Destroyed exergy

Combining with the entropy balance. Equation 23.50, and defining destroyed exergy as irreversibility gives. 

In cases where the heat is generated by combustion, first law efficiencies can also be defined in terms of the heating value of the fuel to account for unbumt fuel. Since, in comparison to the heat produced, relatively little heat is lost to ambient from nuclear reactors, first law reactor efficiencies are close to 100%. However, lowering temperatures associated with the internal heat transfer destroys exergy and reduces second law efficiencies. The exergy destroyed is. 

Thus, the chemical conversion process destroys exergy... 

Because heat loss to the environment is very small in comparison to the heat transferred within an open heat exchanger, the first law efficiency is close to unity. However, mixing streams is an irreversible process that generates entropy and thus destroys exergy. Assuming no heat loss to the environment, using Equation 23.42 yields the energy balance of the open feed heater. 

Accessible work potential is called the exergy that is the maximum amount of work that may be performed theoretically by bringing a resource into equilibrium with its surrounding through a reversible process. Exergy analysis is essentially a TA, and utilizes the combined laws of thermodynamics to account the loss of available energy. Exergy is always destroyed by irreversibilities in a system, and expressed by... 

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 ... 

Energy remains the same in the throttling valve, while exergy is destroyed because of the expansion of the fluid. 

Exergy is destroyed by irreversibilities associated with pressure drops due to fluid friction and stream-to-stream heat transfer due to temperature differences. 

In addition, exergy is destroyed whenever heat is rejected to the environment (energy dissipation). 

The above value is not bad. In reality much more exergy is destroyed because of irreversibilities, particularly due to the cooling. For example, if we take into account only the hot utility for separation (feed preheating and reboiler) the total heat is g, = 138 + 1147 = 1285 kW. This could be extracted virtually from surroundings and converted by means of a Carnot cycle working between 298 and 406.15 K, for which the following work can been obtained ... 

Thermodynamic processes are governed by the laws of conservation of mass and energy. These conser ation laws state that the total mass and total energy can neither be created nor destroyed in a process. However, exergy is... 

A part of the exergy supplied to a real thermal system is destroyed due to irreversibilities within the system. The exergy destruction is equal to the product of errtropy generation Sgen within the system and the temperatrue of the environment To. 

Exergy is always lost (destroyed) in every actual (irreversible) process. 

Change in the total exergy = [Exergy transfer (Total exergy in — Total exergy out)] — Total exergy lost (destroyed). 

The first term on the right-hand side of equation above shows the exergy transfer accompanying heat and work. The second term is the exergy lost (exergy destroyed) due to irreversibilities. 

For a control volume at steady state, the exergy loss (destroyed) becomes... 

The rate of loss exergy (exergy destroyed) Exioss represents the overall thermodynamic imperfections, and is directly proportional to the rate of entropy production due to irreversibilities in a process. As the exergy loss increases, the net heat duty has to increase for the process to occur. Consequently, smaller exergy loss means less waste heat or thermodynamic imperfections. 

The exergy balance equation is very similar to the first law of thermodynamics with the exception that useful energy is considered instead of total energy. An important difference is that exergy is not a conserved quantity it is destroyed as entropy is generated. 

The net rate that heat exergy is transferred into the system The net rate that exergy is destroyed by irreversibility... 

The irreversibility (z) represents exergy destruction within the component, whereas the rejected exergy (( )h ) leaves the component. The distinction between these two terms is that ( )h could potentially be recovered externally, whereas i is destroyed internally. In cases... 

By definition, since energy is conserved, the first law efficiency is unity (rj = l). The second law, whose efficiency (sometimes referred to as effectiveness) accounts for the exergy destroyed, can be calculated using one of two alternate definitions (Li and Priddy, 1985)... 

The greater the difference in temperature between the two inlet streams, the greater the destruction of exergy. The second law efficiency accounts for the exergy destroyed. 

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