Acoustically Driven Sorption Heat Pump

Recent years have seen a dramatic rise in global cooling demand, driven by economic growth and climate change, and resulting in an increasing share of the total electric power consumption. Meanwhile, the ubiquitous vapor-compression air conditioners use refrigerants, which contribute greatly to global emission of greenhouse gases. In order to reduce the strain on electric grids, heat-driven technologies must be developed. Here, an acoustic driven sorption cooling device is examined experimentally and theoretically. The device can potentially utilize heat or electricity as a power input, uses environmentally benign working fluids, and offers simple, reliable construction with little to no moving parts. The results demonstrate the sorption-mediated, time-averaged heat-transfer mechanism, driven by the acoustic field. An unoptimized, proof-of-concept device is operated using a mixture of atmospheric air and either water or methanol, with cordierite and zeolite sorbents. The device is able to achieve temperature differentials $>30{}^{\circ }\mathrm{C}$. Moreover, a coefficient of performance of approximately 3 (based on the acoustic power input) is achieved at a temperature difference of $10{}^{\circ }\mathrm{C}$. Theoretical calculations provide an outlook on the operation of such technology, compared with existing cooling technologies, demonstrating its potential for achieving high efficiencies. Finally, prospects for further development are discussed.

Figure 1

Number of household AC units between 1990 and 2050, adapted from data reported by the International Energy Agency (IEA) [1].

Figure 2

(a) Diagram of the experimental system used to demonstrate the sorption, acoustic driven cooling mechanism. (b) Schematic representation of the sorption-based mechanism that creates a net mass and heat flux along the surface of the stack.

Figure 3

Schematic drawing of the modeled configuration.

Figure 4

Representative experimental run, showing the time evolution of the temperatures $T_m$ (in the hot and cold side of the stack) and the molar fraction of the sorbing species $C_m$ (water). The temperature difference is higher when the mole-fraction difference between the hot and cold side is negative, driving a mass flux from the cold to the hot side. This negative concentration difference is maintained as long as the zeolite plug, acting as the mass sink, is active.

Figure 5

The temperature difference, $\mathrm{\Delta }{T}_m$, and the concentration difference, $\mathrm{\Delta }{C}_m$ as a function of time during operation of the experimental device, as seen in Fig. 4 with an acoustic wave at $f=122\mathrm{Hz}$, $Q_{\mathrm{cooling}}=0\mathrm{W}$, and $p_m=1\mathrm{bar}$, $\dot{E}=1\mathrm{W}$, in a quarter-wavelength resonator with air as the inert gas. Two stages of operation are identified when allowing the use of an active mass sink, where all experiments, with and without an active mass sink, converge to the same temperature differential when the mass sink is inactive.

Figure 6

Mean and standard deviation of experimental measurements made at $f=122\mathrm{Hz}$ and $P_m=1\mathrm{bar}$, demonstrating the connection between the molar fraction and temperature difference in the stack. Dotted lines show the model calculation.

Figure 7

The temperature difference generated by the system, at a given, imposed coeficcient of performance (COP) of the experimental system, at $f=122\mathrm{Hz}$ and $p_m=1\mathrm{bar}$ with a water-air mixture. The heat load, applied through the nichrome wire, changes the COP measured at each acoustic power, $\dot{E}$, and the maximum temperature difference achieved is measured. Each point represents the mean of the set of experiments, with error bars representing the range of data for each point.

Figure 8

Theoretical calculations of (a) the COP versus temperature difference. (b) The relative COP (ratio of the COP to the Carnot limit) versus the temperature difference. Calculations are based on the properties of the experimental heat-pump system described in Fig. 2 calculated with a nominal heat load of $Q_{\mathrm{cool}}=5\mathrm{W}$ and $T_{m,\mathrm{hot}}=40{}^{\circ }\mathrm{C}$, with ${\tau }_{\alpha }\cong 3.3$. The calculations examine different gas mixtures comprised of various combinations of “inert” (air, helium, and argon) and “sorbing” (water, methanol, and ethanol) species. The Carnot COP is also plotted versus the temperature difference. The computed COP is compared with the highest practical and theoretical COP achievable by different cooling technologies in existence today [25], labeled as follows: TO, thermionic; TE, thermoelectric; M, magnetocaloric; CBT, conduction-based thermoacoustics; VC, vapor compression.