"Solar Freakin' Roadways!"

Artist's rendition, courtesy Solar Roadways

A commenter brought up the "Solar Roads" concept with this link. I'd seen it mentioned in the past but had never really dived in to analyze it. In looking around, there are a fair number of youtube videos claiming to debunk the concept, as well as an exuberant video singing the praises of the idea (the one from which the title of this post came). And Julie and Scott Brusaw, the inventors of the concept, have both SBIR grants and a successful Indiegogo campaign. And they mount a defense against the so-called "debunkers" here.

So what is the Solar Roadways concept? The idea is to replace typical pavement materials (asphalt, concrete) with hexagonal tiles that contain solar photovoltaic cells along with circuitry and LEDs. The cells and circuitry are covered with a textured clear glass to provide traction for vehicles and protection for the electronics. Among other things, the solar roadway would harvest solar energy to provide distributed power, heat the tiles to melt snow and ice (though this would be done, as I understand it, with grid power since the energy demands to melt solid water exceed what the solar panels could provide), light the LEDs for lane markings, messages, etc., be sensitive to load and thereby be able to signal through the LEDs that there is a pedestrian or animal in the roadway, and a variety of other ideas.

So, does such a concept have any credibility? To start, Scott Brusaw has significant engineering background and doesn't appear to be either an idiot or a charlatan. The Brusaws recognize that the tiles won't provide electricity at night and thus, like a typical home solar installation, utilize the grid as a "storage medium," that is, they are a source of electrical input to the grid when the sun shines to the extent that their production exceeds their own demand (energizing LEDs, etc.), and utilize electricity from the grid to accomplish their purposes when it does not.

But let's think a bit. At the Renewable Energy Resource Center (a project of the National Renewable Energy Laboratory, RREDC & NREL respectively) there's a very nice page where we can estimate solar irradiance. The page allows us to choose dates, configurations, etc. Using average irradiance, March, horizontal flat plate (to represent a road), I get the map at left. It represents average everything (March at the equinox, average irradiation, etc.).

Let's take an average location, say, Kansas. It looks to me like about 4.5 kWh/m^2/day (kilowatt hours per square meter per day) would be a good number to use. Now, the Brusaws calculate using 18% efficiency in their cells. I'll charitably assume that the textured glass lets 100% of the incoming sunlight through to the underlying photovoltaic cells. Thus, the cells can capture 0.18*4.5=0.81 kWh/m^2/day. Where I live, that's worth a bit under a dime.

But suppose that Kansas (or wherever) has a feed-in tariff for renewable energy. Feed-in tariff rates are quite variable even by state, but let's generously use $0.30/kWh. That makes the electricity produced by a square meter of photovoltaic cells in the Solar Roadway worth a bit under a quarter, but let's call it $0.25. Of course, the DC produced by the cells must be converted to AC to feed the grid. An advanced "micro inverter" to accomplish this task is about 96% efficient, so that the 0.81 kWh is reduced to 0.78 kWh, worth about $0.23. In a year, the square meter would generate about $85.

The Brusaws claim that a tile is designed with a lifetime of 20 years. I doubt this, but let that pass. This would mean that the square meter of tile would produce, in its lifetime, $1,700 worth of electricity. Yes, rates may rise, but I didn't discount the cash flow to represent the time value of money. Further, it's unlikely that feed-in tariffs will endure as solar becomes more widely adopted, and feed-in tariffs aren't universally adopted in any case. Thus, I'm claiming a wash there.

Now, the assumptions here are, without a doubt, generous. No adjustment was made for transmission of the glass wearing layer, for any accumulation of road grime, blown in dust and dirt, portion of pavement shaded by vehicles, etc. But I will adjust for the fact that the LEDs, electronics, etc. take a significant portion of the area of each tile presented to the sun. While no figures are given in Solar Roadway's site, estimating from the various photos, I think that no more than 75% of the area of a Solar Roadway pavement would generate electricity. This reduces that $1,700 per square meter of paving to $1,275 and the annual revenue to a bit under $64. It reduces the electrical input to the grid to 0.585 kWh per day per square meter.

Now, the power required to have the LEDs visible in bright daylight at, say, 17 meters might be on the order of 300 watts/square meter of lighted area. This may seem like a lot, but consider that only a very small portion of any paved area is lit (see the graphic at the top of the post). If we conservatively assume that the worst case is that 5% of paved area may be lit, that would constitute 15 watts. And this is for daylight, the LEDs could (and, no doubt would) be dimmed at night. I'm going to estimate that this would require 240 watt hours/day or 0.24 kWh per day per square meter. Note that this is (.24/.78) = 0.308 or about 31% of the generating capacity of a square meter of tiles. This reduces the $1,275 to about $880 and the $64 to about $44. It reduces the electrical energy fed to the grid each day to 0.40 kWh per day per square meter.

I have not been able to find a cost estimate for a tile on the Solar Roadways web site, but if I use what I can find for tempered glass, photovoltaic panels, and electronics, I can (very roughly) estimate that square meter of these tiles might cost around $250 at wholesale prices. This doesn't include installation. I would estimate that a square meter of installation might take another $100 per square meter for a total of $350. Thus, an installation might pay for itself in eight years or so. That's not really so bad. And note that we don't ask concrete or asphalt pavements to generate any income at all (tolls don't count - the same could be done for a Solar Roadway). Note that this doesn't count the associated infrastructure for cable runs, etc. Add another $150. Since much of the pertinent information with respect to composition, dimensions, etc. isn't given, this is the best that I can do.

The Brusaws go on to claim that replacing the entire paved area of the US with Solar Roadway tiles could generate three times the electricity usage of the Country. Of course, this ignores the intermittency issue which, for a local installation, can use the "grid storage" mentioned above but which is out of the question when trying to replace the entire electrical generating capacity of the Country.

But let's carry on undaunted. I can use numbers from this Wikipedia page to estimate that approximately 74 billion square meters of land in the US is paved. Taking this number times the approximately (on average assumptions for everything) 0.4 kWh/day times 365 days per year, we can estimate that replacing all paving with Solar Roadways might generate a bit over 1.09*10^13 kilowatt hours of electricity. Let's just go with 10 trillion kilowatt hours. Here we find that in 2012, US electricity demand was 3,826 billion kilowatt hours, or 3.8 trillion kilowatt hours. Thus, the Brusaws' contention is in the right ballpark (given the proviso above).

What about the Brusaws' contention that the a Solar Roadway could contribute to the charging of an electric vehicle by induction? Let's assume that the car is traveling at the optimal time of day in cloudless skies. The Roadway can deliver something like 180 watts/meter^2 (18% efficiency * 1000 watts/m^2). The vehicle might be 1.75 meters wide and, at 55 m.p.h., spend about 0.04 seconds to traverse a meter. The Solar Roadway can deliver 1.75*1*180 = 315 watts from this area. In 0.04 seconds, it can deliver 0.0035 watt hours of energy. Suppose that an EV needs 10 horsepower to maintain a speed of 55 m.p.h. This is about 7500 watts. It will spend that same 0.04 seconds on a meter and thus need about .083 watt hours of energy. The Solar Roadway can thus (neglecting all other losses) supply about 4% of the energy requirement of this very efficient car. Of course, only a fraction of the roadway would be drawing power, let's say 15%. Thus, maybe 25% of the energy of the vehicular traffic could be supplied if all EVs were that efficient and every vehicle was an EV. Of course, not every vehicle is not an EV and thus this may actually be a significant contributor. Again, a whole lot of estimates went into this and many numbers were quite optimistic.

Finally, what would be the cost? This is even a harder number to estimate. The Brusaws contend that the tiles could, in many cases, be mounted on existing pavements. I seriously doubt that, overlaying pavement is a very difficult civil engineering problem. Profiles must be maintained, gradients must be within specifications, etc. Nonetheless, let's just take $500*74 billion, for a total of $37 trillion. Keep in mind, however, that these existing pavements will ultimately need to be repaired and replaced as they wear out. And a ballpark figure for construction of a new asphalt or concrete pavement is on the order of $150 per square meter.

Admittedly, much of the above is based on estimates that may or may not closely match the real world. Nevertheless, I see nothing that makes me think that the concept is preposterous. I believe that the most important data to acquire is the performance of the tiles under continuous wheel loading over long durations in varying temperature, moisture, and mounting/subsurface conditions.

And one additional major consideration, completely unaddressed by the Brusaws, is the need for paved surfaces to not simply shed all rainfall to storm water management systems for discharge to a lake, river, or ocean. In this regard, materials scientists are hard at work on pervious concrete and pervious asphalt paving systems that drain water to the subsurface layer for absorption into the underlying earth. There's no possibility of this with the Solar Roadway system, and thus an eco-friendly storm water handling system must be made a part of any such design.

Update: A tweet mentioned that the Brusaws had, in fact addressed drainage. A Google advanced search took me to this page. I haven't had time yet to evaluate the plausibility of their scheme, but I was incorrect in the paragraph above to state that the issue was "completely unadressed." My apologies to the Brusaws.