Some thoughts on window-integrated photovoltaics
Yesterday I tweeted about an innovation from MIT that converts infrared (IR) radiation to electricity, allowing for electricity generation from windows that appear transparent to the naked eye. The paper documenting those findings is here.
My good friend Kurt Brown, who I worked with in collaboration to design the recommendation engine for Wattbot, emailed me to tell me why he thought this was a crazy idea. First, he said vertical windows typically get only half of the insolation of a rooftop PV array, so the total available energy for conversion to electricity is much less than for rooftop panels. Second, he notes that PV panel costs are only 50% of the cost of PV installations nowadays, and while this innovation would substantially reduce panel costs, costs for the wiring and voltage controller to turn the electricity collected into useful power would be significant. He also forgot to mention that the energy from IR radiation is less than half of that contained in total solar insolation, which reduces the potential of this innovation to generate power still further.
I always listen to Kurt, particularly when he tells me I’m crazy, because he’s often right. However, I had some good reasons for highlighting the technology, so I encouraged Kurt to “think outside the box”. In this case, the box is that which says that electricity generated by transparent PVs in windows needs to get fed into the electric power system of the building. I think that’s how most folks read the MIT summary of this study, but it’s the wrong framing. The amount of power generated by this innovation will always be small, and Kurt is absolutely right that the costs of interconnecting the windows to feed their tidbit of electricity into the power system of a building would probably be prohibitive.
But what if the electricity from the windows could be used right then and there to do something useful? I thought of three examples, maybe you can come up with more:
1) A self powered skylight: If you put a small battery inside a skylight and use the IR PVs to charge that battery you wouldn’t need to wire the skylight to the conventional power system (that’s a big cost savings right there). It would be a self contained unit that opened and closed under its own power, and I’m sure it would sell like hotcakes.
2) Ventilation in automobiles: One of the big problems in autos is keeping them cool, and while there’s a lot you can do with window coatings, active cooling is almost always necessary in sunny climates. The electricity from window or sunroof based PVs could be used to run a small ventilation fan to cool vehicles in the sun, without draining the battery. This would be a boon for electric vehicles, for which battery storage is at a premium. There are products like this out there now, but none using this particular PV technology.
3) My favorite–Self powered electrochromic glazings: One of the biggest uncontrolled cooling loads for buildings is sunlight, and while coatings can help with this, having a system that can actively control shading would help buildings become much more flexible in their use of natural energy flows. Electrochromic materials can change their transparency depending on what voltage is applied to them. The IR PVs could generate enough power to activate the electrochromic materials and to run a wireless network through which all the windows in a building could be controlled (similar to the wireless sensor networks that are becoming more common nowadays). Electrochromics are now quite expensive but they are used in some commercial products (like sun roofs for high-end vehicles)–they would have to come down in price before this idea could come to fruition. This application would also require a battery, but would avoid the need for wiring, thus reducing installation costs.
After I told Kurt these ideas, he reconsidered his assessment. What do you think?
