טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentOffner Avshalom
SubjectHeat and Mass Transfer in Acoustic Energy Conversion
DepartmentDepartment of Energy
Supervisor Professor Guy Ramon


Abstract

Low-temperature heat, abundantly available as industrial exhaust streams or non-concentrated solar radiation, is a largely underutilized energy source. Due to intrinsic irreversibilities, the efficiency of converting this heat to accessible power is low, requiring a robust, scalable, low-cost technology to face this challenge. In the present work we propose the use of thermoacoustic devices, i.e. heat engines employing a temperature difference to produce mechanical power through generation of high-intensity sound waves, for tapping this heat. Standard thermoacoustic systems typically operate under large temperature differences (> 200°C) and are therefore not suitable for this purpose. Various works over the past two decades suggested modifications to regular thermoacoustic systems, through which their temperature difference may be dramatically decreased, thus adapting them to waste-heat recovery purposes. These modifications, however, reduce the efficiency and increase system complexity (and cost).

The present work examines a different path towards meeting all the above criteria: we consider a gas mixture as the working fluid, containing a 'reactive' gas, able to exchange mass with a solid boundary, such that heat is primarily transferred at constant temperature, through latent heat. The vast amount of energy associated with the (nearly) reversible process of phase transition lowers the temperature difference in the system by leaving only a small fraction of the heat to be delivered through conduction and increase the temperature. In this manner, massive amounts of energy given at low temperature may be harnessed to produce acoustic power. A theoretical model accounting for this mass exchange was derived to investigate and capture the physical mechanisms of a mass-exchange thermoacoutic interaction; the results revealed not only the expected decrease in temperature difference, compared with a reference system involving no mass transfer, but also an increase in the system efficiency and generated acoustic power. Experimental setups were designed and constructed, demonstrating the first stable operation of a phase-change thermoacoustic engine, employing an air-water vapor gas mixture. The experimental results are in good agreement with the theoretical predictions, showing an increase in power production alongside a decrease in the temperature difference. The increase in efficiency and power generation suggests such systems may be cost-effective for low-grade heat recovery purposes.

Once the theoretical model was validated, we performed a theoretical investigation of the parameters affecting system performance and efficiency. Two fluid properties, namely a large latent heat associated with the phase transition and a low boiling temperature, were found to be key indicators for the choice of a reactive gas in the mixture. The recurrent mass transfer process in phase-exchange systems was discovered to increase acoustic power production by distorting the waveform and thus changing the phase angle between pressure and velocity. Our latest calculations predict that a system consuming heat and providing active cooling to -3°C and containing no moving parts in the process may operate with a total efficiency of more than 15%.