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Polymers in smart windows block solar heat, self-tint with zero electricity

Polymers in smart windows block solar heat, self-tint with zero electricity

Hydrogels in the form of microscopic soft beads suspended in a liquid form can block solar heat when outside temperatures rise. The main component of the hydrogel is a polymer, which has a low critical solution temperature. This has been developed by researchers from State Key Laboratory of Chemical Engineering, East China, East China University of Science and Technology, Shanghai Institute of Ceramics, College of Chemistryy- Chemical Engineering, UC Berkeley and Shihezi University.
In a solvent, it is insoluble at high temperature but soluble at low temperature. The liquid is sandwiched and sealed between two glass panels, which change the optical appearance between opaque when at a low temperature and transparent at high temperatures. This will help moderate temperature within a house, reducing the need for heating or air conditioning. The material designed by the team is a colloid - a substance in which tiny particles or droplets that don't dissolve are spread throughout a larger volume of some other material. 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 700nanometersacross. 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 do not react with each other to form one big lump.
In the gel recipe, the polymer changes shape whenever the temperature rises above 32 degree Celsius (about 90 degree 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 degree Celsius, the polymer molecules coil into small balls. These can not 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.
Under simulation, the new smart window blocked one-fourth, or about 25% of the visible light and infrared energy (heat) emitted by a sun lamp. The smart window reduced the temperature inside that box by 20 degree Celcius-the liquid 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 degree Celcius. When his team added tiny particles of a mineral called vanadium oxide to the polymer, the new smart window blocked 40% of the sun lamp's light.

Another team of scientists from the Nanyang Technological University here have developed a “smart” window which can darken or brighten without the need for an external power source. This unique self-tinting window requires zero electricity to operate and is also a rechargeable battery. The window’s stored energy can be used for other purposes, such as to light up low-powered electronics like a light emitting diode. Currently, the window solutions in the market are either using permanent tinting which cannot brighten at night or are windows that can change its light transmission properties only with an external power source. The “smart” window, however, can be turned into a cool blue tint in bright daylight, cutting light penetration by about half, and then reverts back to clear glass at night or as required. “Our new smart electrochromic window is bi-functional; it is also a transparent battery. It charges up and turns blue when there is oxygen present in the electrolyte – in other words, it breathes,” explained lead researcher professor Sun Xiaowei.
Such an innovative technology can adjust the amount of sunlight coming into buildings in the day which promises significant savings on cooling and lighting costs. The study was published in the journal Nature Communications.

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