Exergy, An International Journal最新文献

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.

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%.

An innovative method for the exergy efficiency calculation of a complex energy-intensive system with arbitrary structures is described in this paper. The method is based on a novel general equation to calculate the total system exergy efficiency, and on an exergy flow graph proposed by the authors. This approach allows a user to obtain not only the exergy efficiency of the total system, but also to show the relationship between the exergy efficiency of an individual element and that of the whole system. An example employing the method to the thermodynamic exergy analysis of a power plant is provided.

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.

The present paper deals with transpiration cooling of two concentric spherical shells. The analysis includes the calculation for the radial distribution of temperature and volumetric entropy generation, and the total rate of entropy generation in the thermal system. Standard air is considered as the cooling fluid. Results showed that the entropy generation increases with increasing temperature difference between the sphere surfaces. Variation of either mass flow rate or radius ratio affects volumetric entropy distribution and the total rate of entropy generation of the processes. The increase of mass flow rate or radius ratio increases the total rate of entropy generation. The performance of the system is analyzed by calculating irreversibility to heat transfer ratio at both inner and outer sphere surfaces. It was found that irreversibility to heat transfer ratio at the inner sphere surface increases with increasing mass flow rate, or decreasing radius ratio. The opposite is true for the outer sphere surface.

Thermodynamic analysis of the thermomechanical coupling in a tangential Couette flow is presented for the Nahme number Na range of 0.5<Na<2.5. In the analysis, the temperature and velocity gradients, obtained from the series solutions in the Brinkman number Br, have been used for Newtonian fluids whose viscosity and thermal conductivity are expressed as linear functions of temperature. The entropy generation and the irreversibility distributions due to the thermomechanical coupling are evaluated and displayed graphically across the gap with asymmetric wall temperatures for the Couette flow of ethylene glycol.

A method is described to quantify the renewability of a biofuel, namely ethanol produced from corn. In the presentation, the ideal CO₂–glucose–ethanol cycle is considered to show that exergy can be potentially produced through the harnessing of natural thermochemical cycles. Then exergy accounting is used to evaluate the departure from ideal behavior caused by non-renewable resource consumption through the concept of restoration work. This procedure leads the authors to propose a renewability indicator. The different cycles and processes involved in ethanol production from corn are described. Based on the renewability indicator calculated for the overall process, for the conditions prevailing in Quebec, Canada, ethanol production is seen to be not renewable.

The thermodynamic optimization of a mechanically driven solar heat pump is presented. A new expression to describe the optimal thermal performance under finite operating conditions considering the internal and external irreversibilities during actual operation is derived. The optimum ratio between the condenser and collector–evaporator conductances (UA) determines the coefficient of performance (COP) for the maximum heating load of the system. An experimental air-R22 heat pump was used to determine the traditional performance parameters (COP and second law efficiency) which are compared with those obtained using the expressions derived in this work. Results show that the new model very closely represents the performance of real systems.

It is shown that Van Mieghems available potential energy for the atmosphere can be derived, in a local formulation, from an extension of the concept of exergy. The available potential energy can be interpreted as exergy applied to each layer of a thermally stratified atmosphere. This interpretation allows the application of well-known exergy theorems to the atmosphere and, therefore, deepens the thermodynamic insight in atmospheric energetics.