Positive Energy Roads – a solution to decarbonise transport?

Whether generation of renewable energy from within the road authority estate is now a feasible option was the question that Maple and three associates from ITEN* (Helen Viner of Enodamus and independent experts Vijay Ramdas and Bill McMahon) have recently addressed in a project for the World Road Association (PIARC).

Transport accounted for 23% of global carbon dioxide emissions in 2010 , and so along with vehicle manufacturers, road administrations have a role to play in reducing emissions in line with the Paris agreement and national targets to reach carbon neutrality. Whilst the majority of the energy use in road transport is used in the propulsion of vehicles, significant energy is used in the construction, maintenance and operation of roads.

The system boundaries defined for the what areas of the road envelope (right of way) would be considered for energy generation included the road surface, embankments, depots, buildings and rest areas, but not buildings outside of the immediate road infrastructure, such as office or headquarters. The energy uses considered were construction, maintenance and operations, but not vehicle propulsion. A matrix was developed that road authorities can use to match the energy requirements with potential sources of renewable energy, and the mechanism for the flow, for example local storage or export to the grid when generating energy and import from the grid when energy required.

Allied to this were four Key Performance Indicators ranging from basic comparison with energy produced, kWh per year investment cost, a return ratio per year and finally, an overall return on investment. Different road authorities have different ambitions for the development of renewable energy, and hence, five progressively higher levels that can be used to understand their performance were defined as a framework that can be built upon.

• Level 1: represents the energy needed to operate a specific asset (e.g. street lighting) which is met by defined renewable energy sources (e.g. local micro solar generators).
• Level 2: considers the annual energy needed to operate the road infrastructure (such as lighting) which is met by renewable energy sources.
• Level 3: considers the annual energy needed to both maintain (e.g. resurface) and operate and the road infrastructure.
• Level 4: the annual energy needed to construct, maintain, operate and decommission the road infrastructure.
• Level 5: the annual energy needs including vehicle propulsion. Without this measure there is potential for perverse outcomes, for example, badly designed and maintained roads will increase vehicle energy use but there is an energy cost associated with the maintenance needed to improve the condition.

A survey was sent to road administrations worldwide using the contacts of the team and with assistance from the World Road Association and Project Oversight Team. The survey was designed to understand the range of interest in relation to PER. Overall, it was found that there was widespread but not universal interest in the generation of renewable energy. Of the road administrations that have installed renewable energy, most of the existing large-scale installations were solar PV, although wind-power and hydropower had been installed, whilst thermal and geothermal were being considered.

An overview and assessment of potentially applicable renewable energy generation is presented in the report supplemented by a selection of case studies selected to provide a range of technologies and levels of maturity. These demonstrate the availability of technically feasible and economically viable options to support the generation of renewable energy and essentially enable PERs.

Most operational energy demand is due to the provision of street lighting, requiring around 8,000 kWh/km/year for a single carriageway (two lanes) and double for a dual carriageway or motorway.

The energy demand for road maintenance is far higher than for operations because it involves far more energy intensive activities such as the manufacture of asphalt and concrete, their transport to site, removal of damaged surface materials, and placement of new materials. Whilst highly site specific, the energy required is in the order of 1 Terajoule (TJ) per lane kilometre, while new construction required between 2 and 3 Terajoules per lane kilometre due to the larger volumes of material being manufactured and moved.

Solar PV panels are a good option for generating renewable energy from the right-of-way as they are a mature technology and provide a modular solution, and they are likely to be able to meet the operational energy for lighting, either directly with a few solar panels on a streetlight, or through a larger array, for example, 40 – 70 panels would provide the energy required for a km of highway street lighting in the UK.

For road maintenance energy demands of 1 TJ/lane km (i.e. 2 TJ/km for a standard single carriageway), would require the equivalent of the annual output of around over 1,100 solar panels, for a resurfacing operation. Achieving Level 3 PER with solar technology within the right-of-way would be difficult, although possible in some locations.


Solar array at I-90, Exit 13 in Massachusetts. Picture courtesy of Massachusetts Department of Transport

Large onshore wind turbines provide a higher energy capacity than solar panels, with an average capacity of 2.5–3 MW can produce more than 6 million kWh in a year. A single turbine could power approximately 375 km of illuminated motorway per year (meeting Levels 1 and 2) or resurfacing around 22 lane km of highway. There are likely to be limits to how close a large wind turbine can be placed from the highway, so they may need to be offsite, e.g. at rest areas or depots. Level 3 PER could be achieved with a mix of solar and onshore wind.

Whilst offsetting construction energy might be achievable with onshore wind turbines at rest areas / depots depending on local conditions, off-site energy would likely be needed for Level 4 cumulative energy of operations, maintenance and construction.

Whilst generating sufficient renewable energy to cover level 5 would not achievable from sources within the right-of-way, using energy generated to cover e.g. street-lighting to power electric vehicle charging stations could be a useful option for local energy use.

PER has the potential to contribute to the decarbonisation of road transport, however, currently most examples of renewable energy generation tend to be small scale and/or ad hoc to meet localised energy needs. The gradual uptake of such schemes can be expected to continue, however, the transformational change that will enable the move to PER in the medium-long term, will require a more determined effort. Factors that can facilitate the transformation include:

• Political support
o Policies / tax regimes to encourage renewable energy and particularly PER.
o Strategic direction to avoid fragmented and scattergun approach.
o Support at DoT level, with PER targets to contribute to national transport emissions targets.
• Legislation support
o Policies to be supported by legislation for successful implementation;
o Long term, corporate Power Purchase Agreements.
o Support for renewable energy projects to cover high capital costs.
o Support for Road Administrations as energy generation is not their ‘day job’.
• Communications and information sharing

Forward thinking road administrations could use the definitions in the report to firstly assess their energy needs, then set targets for achieving PER and/or offsetting emissions from their activities as part of their contribution towards climate targets. These should be short and medium-term targets to stimulate change. Such actions would stimulate demand for renewable technology, give investors’ confidence and ultimately lower the cost. Equally, it will stimulate R&D into new and improved renewable technologies. Here, road administrations could provide support to those developing innovative technologies to overcome the ‘valley of death’.

A key success factor will clearly be a positive business case; engagement with government and energy producers could help develop the strategy and the business case. For example, long-term, corporate power purchase agreements have been shown to be an effective tool in securing a win-win arrangement for both the transport administration and the contractor.
The transformational change required to enable high uptake of PER will need international collaboration. Organisations such as the PIARC can play an important enabling role in supporting further work to understand the wider sustainability impacts of different options and develop panels for sharing advice and experience.

The results of the project were presented by Martin Lamb of Maple Consulting at the World Road Congress in Abu Dhabi in early October, whilst the report is available as a free download in English, French and Spanish here


* Launched in 2019, ITEN (International Transport Experts Network) is a group of independent consultants who have worked collaboratively for many years. It was formed to offer clients a wider range of specialism than could be offered by any one individual and to provide greater capacity for delivery of larger projects.