This story was first published at Green Building Materials. Reprinted with permission.

Imagine if every building in the world had solar power windows that could generate a small amount of electricity from the sun. The technology exists. It has been tested and it is ready for production. In Manhattan alone, there are 47,000 buildings with over 10,700,000 windows, according to a 2013 estimate from The New York Times.

Transparency Versus Efficiency

The first challenge for researchers is to make the solar power windows transparent. “No one wants to sit behind colored glass,” Richard Lunt, an assistant professor of chemical engineering and materials science at Michigan State University. “It makes for a very colorful environment, like working in a disco.”

Lunt and his team have developed a new type of transparent luminescent solar concentrator (TLSC) that creates solar energy when it is placed over a window. MSU’s technology can not only be used on windows for building but also on cell phones and any other device that has a clear, uncolored surface. “We take an approach where we actually make the luminescent active layer itself transparent,” Lunt says.

The problem for MSU at the moment is that its technology is only 1% efficient. A typical solar panel is 15% efficient or more, which means 15% of all the sunlight that strikes the glass is converted into electricity.

“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,” says Solar Window CEO John Conklin. “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.

Perovskite Is A Game Changer

In 2013, Oxford University researchers did 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 their transparent solar cells can be made up to 20% efficient using a simple cell architecture.

The university’s work is being commercialized by a spin-off company, Oxford Photovoltaics, which is planning to produce colored and semi-transparent glass that works as a solar cell. Its glass could be integrated into windows and into the facades of buildings.

Two Companies Ready For Production

Two companies say they are ready to bring solar power windows to the marketplace, according to Computer World. They say the windows will cost about 40% more than ordinary windows, but claim the extra cost will be recaptured in a year through lower utility bills.

One is Solaria, a company that currently manufactures rooftop solar panels. It 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 the human eye works, you don’t even notice them,” says Solaria CEO Suvi Sharma.

As the PV strips absorb light striking a building’s window, they also reduce solar heat gain, which lowers air conditioning costs.  Solaria is targeting its technology for windows that will be installed in newly constructed buildings. It says its windows are about 8% efficient.

A second company is Solar Window Technologies. Its approach uses a different form of transparent photovoltaic cell technology that is suitable for new construction, replacement windows and retrofits to existing windows. Its organic photovoltaics can vary in color and transparency, depending on the needs of the customer.

CEO John Conklin says because his product is based on a photovoltaic film, it can be used on existing windows or incorporated into manufactured products relatively easily. The company has not released data on the efficiency of its technology.

Delivering The Electricity

Once solar power windows generate electricity from the sun, who is that energy connected to the building? Each company uses a different approach. Solaria, embeds its wiring in the window frame. From there, it is connected to a dedicated electrical line that conducts it to a central power inverter. The inverter then converts the direct current from the solar window to alternating current that is fed into the the electric panel for the building.

The Solar Window technology offers more flexibility. It can power a micro DC-to-AC power inverter to provide electricity for a single room. It can also be connected to a distributed microgrid inverter to power a single floor of a building. Finally, it can be fed to a central inverter that supplies elecrticity to an  entire building.

New Clean Power Opportunity

Many buidings are not good candidates for rooftop solar power systems. Skyscrapers and large urban apartment buildings cannot participate in the clean power revolution that rooftop solar is bringing to many homeowners. Solar powered windows could change that dynamic by providing the benefits of electricity from clean solar power to all buildings.

Solaria estimates that a skyscraper with south facing windows could generate up to a third of its annual electrical needs from solar power windows. Apply that to buildings around the world and you have the potential for a significant new source of clean, renewable energy that will pay for itself in a matter of months rather than a period of years.

Photo Credits: Solaria; Solar Window Technologies