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mance. Further, they demonstrated diffusion bonding for nickel- based alloys Haynes N and 242, despite the high chromium content in these materials resulting in protective oxide layers [49]. Ar- gonne National Laboratory has initiated sodium-to-sCO2 heat ex- changer studies with plans to test fill and drain operations to avoid channel blockage [53]. In solar plants where molten salt is the industry standard, it has been long known that molten salts (nitrates and halides) tend to preferentially dissolve and deposit active alloying elements, (e.g. chromium or molybdenum [72–76]), which would need to be ad- dressed for smaller channels where plugging could be an issue [75]. By forcing the salt toward a reducing condition, corrosion be- comes extremely slow and may sufficiently inhibit this behavior [77]. sCO2 corrosion of metals is currently being investigated by several institutions with stable oxides (chrome and nickel oxides, and alumina) as protective barriers [78]. It has been found that high concentrations of chromium and nickel significantly increase the corrosion resistance of steel alloys in CO2 [79]. Current studies include investigations of protective layers exposed to impurities and developing protective barriers [80]. 4.3. Bearings/seals To date, the approach to gas bearings and seals for this system has demanded a disproportionate amount of the total research ef- fort. The closed cycle, small-scale turbo-alternator-compressors developed for the present demonstration loop are a result of cus- tom fabrication, and an iterative design and testing process. This has resulted in a system capable of supporting the necessary thrust loads, on the order of 400 N, at high speeds, within minimal irre- versible losses. However, there is still considerable room for improvement. Modeling results indicate that a bearing with smal- ler diameter and fewer thrust pads could maintain thrust load capacity with reduced frictional losses. In addition, incorporation of geometric features (e.g. chevrons) at the trailing edge of each thrust pad to expel hot fluid would likely increase load capacity by enhancing the thermal wedge effect, while improving thermal management [81]. Experimental work has also demonstrated that using a stamped manufacturing approach rather than assembling the thrust bearing manually from many small pieces can attain tighter engineering tolerances. Smaller engineering tolerances al- low for operation at reduced film thicknesses, increasing load capacity. Finally, it is recommended that shaft and thrust runner of next generation CO2 gas bearings be plasma sprayed with a solid lubri- cant while using bare pads. This improvement is directed at increasing the temperature resistance of the current model, which cannot be heated beyond the dissociation temperature of Teflon. Commercial-scale systems would apply a different approach to bearings and seals. These systems would almost certainly be large enough to operate efficiently at 3600 rpm (60 Hz), eliminating the feasibility of high-speed gas bearings. A commercial generator would be located outside of the high pressure CO2 region, likely by using dry liftoff seals to separate the rotor from ambient condi- tions. Industrial dry liftoff seals use several stages and a buffer or purge gas to isolate the working fluid from the environment, resulting in reduced friction from the present assembly, and use of standard oil-lubricated industrial bearings types. 4.4. Materials Material requirements for thermal solar power applications vary widely depending on the heat transfer fluids under consider- ation and operating conditions imposed. Focus here will briefly dis- cuss the material requirements of CO2, oxoanion salts (nitrate/ nitrite and carbonate), and halide anion salts (fluorides and chlo- rides) that could be used as primary or secondary heat transfer flu- ids depending on the receiver, power cycle and thermal storage subsystem configurations. Nitrate/nitrite salts are currently used in commercial solar applications, but there is concern with the thermal stability above 600 °C, thus other fluids must be consid- ered for higher temperatures. Carbonates and halides have also been selected for consideration based on their high temperature stability and cost. Materials with the ability to form passivated oxide layers, such as a chrome oxide or alumina, have been found to perform well with CO2 [78,82]. Quantification of the presence of impurities (e.g. moisture) and their role in exacerbating corrosion is necessary for long-term power plant operation [82]. While it is understood that an aggressive attack on containment materials will occur in the presence of impurities there are no well-defined limits that currently exist. Oxoanion salts, specifically molten nitrate/nitrites and carbon- ates, have different material considerations than that of CO2. It has been observed that nitrate/nitrite salts and carbonate salts are able to form and maintain passive oxide barriers that are ther- modynamically stable in the melt, which as act as diffusion barri- ers that form following typical parabolic growth rates [83,84]. In contrast to CO2, active alloying elements, such as chromium, are soluble in the melt. Corrosion enhancing impurities typically take the form of chlorides, which act to disrupt passive layers and act as a catalyst for corrosion and must be considered from a systems engineering standpoint [85–87]. Thermal decomposition of oxoan- ion salts into oxides will increase the basicity which, in turn, changes the thermodynamic state of the melt. This decomposition is reflected in potential-oxide (E-pO2-) diagrams (which parallel Pourbaix diagrams for aqueous solutions) and indicates potentially stable phases within the melt, useful in predicting phases that may be used as a protective oxide layer [88]. Questions related to evolved oxide content over time (i.e. thermal decomposition of a given salt) for the long-term stability of the salt need to be ad- dressed, in addition to techniques of online monitoring of salt chemistry. Halide salts differ significantly from oxoanions in that they do not form passive oxide layers, as is the case with chlorides [89] and fluorides [90–93]. In the case of fluorides, a metal fluoride is more stable than the metal oxides. Alloy protection with fluorides must rely on thermodynamic equilibrium between alloys [94] and this approach has largely been used with chloride melts. Due to the lack of a diffusion barrier, corrosion-enhancing impurities in ha- lides take the form of oxygen or oxygen containing molecules, such as water or air [95,96]. Systems’ where initial salt purity and ullage gasses are not controlled experience severe corrosion [77,97]. Sys- tems’ using these salts requires monitoring and purification sys- tems in order to control corrosion of containment vessels. Questions are still outstanding related to chloride systems as to the practical development of thermodynamically and kinetically favorable oxide barriers that might lessen requirements of salt pur- ity, which may preclude the need for a pressure vessel in potential system designs. Information on corrosion rates are incomplete and poorly controlled in many studies, this lack of information on the kinetics of corrosion will be required from a systems standpoint. Diurnal cycling within a CSP plant places an increased emphasis on materials resistance to cycle fatigue failure. Studies on heat exchangers, for nuclear applications, have focused on the overall strength making alloy 617 a logical choice [98,99]. The introduc- tion of thermomechanical stress in a CSP facility motivates evalu- ation of low cycle fatigue (LCF) properties. Haynes 230, a nickel alloy with high tungsten content, has excellent fatigue life charac- teristics. As a comparison, Haynes 230 has been observed to fail around 50,000 cycles at 760 °C, while 617 fails around 15,000 cy- cles [100]. This is a dramatic difference and will be important for B.D. Iverson et al. / Applied Energy 111 (2013) 957–970 967PDF Image | Supercritical CO2 Brayton cycles for solar-thermal energy
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