If you are on Facebook, Twitter, or any other social media network, chances are you have already heard of this project – https://www.indiegogo.com/projects/solar-roadways#home (I obviously do not endorse it). If you haven’t heard of it – it’s a project that wants to replace the surface of all roadways, sidewalks, parking lots, airport aprons, runways, etc, all with these 7″ hexagons that contain about 50 high powered RGB LEDs to be able to make configurable patterns, photovoltaic cells (solar panels) to power them, heating elements to melt snow, and about 1″ thick of specially textured glass on top. The panels can also communicate with each other wirelessly, presumably as well as with a base station of some sort, through the mesh network.
Sounds cool. Looks cool (*1). Is definitely technically plausible. What’s not to like about it?
*1: Though if you look at illustrations on that page, you’ll find something strange – those tiles seem to have much higher resolutions than would be possible using just 50 LEDs per panel.
If you actually read through the Indiegogo page, you’ll find that it has no mentioning of cost at all, except that it supposedly “pays for itself” by generating electricity using those solar panels.
Yeah well, if that’s the case, why aren’t all our roofs covered by solar panels already? Solar panels do not pay for themselves at the present time, in most cases (only in places with extremely high sunlight, and very high electricity prices). That’s why.
If you look at the FAQ, there is a question “How much will your panels cost?”, and the answer essentially boils down to “we are not going to tell you”.
They already have working prototypes, and they can’t even give us an estimate of production price? That’s something all engineers are trained to do from very early on. How can you go as far as having working prototypes, and not having even an estimate of production cost? Or maybe they are hiding something?
But fear not, there is enough details on that page that, with the power of back-of-the-envelope calculations and Wolfram Alpha, we can come up with an estimate that should at least be within an order of magnitude of the actual cost –
How much does one panel cost?
A 7″ hexagon has an area of 0.082 m^2
Judging by the picture, and by the fact that they need to be able to support large trucks, the glass should be at least 3/4″ thick. How much does 3/4″ thick glass cost? I checked a few places, like this one, and they are pretty consistent. $21 per sqft, or $226 per sqm. I’m assuming margins are not very high in the glass material business, so let’s say the cost is $150 per sqm? The glass on a panel would cost $12.30, assuming post-processing is free.
Judging by the picture above, there are about 50 LEDs on one panel. Since they need to be able to change colors, they need to be RGB LEDs. Since they need to be visible under direct sunlight, through 3/4″ thick of textured glass, they need to be at least 1W, possibly much higher.
1W RGB LEDs cost about $8 each at 5k qty. Let’s say $5 at practically infinite qty? That’s $250 per panel.
How much do solar panels cost?
The best solar panels available right now are about 20% efficient, and 1 m^2 gets about 1KW of sunlight on average, when the sun is directly above, with no clouds.
So with a 7″ hexagonal tile, we’d be looking at about 16.4W per tile.
Actual average power output will be 5W or so since it won’t be noon 24/7, but we still need to get 16.4W panels to get 16.4W when it is noon.
Before we go into price… did you notice something doesn’t add up? How the @#$@#% are we going to power 50W worth of LEDs with just 5W average?
This is harder to estimate, but the wireless module will cost at least $5 at qty, and the CPU $2, and there will also need to be at least 3 MOSFETs per LED at maybe 3 cents each, for a total of ~$5.
I’m going to estimate $20 in electronics per tile. Unfortunately I cannot really justify this estimate further, but I believe it’s a very conservative estimate.
I am not going to include the cost of a 50W power supply… because we still don’t know where that power is coming from.
Taking into account those components above, we are looking at a total of $294 per tile, most of which is from the high powered LEDs. The cost of the solar panels turned out to be almost negligible.
How many panels do we need?
First of all, what’s the total length of roads in the US?
Or just over 2 million km if we only count motorways (freeways), highways, and secondary roads (secondary roads are main roads in cities that feed into the highways). Let’s use this number, since it’s smaller.
How wide are the roads? According to the US Standards for Interstate Highways, the minimum lane width for a highway is 3.7m. Or 7.4m both ways. That’s the absolute minimum, and doesn’t include sidewalks. So let’s say the average width of roads is 10m, as a conservative estimate.
How much road surface is that?
How many tiles do we need?
TWO HUNDRED FORTY FOUR BILLION FREAKIN TILES.
But how much will that cost?
Note that the cost is almost entirely proportional to area, so changing the tile size won’t change that number significantly.
Since we usually have trouble taking astronomical numbers like that in perspective, let’s compare it to a few other astronomical numbers –
That means, if we increase taxes to 100%, and shut down education, medicare, the military, and all the other government services, we will be able to make all our roads (well, just the big roads) shiny in 4.5 years… assuming we can still maintain a $16T GDP in those conditions.
Cost of labour, transportation, and disposal not included. They say they are aiming for 20 years life time for those tiles. That’s funding-seeking-speak, so let’s say 15 years as a more realistic estimate.
That’s still $4.7T/year, or almost 7x the military budget.
Even if I am off by an order of magnitude, that’s still 470B/year.
What else is wrong about this project?
Many things, in fact.
I’m not going to get into all of them, since I don’t have all day, but here are few of the best –
Charging EVs with solar panels
Inductive charging is 60% efficient over a gap of 12 cm, and decreases rapidly as the gap increases. Most EVs sit more than 12 cm from the ground. Even if one EV can draw power from 50 tiles, that’s 250W total, and about 125W after inductive loss.
A Tesla Model S base model has a 60kWh battery. That’s almost 21 days to charge at 125W. In those 21 days you can drive 230 miles.
More realistically, if you use it as an extra/bonus thing, if you drive for 4 hours on those tiles, you would have charged your battery 0.7%.
It will cost the same as paving
Sure, you can sell the electricity, if you don’t even need 5W to run a tile. How much can you sell it for?
Assuming a 17c/kWh buyback price (that’s the current solar buyback price in Georgia, and will likely decrease significantly if supply increases significantly), over 15 years, you would be able to sell 5W for $111, likely much much less if all the roads suddenly start selling back power, and government can no longer afford to heavily subsidize selling back solar power (or if the government own those tiles).
Why include solar panels on those tiles?
Why not just put them aside, or get rid of them completely since most of the power is going to have to come from the grid anyways?
The problem with solar power is not space, otherwise all our roofs will be covered by solar panels already.
Putting them in road tiles is solving the wrong problem.
Why not just build solar power plants elsewhere (in a desert), centrally? That would be much more efficient, and with HV power transmission, there is actually not much losses.
We don’t have many solar power plants in use today, mostly because they are still cost prohibitive.
Putting them in road tiles below 3/4″ of glass won’t magically make them any more efficient… quite the opposite in fact.
Of course, the real reason they are doing this is because people have warm and fuzzy feeling about solar power, and we WANT to be convinced that there is a way to use it cost effectively, and we are willing to turn off our common sense for a little while to get that feeling.
How do I identify nonsense like this in the future?
When you see a Kickstarter or Indiegogo campaign that is as technical as this one, yet doesn’t mention any numbers, and gives you everything in qualitative terms, you should be wary.
Just because the tiles cost money and you can sell electricity for money, doesn’t mean the amounts are equal, or even comparable in order of magnitude.
Just because you can theoretically charge EVs through induction, doesn’t mean you’ll actually be able to charge them at a non-negligible rate, and be worth the added cost.
It’s all in the numbers.
Just because 500 media outlets reported on it, doesn’t mean they have actually gone through it with common sense and a little bit of scientific literacy.
I suspect even the scientifically literate among the journalists actually intentionally turned a blind eye to the obvious infeasibility of this idea just so they can write an article and get viewers excited.