Date of Award

2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Mechanical Engineering & Applied Mechanics

First Advisor

Noam Lior

Abstract

Past studies on hybrid power cycles using multiple heat sources of different temperatures focused mainly on case studies and almost no general theory about this type of systems has been developed. This dissertation is a study of their general thermodynamic performance, with comparisons to their corresponding single heat source reference systems. The method used in the dissertation was step-wise: to first analyze the major hybrid power cycles (e.g. Rankine, Brayton, Combined Cycles, and their main variants) thermodynamically, without involving specific operation parameter values, and develop some generalized theory that is at least applicable to each type of system. The second step was to look for commonalities among these theories and develop the sought generalized theory based on these commonalities. A number of simulation case studies were performed to help the understanding and confirm the thermodynamic results. Exergo-economic analysis was also performed to complement the thermodynamic analysis with consideration of externalities, and was compared to the conventional economic analysis method. The generalized expressions for the energy/exergy efficiency differences between the hybrid and the corresponding single heat source systems were developed. The results showed that the energy and exergy efficiencies of the hybrid systems are higher than those of their corresponding single heat source reference systems if and only if the energy/exergy conversion efficiency (defined in the dissertation) of the additional heat source (AHS, can be any heat source that has lower temperature) is larger than that of the original heat source. Sensitivity analysis results showed the relations between the temperature and heat addition rate of the AHS and the energy/exergy efficiency of the hybrid systems. Other big advantages of hybrid systems, i.e. the effects on replacement of fossil fuel by renewable, nuclear and waste energy, lower emissions and depletion of fossil fuel, were revealed in the economic analysis, by considering the cost reduction from fuel saving and carbon tax. Simple criteria were developed to help compare the hybrid and reference systems and determine under which conditions the hybrid systems will have better thermodynamic or economic performance than the reference ones. The results and criteria can be used to help design the hybrid systems to achieve higher energy and/or exergy efficiencies and/or lower levelized electricity cost (LEC) before detailed design or simulation or experiment. So far, 3 archival journal papers and 3 conference papers were published from this dissertation work.

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