Table of Contents
Regular Original Research Article
| Advanced Exergy Analysis for Chemically Reacting Systems – Application to a Simple Open Gas-Turbine System | |
| Tatiana Morosuk, George Tsatsaronis | 105-111 | A conventional exergy analysis has some limitations, which are significantly reduced by an advanced exergy analysis. The latter evaluates: (a) the interactions among components of the overall system (splitting the exergy destruction into endogenous and exogenous parts); and, (b) the real potential for improving a system component (splitting the exergy destruction into unavoidable and avoidable parts). The main role of an advanced exergy analysis is to provide engineers with additional information useful for improving the design and operation of energy conversion systems. This information cannot be supplied by any other approach. In previous publications, approaches were presented that were appropriate for application to closed thermodynamic cycles, without chemical reactions (e.g., refrigeration cycles). Here a general approach is discussed that could be applied to systems with chemical reactions. Application of this approach to a simple gas-turbine system reveals the potential for improvement and the interactions among the system components.
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| On the Reference State for Exergy when Ambient Temperature Fluctuates | |
| Michel Pons | 113-121 | Exergy (availability) is the amount of mechanical work that could be produced by reversible processes. This notion is revisited in the case when ambient temperature fluctuates. Simple examples are first considered, and then a theoretical approach is developed. It results that the most reliable way for combining entropy and total energy into an exergy function is a linear combination where entropy is multiplied by a constant temperature. It results that ambient air has non-zero exergy, but that exergy is surely conserved in ideal reversible processes. This approach is relevant for studying some recent problems in solar energy and building engineering. |
| Analysis of Superimposed Elementary Thermodynamic Cycles: from the Brayton-Joule to Advanced Mixed (Auto-Combined) Cycles | |
| Andrea Lazzaretto, Giovanni Manente | 123-130 | The need for efficiency improvement in energy conversion systems leads to a stricter functional integration among system components. This results in structures of increasing complexity, the high performance of which are often difficult to be understood easily. To make the comprehension of these structures easier, a new approach is followed in this paper, consisting in their representation as partial or total superimposition of elementary thermodynamic cycles. Although system performance cannot, in general, be evaluated as the sum of the performance of the separate thermodynamic cycles, this kind of representation and analysis can be of great help in understanding directions of development followed in the literature for the construction of advanced energy systems, and could suggest new potential directions of work. The evolution from the simple Brayton-Joule cycle to the so called “mixed” cycles, in which heat at the turbine discharge is exploited using internal heat sinks only without using a separate bottoming section, is used to demonstrate the potentiality of the approach. Mixed cycles are named here "auto-combined cycles” to highlight the combination of different (gas and steam) cycles within the same system components.
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| Modelling and Simulation of a Hybrid Solid Oxide Fuel Cell Coupled with a Gas Turbine Power Plant | |
| Valentina Amati, Enrico Sciubba, Claudia Toro, Luca Andreassi | 131-139 | The paper presents a simulation of a hybrid solid oxide fuel cell-gas turbine (SOFC-GT) power generation system fueled by natural gas. In the system considered, the unreacted fuel from a topping solid oxide fuel cell is burnt in an afterburner to feed a bottoming gas turbine and produce additional power. Combustion gas expands in the gas turbine after having preheated the inlet air and fuel and it is used to generate steam required by the reforming reactions. A novel thermodynamic model has been developed for the fuel cell and implemented into the library of a modular object-oriented Process Simulator, CAMELPro™. The relevant plant performance indicators have been analyzed to evaluate the incremental increase in efficiency brought about by the introduction of the gas turbine and heat regeneration system. Simulations were performed for different values of the main plant parameters.
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| Peak Load Electricity Production with Cryogenic Fuels | |
| Giampaolo Manfrida, Duccio Tempesti, Gaetano Zumbo | 141-146 | Natural Gas is often liquefied (LNG) for its transport by ships over long distances. In order to prepare it for further transport by pipelines it has to be reduced again to a gaseous state, normally by heating it with sea water. A similar technology is envisaged for long-distance transport of Hydrogen as an energy carrier. The scope of this article is the thermodynamic investigation of two power plants for peak load energy production. These two power plants use as fuel the fluids obtained by the re-gasification of the cryogenic fluids. The first proposal is a Hydrogen-fired steam power plant, while the second considers the use of LNG in an oxy-combustion arrangement with subsequent CO2 separation, which is obtained by a three-stage intercooled compression train. The power cycle performance was verified in both cases by exergy analysis. Since the size of these power plants is relatively small (10 MWe), they can be easily built inside the area of LNG gasifiers, or inside the area of the plant producing liquid Hydrogen; the cryogenic fuel and oxidizer are thus considered available, and the purpose of the power plant is peak load energy production rather than obtaining high values of conversion efficiency.
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| Thermoeconomic Analysis of Simple Trigeneration Systems | |
| Miguel A. Lozano, M. Carvalho, J. C. Ramos, Luis M. Serra | 147-153 | Trigeneration is the combined production of heating, cooling and power from the same source of energy. In this paper, the operation of a simple trigeneration system is analyzed. The system is interconnected to the electric utility grid, both to receive electricity and to deliver surplus electricity. For any given demand required by the users, a great number of operating conditions are possible. The operational mode with the lowest variable cost is obtained through a linear programming model. Three different approaches to determine the costs of internal flows and final products of the simple trigeneration systems are presented: marginal costs corresponding to optimal operation, costs obtained when production costs are distributed to the final products according to their market prices, and internal costs corresponding to a thermoeconomic analysis of the operation mode of the system. As expected, the costs obtained with the approaches mentioned are different and it can be concluded that there are no general rules to decide which approach is best: it depends on the issue under investigation.
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| Ethanol Production from Banana Fruit and its Lignocellulosic Residues: Exergy and Renewability Analysis | |
| H. I. Velásquez Arredondo, A. A. Ruiz Colorado, Silvio de Oliveira Júnior | 155-162 | Tropical countries such as Brazil and Colombia have the possibility of using their lands for growing vegetable products to produce biofuels such as biodiesel and ethanol. The objective of this work is to apply exergy analysis to evaluate the renewability of anhydrous ethanol production from surplus banana fruit production and its residual biomass. The study takes into account all production stages: growing, feedstock transport, hydrolysis, fermentation, distillation, and dehydration. It also considers the cogeneration plant and residues treatment. Four production routes were analyzed according to the biomass used as feedstock: banana pulp, banana fruit, hanging cluster or banana skin. Based on the exergy concept, performance indicators are proposed and calculated. In order to quantify the renewability of the ethanol production processes, a new indicator called “Renewability Performance Indicator” is defined and applied to the four ethanol production routes studied. The results show that when amilaceous material is used, better results than lignocellulosic material are obtained and four production processes studied must be classified as non-renewable.
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ISSN: 2146-1511