Sunshine Highways

Roads that make their own electricity. Pavement that uses sunlight to produce power for streetlights, signs, and traffic signals. A street surface that can melt ice and snow—and warn drivers of dangers ahead.

Sound like the setup for an April Fools’ Day joke? It’s not. Solar pavement is a real idea. This new concept for streets uses high-tech solar panels in a surface strong enough to support cars and trucks.

Part disco dance floor, part stadium scoreboard, part electric skillet, with bits of cell phone and smart grid technology thrown in for good measure, this 21st-century idea is still in the early stages of development. The U.S. Department of Transportation recently gave $100,000 to a company in Sagle, Idaho, to build some test models.

Drivers of the future may be riding on glass
Instead of asphalt or concrete, the top surface on these roads of the future will feature glass or a glass-like ceramic substance. That’s so sunlight can reach the solar cells embedded in the pavement.

A glass surface that cars and trucks can drive across must be:

• Strong and thick enough to withstand heavy loads.

• Textured to provide traction and prevent glare.

Engineers already know how to make glass and ceramics that can do each of these things. A specially laminated, thick “hurricane glass” used in some buildings in Florida can withstand a lot of pressure from wind, or even the impact of a falling tree. Kitchenware manufacturers already know how to make glass with textured surfaces, such as decorative bumps and grooves, or sandpaper-like etched areas.

The problem is figuring out a way to combine many different characteristics into a single substance. Then, the glass must still be:

• Smooth enough to prevent dust and debris buildup in the textured parts.

• Transparent enough to allow ample sunlight to pass through to the solar cells below.

Below the road surface, the glass within each solar cell presents a different set of problems to be solved. The silicon wafers inside solar cells are getting thinner all the time. Now a skinny, typing paper thickness of 200 microns, wafers produced for the solar manufacturing industry are quickly moving toward an amazing tissue-paper thickness of 160 microns.

However, such ultra-thin wafers are quite brittle and could crack if they flex under the pressure and vibration of traffic.

Putting solar cells flat on the ground in this new kind of pavement could also interfere with their efficiency. Angled solar cells tend to capture more sunlight than solar cells that lie flat. But a bigger problem could involve heat buildup. Air circulating behind angled solar cells mounted on rooftops or in free-standing situations helps keep them cool and operating at peak efficiency. Solar cells embedded within pavement sitting directly on the ground could overheat and not produce as much electricity as expected.

Worries about cost, safety are among the obstacles
Finding the right materials is only part of the difficulty in turning the solar pavement concept into reality. There are other big issues:

• Managing the electricity safely and effectively.

• Figuring the costs of the technology.

One of the most alluring ideas behind solar pavement is that it doesn’t just generate electricity, it uses it right on the spot. Special elements within the pavement could convert electricity into heat to melt snow and ice. Some of the electricity could also be used to power different colors of LED lights embedded throughout the pavement. These subsurface lights could be programmed to show lane markings, no-passing zones, turn arrows, and warnings for upcoming hazards such as pedestrian crossings or curves. Power from the roadbed could also be used for aboveground lights and signals.

Operating lights constantly will require some method of energy storage or connection with the power grid so the road continues to function after sunset. Adding these features will raise the costs.

Electrified roads would also require new safety measures. No matter how carefully constructed, a road surface containing energized electronic circuits would be similar to overhead power lines and could present deadly danger. Special features would likely need to be built in to interrupt power flows during extreme events such as floods or earthquakes. Road crews would need special training to work with this new kind of pavement.

Developing the right combination of solar cells, electric circuits, new kinds of glass, backup power supplies, and safety features for a new pavement will be expensive. Many estimates today are based on making solar pavement in 12′ by 12′ segments that can be laid down end to end.

One idea involves digging up existing asphalt and replacing it with the new panels. But no one knows yet if the below-grade portion of the roadbed under the old asphalt would also need special preparation that would add to the costs.

As the solar pavement experiments continue, engineers may discover new ideas that will help improve the way solar cells are used in other more familiar situations.

POWER IN YOUR DRIVEWAY

Instead of solar highways, consumers are more likely to want electricity right at their driveways. When plenty of time is available, ordinary household current for toasters, coffee pots, and small hand-held tools works fine to recharge the larger batteries in electric vehicles such as the Nissan Leaf and Chevy Volt. But for faster recharging, special power posts with larger outlets like those used for clothes dryers could be the trendy driveway accessory of the future.