Henry Ford’s famous quote, “Any customer can have a car painted any color that he wants so long as it is black,” could almost apply to today’s solar panels. In order to absorb as much sunlight as possible, photovoltaic cells are dark blue - nearly black. Brighter colors would reflect light away from the cells, in effect rejecting the very energy that they use to produce electricity. Building-integrated photovoltaic (BIPV) products such as solar shingles look like shingles with PV cells inside of them, so a customer is stuck with two options: dark solar panels or no solar panels. Engineering and aesthetics appear to be at odds. (As an engineer who’s married to an artist, I’m well aware of this fact!) But the performance vs. appearance duel may not last, at least in the photovoltaic market, thanks to engineers at Centre Suisse d'Electronique et Microtechnique (CSEM).
Any Color You Like (Even If It’s Not Black)
CSEM has developed a solar panel that can be any color, making it suitable for BIPV applications. Imagine an exterior wall made entirely of PV cells that look just like ordinary building materials. They’ve dubbed it the “white solar module” but white is just the base color; color coatings can be added. Here’s what it looks like:
The key to making this work is remembering that visible light is only a tiny slice of the electromagnetic spectrum, and PV cells can absorb light that humans can’t see. CSEM has chosen to use the infrared part of the spectrum, so they use silicon PV cells that convert IR light to electricity and add a filter that blocks visible light, making the panels appear white to the human eye. A slight modification to the filter will allow it to reflect other colors, making the entire color palette available to architects who want to add PV panels to their buildings.
These panels absorb light in the near infrared part of the spectrum, so they aren’t absorbing the far infrared heat waves. This is important, since PV panels lose efficiency at higher temperatures. CSEM’s white panels operate about 20oC to 30oC cooler than standard PV panels.
In this picture of EQE vs wavelength, EQE is External Quantum Efficiency, a ratio of how many free electrons are created by each photon that enters the cell. Visible light has wavelengths in the 400 - 700 nm range; near IR is just above 700 nm. Heat waves are much longer than 1200 nm.
Better Looking but Less Efficient
Of course, the tradeoff of reflecting away some of the light is that the panels are less efficient. Where standard blue-black PV panels are about 20% efficient, white panels are only 10% efficient. On the other hand, non-photovoltaic walls generate no electricity at all, so something is better than nothing. CSEM expects these panels to cost about $1.50 - $4.00 per watt, depending on the configuration. That’s quite a bit more than the average silicon PV panel today, which goes for about $0.75/watt. Over time we could expect that cost to decrease with better manufacturing methods and more competition, but given that white cells are intentionally blocking some of the usable spectrum, I can’t see them ever being on par with regular PV panels. But remember, these aren’t designed to compete with conventional PV; they’re designed to put PV where it wouldn’t otherwise be desirable.
Do the Math
Let’s do the math for one building on the campus where I teach. It’s a two-story building, about 30m x 60m. If we cover 75% of its rooftop with fixed mounted traditional solar panels facing due south with a tilt equal to the site latitude, the array will generate roughly 300 MWh of electricity each year. Adding white panels to all four sides, leaving room for windows, we could generate another 60 MWh of energy - about a 20% increase. A taller building would fare much better, assuming its sides aren’t heavily shaded. The same calculations on a three-story building gives a 30% increase compared to just a rooftop array.
Using colorful PV siding on a building will have a longer payback period than a simple rooftop array. I’ll leave it to the accountants to do a complete financial analysis, but remember that these panels are not added to an existing building - they ARE the exterior siding material. In other words, if a piece of standard siding costs $10 and the same size of white PV panel costs $15, the extra cost is $5, not $15.
This technology may not be an end in itself, but at least it's a stepping stone towards better BIPV technology.
Here’s CSEM’s video introduction to the white solar panel: