Environmentally-sound: An acoustic-driven heat pump based on phase change

Abstract

Thermoacoustic technology is a promising approach for environmentally-friendly, low-cost heat pumping. Here, we present experiments demonstrating an acoustic-driven phase-change heat pump, and a theoretical performance analysis. An experimental setup was constructed and tested, employing a binary mixture of an ‘inert’ and a ‘reactive’ component as the working fluid. In such a system, the reactive component undergoes evaporation and condensation as part of the acoustic cycle, resulting in latent heat transfer that augments the overall heat flux. In order to further characterize the performance of such a system, a mathematical model of the thermoacoustic heat pump was employed. Experiments generally demonstrate that a larger cooling power and a higher COP can be obtained with phase change, compared with a classical system. However, this enhancement is only maintained as long as the temperature difference does not exceed a ’critical’ value. Otherwise, the time-averaged mass flux reverses its direction and thereafter carries heat against the heat pumping direction. This critical temperature difference is proportional to the value of the local acoustic impedance. We found that increasing the acoustic impedance by locally enlarging the cross-sectional area of the stack, resulted in better performance and enabled an increased temperature difference. Further, a high concentration of the reactive component is required for efficient operation. For instance, a COP above 40% of the Carnot COP can theoretically be obtained when the heat pump is operated with isopropanol at a concentration of ~0.8, at a temperature difference of 30°C.