Entropy power plant

The Joule-Brayton (JB) constant pressure closed cycle is the basis of the cyclic gas turbine power plant, with steady flow of air (or gas) through a compressor, heater, turbine, cooler within a closed circuit (Fig. 1.4). The turbine drives the compressor and a generator delivering the electrical power, heat is supplied at a constant pressure and is also rejected at constant pressure. The temperature-entropy diagram for this cycle is also... 

The second law of thermodynamics may be used to show that a cyclic heat power plant (or cyclic heat engine) achieves maximum efficiency by operating on a reversible cycle called the Carnot cycle for a given (maximum) temperature of supply (T ax) and given (minimum) temperature of heat rejection (T jn). Such a Carnot power plant receives all its heat (Qq) at the maximum temperature (i.e. Tq = and rejects all its heat (Q ) at the minimum temperature (i.e. 7 = 7, in) the other processes are reversible and adiabatic and therefore isentropic (see the temperature-entropy diagram of Fig. 1.8). Its thermal efficiency is... 

This author used these equations to explore the present day onsite and power plant investments associated with reducing the entropy change of the process, i.e., with improving its efficiency. Based on simplified investment curves, the average costs of equipment in 1981 dollars were calculated to be ... 

The second application of availability analysis is used to evaluate the nature and magnitude of thermodynamic irreversibilities in a methane reformer plant coupled to a high-temperature nuclear reactor. It is shown that a combination of thermal histograms and availability concepts are helpful not only in evaluating the net impact of irreversibilities in various chemical process steps on the steam power plant, but, more importantly, 1n suggesting process modifications that could improve the overall efficiency by avoiding unnecessary entropy production. 

Keller, A., "The Evaluation of Steam Power Plant Losses by Means of the Entropy Balance Diagram," Trans. ASME, 72, 949, (1959). 

The rate of entropy generation in each of the four units of the power plant calculated by Eq. (16.1), and the lost work is then given by Eq. (16.15). 

Power plants can be built to operate on a cycle that departs from the Rankine cycle only to the extent that the work-producing and work-requiring steps are irreversible. We show in Fig. 8.4 the effects of these irreversibilities on steps 2 - 3 and 4 -> 1. The paths are no longer vertical, but tend in the direction of increasing entropy. The turbine exhaust is normally still wet, but as long as the moisture... 

The. thermodynamic efficiency of the power plant is 27.3 percent, and the major source of inefficiency is the fumace/boiler. The combustion process itself accounts for most of the entropy generation in this unit, and the remainder is the result of heat transfer across finite temperature differences. 

A power plant using a Rankine power generation cycle and steam operates at a temperature of 80° C in the condenser, a pressure of 2.5 MPa in the evaporator, and a maximum evaporator temperature of 700°C. Draw the two cycles described below on a temperature-entropy diagram for steam, and answer the following questions. 

Figure 3.33 presents the possible inner setup of an ideal heat engine in more detail (Fig. 3.33a) as well as the strongly simplified schematic diagram of a thermal power plant (Fig. 3.33b). In the case of such a plant, the energy Pf) (= PFuse) is used which is gained during the transfer of entropy from the steam boiler to the cooling tower. The entropy itself is generated in the boiler by consumption of energy Wi.

A100 MW power plant produces entropy at the rate of 0.7 MW/K. How efficient is this plant ... 

A Rankine power plant using steam operates between pressures 6o bar and 1.013 bar. The turbine expands steam from 500 °C, 60 bar, to 1.013 bar. The liquid at the exit of the condenser is saturated. The efficiency of the turbine and of the pump is 75%. Calculate the energy balances, entropy generation, and thermodynamic efficiency. What are... 

This ratio represents the refrigeration capacity over the refrigeration cost in terms of work. Unlike the efficiency of the power plant, the coefficient of performance is not restricted to be less than one. Its maximum value is, however, limited by the second law. To determine the limiting value of the coefficient of performance, we solve the entropy balance for Qc, substitute into the energy balance and solve the resulting equation for the ratio Qc/W we find ... 

Problem 6.27 A Rankine steam power plant produces 0.5 MW of mechanical power by expanding steam from 60 bar, 700 °C, to 3 bar. The efficiency of the turbine and of the pump is 80%. Calculate the energy balances, determine the flow rate of steam, and determine the entropy generation in each unit. 

Non-isothermal electrochemical cells have been mentioned first in 1858 by Wild [1]. A special variant of them, the thermocells, consist of two half-cell compartments with equal electrodes and equal electrolyte. They play a role in efforts for direct conversion of heat to electric energy (see Chap. 3). An example of sources of dispensable heat is nuclear power plants. If one half-cell of a thermocell is heated by waste heat, whereas the second half-cell keeps at ambient temperature, and if the electrode reaction has a high numeric value of reaction entropy, the resulting voltage between the half-cells may be utilised as source of electric energy. Unfortunately, the efficiency of such thermocells is extremely low. 

Mollier chart The graphical representation of the thermodynamic properties of a pure substance with enthalpy on the y-axis and entropy on the x-axis. Other properties are also included on the chart (see Fig. 29), including pressure and temperamre, and the critical point. They are used to visualize thermodynamic cycles such as in power plants, refrigeration, and air conditioning. It is named after German physicist Richard Mollier (1863-1935). 

We are not ready yet to carry out calculations involving power plants. We will do it after we discuss entropy change calculations. 

Because of entropy increases, no power or industrial plant, no matter how well designed, can completely convert heat from chemical reactions into useful work. Waste heat is inevitable. This kind of loss is easiest to understand in terms of efficiency, which is the amount of work that can be obtained from a process compared to the amount that must go into it. 

It should be mentioned that other methods of design optimization, employing the Second Law for costing, have been used. For example, without explicitly determining the cost of available energy at each juncture of a system, in 1949 Benedict (see 19) employed the Second Law for optimal design. He determined the "work penalties" associated with the irreversibilities in an air separation plant. That is, the additional input of shaft power to the compressors required as a consequence of irreversibilities was determined from the entropy production in each subsystem. Associated with additional shaft power requirements are the costs of the power itself and the increased capital for larger compressors. 

---