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Harnessing solar energy reaching the earth, via photovoltaics, is becoming a major challenge to meet the ever-increasing demand for energy. The answer must be made in a sustainable and ecological way, in the context of climate change. Unfortunately, the most commonly installed panels have an efficiency between 13 and 24%, and require facing the light source. Recently, researchers at Stanford University designed a new pyramid-shaped optical lens that can efficiently collect and focus light, regardless of the angle of incidence and light intensity, onto a cell. solar. It captures up to 90% of the light and the output light intensity is three times greater than that received. Simple, inexpensive, flexible and scalable manufacturing techniques for their implementation, pave the way for a revolution in the world of photovoltaics and laser technology.
Currently, a photovoltaic panel does not transform all the energy it receives into electricity. In addition to the losses during the process, the performance of the panel depends greatly on its composition, its orientation and its inclination in relation to the light source, the Sun.
Indeed, the theoretical yield limit of a panel is 31%. With amorphous silicon solar panels, the efficiency is usually between 6 and 9%, which is quite low. Polycrystalline panels have an efficiency between 13 and 18%. This is the most commonly used type of panel. Finally, with monocrystalline solar panels, the yield can be 16 to 24%. In addition, a southern orientation and a 30° inclination are the optimal conditions for maximum performance. Also, to capture as much energy as possible, many solar panels actively rotate toward the Sun as it moves across the sky. This makes them more efficient, but also more expensive and complicated to build and maintain, than a stationary system.
In this context, at Stanford University, engineering researcher Nina Vaidya, and her thesis supervisor Olav Solgaard, professor of electrical engineering, have designed a pyramid-shaped optical lens that can focus sunlight under n’ any angle on a solar cell, allowing it to collect energy efficiently throughout the day, even under clouds. Their work is published in the journal Microsystems & Nanoengineering.
AGILE: deceptively simple
The device, which the researchers call AGILE — for Axially Graded Index Lens — is deceptively simple. It looks like an upside down pyramid with the tip cut off. Light enters the square, tiled top from any angle and is channeled downward to create a brighter spot as it exits.
The basic principle behind AGILE is therefore similar to the use of a magnifying glass when used to burn a dry leaf for example, which concentrates the Sun’s rays into a smaller, brighter spot. But with a magnifying glass, the focal point moves like the Sun does. Vaidya and Solgaard found a way to create a lens that catches rays from all angles, but still focuses the light at the same exit position, and without having to move the lens to face the Sun.
They determined that, theoretically, it would be possible to collect and focus scattered light using a material that gradually increases the index of refraction — a property that describes the speed at which light travels through a material — causing light to bend toward a focal point. At the surface of the material (entrance), the light would hardly change direction. By the time it reached the other side (exit), it would be almost vertical and focused.
Solgaard said in a statement: The best solutions are often the simplest ideas. An ideal AGILE has, at the very front, the same refractive index as air and it gradually increases – the light follows a perfectly smooth curve “.
For the prototypes, the researchers layered different glasses and polymers that “bend” light to different degrees, creating what’s called a graded-index material. The layers change the direction of the light in steps instead of a smooth curve like the theory predicted. Nevertheless, the authors consider this design to be the best approximation of the “ideal AGILE”. The sides of the prototypes are mirrored, so any light going in the wrong direction is bounced back out.
Vaidya points out: One of the biggest challenges has been finding and creating the right materials “. Effectively, the layers of material in the AGILE prototype allow a broad spectrum of light to pass through, from near ultraviolet to infrared, and bend that light increasingly outward with a wide range of refractive indices, never before seen in the nature, nor in the industrial perspective. These materials used also had to be compatible with each other — if one glass expanded in response to heat at a different rate than another, the entire device could crack — and strong enough to be shaped and remain sustainable.
In their prototypes, the researchers were able to capture more than 90% of the light that hits the surface and create “spots” at the exit, three times brighter than the incoming light. Installed in a layer above solar cells, they could make solar panels more efficient and capture not only direct sunlight, but also diffuse light from the atmosphere, depending on Earth’s weather conditions and seasons.
The top layer of AGILE could replace the existing encapsulation that protects the solar panels. This design could also eliminate or reduce the need to track the Sun, create space for cooling and circuitry between the shrunken pyramids of individual devices, and most importantly, reduce the amount of surface area needed to generate power — and therefore reduce costs. Vaidya hopes the AGILE lenses can be used in the solar industry and other fields, such as laser coupling, display technologies and solid state lighting (more energy efficient than other methods of ‘lighting).
Also, the uses are not limited to terrestrial solar installations: if applied to solar panels sent into space, an AGILE layer could both concentrate light without solar tracking and provide the necessary protection against radiation.
Finally, Vaidya concludes: Being able to use these new materials, these new manufacturing techniques and this new AGILE concept to create better solar concentrators has been very rewarding. Abundant and affordable clean energy is a critical component to addressing pressing climate and sustainability challenges, and we must catalyze engineered solutions to make it a reality “.