Development of a 5 kW traveling-wave thermoacoustic electric generator

Highlights

• A new multi-stage traveling-wave thermoacoustic electric generator was introduced.

• Maximum electric power of 4.69 kW and maximum thermal-to-electric efficiency of 18.4% were obtained.

• Influence of electric capacitance connected with linear alternator on the system performance was investigated.

Abstract

Traveling-wave thermoacoustic heat engine is a new type of external combustion heat engine, which is capable of converting thermal energy to acoustic power with advantage of heat source flexibility, reliability and efficiency. The generated acoustic power will be further converted into electricity by connecting linear alternator with the engine. This power generation system is called traveling-wave thermoacoustic electric generator. In this paper, a new traveling-wave thermoacoustic electric generator is proposed, which consists of a multi-stage traveling-wave thermoacoustic heat engine and linear alternators. The engine has several units connected end-to-end by slim resonance tubes to obtain a traveling-wave acoustic field in the regenerator, which is required by an efficient thermoacoustic heat engine. The alternator is connected as a bypass at the end of each resonance tube. Here, a three-stage traveling-wave thermoacoustic electric generator was developed. In the experiments, the maximum electric power of 4.69 kW with thermal-to-electric efficiency of 15.6% and the maximum thermal-to-electric efficiency of 18.4% with electric power of 3.46 kW were achieved with 6 MPa pressurized helium, 650 °C and 25 °C heating and cooling temperatures. Additionally, the influence of the electric capacitance on the system performance was investigated, which may provide some clue to couple the alternator with the engine. So far, this performance is the best one of such type of machines. It is believed that this technology will be suitable for many applications in the energy area, such as solar energy, industrial waste heat and so on.

Introduction

In recent years, the external combustion engines have caught significant attention, because they can use a variety of heat sources, such as solar energy, industrial waste heat and so on. Thermoacoustic heat engine (TAHE) is a new type of external combustion engine capable of converting thermal energy into acoustic work (i.e. mechanical work). The engine eliminates all moving parts and can achieve high reliability and long lifetime. Additionally, it uses environmental friendly gas as working substance, such as helium and nitrogen. By connecting linear alternator to the TAHE, the produced acoustic power can be further converted into electricity. This type of machine is called thermoacoustic electric generator (TAEG).

According to different acoustic fields in the regenerator or the stack, the TAHEs can be classified into traveling-wave TAHE and standing-wave TAHE. Due to the reversible thermodynamic process in the regenerator, the traveling-wave thermoacoustic heat engine (TWTAHE) can achieve a higher efficiency. The concept of TWTAHE was first proposed by Ceperley in 1979 [1]. However, due to the high work dissipation in the regenerator and high heating power loss caused by the DC flow [2], this concept wasn’t verified until 1998 by Yazaki and Iwata [3]. In 1999, Backhaus and Swift developed a TWTAHE and achieved a thermoacoustic efficiency of 30% [4]. After that, investigations on the traveling-wave thermoacoustic electrical generator (TWTAEG) sprang up around the world. In 2004, Backhaus developed a TWTAEG and obtained an electric power of 58 W with 15% thermal-to-electric efficiency [5]. After further improvement, 70 W electric power with 16.8% thermal-to-electric efficiency was achieved [6]. In their system, the alternator worked as a resonator within a standing-wave acoustic field, which resulted in a large swept volume of the linear alternator to obtain a certain amount of electric power. To solve this problem, our group proposed a type of TWTAEG with a resonance tube to improve the coupling between the TWTAHE loop and the alternator and obtained a maximum power of 481 W with thermal-to-electric efficiency of 15% in 2010 [7]. After further investigation on the coupling rule between the engine and the linear alternator, a maximum electric power of 1043 W with thermal-to-electric efficiency of 19.8% [8] was obtained. In 2013, Sun also developed such a TWTAEG with a maximum electric power of 345.3 W with thermal-to-electric efficiency of 10% [9] and an electric power of 473.6 W with efficiency of 14.5% after improvement [10]. However, due to the big resonance tube and the long feedback tube in these systems, for instance, the resonance tube has diameter tapered from 100 mm to 300 mm with the length of about 5.5 m for 70 Hz working frequency and the feedback tube was about 1 m long and 100 mm diameter in Ref. [8], the power density was very low. In order to improve the system power density, multi-stage TWTAEGs have attracted more attentions. In 2010, de Blok proposed a four-stage TWTAEG as a prototype of cooking device for developing countries [11]. In their experiments, an electric power of 12.6 W with normal pressure atmosphere was achieved [12]. In 2012, Yu proposed a looped-tube traveling-wave thermoacoustic electric generator and obtained an electrical power of 11.6 W [13]. In 2013, Wu developed a double-acting TWTAEG and obtained an electric power of 1.57 kW with a thermal-to-electric efficiency of 16.8% [14] but encountered a big structural problem. In 2014, Kang developed a two-stage TWTAEG and obtained the maximum electrical power of 204 W and the highest thermal-to-electrical efficiency of 3.43% [15]. So far, the output electric power of these TWTAEGs is difficult to be enhanced and is not suitable for real applications. Here, a new multi-stage TWTAEG is proposed, which consisted of a multi-stage TWTAHE and several linear alternators. This system is believed to have advantages of high power rate, high efficiency as well as high power density. In order to verify the idea, a three-stage prototype was developed and introduced in this paper.

In the following sections, the schematic and photograph of the system is presented first as well as the typical structure parameters. Secondly, numerical modeling is briefly introduced and system performance prediction is performed. After that, the experimental investigation is presented in detail with discussions. Lastly, some conclusions are made.