Smart solar harvester inspired by a magnifying glass
Researchers from Nanyang Technological University, Singapore (NTU Singapore) have designed a ‘smart’ device to harvest daylight and relay it to underground spaces, reducing the need to draw on traditional energy sources for lighting. Their innovation has been reported in the journal Solar Energy.
In Singapore, authorities are looking at the feasibility of digging deeper underground to create new space for infrastructure, storage and utilities. Demand for round-the-clock underground lighting is therefore expected to rise in the future.
To develop a daylight harvesting device that can sustainably meet this need, the NTU team drew inspiration from the magnifying glass, which can be used to focus sunlight into one point. They used an off-the-shelf acrylic ball, a single plastic optical fibre — a type of cable that carries a beam of light from one end to another — and computer chip-assisted motors.
Automatic positioning to harvest maximum sunlight
The device sits above ground and, just like the lens of a magnifying glass, the acrylic ball acts as the solar concentrator, enabling parallel rays of sunlight to form a sharp focus at its opposite side. The focused sunlight is then collected into one end of a fibre cable and transported along it to the end that is deployed underground. Light is then emitted via the end of the fibre cable directly. At the same time, 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.
To guarantee the device’s automatic positioning capability, pairs of sensors that measure light brightness are also placed around the sunlight collecting end of the fibre cable. Whenever the sensors detect inconsistencies in the light measurements, the small motors automatically activate to adjust the cable’s position until the values on the sensors are the same. This indicates that the fibre is catching the maximum amount of sunlight possible.
During rain or overcast skies when there is inadequate sunlight to be collected and transported underground, an LED bulb powered by electricity, installed right next to the emitting end of the fibre cable, will automatically light up. This ensures that the device can illuminate underground spaces throughout the day without interruption.
The device overcomes several limitations of current solar harvesting technology. In conventional solar concentrators, large, curved mirrors are moved by heavy-duty motors to align the mirror dish to the sun. The components in those systems are also exposed to environmental factors like moisture, increasing maintenance requirements. The NTU device, however, is designed to use the round shape of the acrylic ball, ridding the system of heavy-duty motors to align with the sun, and making it compact.
The researchers’ prototype weighs 10 kg and has a total height of 50 cm. To protect the acrylic ball from environmental conditions (ultraviolet light, dust, etc), the researchers also built a 3 mm-thick, transparent dome-shaped cover using polycarbonate.
“Our innovation comprises commercially available off-the-shelf materials, making it potentially very easy to fabricate at scale,” said Assistant Professor Yoo Seongwoo, lead author of the study. “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.”
The NTU team believes the device is suited to mounting as a conventional lamp post above ground. This would enable the innovation to be used in two ways: a device to harvest sunlight in the day to light up underground spaces and a streetlamp to illuminate above ground at night using electricity.
Performs better than LED bulbs
In experiments in a pitch-black storeroom (to simulate an underground environment), the NTU researchers found the device’s luminous efficacy — the measure of how well a light source produces visible light using 1 W of electrical power — to be at 230 lm/W. This far exceeds those recorded by commercially available LED bulbs, which have a typical output of 90 lm/W. The quality of the light output of the 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,” said Dr Charu Goel, first author of the study. “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.”
Serving as industry collaborator on the study is Technolite, a Singapore-based design focused agency specialising in lighting. Moving forward, the lighting company is exploring ways to potentially incorporate the smart device or its related concepts into its industrial projects for improved efficiency and sustainability.
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