The material, a new kind of aerogel, is a lightweight material consisting mostly of air with a structure made of silica—the same material used for making glass. As described in the journal ACS Nano, the material lets sunlight pass through, but blocks solar heat from escaping. This mechanism allows the aerogel to generate temperatures higher than 200 C (398 F).
Creating a new, transparent aerogel
To efficiently collect solar heat, the team had to create a material that would be hot on the inside, while remaining cold on the outside. Traditionally, this was done by leaving a vacuum between a layer of glass and a dark, heat-absorbing material. Most concentrating solar collectors use this method. However, while these are easy to install, they are relatively expensive to maintain.
Aerogels, meanwhile, have existed for years as highly efficient and lightweight insulating materials. However, these generally had limited transparency, with most aerogels only having a transmission level of 70 percent. Because of this, the team at MIT had to figure out how to make an aerogel that was transparent enough to efficiently work as a solar hear collector.
To create such an aerogel, the team mixed a catalyst with grains of a silica-containing compound in a liquid solution, forming a gel that they then dried to get all the liquids out. This left a matrix that, despite being composed mostly of air, still retains the original mixture's strength. In the process of creating the material, the team also discovered that creating a mix that dries out much faster than conventional aerogels produces a gel with smaller pore spaces between its grains. These smaller pore spaces scattered much less light, letting as much as 95 percent of sunlight in. (Related: Researchers have recycled clothing waste into an "aerogel" supermaterial that can help stop wound bleeding.)
The team then tested the gel on the rooftop of the MIT campus. There, they created a passive device consisting of a heat-absorbing material covered with a layer of the new aerogel. In the middle of the Cambridge winter with the outside air below zero C (32 F), the new aerogel was able to maintain a temperature of 220 C (428 F). Prior to this, such high temperatures were only possible by concentrating sunlight and heat, using mirrors, onto a central line or point. As it requires no such concentration, a heating set up with the new aerogel costs much less. This makes it more useful for a variety of applications.
Possible applications for higher heat
Simple flat rooftop solar collectors, producing only 80 C (176 F), are often used for heating water for domestic purposes. The higher temperatures achieved by the new aerogel would make it ideal for heating not just water, but possibly an entire home. Beyond domestic uses, larger-scale applications of the new aerogel could also be used for industrial purposes, helping with the manufacturing of food, chemicals and other items that may require high heat.
Aiding this is the fact that all of the materials used to make the aerogel are abundant and inexpensive. According to the researchers, the only costly part of the aerogel's manufacture is the drying process which requires a specialized device called a critical point dryer. This device allows for the very precise drying process that extracts the solvents from the gel while preserving its nanoscale structure.
Additionally, the fact that the aerogel is made in a batch process rather than a continuous one could limit its rate of production. However, a preliminary economic analysis of the process shows that the system can be economically viable, especially when compared to existing, vacuum-based systems.
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