Getting More Heat Out of Sunlight

A test device installed on a rooftop at MIT proved the effectiveness of the new insulating material. When placed in sunlight the device heated up to 220 degrees C., even though the outside temperature at the time was about zero degrees. (Source: E. Wang et al., MIT)

A newly developed material that is so perfectly transparent you can barely see it could unlock many new uses for solar heat. It generates much higher tempera­tures than conventional solar collectors do – enough to be used for home heating or for industrial processes that require heat of more than 200 degrees Celsius. The key to the process is a new kind of aerogel, a lightweight material that consists mostly of air, with a structure made of silica. The material lets sunlight pass through easily but blocks solar heat from escaping.

The key to efficient collec­tion of solar heat, Evelyn Wang, head of the MIT-Department of Mechanical Engineering, explains, is being able to keep something hot internally while remaining cold on the outside. One way of doing that is using a vacuum between a layer of glass and a dark, heat-absor­bing material, which is the method used in many concen­trating solar collectors but is relatively expensive to install and maintain. There has been great interest in finding a less expensive, passive system for collecting solar heat at the higher temperature levels needed for space heating, food processing, or many indus­trial processes.

Aerogels, a kind of foam-like material made of silica particles, have been developed for years as highly efficient and lightweight insulating materials, but they have generally had limited trans­parency to visible light, with around a 70 percent trans­mission level. Wang says developing a way of making aerogels that are trans­parent enough to work for solar heat collection was a long and difficult process involving several researchers for about four years. But the result is an aerogel that lets through over 95 percent of incoming sunlight while maintaining its highly insu­lating pro­perties.

The key to making it work was in the precise ratios of the different materials used to create the aerogel, which are made by mixing a catalyst with grains of a silica-containing compound in a liquid solution, forming a kind of gel, and then drying it to get all the liquid out, leaving a matrix that is mostly air but retains the original mixture’s strength. Producing a mix that dries out much faster than those in conven­tional aerogels, they found, produced a gel with smaller pore spaces between its grains, and that therefore scattered the light much less.

In tests on a rooftop on the MIT campus, a passive device consisting of a heat-absorbing dark material covered with a layer of the new aerogel was able to reach and maintain a tempera­ture of 220 C, in the middle of a Cambridge winter when the outside air was below 0 C. Such high tempera­tures have previously only been practical by using concen­trating systems, with mirrors to focus sunlight onto a central line or point, but this system requires no concen­tration, making it simpler and less costly. That could poten­tially make it useful for a wide variety of appli­cations that require higher levels of heat.

For example, simple flat rooftop collectors are often used for domestic hot water, producing tempera­tures of around 80 C. But the higher tempera­tures enabled by the aerogel system could make such simple systems usable for home heating as well, and even for powering an air condi­tioning system. Large-scale versions could be used to provide heat for a wide variety of appli­cations in chemical, food pro­duction, and manu­facturing processes.

Graduate student Lin Zhao describes the basic function of the aerogel layer as “like a greenhouse effect. The material we use to increase the tempera­ture acts like the Earth’s atmosphere does to provide insu­lation, but this is an extreme example of it.” For most purposes, the passive heat collection system would be connected to pipes containing a liquid that could circulate to transfer the heat to wherever it’s needed. Alter­natively, Wang suggests, for some uses the system could be connected to heat pipes, devices that can transfer heat over a distance without requiring pumps or any moving parts.

Because the principle is essen­tially the same, an aerogel-based solar heat collector could directly replace the vacuum-based collectors used in some existing appli­cations, providing a lower-cost option. The materials used to make the aerogel are all abundant and inex­pensive; the only costly part of the process is the drying, which requires a specia­lized device called a critical point dryer to allow for a very precise drying process that extracts the solvents from the gel while preserving its nano­scale structure.

Because that is a batch process rather than a continuous one that could be used in roll-to-roll manu­facturing, it could limit the rate of production if the system is scaled up to industrial production levels. “The key to scaleup is how we can reduce the cost of that process,” Wang says. But even now, a pre­liminary economic analysis shows that the system can be econo­mically viable for some uses, especially in comparison with vacuum-based systems. (Source: MIT)

Reference: L. Zhao et al.: Harnessing Heat Beyond 200 °C from Unconcentrated Sunlight with Nonevacuated Transparent Aerogels, ACS Nano, online 7 June 2019; DOI: 10.1021/acsnano.9b02976

Link: Device Research Laboratory DRL, Mechanical Engineering Dept., Massachusetts Institute of Technology, Cambridge, USA

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