Harvesting energy from roads

New technologies capture ambient energy and convert it to electric power

Imagine using the millions of kilometres of paved roads around the world to harvest energy. Apart from the initial investment costs required for equipment and installation, this energy source is free to produce and has no adverse effect on the environment. Instead, it uses sunlight or the mechanical vibrations produced by vehicles to generate electrical energy.

Energy harvested from roads captures unused ambient energy and converts it to electric power. This electricity can then be used to power road infrastructure such as lights and signals. It can be stored in batteries for use when needed or fed into the electric power grid. And, because it makes use of the existing road network, no new land space needs to be allocated.

International Standards play a key role in the development of these solutions. IEC Technical Committee (TC) 47 develops International Standards for semiconductor devices including those that harvest energy. Batteries used to store electrical energy rely on the standardization work of IEC TC 21. IEC TC 8 and its Subcommittee (SC) 8A develop Standards for electricity supply systems, including the integration of power generated from renewable energy sources and fed into the electrical grid. A systems group, SyC Smart Energy, has recently been set up to provide systems level standardization, coordination and guidance in the areas of smart grid and smart energy.

Where the road meets the sun

Techniques have been developed to place photovoltaic modules directly on top of a road surface to capture solar power. Energy can be harvested from the over 16 million kilometres of paved roads around the world that are exposed to sunlight.

However, installing glass modules on roadways is no easy feat. Car tyres must be able to grip the road surface and glass panels must be durable and capable of withstanding heavy weights. Shading from trees, buildings and clouds can reduce exposure to the sun.

Despite these challenges, several companies have developed photovoltaic modules that can either replace asphalt or be placed directly on top of existing roadways. While these solutions are proprietary, they rely on International Standards developed by IEC TC 82, which is responsible for solar photovoltaic energy systems.

In the United States, Solar Roadways has developed solar modules that include microprocessors for intelligent communication and light emitting diode (LED) lights for lines and signage as well as heating elements to melt snow and ice. In France, the Wattway Solar Road is currently being trialled across the country, including at the entrance of a motorway toll booth to power the gates and payment machines. In China, a two kilometre road has been built in the Shandong province with the aim of powering street lights and containing a snow-melting system on the road. Roll-out, however, is limited given the high cost of these photovoltaic (PV) modules and the uncertainty regarding the actual levels of energy generated. Alternatives have been suggested: placing PV panels alongside roads or in non-critical areas such as parking areas, bike paths and driveways.

Thermoelectric generators (TEGs) can also be used to harvest energy from roads. Based on the Seebeck effect, TEGs can convert geothermal energy – produced from the heat differential between the road surface and the layers beneath – into electrical energy. As the temperature differential increases, more electrical energy is produced, thus making this technology well suited to areas with extremely hot weather. In 2017, IEC TC 47 prepared the IEC 62830-2 series of Standards which provide methods for evaluating the thermal power of thin films used in thermoelectric energy harvesting devices.

Research is currently underway in the southwest of the United States to test this technology and potentially make use of it in rural areas and in airports to power lights and traffic sensors.

Good vibrations

The vibrations produced by a car driving on the road can be used to generate electricity.

Piezoelectricity is the electric charge produced by certain crystals when a mechanical stress is applied. First demonstrated in 1880 by the brothers Pierre and Jacques Curie, the piezoelectric effect has only begun to have practical applications in the past three decades. Standards for piezoelectric technology are developed by IEC TC 49, which addresses piezoelectric, dielectric and electrostatic devices.

Piezoelectric crystals can also be embedded beneath a layer of asphalt. As cars drive over the road, the wheels exert a force that causes these crystals to deform and generate electrical energy. This energy can then be used to power street lights or can be stored in batteries for later use. The IEC 62830-1 series, prepared by IEC TC 47, includes methods for evaluating the performance of vibration-based piezoelectric energy harvesting devices.

In the United States, the state of California has invested USD 2,3 million to fund two independent projects with the aim of determining the viability of embedding piezoelectric devices in roads to harvest energy. A similar trial is underway at Lancaster University in the United Kingdom. However, challenges remain. Factors that increase piezoelectric road efficiency are inversely related to the durability of a road. Roads with sizeable traffic flows from heavy-duty vehicles travelling at high speeds will generate a greater energy output compared to roads with little traffic, light-weight cars and slow speeds.

It has also not yet been confirmed whether the costs associated with installing and maintaining roads embedded with piezoelectric technology are offset by the electricity generated, given its relatively low energy conversion efficiency. Other energy harvesting solutions, such as PV modules alongside roads, may be cheaper to install and generate higher volumes of electricity.

And down the road…

Energy harvesting solutions making use of roads are still in their early stages. Because much of the current research and development has been undertaken by private companies, there is limited public availability of data. Costs remain high given the lack of mass production.

Some of these solutions may yet gain traction. Additional technologies such as sensors and microprocessors could be incorporated that monitor infrastructure and vehicular traffic conditions in real-time. Such solutions will rely upon Standards developed by the ISO/IEC Joint Technical Committee, JTC 1/SC 25 for microprocessor systems and IEC TC 47 for sensors. It is not yet clear which technologies, if any, will be implemented. But the enthusiasm remains in the pursuit of renewable energies.