Space nuclear energy consists of radioisotope thermoelectric generators (RTGs) and space nuclear reactors (SNRs). Unlike conventional space energy sources like solar panels, space nuclear power exhibits high energy density, larger power range, longevity and absence of orientation to the sun requirements, making it a promising energy source for future space operations. Nearly all the RTGs and SNRs utilizing space satellites and probes use static thermoelectric energy conversions, including thermionic thermoelectric conversion and thermocouples. Unfortunately, these static thermoelectric conversions have relatively low efficiency, below 10%. Therefore, high-efficiency space thermoelectric energy conversion is highly desirable, especially for large-scale space activities.
Among the available dynamic thermoelectric energy conversion methods, the Stirling cycle is highly efficient, simple and reliable. It is suitable for small- and medium-level RTGs and SNRs owing to its compact structure and modular organization. Stirling cycle thermodynamic model is commonly used to describe and predict the Stirling cycle and associated performance. To this end, developing a high-precision Stirling cycle thermodynamic model is of great significance in advancing space nuclear energy.
An ideal Stirling cycle model consists of two constant volumes and two isothermal processes. However, real processes digress from isothermal processes to produce inaccurate isothermal second-order models. Following the introduction of classical simple model to improve the actual processes involved in the Stirling cycle, several modifications have been developed. These models are derived from irreversible factors in the actual Stirling cycle, and the modifications mostly involve physical processes, such as fluid flow, heat transfer and mechanical motions. However, existing modifications are still inadequate to fully describe the irreversible Stirling cycle processes, necessitating further modifications of thermodynamic model loss mechanisms.
On this account, PhD candidate Chenhao Yang, Dr. Nailiang Zhuang, Mr. Weian Du, Dr. Hangbin Zhao and Professor Xiaobin Tang from Nanjing University of Aeronautics and Astronautics proposed a second-order adiabatic modification model, namely, Incorporated Pressure drop-modified Simple Model (IPD-MSM). This model was based on modifying the simple classical model, assuming that the total pressure loss in the Stirling cycle incorporated the friction flow loss and local loss. The thermal properties of different working fluids (He, H2 and He-Xe mixture) were analyzed and discussed. The work is currently published in the journal, Energy Conversion and Management.
The authors showed that IPD-MSM exhibited improved prediction accuracy at high-frequency and high-pressure conditions than other adiabatic models. Theoretically, H2 achieved the highest output power and thermal efficiency, although it was characterized by a severe explosion and leakage problems. Despite having relatively lower output power and thermal efficiency than He under low working pressure, the He-Xe mixture exhibited a superior performance under high working pressure.
The prediction accuracy and overall performance of the IPD-MSM model were validated by comparing it with GPU-3 Stirling engine experimental data and numerical simulation of other models. The prediction capability of the presented IPD-MSM model agreed well with the changing tendency of the data derived from the GPU-3 Stirling engine experiments. The underlying mechanism was also detailed. The added Xe that reduced non-ideal heat transfer loss exceeded the corresponding increase in the pressure loss. Under the working conditions considered in this study, the optimal mixing ratio was 2% by mole ratio of Xe.
In summary, the study and modification of irreversible losses in Stirling cycles based on a new IPD-MSM thermodynamic model was presented. The presented study provided a theoretical improvement in the prediction of free-piston Stirling engines. The characteristics and applications of the working fluids were detailed. In a statement to Advances in Engineering, the authors said their study provided useful insights that would advance the understanding of the impact of space environment and space nuclear reactors on Stirling cycle.
Yang, C., Zhuang, N., Du, W., Zhao, H., & Tang, X. (2022). Modified Stirling cycle thermodynamic model IPD-MSM and its application. Energy Conversion and Management, 260, 115630.