See-Through Windows Make Clean Electricity From Raindrops

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An energy efficiency triple threat has emerged in the form of a new super-smart window that can fulfill its primary function of letting daylight into a room while performing three other energy saving tasks. That includes generating clean electricity without the help of solar cells, which is a pretty neat trick. So, how do they do it?

The Not-So-Secret Semiconductor Behind Windows That Make Clean Electricity From Rain

The new “Plus Energy” window was developed by a research team from the University of Seoul in South Korea. If you’re thinking that ITO (indium tin oxide) is the semiconductor pulling multiple tasks at once, run right out and buy yourself a cigar.

ITO is widely used as a transparent electrode in various electronic devices as well as LEDs and solar cells. To get the triple-threat effect, the Seoul team constructed layers of ITO and silver.

Instead of generating clean electricity like a solar cell, the ITO-silver combination draws electricity from the friction of raindrops on the electrode. As explained by the Seoul team, the entire surface of the window needs to be coated in order for it to work.

“In an experiment simulating rainy conditions, the smart windows generated 8.3 W m-2 of power with just a single raindrop,” the team reported in their study, titled “Energy-saving window for versatile multimode of radiative cooling, energy harvesting, and defrosting functionalities.”

What Else Can This Super-Smart Window Do?

You can find all the details in the study, newly published in the journal Nano Energy. Clearly there are some steps remaining between the labwork and commercial development. Nevertheless, the research team is confident they are on to a new technology that will help boost the supply of clean electricity during warm weather, an essential mission as the global climate warms and people turn up their air conditioners.

In addition to generating clean electricity, the new ITO-silver window coating creates a cooling effect by allowing only the visible part of the light spectrum to pass inside. Other parts of the spectrum are reflected outside.

The study showed that the new window achieved a steady temperature drop of about 7 degrees, when deployed in a hot environment under direct sunlight.

“Unlike conventional air conditioning systems that use refrigerants, this radiative cooling technology offers cooling performance without consuming electrical energy,” the research team emphasizes.

As a third function, the ITO-silver electrode enables the windows to defrost themselves without using energy. That can be a significant concern in some regions.

There’s More Than One Way To Make Clean Electricity From A Window

If this is beginning to sound familiar, you may be thinking of the idea that buildings can — and should — generate electricity instead of just sucking it up from the grid.

Roofttop solar panels and a new generation of building-friendly wind turbines are helping to turn the tables. The next level up is to integrate clean energy harvesting devices with the actual building elements. Windows have attracted much of the attention because they are notorious for interfering with climate control efficiency in buildings.

Solar innovators have long pursued the idea of coating windows with a transparent solar energy harvesting formula. Combining an efficient coating with full transparency has been an elusive goal, but the dam is beginning to break.

One example is the US startup NEXT Energy Technologies, a spinoff from the University of California – Santa Barbara. They have been hammering away at a solution since 2010, with an assist from the US Department of Energy and other funders.

Another interesting example is an early-stages research project that deploys a cellulose base to achieve transparency and clean electricity generation.

More Clean Electricity From The Plus-Energy Building Of The Future

Another development in the buildings-as-electricity-generators field to watch is piezoelectricity, which refers to the ability of certain materials to build up an electric charge when pressure or stress are applied. A number of piezoelectric projects have crossed the CleanTechnica radar over the past 15 years. So far the use cases have been outdoor grill lighters and other small-scale devices, a piezoelectric dance floor being one notable exception.

A significant obstacle to scale-up is the amount of material needed to generate a useful amount of clean electricity. As one research team described the challenges in a study published earlier this year, it’s all about “the challenges of output power with a sharp peak, small bandwidth, and the huge dimensions of the piezoelectric energy harvesters relative to the output power.”

The team, based at the University of Alexandria in Egypt, proposed a new approach consisting of three beams connected at the center. “The proposed design produces an output power of 35 mW between 25 and 40 Hz,” they reported, describing it as more than triple the output of conventional designs.

Clean Electricity From Cement

As for the large scale application of piezoelectric materials in buildings and other structures, cement is a likely candidate due to the massive amount that flows into the global economy every year.

“As the most common construction material, plain cement paste lacks satisfactory piezoelectricity and is not efficient at harvesting the electrical energy from the ambient vibrations of a building system,” warned a multinational research team back in 2019. On the bright side, the same researchers also took note of a considerable amount of progress in improving the piezoelectric performance of cement.

Keep an eye on the Georgia Institute of Technology, which developed electricity-generating footpath tiles for the Kennedy Space Center back in 2017. The $2 million project was supported by Delaware North Corporation, a NASA contractor, as a renewable energy demonstration deploying people power.

If the buildings angle doesn’t pan out, roadways are another potential platform for piezoelectric systems. Last year the California Energy Commission posted the results of a study aimed at assessing efficiency of deploying piezoelectric systems to generate clean electricity from roadways. The team was tasked with designing a system of 333 watts per square foot, costing around $9,000 per kilowatt, and having a lifespan of up to 20 years.

“Based on the laboratory evaluations and road tests, the application of the piezoelectric energy harvesting system in one lane of a one-mile-long roadway has the potential to generate 72,800 kilowatt-hours of energy per year,” the team reported.

That’s just for starters. The researchers also noted that a heavily used truck route yielded a much higher result, coming in at up to 907,873 kilowatt-hours per lane.

Stay tuned for new about other applications. The researchers also suggest that warehouses, seaports, aircraft runways, and railroads can be outfitted to serve as clean electricity generators, too.

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Image: A new super-smart, see-through window can generate clean electricity, keep interior spaces cool and defrost itself, all while letting daylight come through (credit: Seoul National University College of Engineering via Eurekalert).