A team of researchers from NTU (Nanyang Technological UNIVERSITY) in Singapore has designed an "intelligent" device to collect sunlight and transmit it to underground spaces, reducing the need to draw on traditional energy sources for lighting.
In Singapore, the authorities have been studying ways to increase living and productive space for some time. They did it with a huge floor of vertical farms, and evaluated the feasibility of excavating deeper underground to create new spaces for infrastructure, storage and services.
The demand for round-the-clock underground lighting is therefore expected to increase in the future in the Asian state.
Take advantage of sunlight
To develop a sunlight harvesting device that can sustainably meet this need, the NTU team took inspiration from the magnifying glass, which can be used to focus sunlight into one point.
How is the device made?
The researchers used a standard acrylic sphere, a single plastic optical fiber (a type of cable that carries a beam of light from one end to the other), and servo motors.
The device is positioned on the ground and concentrates the sunlight in the acrylic sphere. The system allows parallel rays of light to form a sharp focus on the opposite side. The focused sunlight is then collected at the end of a fiber cable and transported underground.
The servo motors, meanwhile, automatically adjust the position of the end of the fiber cable, to optimize the amount of sunlight that can be received and transported as the sun moves.
Developed by the assistant professor Yoo Seongwoo of the School of Electrical and Electronics Engineering and Dr. Charu Goel, Principal Research Fellow at The Photonics Institute at NTU, the innovation was reported in the scientific journal Solar Energy earlier this month.
An authentic innovation
The device overcomes several limitations of current solar collection technology. In conventional solar concentrators, large curved mirrors are moved by motors to align the mirrors with the sun's rays. The components of these systems are subject to environmental factors such as humidity, increasing the need for maintenance.
The NTU device, however, is designed to use the round shape of the acrylic sphere. This makes it much more compact and easier to manage.
The prototype designed by the researchers weighs 10 kg and has a total height of 50 cm. To protect the sphere from environmental conditions (ultraviolet light, dust, etc.), the researchers built a 3-millimeter-thick transparent “cover” made of polycarbonate.
Lamps for night and day lighting
The lead author of the study, Prof. Charu Goel he thinks it would be useful to integrate the device into existing infrastructure.
The NTU team believes that the device is ideal for mounting like a normal street lamp. It would collect sunlight during the day to illuminate underground spaces and shed light at night using electricity.
In case of rain or overcast skies with little sunlight, an LED bulb installed right next to the emitting end of the fiber cable will automatically light up. This will ensure that the device can illuminate underground spaces throughout the day without interruption.
Sunlight: underground is better than LEDs
In tests in underground environments, NTU researchers found the device's luminous efficacy to be 230 lumens/Watt.
This far exceeds commercially available LED bulbs, which have a typical output of 90 lumens / Watt.
The luminous efficacy of this low-cost device can illuminate parking lots, elevators and underground walkways in densely populated cities with sunlight.
It is also easily scalable: since the light capture capacity of the sphere lens is proportional to its size, it is enough to vary the sphere to vary the power.
