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The efficiency of the actual Stirling engine is much lower than the ideal Carnot cycle efficiency. To obtain more precise efficiency for Stirling engines, this paper proposes a modified Stirling cycle with a more accurate heat transfer process in Stirling engines based on a thermodynamic function called available potential. The finite-time thermodynamic method is used to analyze the model performance under constant heat source temperature, finite temperature difference heat transfer, and incomplete regenerative processes. A new polytropic process is introduced to model the heat transfer between the working fluid and external heat sources in which only heat above ambient temperature is converted into technical work. The regenerator is divided into numerous smaller heat reservoirs with individual temperature to analyze the incomplete regenerative processes. The expressions of the output power and thermal efficiency are obtained based on the modified irreversible Stirling cycle, and the effects of irreversible losses are analyzed to evaluate the performance of the proposed model. Results indicate that the efficiency of the modified cycle is much lower than that of the ideal Stirling cycle with an isothermal process. With the increase of the average heat transfer temperature difference, there exists an optimum value for the power of the irreversible cycle. The optimum power of the model was obtained for varying thermodynamic parameters by optimizing the average heat transfer temperature difference between the hot and cold sides. To optimize the irreversible model, the multi-objective optimization analysis is carried out based on NSGA-II, which results in an optimized output power of 40.87 kW and an optimized thermal efficiency of 44%. [ABSTRACT FROM AUTHOR]

Stirling engines work with all kinds of heat sources thanks to the external heat supply. It has many advantages over internal combustion engines, especially in terms of noise emissions and pollutant emissions. Since the first Stirling engine invented by Robert Stirling, development work continues on it. Considering the problems caused by fossil fuels, Stirling engines are promising in the recovery of solar energy, geothermal energy and waste heat. As a result of the studies carried out from the past to the present, many Stirling engine types, cylinder configurations and drive mechanisms have been designed. In this study, the im-portance, advantages-disadvantages, usage areas and working principles of Stirling engines are explained. The Stirling cycle has been analyzed in detail. Carnot cycle and Ericsson cycle are mentioned and these three cycles are compared with each other in terms of work and efficiency. Stirling engine classifications, cylinder configurations and drive mechanisms are explained in detail. The design differences, operating characteristics, technological details and structural features of these configurations are exam-ined. The advantages and disadvantages of all these different structures in terms of design, production, cost, power, efficiency, friction, wear, sealing, weight, dead volume, noise and number of parts are stated.

Copyright of Journal of Refrigeration is the property of Journal of Refrigeration Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In the article, a quantum Stirling machine is proposed and its behavior as a quantum heat engine (QHE) or a refrigerator is investigated. The working substance of the machine is considered as a 2D and 3D spin chain model under a magnetic field and the Dzyaloshinskii-Moriya interaction (DMI). The effects of a magnetic field, the temperature of a cold bath, DMI, and the system dimension have been studied on the efficiency and performance coefficient of the Stirling heat engine and the Stirling refrigerator. It is deduced that with proper selection of the system parameters, and system dimension the Stirling cycle can be operated as a QHE or quantum refrigerator with sufficient efficiency, and performance coefficient. By decreasing the temperature of the cold bath and considering a 3D model without DMI, the system works as a QHE with a maximum efficiency of 33%. Also, by considering a 2D model with DMI and rising the magnetic field, the system works as a quantum refrigerator with a maximum performance coefficient approximately of 7.1. Therefore, we can say that the maximum efficiency or performance coefficient can be obtained by the appropriate selection of system parameters and dimensions. [ABSTRACT FROM AUTHOR]

For industrial applications, it is important to reliably monitor the condition of critical equipment and to run at optimal performance for varying operating conditions. Often important parameters for monitoring and optimization cannot be reliably measured and must be estimated. For example, there are great difficulties in measuring temperatures inside a Stirling cycle-based heat pump as these fluctuate between heating and cooling cycles at speeds of 10–12.5 Hz. Validation of models using full-scale industrial applications are very rare. We here present and validate a model for simulating cyclic temperature variations inside an industrial alpha-configuration double-acting Stirling cycle-based high temperature heat pump based on measured cylinder pressures and slow-changing temperature data. This will be used for online monitoring of the heat pump and as input to system performance optimization. The heat pump used as a reference is located in Gothenburg, Sweden with a design heat output of 500 kW and steam output at a temperature of 180 °C. The outputs of the model are temperature curves during a full cycle for both cold and hot side water and working fluid temperature variation in different parts of the machine. The model is validated against a different data set from the same system.

Whether at the design stage of a new model or for improvement of an existing microcryogenic gas machine, engineers select the operating cycle and corresponding design for the device. This selection is governed by a set of technical parameters and requirements for the machine and its operation conditions and the object of cooling. This study analyzes various operating cycles of microcryocoolers, namely, the Stirling, Solvay, Gifford–McMahon, and Vuilleumier cycles and modifications thereof. We aim to determine methods of improving the thermodynamic efficiency of future machines at the design stage or enhancing existing models through modernization of the operating cycle; for that, we select the layout scheme with a preliminary assessment of the service life of individual elements and assemblies, based on experimental data from Russian and international companies. We hope that this analysis will allow for selection of the most suitable operating cycle for given parameters and required lifetime at the design stage of future devices, and determination of possible ways of improving the thermodynamic characteristics of existing devices, increasing the lifetime for given technical requirements. [ABSTRACT FROM AUTHOR]

Copyright of Journal of South China University of Technology (Natural Science Edition) is the property of South China University of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Cascade solar thermal systems provide a new direction for solar power generation. This paper focuses on the configuration optimization of a cascade solar system in which a Stirling engine array is applied. The array has multiple configurations. To find out the influence of the configuration on the performance of the engine array, five basic connection types were proposed. A Stirling engine model considering various losses and irreversibilities was developed. The model was evaluated by considering the prototype GPU-3 Stirling engine as a case study. Stirling engine array models were developed based on the Stirling engine model. Global efficiency and power of different connection types of Stirling engine arrays with the same hot and cold flows were evaluated. The effects of different factors on the performance of the Stirling engine arrays were considered. The result shows that flow order, the co-current flow or the counter-current flow, has little influence on the engine array performance. The maximum differences of thermal efficiency and output power of different flow orders are 0.39% and 0.70%, respectively. Serial flow connection type is the best for a Stirling engine array to obtain the best performance and adaptability for given heating and cooling fluids.

Copyright of Journal of King Abdulaziz University: Engineering Sciences is the property of King Abdulaziz University, Scientific Publishing Centre and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Journal of Engineering for Thermal Energy & Power / Reneng Dongli Gongcheng is the property of Journal of Engineering for Thermal Energy & Power and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Efficient engine technologies to convert low-grade heat to electricity are urgently desired. In this work, a conceptual structure of engine for a novel cycle or one-way oscillating flow cycle (OOFC), which consists of two isochoric and two adiabatic processes, is described for low-grade heat utilization. Characteristics of OOFC allows for the working fluid temperature glide to be matched to the decrease in temperature of low-grade heat. Then, thermodynamic model is developed for evaluating the performance. Theoretical simulation results show that maximum specific output works are in the range of 12.2 kJ kg−1 – 79.7 kJ kg−1. Compared to Stirling cycle system, maximum specific output work in OOFC system could be improved by 16.2 %–24.8 %. Compared to ideal Carnot cycle engine system, maximum specific output works in OOFC system is nearly the same and 1.8 %–2.6 % lower. As Carnot cycle engine is ideal while thermodynamic cycle loss and heat transfer loss in cold heat exchangers are considered in OOFC engine, the ratios of maximum specific output work demonstrate that OOFC system could be very promising for low-grade heat utilization as a result of well-matched temperature profile in hot heat exchanger. • Structure of OOFC engine with temperature glide heat addition is described. • Large temperature glide heat rejection is achieved in OOFC engine. • Maximum specific output work of OOFC system is 116 %–125 % of Stirling cycle system. • Maximum specific output work in OOFC system are equivalent to ideal Carnot cycle system. • OOFC engine has well-matched temperature profile for low-grade heat utilization. [ABSTRACT FROM AUTHOR]

Summary: In this contribution, the optimization potential of the seal geometry in Stirling machines is explored both numerically and analytically, leading to a significant reduction of the related losses which are often referred to as appendix gap losses. These are induced by the narrow gap between the displacer and the cylinder and have mostly been underestimated so far. A recent experimental investigation revealed large optimization potentials by reduction of the seal and cylinder wall diameter near the seal, resulting in reduced appendix gap losses and further indirect positive effects. In this work, these experimental findings could be reproduced by a one‐dimensional differential simulation model at a fully satisfyingaccuracy. Furthermore, these investigations reveal that the optimum geometry is largely machine‐dependent. To provide an easily applicable design rule for this optimum geometry, a refined analytical model for the mass flow at the top of the gap is derived, which is based on a phasor analysis and a linearized mass balance that also accounts for changes in the spatial mean gas temperature in the gap. The optimum design predicted by this model is very close to numerical optimization results and sufficiently accurate under practical aspects. Furthermore, this model contributes to a better theoretical understanding of the loss mechanisms in the gap. [ABSTRACT FROM AUTHOR]

In this paper, we introduce the t-extension of the p-Fibonacci matrix and give a Factorization of the Pascal matrix involving the t-extension of the p-Fibonacci matrix. Also, we obtain some results on the relations between the Stirling matrix of the second kind and the 1-extension of the p-Fibonacci matrix. [ABSTRACT FROM AUTHOR]

Summary: Parallel‐plate regenerators (PPR), with flow resistance lower than traditional wire‐mesh regenerators, can improve the thermal efficiency of Stirling engines (SEs). However, as working frequency or plate thickness increase, the heat cannot penetrate into the plate effectively, resulting only the surface part of the plates to have substantial temperature variation, while the internal part fails to store and exchange heat energy. In order to obtain high performance of PPR, the heat storage efficiency and the heat transfer coefficient, as well as their influential factors, are theoretically studied. Three parameters are found to play an important role, which are working frequency, plate thickness, and thermal diffusivity of materials. Their roles can be represented by a dimensionless parameter as a whole, which is the relative thickness, e. By the critical value of relative thickness, ecr = 2.4, two distinct working conditions can be divided, thermally penetrated condition as e < ecr and thermally non‐penetrated condition as e > ecr. Under thermally penetrated condition, the heat storage efficiency is high, and the heat transfer coefficient is high enough when e > 1.6, while under thermally non‐penetrated condition, the heat storage efficiency is low. In conclusion, by comprehensively considering the heat storage efficiency and heat transfer coefficient, it is recommended that the relative thickness e should be chosen within the range [1.6, 2.4]. And the optimal working frequency, plate thickness, and suitable material can be determined accordingly. [ABSTRACT FROM AUTHOR]

Summary: The global cooling demand is one of the fastest growing energy demands and is putting a strain on the electricity infrastructure. Solar‐powered cooling could provide most of the cooling demand due to the coincidence of the cooling demand and the solar irradiance. In particular, the solar‐powered Stirling‐cycle cooler has low maintenance requirement, high theoretical efficiency, and use of environmentally friendly gases. However, Stirling‐cycle coolers are expensive due to high driving temperatures, complex heat exchangers, and expensive solar tracking so that they have so far only been successful at high‐temperature difference applications. This study introduces a novel directly coupled solar Stirling cooler for which the hot engine cylinders are deployed inside evacuated tube collectors. The machine uses air as working fluid, and its driving mechanism is based on the free‐piston, balanced compound technology that was patented by Finkelstein. A second‐order mathematical model is used to investigate the performance of the machine for different cylinder arrangements, gas leakage rates, chilling temperatures, and solar irradiance. In addition, the regenerators are optimised to maximise the cold production. It is shown that mechanical frictions can be reduced to 20% by selecting an appropriate cylinder arrangement. The solar cooler achieves a maximum cold production rate of 367.5 W/m2 without using external heat exchangers at load temperature of 7°C, which is comparable with photovoltaic powered coolers. In addition, the machine is relatively simple, has safe and quiet operation, uses ambient air as working gas, and is able to produce a wide range of chilling including sub‐zero temperatures without changing the working gas. The direct thermal coupling of the Stirling cooler to evacuated tube collectors significantly reduces the complexity of the machine and removes intermediate heat transfer steps which reduce the performance. Thus, the suggested cooling technology has great potential for solar refrigeration, especially for low power and near ambient cooling. [ABSTRACT FROM AUTHOR]