Design and optimization of thermoacoustic devices

Abstract

Thermoacoustics deals with the conversion of heat energy into sound energy and vice versa. It is a new and emerging technology which has a strong potential towards the development of sustainable and renewable energy systems by utilizing waste heat or solar energy. Although simple to fabricate, the designing of thermoacoustic devices is very challenging. In the present study, a comprehensive design and optimization algorithm is developed for designing thermoacoustic devices. The unique feature of the present algorithm is its ability to design thermoacoustically-driven thermoacoustic refrigerators that can serve as sustainable refrigeration systems. In addition, new features based on the energy balance are also included to design individual thermoacoustic engines and acoustically-driven thermoacoustic refrigerators. As a case study, a thermoacoustically-driven thermoacoustic refrigerator has been designed and optimized based on the developed algorithm. The results from the algorithm are in good agreement with that obtained from the computer code DeltaE.

Introduction

Thermoacoustic is a branch of science dealing with the conversion of heat energy into sound energy and vice versa. Device that converts heat energy in sound or acoustic work is called thermoacoustic heat engine or prime mover and the device that transfers heat from a low temperature reservoir to a high temperature reservoir by utilizing sound or acoustic work is called thermoacoustic refrigerator. Although the thermoacoustic phenomenon was discovered more than a century ago, the rapid advancement in this field occurred during the past three decades when the theoretical understanding of the phenomenon was developed along with the prototype devices based on this technology [1], [2]. The thermoacoustic technology has not reached the technical maturity yet, as a result, the performance of thermoacoustic devices is still lower than their convectional counterparts. Thus, significant efforts are needed to bring this technology to maturity and develop competitive thermoacoustic devices. There are several advantages of heat engines and refrigerators based on thermoacoustic technology as compared to the conventional ones. These devices have fewer components with at most one moving component with no sliding seals and no harmful refrigerants or chemicals are required. Air or any inert gas can be used as working fluids which are environmentally friendly. Furthermore, the fabrication and maintenance costs are low due to inherent simplicity of the thermoacoustic devices.

The main components of a typical thermoacoustic engine or refrigerator are a resonator, a stack of parallel plates and two heat exchangers. A half wavelength (or a quarter wavelength) acoustic standing wave is generated in the resonator. The thermoacoustic phenomenon takes place in the stack when a nonzero temperature gradient imposed along the stack plates (i.e. parallel to the direction of the sound wave propagation) interacts with the sound wave oscillations. The heat exchangers are responsible of transferring heat in and out of a thermoacoustic device at their desired temperatures, thus maintaining a given temperature gradient along the stack.

Thermoacoustic refrigerators can be classified based on the source of the acoustic energy input. If the acoustic energy is provided by a thermoacoustic engine, the refrigerator is called thermoacoustically-driven thermoacoustic refrigerator (TADTAR). Whereas, if the acoustic energy is provided by an acoustic driver e.g. a loudspeaker, it is termed as acoustically-driven thermoacoustic refrigerator. During the past decades, several acoustically-driven thermoacoustic refrigerators have been developed [3], [4], [5]. Although the form of energy consumed in these refrigerators is acoustic, the energy source for the acoustic driver is typically electrical from conventional energy resources. During recent years, there is an increased interest in the development of thermoacoustically-driven thermoacoustic refrigerators. These devices are built by coupling a thermoacoustic refrigerator to a thermoacoustic engine. Thermoacoustic engines are capable of producing acoustic energy from any source of heat energy. Thus, the primary energy source to drive the refrigerator could be conventional or unconventional that includes industrial waste heat, solar energy and fossil fuels. If the heat source for the thermoacoustic engine is the industrial waste heat or solar energy then this device has two major advantages. Firstly, it does not require any addition conventional energy resource and secondly, by utilizing the waste heat, the amount of total waste heat rejected to the thermal energy sink will be reduced which will increase the overall performance of the entire system. Thus, a complete thermoacoustic refrigeration system in which the heat engine (which operates on waste heat) drives a refrigerator and the entire system has no harmful affects on the environment can be termed as a “sustainable refrigeration system”. In contrast to the acoustically-driven thermoacoustic refrigerator which has one moving component i.e. the acoustic driver, thermoacoustically-driven thermoacoustic refrigerator has no moving parts thus; chances of mechanical failure are extremely low.

Recently, some efforts have been made to develop heat engines that operate on waste heat. Symko et al. [6] designed and developed a thermoacoustic heat engine that utilizes heat from a microcircuit to produce sound. Hatazawa et al. [7] proposed a heat engine that utilizes waste heat from a four-stroke automobile gasoline engine. Adeff and Hofler [8] developed a prototype thermoacoustic refrigeration system that operates on the solar energy. Babaei et al. [9] have proposed a thermoacoustic refrigeration system for a gas turbine trigeneration system that operates on the waste heat from the gas turbine. It has been demonstrated that the thermoacoustic refrigeration system has the ability to enhance the overall efficiency of a trigeneration system by 5%.

Some recent theoretical studies have demonstrated the strong potential of thermoacoustic devices in energy conservation and reduction of harmful emissions. A study shows that if all the industrial waste heat above 140 °C in Netherlands can be used in thermoacoustic devices, this would save 16 PJ per year which corresponds to the saving of more than 5 billion m³ of natural gas [10]. It is estimated that over 32 billion liters of fuel is consumed annually for the operation of vehicle air-conditioners in the US alone. Modern vehicle refrigeration systems use R-134a, with a global warming potential still 1300 times that of carbon dioxide [11]. Zoontjens et al. [12] theoretically investigated the feasibility of using thermoacoustic devices as the air conditioning system of an automotive by utilizing the automotive waste heat. They concluded that the thermoacoustic refrigerator has a strong potential to replace the existing automotive air conditioning systems.

Although thermoacoustic devices are easy to build and maintain, designing of these devices involves significant technical challenges. These challenges become more substantial when designing a thermoacoustically-driven thermoacoustic refrigerator. This is attributed to the complicated thermoacoustic theory which is not directly applicable for design purposes. Thus, a systematic approach is necessary to design and optimize thermoacoustic devices.

Wetzel and Herman [13] developed a design algorithm for acoustically-driven thermoacoustic refrigerators. They developed the design algorithm by using the simplified linear thermoacoustic model, and normalizing the position and length of the refrigerator stack and the equations of the total power flow and consumed acoustic power in the stack. By applying the algorithm, the designer can decide the stack position and length at the given temperatures of heat exchangers to have the maximum performance of the stack. The geometrical parameters such as stack plate thickness and spacing as well as the cross-sectional area of the resonator can also be estimated. In this study, however, it is not described how the desired cooling power of the refrigerator, the stack consumed acoustic power and the total power flow in the stack are correlated, and under which configuration of the refrigerator stack this correlation is valid.

Tijani et al. [14] also described the design algorithm for acoustically-driven thermoacoustic refrigerators by considering a correlation between the desired cooling power of the refrigerator, the stack consumed acoustic power and total power in the stack, which is different from that of Wetzel and Herman [13]. It is however, not well described how this correlation is derived and at which refrigerator configuration it may be applied.

The above design algorithms are applicable only to design acoustically-driven thermoacoustic refrigerators. These algorithms cannot be used in designing a thermoacoustically-driven thermoacoustic refrigerator (TADTAR), as designing of TADTAR involves more parameters and it is more challenging than the acoustically-driven thermoacoustic refrigerators. Therefore, to design and develop efficient sustainable thermoacoustic refrigeration systems, a detailed design and optimization procedure is necessary. To the best of authors’ knowledge no such design and optimization procedure or algorithm is available.

In this paper, a comprehensive systematic procedure has been developed for the design and optimization of thermoacoustic devices by applying the simplified linear thermoacoustic model. This procedure which is mainly intended to design and optimize a thermoacoustically-driven thermoacoustic refrigerator (TADTAR) can also be used to design and optimize individual thermoacoustic engines and acoustically-driven thermoacoustic refrigerators. It should be noted that the procedure presented in this study provides a more comprehensive discussion on the design and optimization of acoustically-driven thermoacoustic refrigerators than previous studies. The design procedure which is based on the energy and entropy balances applied on different components of the device is a simple and effective tool to design and optimize a thermoacoustic device to meet its requirements. The goal of designing a thermoacoustically-driven thermoacoustic refrigerator is to meet the required cooling power at the desired cooling temperature and at the given heat input temperature while rejecting some heat to the environment.

The developed algorithm not only provides a step by step procedure to design and optimize a thermoacoustic device but also enables to evaluate the influence of different parameters on the behavior and performance of the device.

Finally, a thermoacoustically-driven thermoacoustic refrigerator is designed and optimized based on the developed procedure and simulated by the computer code DeltaE to compare and verify the design parameters.

It is worth mentioning that using DeltaE to design thermoacoustic devices from scratch needs tremendous amount of effort especially in the case of thermoacoustically-driven thermoacoustic refrigerator. The presented procedure significantly reduces the technical challenges associated with the designing of thermoacoustic devices.