Modeling, analysis and optimization of integrated energy systems for multigeneration purposes

Abstract

Energy use is directly linked to well-being and prosperity across the world. Meeting the growing demand for energy in a safe and environmentally responsible manner is an important challenge. There are around seven billion people on Earth and population growth will likely lead to an increase in energy demand, which depends on the adequacy of energy resources. In addition, increasing population and economic development in many countries have serious implications for the environment, since energy generation processes (e.g., generation of electricity, heating, cooling, and shaft work for transportation and other applications) emit pollutants, many of which are harmful to ecosystems. Utilizing advanced technologies to mitigate global warming and increase the efficiency of energy systems are key objectives, with ways to meet them proposed and tested in many countries. Among these technologies, multigeneration processes stand out as a possibility for making important contributions due to their potential for high efficiencies as well as low operating costs and pollution emissions per energy output. In this PhD thesis, three novel multigeneration energy systems are considered, analyzed and optimized. The aim is to consider both renewable- and non-renewable-based multigeneration systems. A non-renewable-based multigeneration system is composed of a gas turbine as a prime mover, a double pressure heat recovery steam generator, a single effect absorption chiller, a domestic water heater, an ejector cooling system and PEM electrolyzer. This proposed multigeneration system can produce electricity, heating, cooling, hot water and hydrogen simultaneously. The overall exergy efficiency of the system is 60%, which is 30% higher than the power generation system. Observations show that shifting from a conventional power generation system to a multigeneration cycle leads to a decrease in CO2 emissions of approximately 120 kg/kWh, providing significant motivation to convert to multigeneration cycles. For renewable-based multigeneration systems, biomass-based and integrated ocean thermal energy conversion (OTEC)-based were selected as candidates to meet the requirements of producing electricity, heating, cooling, hot and fresh water and hydrogen. The biomass-based multigeneration system is composed of a biomass combustor, an ORC cycle for producing electricity, a double-effect absorption chiller for cooling, a heat exchanger for heating, a proton exchange membrane (PEM) electrolyzer for producing hydrogen, a domestic water heater for producing hot water and a reverse osmosis (RO) desalinator for producing fresh water. Pine sawdust is used as the biomass fuel and burned in a biomass combustor. This multigeneration system increases the exergy efficiency by about 20% and reduces CO2 emissions by about 3500 kg/MWh compared to a conventional power generation system.The last multigeneration energy system examined is an ocean thermal energy conversion (OTEC)-based system integrated with a PV/T solar collector and a single-effect absorption chiller to provide the cooling load of the system. An OTEC system utilizes low-grade energy and has a low energy efficiency. This integrated system uses warm surface seawater to evaporate a working fluid like ammonia or a Freon refrigerant, which drives an ORC turbine to produce electricity, which in turn is used to drive a PEM electrolyzer to produce hydrogen. A reverse osmosis (RO) desalination unit is used to produce fresh water. The exergy efficiency of this integrated system is 37%, which is higher than single generation systems and, in addition, this integrated system has no emissions as it uses ocean energy instead of fuel. Multigeneration processes can make important contributions due to their potential for high efficiency as well as low operating costs and pollution emissions per energy output. Issues such as fossil fuel depletion and climate change amplify the advantages and significance of efficient multigeneration energy systems.