We analyzed the power plant using the relations givenabove at the environment reference temperature and pressure of 298.15 K and101.3 kPa, respectively.
The thermodynamic properties of water and air atindicated nodes in Fig. 1 were calculated using MATLAB software andsummarized in Table 4. Exergy-based efficiencies and losses, provide measuresof approach to ideality or deviation from ideality. Exergy and percent ofexergy destruction along with the exergy efficiencies are summarized in Table 6 for all components present in the power plant.
It was seen thatthe highest exergy destruction rate was of boiler is over all other irreversibility’sin the cycle. It alone comprises about 77% of losses in the plant, while theexergy destruction rate of the condenser is only 9%. According to the first lawanalysis, energy losses associated with the condenser are important as theyaccount for 66% of the energy input to the plant.
The major loss is primarily inthe boiler where entropy was produced. Contrary to the first law analysis, thisdemonstrates that significant improvements exist in the boiler system ratherthan in the condenser. The calculated exergy efficiency of the plant iscalculated as 25%, which is pretty low. This shows that there is a room forimprovements. However, not all of the irreversibility could be avoided due to physical,technological, and economic constraints.In order to quantify the exergy of a system, we must specify boththe system and the surroundings. We assume that the intensive properties of theenvironment are not significantly changed by any process. The dead state isdescribed as a state where the system and surroundings are at equilibrium.
Whena system is at the same temperature, pressure, elevation, velocity and chemicalcomposition as its surroundings, no potential differences would be producedthat allows us to extract any useful work. The reference environment state isirrelevant for calculating a change in a thermodynamic property (first lawanalysis). However, it is expected that the dead state will have some effectson the results of exergy (second law) analysis. Although, some researchers assumedthat small and reasonable changes in dead-state properties have little effecton the performance of a given system. To find out how significant this effectwill be on the results, the dead-state temperature was changed from 283.
15 to318.15 K while keeping the pressure at 101.3 kPa. Values of total exergy ratesat different dead states for locations identified in Fig. 1 are summarized in Table7. Results of such analysis show, in Fig. 2, that the major source of exergy destruction is the boiler nomatter what the dead state is. Fig.
3 shows that exergy efficienciesof the boiler and turbine did not change significantly with dead statetemperature; however, the efficiency of the condenser at 318.15 K is almosttwice as muchwhen the ambient temperature was 283.15 K.
This can be explained bynoting the diminution of temperature difference between the steam and thecooling air as the dead-state temperature is increased. This will decrease theexergy destruction and hence, will increase the exergy efficiency.ConclusionIn this study, an energy and exergy analysis as well as the effectof varying the reference environment temperature on the exergy analysis of anactual power plant has been presented. In the considered power cycle, the maximumenergy loss was found in the condenser where 66% of the input energy was lostto the environment. Next to it was the energy loss in the boiler system whereit was found to be about 6% and less than 2% for all other components. Inaddition, the calculated thermal efficiency of the cycle was 26%. On the otherhand, the exergy analysis of the plant showed that lost energy in the condenseris thermodynamically insignificant due to its low quality. If we considerexergy destruction, the major loss was found in the boiler system where 77% ofthe fuel exergy input to the cycle was destroyed.
Next to it was the turbine where20.4MW of exergy was destroyed which represents 13% of the fuel exergy input tothe cycle. 9% of exergy was destroyed in the condenser, while all heaters andpumps destroyed less than 2%.Thecalculated exergy efficiency of the power cycle was 25%, which is low comparedto modern power plants. The major source of exergy destruction was the boilersystem where chemical reaction is the main source of exergy destruction in acombustion chamber.
Exergy destruction in the combustion chamber is mainlyaffected by the excess air fraction and the temperature of the air at theinlet. One way to reduce the inefficiencies of combustion is to preheat thecombustion air and reduce the air–fuel ratio. Even though the percentage ofdestruction and the exergy efficiency at each component in the system changedwith the reference environment temperature, the main conclusion stayed thesame; the boiler is the major source of irreversibility’s in the system.