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Entropy
Published in Kavati Venkateswarlu, Engineering Thermodynamics, 2020
The isentropic process is a process in which entropy is constant. It has been observed that the entropy of a closed system can be varied by heat transfer and irreversibilities. If the process is reversible and adiabatic, the entropy remains constant and it is called an isentropic process. The entropy of a substance will be the same at the end of the process and the beginning, if the process is carried out isentropically. The term efficiency is frequently used by engineers in many engineering applications. Generally, it is defined as the ratio of output to the input. In this section, the isentropic efficiencies for steady-flow devices such as nozzles, turbines, and compressors are presented. Isentropic efficiency is derived by comparing the actual performance of a device and the performance under idealized conditions for the same inlet and the exit conditions. It is defined differently for work-producing and work-consuming devices.
Exergy
Published in V. Babu, Fundamentals of Engineering Thermodynamics, 2019
It was demonstrated in Chapter 8 that, among all engines that operate in a cycle between two thermal reservoirs, the Carnot engine produces the maximum work and its efficiency is the highest possible. Although the Carnot efficiency of simple cycles may be calculated in a straightforward manner, it is not easy to evaluate for complicated and realistic cycles and so it is essential to be able to determine, in some manner, the maximum possible efficiency in such cases. In addition, for devices that execute non-cyclic processes, it is desirable to be able to define an appropriate ideal process between the same initial and final state and evaluate its performance. Although the isentropic efficiency is such a process, its applicability is restricted to adiabatic devices for which the ideal process is an isentropic process, for instance, turbines, compressors, nozzles and diffusers. It would be extremely useful to be able to quantitatively assess the performance of devices such as mixing chambers and heat exchangers, to name a few, or turbines, compressors, nozzles and diffusers when they do not operate adiabatically. With these issues in mind, in the present chapter, a new quantity termed exergy, is defined. It is quite general and allows us to calculate a limiting value against which the actual performance of any device or cycle may be compared.
The Canonical Distribution: The Boltzmann Factor and the Partition Function
Published in Jeffrey Olafsen, Sturge’s Statistical and Thermal Physics, 2019
There is another important result implicit in Equation (6.31). A system undergoes an adiabatic process if the occupancies of its quantum states do not change. The occupancies are simply NPi, and N is fixed, so that the Pi do not change; then, from Equation (6.31), the entropy does not change. Thus, an adiabatic process is necessarily isentropic. Simple cases illustrating this fact are described in Chapter 5 (Section 5.11) and Example 10.2.
Optimal design, sizing and operation of heat-pump liquid desiccant air conditioning systems
Published in Science and Technology for the Built Environment, 2020
Ahmed H. Abdel-Salam, Carey J. Simonson
The main objective of the Detailed Performance Under Different Ambient Air Humidity Ratios section is to present a detailed thermodynamic analysis for the performance of equipment used in the heat-pump membrane LDAC system, in order to provide a clear illustration for how any of the three scenarios proposed in Figure 3 can be achieved and what would be its implications on the thermodynamic performance of the system. In practice, the designer has to evaluate different scenarios for the performance of the used equipment such as the following. (1) Different values can be selected for the NTUs of the dehumidifier, regenerator, evaporator, and condenser. (2) The performance of a heat pump varies according to its isentropic efficiency and the thermodynamics properties of the used refrigerant. (3) Different supply air conditions would be required to be delivered to the conditioned space depending on the space sensible and latent loads.
Energy, exergy and economic analyses on heat pump drying of lignite
Published in Drying Technology, 2019
Ming Liu, Shan Wang, Rongtang Liu, Junjie Yan
Temperature difference of the evaporator, temperature difference of the condenser, and isentropic efficiency of the compressor directly affect the second-law efficiency of the heat pump, thereby affecting the power consumption of the drying system. Figure 9(a) and (b) show that the power consumption of the heat pump in LPPH linearly increases by 2384 and 2930 kW when the temperature differences of the condenser and evaporator decrease by 10 °C, respectively. As a result, the power plant net efficiency linearly decreases by 0.23 and 0.19 percentage points when the temperature differences of the evaporator and the condenser increase by 10 °C, respectively. The power consumption of the heat pump in LPPH decreases with the increase in isentropic efficiency of the compressor. The power plant efficiency then increases by 0.34 percentage points when the isentropic efficiency of the compressor increases from 0.6 to 0.8.
Off-design performance evaluation of Hassi R’Mel ISCC power plant
Published in International Journal of Sustainable Energy, 2019
Abdelaali Benidir, Fouad Khaldi, Fethi Bouras
When operating in hot periods the GTs run every time at inlet air temperature fixed at 15°C (design air temperature). Then, the isentropic efficiencies of the compressor and turbine do not change. However, in extreme cold periods (temperature inferior to 15°C) air temperature varies over a relatively small range, as will be discussed later, therefore the operation of the GT is not so far from the design operation, so, the affection of the isentropic efficiencies of the compressor and the turbine are moderate. Because this fact, while off-design modelling the GT under Cycle-Tempo environment, the isentropic efficiencies of the compressor and the turbine do not change, in hot periods as well as in cold periods, from that of design parameters, see Table 1.