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966 B.D. Iverson et al. / Applied Energy 111 (2013) 957–970 4.1. Turbine development Several improvements in the Brayton turbomachinery could significantly improve the efficiency by reducing losses. A 10 MWe system size has been identified as the minimum needed for dem- onstration of commercial-scale turbomachinery technology for sCO2 [47]. Whereas the present test assembly uses radial compres- sors and turbines, a >10 MWe sCO2 power system would be a mul- ti-stage axial machine much like present-day industrial gas turbines. The 10 MWe size would also allow for high efficiency at rotational speeds on the order of 24,000 rpm, in comparison to the current 75,000 rpm system. This reduction in design speed to 24,000 rpm would allow the turbine to be mated with gear reduc- tion to 3600 rpm (60 Hz) using a commercially-available gearbox and allow for synchronous operation similar to commercial-scale power systems. A significant drawback of the current design is that a simulta- neous high-speed and high-pressure environment at the rotor can cause disproportionately large frictional losses. Also for this small- scale hardware, mere inches separate the high temperature turbine inlet (potentially up to 540 °C) from the motor/generator cooling water flow at room temperature, resulting in thermal losses. Leak- age flow bypasses the turbomachinery wheels to lubricate the gas bearings, reducing productive work as well. To manage leakage flows, the turbine itself and other turbomachinery internals must be designed and built to tolerances on the order of thousandths of an inch. All loss mechanisms would be eliminated or greatly re- duced for a commercial-scale (multi-MWe) Brayton system [47]. Additional objectives include demonstration of high conversion efficiency and control concepts of the sCO2 closed Brayton cycle. The current split flow recompression turbomachinery will be used to investigate system control algorithms for a recompression cycle. Variables of particular interest include responding to heat input variations, changes in load demand, and responding to emergency events. Primary control features will include turbomachinery speed, heat rejection, and possibly circuit mass loading. Developing control algorithms is necessary to maintain a recompression cycle at optimum performance with automatic controls to respond to various transients such as load demand and heat input. The current research and development system requires continual oversight to maintain the system in a stable condition. 4.2. Heat exchangers When targeting high-capacity factors, a secondary thermal transport media for use in the receiver and storage system (indirect system) is a likely approach given the high pressures associated with supercritical working fluids and low specific heat values for gases. In this indirect system, the primary fluid is defined as that employed by the power cycle and the secondary fluid as that used for collection and/or storage. An indirect approach with different receiver and power cycle fluids allows the media used in each sub- system to be optimized for their specific function in the cycle com- ponents. This approach has an additional benefit of being applicable to a variety of power cycles, as the collection and power cycle media are decoupled. However, this requires a heat exchan- ger at the interface of these subsystems. An important consideration relative to indirect systems is the ability to incorporate significant energy storage. In instances where capacity factors are relatively low (25–50%), a direct receiver ap- proach, which incorporates heat exchange to a storage media, can be beneficial because the majority of annual energy generation occurs with the heat-transfer media being sent directly to the power block without the incurred losses in a heat exchanger. For larger-capacity factors, where a significant fraction of the collected energy is sent to storage, it is more efficient to match the storage and receiver media, thus, requiring only a single heat exchanger to interface with a separate power block working fluid (see Fig. 7). For the calculations in Fig. 7 based on a turbine inlet tem- perature of 700 °C and a 98% storage efficiency, the first-law effi- ciency for an indirect liquid receiver is higher than the direct approach, above approximately 10 h. The crossover in second- law efficiency occurs at a lower storage capacity of approximately 7 h. While this amount of storage capacity may appear to be higher than desired in some energy markets, the cost and feasibility of storing supercritical fluids that reach goals applicable to SunShot would prove cost-prohibitive for both large- and small-capacity factors [48]. Further, the effects of thick-walled piping and a poten- tial slower start-up due to a larger thermal capacitance in system components has been neglected for the direct CO2 approach, fur- ther justifying an indirect approach, especially for large capacity factors. In order to successfully implement an indirect system for solar, a heat exchanger to transfer heat between the dissimilar fluids is necessary. Of the heat exchanger designs to consider, a diffusion bonded heat exchanger (such as a PCHE or hybrid-PCHE [49,50]) is a possible candidate as the diffusion bonding process is capable of producing small channel sizes that enables containment of the high pressures required for the supercritical phase [51,52]. A reduction in the channel hydraulic diameter also enables an in- crease in the heat transfer coefficient, as they are inversely related (h 1/Dh). Thus, the small channel size accommodates the two major requirements for heat exchange with sCO2. On the salt side of the heat exchange, the same channel dimen- sions used for sCO2 are not optimal due to the concern for plugging of solidified salt. Therefore a hybrid construction using techniques other than printed circuit methods may be required for salt appli- cations [53]. When selecting salts that have higher operating tem- peratures, typically this also involves a corresponding increase in the melt temperature, making solidification problematic at tem- peratures well above ambient. Similar concerns exist for sodium in PCHEs, with initial investigations recently appearing [33,54,55]. One mitigation strategy is simply to utilize the salt only in a thermal environment where the temperatures never reach solidification temperatures. This is possible for recuperated Bray- ton cycles where the heat addition from an external source is ex- pected to raise the temperature from 531 to 700°C [30]. Common chloride-based salts (e.g., KCl–LiCl–NaCl ternary eutectic melts at 346 °C [56]) or carbonates typically have melting temper- atures well below this range. Start-up procedures, however, may need to involve external thermal input for preheating before salt introduction. The sCO2 Brayton cycle is known to be highly recuperative, with projected capital costs of heat exchangers representing 80% of the total cost of the cycle [31,57]. Highly compact, efficient heat exchangers are, therefore, necessary for power block cost reduction with numerous design and characterization studies in the litera- ture [58–71]. However, there is very little information on heat ex- changer design when considering exchange with a secondary hot working fluid, such as liquid metals or molten salts [54,68]. When considering liquid sodium on the hot side, initial studies have only begun to understand conditions under which freezing conditions may occur [54]. The implementation of a salt-to-sCO2 heat transfer interface must address the differential pressure between the hot and cold fluid which may be as high as 20–25 MPa at temperatures of 600–800 °C; not a trivial matter. Idaho National Laboratory (INL) has initiated work relative to using salt-service heat exchangers to link a sCO2 Brayton cycle to a nuclear reactor. Preliminary studies for fluoride salts indicate that shell-and-tube (helical coil) and PCHE heat exchangers are the most likely to achieve the desired re- sults for their advanced high-temperature reactors [51,52], with the PCHE option preferred for its thermal and structural perfor-PDF Image | Supercritical CO2 Brayton cycles for solar-thermal energy
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