To a general public that is clueless about net energy issues; solar roadways appeared to be a good way to wean ourselves off fossil fuels. As the hype had it, there is abundant sunlight and more than enough road space on which to collect it. So it was that Solar Freakin’ Roadways managed to secure more than $2 million through its crowd funding campaign even though the project was obviously flawed.
The most obvious flaw in the Solar Freakin’ Roadways project was its attempt to do too many things at once. That is, in addition to simply generating solar electricity, the hi-tech hexagonal panels were simultaneously supposed to power LED road markings and generate sufficient hear to melt snow and ice during winter:
“As presently designed, the LEDs and heating elements are disconnected from the photovoltaics, meaning these elements require electricity from the grid 24/7. The heaters alone require more power than the available PV in the hexagon can supply.”
This raised the question of whether a less ambitious solar road project might prove more cost effective. Rather than attempting to illuminate and heat the road, why not simply limit our ambitions to generating electricity? This was what the Anglo-French Colas Wattway promised; and the PR lobbying was slick enough to persuade Ségolène Royal, France’s ecology minister to invest €5.2 million in the construction of a 1km stretch of solar roadway in Tourouvre au Perche in Normandy.
As I pointed out at the time, the French project looked like little more than corporate welfare that had been given a green veneer:
“There is one area where otherwise sensible people have a blind spot to corporate welfare. That area is, of course, so-called ‘green energy’. For the moment, anyone who promises green energy in exchange for a state handout is afforded uncritical public approval. Nowhere is this blind spot more easily exposed than in the recent French decision – against good engineering principles – to spend five million euros building its first kilometre of solar roadway – something that has produced ripples of ecstasy through the ranks of environmentalists who really ought to know better.”
Solar roadways are simply a poor solution to both electricity generation and to road surfacing. In cost terms, asphalt is far cheaper than glass panels. As Andrew Thomson noted in a Conversation article three years ago:
“Per square metre, bitumen/asphalt roads are cheaper (around $5/m²) than centimetre thick slabs of toughened glass (around $15-20/m²), and this will not change. Asphalt is a mixture of waste products from the refining of oil, and fine gravel or aggregate. Glass, on the other hand, is formed by melting silica, and is energy intensive to produce.
“Perhaps recycled glass could be used instead of refining new silica, but with a big performance hit. A centimetre of solar glass will transmit 91% of usable light; typical recycled glass will transmit only 35%…”
A glass surface also presents safety concerns due to both the danger of skidding and of panels becoming dislodged. You might just get away with this on a small country road like the one in Normandy, but you wouldn’t want to risk it on a fast motorway or A-road:
“Imagine driving on a smooth glass surface, in the rain.
“Road surfaces wear with use. As asphalt roads wear, layers of the fine gravel are removed, exposing further layers of rough gravel/aggregate, maintaining the surface consistency and grip.
“Conversely, glass is a malleable network of homogenous material. Hence over time it wears smooth. Because of these material properties, even if the glass road surface were given a textured pattern to provide traction, it would eventually be worn away.”
By far the biggest problem, however, concerns the performance of solar panels placed on a road. Look at any solar farm or rooftop array and the first thing you notice is that the panels are deployed at an angle to optimise the amount of sunlight landing on them. You will also notice that they are placed in such a way that they are not shaded by neighbouring panels or other nearby objects. Solar roadways wilfully do the opposite. They are laid flat on top of a road where they get less than 30 percent of the sunlight that lands on a southwest-facing rooftop array. And, of course, much of that 30 percent is lost because of the shading that occurs as vehicles drive over them. And over time a combination of damage, debris, dust and grease further decrease the amount of sunlight that can be converted into electricity.
As ever, the bright green cheerleaders objected that these issues would be resolved as the technology developed. In any case, many argued, at least they would be generating some renewable energy; surely something we should all celebrate. This argument, however, assumes that we have infinite resources with which to solve (or at least mitigate) our climate and resource depletion crises. We do not. We have a very limited time and a serious problem with declining net energy. What this means is that we cannot afford to waste energy (or money as a proxy for energy) on projects that simply do not deliver. We must concentrate our energy and resources on those energy projects that provide the best return on investment. And by this measure, solar roadways are a non-starter. As Dylan Ryan in a new Conversation article evaluating the results of the Normandy project notes:
“This has a maximum power output of 420 kW, covers 2,800 m² and cost €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 kW 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.”
Given that more than eighty percent of our (global) energy still comes from fossil fuels, we simply cannot afford to waste money and resources on faux technological “solutions” that deliver at best a tenth of the electricity output of an alternative renewable. Indeed, further north than Bordeaux, in Normandy or the UK, the £1,300,000 per MW cost of wind power means that at grid scale, windfarms will outperform even the best solar farms. Insofar as solar panels have a part to play in our energy mix, they will be on rooftops not on the road. Not least because, using UK Department of Transport data, Ryan points out that the amount of road space available to deploy solar panels (even if we were foolish enough to want to) is far less than had been assumed, and amounts to a small fraction of the (far more effective) urban roof space:
“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 solar panels 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.”
If the UK government suddenly became serious about replacing our fossil fuel electricity with a combination of wind and solar (keeping just nuclear for baseload) it can expect to pay around £110.5bn – the equivalent of the combined annual defence and education budget; or just £15bn short of the annual cost of the NHS. Indeed, to achieve this feat rapidly, costs are likely to rise as the declining technology costs are offset by rising costs of labour and materials due to rising demand. In and of itself, this is likely to be politically impossible given the growing populist backlash against a decade of austerity since the 2008 crash. In such circumstances, governments simply cannot afford to squander what resources they still have on projects like solar roadways (or tidal barrages) that deliver such a poor return on investment. Unfortunately, given the lack of understanding of net energy, we may only discover this when it is too late.
As you made it to the end…
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