A two-phase single-reciprocating-Piston heat conversion engine

Abstract

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.This paper considers an energy-conversion heat-engine concept termed ‘Up-THERM’. This machine is capable of converting low- to medium-grade heat to useful positivedisplacement work through the periodic evaporation and condensation of a working fluid in an enclosed space. These alternating phase-change processes drive sustained oscillations of thermodynamic properties (pressure, temperature, volume) as the working fluid undergoes an unsteady thermodynamic heatengine cycle. The resulting oscillatory flow of the working fluid is converted into a unidirectional flow in a hydraulic load arrangement where power can be extracted from the machine. The engine is described with lumped dynamic models constructed using electrical analogies founded on previously developed thermoacoustic and thermofluidic principles, which are extended here to include a description of the phase-change heat-transfer processes. For some sub-components of the engine, such as the gas spring, valves and the temperature profile in the heat exchangers, deviations from the linear theory are nonnegligible. These are modelled using non-linear descriptions. In particular, the results of linear and non-linear descriptions of the gas spring are compared using three important performance indicators — efficiency, power output and frequency. The non-linear description of the gas spring results in morerealistic predictions of the oscillation frequency compared to direct measurements on an experimental prototype of a similar engine. Owing to its mode of operation and lack of moving parts, the Up-THERM engine does offer a much simpler and more cost-efficient solution than alternative engines for heat recovery and solar applications. The results from this work suggest that this technology can be a competitive alternative in terms of cost per unit power in low-power, small-scale applications, especially in remote, off-grid settings, for example in developing countries where minimising upfront costs is crucial.