By BYME Engineering (Hong Kong) Ltd., the Civil Engineering and Development Department, Hyder-Meinhardt Joint Venture

Light Emitting Diodes (LEDs) have been widely adopted in our life: office lightings, architectural lightings, road lightings, and now — tunnel lightings.

There is no doubt that LED lightings are more energy-saving, with a longer lifetime and better illumination performance compared to traditional lamp sources (such as fluorescent tubes or compact fluorescent lamps). LED lamps can achieve better lighting performance with less power consumption, which is one of the reasons why, for road lighting, the HKSAR Government is replacing high-pressure sodium lamps with LED lamps.

Blue light pollution is a vexed issue, since it can disrupt sleep quality and the human circadian rhythm. The potential blue light hazard to the human eyes is also another concern, particularly with prolonged exposure and limited short distance between the eyes and the lamp source. In cases where tunnels are concerned, however, these two concerns could be passed over because we do not expect a long exposure within the tunnel and too close a distance to the luminaires.

Project background

The Trunk Road T2 and Cha Kwo Ling Tunnel Project¹ comprises a dual two-lane sub-sea tunnel of approximately 3.4 km. The tunnel lighting system comprises two kinds of lighting, which are (i) reinforcement lighting and (ii) base lighting.

Design revolution of LED lighting: Point source design

Reinforcement lighting is used to boost the illumination level at the tunnel entrances. By providing a brighter environment and preventing “black hole effect”, it aims to avoid driving accidents. Reinforcement lighting will also be provided at the tunnel exits, as a bi-directional traffic arrangement will be adopted according to operational needs or during emergency cases. The exit of the tunnel will serve as one of the tunnel entrances and the reinforcement lighting serves the same purpose. Traditionally, the lamp source for reinforcement lighting is a high-pressure sodium lamp.

An artist’s impression of the west portal of Trunk Road T2 and Cha Kwo Ling Tunnel

Base lighting serves as general illumination for the entire tunnel. For existing tunnel lightings in Hong Kong, T5 fluorescent tube with 54 W power is adopted traditionally, and the lightings are arranged in a continuous pattern. To conform with the Fire Services Department’s requirements, base lighting also serves as emergency lighting. One out of five base light fittings will be backed up by an Uninterrupted Power System (UPS) to maintain the minimum requirement for illuminance level in traditional design.

For the Trunk Road T2 and Cha Kwo Ling Tunnel Project, LED lighting concepts had been introduced into the tunnel lighting design for reinforcement lighting during the tender stage. Despite the initial costs for LED lightings being higher than traditional lightings, it is calculated that savings on maintenance and running costs will outrun the initial capital costs. Therefore, LED reinforcement lightings had been proposed as an efficient engineering proposal.

To further enhance and enrich the benefits brought about by LED lighting, a study involving the replacement of T5 type fluorescent tube base lighting with LED type base lighting was conducted during the project design development stage. With considerations of the benefits of LED lighting’s wide distribution angle, a new design concept “point source design” of base lighting, with a separation distance of 8.9 m edge-to-edge (10.9 m centre-to-centre), has been investigated. References have been made to other worldwide tunnel projects - for instance, France and Italy. These countries had previously adopted LED point source design from 2012 onwards - in the projects “Marseille Vieux Port Tunnel”² and “Valnerina Tunnel”³ respectively. The spacing between light fittings was 4 m in Marseille Vieux Port Tunnel and 5 m in Valnerina Tunnel. In Australia, the North Connex Tunnel4, which was completed in 2020, adopted point source design with 15 m spacing between light fittings. West Connex Tunnel5 also uses the same design concept and is now under construction and targeted to be opened to the public in 2023.

Although the larger spacing of light fittings could minimise the operation and maintenance costs, because of the heavy traffic density in Hong Kong, the light fittings were designed with an additional “one lighting failure” scenario. The 8.9 m edge-to-edge spacing arrangement was eventually adopted in Trunk Road T2 and Cha Kwo Ling Tunnel Project.

Point source tunnel lighting design in Sluiskil Tunnel, the Netherlands

To make the design application successful, the transversal and longitudinal illumination coverage areas for the LED lights need to be determined. By calculation, the distribution of each LED lighting should cover approximately 4.5 m in the longitudinal direction. In addition, the luminance level of tunnel walls up to a height of 2 m shall not be less than that of the road surface as per Public Lighting Design Manual (PLDM)6 requirement. This constitutes a challenge in point source design (although not a problem with the traditional continuous installation method) because dispersion is a characteristic of LED’s distribution. After repeated research and studies with the manufacturer, LED optics with a specific design for limiting and concentrating the distribution angle have been developed. The design yields a wider distribution angle compared to T5 fluorescent tube in both transversal and longitudinal directions.

Note: RED – Transversal light distribution; BLUE – Longitudinal light distribution

Heat dissipation was another challenge during design. High temperatures could reduce the lifespan of LED chips. To ensure the longest expected lifespan of LED chips, the optimal heat dissipation must be taken into consideration in the light fixture. In-house tests have been conducted and it was concluded that fixing the LED module directly on an aluminum plate with fins designed light fixture was the best solution. The fins design increases the contact surface area, thereby enhancing heat dissipation by ventilation. The LED module’s surface temperature becomes steady at 65°C after 180 minutes of operation. A test in accordance with the standard IES TM-21 has been done to determine the lifespan of LED chips under high temperatures. It is a projection test of the lumen maintenance of an LED source based on data collected according to IES LM-80-207. The test results concluded that the LED chips could operate continuously for at least 150,000 hours under 105°C.

Flickering frequency should also be considered for point source design. Reference has been made to the PLDM6 published by the Highways Department. The flickering frequency of 2.5 Hz to 15 Hz should be avoided by the calculation formula

This is the most critical factor in determining the separation distance between light fittings. The design of tunnel lighting systems should also fully comply with PLDM requirements, for instance, uniformity and glare index. Therefore, the designed separation distance would be affected by these factors and should be adjusted to obtain the most optimum distance, which is 8.9 m edge-to-edge, in Trunk Road T2 and Cha Kwo Ling Tunnel Project. The results were supported by detailed computer simulations for the entire tunnel.

Design enhancement to road safety

Concerns have been raised by government departments on road safety. Should the point source design be adopted, there might be the occurrence of approximately 22 m of “dark zone” in case of power failure. To tackle this situation, the power circuit for all base lightings will be provided with 30-minute UPS for emergency lighting and 6.5-hour generator backup. Should there be a power failure, all the base lightings will remain continuously lit up and will not be disrupted. This design is more beneficial to road safety and more reliable than the traditional T5 type fluorescent tube design, where only one out of five light fittings can serve as emergency lighting. In the case of power failure, nearly 80% of lighting will suddenly fade out and only be resumed after 5 to 6 seconds when the electrical system changeover is completed.

On-site mockup installation (8.9 m spacing)

Apart from power failure, tripping off the power supply will also cause the whole light fitting to switch off. To improve this, the dual-ballast design has been adopted for the 136 W LED type base lighting. Two ballasts are installed in each light fitting, while each ballast supplies power to a “half” LED light section of approximately 68 W. Two power circuits with different power sources will be arranged for each 136 W LED base lighting. In total, four different power sources will be provided for the whole tunnel base lighting, with an alternative connection arrangement. In case of tripping of any one power source, at least 75% of the lighting will be maintained. Group lighting installation, which comprises two 68 W LED base lighting, will be adopted for two lane installation. Each 68 W fitting consists of one ballast and the same circuit arrangement.

A study using computer simulation had been conducted to investigate the effects if one light fitting fails to operate. It is found that the average luminance level for the failure of one LED light fitting would not be worse than one T5 fluorescent failure. Under emergency mode, the average lux level for LED design would provide more than 100 Lux, whereas, by contrast, T5 design only has a maximum of 30 Lux.

Automatic lighting control system

The illuminance inside the tunnel will vary, from time to time, with the degree of brightness outside the tunnel. An intelligent lighting control system with a combination of luminance meter and lux meter was designed. The luminance meter will be installed outside the tunnel 120 m away from the entrance for uni-directional traffic mode measurement. The measured value will be transmitted through the Digital Addressable Lighting Interface (DALI) system to Central Monitor Control System. Lightings inside the tunnel will be dimmed to corresponding lighting stages. The lux meter will then measure the illuminance inside the tunnel after dimming and make corresponding checking to the value measured by the luminance meter. According to PLDM, at least SIX lighting stages and ONE emergency mode shall be provided.

Testing and commissioning for tunnel lighting system

To verify the computer simulation results, actual site measurements for the LED lighting performance will be conducted after completion of installation including builder’s work with wall cladding and road finishing. The measurements will follow the European standard BS EN 13201-3 and the measurement height of meter will be set at 1,500 mm above finish floor level to simulate drivers’ vision. Each driving lane will be divided into 1.5 m (W) x 3.0 m (L) grids for measurement as shown in the figure below.

Measurement setup according to BS EN 13201-3

Actual measurements will be conducted throughout the entire tunnel, with uni-directional traffic mode and bi-directional traffic mode. The average illuminance value from road surface and cladding wall at 2 m above road surface will be recorded and the results will be crosschecked with designed values. Repeated measurements will be performed for the six lighting stages and under emergency mode.

Conclusion

A further study to improve the longitudinal distribution of LED lighting is worth undertaking. The wider the distribution angle in longitudinal direction, the fewer the lightings could be installed—and therefore, the more savings could be achieved. LED point source design will no doubt become the mainstream in the future, including tunnel lighting renovation work for the existing tunnel. The renovation works involve more challenges since the existing tunnel lighting system is already installed at the tunnel’s centre. How the new LED lighting could be installed without affecting the commute? What is the sequence for dismantling the existing lightings and installing the new lightings? These questions should be solved carefully before execution.

About the authors

Three parties contributed to this article’s authorship: From BYME Engineering (HK) Ltd., the co-authors are Ir Lee Hau Ming (Director), Mr Law Ting Sum Roy (Manager), and Ir Cheng Kwok Pui Cicero (Manager); from East Development Office of CEDD, Ir Wong Chi Wai Tommy (Senior Engineer); and from Hyder-Meinhardt Joint Venture, Ir Sung Cheuk Kan Stephen (Senior Resident Engineer) .

References

1. The Civil Engineering and Development Department. Trunk Road T2 and Cha Kwo Ling Tunnel . Available at: https://www.cedd.gov.hk/eng/our-projects/major-projects/index-id-31. html
2. International Database and Gallery of Structures (2022). Marseille Vieux Port Tunnel . Available at: https://structurae.net/en/media/396424-tunnel-du-vieux-port
3. International Database and Gallery of Structures (2018). Valnerina Tunnel . Available at: https://structurae.net/en/structures/valnerina-tunnel 
4. North Connex Tunnel . Available at: https://www.northconnex.com.au/
5. West Connex Tunnel . Available at: https://www.westconnex.com.au/
6. The Highways Department of the HKSAR (2017). Public Lighting Design Manual . Available at: https://www.hyd.gov.hk/en/technical_references/technical_ document/public_lighting_design_manual/index.html
7. Illuminating Engineering Society of North America. IES LM-80-20, Approved Method: Measuring Luminous Flux And Color Maintenance Of LED Packages, Arrays, And Modules .