This study presents energy, exergy, and exergoeconomic assessments for a natural gas-fueled micro-trigeneration unit, which encompasses an internal combustion engine (ICE), an absorption refrigeration system (ammonia-water), and a heat recovery unit. The energy system is designed to meet the electricity and cooling demands of a university building, while heat is directed to a biodiesel plant located on site. The analyses comprehended energy and exergy parameters (first and second laws of thermodynamics). Then, the SPECO methodology was applied for the exergoeconomic assessment, which associates monetary values with exergy flows. The ICE presented the highest irreversibility (61.24 kW) and the highest cost of exergy destruction (21.56 BRL/h), followed by the steam generator (5.52 kW and 6.71 BRL/h, respectively). The exergoeconomic assessment indicated the components that could benefit from improvements: steam generator (cooling of exhaust gases before entry into the generator) and the heat exchanger of the absorber (substitution of the exchanger and/or pre-heating of input air). This study contributes to narrow the knowledge gap regarding the exergoeconomic assessment of compact trigeneration systems, besides making a case for the utilization of ICE as prime movers.
Cost allocation, optimization, and diagnosis are the main application fields of thermoeconomics. Cost allocation allows evaluating the plant cost formation process. For plants containing dissipative equipment, the use of total exergy flows in the conventional productive diagrams is not able to isolate these components. The physical exergy disaggregation, despite increases the complexity, allows the treatment of dissipative equipment and residues. Nevertheless, conventional productive diagrams based on productive flows and fictitious units to connect the subsystems, may not take into account the physical connections existing in the system flowsheet, making the cost formation process arbitrary. The comprehensive diagram uses both flow types, physical and productive, and the subsystems are interconnected according to the physical flows of the flowsheet. This work aims to demonstrate that the combination of exergy disaggregation and comprehensive diagram avoids arbitrariness. The application of the localized exergy disaggregation at the comprehensive diagram reduces the complexity. A gas turbine cogeneration system is chosen as one application example. Productive and comprehensive diagrams are compared with total and localized disaggregation and a systematic procedure to treat waste and dissipative equipment is presented. The reduction of complexity is performed by means of localized disaggregation and the arbitrariness related to the productive diagram is evaluated.
Residues with a large amount of organic content represent a potential for energy recovery. Specifically human feces, given the amount of global production and the environmental appeal, appear as a potential candidate for a new feedstock. The objective of this work is to perform a thermodynamic assessment of human feces gasification considering for the first time all inefficiencies of a downdraft gasifier. A thermochemical characterization was conducted from the sterilized raw material to the products. New data and discussions about the conversion efficiencies for such type of fuel are brought up, such as the influence of the exothermic pyrolysis on the chemical exergy destruction. The results suggest an unfavorable application in energetic terms; however, when the exergy analysis is added with the environmental bias, the process becomes more attractive due to the high physical exergy.
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