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6. Cycle with Reheating Reheating of the working fluid between the turbine stages is another common approach to improve the cycle efficiency for ideal gas Brayton cycles. It is also used for steam cycles. To investigate the effect of reheating on S-CO2 Brayton cycle performance, an additional heat exchanger (reheater) and an additional turbine (low pressure turbine, LPT) are introduced into the cycle. CO2 is still heated in the Na-to-CO2 heat exchanger and expands in the turbine, but then, instead of being sent to the recuperator, it is reheated in another Na-to-CO2 heat exchanger (reheater) and goes through the LPT before returning to a recuperator. It is assumed in this analysis that the CO2 is reheated by intermediate sodium; thus, the sodium flow is split after the IHX between the main Na-to-CO2 HX and the reheater. Figure 18 shows the plant configuration, cycle conditions, and performance for the S-CO2 cycle with a reheat. A 50/50 flow split is assumed for the sodium between the Na-to-CO2 heat exchangers. The CO2 pressure between the turbines is selected such that the pressure ratios for both turbines are about the same. Figure 18 demonstrates that the cycle efficiency is reduced compared to the reference case (Figure 2). Two possible reasons may be identified for the efficiency reduction. First, in the reference cycle configuration the temperature change in the Na-to-CO2 HX is about the same on the Na and CO2 sides (as a result of close specific heats for Na and CO2). When the Na flow rate in the Na-to- CO2 HX is reduced due to a Na flow split between the two heat exchangers, the temperature change on the CO2 side is reduced in order to maintain a heat balance. As a result, CO2 is heated up to a much lower temperature compared to the reference case (400 oC vs. 470 oC). For the same reasons, CO2 heatup in the reheater is not very effective as well – CO2 is reheated to only 415 oC. The other reason for the efficiency loss, although to a lesser degree, is doubling of the turbine exit losses for the two turbines in a serial configuration. CO2 flow speed has to be reduced after the HPT to enter the reheater then the CO2 flow has to be accelerated again to the LPT design speed. Although Figure 18 shows the results of the 50/50 flow split for the intermediate sodium, an attempt to optimize this flow split failed since the cycle efficiency increases with the flow fraction to the main Na-to-CO2 HX such that the most optimal configuration would be with 0 % flow to the reheater; i.e., for the cycle configuration without a reheat. 34PDF Image | Performance Improvement Options for the Supercritical Carbon Dioxide Brayton Cycle
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