Reduce, reuse, recycle. This is the simplest mantra of the environmental movement and the guiding principle for families and small programs across the nation. How does this principle apply to infrastructure, specifically asphalt pavement? We already use as little asphalt pavement as we can, but this is more an economic decision. We recycle asphalt pavement to build new pavement. We reuse it as clean fill. What else can we do with it? Can we use it to collect solar energy?
Energy Recovery from Asphalt Pavement Basics
Some people think so. On the East Coast and in the mountain west, early efforts are underway to see if asphalt pavement can be used, not only to move vehicles, but to also collect solar energy that can be used cost-effectively. We all know how hot asphalt gets in sunlight and how it stays hot after dark, and asphalt pavement is everywhere, in many uses, in many climates.
Worcester Polytechnic Institute (WPI) in Worcester, Massachusetts, has been researching asphalt solar collectors (perhaps better called asphalt pavement solar collectors) since around 2006. In August 2008 they reported on the status of the research to the annual symposium of the International Society for Asphalt Pavements in Zurich, Switzerland, and found that asphalt pavement holds much promise for ongoing energy recovery. With only slight modification to the basic pavement design, energy recovery can be maximized.
“The Worcester Poly report says the maximum heat is two inches below the surface. For some pavements that’s in the wearing course, or at the bottom of it. So each time you resurface you will remove enough inches of deteriorated pavement to reach the solar collector mechanism. Will it be damaged each time? Or do you have to install the mechanism a little deeper, out of the optimum temperature zone, to avoid damage?” Gregory W. Perry, PE, Senior Project Engineer for Flower Mound, Texas
Hindrances to Implementation Are Evident
“I like the idea of using asphalt pavement as a solar collector, but you can’t in any way reduce the primary purpose of the pavement: moving people and goods from place to place in motorized vehicles,” says Gregory W. Perry, PE, senior project engineer for the town of Flower Mound, Texas (a Dallas/Fort Worth area suburb), when he first encountered the idea. “The Worcester Poly report says the maximum heat is two inches below the surface. For some pavements that’s in the wearing course, or at the bottom of it. So each time you resurface you will remove enough inches of deteriorated pavement to reach the solar collector mechanism. Will it be damaged each time? Or do you have to install the mechanism a little deeper, out of the optimum temperature zone, to avoid damage?”
Perry reflects the thoughts of many pavement experts when they consider this new use of existing technologies. “Parking lots are an obvious possibility for this,” Perry says. “Just look at where parking spaces deteriorate due to hot tires and hot engines being parked on hot pavement. A lot of energy is being dissipated and not used.” Major highways and even more heavily traveled city streets may not be good candidates for solar energy recovery. “The critical importance of these arteries for vehicles may preclude their use for any kind of solar energy collection. Parking lots, sure. Neighborhood streets, maybe. Driveways, absolutely. Asphalt sidewalks, why not? But heavily traveled streets and highways, it’s hard to see how it could work,” Perry says.
However, Perry made an alternate suggestion of where this might work: “Perhaps it could be tried in general aviation airports. Not the big airports that carry scheduled traffic, but the smaller ones, used by private and business planes.” These smaller airports almost always have asphalt pavement. The loads from the planes that use the runways, taxiways, and aprons are light, and lots of the pavement stays uncovered by traffic, with day-long sunlight hitting it. “All of these airports have internal power needs, and most are close to the city they serve," Perry adds. "Distance from energy generation to end use is a factor that must be considered.”
Solar Energy Recovery Alternative Methods
What else could you do to asphalt pavement to make it “greener"? A small company in Sagle, Idaho, Solar Roadways, is looking at a different approach. Scott and Julie Burshaw believe that roads can be constructed without any asphalt. Their web site includes this vision:
“The Solar Roadway is a series of structurally engineered solar panels that are driven upon. The idea is to replace all current petroleum-based asphalt roads, parking lots, and driveways with Solar Road Panels that collect energy to be used by our homes and businesses. Our ultimate goal is to be able to store excess energy in or alongside the Solar Roadways. This renewable energy replaces the need for the current fossil fuels used for the generation of electricity. This, in turn, cuts greenhouse gases literally in half.”
In the Solar Roadway model, three layers are used. The top layer, called the Roadway Surface Layer, will be constructed of translucent, high-strength glass. It is designed to be the wearing course, doing everything we expect of pavement for vehicle service but letting sunlight pass to the solar collectors embedded in this layer. Also embedded in the road surface layer are LEDs that can be used as pavement marking or road user messaging. The Electronics Layer contains some high-tech components to let the road sense loads and control the energy recovery. The Base Plate Layer includes the mechanism to move the energy out of the pavement toward the end use. In each of these layers, post-consumer (i.e., recycled) product use is maximized.
The Solar Roadways vision is large and perhaps further away from implementation than the WPI concept. While some small demonstration projects have been installed, much more research is needed.
Research Needed to Prove Feasibility
It’s nice to dream about reusing pavement in place to help the environment, but the small amount of research done so far only demonstrates that energy recovery should be possible. So far it has not addressed the merging of the primary use of pavement with this new secondary use, nor has it demonstrated the feasibility.
“What’s it going to cost me?” asks Perry. “For either new pavement or resurfacing, installation of the solar collector sounds like precision work, probably more precise than the pavement construction itself. That sounds costly. And what about off-pavement equipment to either convert the heat to energy or hot water? That has to be paid for, installed, and maintained. And, if electricity is the end product, you need to transmit or distribute it.”
“Perhaps it could be tried in general aviation airports. Not the big airports that carry scheduled traffic, but the smaller ones, used by private and business planes. All of these airports have internal power needs, and most are close to the city they serve. Distance from energy generation to end use is a factor that must be considered.” Gregory W. Perry, PE, Senior Project Engineer for Flower Mound, Texas
Perry is right. Much research is still needed. The WPI research began with computer simulations, then moved to laboratory scale testing, and then finally went to an outdoor test patch. The results are all promising, but this research didn’t include a vehicle load on the pavement. Normal ranges of weather must be factored in, and various climates must be modeled to determine full feasibility. Although the prospects are exciting, the research looks daunting, as shown by this short list of questions.
- What is the added cost?
- Is the energy recovered sufficient to justify the cost?
- Yes, asphalt pavement holds heat after dark, but if you pull heat out of it during daylight, will there be much heat left when the sun goes down?
- How effective it is on a cloudy day?
- What is the impact on a parking space of frequent coverage by parked vehicles?
- Heat radiates from the vehicle to the pavement, but the sun is blocked. Is solar collection still feasible?
- Is the durability of asphalt pavement changed by the presence of the solar collection mechanism?
- Is the mechanism of sufficient strength to withstand vehicle loads?
- How does this limit the type of mechanism used?
- Will the mechanism be damaged during the next overlay?
- How much does the care needed to protect the mechanism drive up the cost of an overlay?
- Will there be duplication of electrical supply needed, due to times when solar collection is not working?
Is Asphalt Pavement Solar Collection in our Future?
The ongoing development of asphalt pavement solar collectors largely fails to reach the popular news, and not all of this is happening in America. A number of scientific journals have articles about the technology, and demonstration projects have been constructed at a larger scale in Europe than in America. Somewhere in the world, unheralded and without regular press releases, universities and entrepreneurs are working through the problems, figuring solutions, answering questions.
So where does recovery of solar energy from existing and future pavement stand? Is it techno-dreaming, or just a little way away from practical implementation? Might we see some of this in the future? “I’m not about to recommend this to my town,” Perry said, “but I’d like to stay on top of the research and development. I think the future may include asphalt pavement solar collectors, used economically to reduce energy use from other sources.”
A senior engineer and corporate trainer of engineering for CEI Engineering Associates, Inc. David has 36 years of experience as a consulting civil engineer. His experience includes water, wastewater, stormwater, roads, and solid waste infrastructure. For much of the last 20 years he has been involved with stormwater issues. Specifications and construction administration have been a specialty of his within civil consulting engineering . He has BS and MS degrees in Civil Engineering, is a registered engineer in four states, and a Certified Professional in Erosion and Sediment Control.