Operation of a turbine-compound free-piston linear alternator

A free-piston linear-alternator combined with combustion chambers has been examined in many studies. However, only simplified thermodynamic and mechanical models were developed to mimic the actual behavior of the free-piston engine. The purpose of this study is to establish a fully dynamic model that can calculate the energy transformation under the operation of the free-piston engine. The Matlab/Simulink® model uses non-constant-volume combustion event, the piston transient dynamics, flow, heat losses, and thermodynamics as bridges to connect control volumes. The model successfully captured the behavior and measurements of a GS-34 free-piston engine, based on a thermodynamic calculation calibrated with experimental data. The resulting model is used for a series of parametric studies to understand the very complex system behavior, including low load operation. Operation parameters (injection timing and bounce chamber mass) are optimized to generate the engine map for different alternator sizes. At the end, the advantages of the opposed free-piston engine with a linear alternator are presented through the energy analysis.

Numerical Analysis and Parametric Study of a 7 kW Tubular Permanent Magnet Linear Alternator

Free-Piston Stirling Engines (FPSEs) are known for their easy maintenance, longer lifetimes, high reliability, quiet operation due to no crankshafts, and having fewer seals compared to the traditional Stirling engine. Free-piston systems are popular in the conversion of thermal energy into electrical energy and are compatible with many types of heat sources. This research paper concentrates on the development of a Permanent Magnet Linear Alternator (PMLA) and parametrically analyzing it to predict its limitations and performance over variable operable conditions and material choices. Operable conditions including stroke length and frequency of the translator, and material choice for the stator and magnets, are varied in this study to analyze the machine and put it to test for its extreme limitations. Spacing between slots is introduced to reduce the overall mass of the stator and increase the power density. The load test is carried out with varied parameters. It induces a load EMF of 2.4 kV, yields a power of 7 kW, and has a power density of 314 W/kg by FEM analysis in peak variations. This study enumerates the performance variation of a PMLA over these varied conditions and illustrates the limitations of such power-dense machines.

Construction and Performance Analysis of a Two-Stage Traveling-Wave Thermo-Acoustic Generator

Thermo-acoustic technology offers the possibility to convert heat energy into a sound-wave that can potentially be used to generate electricity through a linear alternator. The construction of a two-stage traveling-wave thermo-acoustic generator is described in this paper. The potential of conversion of heat into electricity has been investigated experimentally. The effect of the geometrical configurations of the thermo-acoustic system on its performance has been analyzed. The two thermo-acoustic engine cores were tested separately and subsequently combined to build a two-stage system. Two different configurations of engine cores have been considered namely series and parallel configuration. Hence, the effect of the orientation of the engine core has been investigated to get an insight into its effect on the output of the device. Parallel arrangement was found to be the most efficient configuration. An onset time of 3.15 minutes was recorded for the device to generate a sound wave. This system has achieved 125.7 dB corresponding to an output voltage of 486 mV. This study guides the development of more efficient electricity generators using thermo-acoustic technology.

Experimental investigations of the performance of a thermoacoustic electricity generator

This paper presents the experimental investigation of a two-stage thermoacoustic electricity generator able to convert heat at the temperature of the exhaust gases of an internal combustion into useful electricity. The novel configuration is one wavelength and consists of two identical stages. The identical stages will have out of phase acoustic wave at similar amplitudes which allows coupling a linear alternator to run in push-pull mode. The experimental set-up is 16.1 m long and runs at 54.7 Hz. The working medium is helium at 28 bar. The maximum generated electric power is 73.3 W at 5.64% thermal-to-electric efficiency. The working parameters including load resistance, mean pressure and heating power were investigated.

Advanced combustion free-piston engines: A comprehensive review

A reciprocating engine without a crank-slider mechanism is called a free-piston engine. If the piston is directly connected to a linear alternator, it is called a free-piston linear alternator. Free-piston engines and free-piston linear alternators have the potential to offer solutions for future hybrid electric vehicles and stationary power generation, by enabling direct conversion of mechanical energy to electricity. They benefit from reduced friction losses compared to conventional engines and can have variable compression ratio, which enables combustion control and optimization. Their widespread application has been limited by the necessity for high-speed control strategies. However, their operating characteristics can provide high efficiency, especially when used with low temperature combustion strategies. Low temperature combustion combines the high thermal efficiency of diesel engines, with the low soot emissions of spark-ignition engines, and low NO x emissions because of low burned gas temperatures. This article provides a comprehensive review of free-piston engine technology, with a focus on advanced combustion processes and their potential for use in future powertrain systems.