Manhattan has approximately 47,000 buildings with around 10.7 million windows, according to a 2013 estimate from The New York Times.
Now imagine if just 1% -- or 100,700 -- of those windows could generate electricity through transparent photovoltaics.
That's the idea behind solar power windows, and at least two companies are hoping to sell the technology to window manufacturers, saying once installed in a building the technology will pay for itself in about a year.
"If you look at the glass that's manufactured worldwide today, 2% of it is used for solar panels; 80% of it is used in buildings. That's the opportunity," said Suvi Sharma, CEO of solar panel maker Solaria.
Solaria uses existing photovoltaic (PV) cells and slices them into 2.5mm strips. It then sandwiches those thin PV strips between glass layers in a window. "The way human eye works, you don't even notice them," Sharma said.
An additional benefit? As the PV strips absorb light striking a building's window, they reduce the "solar heat gain coefficient"; in other words, the windows reduce the sunlight's effect on a building's internal air temperature and thereby lower air conditioning costs.
Solaria is targeting its technology for windows that will be installed in newly constructed buildings.
Another company, SolarWindow Technologies, is pitching a different form of transparent PV cell technology for new construction, replacement windows and retrofits to existing windows.
SolarWindow is using what it calls organic photovoltaics, which can vary in color and transparency. The company is planning to announce its product in a couple of weeks.
SolarWindow CEO John Conklin said what sets his company's technology apart is its ease of integration. Because it's based on a PV film, it can be adhered to existing windows or incorporated into manufactured products relatively easily.
Depending on the number of south-facing windows, which receive a majority of the sun's light, and the building's location in the U.S., Solaria's technology could provide from 20% to 30% of a skyscraper's energy needs, Conklin said.
Conklin would not disclose exactly which organic material SolarWindow uses.
In 2013, however, Oxford University researchers released the results of a study on how neutral-colored, semi-transparent solar cells made of perovskite could be used in building and car windows to generate electricity. Perovskite is an oxide used in ceramic superconductors.
The Oxford researchers said they could create transparent solar cells with comparatively high efficiencies. For example, the researchers were able to drive PV efficiencies up to 20% in a "remarkably short period of time" using a simple cell architecture.
The university's work is being commercialized by Oxford Photovoltaics (a spin-out company), which is planning to produce attractively colored and semi-transparent glass, which works as a solar cell and could be integrated into the facades of buildings and windows.
Similarly, a team of researchers at Michigan State University (MSU) has developed a new type of transparent solar concentrator that when placed over a window creates solar energy.
Called a transparent luminescent solar concentrator (TLSC), MSU's technology can not only be used on building windows but also on cell phones and any other device that has a clear, uncolored surface.
Richard Lunt of MSU's College of Engineering said the key to the TLSC technology is that it's completely transparent.
"No one wants to sit behind colored glass," Lunt, an assistant professor of chemical engineering and materials science, said in a statement. "It makes for a very colorful environment, like working in a disco. We take an approach where we actually make the luminescent active layer itself transparent."
MSU's solar harvesting technology uses small organic molecules developed by Lunt and his team to absorb specific nonvisible wavelengths of sunlight.
One problem with MSU's technology is that more work is needed to improve its energy efficiency. Currently it is able to produce a solar conversion efficiency close to 1%, but the researchers hope to achieve efficiencies beyond 5% when fully optimized.
Today, traditional solar power panels that reside in solar farms or on building rooftops can achieve a PV efficiency of about 15% to 20%. The efficiency rating refers to how much of the photons striking a solar cell are converted into energy.
Solaria's solar window technology can achieve a solar effiency of about 8% to 10%. SolarWindow's Conklin would not disclose his company's technology efficiency rating, but did say it was less than standard PV panels.
"Obviously when you're looking at absorbing visible light and it's transparent, it's not as efficient as an opaque panel," Conklin said.
When it comes to solar windows, however, efficiency matters less than transparency, Conklin said.
"When you're looking at transparent or clear photovoltaics, it's not necessarily a function of power conversion efficiency as it is about using the vast amount of space available for that tech," Conklin said. "We're making use of the space that right now is not available for solar energy production. Passive windows are turned into active energy generating windows."
In other words, transparent solar PV is about not wasting perfectly good real estate in order to supplement a building's power requirements.
The solar window technologies utilize varying methods of transmitting the energy that the PVs produce to a building's internal power infrastructure.
Solaria, for example, hides its wiring in the window's frame, and the connectors are wired into a newly constructed building's electrical conduits. Those conduits lead to a central power inverter, which converts the solar windows direct current to alternating current that's usable in the electric grid.
SolarWindow's technology can come with micro DC-to-AC power inverters, allowing the electricity to be used only in one room with a solar window. Alternately, it can be connected to a distributed microgrid inverter to power a single floor of a building or to a central inverter from which the entire building can draw power.
Solaria is already piloting its windows in "a few" buildings and it is working on the first large-scale commercial projects in California and Europe, according to Sharma. Sharma did not disclose the projects.
Solaria has also partnered with Tokyo-based Asahi Glass Co., a global glass manufacturer. Sharma said Asahi intends to sell both windows and a bamboo shade called Sudare with embedded solar cells from Solaria.
"So we're enabling different glass and curtain wall providers in North America, Europe and Asia to provide products," Sharma said.
Solar windows will cost about 40% more than conventional windows, but the ROI is achievable in under a year and there's big demand even though products have yet to ship, the manufacturers say.
"It's actually very viable and will be even more viable as we approach our product launch," Conklin said. "It's in very high demand because right now skyscrapers... don't have a good way of offsetting energy through renewable energy generation."