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Energy: Solar roads take off

Trials in the Netherlands and US are looking at the possibility of using roads as a source of solar energy, as Dave Parker discovers.

Solar road section

Going Dutch: The prefabricated solar panels

Prefabricated photovoltaic (PV) road units have gone into production in the Netherlands, in advance of a full scale trial on a cycleway starting this autumn.

The 5t, 2.5m x 3.5m units have silicon PV cells underneath a 10mm thick tempered safety glass surface with a skid resistant coating. The electricity they generate will be fed into the Dutch national grid.

The system, dubbed “SolaRoad”, has been developed by a consortium of Dutch research institution TMO, contractor Ooms Civiel, multidisciplinary consultant Imtech and the province of Noord-Holland.

According to SolaRoad, the existing road network could become a major source of sustainable electricity. If every suitable rooftop in the Netherlands were covered in PV cells, the power generated could satisfy no more than 25% of national demand, SolaRoad claims. But Dutch roads cover a significantly greater area, at nearly 50,000ha.

“Traffic is heavy on the cycleway, so we will be monitoring the effects of shading from cyclists on energy production”

Stan Klerks, TNO

The 100m long trial, in the small town of Krommenie, north west of Amsterdam, is intended to test the durability and safety of the glass road surface as well as energy generation performance.

“Traffic is heavy on the cycleway, so we will be monitoring the effects of shading from cyclists on energy production,” says TNO lead architect and systems engineer Stan Klerks. “This will be a ‘living lab’, so the reaction of the public to the new surface will be interesting.”

How the translucent glass surface will be kept clean to maintain generation efficiency is another key area to be investigated, as is skid resistance during wet and icy conditions.

The units are made up of two layers of glass with the PV cells sandwiched in between, mounted in a precast concrete frame. Their resistance to weathering, wear and tear and impact will be under close scrutiny, Klerks says.

“If the glass should shatter for any reason, there will be no dangerous sharp shards, as tempered glass breaks up into small granular chunks, which are much safer,” he adds. “And the (low voltage) electrical systems are also designed for safety in such circumstances.”

“This will be a ‘living lab’, so the reaction of the public to the new surface will be interesting”

Stan Klerks, TNO

Other issues considered during the development of the units include potentially distracting reflections from the glass surface, and ride quality. The units will be bedded on compacted sand and gravel and locked together into a smooth surface to maximise ride quality, Klerks says.

SolaRoad acknowledges that, if the technology was installed on a national scale, the major challenge would be storing surplus electricity, as peak output would rarely coincide with peak demand. But this is a problem for most forms of sustainable energy, and considerable development work is in progress on vanadium redox batteries and other improved forms of electricity storage. SolaRoad expects to take advantage of these new technologies as they come on line.

In the longer term it is hoped that one significant storage option will be the batteries of electric vehicles and bicycles. It is even possible that later generations of electric vehicles will be able to draw current directly from the solar road surface.

Klerks says that, in the first phase of the trial, 70m of the cycleway will have the first generation units installed, with a further 30m of second generation units due to be placed after six to nine months.

Small trials

Idaho solar road

Private Idaho: Computer generated design for an application on a suburban street in the United States

Small scale trials have also begun on a competitive, more complex, solar roads technology in Idaho, United States.

Hexagonal units containing solar PV cells, light emitting diodes (LEDs), microprocessors and heating units have been installed on a 3.7m by 11m area next to the home of the husband and wife team promoting the system.

Solar Roadways, as the system is called, is the brainchild of Scott and Julie Brusaw, who have resorted to crowd funding to finance the next stages of development. The Brusaws hope that the next trial will be on a visitor centre car park in their hometown of Sandpoint next spring.

They claim that tests show the 12mm tempered glass surface can cope with loads of up to 110t.

The combination of LED lighting and microprocessors will enable Solar Roadways to display warning messages, or change road markings in emergencies. Heating elements will clear the road surface of winter ice and snow, while drainage channels will take the snow melt away, eliminating the risk of it refreezing next to the road and causing frost heave.

UK applications

Conventional asphalt roads can also be used to harvest solar energy, in the form of heat. Even in northern European sunshine, surface temperatures on black asphalt can reach 50ºC.

To collect this sustainable energy, pipes can be buried beneath the road surface. Water circulates through them, transferring the thermal energy to an underground energy store.

This significantly reduces the surface temperature in the asphalt in summer, which in turn could reduce the risk of rutting. In icy winter conditions the heat can be recycled back through the network of pipes, eliminating the need for de-icing salts.

There have been full scale trials in the UK and the Netherlands that are said to have been successful.

In 2005, part of the access road to the Toddington service area on the M1 had a system installed by British technology company Icax (NCE 19 May 2005). A two year trial was carried out by transport research body TRL, which finished in 2007.

Two thermal collector arrays, each 5m wide by 30m long, made up of 25mm polyethylene pipes at 250mm spacing, were installed 120mm below the road surface. These were connected to similar arrays at 875mm depth, one below the road, one beside it. They were topped with a 200mm layer of expanded polystyrene. The insulated earth acts as thermal energy stores, dubbed “ThermalBanks” by Icax.

During the summer of 2006, TRL recorded peak temperatures in these stores close to 300C.

Heat was transferred back to the roadway whenever the surface temperature fell below 20C.

During the trial, the system kept the heated section above freezing apart from a few hours during extremely cold weather, when unheated surface temperatures plunged to -60C.

In its report on the trial, TRL suggested that an even better performance could have been achieved if the collector arrays were at a shallower depth than 120mm, and that the system could be cost effective in well known cold spots on the highway network or on slip roads and interchanges.

Dutch contractor Ooms, part of the SolaRoads consortium, trialled its similar Road Energy System in Avenhorn, north Holland, from 1997. A 200m stretch of asphalt road and a small car park yielded enough energy to heat 70 apartments in a four storey building.

Unlike Icax, the Dutch system normally uses aquifers as the thermal store, as these are much more common in the Netherlands.

A 1ha array has been installed on the Vondelingen viaduct in the Port of Rotterdam, but in the UK the most popular enduse so far is as a renewable energy source for major buildings, using car parks, school playgrounds and the like as the solar collector.

In such applications, with space heating as the main end use, heat pumps are used to raise the temperature of the water in the thermal store before it enters the building’s heating system.

Icax has completed a number of school, supermarket and prison projects, and is actively marketing its system for airport runways and hard standings.

“Solar incident energy is one of the great renewable sources of energy,” says Icax managing director Mark Hewitt.

“Collecting and using this renewable heat makes complete sense, and we will need to grow our knowledge and use of this resource.”

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