A New Way to Store Solar Heat

A new way to store solar heat. Material could harvest sunlight by day, release heat on demand hours or days later.

The layer-by-layer solar thermal fuel polymer film comprises three distinct layers (4 to 5 microns in thickness for each). Cross-linking after each layer enables building up films of tunable thickness.
Courtesy of the researchers

Scientia — Imagine if your clothing could, on demand, release just enough heat to keep you warm and cozy, allowing you to dial back on your thermostat settings and stay comfortable in a cooler room. Or, picture a car windshield that stores the sun’s energy and then releases it as a burst of heat to melt away a layer of ice.

According to a team of researchers at MIT, both scenarios may be possible before long, thanks to a new material that can store solar energy during the day and release it later as heat, whenever it’s needed. This transparent polymer film could be applied to many different surfaces, such as window glass or clothing.

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Although the sun is a virtually inexhaustible source of energy, it’s only available about half the time we need it — during daylight. For the sun to become a major power provider for human needs, there has to be an efficient way to save it up for use during nighttime and stormy days. Most such efforts have focused on storing and recovering solar energy in the form of electricity, but the new finding could provide a highly efficient method for storing the sun’s energy through a chemical reaction and releasing it later as heat.

The finding, by MIT professor Jeffrey Grossman, postdoc David Zhitomirsky, and graduate student Eugene Cho, is described in a paper in the journal Advanced Energy Materials. The key to enabling long-term, stable storage of solar heat, the team says, is to store it in the form of a chemical change rather than storing the heat itself. Whereas heat inevitably dissipates over time no matter how good the insulation around it, a chemical storage system can retain the energy indefinitely in a stable molecular configuration, until its release is triggered by a small jolt of heat (or light or electricity).

In the researchers’ platform for testing macroscopic heat release, a heating element provides sufficient energy to trigger the solar thermal fuel materials, while an infrared camera monitors the temperature. The charged film (right) releases heat enabling a higher temperature relative to the uncharged film (left).
Courtesy of the researchers

Molecules with two configurations

The key is a molecule that can remain stable in either of two different configurations. When exposed to sunlight, the energy of the light kicks the molecules into their “charged” configuration, and they can stay that way for long periods. Then, when triggered by a very specific temperature or other stimulus, the molecules snap back to their original shape, giving off a burst of heat in the process.

Such chemically-based storage materials, known as solar thermal fuels (STF), have been developed before, including in previous work by Grossman and his team. But those earlier efforts “had limited utility in solid-state applications” because they were designed to be used in liquid solutions and not capable of making durable solid-state films, Zhitomirsky says. The new approach is the first based on a solid-state material, in this case a polymer, and the first based on inexpensive materials and widespread manufacturing technology.

“This work presents an exciting avenue for simultaneous energy harvesting and storage within a single material,” says Ted Sargent, university professor at the University of Toronto, who was not involved in this research.

Manufacturing the new material requires just a two-step process that is “very simple and very scalable,” says Cho. The system is based on previous work that was aimed at developing a solar cooker that could store solar heat for cooking after sundown, but “there were challenges with that,” he says. The team realized that if the heat-storing material could be made in the form of a thin film, then it could be “incorporated into many different materials,” he says, including glass or even fabric.

To make the film capable of storing a useful amount of heat, and to ensure that it could be manufactured easily and reliably, the team started with materials called azobenzenes that change their molecular configuration in response to light. The azobenzenes can then can be stimulated by a tiny pulse of heat, to revert to their original configuration and release much more heat in the process. The researchers modified the material’s chemistry to improve its energy density — the amount of energy that can be stored for a given weight — its ability to form smooth, uniform layers, and its responsiveness to the activating heat pulse.

A spin-coating process enables deposition of the solar thermal fuel polymer material from solution. The film can then be readily charged with ultraviolet light. This approach can be extended to a variable thickness in a layer-by-layer process.
Courtesy of the researchers

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Shedding the ice

The material they ended up with is highly transparent, which could make it useful for de-icing car windshields, says Grossman, the Morton and Claire Goulder and Family Professor in Environmental Systems and a professor of materials science and engineering. While many cars already have fine heating wires embedded in rear windows for that purpose, anything that blocks the view through the front window is forbidden by law, even thin wires. But a transparent film made of the new material, sandwiched between two layers of glass — as is currently done with bonding polymers to prevent pieces of broken glass from flying around in an accident — could provide the same de-icing effect without any blockage. German auto company BMW, a sponsor of this research, is interested in that potential application, he says.

With such a window, energy would be stored in the polymer every time the car sits out in the sunlight. Then, “when you trigger it,” using just a small amount of heat that could be provided by a heating wire or puff of heated air, “you get this blast of heat,” Grossman says. “We did tests to show you could get enough heat to drop ice off a windshield.” Accomplishing that, he explains, doesn’t require that all the ice actually be melted, just that the ice closest to the glass melts enough to provide a layer of water that releases the rest of the ice to slide off by gravity or be pushed aside by the windshield wipers.

A heating element is used to provide sufficient energy to trigger the solar thermal fuel materials, while an infrared camera monitors the temperature. (View full screen to see animation.)
Courtesy of the researchers

The team is continuing to work on improving the film’s properties, Grossman says. The material currently has a slight yellowish tinge, so the researchers are working on improving its transparency. And it can release a burst of about 10 degrees Celsius above the surrounding temperature — sufficient for the ice-melting application — but they are trying to boost that to 20 degrees.

Already, the system as it exists now might be a significant boon for electric cars, which devote so much energy to heating and de-icing that their driving ranges can drop by 30 percent in cold conditions. The new polymer could significantly reduce that drain, Grossman says.

“The approach is innovative and distinctive,” says Sargent, from the University of Toronto. “The research is a major advance towards the practical application of solid-state energy-storage/heat-release materials from both a scientific and engineering point of view.”

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– Credit and Resource –

Written by: David L. Chandler | MIT News Office

Provided by: The work was supported by a NSERC Canada Banting Fellowship and by BMW.

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A New Way to Store Solar Heat

New Material for Solar Cell Technology

Scientia — The University of Liverpool is part of an international research team that have demonstrated a new semiconductor material made from abundant elements which can be “tuned” for use in solar cells instead of rare elements.

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Semiconductors are vital to the electronics industry and are used in everything from smart phones to solar panels but they rely on rare elements such as tellurium, gallium and indium and there is increasing concern over their cost and availability.

Researchers focussed on the compound zinc tin nitride (ZnSnN2) which has been recently synthesized by research groups around the world, using zinc and tin – metals which are readily available through mature recycling facilities – rather than expensive and rare metals.

Working with the compound, they discovered an innovative tuning process which means it could provide a possible replacement for semiconductor material currently used in solar cells.

It had been thought that the band gap (a semiconductor’s defining characteristic) was too large for device applications such as solar cells. However, the researchers found that the alloy’s band gap can be “tuned” by altering the perfectly ordered crystalline lattice by introducing disorder in the form of randomly distributed zinc and tin atoms.

Dr Tim Veal, a reader in Physics and researcher in the University of Liverpool’s Stephenson Institute for Renewable Energy, said: “Such tuneability is typically achieved in other material systems by alloying, or blending in other elements, to obtain the desired result, However, this is not necessary with ZnSnN2, given the recent discovery.”

Dr Steve Durbin, Professor and Chair of Electrical and Computer Engineering at Western Michigan University, added: “We use a sophisticated crystal growth technique known as molecular beam epitaxy. It allowed us to control crystal quality by carefully adjusting parameters such as temperature and the ratio of incident atomic (or molecular) beams.”

Dr Veal added: “By doing so, the team has been able to observe a wide range of disorder in a number of samples, and correlated this with a significant reduction in band gap energy – paving the way for this material to be considered for solar cell applications.”

The research, funded by the UK Engineering and Physical Science Research Council and US National Science Foundation, is published in the journal Advanced Energy Materials.

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– Credit and Resource –

More information: Tim D. Veal et al. Band Gap Dependence on Cation Disorder in ZnSnN2 Solar Absorber , Advanced Energy Materials (2015). DOI: 10.1002/aenm.201501462

Journal reference: Advanced Energy Materials

Provided by: University of Liverpool

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New Material for Solar Cell Technology

Quest for organic solar panels and flexible electronics

University of Vermont scientists have invented a new way to create what they are calling an electron superhighway in an organic semiconductor that promises to allow electrons to flow faster and farther — aiding the hunt for flexible electronics, organic solar cells, and other low-cost alternatives to silicon. To explore these organic materials, UVM graduate students (from left) Naveen Rawat and Lane Manning, and professors Randy Headrick and Madalina Furis, deployed this table-top scanning laser microscope. Their latest finding is reported in the journal Nature Communications — and may, someday not too far off, let you roll up your computer like a piece of paper. Credit: Joshua Brown, UVM

Scientia — TV screens that roll up. Roofing tiles that double as solar panels. Sun-powered cell phone chargers woven into the fabric of backpacks. A new generation of organic semiconductors may allow these kinds of flexible electronics to be manufactured at low cost, says University of Vermont physicist and materials scientist Madalina Furis.

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But the basic science of how to get electrons to move quickly and easily in these organic materials remains murky.

To help, Furis and a team of UVM materials scientists have invented a new way to create what they are calling “an electron superhighway” in one of these materials—a low-cost blue dye called phthalocyanine—that promises to allow electrons to flow faster and farther in organic semiconductors.

Their discovery, reported Sept. 14 in the journal Nature Communications, will aid in the hunt for alternatives to traditional silicon-based electronics.

Hills and potholes

Many of these types of flexible electronic devices will rely on thin films of organic materials that catch sunlight and convert the light into electric current using excited states in the material called “excitons.” Roughly speaking, an exciton is a displaced electron bound together with the hole it left behind. Increasing the distance these excitons can diffuse—before they reach a juncture where they’re broken apart to produce electrical current—is essential to improving the efficiency of organic semiconductors.

Using a new imaging technique, the UVM team was able to observe nanoscale defects and boundaries in the crystal grains in the thin films of phthalocyanine—roadblocks in the electron highway. “We have discovered that we have hills that electrons have to go over and potholes that they need to avoid,” Furis explains.

To find these defects, the UVM team—with support from the National Science Foundation—built a scanning laser microscope, “as big as a table” Furis says. The instrument combines a specialized form of linearly polarized light and photoluminescence to optically probe the molecular structure of the phthalocyanine crystals.

“Marrying these two techniques together is new; it’s never been reported anywhere,” says Lane Manning ’08 a doctoral student in Furis’ lab and co-author on the new study.

The new technique allows the scientists a deeper understanding of how the arrangement of molecules and the boundaries in the crystals influence the movement of excitons. It’s these boundaries that form a “barrier for exciton diffusion,” the team writes.

And then, with this enhanced view, “this energy barrier can be entirely eliminated,” the team writes. The trick: very carefully controlling how the thin films are deposited. Using a novel “pen-writing” technique with a hollow capillary, the team worked in the lab of UVM physics and materials science professor Randy Headrick to successfully form films with jumbo-sized crystal grains and “small angle boundaries.” Think of these as easy-on ramps onto a highway—instead of an awkward stop sign at the top of a hill—that allow excitons to move far and fast.

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Better solar cells

Though the Nature Communications study focused on just one organic material, phthalocyanine, the new research provides a powerful way to explore many other types of organic materials, too—with particular promise for improved solar cells. A recent U.S. Department of Energy report identified one of the fundamental bottlenecks to improved solar power technologies as “determining the mechanisms by which the absorbed energy (exciton) migrates through the system prior to splitting into charges that are converted to electricity.”

The new UVM study—led by two of Furis’ students, Zhenwen Pan G’12, and Naveen Rawat G’15—opens a window to view how increasing “long-range order” in the organic semiconductor films is a key mechanism that allows excitons to migrate farther. “The molecules are stacked like dishes in a dish rack,” Furis explains, “these stacked molecules—this dish rack—is the electron superhighway.”

Though excitons are neutrally charged—and can’t be pushed by voltage like the electrons flowing in a light bulb—they can, in a sense, bounce from one of these tightly stacked molecules to the next. This allows organic thin films to carry energy along this molecular highway with relative ease, though no net electrical charge is transported.

“One of today’s big challenges is how to make better photovoltaics and solar technologies,” says Furis, who directs UVM’s program in materials science, “and to do that we need a deeper understanding of exciton diffusion. That’s what this research is about.”

– Credit and Resource –

More information: Polarization-resolved spectroscopy imaging of grain boundaries and optical excitations in crystalline organic thin films, Nature Communications, 6, Article number: 8201 DOI: 10.1038/ncomms9201

Provided by: University of Vermont

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Quest for organic solar panels and flexible electronics