Making Windows Smarter

New tech affords manufacturers a path to creating thermochromic smart windows
Jie Li
March 4, 2019
COLUMN : In the Trenches

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It is estimated that billions of dollars of wasted energy flows through inefficient windows every year, allowing a big opportunity for manufacturers to make smart windows that offer some relief to dam the flow of this wasted money and energy. However, the high cost and limited performance of thermochromic windows has prevented them from achieving their full market potential.

Conventional thermochromic technology performance has been considered limited based on how it incorporates vanadium dioxide particles—it is this VO2 content of thermochromic windows that makes them smart, allowing windows to pass infrared heat in winter and block it during summer. The use of smaller VO2 particles, nanoparticles, is the key to increasing the material performance. Unfortunately, the standard method for manufacturing VO2 nanoparticles is slow, expensive and even potentially dangerous.

A team of process engineers and scientists who specialize in materials, energy and buildings have been working at the U.S. Department of Energy’s Argonne National Laboratory, anl.gov, to change all of this. In May of 2018, the team received a U.S. patent on its “Continued flow synthesis of VO2 nanoparticles or nanorods by using a microreactor.” This technology is now available for licensing. 

How it works

Manufacturers typically prepare VO2 nanoparticles in an autoclave batch reactor. This method has a limited ability to manufacture high-quality nanoparticles in large quantities with stable quality, and the process takes two or three days while operating at high temperatures and pressures the entire time. The batch process also carries the risk of explosions unless proper precautions are taken, which adds to the cost. 

The new system uses a continuous-flow hydrothermal reactor process that can produce VO2 nanoparticles in minutes and will cost an estimated five times less than the batch method. The process uses a small, tabletop reactor in the laboratory to produce seven-tenths of a gram of VO2 nanoparticles per hour at an estimated cost of about $5 per gram. With further refinements to the process, the Argonne team expects to increase that rate to 100 grams per hour (more than a kilogram, or 2.2 pounds, per day), with another year of development.

Think of the continuous-flow reactor as an oven with a pipe running through it. Manufacturers can connect scores of such reactors to process VO2 nanoparticles in parallel, which can keep the pressure and temperature unchanged with an increasing throughput. 

The process is also designed to offer precise control of the temperature, pressure and flow rate. Such control, and because only a fraction of the reactant is heated at a given time, can make the process safer and provide the control needed to make the nanoparticles in a uniform size, the engineers and scientists found. And, they report the system can readily expand to an industrial scale. 

Why it works

Smaller is better for two reasons. First, the smaller nanoparticles will scatter less light, which enhances window-film transparency. This is an issue because conventional thermochromic windows are tinted to increase their energy efficiency. Second, the smaller nanoparticles would increase energy efficiency through improved heat modulation.

Despite their vanishingly small size, the Argonne team was able to measure nanoparticles thanks to the advanced instrumentation available at the Center for Nanoscale Materials at Argonne. Thus far, the team managed to produce 100-nanometer particles with the process and aims to further reduce the particle size to 50 nanometers or smaller—so small that 5,000 of them would fit along the edge of a piece of paper. 

The next step for the team is to observe the formation and growth of the particles in-situ and in real time at the micro-second resolution as the process plays out on the nanoscale by using the unique capability of high-energy X-rays of the Advanced Photon Source at Argonne. This will provide insights into reaction kinetics and, eventually, how to improve the technology.

Even without further improvement, the technology readily lends itself to immediate use in new window construction. It could also be manufactured as a film that both professional installers and do-it-yourselfers could apply to windows for an immediate 20-percent reduction in energy costs. 

The technology can achieve such energy savings because thermochromic windows that incorporate VO2 nanoparticles doped with tungsten begin blocking infrared heat at about 77 degrees Fahrenheit (tunable), like low-emissivity film. However, they also allow infrared heat in to heat the room, which is not available for a low-E film. Robust oxide allows for a long lifetime as well.

The smart window market is sure to grow. In 2014, NanoMarkets, an advanced materials and emerging energy and electronics market analyst, valued the smart window market at $1 billion. The NanoMarkets smart window forecast for 2021: nearly $3 billion. Another market analyst, IndustryARC, pegged thermochromic windows as the fastest-growing market segment, with an anticipated compound annual growth rate of 38 percent from 2015 to 2020. This new technology could allow manufacturers to realize some of the smart window growth. 

Jie Li is a chemical engineer at Argonne National Laboratory. He was part of the team that developed the continued flow synthesis of VO2 nanoparticles or nanorods by using a microreactor process. To inquire about the technology, email partners@anl.gov.