Parking lot gold

by Stewart McCoy

You know how hot parking lot pavement gets during the summer? Well, it turns out all that heat can do more than make us run for air conditioning.

Harvesting parking lot heat and converting it to a sustainable energy source has been the business of researchers at Worcester Polytechnic Institute (WPI), Worcester, Massachusetts, since 2006.

Combating heat islands

The heat that radiates from pavement increases air temperature artificially, creating what’s called a "heat island."

Urban environments with lots of pavement use more energy (e.g. air conditioning) to combat heat islands. Higher temps also cause pavement to expand, forming ruts and reducing the life cycle of the pavement. A system to harvest heat from asphalt pavement would not only create energy but also reduce energy consumption, heat islands, and rutting.

Harvesting heat

The research team at WPI is devising a system for harvesting heat from asphalt parking lots. They’re focusing on asphalt because its darker colored and absorbs more sunlight than other kinds of pavement.

The experiments are centered on the best way to flow water through a network of pipes laid beneath asphalt. Solar energy absorbed by the asphalt would be transferred to the piping and heat the water.

The most likely application would be using the heat in hot water systems in nearby buildings. But another potential application is passing the heated water through a thermoelectric generator to produce electricity. A third option is using the harvested heat in absorption chillers for the air conditioning of buildings.

Possible obstacles

As with most projects, it’s not always smooth sailing. A solar energy collection system like this one would have to contend with the issues of heavy loads and climate variability.

Curriculum connection

The researchers wanted to discover if using different materials, called aggregates, in the asphalt mix would affect how well asphalt collects and transfers heat to the copper piping. In scientific terms, heat transfer is referred to as thermal conductivity.

Gravel, limestone, granite, quartzite, and sand are just a few examples of aggregates that can be used to make asphalt.

Instead of determining the thermal conductivity of the aggregates on his own, Dr. Mallick was able to borrow work from geophysicists who already determined the average thermal conductivity of limestone and quartzite.

The geophysicists found that quartzite is a more conductive aggregate than limestone.

With this information in mind, Dr. Mallick’s team tested limestone and quartzite to see which would increase the conductivity of the asphalt. They chose these 2 aggregates because they represent the low- and high-end of aggregate conductivity. Other aggregates, such as gravel, granite, and sand, fall somewhere in between limestone and quartzite in terms of conductivity.

The test results showed that quartzite significantly boosts thermal conductivity. They found that the quartzite mix can hold 1.5 more heat and transfer it 2.5 times better than the limestone mix.

This means that if solar energy collection systems are eventually installed in parking lots, it’s likely the limestone typically used in asphalt mixes would be replaced with a mix containing quartzite.

Heavy loads

Can a parking lot with a network of pipes underneath hold the weight of a semi-tractor trailer? Dr. Rajib Mallick, the principal investigator of the research team at WPI, hopes it wouldn’t have to.

Still, Dr. Mallick says they’re considering alternative piping materials capable of carrying heavy loads. The team recently began a project funded by the National Science Foundation to research the possibility of replacing the copper piping with flexible heat conducting systems, similar to geotextiles.

Geotextiles, often made from polyester, are normally used to cover the ground between pavement and soil. But Dr. Mallick’s team is researching the possibility of inventing a similar product that would be a highly flexible alternative to the copper piping.

Using a flexible system instead of copper means the heat collection system will be less rigid and less likely to break under the stress of heavy loads.

Extreme climates

A second obstacle is whether the technology can be used in climates that have harsh winters. Midwestern states, such as Colorado, Minnesota, and Iowa, have hot summers and very cold winters.

If a new flexible system is invented, then this will not be as much of a problem; the material will be less susceptible than copper piping to the contraction and expansion caused by extreme changes in climate.

Still, the solar energy collection system would be best for areas with long, hot summers, according to Dr. Mallick

Re-paving the way

This technology could be coming to a parking lot near you.

If Dr. Mallick’s research shows that the system will work and be profitable, there’s a company waiting in the wings to sell and market it.

And, in case you’re worried all the asphalt parking lots near you would suddenly be ripped up, don’t worry. Parking lots are usually resurfaced every 10 to 12 years, so the solar energy collection systems could be added when parking lots need a fresh coat of asphalt.

Learn more

Read more about solar energy at the Energy Kids website, thanks to the Energy Information Administration. There are short articles on the source of solar energy, where it’s most prevalent, how it’s processed, and its impact on the environment.

Stewart McCoy is a writer for Go!.