Heat of conversion

As with the steam turbine, if there was no stack loss to the atmosphere (i.e., if Qloss was zero), then W heat would he turned into W shaftwork. The stack losses in Fig. 6.34 reduce the efficiency of conversion of heat to work. The overall efficiency of conversion of heat to power depends on the turbine exhaust profile, the pinch temperature, and the shape of the process grand composite. 

Carnot s cycle A hypothetical scheme for an ideal heat machine. Shows that the maximum efficiency for the conversion of heat into work depends only on the two temperatures between which the heat engine works, and not at all on the nature of the substance employed. 

Calculate also the activation energy for the reaction, again in kcal/mol, assuming that the Coulomb repulsion maximizes at 3 -y 10 cm separation of the nuclear centers. Assuming a successful cold-fusion device, how many fusions per second would generate one horsepower (1 hp) if the conversion of heat into work were 10% efficient ... 

In a numerical exercise described in section 4.2.2, it was shown that, for a stoichiometric, hydrocarbon-air detonation, the theoretical maximum efficiency of conversion of heat of combustion into blast is equal to approximately 40%. If the blast energy of TNT is equal to the energy brought into the air as blast by a TNT detonation, a TNT equivalency of approximately 40% would be the theoretical upper limit for a gas explosion process under atmospheric conditions. However, the initial stages in the process of shock propagation in the immediate vicinity of... 

Thermodynamic It is the scientific principle that deals with the inter-conversion of heat and other forms of energy. Thermodynamics (thermo = heat and dynamic = changes) is the study of these energy transfers. The law of conservation of energy is called the first law of thermodynamics. 

Electricity is produced by the conversion of heat energy (produced in a boiler and transmitted as steam) into mechanical work by use of a turbine, which is connected to an electrical generator (turbine generator). 

As we have seen, statements of the laws in terms of the conversion of heat into work in a cyclic engine are equivalent to the statements involving energy and entropy. The conversion from one to the other is not always obvious, but the relationships predicting the efficiency of the conversion of heat into work can be derived from the U and S statements. 

The reversible process (for which the equal sign applies) gives the maximum efficiency for the conversion of heat into work, but even the reversible engine is limited in the extent to which heat can be converted into work. 

Substitution for K4 jVy from this equation into equation (3.84) and dividing by <72 given by equation (3.79) gives the efficiency 77 of conversion of heat into work for the Carnot cycle as... 

SF is then a measure of the degree of irreversibility of the process. It represents the amount of mechanical energy converted into heat or the conversion of heat energy at one temperature to heat energy at another temperature. For a finite process ... 

One possibility to avoid this limitation is the conversion of heat into another kind of energy like mechanical or electrical energy. In this case (see Figure 231) the converter is producing entropy free work, which can be stored without theoretical limitations. Examples are pump storages, where water is pumped to a higher level, or flywheels, where kinetic energy can be stored. 

Figure 16.33 shows a schematic of a simple gas turbine. The machine is essentially a rotary compressor mounted on the same shaft as a turbine. Air enters the compressor where it is compressed before entering a combustion chamber. Here the combustion of fuel increases its temperature. The mixture of air and combustion gases is expanded in the turbine. The input of energy to the combustion chamber allows enough power to be developed in the turbine to both drive the compressor and provide useful power. The performance of the machine is specified in terms of the power output, airflow rate through the machine, efficiency of conversion of heat to power and the temperature of the exhaust. Gas turbines are normally used only for relatively large-scale applications, and will be dealt with in more detail in Chapter 23. 

Work can be completely converted to heat, but—and this is important—a complete conversion of heat to work is not possible in an isothermal system. This problem is dealt with by the second law of thermodynamics, with its statement on entropy The entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium. ... 

The most concentrated and the most easily used fuels have been those most sought for and most used. The discovery of a new and more concentrated fuel has been followed each time by a notable increase in the number and importance of devices for the conversion of heat into other forms of energy valuable to industry and life. ... 

The Carnot cycle engine achieves what we are looking for, a conversion of heat into work, with return of the engine to its initial state. We note, however, that in order to complete the cycle, we have paid a price. In the isothermal compression at Tc, some of the work produced in the expansion has to be used up to compress the system, finding its way into heat at the cold reservoir temperature. 

Eq. (11) becomes for the electrolyser, to take into account the exergy loss for the conversion of heat to work ... 

Fig. 10.1. Conversion of heat Q into work through a reversible heat engine between a high temperature T and the temperature T0 of our environment.

On the other hand, all efforts to devise a process for the continuous conversion of heat completely into work or into mechanical or electrical energy have failed. Regardless of improvements to the devices employed, conversion efficiencies do not exceed about 40 percent. These low values lead to the conclusion that heat is a form of energy intrinsically less useful and hence less valuable than an equal quantity of work or mechanical or electrical energy. 

The second law does not prohibit the production of work from heat, but does place a limit oq. the fraction of the heat that may be converted to work i any cyclic process. The partial conversion of heat into work is the basis for nearly all commercial production of power (water power is an exception). The develop ment of a quantitative expression for the efficiency of this conversion is the nex step in the treatment of the second law. 

However, according to statement 1 a of the second law, Q v cannot be directed into the system, for the cycle would then be a process for the complete conversion of heat into work. Thus, j dQnv is negative, and it follows that SA - SB is also negative whence SB > SA. Since the original irreversible process is adiabatic, the total entropy change as a result of this process is AStota, = SB - SA > 0. 

CONVERSION OF HEAT INTO WORK BY POWER CYCLES 2S7... 

Conversion of Heat into Work by Power Cycles 247... 

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