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Intelligent solutions for energy efficient tunnel lighting

Control systems and LED luminaires can improve the energy efficiency of tunnel lighting solutions while offering better visibility to increase the feeling of safety in tunnel environments.

Intelligent solutions for energy efficient tunnel lighting

The tunnel lighting environment has evolved significantly in recent years thanks to LED technology and control systems. Despite these recent evolutions, the vast majority of tunnels are still lit today by 100% HID solutions which consume a lot of energy; a total power consumption of more than 100kW for a 300m long tunnel!
We will illustrate how tunnel lighting can be more efficient, using control systems, better lamp lifetime management and LED lighting.


We consider that there are 5 areas where savings can be made in tunnel lighting while increasing efficiency:

  1. develop a precise management of the lighting stages
  2. increase the number of lighting stages
  3. adjust the levels according to traffic speed
  4. integrate control systems
  5. use LED technology.


The tools to improve the efficiency of a tunnel lighting design must be defined before elaborating the first tunnel lighting design.

 

1. Lighting stages management
In most tunnels, 4 different lighting stages are used in the entrance zone.
In table 1 attached, we propose 3 alternatives for managing the different lighting stages.
The “Simple” lighting stage is the typical lighting stage management; the same luminaire is switched on in each lighting stage. Therefore the light source of the luminaires in circuit 1 (C1) reaches the end of its life before the other light sources.
The “Dimming” lighting stage can be implemented with a simple control system using not only ON/OFF but also 50% dimming to optimise the replacement of the light sources and installation costs.
The third alternative is the optimum solution because the management system can choose several groups of luminaires to light the same lighting stage.
For HID solutions, the period between the lamp replacement is extended from 3 years to 8 - 10 years, significantly reducing maintenance costs. The maintenance lifetime of an HID solution can thus reach the maintenance lifetime of the LED solution.
These three alternatives for managing the different lighting stages are not the only ones, other solutions do exist.


2. Closer to the CIE 88 curve
A tunnel is always over lit in the entrance zone. With a better use of the CIE 88: 2004 curve, taking into consideration the lighting levels outside, the losses due to this excess lighting can be reduced. These savings can reach up to 20% by increasing, for example, the number of lighting stages from 4 to 8.

In a tunnel with 4 lighting stages, if the luminance in the access zone increases to 60% of the L20 [Figure 1a attached], the management system will switch on the 100% lighting stage.
In this case, the power is 40% higher than needed.
If we add, for example, a 75% lighting stage [Figure 1b attached], the losses will be reduced (15% instead of 40%).
With a 100% LED installation in the entrance zone, the lighting could be permanently closer to the CIE curve 88.


3. Adjusting the levels to traffic speed
Traffic speed is a critical element to determine the luminance level needed at the tunnel entrance. The speed criterion should be taken into account in the tunnel lighting design to generate the first energy savings.
In the case of an urban tunnel, by measuring the speed of traffic at rush hour, we can further adapt the levels at the entrance of the tunnel. Energy savings of 5 to 10% can be achieved this way.


4. Control systems
A flexible control system makes it possible to easily integrate extra features like adapting the lighting according to traffic density, pollution environment adjustment, dynamic lighting evacuation systems and more. Monitoring and reporting the energy consumption and failures of luminaires, meters or other systems, provides tunnel managers with additional information and visibility concerning the network.


5. LED technology
While in the past, the entire tunnels was lit with HID luminaires, today we have moved towards HID or LED solutions for the entrance and almost completely to LED technology for the interior zone, where the requirements are very different.

The entrance requires 150 - 350cd/m², depending on the country, type of tunnel, orientation, speed, etc. With a HID solution, sources of up to 400W were needed for the entrance zone.
Nowadays, the luminous flux of LEDs can compete with these high-power HID sources. However, the ease of adaptation of the LED photometry to the tunnel’s geometry ensures that LED installations reach the same results with lower power consumption.

For the interior zone, we only need 2 lighting stages: day and night. In this zone it is easy to move completely towards an LED solution because the requirements are much lower than the entrance zone, only 1 - 10cd/m².

The long lifetime of an LED installation is another advantage. Without any management, we can consider 12 years with no light source replacement.

 

HID, Hybrid ou LED lighting solution?
To fully analyse the most efficient solution, we have compared 3 types of installation:

  • 100% HID
  • Hybrid (a combination of LED and HID)
  • 100% LED.

For each installation, 2 types of tunnel were analysed:

  • Luminance threshold (Lth) of 150cd/m² for a length of 220m and
  • Luminance threshold (Lth) of 350cd/m² for a length of 350m.


The chosen tunnel is just long enough to ensure that we reach the end of the entrance zone. The interior zone is repeated along the tunnel. Four daytime lighting stages were taken into consideration: 100%, 50%, 25%, 12.5%; and one night time level: 33% of the basic daytime level.

For the 100% HID installation, the luminaires are equipped with high-pressure sodium (HPS) and bi-power ballasts. The interior zone is lit by HPS or fluorescent lamps.
For the Hybrid installation, the interior zone and the first 2 lighting stages are lit by LEDs. The remaining lighting stages are with luminaires fitted with HPS lamps. The LED luminaires can be dimmed to switch between interior and lighting stages.
For the 100% LED installation, all fixtures are equipped with LEDs and can be dimmed.

The table below shows the results of the different lighting solutions.

 

100% HID installation

Hybrid installation

100% LED installation

Energy savings per year compared to HID installation

n/a 

15 - 20%

20 - 25%

Maintenance: lamp replacement

Every 3 years

Every 12 years

Every 12 years

Quantity of luminaires installed compared to HID installation

n/a

5 - 10% more luminaires 

10 - 15% more luminaires 

Conclusion

Even more than in road lighting, lighting management is necessary in tunnel lighting. Simple lighting management makes it possible to obtain interesting savings, even with HID.
To obtain efficient solutions, which can provide significant energy savings and lower maintenance costs, LED luminaires combined with dimming options are necessary.
The improvements in LEDs and drivers means that they now offer a high lumen package to cover all the tunnel applications needs.
Moreover, the white light provided by an LED solution (due to a much higher CRI than HPS lamps) offers better visibility when driving and improves the overall feeling of safety in tunnel conditions.

 

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