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EDITORIAL

J. Energy Resour. Technol. 1999;121(4):233. doi:10.1115/1.2795987.
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Abstract
Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS

J. Energy Resour. Technol. 1999;121(4):234-240. doi:10.1115/1.2795988.

Plunger lift and velocity strings are two common methods of attacking liquid loading problems in gas wells. This paper shows how to compare the calculated performance of both methods at downhole producing conditions on one plot.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(4):241-246. doi:10.1115/1.2795989.

From the second law of thermodynamics, the concepts of irreversibility, entropy generation, and availability are applied to counterflow, parallel-flow, and cross-flow heat exchangers. In the case of the Cross-flow configuration, there are four types of heat exchangers: I) both fluids unmixed, 2) both fluids mixed, 3) fluid of maximum heat capacity rate mixed and the other unmixed, 4) fluid of minimum heat capacity rate mixed and the other unmixed. In the analysis, the heat exchangers are assumed to have a negligible pressure drop irreversibility. The Counterflow heat exchanger is compared with the other five heat exchanger types and the comparison will indicate which one has the minimum irreversibility rate. In this comparison, only the exit temperatures and the heat transfer rates of the heat exchangers are different. The other conditions (inlet temperatures, mass flow rates, number of transfer units) and the working fluids are the same in the heat exchangers.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(4):247-253. doi:10.1115/1.2795990.

This paper presents the analysis of two-dimensional heat transfer in an annular finned tube assembly during the process of dehumidification. All possible fin surface conditions, namely, dry, fully wet, and partially wet, have been studied and fin efficiency under these conditions have been modeled. Computations have been carried out using a control volume-based finite-difference method and compared with past one-dimensional analytical studies and available experimental data. The parameters that influenced the heat transfer rate in the finned tube structure are ratio of fin and wall thermal conductivities, ratio of fin thickness to fin pitch, ratio of wall thickness to fin pitch, ratio of fin length to fin pitch, cold fiuid Biot number, ambient Blot number, the relative humidity and dry bulb temperature of the incoming air, and the cold fluid temperature inside the coil. It was found that the heat transfer increased with increment in both dry bulb temperature and the relative humidity of the air. The fin efficiency changed rapidly with relative humidity under partially wet condition. The results suggest that coil performance can be very significantly altered by the condensation phenomenon on the fin surface and designs with dry fin data may not be adequate. The present results are expected to be very useful for the design of dehumidifier (cooling) coils for air conditioning applications.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(4):254-261. doi:10.1115/1.2795991.

A rational method of determining the operational strategy of energy supply plants in consideration of equipment startup/shutdown cost is proposed. The operational planning problem is formulated as a large-scale mixed-integer linear programming one, in which on/off status and energy flow rates of equipment are determined so as to minimize the sum of energy supply and startup/shutdown costs over the period considered. By utilizing a special structure of the problem, an algorithm of solving the problem efficiently is proposed. Through a numerical study on the daily operational planning of a gas turbine cogeneration plant for district heating and cooling, the effectiveness of the proposed algorithm, is ascertained in terms of computation time, and the influence of equipment startup/shutdown cost on the operational strategy and cost is clarified.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(4):262-267. doi:10.1115/1.2795992.

An optimal planning method is presented for a cascade-type multistage refrigeration system. Heat exchange areas of evaporator, condenser, and beverage cooler are determined optimally so as to minimize the annual total cost and input energy consumption subject to constraints concerning annual equipment operation. This problem is considered as a multiobjective optimization one, and a discrete set of Pareto optimal solutions is derived numerically by a weighting method. Through a numerical study, it is investigated how the heat exchange areas influence the long-term economics and energy conservation. Cascade-type multistage refrigeration systems are compared with single-stage systems.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(4):268-276. doi:10.1115/1.2795993.

The values of absolute exergy and exergetic efficiency depend on the assumed values of the “reference states,” which are essentially a model of the environment of the processes and components considered. The environmental model plays an important role in any optimization process that uses any form of exergy-based analysis. This paper examines the influence of eleven previously proposed models for the environment on the absolute values of the exergy and on the exergetic efficiencies, associated with the operation of three different power plants: a methane cogeneration plant, a coal-fired plant, and a dual-flash geothermal power plant. It is observed that although there are some common characteristics in most of the values obtained from all the models, the absolute exergies and exergetic efficiencies calculated using some of these models deviate substantially from those obtained with the others. The reasons for these deviations are also examined here.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(4):277-285. doi:10.1115/1.2795994.

A proposed molten carbonate fuel cell power plant design, intended for commercial production by the end of the 1990s and developed under the auspices of the U.S. Department of Energy, the Gas Research Institute, and Energy Research Corporation, has been analyzed with exergy and pinch analysis. The commercial production units, targeted for dispersed power generation markets, are based on an existing demonstration molten carbonate fuel cell power plant design which was demonstrated from 1996–1997. Exergy analysis of the commercial plant design shows the overall, second-law system efficiency to be 53 percent. The principal inefficiency, 17 percent of the total, lies in the catalytic combustor. Another major inefficiency is the stack loss, 14 percent. Heat transfer accounts for approximately 6 percent of the loss. System reconfigurations, incorporating a steam cycle with reheat (System I) and a gas turbine cycle (System II), both with revised heat exchanger networks, for significant improvement are proposed and evaluated. The second-law system efficiency is raised to 66 percent in System I and to 70 percent for System II.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(4):286-294. doi:10.1115/1.2795995.

An analytical model for the second-law-based thermoeconomic analysis and optimization of a sensible-heat-storage system is derived and discussed, in which the storage element is both heated and cooled by flowing streams of gases. In this analysis, monetary values are attached to the irreversible losses caused by the finite temperature difference heat transfer and pressure drop in the storage system. Important dimensionless parameters are identified and the results are presented in terms of the optimum dimensionless charging time θS ,opt as a function of a dimensionless temperature difference τ, as well as the optimum number of heat transfer units NTUS ,opt , as a function of the dimensionless unit cost per unit heat conductance γUA and τ of the storage systems. The systems analyzed are optimized by introducing a new performance criterion described as the cost rate number , Γ*. Several example problems are also presented and the results are compared with that obtained from Krane’s analysis to illustrate the usefulness of the present approach. The influence of important unit cost parameters on NTUS ,opt and θS ,opt , are also studied in somewhat more detail.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(4):295-301. doi:10.1115/1.2795996.

Performance evaluation of a 12.8-MW single-flash design geothermal power plant in Northern Nevada is conducted using actual plant operating data, and potential improvement sites are identified. The unused geothermal brine reinjected back to the ground is determined to represent about 50 percent of the energy and 40 percent of the exergy available in the reservoir. The first and second-law efficiencies of the plant are determined to be 6 percent and 22 percent, respectively. Optimizing the existing single-flash system is shown to increase the net power output by up to 4 percent. Some well-known geothermal power generation technologies including double-flash, binary, and combined flash/binary designs as alternative to the existing system are evaluated and their optimum operating conditions are determined. It is found that a double-flash design, a binary design, and a combined flash/binary design can increase the net power output by up to 31 percent, 35 percent, and 54 percent, respectively, at optimum operating conditions. An economic comparison of these designs appears to favor the combined flash/binary design, followed by the double-flash design.

Commentary by Dr. Valentin Fuster

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