International Journal of Thermodynamics

Exergy-based methods can be used as a tool for examining, comparing and assessing thermodynamic systems. In this paper, an exergoeconomic analysis is used to evaluate a power plant with chemical looping combustion (CLC) for CO₂ capture. This oxy-fuel plant is compared, from an exergetic and an economic perspective, to a conventional, reference power plant without CO₂ capture. The exergetic analysis shows decreased exergy destruction in the CLC reactors, compared to the exergy destruction in the conventional combustion chamber of the reference case; thus, the irreversibilities caused by combustion in the CLC are reduced. However, due to the addition of the CO₂ compression unit, the overall exergetic efficiency of the plant with CLC is lower than that of the reference plant by approximately 5 percentage points. The economic analysis confirms a significant increase in the investment cost of the CO₂ capture plant, due to the addition of the units for CO₂ compression and CLC. Thus, the cost of electricity is 24% higher for this plant in comparison to that of the reference case. Nevertheless, when compared to the reference plant with CO₂ capture with monoethanolamine, the plant with CLC was found to be a more economical option. Since CO₂ abatement must be realized in the future, given expected environmental or tax measures, CLC provides relatively low cost carbon dioxide capture and it, therefore, appears to be a promising option for reducing greenhouse gases emitted by power plants using fossil fuels.

A novel hybrid configuration of a coal-based polygeneration system, which bears the configuration characteristics of both parallel and sequential polygeneration systems, is proposed and investigated. Due to its configuration characteristics, the new system performs flexible production distribution (power and methanol) to coordinate the performance and capacity ratio of chemical to power and facilitate peak shaving of power generation. The performance of the new system is simulated by means of Aspen Plus. The new system has a wider range of product capacity than the sequential polygeneration system, with a range of energy saving rate of 2~14%. Compared with the two other polygeneration systems the new system has better performance at each capacity ratio. Especially at a capacity ratio of chemical production to power generation of 0.5, it is about 2% higher than sequential polygeneration system and 4.5% higher than the parallel polygeneration system. Exergy analysis is performed. Better chemical energy utilization of the syngas is obtained without component adjustment but with proper recycling of the unreacted gas.

In the presented paper possible design modifications in tubular solid oxide fuel cell (SOFC) geometry are investigated in order to increase its performance. The analysis of entropy generation mechanism is very important to optimize the second-law performance of these energy conversion devices. The use of this technique makes it possible to identify the main irreversibilities, understand their causes and propose changes in the system design and operation. The various contributions to the entropy generation are analyzed separately in order to identify which are the main geometrical parameters to be considered as the independent variables in the optimization procedure. The optimization is applied to a CFD model of the fuel cell which accounts for energy equation, fluid dynamics in the channels and in porous media, current transfer, chemical reactions, electrochemistry and raditive heat transfer. The entropy generation is computed as a post-processed quantity with the data obtained for the CFD model. The geometrical parameters of the fuel cell are modified to minimize the overall entropy generation. Boundary conditions to the CFD model are provided with the aid of a reduced thermal model to take in account stack operating condition.

This paper presents an exergy analysis of the human body under physical activity. A model of the respiratory system and a model of the thermal system were used for this purpose. These models consider heat and mass transfers in lungs, tissues and blood. Each component of these models is represented by a uniform compartment governed by equations for diffusion, convection, O₂ consumption, CO₂/heat generation and heat and mass transfer with the environment. The models allow the calculation of the exergy destruction in the lung and tissues, and the contribution of each entropy generation mechanism in the total generation. Furthermore, a discussion is proposed regarding the efficiency of the human body under physical exercise.

Ethanol fermentation is an exothermic process, where the kinetics depends on temperature. This study proposes an alternative cooling system for use in ethanol fermentation using a single-eect water/lithium bromide absorption chiller, powered by waste heat from sugar and ethanol production processes, with a temperature range of 80 to 100 oC. The aim of this study is to model, simulate and analyze the behavior of an absorption refrigeration machine, according to the required cooling capacity of the fermentation system. A comparative analysis with and without the chiller is performed. The introduction of a chiller allowed a reduction in the temperature of the medium of around 1 oC and an increase of around 0.8 % in the fermentation efficiency. Under these conditions less cellular stress occurs and cellular viability is kept at higher levels. The results show that this reduction in temperature can increase the ethanol content of the wine. In the recovery of ethanol, a lower thermal load will be needed at the distillation, with a smaller amount of vinasse produced and consequently the energy efficiency of the plant will increase.