Utilization of waste heat from industries and low-grade heat from renewable energy sources, such as solar and geothermal, is particularly important for global energy efficiency. Recovering low-grade heat helps cut down on energy demand, energy costs, and carbon emissions, and also helps improve overall system efficiency. The ability to recover low-grade heat for cooling purposes would be attractive for cooling in buildings, preservation of food and medical supplies, among other applications.
Cooling using thermoacoustic wave-driven thermodynamic cycle presents an emerging heat-driven cooling technology involving no mechanical moving parts or electrical power, therefore no harmful carbon and other ozone-depleting emissions. A thermoacoustic refrigerator is made up of a thermoacoustic cooler and a thermoacoustic engine. It works by converting heat energy into acoustic work required to move heat from lower to higher temperatures.
Onset temperature difference at two ends of a well-designed thermoacoustic engine of a thermoacoustic refrigerator should exceed a critical value so that acoustic gains exceed loses to initiate a spontaneous gas oscillation. Therefore, to better utilize low-grade heat, the onset temperature should be kept to the minimum in a thermoacoustic engine.
Despite the promising results posted in previous studies, existing thermoacoustic systems using gaseous phase-matching elements still show high onset temperatures and unacceptable cooling efficiencies, which limit their low-grade heat utilization capacity. Fortunately, a gas-liquid resonator may provide an effective solution, with previous studies showing that it can reduce the onset temperature significantly, and improve cooling efficiency.
Scientists at University of Cambridge, Dr. Jingyuan Xu and Professor Simone Hochgreb together with Professor Ercang Luo at Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences explored the use of a gas-liquid resonator in a thermoacoustically-driven refrigerator with an objective of resolving high onset temperature and low cooling efficiency affecting the implementation of thermoacoustic cooling systems in low-grade heat recovery. They were looking to improve cooling efficiency and cooling power density, and extend the required heating temperature to the lowest. The original research article is published in the journal, Applied Energy.
First, theoretical analyses on multi-stage systems were performed to investigate steady performance and onset features. Then the research team studied carefully the effects of liquid volume ratio, mean pressure, and expected liquid mechanical damping coefficient on working frequency and onset temperatures in systems with different stages. A comparison of the system’s onset performance was also made with conventional gas-only resonator systems.
The investigators observed that a thermoacoustic refrigerator using gas-liquid resonators outperformed a conventional gas resonator system to a greater extent on both onset and steady performance. Using a gas-liquid resonator to replace a gas-only resonator it was observed that the onset heat temperature difference was reduced from 144.1 K to 35.5 K while cooling power and efficiency were improved by a factor of 5.6 and 1.5, respectively. It was also observed that higher pressure amplitudes and low working frequencies created by the gas-liquid resonator contributed to an improvement in performance.
The new system’s cooling steady performance was mainly affected by liquid volume ratio and the number of stages. Increasing the number of stages lead to a higher cooling power but didn’t significantly affect cooling efficiency. The liquid volume ratio optimum value fell within a range of 0.3 and 0.5 for both cooling efficiency and cooling power. Onset temperature difference is closely related to liquid damping coefficient, liquid volume ratio, and mean pressure. Most systems considered for the study were observed to begin oscillation at onset temperature differences below 50 K.
When implemented, the novel thermo-acoustically driven refrigerator reported in the study will enable for efficient low-grade recovery from renewable energy sources and waste heat from industrial processes.
Jingyuan Xu, Ercang Luo, Simone Hochgreb. Study on a heat-driven thermoacoustic refrigerator for low-grade heat recovery. Applied Energy, issue 271 (2020), pages 115-167.