Opening Plenary Lecture

Recent Progress on Some Fundamental and Technical Issues of Thermoacoustic and Stirling Machines

Prof. Ercang LUO

University of Chinese Academy of Sciences, Beijing, China
Technical Institute of Physics and Chemistry, Beijing, China


The Stirling heat engine prototype for power generation was first proposed and developed by Robert Stirling in 1816. More than 200 years has already gone, and numerous different configurations and protypes originating from that oldest Stirling heat engine have been developed. Due to the very complicated heat transfer, flowing and thermodynamic process, some fundamental problems have not been well understood yet. Consequently, key techniques for achieving high capacity, high reliability and low cost have not been solved and developed. In recent years, the requirement of decarbonization and carbon neutrality of energy is revitalizing this old heat engine technology because of its many potential advantages. Important progresses on some fundamentals and key technologies have been made in recent years, leading to a very promising future of this technology and its close relatives, thermoacoustic technology. In this plenary talk, I would like to deliver my talk focusing on the following two aspects:

Theoretical aspect

Novel viewpoint and understanding on thermodynamic processes and cycle

All so-called Stirling and thermoacoustic machines operate on complex interaction between compressible oscillating gas and solid regenerator, and heat exchangers. Traditionally, these regenerative machines have been thought to undergo Stirling cycle which mainly incorporate two isothermal processes and two isovolumic processes. However, the advance of thermoacoustics in recent decades has demonstrated a totally different micro/macro-scale thermodynamic processes for the regenerative machines. Hereby, detailed study on the thermodynamic processes of micro-scale single gas parcels and macro-scale all gas envelope has been implemented in the past decades. As a result of the study, a quite novel perspective on thermodynamic cycle on the regenerative machines has been proposed, in which a micro-scale gas parcel in the so-called regenerator undergoes Lorentzs-like thermodynamic cycle, while the envelope of all gas parcels forms Ericsson-like cycle. Effects of non-ideal heat transfer and viscous flowing only add some little shaping effect on the unique thermodynamic cycle. In other word, each gas parcel finishes its own thermodynamic cycle under different temperature level and in different locations, etc. The regenerator is not only regenerative heat exchanger, but it is really a micro-scale heat-to-power convertor. Several examples for the kinematic Stirling configuration, free-piston Stirling and some thermoacoustically-driven heat engine and refrigerator are given for supporting the novel viewpoint. Furthermore, a 3D description and characterization for such unique thermodynamic cycle, e.g., P-v-x and T-s-x diagrams, is proposed and reported in this talk for the first time.

This work has led to a novel LEC thermodynamic cycle theory for deeply understanding Stirling-type and thermoacoustic-type prime movers and refrigerators.

In-depth physical understanding of thermoacoustic effect and advanced weakly nonlinear thermoacoustic model

Historically, there have been several quantification mathematical-physical models for explaining and quantifying the Stirling and the like regenerative machines. Sometimes, they are called zeroth-order, first-order, second-order, and third-order. However, all most of these models have not clearly shown the fundamental physical essence of the complex interaction of oscillating gas and solid nearby. In the recent past decades, thermoacoustics really reveals the physical fact, leading to a very clear physical picture of the regenerative thermoacoustic and Stirling machines’ operation. However, in the beginning period and even up to now, many researchers have no correct understanding for dynamic and time-averaged thermoacoustic effects. Thus, a so-called linear thermoacoustic model has been developed and used for quantifying various thermodynamic parameters and performance under linear acoustic approximation. Based on the misleading understanding, two fundamental questions have puzzled people for a long time. Hereby, an in-depth viewpoint on how to understand various thermoacoustic effects including dynamic and time-averaged thermoacoustic effects has been shown. As a result, an advanced thermoacoustic theory has been founded. With the help of the novel theory, two classic puzzled questions have been successfully solved. Particularly, the advanced theory has provided a powerful guidance for us to design numerous high-performance thermoacoustic and Stirling prototypes and commercial products. This work has been summarized to form a set of weakly nonlinear locally-analytical thermoacoustic models for qualitatively understanding thermoacoustic effect and quantitively predicting thermodynamic performance of the Stirling-like and thermoacoustic-like machines.

Time-domain dynamic evolution process and non-linear thermoacoustic network models for thermally self-exciting thermoacoustic and Stirling machines

There are two kinds of thermally-driven mechanical power generator, one is non-resonant system and the other is resonant system. The kinematic Stirling heat engine is actually a kind of non-resonant mechanical power system, while the free-piston Stirling and thermoacoustic systems are basically belong to resonant thermal-mechanical system. For the later type, the onset temperature and resonant frequency are very important parameters to evaluate its basic feature. In particular, the dynamic evolution process from the onset moment to saturation state (e.g., steady periodic oscillation) is critical and valuable to understand the operating state and global thermodynamic performance. There are several mathematical-physical ways to answer the question. In this section, we introduce a nonlinear electrical-like network way to solve the above-mentioned issue. First, the network models based on lumped-parameter are derived for different thermodynamic and mechanical components such as regenerator, heat exchangers, free-piston displacer, linear motor, etc. Then, several representative systems such as free-piston Stirling heat engine, thermoacoustically-driven heat engine and refrigerators are modeled to show their dynamic evolution processes including the dynamic pressure amplification and limit cycle, etc. This nonlinear network method is very straightforward and effective to evaluate the dynamic and steady performance of thermally-driven Stirling and thermoacoustic systems.     

Technical aspect

The kinematic Stirling machines has long development history, but they have very limited application so far. The obstacle of hindering wide and large-scale commercial application may exist various reasons, among which the low reliability and lifetime, due to serious mechanical friction of kinematic piston-cylinder pair, is one of the most serious problems. Therefore, free-piston type and thermoacoustic type of systems have been studied intensively in the recent decades. Inside these systems, free solid piston, liquid piston, and gas resonator, etc., have been adopted to ease or eliminate the kinematic crank-shaft connection problem. In this part, several key technical progresses on free-piston Stirling power generator including our 100kWe-class prototype, our commercialized high-frequency liner-motor driven pulse tube cryocoolers, and thermoacoustically-driven high-efficiency air-conditioning protype, have been successfully made. These important technological progress substantially have explored the advantages of the new-generation Stirling machines.

All in a word, significant progress including several fundamental and technical work have been made in the past decades. The development trend of decarbonization and electrification of energy exploration and utilization provide a huge opportunity for the Stirling and thermoacoustic technology. Certainly, the wonderful dream of those exciting application scenes still needs a huge effort and innovative researches from the Stirling community worldwide. Let’s work together!   

Ercang LUO’s CV

Prof. Ercang Luo is now working at the University of Chinese Academy of Sciences (UCAS), and the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (TIPC/CAS). He obtained his Bachelor degree from the Department of Thermal Energy of Tsinghua University in 1990 and his Ph.D. degree from the Cryogenic Laboratory of Chinese Academy of Sciences in 1997. Then, he joined the Cryogenic Laboratory of CAS as a research scientist, and was promoted a full professor of TIPC/CAS in 2001. In 2006, he was selected the outstanding young scholar of the National Natural Sciences Foundation of China. Since 2009 he has been the head of CAS Key Laboratory of Cryogenics. In 2015, he was elected to be the deputy director of TIPC/CAS. His R&D activities are mainly involved with various refrigeration modeling and technologies including mixed-gas Joule-Thomson refrigerator, pulse tube cryocooler and thermoacoustically-driven refrigerator, Free-piston Stirling generators and heat pumps.etc. Also, he has been investigating and developing thermoacoustic electrical generator by using solar energy and industrial waste heat in recent years. Prof.Luo has published over 400 peer-reviewed international journal and conference papers and has been issued over 200 patents. He has received several awards, including a Silver Medal of China National Technology Invention Prize in 2006 and the Hugangfu Prize of Chinese Association of Physics in 2007. Nowadays, he has been severing for the Chinese Association of Refrigeration (CAR) as the vice president, the International Institute of Refrigeration (IIR) as the Commission A1 member, the International Cryogenic Engineering Conference Board member, and the member of the International Stirling Conference Committee, etc.