The predicted efficiency of a simple thermoacoustic waste heat power conversion device has been investigated as part of a collaborative effort combining a thermoacoustic engine with a piezoelectric transducer. Symko et al. [Microelectron. J. 35, 185-191 (2004)] at the University of Utah built high frequency demonstration engines for this application, and Lynn [ASMDC report, accession number ADA491030 (2008)] at the University of Washington designed and built a high efficiency piezoelectric unimorph transducer for electroacoustic conversion. The design presented in this paper is put forward to investigate the potential of a simple high frequency, air filled, standing wave thermoacoustic device to be competitive with other small generator technologies such as thermoelectric devices. The thermoacoustic generator is simulated using a low-amplitude approximation for thermoacoustics and the acoustic impedance of the transducer is modeled using an equivalent circuit model calculated from the transducer's mechanical and electrical properties. The calculations demonstrate that a device performance of around 10% of Carnot efficiency could be expected from the design which is competitive with currently available thermoelectric generators.

Thermoacoustic power conversion using a piezoelectric transducer.

https://dr.ntu.edu.sg/bitstream/10356/83361/1/Acoustic%20matching%20of%20a%20traveling-wave%20thermoacoustic%20electric%20generator.pdf

Acoustic matching of a traveling-wave thermoacoustic electric generator

This article reports experimental and modelling results obtained with a small-scale standing-wave thermoacoustic engine. Reticulated vitreous carbon is used as the stack material and atmospheric air as the working fluid. The engine is tested with resonators of variable lengths in the range 57–124 mm. The engine starts generating sound at temperature differences of 200–300 °C between the hot and cold parts of the system. The acoustic pressure amplitudes up to 2 kPa are measured inside the resonator in the excited regimes. A simplified energy-balance theory adequately predicts a trend in the temperature difference for the sound onset, while underestimating actual values. Model estimations show that the stack-generated acoustic power reaches 100 mW at the stack-based efficiencies of several per cent.

Study of a small-scale standing-wave thermoacoustic engine

A thermoacoustic refrigerator was designed using a dimensionless parameter approach. Some basic insight into thermoacoustic design principles was obtained. The resulting device was used as a test bed for three different control schemes. The first was a phase-locked loop, which is the control method most often used in the literature; the second controller utilized a gradient ascent algorithm to track the operating frequency of maximum acoustic pressure; and the third utilized the same gradient ascent architecture to track the operating frequency corresponding to maximum acoustic power transfer to the resonator. The three controllers, tracking different parameters associated with a strong thermoacoustic effect, were compared in simulations and experiments. Difficulties in collecting data for the power controller resulted in unreliable data. Therefore, the power controller was not compared quantitatively with the other two. The PLL performed best in terms of thermoacoustic efficiency, but the acoustic pressure controller was able to produce more cooling power and converted electrical power to cooling power more efficiently due to the amplitude of the input voltage to the driver being held constant. The major short-coming of the gradient ascent approach was the relatively long convergence time. However, convergence time is not always relevant to refrigerator operation. The maximum acoustic pressure control scheme was determined to be the best controller considered because it has fewer sensors than the other two controllers, involves less computational effort than the power controller, and yielded better electrothermal performance than the PLL.

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Design and Control of a Standing-Wave Thermoacoustic Refrigerator

Standing wave thermoacoustic-piezoelectric (TAP) energy harvesters convert thermal energy, such as solar or waste heat energy, directly into electrical energy without the need for any moving components. The input thermal energy generates a steep temperature gradient along a porous medium called “stack.” At a critical threshold of the temperature gradient, self-sustained acoustic waves are developed inside an acoustic resonator. The associated pressure fluctuations impinge on a piezoelectric diaphragm, placed at the end of the resonator, to generate electricity. The behavior of this multi-field system is modeled using the electrical analogy approach. The developed model combines the descriptions of the acoustic resonator and the stack with the characteristics of the piezoelectric diaphragm. The equivalent electric network is analyzed to determine the system's stability and predict the temperature gradient necessary to developing self-sustained oscillations inside the harvester. The developed network is utilized also to investigate the transient performance of the harvester by employing the network theory and Simulation Program with Integrated Circuit Emphasis software package. The established stability boundaries are validated against the predictions of the root locus technique. Furthermore, the obtained results are compared with experimental results extracted from testing a prototype of the harvester. The developed approach presents an innovative tool for the design of TAP energy harvesters.

Transient characteristics and stability analysis of standing wave thermoacoustic-piezoelectric harvesters

In order to miniaturize thermoacoustic refrigerators, piezoelectric loudspeakers are used as acoustic sources for operating at high resonance frequencies. An integrated equivalent circuit model of a thermoacoustic resonance pipe driven by a piezoelectric loudspeaker is presented to investigate the coupling conditions between the piezoelectric loudspeaker and thermoacoustic resonance pipe. Using the equivalent circuit model the optimized coupling conditions can be obtained, with which the highest electroacoustic transfer efficiency and the largest acoustic output power of a piezoelectric acoustic source can be achieved. The theoretical simulations are taken to optimize the structures and the operating frequencies of the thermoacoustic systems.

Coupling between thermoacoustic resonance pipes and piezoelectric loudspeakers studied by equivalent circuit method

The selection of operating frequencies is a key step to optimize the performance of thermoacoustic systems, and a minor frequency mismatch can cause a serious drop of the efficiency. Generally, the characteristic frequency of the resonance pipe is a significant factor to determine the operating frequency, however, in a thermoacoustic refrigerator driven by an electrodynamic loudspeaker, the match between the loudspeaker and resonance pipe cannot be ignored. The two-microphone method is used to determine the operating frequency of a thermoacoustic system, meanwhile, the proper position for the stack is also determined. It is demonstrated that the two-microphone method, which can be used to obtain the sound pressure, particle velocity and acoustic impedance distributions in the pipe, is proper to determine the working conditions of a thermoacoustic system driven by an electrodynamic loudspeaker.

Li Fan

Papers

Coupling between thermoacoustic resonance pipes and piezoelectric loudspeakers studied by equivalent circuit method

Performance optimization of thermoacoustic systems by measurement with two-microphone method