Sunshine highways: Roads of the future

The roads of the future may forego the usual tarmac in favour of solar panels, harvesting energy to defrost winter ice, communicate danger and generate electricity. Meet the engineers hoping to plot a new route for our cars.

Walk barefoot across an asphalt road on a hot sunny day and you'll soon realise how well the material absorbs heat. Road temperatures in direct sunshine often reach 15°C higher than the ambient air temperature while summer surface temperatures in cities easily top 50°C.

One way to harness this solar energy is to install a solar collector – typically an array of pipes – and thermal stores beneath the road. In the summer, fluid circulates through the pipes to absorb heat from the road surface and is then piped to an insulated tank. Come winter, the warm fluid is pumped back along the pipes to clear ice from the road or even to a connecting building to provide under-floor heating.

Simple and effective, a handful of engineers are building on this concept of seasonal heat transfer for new applications. Rajib Mallick, professor of civil and environmental engineering at Worcester Polytechnic Institute, US, wants to extract heat from asphalt roads to reduce the 'heat-island' effect in towns and cities.

'Pavements soak in heat and radiate it back to the environment, raising the temperature of near-surface air. This increases the power required for cooling nearby buildings and deteriorates air quality,' he says. 'With the potential doubling of the human population in the next five decades... unmodified urban heat-island effects could significantly affect sustainability.'

A blend of surface elements

To tackle this issue, Mallick and his team have designed a pipe network to best harvest heat from an asphalt surface, and reckon they could build a system with a heat capture efficiency of up to 10 per cent. Finite element modelling and laboratory experiments on asphalt samples support the concept and indicate that blending asphalt with a high thermal conductivity aggregate, such as quartzite, increases the amount of heat that can be extracted from the road.

Meanwhile, painting the road surface with a black sealer 'traps' more heat inside the road, boosting the temperature of the fluid passing through the pipes.

Professor Kevin Wayne Lee and Andrew Correia from the civil and environmental engineering department at the University of Rhode Island, US, have built a prototype pipe system to extract heat from roads. Their tests on different asphalt-mixes, pipes and pipe arrays suggest the technology is practical and Lee is now considering the best ways to use the extracted heat.

'We can heat the water by about 60°C so how can we use this?' asks Lee. 'We can send the water to bridges to keep them ice-free or to buildings for showers... but we are also working on generating electricity. If we can get steam we could use this to turn a turbine in a small power plant.'

But while Mallick is sure such systems will reduce road temperatures, combat heat island effects and even generate electricity, he admits an extensive pipe network just beneath an asphalt surface could be impractical. 'From a design, construction and maintenance viewpoint you don't want to put a lot of pipes under the road,' he explains. 'Challenges include the effect of pipes on [road] performance, the effect of traffic and load on the pipes and maintaining the piping system.'

Graphene alternative

Instead, as part of a National Science Foundation-funded project, Mallick is looking to replace the pipes with a sheet of high-conductivity material, such as graphene. His alternative system could comprise just two pipes running beneath the sides of a road, connected by the sheet.

'The flexible sheet would transmit heat away from roads to the pipes at the sides,' he says. 'We would use less pipe as well as avoid putting them underneath the main travel-ways. If we start putting zillions of pipes under roads, people will become very concerned about the effects.'

Mallick and his team are now considering the best material to use. They need a sheet that is flexible, thin, has a high conductivity but also the heat capacity to extract the heat from the road surface and transmit it to the pipes. 'This is a challenge, but it can be done,' Mallick adds. 'I do believe in my heart this technology will be on the ground within the next five years.'

Mallick is also developing electronic systems to monitor and control the flow of fluids through a solar collector, beneath an asphalt surface, to prolong the lifetime of the road. As he explains, asphalt roads are more prone to damage from traffic when hotter so control systems would 'adjust' road temperature according to the ambient temperature as well as incoming traffic.

'Trucks could be fitted with sensors that emit the load and wheel configuration to another sensor at an intersection,' he explains. 'The control system decides at what temperature the road should be to take the load without failing and switches on a system that flows cold fluid to the road... if the road temperature cannot be reduced enough, the control system will flash a warning sign to tell the truck to change its path.'

Intelligent road

While such a system will not be in practice for many years, Mallick is enthusiastic about its potential. 'If we can make a road adaptive to the environment, then we can come up with smarter roads without putting down energy intensive, expensive materials,' he says.

American electrical engineer Scott Brusaw, co-founder of US-based Solar Roadways, has also designed an intelligent road system. Like Mallick's effort, it can melt ice, warn of maintenance needs and generate electricity. Unlike Mallick's, there isn't a single granule of asphalt in sight.

Speaking at a recent TEDx conference in Sacramento, US, Brusaw said: 'When the phrase 'global warming' began gaining popularity, we thought about replacing asphalt with solar panels that could be driven on. We asked, what if we could make a structurally engineered case that could withstand the forces of 35-tonne truck flying over it at 80mph?'

As Brusaw concluded, you could put anything in such a case, which he did. The electrical engineer has built a 3.5m2 slab, or 'Solar Road Panel', that contains photovoltaic cells, heating elements, light emitting diodes, microprocessors and sensors. To boost green credentials, he intends to fabricate the panel's support structure from plastics recovered from landfill.

Brusaw also believes the road could re-charge electric vehicles. As he asserts, the Solar Roadway will carry electricity so vehicles could be recharged at any convenient stop. Outlandish? Yes. Implausible? Maybe not.

In August 2009, the company won a $100,000 grant from the US Department of Transportation to construct a prototype road panel. Satisfactorily built, Brusaw is now awaiting further public funding to install several panels into car parks, to 'learn lessons' from slow-moving vehicles.

Two questions have to be asked; how much power would a solar road generate and how much will it cost? Brusaw's calculations indicate that one mile of a four-lane highway would require 1,760 panels and could generate 13.376MWh daily, providing electricity to 428 US homes. And while the target price of a single panel is $10,000, Brusaw argues this is getting close to the asphalt equivalent as his solar road will last 20 years compared to the seven years expected of an asphalt road, and generates revenue-earning electricity.

Brusaw isn't alone in his passion for solar roads. In October 2010, out of 3,795 ideas submitted to the GE Ecomagination Challenge – in which 'inventors' submit ideas on how to build the next-generation power grid – the Solar Roadway concept earned the most public votes, winning a $50,000 slice of the total $200m award.

But while the Solar Road has captured public imagination, will Brusaw snare enough funding to build this asphalt-free highways? You'll probably be burning your feet on the black stuff for some time yet.

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