‘Smart’ windows could save energy

Tiny droplets sandwiched between glass panes turn cloudy when it’s hot outside; this filter out some warming sunlight

smart windows

At temperatures above 32 °C (90 °F), a new “smart window” (left) blocked some of the light streaming into a box from a sun lamp. That kept the box cooler than the one at right, which had a more conventional window.

Wang et al., Ind. & Eng. Chem. Res. (2014)

Sunlight streaming through a window can really heat up a room. In winter, when heating bills can soar, people tend to welcome that extra warmth. But in summer, that heat just boosts cooling costs. A homeowner could keep out some of that warming light by drawing the curtains or lowering the blinds. Or the window could change its transparency — blocking out some light, as needed — all by itself. That’s the idea behind new “smart” windows.

Some smart windows already exist. They work just like large versions of the LCDs (liquid crystal diodes) found in watches and other electronic devices. When an electric current flows through an LCD window, a coating on the panes of its glass darken. That blocks out some of the light. A homeowner can control the window’s light-blocking ability — or opacity — simply by flipping a switch. Or, a sensor connected to the window can automatically control the current, just like the thermostat used to control a furnace or air conditioner.

But the new smart window does not require such electronics. It depends only on the temperature outdoors, says Xuhong Guo. He’s a chemical engineer at the East China University of Science and Technology in Shanghai. His team designed a new liquid that it sandwiches between two panes of window glass. The researchers describe how this makes their window “smart” in the December 3 issue of Industrial & Engineering Chemistry Research.

The key: A heat sensitive gel

The material that Guo’s team designed is a colloid. That’s a substance in which tiny particles or droplets that don’t dissolve are spread throughout a larger volume of some other material. (Smoky air is one type of colloid. Milk is another.) The larger part of the new mix is a blend of water and alcohol. Floating inside are tiny globs of a gel.

Each glob is only between 200 and 700 nanometers across. That makes the diameter of the thinnest human hair about 24 to 85 times wider than each glob. The gel contains a heat-sensitive polymer (a chemical made from chain-shaped molecules). It also contains water and glycerol, a type of alcohol. The water and glycerol attach loosely to the polymer. This keeps the gel from dissolving into the larger volume of liquid. This also ensures that the gel globs don’t react with each other to form one big lump of goo.

In the gel recipe that Guo and his colleagues use, the polymer changes shape whenever the temperature rises above 32° Celsius (about 90° Fahrenheit). At lower temperatures, the polymer’s molecules remain long and straight. This allows them to dissolve throughout the gel. Now, lots of light can pass through the gel, making it appear clear. But once the gel’s temperature rises above 32 °C, the polymer molecules coil into small balls. These can’t dissolve into the gel. That makes the gel look cloudy. When dispersed throughout the liquid in between the window panes, these globs now block some light. 

For their tests, the engineers built small boxes to simulate rooms in a house. In one box, they installed a smart window. A second box had the same sort of liquid-filled window, but its liquid didn’t contain any globs of the light-blocking polymer.

The new smart window blocked one-fourth, or about 25 percent, of the visible light and infrared energy (heat) emitted by a sun lamp. “That made a big difference in the temperature inside the box,” Guo told Science News for Students. The plain window reduced the temperature inside the lamp-lit box by 10 °C (18 °F). That’s largely because the liquid between the panes of glass absorbed some of the light’s energy, he explains. But his team’s smart window reduced the temperature inside that box by 20 °C — fully twice as much. Here, too, the liquid in between the window panes absorbed some of the lamp’s energy. But as the polymer-filled globs turned cloudy, more energy was blocked.

The globs turn clear again as soon as their polymer molecules uncoil. This occurs when they cool below 32 °C.

How smart can a window get?

It’s possible to design globs that block even more light, says Guo. When his team added tiny particles of a mineral called vanadium oxide to the polymer, the new smart window blocked 40 percent of the sun lamp’s light.

It also might be possible to essentially choose the temperature at which the polymer changes its shape, Guo says. Experiments show that increasing the proportion of glycerol in the gel globs, for instance, lowers the temperature at which the polymer changes shape.

The new windows are “a great example of researchers finding a new behavior for a material and then taking advantage of it,” says Robert Prud’homme. He’s a chemical engineer at Princeton University in New Jersey.

But further study will be needed to see if the team’s “smart” window is really a smart idea after all, Prud’homme adds. While a cloudy window blocks radiation, that’s not the only way energy gets transferred. Conduction is another way. In that process, energy is transferred when atoms and molecules bump into one another. During such collisions, the slower, colder particles gain energy from the faster, warmer ones slamming into them.

So it’s possible, Prud’homme says, that the liquid-filled layer might actually increase the total amount of heat transferred through the window. Only more research can settle that question. “It’s up to scientists to find out what is possible,” he says. Then, he adds, engineers must work out “what is practical.”

Power Words

(for more about Power Words, click here)

atom   The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

chemical engineer  A researcher who uses chemistry to solve problems related to the production of food, fuel, medicines and many other products.

colloid   A material in which tiny insoluble particles are spread throughout a larger volume of another substance. Colloids take many forms. Smoky air is a colloid. So is fog. Milk is a colloid, with tiny globs of butterfat suspended throughout the liquid. Whipped cream is a colloid too. Colloids typically don’t separate into their individual components over time.  

conduction   One of three major ways that energy is transferred. (The other two are convection and radiation.) In conduction, energy is transferred when atoms and molecules bump into each other, with slower, colder particles gaining energy from the warmer, faster ones that slam into them.

crystal  A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.

dissolve  To turn a solid into a liquid and dispersing it into that starting liquid. For instance, sugar or salt crystals (solids) will dissolve into water. Now the crystals are gone and the solution is a fully dispersed mix of the liquid form of the sugar or salt in water.

electromagnetic radiation Energy that travels as a wave, including forms of light. Electromagnetic radiation is typically classified by its wavelength. The spectrum of electromagnetic radiation ranges from radio waves to gamma rays. It also includes microwaves and visible light.

engineering   The field of research that uses math and science to solve practical problems.

glycerol       A colorless, odorless, sticky syrup that can be used as an antifreezing agent.

hydrogel    A “smart” material that can change its structure in response to its environment, such as the local temperature, pH, salt or water concentration. The material is made from a polymer — a chain made from links of identical units — that have free, water-attracting ends sticking out. So in the presence of water, it may hold (bond) those water molecules for quite a while. Some hydrogels are used in baby diapers to hold urine, in potting soils to hold water near to plants until they need it and in wound dressings to keep a sore from drying out.

infrared light  A type of electromagnetic radiation invisible to the human eye. The name incorporates a Latin term and means “below red.” Infrared light has wavelengths longer than those visible to humans. Other invisible wavelengths include X rays, radio waves and microwaves. It tends to record a heat signature of an object or environment.

insoluble Incapable of being dissolved into a fluid or gas. Salt and sugar can dissolve in water, for example, but some other substances, including some of those with large molecules such as proteins, do not.

liquid crystal diode  Commonly abbreviated as LCD, these materials often are used in digital watches and other electronics. In general, the material’s structure affects how much light is transferred through it. When electricity is applied to an LCD, its molecules change shape and block some light.

mineral  The crystal-forming substances, such as quartz, apatite, or various carbonates, that make up rock. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in certain regular three-dimensional patterns). (in physiology) The same chemicals that are needed by the body to make and feed tissues to maintain health.

molecule  An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

nanometer    A tiny unit of length equal to 1 billionth of a meter.

opacity  The degree to which some normally transparent substance (such as glass or air) blocks light. When it fully blocks out light, the substance is described as opaque.

oxide  A compound made by combining one or more elements with oxygen. Rust is an oxide; so it water.

polymer  Substances whose molecules are made of long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example).

radiation (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

simulate  To deceive in some way by imitating the form or function of something. A simulated dietary fat, for instance, may deceive the mouth that it has tasted a real fat because it has the same feel on the tongue — without having any calories. A simulated sense of touch may fool the brain into thinking a finger has touched something even though a hand may no longer exists and has been replaced by a synthetic limb. (in computing) To try and imitate the conditions, functions or appearance of something. Computer programs that do this are referred to as simulations.

vanadium  A soft metallic element (number 23 on the periodic table of elements) that is structurally strong.


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