Why roads paved with so­lar pan­els are not such a bright idea

Tehran Times - - SOCIETY -

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

A so­lar panel ly­ing un­der a road 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 per cent of the sur­face of a panel can re­duce power gen­er­a­tion by 50 per cent.

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, it’s in­evitable these pan­els will heat up more than a rooftop so­lar panel too. For ev­ery 1 de­gree Cel­sius over op­ti­mum tem­per­a­ture you lose

0.5 per cent of en­ergy ef­fi­ciency.

As a re­sult, a sig­nif­i­cant drop in per­for­mance for a so­lar road, com­pared with 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 in France. This has a max­i­mum power out­put of 420 kW, cov­ers 2,800m² and cost €5m (£4.5m) to in­stall. This im­plies a cost of €11,905 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 about 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 per cent.

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 per cent. And at a cost of €360m, or €1,200 per in­stalled kW, one tenth the cost of our so­lar road­way, it gen­er­ates three times more power.

In the U.S., 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 its SR3 pan­els have been in­stalled in a small sec­tion of pave­ment in Sand­point in Idaho. This is 13.9 m² in area, with an in­stalled ca­pac­ity of 1.529 KW. The in­stal­la­tion cost is given as $48,734 (about £37,482), which im­plies a cost per in­stalled kW of €27,500, more than 20 times higher than the Ces­tas power plant.

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 per cent 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 data from the Sand­point in­stal­la­tion, which gen­er­ated 52.397 kWh in six months, or 104.8 kWh over a year. From this we can es­ti­mate a ca­pac­ity fac­tor of just 0.782 per cent, 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 rep­re­sent as large an area as we as­sume. The Depart­ment for 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 and B roads and half a lane for C and U roads (a lot are sin­gle track roads and just won’t be suit­able) we come up with a sur­face area of two bil­lion m².

Which sounds like a lot, un­til you re­al­ize that build­ings in 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.

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

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