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960 B.D. Iverson et al. / Applied Energy 111 (2013) 957–970 Fig. 3. Schematic of the internals for the Sandia sCO2 turbo-alternator-compressor. Radial Inflow Turbine Turbine Inlet Flow Shaft Seals Permanent Magnet Motor Suction via Scavenging Pump Journal Bearing Compressor Inlet Flow Centrifugal Compressor Thrust Bearing market conditions that drive power plant operation, availability of storage and the short-term forecasting of the cloud transient. Due to diurnal cycling, even without cloud transients, power cycles dri- ven by a solar resource are inherently transient. Stability may be improved through usage of other power sources such as nuclear, geothermal, etc. However, characterization of transient responses is still required for off-normal operation especially as one considers the fluctuations around the CO2 critical point. Test runs of the recompression Brayton cycle are time-inten- sive. Test preparation includes evacuating the system over night to near-zero pressure followed by filling with CO2 to the desired mass loading, and then elevating the system temperature from a cold state to the selected steady state operation. The current max- imum thermal ramp-up rate is approximately 5 °C per minute. At this rate, increasing the operating temperature from 17°C to 477 °C requires approximately 2 h. This ramp rate is derived from experience and by piping stress limitations; alternate heating rates and associated designs can be optimized for particular system needs. In order to simulate the system response to a fluctuating heat source, several test runs were conducted where the heater power settings were reduced by 50% and 100%. The time periods for 50% reduced power setting (Fig. 4) are from 7012 to 7073 s, and from 7349 to 7529 s (61 s and 170 s, respectively). The time peri- ods for 100% reduced power setting (Fig. 5) are from 4082 to 4148 s, and from 5332 to 5465 s (66 s and 134 s, respectively). Be- fore adjustment of the power setting, the nominal heating power inputs for the 50% and 100% cases are 280 kW and 160 kW, respec- tively. It is difficult to maintain a perfectly steady state condition prior to transient excursions due to variations in TAC speeds and system cooling, among other perturbations. A best effort was made to establish conditions prior to an excursion that would produce accurate indications of a true system response to simulated solar resource transients. System response is characterized in four separate plots for each heater power setting reduction, namely: pressure response in the low-pressure and high-pressure legs, temperatures at the heater inlet and discharge, and the system net power generation re- sponse. Negative power indicates power production by the system. The large change in net power generation indicated at 4870 s in Fig. 5d are due to a controlled reduction in cooling water temperature, which caused the compressor inlet flow to become more dense, less compressible, and therefore requiring less com- pressor work. The main compressor experienced an increase in mass flow, at the expense of the recompressor, but at a higher den- sity. The greater mass flow at lower compressibility resulted in a roughly net-zero change in power for the main compressor. How- ever, the reduced mass flow to the recompressor resulted in a sharp decline in required compressor power. The mass associated with the heat input system results in a thermal capacitance effect. Despite changing the heater power set- ting by a 50% or 100% reduction in power, the thermal input to the cycle fluid does not necessarily reduce by 50% or 100%. Instead, for the 50% reduction, the thermal input (due in part to thermal capac- itance of the piping and heater array) declined to minimum values of 210 kW in the first excursion, and 200 kW in the second. This thermal input was determined by using the enthalpy change in the fluid across the heaters and the mass flow rate. For the 100% power reduction, the thermal input (all due to thermal capaci- tance) declined to minimum values of 60 kW and 56 kW in the first and second excursions, respectively. From plots (a) and (b) in Figs. 4 and 5, it is apparent that system pressures decline in response to the loss of thermal input. An inherent characteristic of a closed Brayton system is that as the heated cycle fluid increases in temperature and decreases in den- sity during startup, it pushes fluid to the colder components, effec- tively increasing the whole system pressure. Thus, when the hot side declines in temperature, so too will the system pressure. The low-pressure leg response to thermal power input reduc- tion is a modest decline, approximately 50 kPa or less. The high- pressure leg response (plot b, Figs. 4 and 5) is greater, with a max- imum reduction approaching 100 kPa. Thus, the high-pressure leg responds to thermal input changes with greater fidelity. The cycle fluid temperature response is presented in plot (c) of Figs. 4 and 5. These plots for the 50% and 100% reductions show the temperatures immediately downstream of the power perturbation (heater discharge), and immediately upstream (heater inlet). As one would expect, the temperature immediately downstream shows a much more dramatic and immediate change than the up- stream temperature. Downstream and upstream temperature reductions for the 50% power reductions are 20 °C and 10 °C for the first transient, and 35 °C and 15 °C for the second transient.PDF Image | Supercritical CO2 Brayton cycles for solar-thermal energy
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