By Kayt Sukel
Coming off the hottest summer on record, the need to curb energy consumption has never been greater.
Breakthroughs in building materials, including those in windows, provide tremendous opportunities to do just that, said Jin Wen, Ph.D., professor of civil, architectural, and environmental engineering at Drexel University in Philadelphia.
In New York City – or any big city known for its glass and steel skyscrapers – the estimated number of windows across the urban landscape easily reaches into the millions. This provides an opportunity for researchers to develop new, smarter materials that maximize energy efficiency in the face of rising temperatures (and the corresponding increase in energy costs).
“We could drastically lower the load, or the amount of heat you have to remove or provide to keep the temperature at the right level, with the right building materials,” Wen said. “And if these materials are passive and perform well under real weather over a long period of time, we can see even greater savings.”
A durable option at the right price point
The first so-called smart window, or glass panes with transparent conductors placed between them that changed tint in response to different wavelengths of light, hit the market in the 1980s. Researchers have since worked to create more durable and cost-effective prototypes.
While there are several smart window options on the market, they have been plagued by high costs, manufacturing challenges, additional energy requirements, and – often – decreased performance (as well as unattractive discoloration) over time.
Now, researchers at Rice University’s Nanomaterials Laboratory, led by professor of materials science and nanoengineering professor Pulickel Ajayan, Ph.D., have developed a new thermochromic material that meets these challenges.
“With window-to-wall ratios dramatically increasing over the last few years – it’s now at 75% of buildings – we need windows that can passively cool interiors,” said Sreehari Saju, a graduate student in Ajayan’s lab. “Current smart window solutions have less than a 0.3% adoption. Our thermochromic material, however, can reduce energy usage and improve thermal comfort. It also has simulated durability with resilience to extreme weather conditions for more than 50 years.”
The new material is made from a salted polymer blend. When the temperature goes up, it initiates a phase change that makes the material grow more opaque in a dose-dependent manner. This blocks infrared radiation from the outdoors and keeps the interior of the room cool. When temperatures go down, it works in the reverse manner. The window is more transparent and allows the radiation to come in and heat things up.
Saju and colleague Anand Puthirath, Ph.D., a research scientist in Ajayan’s group, conducted experiments and simulations to better understand how it might work in a real-world setting. They learned that the material easily outperforms other thermochromic materials in a variety of ways.
“It is very easy to apply. You can just paint the material on windows, so there is no need to retrofit the windows in buildings. You can put it on the existing windows,” Saju said. “Because our material is hydrophobic, it performs well in bad, even extreme weather. And the material is clear and does not have the same tinting problems that other thermochromic materials have.”
Perhaps most importantly, however, is that the new material is much more cost effective than other smart window technologies.
“We see a superior durability of over 50 years and the cost for lab-scale materials, which are expensive, per square foot is about $2.60, compared to $25-150 for smart windows,” he explained. “Based on our preliminary investigations of industrial scale materials, we believe we could potentially bring the cost down to just 50 cents per square foot. That is comparable to normal paint.”
Entering the mainstream?
Puthirath said he and his team have a lot of work to do before they can make the material commercially available. They are looking to take the material out of the lab and into the real world – where they can apply their thermochromic material on a large scale. This will allow them to validate their simulation studies and determine “actual HVAC energy efficiency numbers.”
Wen said such testing is what is needed to bring such a material into mainstream use. The building industry, in general, is both “cost sensitive and risk sensitive” and needs to see the value in any new material before investing, she said. Far too often, new advances in thermochromics, or any other passive cooling materials, simply do not have large enough simulation studies or real system demonstrations to foster commercialization – and then, once solutions are available to the market, widespread adoption.
“If I’m a designer and adopting this new technology, I need to make sure any additional costs are justifiable – and that it works outside of just a simulation,” Wen said. “So having demo sites where designers and builders can visit to see those benefits firsthand – and see that the numbers check out – is important.
“The new solution then becomes something they can get excited about. They can tell the architect to look into it and raise more awareness. That’s what it takes to get these kinds of new technologies into use.”
