Solar cells are an increasingly important source of renewable energy, but there is still room for improvement. Stanford engineers have now developed a pyramid-shaped lens that can focus sunlight from any angle onto a solar cell, keeping it capturing energy from sunrise to sunset.
Solar cells work best in direct sunlight, which means many of them only generate a good few hours of power per day. Others maximize their working time by actively moving to follow the Sun across the sky, but this consumes energy and adds mechanical complexity.
For the new study, the Stanford team set out to develop passive technology that could collect sunlight that strikes it from any angle and focus it toward the solar cell below. The end result is an array of inverted pyramid-shaped structures the team calls axially-graded-index (AGILE) lenses, which would form a layer that would replace the protective top surface of a solar cell.
In testing, the AGILE prototypes were able to capture more than 90% of the light that hit its surface, focusing it so that it was three times brighter by the time it hit the solar cell. The team says this system could improve the efficiency of solar cells by allowing them to collect indirect sunlight, as well as increase their output in less than ideal weather and conditions.
AGILE sounds simple enough, but the engineering behind it is quite complicated. Each little pyramid is made up of a stack of different glasses and polymers with different indices of refraction – essentially, each layer bends incoming light to a different degree. The top layer has a low index of refraction to allow light to enter from any angle, but each step down bends it a little more, until it focuses on the solar cell in below. The sides are mirrored to direct any wayward light where it needs to go.
These multiple materials also allow the device to capture a broad spectrum of light, from near ultraviolet to infrared. The team also had to make sure that the materials worked well together, for example, that they expanded under heat at similar speeds so as not to crack the device. Even using multiple materials, the team demonstrated that AGILE can be 3D printed.
The team says the new system could help expand where solar power can be used, reducing both the cost and the land needed. AGILE could even improve the solar cells of spacecraft.
The research was published in the journal Microsystems & Nanoengineering.
Source: Stanford University