.

Solar Roadway's own estimates are 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 is consumed by the LEDs. This would reduce performance even further. The heating plates are also quoted as drawing 2.28 MW per lane mile, so running them for just six days would cancel out any net gain from the solar panels.

And this is before we look at the actual data from the Sandypoint installation, which generated 52.397 kWhs in 6 months, or 104.8 kWhs over a year. From this we can estimate a capacity factor of just 0.782%, which is 20 times less efficient than the Cestas power plant.

That said, it should be pointed out that this panel is in a town square. If there is one thing we can conclude, it's that a section of pavement surrounded by buildings in a snowy northern town is not the best place to locate a solar installation. However, perhaps there's a bigger point – solar roads on city streets are just not a great idea.

Running out of road

Roads don't actually represent as large an area as we assume. The UK department of transport gives a breakdown of the length of the UK's different road types.

Assuming we can clad these in solar panels, four lanes of every motorway, two lanes on the A & B roads and half a lane for C & U roads (a lot are single track roads and just won't be suitable) we come up with a surface area of 2 billion m².

Which sounds like a lot, until you realise that buildings in the UK's urban areas occupy an area of 17.6 billion m². So just covering a fraction of the UK's rooftops with would immediately yield more power than putting them on roads. That's quite apart from the benefits that a more elevated position would yield for greater power generation.

All of this suggests that only a small fraction of the road network would actually be suitable. And, given the relatively small size of the road network, solar roads could only ever become a niche source of and never the shortcut to our future energy supply.


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Road paved with solar panels powers French town

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Sep 21, 2018
All these problems would have been seen with just a small amount of study and foresight before this boondoggle. Another example of a knee-jerk reaction to climate change and thinking with the emotions (or politics) instead of using one's brain.

Sep 21, 2018
Solar has low ERoI, it consumes more energy than produces, worse yet when "batteries included", produces more ecological impacts than energy; energy from fossil fuels used to manufacture/mine/transport/install/maintain/repair/recycle the solar panels.
http://blogs-imag...gure.jpg
https://pbs.twimg...QhYP.jpg

Sep 21, 2018
'The installation cost is given as $48,734 '
A boondoggle is spending $109 million on golfing trips a year. This is a relatively cheap experiment that gave useful data.

Sep 21, 2018
The astonishing stupidity of the AGW Cult, driven by dogma, founded on pathological LIES.
These are the jackasses, that the ignorant Chicken Littles believe will save the world.

Sep 21, 2018
Gosh. Are you chickenshit deniers still mad? That no residential community volunteered for you nukie junkies experiment to spread radioactive salts on iced roads?

Sep 21, 2018
"until you realise that buildings in the UK's urban areas occupy an area of 17.6 billion m²"

-This is gross, not net. Includes curbs, rooftop equipt, antennae, chimneys, circulation, etc. A more realistic figure might be 60%.
Gosh. Are you chickenshit deniers still mad? That no residential community volunteered for you nukie junkies experiment to spread radioactive salts on iced roads?
Another empty post from an empty head

Sep 21, 2018
The idea is moronic. The promotion is moronic. The production is moronic. The support is moronic.

Too many adults playing childish video games, who - as a result - are incapable of coherent, rational, adult thoughts.

In the failed American state this has resulted in the election of a low grade moron to the office of the presidency.


Sep 22, 2018
it's not a boondoggle - it's a process, an experiment. In some remote areas having smart road signs or emergency communication is very valuable. How much energy is required vs storage vs cost vs lifespan has to be evaluated, then compared later as technology evolves

Sep 22, 2018
First do the most efficient and least expensive total envelope cost areas....the low hanging fruit. In solar productioin, there is quite a lot of this. The Sahara Desert, for example. Then and only then do the more difficult. IT will be many decades before we as a society have to do this. Long before than we will have other energy, chirfly fusion.

Sep 22, 2018
First do the most efficient and least expensive total envelope cost areas....the low hanging fruit. In solar productioin, there is quite a lot of this. The Sahara Desert, for example. Then and only then do the more difficult. IT will be many decades before we as a society have to do this. Long before than we will have other energy, chirfly fusion.

Sep 23, 2018
Love me a solar roads article. It's amazing to see how some people are consumed by pure rage and yet others vehemently support the idea. Not too many topics are capable of such a feat.

Sep 23, 2018
They're doing it wrong, the most expensive approach rather than turning to some other methods of making the solar cells. For instance: https://phys.org/...lls.html as just one example and on this site. Try searching on "solar cell paint" on DDG, google, whatever.

Sep 23, 2018
If these projects were funded by private foundations or corporations then there is no reason to complain. BUT.....when an obviously unfeasible project is funded by taxpayer dollars and enriches the politically connected we should all complain. Spend the money on meaningful research instead like battery development or health studies. When these projects were first announced it was obvious to any critical thinker that they were boondoggles.

Sep 23, 2018
"and costed €5m to install."

My little brother used to say things like that, as a young child.

Sep 23, 2018
Love me a solar roads article. It's amazing to see how some people are consumed by pure rage and yet others vehemently support the idea. Not too many topics are capable of such a feat.


Don't get out much, do you? Read anything about black holes on this site lately?

Sep 24, 2018
Hey, whaddya know, Thunderfoot was right...

Sep 24, 2018
I like the idea, but it's not practical, or cost efficient, so it's a definite lead balloon with a few holes in it.
By the way, the test section of "smart roadway" that was installed in a sidewalk has NOT fared well. Mind you that's only with pedestrian traffic. Could you imagine how torn up it would be if actual vehicles were driving over it? Should I even mention vehicles with tire chains or studded tires in winter months?
Perhaps some day in the future they'll be feasible if cost is sufficiently decreased, efficiency increase, and durability greatly increased. But don't bet on it being any time soon.

Sep 24, 2018
Needs more alternative comparisons to consider the data, but yeah, looks very disappointing. OTOH, is the cost the difference between repaving the road vs. putting in the solar panels, or is it just the cost of the particular projects where they installed the panels.
In other words, are these cost numbers gross or net cost? Roads are always getting repaved and it's expensive. The wear-and-tear factor isn't covered here. We might find out that glass roadways aren't such a bad idea, regardless of the solar panels.
No, I don't have a lot of faith in the idea, but as long as the experiment is going, collect the data.
For comparison, there are some freeways in the U.S. that could put solar collectors in the right-of-way areas without the cost of land acquisition. Not to mention commuter corridors that need more trains in the medians.

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