Overall efficiency recuperation

Fig. 9.2 shows how a simple open circuit gas turbine can be used as a cogeneration plant (a) with a waste heat recuperator (WHR) and (b) with a waste heat boiler (WHB). Since the products from combustion have excess air, supplementary fuel may be burnt downstream of the turbine in the second case. In these illustrations, the overall efficiency of the gas turbine is taken to be quite low ((tjo)cg = ccJf ca 0.25), where the subscript CG indicates that the gas turbine is used as a recuperative cogeneration plant. 

It is the aim of this paper to present a comparison of thermal and chemical recuperation options 1n a thermodynamic framework. The paper will begin by identifying the major irreversibilities in a simple gas-turbine cycle with liquid methanol fuel continue with a comparison of thermodynamic losses and overall efficiencies among various options utilizing thermal and/or... 

One last point to be noted pertains to a comparison between the steam-reforming reaction (Case 4) and the methanol cracking reaction (Case 5). From the exergy ratio calculations, the reforming reaction appears to be superior in its ability to produce lower quality fuel. However, the overall efficiency calculations show a lower value for Case 4 than that for Case 5. The main reason for this reversal is due to the fact that nearly 25% of the recuperated energy for Case 4 is in the form of the heat of evaporation of H 0 and is not recovered from the exhaust gases. The result 1s an increase in the stack losses. 

Figure 8.4 compares the recuperated cycle with and without intercooling. Intercooling will reduce the overall compressor power since the average density is reduced. Since there is recuperation, the lower compressor outlet temperature should not hurt system efficiency (intercooling is not shown for the simple cycle since the... 

Figure 1 Overall HTE hydrogen production efficiencies for the VHTR/recuperated direct Brayton cycle, as a function of per-cell operating voltage...

This component allows the core to add a lesser amoimt of heat to the flow at a higher temperature and thus increase overall thermal efficiency. The recuperator parameter of interest is the efiectiveness, which is a measure of the actual heat transferred to the maximiun possible heat that could be transferred. A low value of effectiveness will decrease cycle thermal efficiency but have a small physical size. A high value of effectiveness will increase cycle efficiency at the expense of a larger physical size. An appropriate value balancing the cycle efficiency and the physical size is 95%. 

Since both the recuperator and gas cooler use gas as the working fluid on at least one side of the heat exchanger, they both employ offset strip fins (Kays London, 1984) common to compact heat exchangers to enhance heat transfer. Rather than trying to explicitly represent the complex geometry of offset strip fins, a multiplier was developed to account for the overall temperature effectiveness of the heat transfer surfaces on both sides of the heat exchanger. The temperature effectiveness ( 7o) is a combination of both the ratio of fin area (Af) to total heat transfer area (A) and the efficiency of the fins themselves. 

None of the component reliabilities based on area reflect differences due to variations in system efficiency with system configuration. For example, although the three and four Brayton system recuperators are nominally the same size <100 kWe), differences in overall system efficiency would require the recuperators for the four Brayton system to have a larger heat transfer area, according to the system heat balances of Section 6. 

Figure 9-22 provides a plot of overall system efficiency and power output as a function of recuperator effectiveness. If after constructing a recuperator unexpected reductions in overall heat transfer coefficient result in reduced recuperator effectiveness, three options are available (1) redesign the recuperator to add heat transfer area to accommodate the reduced heat transfer coefficient/effectiveness resulting in an increase in overall system mass, (2) redesign the remainder of the system to accommodate the reduced effectiveness resulting in an increase in system mass, (3) accept the overall reduction in system performance associated with the lower recuperator effectiveness. The selected option would be driven by the system margin in the as-built Brayton cycle. 

The impact of the changes in recuperator inlet temperatures described above is a significant reduction in the amount of heat transferred between the two gas streams and a reduction in overall plant efficiency. The new recuperator high pressure exit temperature (reactor inlet temperature) of 865 K is significantly lower than the initial temperature of 889 K. It is this reduction in reactor inlet temperature, combined with increased flow rate that produce a lower average coolant temperature. The lower average coolant temperature results in a core block temperature change from the initial 1072 K to a final 1059 K. This temperature deficit produces the reactivity surplus that increases reactor power from 100% to a final value of about 110% (5000 sec). 

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