Promoting energy efficiency
Energy efficiency is a major challenge in underground structures. High energy consumption can have significant financial implications, impacting both the construction and operation of underground structures. Energy consumption can also have a considerable impact on the environment, either directly or indirectly depending on the methods and locations of production (e.g. electricity), or even depending on fossil fuel emissions. This is why, when considering energy efficiency, we must not only consider a simple reduction in energy consumption, but also a reduction in its impact on the environment.
Measures to promote energy efficiency must be implemented at all stages of an underground structure’s life cycle: design, construction and operation.
Use less energy-intensive and low-emission vehicles and modes of transport.
During the design and construction phase, it is important to promote the use of construction machinery and vehicles that are more energy efficient and ‘cleaner’. This implies a certain mastery of construction methods (with optimised travel), appropriate maintenance, and even early renewal of the fleet in order to promote the use of newer machinery and vehicles that consume less fossil fuels or use alternative propulsion technologies. These considerations also apply to transport used in supplying the construction site and removing waste, including excavated materials.
Choosing more energy-efficient equipment and optimising its maintenance
Although not all tunnels are lit or ventilated, these types of equipment consume the most energy, especially in road tunnels.
Regardless of whether the equipment is initial equipment or equipment that is being replaced during the operational phase, careful consideration must be given toits choice. This choice should not be based on a single criterion, but on several: cost, reduced maintenance requirements (particularly important if there is likely to be significant disruption to operations), recyclability, low energy consumption, etc. Energy efficiency is therefore one of the selection criteria, but it should not be the only one.
It should also be noted that energy savings can be achieved through the design and operation of facilities, as well as through the optimisation of maintenance and renewal operations. However, these measures must ensure a consistent level of safety and remain compliant with regulations.
Recovering energy from the ground and producing renewable energy
While it is possible to choose ‘greener’ energy suppliers for a tunnel in operation, with a higher proportion of renewable energy than in a conventional supply, the question of capturing or producing energy underground also arises.
Studies and experiments have been conducted in recent years on the integration of geothermal sensors into tunnel segments. However, questions remain at present about economic balance, energy efficiency, the reliability of the installations, the risk of increased maintenance and disruption to operations.
Other avenues are being explored, such as the recovery of drainage water, whose temperature can also be exploited, or the installation of solar collectors above the tunnel where possible. The energy produced can then be used to power equipment in the tunnel or nearby facilities.
CETU research activities
The development, adoption and integration of technological innovations are a key driver for improving energy performance in tunnels. The CETU itself contributes to the development of certain solutions as part of its research programme.
The CETU also plans to conduct a study to assess the economic and environmental impacts of the choice of road surface type (dark, lightened or light), on lighting design. There is indeed a strong expectation on the part of tunnel owners and operators for information on the potential benefits of a light-coloured road surface used with LED lighting.
The CETU is also conducting tests on the reliability of vehicle detection in tunnels, in order to verify the technical feasibility of activating lighting only when users are present. This provision could only be applied in tunnels with very low traffic.
The latest work on Life Cycle Assessment (LCA) applied to lighting and ventilation equipment in road tunnels has made it possible to quantify their impact on the environment. The results show the significant impact of energy use. They also highlight the importance of ensuring that equipment is properly recycled in order to conserve resources.
The continuation of this CETU research should make it possible to extend the approach to other types of equipment, integrate new equipment technologies, and move towards the development of impact assessment tools for the profession, in order to support the various stakeholders in their choices. The aim is to raise awareness among professionals of the main environmental issues surrounding equipment (resource consumption and energy use, waste management). This work will lead to a CETU publication.
Consideration may also be given to energy production in tunnels, as well as the actual energy consumption of tunnel equipment compared to theoretical consumption based on operating scenarios.
CETU publications
This document is primarily intended for road tunnel operators, but it can also serve as a reference for tunnel owners and design offices responsible for designing new tunnels or studying major equipment renewal projects.
It proposes a list of measures to reduce energy consumption for lighting and ventilation, as well as ways to reduce consumption for other equipment whose consumption is not negligible. The aim is to give operators the means to take short-term action to minimise the environmental impact of tunnel operation, while enabling them to make savings, with equivalent levels of safety and functionality.
To take this further, the document proposes a general approach to be adopted in order to initiate a comprehensive process of reducing a tunnel’s energy consumption in the medium to long term. Such an approach requires a preliminary analysis to identify key areas of consumption in order to prioritise the actions to be implemented. This overview of key areas is essential in order to anticipate as far as possible the measures to be taken to minimise energy consumption and thus pursue a genuine energy efficiency policy in the long term.
Some of the measures presented in this document are immediately applicable, while others are still at the research stage. However, the latter are mentioned, at least for information purposes, as they may guide operators who wish to experiment with new technical measures.
Conference articles related to energy efficiency
‘Life Cycle Analysis of Ventilation and Lighting Equipment in Road Tunnels: Modelling, Results, Ways to Reduce Environmental Impact and Energy Consumption,’ E. CHARLES, M. YAGHZAR, L. D’ALOIA SHWARTZENTRUBER, S. BESSON, J.F. BURKHART (CETU), 22nd Congress on Risk Management and Operational Safety, 2020
‘LED lighting in road tunnels: Simulation of energy-efficient adaptive lighting scenarios’, D. TALON, S. BESSON (CETU), D. DUMORTIER (ENTPE), 30th Quadrennial Session of the CIE, Ljubljana (Slovenia), September 2023
Publications in journals
‘Life cycle analysis of lighting and ventilation in road tunnels’, E. CHARLES, M. YAGHZAR, L. D’ALOIA (CETU), Tunnels et Espace Souterrain journal No. 278, 2021.