Analytical model for an overdriven free-displacer thermocompressor. (25th February 2021)

• Record Type: Journal Article
• Authors: Fischer, Fabian; Kuehl, Hans-Detlev
• Is Part Of: Applied thermal engineering. Volume 185(2021)
• Publication Date: 2021-02-25
• Subjects: Reciprocating thermocompressor -- Thermal compression -- Stirling cycle -- Compressed air -- Waste heat utilization -- Overdriven mode; Heat engineering -- Periodicals; Heating -- Equipment and supplies -- Periodicals; Periodicals; 621.40205
• DOI: 10.1016/j.applthermaleng.2020.116251
• Languages: English
• ISSNs: 1359-4311

Abstract

Highlights: A thermocompressor cascade composed of identical stages may generate compressed air. Any heat source, especially waste heat of sufficient temperature, can be used. The developed simplified analytical model demonstrates the principal functionality. Overdriven free displacers allow a self-controlled speed adjustment of the stages. Drive power limitations force minimizing friction with priority over thermal losses. Abstract: Compressed air is an important but costly and inefficient industrial energy source. Coincidently, large amounts of waste heat are mostly discarded or used as low-grade heat only. To utilize such waste heat at low manufacturing and almost zero operating costs, this contribution proposes a novel concept based on reciprocating thermocompressors. It focuses on maximum constructional simplicity by arranging identical stages in a cascade and resorting to self-excitated, overdriven free displacers, the frequencies of which may adapt inversely to the pressure to ensure an optimum operation of all stages, simultaneously preserving thermal similarity. The principal functionality of this system is demonstrated by derivation and solution of a set of simplified analytical equations of motion for the displacer, based on an idealized isothermal cycle. The results show that both the displacer mass and stroke must be sufficiently large to ensure the desired reciprocating motion at an acceptable frequency and an almost constant velocity, reducing both thermal and viscous flow losses. Limitations of the available power to drive the displacer suggest a design primarily further minimizing the latter rather than the former. These results open up a new perspective for a direct, both cost- and energy-efficient generation of compressed air from waste heat.