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Scientists Simplify Spray-On Solar

Wednesday, December 17, 2014

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Canadian engineers say they have developed a new method of spray-coating solar cells.

The technology could eventually be used to coat roofs, patio furniture and even airplane wings, according to scientists at the University of Toronto.

“My dream is that one day you’ll have two technicians with Ghostbusters backpacks come to your house and spray your roof,” said Dr. Illan Kramer, a post-doctoral fellow in the Edward S. Rogers Sr. Department of Electrical & Computer Engineering.

The university announced the research Dec. 5.

The New System

Kramer and his team call the new system sprayLD—a play on the manufacturing process called ALD, short for atomic layer deposition. During the process, materials are laid down on a surface one atom-thickness at a time, according to the university.

The system involves miniscule light-sensitive materials called colloidal quantum dots (CQDs).

A surface the size of a car’s roof wrapped in a CQD-coated film would produce enough energy to power three 100-watt light bulbs or 24 compact fluorescents, the university says.

Previously, CQDs could be incorporated onto surfaces only through batch processing—an inefficient, slow and expensive assembly-line approach to chemical coating, according to the scientists.

How it Works

SprayLD, on the other hand, blasts a liquid containing CQDs directly onto flexible surfaces like film or plastic, in the same way a newspaper is printed or ink applied to a roll of paper.

solar spray
University of Toronto

“My dream is that one day you’ll have two technicians with Ghostbusters backpacks come to your house and spray your roof,” said Dr. Illan Kramer, who is leading the research.

This roll-to-roll coating method makes incorporating solar cells into existing manufacturing processes much simpler, the team relates.

The team has shown that the sprayLD method can be used on flexible materials without any major loss in solar-cell efficiency.

Parts from Junk

The sprayLD device was constructed with parts that are easily accessible and rather affordable.

Kramer sourced a spray nozzle used in steel mills to cool steel with a fine mist of water and a few regular air brushes from an art store.

“This is something you can build in a Junkyard Wars fashion, which is basically how we did it,” he said. “We think of this as a no-compromise solution for shifting from batch processing to roll-to-roll.”

The team’s research is detailed in two recent papers published in the journals Advanced Materials and Applied Physics Letters.

Related, but separate, research regarding solar cell technology is described in “Scientists Spray Paint Solar Power.”

   

Tagged categories: Air spray; Coatings technology; Photovoltaic coatings; Research; Solar; Solar energy; Spray Paint

Comment from Tom Schwerdt, (12/17/2014, 8:18 AM)

Something is wrong with those numbers, based on a quick BoE calculation. Lets be generous and say that a car rooftop is one square meter. Ballpark daytime average (across 11-12 hours of daylight) solar insolation in the US at 600 watts per square meter for an optimally-angled solar panel. Standard crystalline silica solar panels have an efficiency of something like 15%, meaning we would get around 90 watts from that panel (average across the day.) This guy is apparently claiming 300 watts(!!) per square meter. This is 50% efficiency - better than the best high-end solar panels used exclusively for satellites (where it costs $10,000 or more to lift a kilogram into orbit.) And that's not even considering that a car roof is NOT optimally angled. Those numbers in the example just don't add up. If we're generous and use peak solar insolation (ie - measured at high noon) we can start with maybe 1,000 watts. Use an large SUV roof instead of a car and we have two square meters. Then we park our SUV on a hill tilted due South to optimize that peak noon sun. This gets us to a claimed installed efficiency of 15% - which is plausible, but certainly out-of-the ordinary for a spray-on application.


Comment from Gerald Curtis, (12/17/2014, 10:33 AM)

You certainly cannot argue with Mr. Schwerdt's careful analysis. The spray-application concept is interesting, nonetheless, and for gabled, gambreled, and shed roofs tilted at the correct angle(s) to the sun, would be of considerable interest.


Comment from Mary Chollet, (12/17/2014, 1:57 PM)

We reached out to Dr. Kramer for additional information. While calling the figures "back-of-the-envelope" estimates, he explains: "A Honda Civic(for example)is ~ 1.75 m wide, and if you assume the roof of the car is roughly a square, that translates to about 3 square meters as the area we are talking about(for a relatively small car). In the solar research world, the standard by which we measure and discuss our cells is relative to the global AM 1.5 standard of 1000 W/m^2. With our best solar cells (8.1%), that translates to 243 W (3*1000*0.081). Obviously the average car roof would be bigger than a Honda Civic, so I estimated it would be slightly boosted up from there. The discussion was meant to be aspirational and not a measure of what can be actually achieved now. The point of the research papers and the story was not about how big a car roof is or how bright the sun shines, but that we could make spray-coated solar cells that were quite efficient at all."


Comment from Tom Schwerdt, (12/18/2014, 8:48 AM)

Thanks for reaching out to Dr. Kramer. I don't have a Civic handy, but 1.75M is NOT the roof width, it is the width of the entire car.http://automobiles.honda.com/civic-sedan/specifications.aspx. Mirrors, doors and frames extend the width much beyond the usable area. I don't have a Civic handy, so I have used my Saturn with a nearly identical width as a standin. The Saturn has a notional published width of 1.73m but the actual painted roof size is 1.12m (width) x 1.27m, giving a resulting usable area of 1.42 square meters. So, larger than the low end of my quick BoE - but less than half of Dr. Kramer's optimistic sizing. His email confirms the optimism I presumed he used with solar insolation and angle. With all that, his best cell efficiency is slightly over half my optimistic example. I appreciate that actual efficiency is now provided, even though that is lab cell efficiency and not actual panel-scale efficiency. In the solar research world, it is well understood that lab cell efficiency is less than real-world panel-size efficiency. As an example, First Solar has achieved thin film solar efficiency of 21% in the lab, but their average commercial thin film efficiency is 14%. While roll-to-roll is interesting, Nanosolar had successful roll-to-roll commercial production years ago, at a notably higher efficiency - but they still went under. Perhaps the simplified application method presented here will be enough to overcome the issue of low efficiency. Installation costs have started to dominate the economics of solar due to falling panel prices, making either higher efficiency or dramatically reduced installation costs vital for success.


Comment from M. Halliwell, (12/18/2014, 10:52 AM)

Tom, I'm not going to argue against your take on the roof size and the math involved...but if you can get the hood, roof and trunk going and this coating performs as efficiently (both in cost and in power conversion) as Dr. Kramer indicates it could, it means there is some definite potential here. With more and more electric vehicles and hybrid vehicles out there, this could be the start of reducing the use of the grid or fuel to recharge the on-board batteries. Sure, it's not going to eliminate the need overnight....but if it can be converted to a commercial process that delivers as expected, it would be a decent step forward. IF there is a good step forward in cell efficiency, who knows....it could be that you launch an electric vehicle from the factory and then only have to plug it in as an exception or if you are in northern climates where there is less light. Pipe dream, perhaps...but it sounds like the stepping stones are there to eventually make it a reality. Cheers, Mike.


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