Graduation Year

2010

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Chemical and Biomedical Engineering

Major Professor

D. Yogi Goswami, Ph.D.

Co-Major Professor

Elias K. Stefanakos, Ph.D.

Committee Member

Muhammad M. Rahman Ph.D.

Committee Member

John T. Wolan, Ph.D.

Committee Member

Dale Johnson, Ph.D.

Keywords

Organic Rankine Cycle, Working Fluids, Heat Recovery, Efficiency, Optimization

Abstract

Low-grade heat sources, here defined as below 300 ºC, are abundantly available as industrial waste heat, solar thermal, and geothermal, to name a few. However, they are under-exploited for conversion to power because of the low efficiency of conversion. The utilization of low-grade heat is advantageous for many reasons. Technologies that allow the efficient conversion of low-grade heat into mechanical or electrical power are very important to develop.

This work investigates the potential of supercritical Rankine cycles in the conversion of low-grade heat into power. The performance of supercritical Rankine cycles is studied using ChemCAD linked with customized excel macros written in Visual Basic and programs written in C++.

The selection of working fluids for a supercritical Rankine cycle is of key importance. A rigorous investigation into the potential working fluids is carried out, and more than 30 substances are screened out from all the available fluid candidates. Zeotropic mixtures are innovatively proposed to be used in supercritical Rankine cycles to improve the system efficiency.

Supercritical Rankine cycles and organic Rankine cycles with pure working fluids as well as zeotropic mixtures are studied to optimize the conversion of lowgrade heat into power. The results show that it is theoretically possible to extract and convert more energy from such heat sources using the cycle developed in this research than the conventional organic Rankine cycles. A theory on the selection of appropriate working fluids for different heat source and heat sink profiles is developed to customize and maximize the thermodynamic cycle performance.

The outcomes of this research will eventually contribute to the utilization of low-grade waste heat more efficiently.

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