• Energy Research

As an environmentally friendly refrigeration system, the heat-driven Stirling refrigerator, which has demonstrated high efficiency and promising application prospects, is receiving significant attention for utilizing the waste heat to generate the cooling capacity. In this study, a heat-driven direct-coupled Stirling refrigerator, featuring an engine unit and a refrigeration unit directly coupled through a thermal buffer tube rather than utilizing a piston-based mechanism, is designed and tested. In comparison to the conventional heat-driven Stirling refrigeration systems, the proposed system exhibits the potential for significantly increased reliability and simplicity. Simulations and experiments were carried out to investigate the output characteristics of the system under different heating temperatures and mean pressures. The results show that higher heating temperature is beneficial for producing the acoustic power, thereby increasing the cooling capacity, with the mean pressure among 2.7–3.2 MPa. In the experiments, the system can provide a cooling capacity of 363W with a coefficient of performance of 0.17 when the heating, ambient, and cooling temperatures are 250, 35, and 7 °C, respectively. The results have validated the feasibility of the heat-driven direct-coupled Stirling refrigerator, which is a potential alternative for air-conditioning through waste-heat recovery.

Thermoacoustic technology emerges as a sustainable and low-carbon method for energy conversion, leveraging environmentally friendly working mediums and independence from electricity. This study presents the development of a multimode heat-driven thermoacoustic system designed to utilize medium/low-grade heat sources for room-temperature cooling and heating. We constructed both a simulation model and an experimental prototype for a single-unit direct-coupled thermoacoustic system, exploring its performance in heating-only, cooling-only, and hybrid heating and cooling modes. Internal characteristic analysis including an examination of internal exergy loss and a distribution analysis of key parameters was first conducted in the hybrid cooling and heating mode. The results indicated a positive-focused traveling-wave-dominant acoustic field within the thermoacoustic core unit, enhancing energy conversion efficiency. The output system performance was subsequently tested under different working conditions in the heating-only and cooling-only modes. A maximum output heating power of 2.3 kW and a maximum COPh of 1.41 were observed in the heating-only mode. Meanwhile, a cooling power of 748 W and a COPc of 0.4 were obtained in the typical cooling condition at 7 °C when operating in cooling-only mode. These findings underscore the promising potential of thermoacoustic systems for efficiently utilizing medium/low-grade heat sources for cooling and/or heating applications in the future.

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.

Abstract In this paper, a double-acting thermoacoustic Stirling electric generator is proposed as a new device capable of converting external heat into electric power. In the system, at least three thermoacoustic Stirling heat engines and three linear alternators are used to build a multiple-cylinder electricity generator. In comparison with the conventional thermoacoustic electricity generation system, the double-acting thermoacoustic Stirling electric generator has advantages on efficiency, power density and power capacity. In order to verify the idea, a prototype of 3 kW three-cylinder double-acting thermoacoustic Stirling electric generator is designed, built and tested. Based on the classic thermoacoustic theory, numerical simulation is performed to obtain the thermodynamic parameters of the engine. And distributions of key parameters are presented for a better understanding of the energy conversion process in the engine. In the experiments, a maximum electric power of about 1.57 kW and a maximum thermal-to-electric conversion efficiency of 16.8% were achieved with 5 MPa pressurized helium and 86 Hz working frequency. However, we find that the mechanical damping coefficient of the piston is dramatically increased due to the deformation of the cylinder wall caused by high thermal stress during the experiments. Thereby, the system performance was greatly reduced. Additionally, the performance differences between three engines and three alternators are significant, such as the heating temperature difference between three heater blocks of the engines, the piston displacement and the output electric power differences between three alternators. These problems need further investigation. This work presents a new thermal-to-electric conversion technology, which can be utilized in many energy area, such as solar energy, industrial waste heat and so on.

Abstract As a new type of refrigeration technology, the traveling-wave thermoacoustic refrigerator offers advantages that include high efficiency, reliability and environmental friendliness. To date, because of problems such as low power utilization and high power recovery losses, traveling-wave thermoacoustic refrigerators for use in room temperature applications have not been widely studied. In this paper, following an investigation of the traditional single-stage traveling-wave thermoacoustic refrigerator, a multi-stage traveling-wave thermoacoustic refrigerator is proposed and the working mechanism of this refrigerator is studied numerically using SAGE software. The calculation results show that the proposed multi-stage traveling-wave thermoacoustic refrigerator can enhance the utilization of the input acoustic work effectively, thereby improving the cooling power of the refrigerator with high cooling efficiency. As a result, the cooling power increases from 2.17 kW for a single-stage refrigerator to 6.42 kW for a seven-stage refrigerator, while the acoustic work utilization rate increases from 0.26 to 0.82, and the coefficient of performance changes from 2.60 to 3.19. The calculation results also indicate that three to five stages may be most suitable for the multi-stage traveling-wave thermoacoustic refrigerator when working within the temperature range of interest here by striking a balance between cooling efficiency and cooling power.

Abstract In recent years, combined heat and power (CHP) systems have attracted increasing attention worldwide. Owing to their advantages of high overall thermal efficiency, fuel flexibility, low noise and vibration, and low emissions, Stirling engines, especially dynamic Stirling engines (i.e., free-piston Stirling engines, FPSEs) are promising candidates for micro-CHP systems. In this paper, recent progress in Stirling engine-based micro-CHP systems and FPSE modeling and analysis is first briefly reviewed, and then a hybrid calculation model based on thermoacoustic theory is proposed and developed to simulate the entire micro-CHP system. Finally, the construction and testing of a pilot setup is described in detail. The obtained experimental results clearly validate the numerical model and scheme, with the primary deviation within approximately 10%. CHP performance tests revealed a maximum CHP efficiency of 87.5% and an output electrical power of 2.9 kW, corresponding to a 28% thermal-to-electric efficiency, when the delivering temperature was above 60 °C. Furthermore, acoustic impedance analysis indicated that the CHP efficiency remains high over a large temperature lift, which was also confirmed experimentally.

Abstract We propose a novel cooling and power cogeneration system based on duplex free-piston thermoacoustic-Stirling cycles and an oscillating linear alternator. In such a cogeneration system, a free-piston Stirling heat engine produces acoustic power via the thermoacoustic power cycle, and a free-piston Stirling refrigerator produces cooling power via the thermoacoustic refrigeration cycle; an oscillating linear alternator is used to couple the free-piston heat engine and refrigerator and simultaneously produces electricity. The configuration and operating principles of the cogeneration system are described along with the thermoacoustic energy transformations and transmission mechanisms. An experimental prototype system was designed, built and successfully operated for the first time. In the preliminary experiments, thermoacoustic oscillation startup typically occurred at heating temperatures around 50 to 60 °C, and a cooling power of about 230 W at -20 °C and an electrical power of more than 6 W were obtained at a heating temperature below 190 °C. The alternator-coupled system has potential applications in heat-driven cogeneration systems for low-grade waste heat recovery and solar energy utilization.

Abstract In recent years, the share of natural gas in total primary energy consumption has gradually increased around the world. In China, almost half of imported natural gas is in the form of LNG. Thus, how to effectively use LNG’s cold energy or exergy has gained increasing attention. In this paper, a double-acting thermoacoustic Stirling heat electrical generator capable of using LNG cold exergy is introduced. The system consists of a four-unit thermoacoustic Stirling engines and four linear alternators connected end-to-end to construct a loop configuration. The engine converts the external thermal energy to acoustic work by completing the thermoacoustic Stirling cycle between the low temperature provided by the LNG and that of the ambient environment. Then, the alternator converts the acoustic work to electrical power. To understand the system’s operating mechanism, numerical simulation is performed based on the classic thermoacoustic theory. Besides the distributions of key parameters, the influences of the electrical parameters on the system performance and the optimization of the regenerator in low temperature are presented in detail. According to the simulation results, the regenerator of the engine prefers a higher porosity to achieve high power and efficiency. The maximum acoustic work of 17.6 kW and electrical power of 12.4 kW for the whole system is obtained with a porosity of 0.9 and a hydraulic radius of 53 μm when the electrical resistance and capacitance are 160 Ω and 80 μF, respectively. The cooling and heating temperatures are 110 K and 303 K. This study presents a new way to efficiently use the cold exergy of LNG and may be especially relevant for distributed small-scale applications.