Transparent Solar Windows: How Michigan State's Concentrator Works

Michigan State’s Transparent Solar-Concentrator Concept

Can a window make electricity without looking like a solar panel? Researchers at Michigan State University showed that it can, at least in principle. In 2014 a team led by Richard Lunt, a chemical engineering professor, unveiled a transparent luminescent solar concentrator (TLSC): a clear plastic sheet that harvests light we cannot see and sends it to thin solar cells hidden at the edges.

The short answer for anyone wondering whether every window will soon power the house: not yet. The idea is real and has been developed further since, but early versions converted only a small fraction of sunlight into electricity. Here is how the technology works, what it can and cannot do, and where clear solar glass stands today.

The problem: solar cells need to absorb light

A conventional solar panel looks dark because it is designed to absorb as much visible light as possible. Photons hit a semiconductor such as silicon, knock electrons loose and create a current. A window has the opposite job: it must let visible light pass through so people can see. For a long time those two goals seemed incompatible, and earlier "see-through" solar products were really tinted glass or panels with gaps between opaque cells.

The way out is to remember that sunlight contains much more than visible light. A large share of its energy arrives as ultraviolet (UV) and near-infrared (IR) radiation, which our eyes cannot detect. A material that captures only those wavelengths and lets visible light through can, in principle, look almost perfectly clear.

How the Michigan State concentrator works

The MSU team embedded small organic salt molecules in a transparent plastic layer. These molecules absorb specific UV and near-infrared wavelengths and then re-emit the energy as another infrared wavelength, a process called luminescence. The emitted light is guided through the sheet, bouncing internally much as light travels along an optical fibre, until it reaches the edges.

Narrow strips of conventional photovoltaic cells sit along those edges and turn the concentrated infrared light into electricity. Because the absorbing and emitting happen outside the visible range, the panel does not glow or carry a coloured tint, which had been the main drawback of older luminescent concentrators.

Why "concentrator" matters

Luminescent solar concentrators are not new; researchers have studied them since the 1970s. The appeal is that a large, cheap sheet can collect light while only a small area of expensive solar cell is needed at the edges. Earlier designs used brightly coloured dyes, which made them unsuitable for ordinary windows. The MSU advance was making the concentrator colourless.

How efficient is it?

This is where the headlines ran ahead of the science. At the time of the announcement, the prototype converted roughly 1% of incoming solar energy into electricity, according to reports on the research. The team said it aimed for efficiencies above 5% for a fully transparent device. For comparison, typical rooftop silicon panels convert around 18–22% today.

There is also a physical ceiling. Since a transparent device deliberately lets visible light pass, it gives up much of the available energy. Lunt has calculated a theoretical limit of about 21% for a fully transparent single-junction solar cell, compared with about 33% for an opaque one. Real devices sit well below these ceilings.

Low efficiency does not make the idea pointless. Buildings have enormous areas of glass, and windows are being installed anyway. The question is whether the extra power justifies the extra cost, durability requirements and wiring. Vertical windows also receive less direct sun than tilted rooftop panels, which further reduces output.

Correcting the early claims

Some 2015 coverage, including the version originally shared here, promised a solar cell that could replace windows and give phones and tablets a much longer battery life. Both claims need tempering:

Windows: the TLSC is a coating or layer that could be built into window glass, not a replacement for the window itself. It was a laboratory demonstration, not a product on sale.
Devices: a phone screen is small and often indoors, so even a perfect clear solar layer would harvest only a trickle of power. At best it could slow battery drain slightly or power low-energy gadgets such as e-readers or sensors.
"Once impossible": transparent and semi-transparent solar designs existed before 2014; the MSU work stood out because it was genuinely colourless with a clear path to scale.

What has happened since

Research on transparent photovoltaics has continued at Michigan State and elsewhere, with groups testing new organic molecules, quantum dots and perovskite materials. In a widely cited 2017 analysis in Nature Energy, Lunt's team estimated that transparent solar on the roughly 5 to 7 billion square metres of glass in the United States could supply around 40% of the country's energy demand, about as much as rooftop solar, and that the two together, with better storage, could come close to covering it. That is a theoretical potential, not a forecast.

Several companies, including Ubiquitous Energy, a start-up Lunt co-founded, have worked on bringing clear solar glass to market, and pilot installations have appeared on a handful of buildings. Costs, long-term durability under sunlight and efficiency remain the main hurdles. For now, semi-transparent and coloured building-integrated panels are more common than fully clear ones.

Why it still matters

Even at modest efficiencies, clear solar layers could make use of surfaces that today produce nothing: skyscraper façades, greenhouse roofs, bus shelters and car sunroofs. Because they block much of the UV and infrared, they can also reduce heat gain inside a building, cutting cooling costs, which adds value beyond the electricity they make.

If you are considering solar for your own home, conventional rooftop panels remain far more cost-effective. Transparent solar is one to watch, a good example of a clever idea making steady, unspectacular progress from the lab towards real buildings.

Sources: IEEE Spectrum: Can solar power go truly transparent?; Sci.News: Scientists develop transparent solar concentrator; World Economic Forum: These windows could soon generate solar power (2017); APS March Meeting 2013: Lunt, Theoretical and practical limits for transparent photovoltaics; Wikipedia: Luminescent solar concentrator; Higher Perspectives, via True Activist (where the story circulated in 2015).

Related: