With a transition required from fossil to renewable energy and recognising the role that transport plays in generation of carbon dioxide, there are a number of transport authorities and private companies looking to use the road surface to generate energy in one form or another; either heat or electricity.

On the one hand, there is a clear rationale in that there is a large surface area available which remains clear for the majority of the time; there is no additional land use, less likelihood of local objection and it can combine two functions in one. Conversely, the primary function of the transport infrastructure is to convey vehicles and so this must take precedence in terms of structural stability, safety, operational and maintenance requirements. Equally, transferring a renewable energy system into a transport environment can reduce its efficiency and hence its economic viability.

Examples of renewable technology include piezoelectricity where the weight and force of passing vehicle wheels, heat exchange through buried underground pipes and an early stage idea to use hollow plastic reinforcing bars and even using bridge piers to anchor tidal energy generation devices.

Perhaps the greatest coverage and research effort recently has been focussed on the potential of solar photovoltaic panels to be integrated into the transport system. There are a number of sites where solar panels have been placed either on embankments or on noise barriers at the side of the road, on the roof of railway stations, or in one case, a tunnel topped with solar panels. The advantages of this approach are that standard panels can be used and they can be orientated for maximum solar gain. They are particularly well suited to south facing embankments.

Their visual impact means that they are unlikely to be acceptable in all situations, and there is also a risk of theft, damage or vandalism.

There have been some high profile examples of engineers looking to go further by integrating solar panels in the road surface itself. Solar Roadways of the USA has developed a prototype system where the solar panel is covered by a tempered glass covering, with added functionality to include integrated LED lights for provision of road messages or markings and the facility to melt snow. SolaRoad of the Netherlands installed a small section of prefabricated cycle path in 2015, manufactured with integrated solar panels. Finally, French technology company, Colas has recently announced its Wattway product, which is a thin layer that is placed on top of an existing road surface and claims to have a yield only slightly lower than conventional solar panels, yet is capable of withstanding all traffic types. So far, it has been deployed on a small number of pilot sites.

The upside of all of these innovations is they take no additional space, there is no requirement to purchase or rent farmland and their visual impact is no worse than the road surface it replaces. In all cases however, there will be a loss in efficiency, for a number of reasons including, ruggedizing the panels and reducing the light the photovoltaic cells, the cells being horizontal rather than angled towards the sun, by the occasional shading of passing vehicles and the build-up of dust and dirt on the surface.

Ultimately, widespread adoption will come down to a number of factors, but primarily safety, whole life cost including any additional maintenance requirements, and an acceptable return on investment. Any additional functionality such as integrated sensors or induction charging of electric vehicles might improve the overall economics if they do not add significantly to the overall cost.

Whilst existing systems are developed, improved and evaluated, in the short term some of the early potential could lie in the use of conventional systems on transport sites, such as solar canopies on car parks or at maintenance depots. As the cost of solar power continues to fall, the large land area occupied by transport infrastructure certainly offers the opportunity to generate solar energy in one form or other.

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