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Thermoacoustic fridge achieves greater efficiency

CHINA: Chinese scientists claim to have developed a heat-driven thermoacoustic refrigerator (HDTR) with a COP of 1.12, nearly three times higher than previously achieved.

The improvement is said to rely on a novel bypass configuration that significantly improves the efficiency of this technology. 

HDTRs are recognised as a promising cooling technology with the advantages of having no moving parts, while using climate-friendly substances like helium and nitrogen. However, its relatively low efficiency has hindered its commercial application.

A thermoacoustically-driven refrigerator consists of a thermoacoustic engine and a thermoacoustic cooler, and works by converting thermal energy into acoustic work necessary to pump heat from lower to higher temperatures.

Researchers led by Prof Lou Ercang from the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences in Beijing found that existing advanced direct-coupled HDTRs experienced an unexpected decrease in COP with increased heating temperature. 

They discovered a temperature matching constraint in these systems that could be overcome by allowing part of the acoustic power to be bypassed. As a result, the system achieves greater efficiency through what the researcher’s describe as energy flow field synergy. 

The researchers then built a kilowatt-scale HDTR prototype. Using helium as the working gas, they achieved a record high experimental COP of 1.12 with a cooling capacity of 2.53kW, at the heating, ambient, and cooling temperatures of 450°C, 35°C, and 7°C, respectively. This COP is said to be 2.7x higher than the best result ever achieved for existing HDTRs. 

According to the researchers, in the field of room-temperature, heat-driven refrigeration, their new HDTR technology not only outperforms adsorption refrigerators and single-effect absorption refrigerators but is comparable to double-effect absorption refrigerators. 

These results highlight the great promise of this novel HDTR technology in commercial air conditioning applications.

The researchers claim that the novel HDTR system has an intrinsically higher COP than absorption refrigerators due to its efficient utilisation of high-temperature thermal energy. Their numerical analysis suggests an outstanding potential COP value of 2 for the proposed HDTR system at a heating temperature of more than 800℃. 

The researchers stated that they would make some modifications to the current system and increase the heating temperature to further improve the cooling performance of the system. After making that change, they anticipate achieving a COP that exceeds double-effect absorption refrigerators in the foreseeable future.

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