Wouldn’t it be great if somebody invented a free source of power that anyone could use to power their lives? Ideally it would be an entity of pure energy positioned in the sky so we could all make use of it. If you made it big enough and powerful enough you could even put it 150,000,000 km’s away.
Fortunately for humans, the laws of physics have done the job for us and the bright, hot, skin bronzing power plant we call the Sun has been powering our planet since its fusion switch was first flipped on around 5 billion years ago. Unfortunately, we humans really suck at taking advantage of it.
Give a plant a steady stream of sunlight and it will work wonders. Plants can take the particles of light given off by the sun (photons) and directly use them as food. Photons power plant growth as well as photosynthesis, which is something all of us air-breathers should be thankful for. Since every food chain on the planet begins with plants, it is easy to see how the whole notion of “food” is based on transferring the sun’s energy from one body to another. Compared to plants, we humans are solar sissies.
The trouble with people is that having enough energy to power our bodies just doesn’t cut it anymore. We also need to power our houses, cars, computers, iPods, robot vacuums, dancing Santa’s and all the other things that modern life would be impossible without. It turns out that coaxing the sun into juicing our electronics for us is trickier than it might at first seem.
Conventional solar cells do a remarkably bad job turning photons into useful energy. Your average solar panel has an efficiency of about 20% - meaning that only 20% of the energy input (light) is converted into useful output. The way they work is complicated but basically you take some semi-conductive material (usually silicon (Si)), form it into a sheet, wire it up, and place it in the sun. Photons excite the electrons in the material, knocking them loose and sending them off to do useful things. To get electrons to do what you want, you need to create a current for them to flow along.
Where you will run into trouble is primarily in 2 places: the nature of light itself, and willfully moving electrons. As anyone who has ever seen a rainbow can tell, light comes in different wavelengths – each represented by a different colour. Different wavelengths have different amounts of energy. Only a small proportion of the Sun’s light has the exact amount of energy needed to excite electrons in a given material (ex. silicon). If a photon doesn’t have enough energy, it won’t knock any electrons loose. If it has too much energy, it uses just the amount is needs to nudge an electron and the rest is wasted. In the end, you lose about 70% of the possible energy in a beam of light to this problem. Even when you manage to get your electrons going and try to move them from one place to another, you lose energy if the distance is too far or if they are flowing through something that isn’t a very good conductor.
The upshot of these limitations is that you usually need a whole lot of solar panels if you want to power anything beyond a calculator with reliability. Recently, though, there has been some cause for optimism. Researchers at Caltech led by Dr. Harry A.Atwater have created a commercially viable solar panel that is up to 50% efficient. That still may not sound great, but it represents a massive leap forward in solar energy.
Dr. Atwater and his team, working under the company name Alta Devices, have done away with silicon and instead build their cells from gallium arsenide (GaAs). The researchers argue that this material builds purer crystals than silicon (increasing efficiency), absorbs more photons, and produces more energy than conventional solar panels. On top of that, they have engineered their devices to recycle unused photons and be 1/40 the thickness of a human hair, making them doublely useful and ridiculously flexible. A few other innovations offer promise in creating more efficient solar cells including organic polymers and dye-sensitized materials that can absorb more photons.
Arun Das, PhD student at the University of Waterloo’s Autonomous Vehicles Lab, told Sketchy Science that, “low weight solar cells are very exciting. We would no longer be constrained to using [solar panels] on rooftops and solar farms. We could soon see solar cells as a viable option for charging our smartphones, extending the range of electric cars, and powering small unmanned rescue drones in disaster scenarios.”
As you can imagine, the impact all this would have on fossil fuel emissions would be immense. But that pales in comparison to the benefits of being able to watch The Biggest Loser in the middle of the woods.
Sketchy Science is a weekly blog written by Steve Kux and illustrated by Geoff Lee. Sketchy Science articles relating to sustainability and conservation are published in partnership with The Starfish.