Singapore’s limited land availability is driving local authorities to explore the feasibility of digging deeper underground to create new space for infrastructure. Lighting up these underground spaces round-the-clock in an energy-efficient and sustainable way remains a major challenge.

Researchers at the Nanyang Technological University, Singapore (NTU Singapore) have designed a smart device inspired by the simple magnifying glass to harvest daylight and transmit it to underground spaces, reducing the need for traditional energy sources of lighting.

Using an off-the-shelf acrylic ball, a single plastic optical fibre and computer chip-assisted motors, the team created the device that sits aboveground and – similar to the lens of a magnifying glass – utilises the acrylic ball to act as the solar concentrator, enabling parallel rays of sunlight to form a sharp focus at its opposite side. The focused sunlight is collected into one end of the fibre cable and transported to the other end deployed underground to emit light directly.

The small motors – assisted by computer chips – automatically adjust the position of the fibre's collecting end to optimise the amount of sunlight that can be received and transported as the sun moves across the sky.

This innovative device was developed by assistant professor Yoo Seongwoo from the School of Electrical and Electronics Engineering and Dr Charu Goel, principal research fellow at NTU's The Photonics Institute, and reported in the peer-reviewed scientific journal Solar Energy in March 2021.

Conventional solar concentrators use large, curved mirrors moved by heavy-duty motors to align the mirror dish to the sun. The components in those systems are also exposed to environmental factors such as moisture, increasing maintenance requirements. These limitations have been overcome by the NTU device, which uses the round shape of the acrylic ball, eliminating the need for heavy-duty motors to align with the sun, while also providing a compact solution.

The prototype device weighs 10kg and has a total height of 50cm. A 3mm thick transparent dome-shaped cover made using polycarbonate protects the acrylic ball from environmental conditions.

"Our innovation comprises commercially available off-the-shelf materials, making it potentially very easy to fabricate at scale. Due to space constraints in densely populated cities, we have intentionally designed the daylight harvesting system to be lightweight and compact. This would make it convenient for our device to be incorporated into existing infrastructure in the urban environment," Seongwoo said.

According to the NTU researchers, the device can be mounted as a conventional lamppost aboveground, allowing it to serve two purposes: a device to harvest sunlight in the day to light up underground spaces, and a streetlamp to illuminate the aboveground space at night using electricity.

To ensure that maximum sunlight is collected and transported down the fibre cable throughout the day, the system uses a computer chip-based mechanism to track the sun’s rays.

GPS coordinates of the device location are pre-loaded into the system, allowing it to determine the spot where maximum sunlight should be focused at any given time. Two small motors are then used to automatically adjust the position of the fibre to catch and transport sunlight from the focused spot at one-minute intervals.

During inclement weather when sufficient sunlight cannot be harvested for transporting underground, an LED bulb powered by electricity installed right next to the emitting end of the fibre cable will automatically light up, ensuring the device can illuminate underground spaces throughout the day without interruption.

Experiments in a pitch-black storeroom (to simulate an underground environment) revealed that the device's luminous efficacy – the quantity of visible light produced by a light source using 1 Watt of electrical power – to be at 230 lumens/Watt.

This far exceeds those recorded by commercially available LED bulbs, which have a typical output of 90 lumens/Watt. The quality of the light output of the NTU smart device is also comparable with current commercially available daylight harvesting systems, which are far more costly.

"The luminous efficacy of our low-cost device proves that it is well-suited for low-level lighting applications, like car parks, lifts, and underground walkways in dense cities. It is also easily scalable. Since the light capturing capacity of the ball lens is proportional to its size, we can customise the device to a desired output optical power by replacing it with a bigger or smaller ball," Dr Goel said.

Technolite, a Singapore-based design focused agency specialising in lighting, collaborated in this research study. Managing director Michael Chia said, "It is our privilege and honour to take this innovation journey with NTU. While we have the commercial and application knowledge, NTU’s in-depth know-how from a technical perspective has taken the execution of the project to the next level that is beyond our expectations."

Technolite is now exploring ways to potentially incorporate the smart device or its related concepts into its industrial projects for improved efficiency and sustainability.

Images credit: NTU Singapore

Caption: Assistant Professor Yoo Seongwoo (left) and Dr Charu Goel with their daylight harvesting invention.