A recent study by the Chinese Academy of Sciences, published in the journal Earths Future, claims that if the global road network were covered with electricity-generating photovoltaic panels, offset 28% of global CO2 emissions. Their calculations add up the main, secondary and local roads of 222 countries; 3.2 million kilometres of land that could provide 17,580 TWh of energy with photovoltaic cells. It sounds good, but is it possible to cover roads for solar production? Does it make sense in terms of efficiency and costs to convert the road network into a power generator? In that same study, the researchers already find a drawback: the initial cost of carrying out the installation would raise the price of the MWh to 120 dollars (depending on the land and the cost of energy in each region of the world).
The truth is that, to date, several pilot tests of solar roads have been carried out, but some proposals still raise doubts regarding, for example, accidents. What happens to the reflection of the photovoltaic cells if the panels are installed taking advantage of the slopes or empty spaces at the intersections of the road network? There is already a lot of photovoltaic energy integrated into different infrastructures, for example, in streetlights, and at first glance it seems like a good solution because it avoids running cables to bring electricity to the point of consumption. In the case of roads, the consumption would be made by electric cars in transit. “There are proposals that look similar to each other, but that are giving very different results. For example, solar car parks are being a success both in Spain, where they are multiplying, and in France, where it is already mandatory for car parks with more than 250 spaces to have these solar roofs. However, road pavements that integrate photovoltaics under the asphalt have not given good results, wherever they have been tested they have worked poorly. “They have turned out to be four times more expensive and less efficient than solar parks and they drive up the price of electricity. Covering roads does not seem like a good idea either, given the increased costs involved in setting up the entire structure,” says Héctor de Lama, technical director of the Spanish Photovoltaic Union (UNEF).
One of the advantages of using the road network for energy production has to do with the occupation of land; it would avoid consuming new land for photovoltaic installations. If they are used to power electric cars, the weight of batteries and these would also be reduced.
would gain autonomy. In Sweden An analysis of a 21 km installation between the towns of Hallsberg and Örebro claims that weight could be reduced by 70%. and highlights a succession of derived advantages such as lower prices for electric cars and savings in subsidies for the purchase of these vehicles that could be used to invest in modifying the road network. The recent study also highlights a possible reduction in the number of accidents during episodes of rain, because the panels would divert rain to the sides, avoiding flooding. In addition, as Enrique Miralles, technical director of the Spanish Road Association (AEC), says, “it would serve to offset emissions where they are generated.”
Technologies and uses
«The news of the pergola full of photovoltaic panels is one of the possible proposals, but not the only one. This one in particular has theat the disadvantage of cost or visual impact. However, asphalt is only one part of the road network, which also has slopes and links between roads and a lot of lost surface area that could be used. As for the integration of cells under the asphalt, pilots have been carried out in Belgium and China, but these experiments have proven to be inefficient because to support the weight of the cars you need very thick glass and surfaces that are also sufficiently adherent. In the end, the studies have shown that the radiation that reaches the photovoltaic cells, several centimetres below the ground, was very low,” explains Alfredo García, director of the Road Engineering Research Group and of the new Torrescámara chair at the Polytechnic University of Valencia (UPV) dedicated to the study of solar solutions for roads.
As for their use, they could be different, although the one that makes the most sense, say the experts consulted, is to power electric cars. «Europe has a decided commitment to this type of mobility, but, today, there is no electricity grid with enough capacity to power all the cars that are expected to be on the road, especially in rural areas. It is a vicious cycle; there is now a lack of charging points, because there are not enough cars and without cars, the big operators do not invest in new points or in bringing energy to the roads,» explains García.
Generating energy on the road itself could supply fixed charging points at service stations along motorways or charge batteries while cars are moving. In this regard, there are also several proposals that have been tested in real-life situations. One of them consists of installing pantographs at the top of one of the lanes and connecting the vehicles to them, as if they were trains. Germany has been the country within Europe that has invested the most in testing this solution, which is especially designed for heavy vehicles. In 2019, the A-5 highway near Frankfurt had 10 km of overhead power lines on which up to 15 trucks traveled. routinely. The section required an investment of 15 million euros.
Another alternative is induction charging, where energy is transmitted electromagnetically from the transmitter base located under the asphalt to the vehicle, as is already the case with certain models of electric toothbrushes and some mobile phones. The future of this solution seems promising even for the World Road Association (PIARC), which dedicated a comprehensive study to it in 2023. In it, it mentions some projects already in development, such as the one in Visby in Sweden. An electric bus covers the route between the city and the airport (4 km away) and is, according to the organisation, the first induction road on which several vehicles have been charged at the same time. Likewise, Coventry University (United Kingdom) has just announced its intention to cover a section of the Kenilworth road, about 10 km from Coventry, with induction charging to power buses and logistics vehicles, previously equipped with induction plates to absorb the electrical charge from the coils.
There is talk of powering electric cars, but the electricity could also be used to gain connectivity on the road, a trend that is beginning to emerge and which represents the bulk of investments by both administrations and companies. «Since 2018, cars in the EU have to have an eCall system, which allows the car to send a warning signal to emergency centres in the event of an accident. Manufacturers receive data from more than 300 sensors “We have anonymous information that allows us to know all kinds of details, such as when the headlights are on or if the cars are skidding. This data is processed and can be sent to the administrations. For example, the data on vehicle vibrations can provide information on the state of deterioration of certain stretches of road. If you have information on thousands of cars on that road, you can warn the manager that that stretch is not in optimal condition so that they can fix it. Before, the administrations had to carry out inspection campaigns with trucks to find out if the roads were in good condition, which took a long time to make measurements on site,” says the director of the AEC.
Four-color traffic lights
Roads are undergoing transformation due to the future arrival of autonomous cars, electric cars and climate change. This week we saw the appearance of the first four-colour traffic light in Spain. It was launched in Madrid and is used to indicate special situations, such as allowing emergency vehicles to pass, and to give priority to autonomous cars in the future. Autonomous mobility is part of the R&D of the sector, which is also working on self-repairing pavements or photoluminescent signs that absorb light during the day and emit it at night, which would reduce “the need for lighting,” says Miralles of the AEC.