When the heat of summer hits, air conditioners kick in and energy demand skyrockets, straining the electrical grid. In a warmer world, more efficient cooling options will play an important role in curbing the increase in cooling-related energy demand. This is especially true for the nearly 80 percent of the world’s population living in countries near the equator, where even small increases in temperature can be life-threatening.
New research from Pacific Northwest National Laboratory (PNNL) provides a roadmap showing how more efficient refrigeration systems are achievable with industry development and support. The invited research study appeared in the magazine, Chemical research accounts.
“Right now this is basic science. However, this could be a game changer for the industry,” said Radha MotkuriPNNL chemical engineer and corresponding author.
The chemistry of cool
Motkuri and the research team examined one approach that could yield significant energy savings: adsorption cooling. These systems can run on small amounts of waste heat from a building or industrial facility to drive reactions between a vapor refrigerant and a solid material.
“Once we enter power the first time, that’s it,” explains Motkuri. “Then the system continues to cycle – adsorption, desorption, adsorption, desorption – with very little power input.”
This is in stark contrast to conventional refrigeration systems that use a compressor and require regular energy.
In adsorption refrigeration systems, molecules of vapor refrigerant (the guest) are adsorbed into the nanopores of a solid material (the host). Motkuri and his collaborators investigated how changing the geometry of the nanopores (pore engineering) and the rate of host-guest chemical interactions influence the cooling capacity and energy efficiency of adsorption refrigeration systems. Image by Rose Perry | Pacific Northwest National Laboratory
Tuning an adsorption cooling system to achieve ideal cooling capacity and energy efficiency requires an understanding of the complex chemistry between the system’s vapor refrigerant, called the guest, and the solid absorbing material, called the host. Motkuri and his collaborators dug into these details – adjusting the pore geometry of the solid sorbent, the rate of chemical interactionsand even the impact of minor defects in the solid material — to understand how they affect the whole system. Recently, the team was invited to merge their work into an efficient ensemble that can help developers in the refrigeration industry meet the demand for more energy-efficient options..
“Refrigerant-based adsorption refrigeration eliminates the major cost, efficiency and reliability issues that have limited the adoption of current water-based adsorption refrigeration systems in commercial and residential buildings,” said Pete McGrail, lab worker and chemical engineer who led PNNL’s adsorption refrigeration efforts for many years. “This journal article represents a summary of years of research into new sorbent-refrigerant pairs that have significantly improved adsorption refrigeration technology.”
Environmentally conscious components
With global heat waves on the rise and cooling-related energy needs that are expected to triple by 2050, there is a push for cooling systems with a smaller ecological footprint. In addition to more energy-efficient systems, this also includes changing standards for refrigerants.
Commonly used hydrofluorocarbon refrigerants will be phased out in the coming years in favor of more environmentally friendly hydrofluoro-olefins (HFOs). HFOs have a near-zero global warming potential, which means that emissions from HFOs trap much less relative heat in the atmosphere than emissions from fluorocarbon refrigerants.
In an invited article for the Accounts of Chemical Research, Motkuri and his co-authors provided this cover illustration showing how the chemistry between the refrigerant and the solid sorbent materials in an adsorption refrigeration system can be refined to affect refrigeration capacity and energy efficiency. Reprinted with permission from Accounts of Chemical Research 2022 55 (5). Copyright 2022 American Chemical Society. Image by Mike Perkins | Pacific Northwest National Laboratory
Motkuri and his collaborators were aware of this transition and conducted their tests with the readily available, inexpensive fluorocarbon refrigerant, R-134a. This fluorocarbon refrigerant has a high global warming potential, but exhibits chemical behavior similar to HFOs, making it a suitable alternative for studying the molecular interactions of adsorption refrigeration systems that will use HFOs in the future. The researchers look forward to integrating HFOs into future research on adsorption refrigeration as the next step in green refrigeration systems.
PNNL researchers Dushyant Barpaga, Jian Zheng, Peter McGrail and Radha Motkuri contributed to the article in Accounts of chemical research. This work was supported by ARPA-E, the US Military Sealift Command, the Department of Energy Geothermal Technologies Office, and PNNL’s Laboratory Directed Research and Development program.
Through Alexandra Freibott, Pacific Northwest National Laboratory (PNNL)
Related story: Pioneering refrigerant work delivers big gains in carbon footprint reduction
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