The Pacific Northwest National Laboratory research team investigated an absorption cooling system, which they believed could offer significant energy savings. The system uses modest amounts of waste heat from a building or industrial plant to power reactions between a vapor refrigerant and a solid material. The adsorption cooling system is in direct contrast to conventional cooling systems that use a compressor and require regular energy inputs.
"Once we input to power the first time, that's it. Then the system keeps cycling-adsorption, desorption, adsorption, desorption-with very little power input," Radha Motkuri, PNNL chemical engineer and study's corresponding author, said.
Adsorption Cooling System Mechanism
Understanding the complex chemistry between the system's vapor refrigerant, known as the guest, and solid absorbent material, known as the host, is required for tuning an adsorption cooling system to achieve optimal cooling capacity and energy efficiency.
To better understand how they affect the overall system, Motkuri and his team investigated these subtleties, adjusting the pore geometry of the solid sorbent, the speed of chemical interactions, and even the impact of minute imperfections in the solid material. Recently, the team was encouraged to combine their efforts into an efficient ensemble that can assist cooling sector innovators in meeting the demand for more energy-efficient solutions.
The research focuses on an ongoing strategy to boost sorption cooling through a sorbent/refrigerant pair. The study targets the interactions of hydrofluorocarbon refrigerants with MOF/COP materials by analyzing host-guest chemistry and the significance of framework pore structure. The chemical variations translate into cooling performance.
The strategies include engineering framework porosity (pore size, pore volume) through elongated organic linkers, stereochemistry control during synthesis, and varying pore topology and morphology to impact adsorption isotherm behavior. It also includes manipulating the sorbate/sorbent interaction by introducing functional moieties or unsaturated metal centers to enhance working capacities in narrow pressure ranges and leveraging defective sites within the frameworks to improve working capacities further.
The atomic level understanding of sorbate-sorbent interactions is achieved through in situ experimental techniques such as synchrotron-based X-ray diffraction, in situ Fourier transform infrared spectroscopy, X-ray absorption spectroscopy, and direct sorption energy determination via calorimetry.
Furthermore, computational analyses employing density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations support the experimentally investigated interactions and adsorption mechanisms.
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Hydrofluoro-olefins and Adsorption Cooling System
In the coming years, common hydrofluorocarbon refrigerants will be phased out in favor of more environment-friendly hydrofluoro-olefins (HFOs). HFOs have a global warming potential close to zero, which means that HFO emissions contain substantially less relative heat in the atmosphere than hydrofluorocarbon refrigerant emissions. Due to this transition, the team did their testing with the easily available, low-cost hydrofluorocarbon refrigerant R-134a.
As the next stage in green cooling systems, the researchers hope to incorporate HFOs into future adsorption cooling studies.
Pete McGrail, the chemical engineer who led PNNL's adsorption cooling effort for several years, said that the journal article represents the major cost, efficiency, and reliability issues that have limited the adoption of current water-based adsorption cooling systems in residential and commercial buildings.
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