Human beings tend to associate tunnels with transit into the unknown. When we go down to the prosaic world of infrastructures, this transcendental concept is not only maintained, but also acquires an economic, energy, environmental and, of course, security impact of colossal proportions.
Entering a tunnel means entering absolute darkness, so its lighting is a challenge of the first magnitude that the driver does not even imagine when approaching those illuminated mouths that lead us into the depths of the mountains.
Injecting sunlight into the tunnels is a bold proposition, and it’s well underway.
Researchers from the University of Granada and the University of Seville have designed an innovative model to inject sunlight into highway tunnels: it is a coupled system in which the capture of the luminous flux is achieved thanks to light collectors (which work as drains) located on the outside, before the tunnel entrance, and its guidance is carried out through lumiducts at ground level or buried under the shoulders, to the inside of the tube, where an opening mechanism and distribution of the luminous flux projects the light towards the vault, in which a new surface with a complex geometry achieves a homogeneous distribution of lighting on the tunnel road.
Elsevier, CC BY
But why is tunnel lighting and the use of sunlight so relevant?
Danger, death trap
When talking about transport infrastructure such as roads, bridges or tunnels, we start from a premise: its users cannot lose their lives due to poor visual perception.
Accidents in tunnels are more serious because an exit from the road leads to a collision with a concrete wall and, as if that were not enough, they become deadly traps in the event of a fire. Therefore, the visual and psychological demands are very high.
These two reasons are not independent of each other, but the first is usually a consequence of the second. For all this, the quality of tunnel lighting is a fundamental factor on which our reaction to any dangerous situation depends.
More light is needed during the day than at night
We start with a paradox: in tunnels, drivers require much higher lighting levels during the day than at night. We talk about lighting levels although in reality we work with a physical quantity called luminance.
This paradox responds to the characteristics of the retinal cells that transform light into nerve impulses and that our brain interprets as visual sensations. They are of three types: cones, which work at high light, rods, at low light, and ipRGC cells, about which much remains to be learned and which channel non-visual pathways such as circadian rhythms, sleepiness, stress, etc.
During daytime driving, our brain receives information from the cones, while the rods remain saturated because they cannot work with so much light. However, when moving to dimly lit environments, the brain processes signals from cones and rods. Upon entering the tunnel, the cones do not work because they need a lot of light and the rods are saturated. It is clear that a driver who travels 28 meters every second cannot afford this very slow visual adaptation. For this reason, during the day it is necessary to provide high luminance, especially in the first 100-120 meters of the tunnel (called the threshold zone) so that the cones can function as outside. This entails high energy consumption, projectors, cabling and auxiliary devices, costly maintenance, recycling, etc.
The question is whether we could make tunnel lighting more sustainable without compromising safety. The answer is yes.
We have different strategies to achieve this.
Reduce light needs
We can place elements that reflect little sunlight around the tunnel entrance portal, such as vegetation or even photovoltaic panels, which can also power beacon or emergency devices for the tunnel itself. Thus the exterior light decreases and the driver’s adaptation to the interior requires less light. It has also been proposed to reduce the maximum speed of circulation below 100 km/h as this is proportional to the luminance required inside.
Elsevier, Author provided
Taking advantage of sunlight: an injection of energy
It is also possible to move the threshold zone outside the tunnel by means of structures that allow the passage of a certain amount of light, or by injecting it inside by means of optical systems such as periscope-type light guides (light ducts), diffusers, etc.
The first option consists of installing pergolas or semi-transparent structures at the tunnel entrance as a lighting complement to the projectors. Although the savings are notable, the lengthening of the tunnel with the consequent danger, the possible glare and the lack of uniformity at certain times of the day are drawbacks to consider.
International Journal of Photoenergy, CC BY
Finally, sunlight can enter tunnels through more or less sophisticated optical systems without the need to lengthen them. In the past decade, a tunnel in Huashuyan (China) incorporated a solar light distribution system with fiber optic light ducts.
The dilemmas of the injection of sunlight
One problem with injection is its high cost as it requires tunnels with a non-standard height (gauge). In addition, the Sun is not permanently aligned with the entrance of the light ducts, which drastically reduces the luminous flux captured and causes reflections that lower performance even more.
To overcome these drawbacks of the injection of sunlight, it has been considered to install external heliostats that capture the rays and inject them into the light ducts in optimal conditions, a solution that is not without problems.
The novelty of the system that we have just proposed for injecting sunlight into tunnels is that it couples collectors, light ducts and a distributor vault. The collectors, located on the sides of the road, capture the light and take it to some light ducts with a set of lenses inside. Finally, a vault with special reflective properties projects it onto the road. The advantage, compared to other systems described, is that it offers the possibility of basing the light ducts on the ground without suspending them from a height. Thus, no special gauge is required and they can be installed in existing tunnels.
The enormous impact of tunnel lighting on safety and sustainability leads us to look for solutions from which we are gradually beginning to benefit and whose prospects are truly promising.