Kinematic Stirling Engine Research Articles

Assessment of the integration of a switched nuclear isomer material with a kinematic Stirling engine

Presently, power systems are limited by the source and energy density of the chemical fuels that are currently in use. In response to this challenge, nuclear energy sources are being developed. Metastable nuclear states, or isomers, have an intrinsic energy density that is many orders of magnitude larger than chemical fuels. Through isomer depletion (or switching), these materials can be transformed from a long-lived half-life energy-storage state to a shorter-lived half-life energy-releasing state. The abundance of energy released by switched nuclear isomer (SNI) materials introduces unique engineering challenges for the power conversion system. Once an SNI material has been switched, its heat output decays with time. In contrast to hydrocarbon-based fuels, which can be throttled, generating constant power from an SNI system requires sufficient material based on duration of use and system efficiency. Despite these challenges, SNI materials could be used to replace chemical fuels for relatively high-power applications providing months of power from a compact energy source.The purpose of this work is to explore the coupling of an SNI heat source to a kinematic Stirling engine in order to assess the potential of such a system. First, the characteristics of a reference SNI material are discussed in the context of the conversion efficiency and power density of the power conversion system that it is coupled to. Second, the integration of an SNI with a kinematic Stirling engine is explored at two power levels, 6 kWe and 60 kWe, in order to explore the feasibility and efficiency of such a power system. In order to carry out this study, a second-order Stirling engine model (the Stirling Engine Trade-space Tool, or SETT) has been developed. The potential of the SETT tool to predict performance trends has been shown through comparison of the predicted to the measured performance for two baseline, well-documented Stirling engines: the GPU-3 (nominally 6 kW) and Mod II (nominally 60 kW) engines. The SETT model is then used to optimize the design of the integration heat exchanger that is used to thermally couple a Stirling engine to the SNI and evaluate the performance characteristics of this energy conversion system. The resulting model is also used to show that the kinematic Stirling engine has advantages related to its inherent ability to modulate its output through speed or inventory control, allowing it to effectively deal with the time varying rate of heat transfer that is inevitably provided by the SNI. This work shows that the kinematic Stirling engine is an attractive power conversion system to use with SNI material as it can achieve high conversion efficiency, high power density, and provides inherent modulation capability.

Numerical and experimental investigation on the drive mechanisms of Alfa type Stirling engines

ABSTRACT Stirling technology has applications in both renewable energy and waste heat recovery systems. The applicability of Stirling engines in these fields depends on the development of more efficient external combustion machines using renewable energy sources or waste heat. In this study, a new drive mechanism is proposed for the low cost, low weight, low volume and high-efficiency Stirling engines. The effects of the new drive mechanism on the performance parameters of the alpha-type kinematic Stirling engine are investigated, experimentally and theoretically. The mathematical model is validated by using the experimental results. The torque values for different piston forces are presented and compared with that of the conventional slider-crank mechanism. The results show that the torque value of the engine using the new drive mechanism increases by up to 3.66 times compared with that of the conventional slider-crank mechanism.

Development of a low-temperature-difference indirect-heating kinematic Stirling engine

Stirling engines can become a key component of distributed combined heat and power systems using renewable energy; however, their heat transfer surfaces suffer from overheating. To solve this, a low-temperature-difference indirect-heating kinematic Stirling engine employing a heat-transfer medium was developed. A new basic design method is devised by returning to the ideal Stirling cycle. The basic design of an engine optimized for temperature differences can be carried out using these criteria. To realize this basic design as an actual engine, an α+-type engine—the basic type suitable for low-temperature difference conditions—was developed. We developed a 10 kW-class engine using the basic design and type. Heat transfer oil was heated to 300 °C by burning wood biomass and circulated to the engine, which attained a maximum output of 14.3 kW, and a thermal efficiency of 15%. The engine was installed at the Minamisōma city community center and operates as a micro-combined heat and power system connected to the grid and independently during emergencies. The results show that the concept of the indirect heating low-temperature difference-type Stirling engine solves the main problem of its practical use, which can be used as a new distributed energy system.

A novel unified dynamic-thermodynamic method for estimating damping and predicting performance of kinematic Stirling engines

This paper presents a novel unified dynamic-thermodynamic (UDT) method to achieve the damping coefficient and overall performance of a kinematic Stirling engine. First, dynamic, kinematic, thermodynamic, and heat transfer equations are used to present a comprehensive model of a gamma-type Stirling engine. The equivalent inertia method is proposed to present a compact form of motion equation of the engine. Then, the proposed mathematical scheme is further employed to predict the engine speed corresponding to different values of the damping coefficient. Subsequently, a prototype Stirling engine is developed and tested to achieve the practical engine speed. Therefore, the damping coefficient of the developed gamma-type Stirling engine is identified through the proposed dynamic model taking into account the measured engine speed. Next, the overall performance of the Stirling engine in terms of engine frequency, output power, work, efficiency, gas pressure, and gas temperature is simulated via the proposed model considering finite heat transfer, imperfect regeneration, and frictional damping. Finally, the prototype Stirling engine is tested to validate the proposed mathematical procedure as well as to verify the identified damping coefficient. The good agreement between the simulation and experimental results reveal the effectiveness of the proposed modeling scheme.

Stirling Engine and absolute sealing

Referring to the sketches, the alternative rotation of shaft (101) allows for absolute sealing between the left side and the right side of seal (1020). A device for total sealing around a shaft in alternating rotation is characterized in that a seal (1020), the main part of the device, called a torsion disc, constitutes the material insulator and is fixed on a shaft (101) by means of two spacers (610) and (620), where the spacer (610) is held in position by the bearing (710) and the spacer (620), which clamps the seal (1020), is held in position by the bearing (720). Along the shaft (101) on both sides of the seal (1020), O-rings (510) and (520) and possibly temperature insulating rings (410) and (420) can be arranged. On both sides of the seal (1020) there are possibly two pressure isolating stops (210) and (220), which slide in rotation on two other stops (110) and (120). The two bearing housings (228) and (8) are positioned relative to each other by means of shims (7) in order to clamp the seal (1020) at the bore and at the shaft. Connecting screws connect the bearing housings (228) and (8) and further connecting screws connect the housing (8) and the wall (22). Absolute sealing of a kinematic Stirling engine is now achievable. A French patent was taken out on August 27th, 2020.

Flywheel-based traveling-wave thermoacoustic engine

Motivated by the success of a traveling-wave thermoacoustic engine with a linear load, we built a thermoacoustic engine with a flywheel and a reciprocal piston. The engine has a looped tube with a regenerator and heat exchangers at the ends. When the temperature difference between the ends of the regenerator is increased above a threshold, the flywheel undergoes steady rotation. From simultaneous measurements of the pressure and velocity of the working gas, we demonstrated experimentally that this flywheel-based thermoacoustic engine is a kinematic Stirling engine.

Modeling of a 1kW free piston Stirling engine: Opportunity for sustainable electricity production

As part of the many alternatives for the development of new methods for sustainable energy transformation, the Stirling engine is distinguished due to the characteristics of an external combustion engine, with many advantages. These engines can be activated with heat, representing an additional option for the use of renewable energy. The Free Piston Stirling Engine (FPSE) is special due to the elimination of all wearing mechanisms of a typical kinematic Stirling engine. A linear mechanical storage device replaces the crank device of the kinematic one, making possible long operating life, higher efficiency and zero maintenance. This document presents a theoretical model of a Free Piston Stirling Engine (FPSE) based on a dynamical and thermal analysis. As the core of the study, the “first order analysis” of the thermal machine is implemented by taking some ideal assumptions. A technique of evolutionary computation is presented as a feasible solution for finding functional correct design parameters. For the dynamical part, the mathematical model is obtained through energy balances, and then a stability study is implemented to assure the oscillations of the machine. Results include several cases, which offers parameters such as cylinder and rod area, cooler temperature, heater temperature, piston and displacer mass, piston and displacer stiffness, phase angle, frequency and power. It is critical to consider the sensibility of the system, and therefore, it is essential to apply a stability analysis in order to fully understand the behavior of the thermal machine. Results are compared are compared in terms of a calculated error considering first order analysis and the stability study developed with the Schmidt analysis.

Kinematic Beta-Type Stirling Motor-Driven Compressor

The theoretical functioning of a kinematic one-stage beta-type Stirling motor-driven compressor was analyzed. This machine comprises a kinematic Stirling engine and a reciprocating compressor in one single block. A double acting cylinder houses a piston that plays both the roles of the power piston of the Stirling engine and of the compressor’s piston. The maximum performances of this machine are determined through numerical modelling. For a certain compression ratio of the compressor, a functioning regime for which the work produced by the Stirling engine is equal to the absolute value of the work consumed by the compressor can be found.

Numerical simulation for the design analysis of kinematic Stirling engines

The Stirling engine is a closed-cycle regenerative system that presents good theoretical properties. These include a high thermodynamic efficiency, low emissions levels thanks to a controlled external heat source, and multi-fuel capability among others. However, the performance of actual prototypes largely differs from the mentioned theoretical potential. Actual engine prototypes present low electrical power outputs and high energy losses. These are mainly attributed to the complex interaction between the different components of the engine, and the challenging heat transfer and fluid dynamics requirements. Furthermore, the integration of the engine into decentralized energy systems such as the Combined Heat and Power systems (CHP) entails additional complications. These has increased the need for engineering tools that could assess design improvements, considering a broader range of parameters that would influence the engine performance when integrated within overall systems. Following this trend, the current work aimed to implement an analysis that could integrate the thermodynamics, and the thermal and mechanical interactions that influence the performance of kinematic Stirling engines. In particular for their use in Combined Heat and Power systems.The mentioned analysis was applied for the study of an engine prototype that presented very low experimental performance. The numerical methodology was selected for the identification of possible causes that limited the performance. This analysis is based on a second order Stirling engine model that was previously developed and validated. The simulation allowed to evaluate the effect that different design and operational parameters have on the engine performance, and consequently different performance curves were obtained. These curves allowed to identify ranges for the charged pressure, temperature ratio, heat exchangers dimensions, crank phase angle and crank mechanical effectiveness, where the engine performance was improved. In addition, the curves also permitted to recognise ranges were the design parameters could drastically reduce the brake power and efficiency. The results also showed that the design of the engine is affected by the conditions imposed by the CHP interactions, and that the engine could reach a brake power closer to 832W with a corresponding brake efficiency of 26% when the adequate design parameters were considered. On the other hand, the performance could also be very low; as the reported in experimental tests, with brake power measurements ranging 52–120W.

Dimensional analysis of leakage and mechanical power losses of kinematic Stirling engines

Dimensional analysis is applied to both leakage and mechanical power losses in order to obtain the set of parameters that influence power performance of the kinematic Stirling engine. An empircical model of mechanical power losses is proposed to complete the indicated power model previously introduced by Prieto and co-workers. That approach is mainly based on the characteristic Stirling number and allows the performance of known prototypes to be analysed from the viewpoint of indicated power, brake power and mechanical efficiency. After showing that similarity criteria can be extended to brake power performance, scaling factors have been derived for typical scaling cases. Some examples show that quasi-static simulation and scaling techniques may be combined to support preliminary design procedures.

A new equation representing the performance of kinematic Stirling engines

Previous work carried out for the last eight years resulted in the proposal of a complete system of dimensionless groups in order to represent the performance of different kinematic Stirling engine configurations. When looking for experimental support for the proposed model, some differences between the performances of several prototypes were observed. In this paper an equation is introduced to be applied to all known kinematic engines and to their whole range of performance. The coefficients appearing in this equation can be computed from temperature and geometrical dimensionless ratios and from experimental measurements at the maximum indicated power operating point. The meaning of those coefficients is interpreted and their usefulness to provide an engine performance overview is shown. The quasi-static simulation and the characteristic Mach number at the maximum indicated power operating point appear to be interesting criteria in order to evaluate the performance of prototypes.

Preliminary design of the kinematic Stirling engine using dynamic similarity and quasi-static simulation

This paper presents a preliminary design method based on dynamic similarity and quasistatic simulation. The proposal improves methods based on Beale and West correlations and provides the engine speeds corresponding to the maximum indicated powers and the achievement of indicated power maps. The dimensionless factor of indicated power losses provides a new viewpoint for engine analysis and design. In support of the method, experimental GPU-3 test results have been reanalysed, proving the accuracy of maximum indicated power predictions and providing values of the dimensionless factor of indicated power losses. Scaling factors corresponding to strict dynamic similarity are supplied for several typical cases. Similarity relaxation criteria are also recommended to be used when strict dynamic similarity leads to inadequate specifications for the derivative. Some numerical examples illustrate the method.

Application of Discriminated Dimensional Analysis to the Kinematic Stirling Engine

By means of spatially discriminated dimensional analysis, a complete system of dimensionless groups is proposed to describe the thermodynamic performance of the kinematic Stirling engine From experimental results, homogeneous dimensionless indicated power equations are deduced, as well as equations corresponding to the maximum indicated power operating point. Spatial discrimination evidences conceptual differences between quantities, reduces the number of dimensionless parameters and improves the design method based on dynamic similarity.

Isothermal Simulation with Decoupled Losses for Kinematic Stirling Engine Design.

Dimensional analysis has been employed to review the isothermal or quasi-static simulation of kinematic Stirling engines and to study the influence of the engine and drive mechanism types, and other parameters, on the thermodynamic engine cycle. The use of isothermal simulation with decoupled losses to obtain reliable predictions at practical levels of rpm is experimentally justified. The method seems to be an adequate tool to study a large number of configurations, which the engineer needs at the initial design stage. At the same time it provides a intuitive knowledge of and similarity criteria for the thermodynamic engine performance.

Noise and Vibration Characteristics of a 3 kW-class Stirling Engine

In recent years, a wide variety of papers on the Stirling engine and its unique properties have been published. However, the available knowledge on noise and vibration characteristics of Stirling engines is very limited. This paper presents experimental results which are useful for modifying the engine parts influencing the noise and vibration characteristics of the reciprocating kinematic Stirling engines. In order to clarify the noise and vibration sources of a 3 kW-class Stirling engine, the modal analysis for the engine parts and the measurement of sound intensity were examined. As a result, it was clarified that modifications for the combustion system and the driving system, particularly with respect to the crankshaft, can be made for decrement of the engine noise and vibration. Through the application of the proposed modifications, the 1987 engine noise was greatly decreased, and the 59 dB(A) noise level was measured under normal conditions.

Noise and vibration characteristics of a 3kW class Stirling engine.

In recent years a wide variety of papers on the Stirling engine and its unique properties have been published. However, the available knowledge on noise and vibration characteristics of the Stirling engines is very limited. This paper presents experimental results which are useful in modifications for the engine parts influencing the noise and vibration characteristics of the reciprocating kinematic Stirling engines. In order to clarify the noise and vibration sources of a 3 kW class Stirling engine, the modal analysis for the engine parts and the measurement of sound intensity were examined. As a result, it became clear that the modifications for the combustion system and the driving system, particularly with respect to the crankshaft, are available for decrement of the engine noise and vibration. Through use of the proposed modifications, the 1987 engine noise was greatly decreased, and the 59 dB(A) noise level was measured under a normal driving condition.