In the world of solar panels, the general approach has been to improve the efficiency of conversion from the electromagnetic flux to electricity. This is generally attempted in the visible part of the flux spectrum. However, some Michigan State University researchers have decided on a different approach: harvesting from the ultraviolet and infrared parts of the spectrum. What’s the result?
A Michigan State University research team has finally created a truly transparent solar panel — a breakthrough that could soon usher in a world where windows, panes of glass, and even entire buildings could be used to generate solar energy. Until now, solar cells of this kind have been only partially transparent and usually a bit tinted, but these new ones are so clear that they’re practically indistinguishable from a normal pane of glass. …
Versions of previous semi-transparent solar cells that cast light in colored shadows can usually achieve efficiency of around seven percent, but Michigan State’s TLSC is expected to reach a top efficiency of five percent with further testing (currently, the prototype’s efficiency reaches a mere one percent). While numbers like seven and five percent efficiency seem low, houses featuring fully solar windows or buildings created from the organic material could compound that electricity and bring it to a more useful level.
The strategy is to cover everything in this material and beat the efficiency challenge by going around the barrier – put up enough of this glass and you don’t have to worry about the efficiency. You may wonder how efficiency is defined, so I asked Wikipedia:
Solar cell efficiency is the ratio of the electrical output of a solar cell to the incident energy in the form of sunlight. …
By convention, solar cell efficiencies are measured under standard test conditions (STC) unless stated otherwise. STC specifies a temperature of 25 °C and an irradiance (G) of 1000 W/m2 with an air mass 1.5 (AM1.5) spectrum. These conditions correspond to a clear day with sunlight incident upon a sun-facing 37°-tilted surface with the sun at an angle of 41.81° above the horizon.[2][3] This represents solar noon near the spring and autumn equinoxes in the continental United States with surface of the cell aimed directly at the sun. Under these test conditions a solar cell of 20% efficiency with a 100 cm2 ( (10 cm)2 ) surface area would produce 2.0 W.
Incident energy is the amount of energy at all wavelengths encountered at the specific geographic location. If the solar cell is transparent, this implies that the visible wavelengths are not harvested – i.e., that energy is lost. So how much energy is carried on those wavelengths? Windows to the Universe provides an explanation aimed at science teachers, and a nifty chart:
The peak of the Sun’s energy output is actually in the visible light range. This may seem surprising at first, since the visible region of the spectrum spans a fairly narrow range. And what a coincidence, that sunlight should be brightest in the range our eyes are capable of seeing! Coincidence? Perhaps not! Imagine that our species had “grown up” on a planet orbiting a star that gave off most of its energy in the ultraviolet region of the spectrum. Presumably, we would have evolved eyes that could see UV “light”, for light of that sort is what would be most brightly illuminating our planet’s landscapes. The same sort of reasoning would apply to species that evolved on planets orbiting stars that emit most of their energy in the infrared; they would most likely evolve to have IR sensitive eyes. So it seems that our eyes are tuned to the radiation that our star most abundantly emits.
The graph below shows a simplified representation of the energy emissions of the Sun versus the wavelengths of those emissions. The y-axis shows the relative amount of energy emitted at a given wavelength (as compared to a value of “1” for visible light). The x-axis represents different wavelengths of EM radiation. Note that the scale of the y-axis is logarithmic; each tick mark represents a hundred-fold increase in amount of energy as you move upward.
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So now we can understand why 5% is their efficiency goal. With most of the energy unharvested, they have to go for lots of coverage rather than concentrated collection.
This raises two questions for me:
- How will the removal of UV and infrared energy affect the environment? If we install these everywhere, are we going to regret it?
- How will this play with the nascent move towards building with wood? Coating a wood building in this stuff may not be an optimal strategy, technologically nor aesthetically. I have to wonder if this will be anything more than a niche product as the monster concrete (environmentally unsafe) and glass buildings become passé. Or will burgeoning population force the creation of more such buildings, and since this energy collection material can be designed in from the ground up, it’ll become popular?
(h/t Sydney Sweitzer)
