Free-Piston Stirling Engines

“…Clearly, reciprocating and rotary piston compressors and expanders are of interest in the applications just mentioned. The present study focuses on reciprocating piston machines; however, the results are transferable to rotary piston equivalents, as well as related engines and devices (e.g., [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]). …”

Section: Piston Compressors and Expandersmentioning

confidence: 99%

“…In many cases the unsteady (i.e., time-varying) heat transfer can take place simultaneously with pressure and density variations in the fluid, for example, in thermoacoustic engines [1][2][3], dry free-liquid-piston (Fluidyne) engines [4][5][6] or two-phase thermofluidic oscillator equivalents [7][8][9], free-piston Stirling engines [10][11][12], and pulse tubes [13][14][15], as well as in standalone compressors and expanders or when these units appear as components within larger systems. In some cases the conditions can give rise to conjugate heat transfer in the presence of the time-varying variations of pressure, density, and temperature.…”

Section: Introduction Unsteady and Conjugate Heat Transfermentioning

confidence: 99%

A Framework for the Analysis of Thermal Losses in Reciprocating Compressors and Expanders

Mathie

Markides

White

2014

Heat Transfer Engineering

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This article presents a framework that describes formally the underlying unsteady and conjugate heat transfer processes that are undergone in thermodynamic systems, along with results from its application to the characterization of thermodynamic losses due to irreversible heat transfer during reciprocating compression and expansion processes in a gas spring. Specifically, a heat transfer model is proposed that solves the one-dimensional unsteady heat conduction equation in the solid simultaneously with the first law in the gas phase, with an imposed heat transfer coefficient taken from suitable experiments in gas springs. Even at low volumetric compression ratios (of 2.5), notable effects of unsteady heat transfer to the solid walls are revealed, with thermally induced thermodynamic cycle (work) losses of up to 14% (relative to the work input/output in equivalent adiabatic and reversible compression/expansion processes) at intermediate Péclet numbers (i.e., normalized frequencies) when unfavorable solid and gas materials are selected, and closer to 10-12% for more common material choices. The contribution of the solid toward these values, through the conjugate variations attributed to the thickness of the cylinder wall, is about 8% and 2% points, respectively, showing a maximum at intermediate thicknesses. At higher compression ratios (of 6) a 19% worst-case loss is reported for common materials. These results suggest strongly that in designing high-efficiency reciprocating machines the full conjugate and unsteady problem must be considered and that the role of the solid in determining performance cannot, in general, be neglected.

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“…This device weighs just under a kilogram and produces a nominal power of 1 Watt. A simple thermodynamic analysis of this type of device is offered in [2], and relies mostly on equations of state and intuitive notions of gas dynamics. This engine configuration was invented by William Beale in the 1970s and set the standard for future devices.…”

Section: Not Shown Inmentioning

confidence: 99%

Control-based design of free-piston stirling engines

Riofrio

Al-Dakkan

Hofacker

et al. 2008

2008 American Control Conference

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A control-based analysis and characterization of a free-piston Stirling engine is presented, and proposed as a lightweight power supply for untethered robots. Typically, such devices are designed from the point of view of a thermodynamic cycle in terms of traditional thermodynamic equations of state. Such equations of state are independent of time and therefore lend little insight when dynamic elements are incorporated into the design. The approach presented here is from a system dynamics and control perspective. Equations of state are replaced by dynamic system modeling elements. Utilizing these dynamic elements, control concepts are applied to evaluate a given configuration and ensure an unstable oscillatory response and therefore transform heat into useful work. A simulation of a commercially available free-piston engine is presented, and standard control design tools are applied to its linearized model. The results show promising potential in utilizing small-scale free-piston Stirling engines as portable power supply for robotic systems.

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Free-Piston Stirling Engines

Cited by 36 publications

References 2 publications

A novel small dynamic solar thermal desalination plant with a fluid piston converter

A novel small dynamic solar thermal desalination plant with a fluid piston converter

A Framework for the Analysis of Thermal Losses in Reciprocating Compressors and Expanders

Control-based design of free-piston stirling engines

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