Exergy, An International Journal最新文献

In this study, an exergoeconomic analysis of condenser type parallel flow heat exchangers is presented. Exergy losses of the heat exchanger and investment and operation expenses related to this are determined with functions of steam mass flow rate and water exit temperature at constant values of thermal power of the heat exchanger at 75240 W, cold water mass flow rate and temperature. The inlet temperature of water is 18 °C and exit temperatures of water are varied from 25 °C to 36 °C. The values of temperature and pressure of saturated steam in the condenser are given to be T_con=47 ° C and P_con=10.53 kPa. Constant environment conditions are assumed. Annual operation hour and unit price of electrical energy are taken into account for determination of the annual operation expenses. Investment expenses are obtained according to the variation of heat capacity rate and logarithmic mean temperature difference and also heat exchanger dimension determined for each situation. The present analysis is hoped to be useful in determining the effective parameters for the most appropriate exergy losses together with operating conditions and in finding the optimum working points for the condenser type heat exchangers.

The building of an industrial society can be viewed as a process of self-organisation with a decrease in entropy in society and a corresponding increase of entropy through dissipation of energy into the environment. The process is driven by the “degradation” of high quality energy to low-quality heat as energy flows down potential gradients at the same time creating a favourable potential gradient driving the reaction. The post-industrial society is characterised by an increase in complexity, which can be monitored by the exergy consumption. Here a first attempt is made to relate the complexity of a number of products, as represented by the number of their functional parts, to their actual economic value.

In general the field of exergy analysis is both well formulated and well understood. However, the exergy flux, or maximum work obtainable, from thermal radiation (TR) heat transfer has not been clearly formulated. In a previous article it was shown that Petela's result, from his thermodynamic approach, does in fact represent the exergy flux of blackbody radiation (BR) and the upper limit to the conversion of solar radiation (SR) fluxes approximated as BR. This conclusion was obtained by resolving a number of fundamental issues including questions relating to: inherent irreversibility, definition of the environment, the effect of inherent emission and the effect of concentrating source radiation. In this paper, a new expression based on inherent irreversibility is presented for the exergy flux of TR with an arbitrary spectrum. It is shown that previous approaches by Petela and Karlsson are equivalent and assume that reversible conversion of non-blackbody radiation (NBR) is theoretically possible. However, evidence is presented indicating that the conversion of NBR is inherently irreversible. The analysis in this paper emphasizes the proper formulation for TR exergy by re-stating the general exergy balance equation for thermodynamic systems so that it correctly applies to NBR heat transfer. Finally, it is shown that the exergy flux of NBR, or the maximum work obtainable from NBR conversion, can be a small fraction of the energy flux.

A computational model based on the exergy analysis is presented for the investigation of the effects of the evaporating and condensing temperatures on the pressure losses, the exergy losses, the second law of efficiency, and the coefficient of performance (COP) of a vapor compression refrigeration cycle. It is found that the evaporating and condensing temperatures have strong effects on the exergy losses in the evaporator and condenser, and on the second law of efficiency and COP of the cycle but little effects on the exergy losses in the compressor and the expansion valve. The second law efficiency and the COP increases, and the total exergy loss decreases with decreasing temperature difference between the evaporator and refrigerated space and between the condenser and outside air.

This paper shows how the heat reclaimed from the exhaust gas of a gas turbine engine can be used to convert dilute ethyl alcohol (“beer”) to a fuel that can used to power the engine. Ordinarily, there is not enough heat available to distil the weak liquor to sufficient amounts of a strong product (>90%), due to high exergy destruction. In this design, a combination of destructive distillation and ordinary distillation is used, the wet product then being sent to a reformer for an endothermic shift reaction. The high temperature gas is now ready to burn in a gas turbine engine (or high temperature fuel cell). Water in the low temperature stack gas can be reclaimed in a cooling tower, if desired, so as to have no net loss of water for the system. The exergetic (Second Law) efficiency for power production is nearly 50%.

The author explains his views that aspects of exergy relate to government policies in a variety of fields, including natural resources, energy, environment and industrial development, and that our governments need to use—or be encouraged to use—exergy in establishing public policies, to increase the benefits they bring.

A typical ideal distillation process is proposed and analyzed using the first and second-laws of thermodynamics with particular attention to the minimum work requirement for individual processes. The distillation process consists of an evaporator, a condenser, a heat exchanger, and a number of heaters and coolers. Several Carnot engines are also employed to perform heat interactions of the distillation process with the surroundings and determine the minimum work requirement for processes. The Carnot engines give the maximum possible work output or the minimum work input associated with the processes, and therefore the net result of these inputs and outputs leads to the minimum work requirement for the entire distillation process. It is shown that the minimum work relation for the distillation process is the same as the minimum work input relation for an incomplete separation of incoming saline water, and depends only on the properties of the incoming saline water and the outgoing pure water and brine. Also, certain aspects of the minimum work relation found are discussed briefly.

Classical approaches to the formulation of the defining equation for entropy are presented in this paper that eliminate dependence on the arbitrary treatment of the Carnot function f(θ) that has long existed and is featured in modern thermodynamic textbooks.

An irreversible air standard Dual cycle model is proposed in this paper. The finite-time evolution of the cycle's compression and power stroke is taken into account and its global losses lumped in a friction-like term is also considered. The analytical formulas of power output versus compression ratio and efficiency versus compression ratio of the cycle are derived. They are generalized formulas for internal combustion engines because they include the performance characteristic of special cases of Diesel and Otto engines. The effects of various design parameters on the performance of the cycle are demonstrated by one numerical example. The model leads to loop-shaped power-versus-efficiency curve as is common to almost all real heat engines.