January 20, 2012
Lab Report XVIII
Those who occupy commercial and residential buildings outfitted with photovoltaic panels are a hopeful lot. For some time now they have looked forward to a moment when their PVs will ultimately pay for themselves. Beyond that, they’re looking forward to getting to the point when they can sell back the extra electricity generated to the […]
Those who occupy commercial and residential buildings outfitted with photovoltaic panels are a hopeful lot. For some time now they have looked forward to a moment when their PVs will ultimately pay for themselves. Beyond that, they’re looking forward to getting to the point when they can sell back the extra electricity generated to the local utility. Solar advocates see themselves on the vanguard of promoting an ecologically conscientious way of working and living.
Costs are an obstacle to their dreams. The panels are expensive. Then there’s the issue of incorporating them—both design and installation—into a structure, also expensive. But now two teams at MIT, one from the engineering department and one from the physics department, think they may have discovered how to build a better, cheaper solar trap.
The Joannopoulos Research Group in the physics department believes the road to success is using photonic crystals and tungsten thin films to construct a better solar panel. For this team, innovation comes in the form of metamaterials, or artificial materials that have supra-natural properties. Why are these necessary? Well, there is one big problem with conventional PVs. At a certain temperature, they lose their effectiveness, experiencing re-radiation loss and, therefore, can no longer continue collecting heat from diffuse sunlight. As they explained in one article: “devices that turn sunlight into heat and then into electricity in this way do not get much warmer than boiling water when they are exposed to direct, un-concentrated sunlight. The reason is that at temperatures significantly higher than this the laws of thermodynamics dictate that they shed heat as fast as they absorb it.” That means that the material composition of conventional PV panels has an efficiency rate of just 10.5%.
Sun image via science.nationalgeographic.com, mousetrap image via onpath.com
However, using meta-materials can achieve an efficiency of up to 37% of photovoltaic conversion under sunlight, which, as the authors remind us, is a 250% increase in efficiency! Moreover, this is done without deploying expensive technologies like parabolic mirrors used to concentrate light. That’s because these meta-materials—tungsten thin films and photonic crystals—limit the “radiation exchange” we’ve already described. In short, the team has discovered that “materials that could be employed in fabricating a high-performance design, and suggested a combination of tungsten and silica is optimal.” What’s more, fewer layers of these materials need to be assembled than are normally used in traditional PVs, so they are more efficient and therefore more cost-effective to fabricate.
The other team, engineers at MIT’s NanoEngineering Group also use thin films, but they take a different approach. They’re working with thermoelectric devices that are normally used for capturing waste heat. Remember those meta-materials? Well, this team is focused on electromagnetic meta-materials and photonic crystals. The benefit of these materials is that they have a lower density therefore they can be used to make lighter and smaller PV panels. And, because they have supra-natural properties, the panels perform better. In addition, electromagnetic meta-materials are preferred for their structure, for their ability to convert a wide range of thermal emissions into electrical power. Using electromagnetic meta-materials combined with photonic crystals improves the conversion efficiency of PV panels, and it makes them lighter, smaller, and cheaper to make.
If these two research groups are successful we may, at long last, see the proliferation of better, cheaper PV panels in the near future.
Sherin Wing writes on social issues as well as topics in architecture, urbanism, and design. She is a frequent contributor to Archinect, Architect Magazine and other publications. She is also co-author of The Real Architect’s Handbook. She received her PhD from UCLA. Follow Sherin on Twitter at @xiaying.
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