Recuperator effect

Fig. 3.16 showed carpet plots of efficiency and specific work for several dry cycles, including the recuperative [CBTX] cycle, the intercooled [CICBTX] cycle, the reheated [CBTBTX] cycle and the intercooled reheated [CICBTBTX] cycle. These are replotted in Fig. 6.17. The ratio of maximum to minimum temperature is 5 1 (i.e. T nx 1500 K) the polytropic efficiencies are 0.90 (compressor), 0.88 (turbine) the recuperator effectiveness is 0.75. The fuel assumed was methane and real gas effects were included, but no allowance was made for turbine cooling.

First, it is instructive to examine the performance of a recuperated system that has only one compressor (i.e., remove the IC and C2 from Figure 8.2) and compare this to a simple cycle GT (i.e., also remove the recuperator from the diagram). Consider an isentropic compressor efficiency of 85%, isentropic turbine expander efficiency of 90%, recuperator effectiveness of 88% and no pressure losses. A fixed turbine inlet temperature of 1200 K will be assumed for various pressure ratios. This value is based on an assumed 1000 K SOFC inlet temperature, and a 200 K temperature rise from the SOFC inlet to the turbine inlet. The 200 K temperature increase from the cathode inlet to the turbine inlet is reasonable to assume given a cathode temperature difference across the cell of 150 K, and another 50 K temperature increase from anode exhaust combustion. Thus, 1200 K will be used as a base case for the turbine inlet temperature, and for sensitivity, values of 1100 and 1300 K will also be analyzed. 

The cycle is not optimized yet, especially with respect to intermediate pressures. Also, the expanders effectivenesses might be higher than those used in the calculation (0.85). That means, that some potential still exists for efficiency increase which could offset some small losses, not taken into account. The energy balance in the recuperators is fulfilled when the recuperator effectiveness Sfgg is taken equal to 0.98. Indeed, the available heat is ... 

Gas turbine pressure ratio Gas turbine inlet temperature Turbine polytropic efficiency Compressor polytropic efficiency Recuperator effectiveness... 

The recuperator effectiveness (e) is the fraction of heat transferred that would be transferred in a counterflow heat exchanger of infinite heat transfer area. In this case... 

One parameter of choice is the recuperator effectiveness. Effectiveness is defined as the actual heat transferred divided by the maximal possible heat transferred with an infinite area heat exchanger. As might be expected, the higher the effectiveness, the higher the resultant cycle thermal efficiency. The disadvantage of a higher effectiveness is that the heat transfer area and volume, as well as pressure drop, increase as the effectiveness increases. Because recuperators with a high effectiveness of 95% are currently reasonably achievable in hardware, this parameter will be used in this study. 

The expression for cycle thermal efficiency can now be calculated by varying the pressure ratio. Other curves representing the thermal efficiency as a function of pressure ratio for different recuperator effectiveness were also calculated. The results are shown in Fig. 4.45. 

From inspection of Fig. 4.45, the optimum cycle thermal efficiency is 28.2%, occurring at a pressure ratio of 1.65 for a recuperator effectiveness of 95%. It is clear from Fig. 4.45 that the higher the effectiveness, the higher the cycle thermal efficiency. Also, note that the maximal efficiency occurs at a lower pressure ratio as the effectiveness increases. The cycle heat balance showing the temperatures throughout the cycle are shown in Fig. 4.46. 

The recuperator effectiveness is a measure of the actual heat transferred to the maximum possible from the hot side to the cold side of the recuperator. A reasonably achievable effectiveness is 95%, and 95% recuperators are within the existing industrial capabilities for manufacturing. 

Recuperator effectiveness Based on heat balance in section 6 and Recuperator Section 9.2. 

Figure 9-22 System efficiency and power output as a function of recuperator effectiveness.. ..66...

Figure 9-28 Recuperator effectiveness as a function of various flow distribution maps (see...

The baseline recuperator effectiveness has been increased from 0.90 to 0.92 and the thermal load has been increased from 514 IdA/ to 761 kW relative to the Hamilton Sundstrand recuperator configuration in Reference 9- 54. 

Literature (Reference 9- 58) suggests that for the low Prandtl number HeXe fluid being considered for the direct Brayton cycle, the heat transfer coefficients may be lower resulting in larger heat transfer area requirements to meet the desired recuperator effectiveness. 

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. 

Effects of Reduced Recuperator Effectiveness on System Performance (3.3.3 Configuration. 2 Braytons Normally Running with One Spare)... 

Turbine inlet temperature is maintained at 1150K Maintaining this temperature while reducing recuperator effectiveness results in reduced eletncal output and increased reactor power. 

The flow maldistribution evaluations on recuperator effectiveness assumed that the integrated mass flow rate of the uniform flow case was conserved for the nonuniform cases. Additional system performance reductions will result due to the larger recuperator pressure drops associated with the nonuniform flow conditions. The increased pressure drop for the nonunifoim flow condition was not modeled. The effects of reduced effectiveness and increased pressure drop were also not iterated on in a system model to establish the reduced flow conditions for the nonuniform distribution cases relative to the uniformly distributed case. 

Table 9 13 Recuperator flow distribution maps generated to assess the sensitivity of recuperator effectiveness to maldistribution.

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