Pavement, it turns out, is a pretty good place to look for free energy.
A handful of Massachusetts researchers on Monday published a paper detailing a technique for using water-carrying pipes to convert the built-up heat in asphalt roads into usable energy.
Researchers measure ways to transfer heat from a source, such as this lamp shining over asphalt, to water.
(Credit: Worcester Polytechnic Institute)Released at the International Symposium on Asphalt Pavements and Environment in Zurich, Switzerland, the paper argues that asphalt roads have a number of advantages over solar-electric panels as a source of distributed energy.
"The significance of this concept lies in the fact that the massive installed base of parking lots and roadways creates a low-cost solar collector an order of magnitude more productive than traditional solar cells. The significantly high surface area can offset the expected lower efficiency (compared to traditional solar cells) by several orders of magnitude, and hence result in significantly lower cost per unit of power produced," according to the paper.
Blacktops can continue to generate energy after the sun goes down, and upgrades with heat exchangers could be fit into road constructions, which are done every 10 to 12 years, Rajib Mallick, associate professor of civil and environmental engineering at Worcester Polytechnic Institute, said in a statement.
Also, wicking heat away from roads could reduce the "heat island" effect in densely populated areas where temperatures rise when buildings and pavement release heat accumulated during the day.
The researchers used computer modeling and small-scale prototypes to test alternatives to pipes for transferring asphalt heat to water.
It found that the depth of the heat exchanger was critical and that a material with higher heat conductivity, such as quartzite, can be added to asphalt to improve heat transfer.
The hot water from the roads could be used in neighboring buildings, something that has already been done in the Netherlands. A more sophisticated approach would be to convert the heat into electricity using thermoelectric modules.
"Our preliminary results provide a promising proof of concept for what could be a very important future source of renewable, pollution-free energy for our nation. And it has been there all along, right under our feet," Mallick said.
Most efforts to "green" transportation focus on car technology, but roads can also be revamped to reduce carbon emissions. A national effort to improve millions of miles of highways and streets seeks to make asphalt more eco-friendly and less expensive.
The Asphalt Research Consortium aims to increase the use of recycled materials and improve energy efficiency of asphalt, which makes up more than 90 percent of U.S. roads.
Sand and other binder materials are being explored to make asphalt at the Modified Asphalt Research Center (MARC) at the University of Wisconsin-Madison.
(Credit: Jeff Miller/University of Wisconsin-Madison)"It has been a challenge to get the industry to look at this seriously just because our pavements have been performing relatively well and there have not been many complaints about failures," said Hussain Bahia, a professor of civil engineering at the University of Wisconsin at Madison, which is using $5 million for the studies.
The research project includes five institutions and is backed by the Federal Highway Administration. Increasing costs of energy and materials are driving up interest.
Until recently, asphalt has been relatively cheap to make in the United States. But the price of paving and repairing roads is rising along with that of gasoline. Asphalt costs of $568 per ton have risen from $315 last May, according to the California Department of Transportation.
Asphalt is a byproduct of the process of refining crude oil for fuel and lubricants. To prepare the gooey substance for application on roadways, Americans and Europeans may heat asphalt up to 300 degrees Fahrenheit, creating hefty emissions of greenhouse gases.
Less-wealthy nations including South Africa and India have many years of headway in using so-called cold or warm mixes of asphalt, which require less heat and energy.
South Africans shave asphalt into smaller bits and mix it in water and soap-like surfactants, which don't harden until after being laid on the road.
Bahia is interested in exploring modified, cold mix asphalts that might use plastics to achieve a longer-lasting, quieter, and safer end result.
And cold mixes also can use more recycled materials. Asphalt is the most frequently recycled material in the nation, according to the industry. Still, recycled asphalt contains only about about 15 percent reused materials, which Bahia wants to help expand.
"My best hope is to get the paving industry to recognize that they can save tremendous amounts of energy and impact on the environment by using different ideas in building our roads," Bahia said.
Porous asphalt, used on Italian toll roads, is also considered more eco-friendly because it allows rainwater to seep into the ground and reduces noise. And it helps to reduce skidding and accidents. Bahia said cold mix, porous asphalt might be a possibility to explore.
In the remaining four years of the U.S. asphalt project, Bahia said he hopes cold mixes from the labs will be good enough for the asphalt industry to test in the field. Down the road, getting approval for any new material from state highway agencies will be another challenge.
Also involved in the Asphalt Research Consortium are the Western Research Institute, Advanced Asphalt Technologies, Texas A&M University, and the University of Nevada at Reno.
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