What if we could turn our urban landscape into a giant solar factory that met all of our nation’s energy needs? The idea isn’t as far fetched as it seems. Photovoltaic glass solar panels are transparent solar panels that can generate energy from windows. Instead of treating skyscraper windows to shed energy, wasting it, sunlight can be harvested to reduce a building’s energy needs. New advancements means the cutting edge technology is closer than ever before.
The single greatest source of energy in the entire solar system is free and accessible to all. Every single second the sun produces the equivalent of 384.6 septillion watts of energy. Learning how to effectively harness even a minuscule fraction of that energy has the potential to unlock a brighter, cleaner future for the energy sector. Researchers at Michigan State University created the first transparent solar panels in 2014, capable of replacing practically any glass sheet, from the one in your smartphone to the one in your sunroof. The new material changes the potential of solar power. Instead of building a sea of solar cells on rooftops and the sunniest spots on earth, our existing buildings, windows and glass surfaces can be converted into energy producers.
The idea seems counter-intuitive. Solar cells capture light. Transparent material lets light pass through. Fundamentally, the two forces work against each other. The researchers found a new way to harness a portion of the solar spectrum invisible to the naked eye. Visible light passses through the organic material while the high energy form of near infrared light (NIR) is harvested. To achieve the separation, researchers used organic salts that absorb specific UV and infrared light wavelengths, creating a glow that is subsequently guided to the edge of the window, where thin PV solar cells convert the small amount of luminescence into electricity.
If that sounds a little convoluted, that’s because it is. The transparent solar panels are nowhere near as efficient as traditional solar cells. This year a team at the University of Michigan achieved 8.1 percent efficiency and 43.3 percent transparency with organic, carbon-based glass that had a slight green tint. In the world of traditional opaque solar panels, researchers in Europe hit an efficiency of 29.15 percent in the perovskite/silicon tandem solar cell category. Researchers at the National Renewable Energy Laboratory (NREL) have developed a new solar cell with an efficiency of 47.1 percent by combining six types of photoactive layers. Conventional, commercially available opaque solar cells range from 20-30 percent efficiency.
It’s the scale of transparent solar panels that has the potential to be transformative. Solar cells don’t necessarily need to be efficient if they have enough scalability. A less efficient panel means the panel needs to be larger in size compared to more efficient panels to generate the same amount of energy—like the size of an entire building. The ability to convert practically any window or glass surface into a solar cell offers an endless list of possibilities for harvesting solar energy, none more enticing than fully converting glass curtain wall skyscrapers into giant solar panels.
“Windows, which are on the face of every building, are an ideal location for organic solar cells because they offer something silicon can’t, which is a combination of very high efficiency and very high visible transparency,” added Stephen Forrest, the Peter A. Franken Distinguished University Professor of Engineering at the University of Michigan, who led the research.
The technology in hand, the problem is now economic. Currently teams are working on improving light utilization efficiency and extending the life of the cells to 10 years. Manufacturing and installation improvements in new and existing buildings are also being explored. To be commercially viable, the cost and installation will need to be amortized in a reasonable time frame. The race to seize this enormous opportunity is on.
Researchers at Michigan, Michigan State, MIT and several manufacturers are working to bring the first transparent solar panels to the mass market. A small caveat to photovoltaic glass is that indoor plants will struggle to survive. Letting only visible light through is sufficient for humans to see but photosynthesis requires the full spectrum of the sun’s rays. Research is still being done to promote plant health by finding the right balance in transparency.
“As we think about building in cities, where we’re building vertically, the rooftop area is so much smaller than the glazed area on the walls of the building,” Ubiquitous Energy CEO Susan Stone told listeners on the Business for Good Podcast with Paul Shapiro. After spending nearly a decade in development, Stone is executing an aggressive go-to-market strategy for Ubiquitous Enegy’s patented transparent solar panels. Stone said in a quick analysis of Salesforce Tower in San Francisco, there’s 700 times more surface area available for transparent solar panels than for traditional panels on the rooftop. Ubiquitous Energy’s best transparent solar cell registers an energy efficiency rating of 9.8 percent.
Even though transparent energy panels may be 2 to 3 times less efficient than opaque panels, they can cover 700 times more surface area in the Salesforce Tower example, more than compensating for the panel’s diminished efficiency over traditional rooftop solar cells.
To get down to brass tax, photovoltaic glass is not cheap. It can’t be retrofitted onto existing glass, so in the commercial real estate market, new construction is the primary target until replacement costs come down. Stone said Ubiquitous Energy’s panels add about 30% to the cost of glass in a new construction project. That cost can be offset by tax credits in many areas. The steep price is also offset by energy production, which depends on building location and size. Stone said energy simulation models ran on two buildings in Asia saw payback periods between 2.5 and 4 years.
Transparent solar panels are not a substitute for traditional opaque panels, they’re compliments. Where higher efficiency transparent solar panels can be used, they should be. Transparent solar panels’ most transformative aspect is the prospect of increasing the scale of solar energy usage exponentially. Experts estimate the U.S. alone has about 16 to 22 billion square feet of glass surfaces. Converting that much glass into transparent solar panels could meet an estimated 40 percent of energy demand in the United States, roughly the same potential as converting every rooftop to a traditional solar panel. Transparent solar sides, traditional opaque rooftops, and improved energy storage could get the United States “close to 100 percent of our demand.”