Thermoacoustic Energy Conversion Research Articles

Standing-wave and traveling-wave thermoacoustics in solid media

Thermoacoustics is a multi-physics process in which heat and mechanical power (associated with waves) can be converted into one other. This process has been exploited to design different types of modern-day energy conversion devices, such as thermoacoustic engines (TAE) and refrigerators (Swift, 1988), since the first experimental assessment of the phenomenon by Soundhauss (1850) in the glass blowing process. To date, all the thermoacoustic devices are fluid-based, using mostly air or Helium as the working medium. Our study explores for the first time the possibility of achieving thermoacoustic energy conversion in solid media, by laying out the fundamental theory and showing numerical evidence of the existence of both standing and traveling thermoacoustic waves in solids. Consistent with established results for fluid-based TAEs, the growth-rate-to-frequency ratio of traveling waves in solids is found to be significantly higher than that of standing waves. While sold-state thermoacoustics (SSTA) share some commonalities with their fluid counterpart, some important and noteworthy distinctions are present. For example, solids have the potential to be highly engineered (e.g., metamaterials), with their properties tuned to enhance thermoacoustic energy conversions. The theoretical investigation of this mechanism may motivate novel ideas designing a new generations of ultra-compact, highly efficient thermoacoustic devices.

Experimental investigation of the thermoacoustic traveling-wave engine

Thermoacoustic engines are devices with external heat input, in which thermal energy is converted into energy of acoustic oscillations. Their main advantages include high energy conversion efficiency (up to 35%), minimal number of moving parts, high reliability, absence of contact seals and operation from a variety of heat sources (hydrocarbon fuels, thermal emissions, nuclear sources, solar radiation, etc.). The main goal of this work is to develop, manufacture and carry out the experimental investigations of the thermoacoustic traveling wave engine with a maximum heat input from the electric heating element up to 1000 W. The thermoacoustic engine includes an inertance tube, three heat exchangers, a regenerator, a thermal buffer tube and an acoustic resonator. Helium acts as the working fluid in the engine at an average pressure of 1.0 to 3.0 MPa. The frequency of acoustic oscillations on the steady-state operating mode of the engine is 96-98 Hz. The regenerator is made in the form of a package of metal screens and is the main element of the engine, in which thermoacoustic energy conversion is carried out. In experimental research the following tasks were solved: - determination of the optimal conditions for the excitation of acoustic oscillations in the internal contour of the thermoacoustic engine; - determination of the minimum onset temperature at which acoustic oscillation are excited; - determination of the dependences of the onset temperature, the frequency and the amplitude of acoustic oscillations from the input thermal power; - determination of the efficiency of the rmoacoustic energy conversion; In experimental studies, an optimal mean helium pressure is determined, which ensures the minimum heat exchanger temperature 436 К (163 C) required to start the engine. The maximum value of the acoustic power generated by the regenerator was 90 W. At the same time, the efficiency of conversion of thermal energy into acoustic energy reached 22.5% at a heat exchanger-heater temperature of 317 C. Also during the tests, it was possible to establish the dependences of the onset temperature, the frequency of acoustic oscillations and the amplitude of the acoustic pressure on the supplied thermal power. This type of engine described in this paper can be used in many practical applications.

Using a side-branched volume to tune the acoustic field in a looped-tube travelling-wave thermoacoustic engine with a RC load

Travelling-wave thermoacoustic engine utilises a compact acoustic network to obtain a right time-phasing between the acoustic velocity and pressure oscillations within the regenerator to force gas parcels to experience a Stirling-like thermodynamic cycle. As such, thermal energy can be converted to mechanical work (i.e., high-intensity pressure waves). It is therefore crucial to control the time-phasing carefully to improve the performance of thermoacoustic engines. Various ways have been proposed and demonstrated for adjusting time-phasing, including both passive and active methods. The aim of this study is to introduce a new passive phase tuning method (i.e., a side-branched acoustic volume) to tune the time-phasing within a looped-tube travelling wave thermoacoustic engine. The proposed concept has been investigated both numerically and experimentally in this research. An experimental rig was simulated and designed using DeltaEC software (Design Environment for Low-amplitude ThermoAcoustic Energy Conversion). It was then constructed according to the obtained theoretical model. The result of this study showed a qualitative agreement between experimental measurement and numerical simulations, demonstrating that the proposed technique can effectively adjust the phase angle between the acoustic velocity and pressure oscillations within the loop-tube thermoacoustic engines, and improve its performance.

Optimization of thermoacoustic engine driven thermoacoustic refrigerator using response surface methodology

Thermoacoustic engines (TAEs) are devices which convert heat energy into useful acoustic work whereas thermoacoustic refrigerators (TARs) convert acoustic work into temperature gradient. These devices work without any moving component. Study presented here comprises of a combination system i.e. thermoacoustic engine driven thermoacoustic refrigerator (TADTAR). This system has no moving component and hence it is easy to fabricate but at the same time it is very challenging to design and construct optimized system with comparable performance. The work presented here aims to apply optimization technique to TADTAR in the form of response surface methodology (RSM). Significance of stack position and stack length for engine stack, stack position and stack length for refrigerator stack are investigated in current work. Results from RSM are compared with results from simulations using Design Environment for Low-amplitude Thermoacoustic Energy conversion (DeltaEC) for compliance.

Performance Evaluation of Ceramic Substrates for Cooling Applications in Thermo-acoustic Refrigerators

Thermo-acoustic refrigerators have recently drawn more attentions because of its eco-friendlier potential to address the current environmental issues resulting from the use of traditional vapour compression refrigerators. This paper aims at evaluating different selected ceramic substrates, with square pores, from the point of view of their performance as stack materials in the design of thermo-acoustic standing wave refrigerators. A 465mm standing wave thermo-acoustic refrigerator was designed using numerical approximation provided by a modeling code called DELTAEC (Design Environment for Low-amplitude Thermo-Acoustic Energy Conversion). The design developed focuses in particular, on the effects of different ceramic substrate configurations (diameter, length, porosity and position) on the performance of the device. Meaningful comparison on the effect of the ceramic substrates configuration is provided in order to assess the performance of the device. Guidance on the identification and the selection of the best geometrical configurations of ceramic substrates are the main contributions of this work.

Investigation of carbon nanotubes for acoustic transduction applications

Traditional acoustic transduction sources typically begin with the generation of an electrical excitation pulsed through an amplifier into an electroacoustic material to create a mechanical vibration which is then converted into an acoustic wave to produce sound. The lower the preferred transmitting frequency (and hence, longer acoustic range) desired, the larger the required size of the source. Often this means that for acoustic projectors producing sound at frequencies below a few kHz, that the electroacoustic device will need to be very large in order to produce very long sound waves. This has a limitation for incorporating low frequency sonars on smaller autonomous underwater vehicles (AUVs). The topic of our presentation is an acoustic source technology that relies on the conversion of thermal energy to acoustic energy. Though the concept was first demonstrated in 1917, the recent advent of carbon nanotubes (CNT) now makes it possible to transmit acoustic waves in small and affordable packages using the thermophone approach. The presentation begins with an overview of thermophones and a method for incorporating the CNTs into a useable transduction device. Discussions will include detailing on-going efforts for the encapsulation, packaging and further insight of the thermoacoustic energy conversion process. Recent 2016 experimental data will be shown. New start numerical and analytical modeling efforts and future development thrusts will be discussed.

Numerical model of the onset of acoustic oscillations in a pulse tube engine

A numerical model of the thermal excitation process of acoustic oscillations in the duct of a pulse tube engine is presented. Theoretical data on induced thermoacoustic heat transport and generation of acoustic power in the regenerator are evaluated. The results of numerical simulation demonstrate interesting features in the onset of acoustic oscillations in the engine. The model includes non-linear losses accounting for variable transduction loads. The onset temperature, operating frequency, and heat transfer rates in the regenerator and heat exchangers, as well as generated acoustic power, are calculated. This study extends the understanding of mechanisms of thermoacoustic energy conversion.

Feasibility Study on Thermoacoustic Cooling for Low-Power Handheld Electronic Devices

A feasibility study on developing a small-scale thermoacoustic cooler based on form and size factors for a typical cell phone is presented. First, an approximate analytical model for the temperature difference was derived using the linear theory of thermoacoustics. Cooling performance could be reasonably predicted with the analytical model proposed in this study. Air and helium as the working gases and the operating frequencies of 3 kHz for air and 9.2 kHz for helium are considered within the scope of typical cell phone configurations. A stack as a core of thermoacoustic cooler is designed to accomplish the most effective performance based on normalized parameters. For the 57 mm thermoacoustic cooler operating at 3 kHz with air, the maximum temperature difference of 23.13 °C across the stack in the resonance cavity is achieved with a drive ratio of 2% with air as the medium and Mylar as a stack material. This temperature difference varies depending on the stack placement along the length of the resonance cavity, but the maximum difference was achieved when the center of stack is placed at around 7 mm away from the driver end. The drive ratio, which is proportional to the power required to produce the thermoacoustic effect, is shown to be directly related to the cooling performance achieved by thermoacoustic drivers. For example, while a drive ratio of 2% results in a temperature difference of over 20 °C at its maximum, a drive ratio of 0.2% causes a temperature difference less than 1 °C. This will be one of hardware issues to be considered in making commercially viable products. The possibility of omitting heat exchangers in the thermoacoustic cooler is investigated considering their manufacturing cost and the relatively minute improvement they bring to overall cooling for small-scale systems. The numerical result of the thermoacoustic cooling system based on design environment for low-amplitude thermoacoustic energy conversion (DeltaEC) is compared to the theoretical result. Discrepancies between the two results exist in the range of 10–15% mainly due to the limitation imposed by short stack considerations and the linear theory of thermoacoustics.

Studies on Performance of Thermoacoustic Prime Mover

Thermoacoustic engines are the devices that convert thermal energy into acoustic energy without moving parts. The main objective of this study is to analyze the performance of a thermoacoustic prime mover measured in terms of onset temperature difference, frequency, and pressure amplitude by varying resonator, stack length, and plate thickness. From the experiments, it is observed that onset temperature difference and pressure amplitude increases with increase in resonator and stack length with minimum plate thickness, whereas the frequency increases with decrease in resonator and stack length with higher plate thickness. The experimental results are compared with simulated results via Design Environment for Low Amplitude Thermoacoustic Energy Conversion software (Los Alamos National Laboratory, Los Alamos, New Mexico, USA).

Onset of Oscillations in Traveling Wave Thermo-Acoustic-Piezo-Electric Harvesters Using Circuit Analogy and SPICE Modeling

Equations governing different physical fields such as mechanical, acoustical, and electrical are inherently similar. This enables mechanical, thermal, and acoustical networks to be fully described with analogous electric networks. For thermo-acoustic-piezo-electric (TAP) harvesters, such a modeling approach allows the whole system to be characterized in the electrical domain and facilitates the understanding of the underlying physics. In this paper, a traveling wave thermo-acoustic-piezoelectric (TWTAP) energy harvester is considered which converts 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 regenerator. 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 piezo-electric diaphragm, placed at the end of the resonator, to generate electricity. The acoustic pressure oscillations are amplified by a specially designed acoustic feedback loop that introduces appropriate phasing to make the pulsations take the form of traveling waves. The behavior of this TWTAP is modeled using electrical circuit analogy. The developed model combines the descriptions of the acoustic resonator, feedback loop, and the regenerator with the characteristics of the piezo-electric diaphragm. With the help of a simulation program with integrated circuit emphasis (SPICE) code, the developed electric circuit is used to analyze the system’s stability with regard to the input heat and hence predict the necessary temperature ratio required to establish the onset of self-sustained oscillations inside the harvester’s resonator. The predictions are compared with published results obtained using root locus and numerical methods and validated against experiments. This approach provides a very practical approach to the design of TAP energy harvesters both in the time and frequency domain. Such capabilities do not exist presently in the well-known design tool design environment for low-amplitude thermo-acoustic energy conversion (DeltaEC) developed at Los Almos National Laboratory which is limited to steady-state analysis. This is in contrast to the present approach which can be applicable to both steady as well as transient analysis.

D112 テーパ型スタックによる熱音響装置の性能向上(OS8 外熱機関・廃熱利用技術(1))

We are developing a highly efficient thermoacoustic system that is capable of using industrial waste heat efficiently. In this study, we have designed a taper stack, for the thermoacoustic system, in which the cross sectional area increases toward the axial direction. Theoretically, the smaller the taper ratio, the prime mover efficiency improves compared with the efficiency of a straight prime mover. The efficiency of thermoacoustic energy conversion was examined for a taper prime mover and cooler with a taper ratio of 0.75. The specific Carnot efficiency of prime mover improved to 1.85 times whereas that for a straight by experiment.

Computational fluid dynamics simulation of open ended thermoacoustic prime mover with straight and tapered resonator

A thermoacoustic refrigerator driven by a thermoacoustic primemover is an effective way to produce durable and long lasting refrigeration due to high reliability, no exotic materials, and no moving parts. Resonator geometry is also one of the important factors that influence the performance of a thermoacoustic prime mover, namely, frequency. Computational fluid dynamics simulation of performance comparison of thermoacoustic prime mover with a straight and tapered resonator is chosen for the present study under an identical stack condition with the air as a working fluid. The frequency and pressure amplitude of oscillations obtained from simulation results were found to be more in the tapered resonator than the straight resonator. Apart from computational fluid dynamics simulation, the simulation studies have also been conducted using design environment for low-amplitude thermoacoustic energy conversion, which predicts the performance of thermoacoustic primemover comparatively well. Simulation results from computational fluid dynamics and design environment for low-amplitude thermoacoustic energy conversion were compared and found to be matching well, representing the good validity of computational fluid dynamics modeling.

Optimization of thermoacoustic refrigerator using response surface methodology

Thermoacoustic refrigerator (TAR) converts acoustic waves into heat without any moving parts. The study presented here aims to optimize the parameters like frequency, stack position, stack length, and plate spacing involving in designing TAR using the Response Surface Methodology (RSM). A mathematical model is developed using the RSM based on the results obtained from DeltaEC software. For desired temperature difference of 40 K, optimized parameters suggested by the RSM are the frequency 254 Hz, stack position 0.108 m, stack length 0.08 m, and plate spacing 0.0005 m. The experiments were conducted with optimized parameters and simulations were performed using the Design Environment for Low-amplitude ThermoAcoustic Energy Conversion (DeltaEC) which showed similar results.

F222 熱音響装置におけるテーパ型スタックの性能(OS10 外燃機関・廃熱利用技術(3))

We are developing a highly efficient thermo-acoustic system that is operatable below 200 deg. Celsius and is capable of using industrial waste heat efficiently. In this study, we have designed a taper stack, for the thermo-acoustic system, in which the cross sectional area increases toward the axial direction. Theoretically, the smaller the taper ratio, the stack efficiency improves compared with the efficiency of a straight stack. The efficiency of thermoacoustic energy conversion was examined for a taper stack with a taper ratio of 0.75. The specific Carnot efficiency improved to 2.95%, whereas that for a straight stack was 1.37%.

Energy harvesting from a standing wave thermoacoustic-piezoelectric resonator

In this paper, a one-dimensional thermoacoustic-piezoelectric (TAP) resonator is developed to convert thermal energy, such as solar or waste heat energy, directly into electrical energy. The thermal energy is utilized to generate a steep temperature gradient along a porous stack which is optimally sized and placed near one end of the resonator. At a specific threshold of the temperature gradient, self-sustained acoustic waves are generated inside the resonator. The resulting pressure fluctuations excite a piezoelectric diaphragm, placed at the opposite end of the resonator, which converts the acoustic energy directly into electrical energy without the need for any moving components. The theoretical performance characteristics of this class of thermoacoustic-piezoelectric resonators are predicted using the Design Environment for Low-amplitude Thermoacoustic Energy Conversion Software. These characteristics are validated experimentally on a small prototype of the system. Particular emphasis is placed on monitoring the temperature field using infrared camera, the flow field using particle image velocimetry, the acoustic field using an array of microphones, and the energy conversion efficiency. Comparisons between the theoretical predictions and the experimental results are also presented. The developed theoretical and experimental techniques can be invaluable tools in the design of TAP resonators for harvesting thermal energy in areas far from the power grid such as nomadic communities and desert regions for light, agricultural, air conditioning, and communication applications.

Design and experimental validation of looped-tube thermoacoustic engine

The aim of this paper is to present the design and experimental validation process for a thermoacoustic looped-tube engine. The design procedure consists of numerical modelling of the system using DELTA EC tool, Design Environment for Low-amplitude ThermoAcoustic Energy Conversion, in particular the effects of mean pressure and regenerator configuration on the pressure amplitude and acoustic power generated. This is followed by the construction of a practical engine system equipped with a ceramic regenerator — a substrate used in automotive catalytic converters with fine square channels. The preliminary testing results are obtained and compared with the simulations in detail. The measurement results agree very well on the qualitative level and are reasonably close in the quantitative sense.

Modeling of Thermoacoustic Resonators With Nonuniform Medium and Boundary Conditions

The subject of this paper is modeling of low-amplitude acoustic fields in enclosures with nonuniform medium and boundary conditions. An efficient calculation method is developed for this class of problems. Boundary conditions, accounting for the boundary-layer losses and movable walls, are applied near solid surfaces. The lossless acoustic wave equation for a nonuniform medium is solved in the bulk of the resonator by a finite-difference method. One application of this model is for designing small thermoacoustic engines. Thermoacoustic processes in the regular-geometry porous medium inserted in resonators can be modeled analytically. A calculation example is presented for a small-scale thermoacoustic engine coupled with an oscillator on a flexing wall of the resonator. The oscillator can be used for extracting mechanical power from the engine. A nonuniform wall deflection may result in a complicated acoustic field in the resonator. This leads to across-the-stack variations of the generated acoustic power and local efficiency of thermoacoustic energy conversion.

CFD simulation of thermoacoustic cooling

In the field of thermoacoustic energy conversion, the application of numerical analysis techniques, specifically computational fluid dynamics (CFD) simulations, have gained ground in recent years. Previous efforts have focused on single thermoacoustic couples that were subjected to the thermoacoustic effect through an oscillatory boundary condition. CFD simulations of an entire thermoacoustic device are computationally expensive and few examples exist. The present work presents an extension of a simulation of a whole thermoacoustic engine that also includes a refrigeration stack. Through interaction of thermally generated sound waves, cooling of the working gas in this stack is demonstrated.

C06 多段階音響スターリングエンジン発電機(パルス管冷凍機,熱音響機器及び関連要素と応用システム(2))

We designed and built a multistage thermoacoustic Stirling engine electric generator prototype, where three regenerators aligned in series, not in parallel, enables the power gain of 5.1 with the temperature ratio of 1.95. In this study, we increased the acoustic power level to 17 W from 20 mW in the previous experiment. Additionally, we studied the thermal efficiency of thermoacoustic energy conversion by controlling specific acoustic impedance, in order to verify the multistage thermoacoustic Stirlung engine as a useful energy conversion device.

CFD simulation of a thermoacoustic engine with coiled resonator

Thermoacoustic energy conversion is based on the Stirling cycle. In their most basic forms, thermoacoustic devices are comprised of two heat exchangers, a porous medium, both placed inside a resonator. Work is created through the interaction of strong sound waves with the porous medium that is subject to external heating. This work explores the effect of resonator curvature on the thermoacoustic effect. A CFD analysis of a whole thermoacoustic engine was developed and the influence of a curved resonator on the thermoacoustic effect is discussed. The variation of pressure amplitude and operating frequency serves as metrics in this investigation. It was found that the introduction of curvature affects the pressure amplitude achieved. Severely curved resonators also exhibited a variation in operating frequency.