Author: Mehmet Fatih Orhan

Publisher:

Published: 2011

Total Pages:

ISBN-13:

The world faces problems with depleting energy resources and the harmful impact of present energy consumption patterns on the environment, and consequently on the global climate and humanity. The concerns regarding global climate change are serious and have resulted in extensive research and developments on alternative, clean energy sources. While many of the available natural energy resources are limited due to their reliability, quality, quantity and density; nuclear energy has the potential to contribute a significant share of large scale energy supply without or little contributing to climate change. Hydrogen production via thermochemical water decomposition is one of the key potential processes for direct utilization of nuclear thermal energy. Thermochemical water splitting with a copper-chlorine (Cu-Cl) cycle is a promising process that could be linked with nuclear reactors to decompose water into its constituents, oxygen and hydrogen as a net result, through intermediate copper and chlorine compounds with a net input of water and heat. The process involves a series of closed-loop chemical reactions that does not contribute to any greenhouse gas emissions into the environment. Although some preliminary technical studies of the Cu-Cl cycle have been reported and some small lab scale experiments of individual reactions in the cycle have been carried out, there is still a need to link all the sub-steps of the cycle and build a pilot plant, to facilitate eventual commercialization. Such an experimental set up of overall cycle is lacking, especially to evaluate characteristics of the complete cycle such as energy, exergy and cost effectiveness. Simulation packages, such as Aspen Plus, are useful tools to provide the system designer or operator with design, optimization and operation information before building a pilot plant. In this thesis, process analysis is performed and simulation models are developed using the Aspen Plus simulation package, based on experimental work carried out at the University of Ontario Institute of Technology (UOIT), the Argonne National Laboratory (ANL), the Atomic Energy of Canada Limited (AECL) and other sources. The energy and mass balances, stream flows and properties, the heat exchanger duties and shaft work are calculated. Heat recovery options are assessed to improve thermal management and hence overall efficiency of the Cu-Cl cycle. An integrated heat exchange network is designed to use heat from the process streams efficiently and decrease the external heat demand. The efficiency of the process, based on three, four and five-step cycles, is examined in this thesis. The thermal efficiency of the five-step thermochemical process is calculated as 44%, of the four-step process is 43% and of the three-step process is 41%, based on the lower heating value of hydrogen. Sensitivity analyses are performed to study the effects of various operating parameters on the efficiency, yield, and cost. A parametric study is conducted, and possible efficiency improvements are discussed. The manner is investigated in which exergy-related parameters can be used to minimize the cost of a Cu-Cl thermochemical cycle for hydrogen production. The iterative optimization technique presented requires a minimum of available data and provides effective assistance in optimizing thermal systems, particularly in dealing with complex systems and/or cases where conventional optimization techniques cannot be applied. The principles of thermoeconomics, as embodied in the specific exergy cost (SPECO) and exergy-cost-energy-mass (EXCEM) methods, are used here to determine changes in the design parameters of the cycle that improve the cost effectiveness of the overall system. It is found that the cost rate of exergy destruction varies between $1 and $15 per kilogram of hydrogen produced; and the exergoeconomic factor between 0.5 and 0.02 as the cost of hydrogen rises from $2.8 to $20 per kg of hydrogen produced. The hydrogen cost is inversely related to the exergoeconomic factor, plant capacity and energy/exergy efficiencies. Based on the cycle's design parameters and conditions the hydrogen production cost is calculated as $3.8/kg hydrogen. Also, an integrated Cu-Cl cycle hydrogen production system, based on nuclear and renewable energy sources, is investigated. Nuclear and renewable energy sources are reviewed to determine the most appropriate option to couple with the Cu-Cl cycle. An environmental impact assessment is conducted and compared to the conventional methods using fossil fuels and other options. Some cost assessment studies of hydrogen production are presented for this integrated system. The results show that hydrogen production cost could drop down to as low as 2.8 $/kg. The results are expected to assist ongoing efforts to increase the economic viability of the Cu-Cl cycle, and to reduce product costs of potential commercial versions of this process.