December 27, 2016, 1:32 pm

I have written about the horribly stupid but oddly appealing idea of solar roads many times before, most recently here. As a quick review, here are a few of the reasons the idea is so awful:

Even if they can be made to sort of work, the cost per KwH has to be higher than for solar panels in a more traditional installations -- the panels are more expensive because they have to be hardened for traffic, and their production will be lower due to dirt and shade and the fact that they can't be angled to the optimal pitch to catch the most sun. Plus, because the whole road has to be blocked (creating traffic snafus) just to fix one panel, it is far more likely that dead panels will just be left in place rather than replaced.

But the environmentalists are at it again, seem hell-bent on building solar roads with your tax money; (hat tip to a reader, who knew these solar road stories are like crack for me)

France has opened what it claims to be the world’s first solar panel road, in a Normandy village. A 1km (0.6-mile) route in the small village of Tourouvre-au-Perche covered with 2,800 sq m of electricity-generating panels, was inaugurated on Thursday by the ecology minister, Ségolène Royal. It cost €5m (£4.2m) to construct and will be used by about 2,000 motorists a day during a two-year test period to establish if it can generate enough energy to power street lighting in the village of 3,400 residents.

The choice of Normandy for the first solar road is an odd one, given that:

Normandy is not known for its surfeit of sunshine: Caen, the region’s political capital, enjoys just 44 days of strong sunshine a year

Wow, nothing like a 12% utilization to really bump up those returns on investment.

The article follows the first rule of environmental writing, which is to give the investment required or the value of the benefits, but never both (so the return on investment can't be calculated). This article follows this rule, by giving the investment but stating the benefits in a way that is impossible for the average person to put a value on, e.g. "enough energy to power street lighting in the village of 3,400 residents". Since we have no idea how well-lighted their streets are or how efficient the lighting is, this is meaningless. And by the way, they forgot to discuss any discussion of batteries and their cost if they really are going to run night-time lighting with solar.

But, the article does actually give something close to the numbers one would like to have to evaluate another similar investment, and oh boy are the numbers awful:

In 2014, a solar-powered cycle path opened in Krommenie in the Netherlands and, despite teething problems, has generated 3,000kWh of energy – enough to power an average family home for a year. The cost of building the cycle path, however, could have paid for 520,000kWh.

As a minimum, based on these facts, the path has been opened 2 years and thus generates 1500 kWh a year (though probably less since it likely has been open longer than 2 years). This means that this investment repays about 0.29 percent of its investment every year. If we ignore the cost of capital, and assume unlimited life of the panels (vs a more likely 5-10 years in this hard service) we get an investment payback period of only 347 years. Yay!