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958 B.D. Iverson et al. / Applied Energy 111 (2013) 957–970 Over the past decade, there has been a significant amount of re- search on sCO2 power cycles and heat transfer. Turbine and com- pressor performance characterization and prototype system testing has been a primary focus at Sandia National Laboratories (SNL) within the Advanced Nuclear Concepts group [17–23]. Sys- tem control and transient analysis on sCO2 has been a large focus at Argonne National Laboratory (ANL), specifically for Lead Fast Reactors (LFRs) and sodium-cooled reactors [24–29]. Various cycle configurations have been investigated for specific reactor designs [8,9,30–33]. Echogen has considered sCO2 cycles for waste heat recovery, utilizing smaller power systems [34] and views sCO2 as a valid competitor to steam technology [35]. While some work has been done pertaining specifically to solar applications, the literature for sCO2 is introductory by comparison [11,36]. The Southwest Research Institute is also active in enabling sCO2 for so- lar energy and is pursuing turbo-expander and heat exchanger development for this purpose [37]. Interfacing the solar resource with a sCO2 Brayton cycle re- quires a receiver to absorb the solar-thermal energy from the inci- dent concentrated flux and transfer the energy to a transport media. The transport media in the receiver can either be the same as the power cycle working fluid (direct receiver) or employ a secondary media, either fluid or solid, that would experience heat exchange with the power cycle working fluid (indirect receiver). A direct receiver approach can leave the power cycle exposed to potential issues with a transient heat source whereas an indirect approach provides a buffer from transients. This paper demonstrates the response of a prototype sCO2 Bray- ton cycle under transient operating conditions similar to that expe- rienced in a typical solar plant with a direct receiver. While the operating conditions of temperature and pressure for the experi- ments are lower than that desired for high-efficiency operation, this data serves to validate modeling efforts that can be used to evaluate higher-temperature systems. A discussion of primary mechanical and thermal losses is provided as well as areas of advancement required for adoption of sCO2 Brayton turbomachin- ery for solar applications. 2. Experimental loop 2.1. Layout The experimental loop installed at Sandia National Laboratories (Fig. 1 and Table 1) is a split flow recompression cycle. The ‘split flow’ indicates that two separate turbines receive separate, dedi- cated flows. These two flow streams are expanded and then recom- bine after the turbines. A second flow split is located prior to the cooling and compression stages. One stream of the low-pressure flow is ‘recompressed’ without rejecting heat and is designed to operate at temperatures above the critical point. The main com- pressor operates near the critical point and the flow stream through this compressor experiences heat rejection. This configuration is expected to have improved cycle efficiency relative to a simple Brayton cycle. First, there is less heat rejection in the pre-cooler resulting in smaller heat loss as only a fraction of the flow passes through this component. Second, the thermal capacities of the hot and cold flows in the low temperature recuperator (LT recuperator) are better matched to optimize heat recuperation [38] with a resulting mass flow ratio of hot to cold that is close to 2:1. This is because the low temperature fluid exiting the main compressor is much closer to the critical point and therefore has a specific heat that is approximately double that of the higher temperature flow. Matching thermal capacities optimizes the heat transfer. Fig. 2a presents the thermodynamic state points for a represen- tative recompression Brayton cycle capable at Sandia. The expan- sion process from points 5 to 6 is the same regardless of the number of turbines, assuming equal speeds for separate shafts. The final loop design with two separate turbo-alternator-compres- sors (TAC) was determined, in part, by a staged approach due to anticipated incremental government funding. The modular nature of the design allows for multiple configurations, a feature that en- ables proprietary and novel configurations by independent institutions. The thermal input for the system is 780 kW and was selected based on key control and stability issues of the sCO2 Brayton cycle while small enough to be affordable over several years of develop- ment. The main disadvantage of the relatively small size, and the resultant high turbomachinery rotational speeds, is that the sys- tem requires bearing, seals and motor alternator approaches that are not necessarily representative of a commercial-scale system. A compromise between fidelity and cost was achieved, while addressing the underlying questions for the technology to reduce risk for future industrial efforts in sCO2 power systems. 2.1.1. Cycle components A complete description of the major components that constitute the Sandia split flow recompression test assembly is presented in Sandia report SAND2012-9546 [39]. The following is a summary description of the major components. The TACs are hermetically sealed pressure vessels, rated for the maximum pressure and temperature conditions anticipated in the flow system. Within the vessel, the compressor wheel, gas bear- ings, and turbine are laid out along the shaft as shown in Fig. 3. CO2 enters the compressor on the right hand side of the compres- sor, and is discharged radially. Likewise, hot CO2 enters down from the top of the turbine, and is expanded radially to the left-hand side of the figure. At design conditions, both turbomachinery wheels are subjected to pressures in excess of the critical point. During operation, leakage flow passes around the compressor and turbine through abradable labyrinth shaft seals to provide lubrication to thrust and journal bearings. The leakage flow is con- tinuously pumped out of this region using scavenging pumps, driv- ing a cooling flow and maintaining reduced film pressures (ideally around 1.4 MPa) in the central cavity surrounding the permanent magnet shaft and bearings. The split-flow Brayton cycle uses TACs to compress the low- pressure and low-temperature CO2 to a high pressure at the com- pressors, and then expand the high-pressure and high-temperature CO2 in the turbines. At and near design conditions, the turbines generate more power than the compressors and inefficiencies con- sume, and the remaining power is used to make electricity in the motor alternator. In the power generation mode, the alternator ap- plies an electrical load to the TACs’ rotating shaft that is sufficient to maintain the commanded rotational speed. The applied electri- cal load represents the power that the TAC would produce for con- sumer use. TAC-A takes, as input to the compressor, the flow that discharges from the gas chiller, which is the coldest point in the circuit. As such, it is also the least compressible. Therefore, TAC-A consumes less energy per unit mass to compress the fluid than the recompressor in TAC-B. A single low-pressure flow discharges from the LT recuperator, where it splits into two flow paths, one path to each compressor. The fraction of the total flow going to each compressor is a function of the relative speeds of the two TACs and the thermodynamic state of the fluid at each inlet. These factors combine to determine each compressor’s discharge pressure. When the two flows recom- bine (at the main compressor flow discharge from the LT recuper- ator) they must be at the same pressure. Pressure mismatch at this point can put a compressor into a potentially damaging state of surge. The primary control to avoid surge is the speed of each TAC, with the magnitude of heat rejection in the gas chiller beingPDF Image | Supercritical CO2 Brayton cycles for solar-thermal energy
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