Transfer of exergy

This is the criterion for the transfer of exergy between the donor and the acceptor process. 

In the theoretical limit, any amount of exergy contained in one or more given feed streams (call them fuels and feedstocks) could be completely transferred or transformed to any other commodities (call them products). For example, the exergy transported in with Feeds 1 and 2, P + in Figure 1, in theory could be completely delivered—by transformation of Feeds 1 and/or 2 into Product A, yielding P, and by transfer of exergy to stream B... 

For an isolated system, there is no transfer of exergy between the system and its surroundings, and hence the change of exergy is equal to exergy loss... 

Equation 6.33 provides the definition of exergy if state 1 is chosen as the state at ambient condition, namely, P, = P0 and = T0 the minimum amount of work required to transfer the system from environmental conditions to those at P2 and T2. At these conditions, this is the maximum amount of work available for the reverse process. That is the valuable idea behind the exergy concept to be able to assign to any process stream a value, its exergy, that expresses the confined work available in the stream. For the general change in state from P0r T0 to P, T, we can write the net energy input as... 

Coal gasification is essentially the transfer of the exergy from solid coal to an excellent chemical gaseous fuel and base chemical. Suppose gasification is shown by... 

In actuality, however, there will always be a consumption of some exergy to drive the various transformation and/or transfer processes. Furthermore, there may be effluent losses of exergy. Then, for real operation, an exergy balance says... 

Analysis of Sub-processes. To determine the locations and magnitudes of the various different consumptions which comprise ac one need only subdivide the system appropriately into sub-systems, and then repeat the foregoing procedure. Thus, in this problem, the consumptions within the boiler can be broken down into. 1) combustion, and 2) heat transfer. Each can be analyzed for its second law efficiency and the amount of exergy it consumes. 

To calculate the consumptions of exergy in the combustion process and the heat transfer process, it is supposed that the boiler may be separated into two distinct hypothetical entities an "adiabatic combustor" and a "heat exchanger." After determining the state of the combustion products, using an energy balance, the transport of exergy from the combustion process with product gases can be determined with the same procedures as above ... 

Here, the difference between the denominator and the numerator is the destruction by the heat transfer process plus the loss of exergy with the stack gases. 

Figure 4.16. Mechanism of exergy transfer for a general system.

In accordance with the second law, the exergy loss is positive in an irreversible process and vanishes in a reversible process. The change in exergy of a system can be positive, negative, or zero. When the temperature of a process where heat transfer occurs is less than the temperature of the environment, the transfer of heat and exergy flows in opposite directions. Work and the accompanying exergy transfer can be in the same or opposite directions. 

This equation shows that the rate of exergy transferred into the control volume must exceed the rate of exergy transferred out, and the difference is the exergy destroyed due to irreversibilities. Exergy concepts for some steady-state processes are ... 

Table 4.3a shows the state properties of the ideal regenerative Rankine cycle. Table 4.3b shows the distribution of exergy losses at each process. As seen from this table, the highest exergy loss occurs due to heat transfer in the boiler. 

The use of an augmentation device results in an improved heat transfer coefficient, thus reducing exergy destruction due to convective heat transfer however, exergy destruction due to frictional effects may increase. The exergy destruction number Nx is the ratio of the nondimensional exergy destruction number of the augmented system to that of the... 

The exergy transfer associated with the transfer of energy by work W is given by... 

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