Solar cell technology has been moving so fast that two or three years is old hat, let alone 10 or 20. Now consider that scientists at the National Renewable Energy Laboratory are applying 50-year-old methods to bring the high cost of ultra-efficient solar cells down to Earth, and you’re practically treading on ancient burial grounds.
Nevertheless, the NREL team is confident they’re on the right track. If all goes according to plan, those same super-expensive solar panels that bedecked the Mars rovers back in 2003 could be coming to a rooftop near you, at a fraction of the cost.
The Solar Cell Cost Conundrum
Developing a solar cell for the commercial rooftop market is a delicate balancing act between efficiency and cost. With more expensive materials, you get far greater efficiency, but then people like NASA would be your only customers.
On the other end of the scale, you can’t go too low on the efficiency side for the sake of cutting costs, because then you’ll need a wider area for your solar panels. So, your market will be limited to bigger, stronger roofs.
A wider area will also bump up all kinds of other costs. That includes manufacturing and transportation as well as hardware, labor and other costs related to installation. In other words, it’s a wash.
Why Solar Cell Technology From Mars?
That’s where the photovoltaic panels on the Mars rovers come into the picture.
The NREL team was intrigued by the III-V solar cell technology used for the 2003 rovers, which were based on indium and gallium. These elements are on the third and fifth columns of the periodic table of elements, and that explains why this type of solar cell is called III-V.
The III-V solar panels sported by the 2003 rovers had a high conversion efficiency of 27%, but they also cost a fortune.
How much? Well, former President Obama’s “SunShot” goal was to bring the average cost of utility scale photovoltaic technology down to $1.00 per watt, and the rover-type panels came in between $100.00 and $300.00 per watt.
So, no.
The interesting thing is that the US already made the $1.00 per watt goal back in 2017, so why are we still trying to push past that mark?
That’s a good question. CleanTechnica is reaching out to NREL for some insights on that score, so stay tuned for followup.
How Low Can Solar Go?
In the meantime, here’s one answer: because we can, that’s why.
The NREL team skipped over the conventional method for making III-V solar cells, which is very precise and time-consuming, therefore expensive.
Instead, they took a look at a method called hydride vapor-phase epitaxy (HVPE), which fell out of favor back in the 1960s.
Part of the reason people stopped using HPVE was because it did a really poor job of fabricating complex structures with multiple layers, and the whole process was a cumbersome routine of swapping layers and chemicals in and out of the same chamber.
The NREL team redesigned HPVE to use two chambers. That chopped the time for making a III-V solar cell down to two minutes, compared to one to two hours for a single chamber.
NREL senior scientist Aaron Ptak explains the initial mindset for the method, now re-named D-HPVE (the D is for Dynamic):
When we started this program, we thought, ‘OK, we’re going to make cheap solar cells and they’re going to be kind of bargain-basement solar cells, but they’re going to be good.’
As often happens in science, the results exceeded expectations. So far, the team has deployed D-HPVE to make a III-V solar cell with a conversion efficiency of 25.3%.
That compares favorably with the 28.8% mark achieved for the same solar cell, but using the conventional (and more expensive) fabrication method. Here’s Ptak again on that score:
What we have learned during the course of this project is we need to shoot higher. We can shoot higher because the material quality that we’re seeing, the device quality that we’re seeing, is way better than we expected.
So, how low can solar go? Mass-produced, NREL’s III-V solar cell could get down to 80 cents per watt, and possibly even 20 cents.
Go back up to that solar cell cost balancing act, and you can see where this is going. Greater efficiency translates into smaller area and lighter weight, and that expands your rooftop market into new territories.
Don’t get too excited! There are many (many) steps before that new D-HPVE factory starts churning out high efficiency III-V solar cells at rock bottom prices.
In the meantime, though, the lab anticipates that getting costs down to the $70-to-$100 range would be low enough to open up more specialty applications on Earth.
The US Department of Defense is one customer that comes to mind. It is already all over PV tech, and not just utility scale PV. It is also pursuing portable and transportable systems for use in the field.
If you can think up some others, drop us a note in the comment thread.
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Image: 2003 Mars rover with PV panels via NASA.
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