Sunnel

Meet Sunnel, a solar panel who soaks up sunlight and turns about a fifth of it into electricity.

A solar cell is a semiconductor junction. Each photon with enough energy frees an electron, and the junction's built-in electric field pushes the freed electrons one way, creating a current. The efficiency is the electric power out divided by the light power falling on the panel.

On a bright day the irradiance G is about 1000 W/m². A panel with an area of 1.7 m² and an efficiency of 20 percent delivers about 340 W.

Not every photon helps. Infrared photons have too little energy to free an electron. Blue photons have more than enough, and the extra becomes heat. For a single silicon junction this sets a theoretical limit near 33 percent, known as the Shockley–Queisser limit.

Cells stacked with several layers, each tuned to a different colour, beat that limit. Multi-junction cells on spacecraft have passed 30 percent, and laboratory cells under concentrated light have gone above 45 percent.