Exergy, An International Journal最新文献

Certain thermodynamic aspects of cryogenic turbines are investigated based on operational data provided by a cryogenic test facility. The cryogenic turbine is intended to produce power by replacing the throttling valve used in natural gas liquefaction plants. The turbine operates at very low temperatures; it admits liquefied natural gas (LNG) at a high pressure and discharges it at a low pressure. The temperature change of LNG in the expansion process through the cryogenic turbine is studied and compared with the temperature change through the throttling valve. It is found that the temperature will be about 2 ^∘C smaller at the turbine exit than that at the throttling valve exit for the same inlet state and exit pressure. To establish a suitable model for the assessment of cryogenic turbine performance, the isentropic efficiency, the hydraulic efficiency, and the exergetic efficiency are studied and compared. The hydraulic efficiency is determined to be the only feasible method to assess the performance of cryogenic turbines.

An exergy analysis has been carried out for an irreversible Braysson cycle. The analytical formulae of power output and exergy efficiency are derived. The influences of various parameters on the exergy performance are analyzed by numerical calculation, and the results obtained have been compared with those of Brayton cycle under the same conditions. It is shown that the exergy loss in the combustion is the largest in the Braysson cycle, and both specific work and exergy efficiency of the cycle are larger than those of Brayton cycle.

A procedure to establish the optimal performance parameters for the minimum entropy generation during the collection of solar energy, is presented. The Entropy Generation Number, N_s, and the criterion for the optimal thermodynamic operation of a collector under nonisothermally, finite-time conditions, are reviewed. The Mass Flow Number, M, corresponding to the optimum flow of working fluid as a function of the solar collection area, is also considered. A general method for the preliminary solar collector design, based on N_s, M and the “Sun–Air” or stagnation temperature, is developed. This last concept is defined as the maximum temperature that the collector reaches at nonflow conditions for given geographic location, geometry and construction materials. The thermodynamic optimization procedure was used to determine the optimal performance parameters of an experimental solar collector.

A thermodynamic for the effect of the annualized cost of a component on the production cost in 1 000 kW gas-turbine cogeneration system was studied by utilizing the generalized exergy balance and cost-balance equations developed previously. Comparison between typical exergy-costing methodologies were also made by solving a predefined cogeneration system, CGAM problem. It was successful to identity the component which affects the unit cost of system's products decisively. It has been found that the cost of products are crucially dependent on the change in the annualized cost of the component whose primary product is the same as the system's product. On the other hand, the change in the weighted average cost of the product is proportional to the change in the annualized cost of the total system.

This paper presents second-law based performance evaluation criteria to evaluate the performance of heat exchangers. First, the need for the systematic design of heat exchangers using a second law-based procedure is recalled and discussed. Then, a classification of second-law based performance criteria is presented:

• 1.

  criteria that use entropy as evaluation parameter, and

• 2.

  criteria that use exergy as evaluation parameter.

This review draws attention to an emerging body of work that relies on global thermodynamic optimization in the pursuit of flow system architecture. Exergy analysis establishes the theoretical performance limit. Thermodynamic optimization (or entropy generation minimization) brings the design as closely as permissible to the theoretical limit. The design is destined to remain imperfect because of constraints (finite sizes, times, and costs). Improvements are registered by spreading the imperfection (e.g., flow resistances) through the system. Resistances compete against each other and must be optimized together. Optimal spreading means spatial distribution, geometric form, topology, and geography. System architecture springs out of constrained global optimization. The principle is illustrated by simple examples: the optimization of dimensions, spacings, and the distribution (allocation) of heat transfer surface to the two heat exchangers of a power plant. Similar opportunities for deducing flow architecture exist in more complex systems for power and refrigeration. Examples show that the complete structure of heat exchangers for environmental control systems of aircraft can be derived based on this principle.