October 13, 2008

Solar asphalt energy technology heats up

Korky Koroluk

Construction Corner

We’ve all seen bad old movies in which an obscure scientist, labouring by himself to develop an idea, suddenly sees it unfold before him with astonishing clarity.

It’s a “eureka moment,” and, except in Hollywood, it almost never happens.

What we see instead are ideas hatched and discarded, a lot of discussion over a lot of bad coffee, until something emerges that looks promising.

Then a relatively cheap and simple experiment is designed to prove (or disprove) the concept.

If it works, another experiment is designed to flesh out the original idea.

And so it goes: Step by patient step. There is rarely one big eureka moment; but there are likely to be many tiny little eureka moments along the way.

So it is with what is coming to be known as solar asphalt. A couple of years ago I wrote about an idea some British scientists had for using a paved road surface as a solar collector.

The sun warms the pavement, which in turn warms a water/glycol solution in a pipe array beneath the surface. The heat is then sent to a roadside storage area, which can be as simple as a generous sub-surface layer of rocks, where it is stored until it’s needed in the wintertime, when it’s sent back to the piping beneath the road’s running surface, warming it to help prevent accumulation of snow and ice.

It works in the relatively benign climate of southern England, but it adds to the road’s cost. And it’s effectiveness has yet to be proven in the harsher climate of the north.

But there are already offshoots from the original idea.

The paved playground of a new school in the south of England acts as a solar collector which heats the fluid in a sub-surface pipe system.

The heat is then piped to a storage bed beneath the building. In the winter, a valve isolates the collector from the rest of the pipe system, and the stored heat can then be sent to the building’s hollow-core floors and ceilings.

It works, and has already seen the school through one winter without using any auxiliary heating.

The Dutch have taken the idea a step farther, using the parking lots as collectors to heat buildings in a shopping complex. The Japanese reversed the idea, and have pumped river water through embedded pipes to reduce the temperature and heat deformation of the pavement.

The concept has finally reached the United States where researchers at the Worcester Polytechnic Institute in Massachusetts, have lab-tested a system that cools pavement while heating the fluid in the pipe field embedded in it.

The fluid can then be used for heating by running it through a heat exchanger, or for power generation using a thermal-electric generator.

The leader of the research team, Rajib Mallick, estimates that the system will cost between $20 and $50 per square metre installed.

Mallick’s tests show that the greatest heat absorption occurs just a few centimetres below the surface, which has started him experimenting with asphalt mixes using different aggregates to improve conductivity.

Quartzite is one aggregate material that shows promise.

There is a potential benefit the system offers that no one has said much about yet.

The immense amount of pavement in modern cities contributes significantly to the “urban heat island” effect. Green roofs help counteract that, helping keep the air a bit cooler, and thus reducing the cooling load during the summer.

If a lot of urban paving were to be done with solar asphalt, collecting solar heat would keep the pavement cooler, and help reduce the urban air temperature.

Green roofs combined with solar asphalt working together to help heat and cool our cities: Another little eureka moment.

Korky Koroluk is an Ottawa-based freelance writer. Send comments to editor@journalofcommerce.com.

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