Study of free-piston Stirling engine driven linear alternators

The analysis, design and operation of single phase, single slot tubular permanent magnet linear alternator is presented. Included is the no-load and on-load magnetic field investigation, permanent magnet's leakage field analysis, parameter identification, design guidelines and an optimal design of a permanent magnet linear alternator. For analysis of the magnetic field, a simplified magnetic circuit is utilized. The analysis accounts for saturation, leakage and armature reaction

A free-piston Stirling engine/linear alternator controls and load interaction test facility

A test facility at LeRC was assembled for evaluating free-piston Stirling engine/linear alternator control options, and interaction with various electrical loads. This facility is based on a 'SPIKE' engine/alternator. The engine/alternator, a multi-purpose load system, a digital computer based load and facility control, and a data acquisition system with both steady-periodic and transient capability are described. Preliminary steady-periodic results are included for several operating modes of a digital AC parasitic load control. Preliminary results on the transient response to switching a resistive AC user load are discussed

Integration of linear alternators in thermoacoustic heat Engines

A thermoacoustic power converter consists of a thermoacoustic heat engine driving a linear alternator connected to a matched electric load. Accordingly, linear alternators are essential parts of thermoacoustic power converters. However, integration of a linear alternator in a thermoacoustic power converter is complicated since it requires acoustic matching with the thermoacoustic engine as well as electrical matching with the electric load connected to it and fast protection against piston over-stroking. In order to simplify the integration process, an experimental setup designed and built, in which the acoustic power generated by a thermoacoustic engine simulated by an acoustic driver. This setup provides a platform to test and evaluate the performance of a linear alternator in a controlled environment before integrated into thermoacoustic heat engines that allows identification and resolution of potential problems only related to linear alternators. A control circuit designed and built to protect the alternator’s piston against over-stroking. A non-linear electric load connected to the alternator to provide a stable operating point of the complete system. In this setup, instrumentation is used to monitor the main variables (input and output current, input and output volt, dynamic gas pressure at exit of acoustic driver and inlet of linear alternator, dynamic gas pressure in the enclosure volume of the acoustic driver and linear alternator, acoustic driver stroke, linear alternator stroke, air and coil temperatures). The setup allows use of different resonators to simulate the effects of different front volumes on the performance of linear alternators and allows alterations in the enclosure volumes housing the acoustic driver and/or alternator to control their resonance frequencies. Results show the performance of a given linear alternator under different operating frequencies, mean gas pressure, gas mixtures, input voltage, electrical resistance and zener break-down voltage

Characteristics of linear alternator performance under thermoacoustic-power-conversion conditions

Recently, immense research work was done on the thermoacoustics power converters for their great potential in generating electricity by different types of heat sources, including solar energy, waste heat as well as conventional fuels. This work studies the performance of the linear alternator which is the part responsible for converting the acoustic power generated by the thermoacoustic engine into electric power. This work encompasses three parts: the first part is an analytical model that consists of algebraic equations that estimate the main acoustic, mechanical and electrical performance indices of the linear alternator and the relationships between them under linear loading case. These equations are experimentally validated under different conditions. These equations are used to analyze the effects of the operation conditions such as operation under mechanical resonance and electrical resonance, the values of linear alternator parameters and the load parameters on the performance of the linear alternator. This part of work introduces the relationship between the effective inductance of the linear alternator and the mechanical stroke. Additionally, this part introduces an optimization for the piston area to achieve the minimum sum of the mechanical motion loss and the Ohmic loss and an optimization for the load resistance to achieve the maximum electric power in the load and to achieve the maximum acoustic-to-electric conversion efficiency. The second part of this work is an experimental parametric study using an experimental setup that was built for testing the linear alternator over a wide range of the thermoacoustic-power-conversion conditions that cannot be experimentally achieved in the case of testing the linear alternator with thermoacoustic engine. The parametric study covers the performance of the linear alternator under linear loading case and non-linear loading case. The effects of operating frequency, input dynamic pressure ratio, mean gas pressure, gas mixture, electric load value and the value of the power-factor-correcting capacitor on the performance indices are studied under the two types of loads. The results of the parametric study in the linear load case are compared to results of the analytical model and DELTAEC simulations and good agreement was found. The third part of this work is a sensitivity analysis that utilizes design-of-experiment methodology to estimate how the factors and their combined interactions affect the performance indices of the linear alternator under linear loading. The results of the study build a comprehensive study about the linear alternator performance under thermoacoustic-power-conversion conditions. The result are useful to properly select a linear alternator for an existing engine, or vice versa and to match the resulting system to an electric load. The results can be used to select the operating conditions that result in large generted power and or large efficiency. The results can be used to control the operating conditions in the correct proportions to achieve a certain required performance index, while observing the effects on other indices

Overview of NASA Magnet and Linear Alternator Research Efforts

The Department of Energy, Lockheed Martin, Stirling Technology Company, and NASA Glenn Research Center are developing a high-efficiency, 110 watt Stirling Radioisotope Generator (SRG110) for NASA Space Science missions. NASA Glenn is conducting in-house research on rare earth permanent magnets and on linear alternators to assist in developing a free-piston Stirling convertor for the SRG110 and for developing advanced technology. The permanent magnet research efforts include magnet characterization, short-term magnet aging tests, and long-term magnet aging tests. Linear alternator research efforts have begun just recently at GRC with the characterization of a moving iron type linear alternator using GRC's alternator test rig. This paper reports on the progress and future plans of GRC's magnet and linear alternator research efforts

Demagnetization Tests Performed on a Linear Alternator for a Stirling Power Convertor

The NASA Glenn Research Center (GRC) is conducting in-house research on rare-earth permanent magnets and linear alternators to assist in developing free-piston Stirling convertors for radioisotope space power systems and for developing advanced linear alternator technology. This research continues at GRC, but, with the exception of Advanced Stirling Radioisotope Generator references, the work presented in this paper was conducted in 2005. A special arc-magnet characterization fixture was designed and built to measure the M-H characteristics of the magnets used in Technology Demonstration Convertors developed under the 110-W Stirling Radioisotope Generator (SRG110) project. This fixture was used to measure these characteristics of the arc magnets and to predict alternator demagnetization temperatures in the SRG110 application. Demagnetization tests using the TDC alternator on the Alternator Test Rig were conducted for two different magnet grades: Sumitomo Neomax 44AH and 42AH. The purpose of these tests was to determine the demagnetization temperatures of the magnets for the alternator under nominal loads. Measurements made during the tests included the linear alternator terminal voltage, current, average power, magnet temperatures, and stator temperatures. The results of these tests were found to be in good agreement with predictions. Alternator demagnetization temperatures in the Advanced Stirling Convertor (ASC-developed under the Advanced Stirling Radioisotope Generator project) were predicted as well because the prediction method had been validated through the SRG110 alternator tests. These predictions led to a specification for maximum temperatures of the ASC pressure vessel

Field analysis and design of a moving iron linear alternator for use with linear engine

The previous research at West Virginia University has developed a permanent magnet linear alternator and linear combustion engine for testing and studying the performance. In this prototype, magnets are part of the moving part.;This research will present a new type of linear alternator with permanent magnets installed on stationary called Moving Iron Linear Alternator (MILA). MILA offers several advantages over other types such as rugged structure, and low cost production. First, MILA will be designed for use with the existing linear engine. An optimization methodology will be applied to obtain optimal design parameters. Next, a MILA model will be created in EMAS, field analysis software, to determine the machine flux. Later, the simulation will be performed for calculating the back emf and current of MILA. Finally, the simulation results will be discussed and explanations will be given

Development of a Low Inductance Linear Alternator for Stirling Power Convertors

The free-piston Stirling power convertor is a promising technology for high efficiency heat-to-electricity power conversion in space. Stirling power convertors typically utilize linear alternators for converting mechanical motion into electricity. The linear alternator is one of the heaviest components of modern Stirling power convertors. In addition, state-of-art Stirling linear alternators usually require the use of tuning capacitors or active power factor correction controllers to maximize convertor output power. The linear alternator to be discussed in this paper, eliminates the need for tuning capacitors and delivers electrical power output in which current is inherently in phase with voltage. No power factor correction is needed. In addition, the linear alternator concept requires very little iron, so core loss has been virtually eliminated. This concept is a unique moving coil design where the magnetic flux path is defined by the magnets themselves. This paper presents computational predictions for two different low inductance alternator configurations, and compares the predictions with experimental data for one of the configurations that has been built and is currently being tested

Megawatt Class Nuclear Space Power Systems (MCNSPS) conceptual design and evaluation report. Volume 1: Objectives, summary results and introduction

The objective was to determine which reactor, conversion, and radiator technologies would best fulfill future Megawatt Class Nuclear Space Power System Requirements. Specifically, the requirement was 10 megawatts for 5 years of full power operation and 10 years systems life on orbit. A variety of liquid metal and gas cooled reactors, static and dynamic conversion systems, and passive and dynamic radiators were considered. Four concepts were selected for more detailed study. The concepts are: a gas cooled reactor with closed cycle Brayton turbine-alternator conversion with heat pipe and pumped tube-fin heat rejection; a lithium cooled reactor with a free piston Stirling engine-linear alternator and a pumped tube-fin radiator; a lithium cooled reactor with potassium Rankine turbine-alternator and heat pipe radiator; and a lithium cooled incore thermionic static conversion reactor with a heat pipe radiator. The systems recommended for further development to meet a 10 megawatt long life requirement are the lithium cooled reactor with the K-Rankine conversion and heat pipe radiator, and the lithium cooled incore thermionic reactor with heat pipe radiator

Reliability of Radioisotope Stirling Convertor Linear Alternator

Onboard radioisotope power systems being developed and planned for NASA s deep-space missions would require reliable design lifetimes of up to 14 years. Critical components and materials of Stirling convertors have been undergoing extensive testing and evaluation in support of a reliable performance for the specified life span. Of significant importance to the successful development of the Stirling convertor is the design of a lightweight and highly efficient linear alternator. Alternator performance could vary due to small deviations in the permanent magnet properties, operating temperature, and component geometries. Durability prediction and reliability of the alternator may be affected by these deviations from nominal design conditions. Therefore, it is important to evaluate the effect of these uncertainties in predicting the reliability of the linear alternator performance. This paper presents a study in which a reliability-based methodology is used to assess alternator performance. The response surface characterizing the induced open-circuit voltage performance is constructed using 3-D finite element magnetic analysis. Fast probability integration method is used to determine the probability of the desired performance and its sensitivity to the alternator design parameters