Schematic drawings of a window retrofitted with a SiCellA film (a) and an IGU with a SiCellA film inserted between glass panes, image via Nature Energy

See-Through, Energy Efficient Windows Made From Paper

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The idea of recycling cellulose from beer wort into a see-through, energy efficient application for window glass didn’t quite catch on after it made headlines back in 2018, but it has been simmering on the back burner all this time. Researchers have just achieved another breakthrough by using paper pulp, and their patent applications are pending. This could be a hold-my-beer moment for the building industry, which is under pressure to cut its share of global greenhouse gas emissions.

The Search For An Energy Efficient Window, Beer Edition

Window glass is responsible for an outsized share of energy loss. According to one estimate, climate control systems in buildings account for 14% of overall energy consumption in the US and about 25% of that goes to waste through window glass in cold weather.

The beer wort angle surfaced in 2018, when researchers at the University of Colorado in Boulder discussed their search for a transparent aerogel made from cellulose. They derived the cellulose from a handy source in their area, the leftover liquid waste from breweries (aka beer wort).

To cultivate the cellulose, they introduced specialized bacteria to the waste for two weeks. Once they had a sufficient amount, they harvested it and fabricated an aerogel consisting of about 90% gas.

With only 10% solids, an aerogel forms an intricate structure with superior insulating qualities, which raises the potential for use as a lightweight, energy efficient insulator. The catch is that typical aerogels have a jumbled-up nanostructure that produces a smoky, translucent appearance, limiting their application to window glass in buildings.

The CU Boulder team set a higher bar, because they were aiming to make a material suitable for use on Mars. They tweaked the structure of their aerogel to smooth out the irregularities and enable more light to pass through.

“You could harvest sunlight through that thermally insulating material and store the energy inside, protecting yourself from those big oscillations in temperature that you have on Mars or on the moon,” explained Ivan Smalyukh of the school’s  Department of Physics.

If this sounds like a job for the Energy Department’s cutting edge research funding office ARPA-E, it was. In 2016 ARPA-E awarded funding to the research team to develop a see-through, nanostructured cellulose film that can be retrofitted onto windows.

“The team will be able to economically harvest cellulose needed for the films from food waste using a bacteria-driven process. The cellulose will self-assemble into liquid crystal type structures that selectively reflect infrared light (or heat) while transmitting visible light,” ARPA-E explained, adding that “the technology is related to liquid crystals that are used in display screens ranging from smart phones to flat-panel HDTVs.”

energy efficient transparent wood windows
See-through, energy efficient transparent window film can be made from wood, just like they did in the olden days.

An Energy Efficient Beer Wort Window That Really Works

The team was quite satisfied with their results. They described their aerogel as “a thin, flexible film that is roughly 100 times lighter than glass.”

Between the energy efficient angle and the light weight, the applications for windows come into sharper focus. The heat-resisting quality of the film is another benefit. “This gel is so resistant to heat that you could put a strip of it on your hand and light a fire on top — without feeling a thing,” CU Boulder reported.

Meanwhile, a research team from the Forest Products Laboratory of the US Department of Agriculture also picked up the cellulose angle.

Instead of relying on bacteria to do the heavy lifting, the USDA team treated balsa wood to an oxidizing bath that rendered it nearly transparent, then injected it with the synthetic polymer polyvinyl alcohol.

If this is beginning to sound familiar, you may be thinking of the oiled paper commonly used in rustic buildings before window glass became inexpensive and widely available.

In a study published in the journal Advanced Functional Materials, the USDA team reported the following attributes for their energy efficient window:

1) high optical transmittance (≈91%), comparable to that of glass;

2) high clarity with low haze (≈15%)

3) high toughness (3.03 MJ m-3) that is 3 orders of magnitude higher than standard glass (0.003 MJ m-3)

4) low thermal conductivity (0.19 W m-1 K-1) that is more than 5 times lower than that of glass.

They also pointed out that transparent wood can be handled with standard industry equipment. “The scalable, high clarity, transparent wood demonstrated in current work can potentially be employed as energy efficient and sustainable windows for significant environmental and economic benefits,” they concluded.

One Step Closer To The Paper Window Of The Future

In the latest development, researchers from the CU Boulder team have introduced a paper-based concoction for an energy efficient window, working with affiliates at the International Institute for Sustainability with Knotted Chiral Meta Matter at Hiroshima University in Japan and the Energy Department’s National Renewable Energy Laboratory.

On March 16, the journal Nature published their research under the title, “Highly transparent silanized cellulose aerogels for boosting energy efficiency of glazing in buildings.

This time around, the team used paper pulp instead of cultivating bacteria. They reported a “scalable manufacturing of highly transparent silanized cellulose aerogels (SiCellAs) with material characteristics adequate for glazing applications,” which could be sandwiched between glass panes or used to insulate walls.

If you have the right equipment, you can DIY this at home. Just order a some never-dried hardwood pulp from the Rumsford Division of Nine Dragons Paper, which is where the CU Boulder team got their supply.

After that it gets a little more complicated. It helps if you have a blender, a centrifuge, a grinder, a sonifier, some other equipment, and a lot of patience, because some of the steps need to be repeated multiple times to produce a batch of pure, oxidized cellulose fibers dispersed in water. That gets you to the starting point for fabricating the aerogel, which is a whole ‘nother kettle of fish. So on second thought, it would probably be easier to wait for the new research to find its way into the market.

That could take some time, but patent applications have already been submitted. The CU Boulder team also notes that the source material for their new energy efficient aerogel is Earth-abundant, and that it is compatible with industry standard roll-to-roll fabrication methods.

“The aerogels have visible-range light transmission of 97–99% (better than glass), haze of ~1% and thermal conductivity lower than that of still air,” they point out.

CleanTechnica is also keeping an eye on see-through solar cells that could help convert buildings into distributed power generation units. Stay tuned for more on that.

UPDATE: We reached out Ivan Smalyukh at CU Boulder, asking if the team was still working on the beer wort formula, and received the following response:

A: Yes, the aerogel, including the one reported in the recent article, can be made from cellulose derived from the waste like wort with the help of bacteria, though such new SiCellA aerogels can be also made from wood or waste of wood industry.

UPDATE: We also reached out to the USDA Forest Products Laboratory with the following questions:

Q: What is the next step in the research? Has the team explored other sources for cellulose?

A: We are engaged in ongoing research, and researchers are working on the problem of scalability. The challenge is delignification, the process used to remove lignin (a complex organic polymer) from the wood to make it translucent. The process requires chemicals and creates undesirable waste products that can be problematic at larger scales. The team has not yet found other suitable sources that work the same way as wood.

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Photo: Energy efficient film for windows made from balsa wood, courtesy of USDA.

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Tina Casey

Tina specializes in advanced energy technology, military sustainability, emerging materials, biofuels, ESG and related policy and political matters. Views expressed are her own. Follow her on LinkedIn, Threads, or Bluesky.

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