It could soon be more practical to cool buildings using solar water heaters and waste heat from generators. That’s because of new porous materials developed by researchers from the Pacific Northwest National Laboratory. These materials can improve a process called adsorption chilling, which can be used for refrigeration and air conditioning.
Adsorption chillers are too big and expensive for many applications, such as use in homes. Peter McGrail, who heads the research effort, predicts that the materials could allow adsorption chillers to be 75 percent smaller and half as expensive. This would make them competitive with conventional, compressor-driven chillers.
All refrigerators and air conditioners cool by evaporating a refrigerant, a process that absorbs heat. They differ in how that refrigerant is condensed so that it can be reused for cooling. Unlike the technology inside most air conditioners, which employs electrically driven compressors to mechanically compress the vaporized refrigerant, adsorption chillers use heat to condense the refrigerant. Adsorption chillers are typically far less efficient than chillers that use electrical compressors, and are bulky and expensive. But they have the advantage of being cheap to operate, since they require very little electricity. “If you have waste heat, you can run it for free,” McGrail says.
So far these chillers have been limited to applications where there is a lot of waste heat—such as industrial facilities and power plants—or where electricity isn’t always available. Cutting their size and cost could make them attractive in more applications, including in homes, where they could be run using hot water from solar heaters, McGrail says.
The key is improving the solid adsorbent material. In an adsorption chiller, evaporated refrigerant is adsorbed—it adheres to a surface of a solid, such as silica gel. The silica gel can hold a large amount of water in a small space—it essentially acts as a sponge for the water vapor. When the gel it heated, it releases the water molecules into a chamber. As the concentration of water vapor in the chamber increases, the pressure rises until the water condenses.
McGrail is replacing silica gel with an engineered material made by creating nanoscopic structures that self-assemble into complex three-dimensional shapes. The material is more porous than silica gel, giving it a larger surface area for water molecules to cling to. As a result, it can trap three to four times more water, by weight, than silica gel, which helps reduce the size of the chiller.