Solar Cells, Quantum Dots and Energy Efficiency

Jonathan Strickland

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This morning, I saw a Facebook post from Dan Bush, the guy who directs the Fw:Thinking videos (hi, Dan!). Dan shared an article about researchers who have developed a new approach to building solar cells. They've worked with colloidal quantum dots, a type of nanoparticle, to make a substance that we could hypothetically add to ink or paint. Then, we could apply this mixture to flexible materials, like roof shingles. Bam! Super cheap, low-profile solar panel city.

It's a great idea -- it could bring the cost of installation down dramatically and bring solar power to a larger customer base. And the tech doesn't stop there -- these particles may play a part in multiple other applications from infrared-emitting diodes to gas sensors. The real breakthrough here is that the researchers found a way to keep the colloidal quantum dots stable when exposed to air.

So will you be painting your roof with this stuff next week? Probably not. There are still practical problems to overcome. You still have to build a system to channel the electricity the colloidal quantum dots generate. And the solar material is only 8 percent efficient, which means more than 90 percent of the energy hitting the cell is lost. Low efficiency is a problem across the board with solar cells. We've talked about it before on the podcast but never really explained what we mean. Here's a breakdown.

You determine a solar cell's efficiency by taking the amount of electricity it generates at peak performance (in watts). You divide that number by the product of the amount of light hitting the cell times the surface area of the cell itself.

We're not talking about measuring a solar panel on any given day -- the amount of sunlight a panel might receive varies greatly by location and time of year. Instead, we use what's called the standard test condition, or STC. This eliminates one of the variables -- the amount of light hitting the cell -- by standardizing it at 1,000 watts per meter squared. So now we can determine any solar cell's efficiency as long as we know its size and how much power it generates with that much light.

Here's an example: You've got a solar cell that generates 500 watts under standard test conditions. The cell is 5 meters square (OK, so it's a big cell). We know that the amount of light hitting the cell is 1,000 watts/meter squared because we're operating under standard test conditions. Doing the math shows that the solar cell operates at 10 percent efficiency.

An 8 percent efficiency means that a square meter of solar cell will produce 80 watts under STC. To apply that to the real world, you need to know the average amount of sunlight energy (in watt-hours per meter squared) you'll get for any particular region. A solar cell near the equator is going to get more uniform sunlight throughout the year than one closer to the north pole.

I expect we'll see improvements in colloidal quantum dot solar panel efficiency over time. Solar panel efficiency in general is something that has improved slowly as the technology has matured. Remember my post about solar roadways? Those panels have a projected efficiency of 15 percent. But because there'd be so many of them covering a huge area, you'd end up with a lot of electricity (if it worked). Even cutting-edge solar panels used by the space industry top out around 44 percent or so. Then there's the theoretical limit to solar panel efficiency -- that's at 86 percent -- but to hit that efficiency would require an infinite number of perfect solar cells.

Okay, so we'll never be able to capture all the energy hitting a solar cell and convert that into electricity. Does that mean solar cells are a dead end? Not at all! The sun has been a source of energy for our planet for billions of years. It's a renewable, clean resource (though producing solar panels can be a dirty business). Again, the application of solar cells is what's important.

The two things we can address with solar cells are the size of the solar arrays and their location. Efficiency is still important but we can balance it out. If you have a large roof that gets a lot of sun, coating the whole thing with an 8-percent-efficient solar panel ink might be better than installing a few expensive panels of solar cells that have higher efficiency. You make it up in volume (OK, you make it up in area, but you get what I mean).

The bottom line is this -- whenever you look at any potential source of energy, it's important to look at the big picture. A couple of facts and figures up close might look promising (or discouraging), but it's possible the whole thing turns around when you step back to get some perspective.