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Continuously increasing offshore wind turbine scales require rotor designs that maximize power and performance. Downwind rotors offer advantages in lower mass due to reduced potential for tower strike, and is especially true at large scales, e.g., for a 25 MW turbine. In this study, three 25 MW downwind rotors, each with different prescribed lift coefficient distributions were designed (chord, geometry, and twist) and compared to maximize power production at unprecedented scales and Reynolds numbers, including a new approach to optimize rotor tilt and coning based on aeroelastic effects. To achieve this objective the design process was focused on achieving high power coefficients, while maximizing swept area and minimizing blade mass. Maximizing swept area was achieved by prescribing pre-cone and shaft tilt angles to ensure the aeroelastic orientation when the blades point upwards was nearly vertical at nearly rated conditions. Maximizing the power coefficient was achieved by prescribing axial induction factor and lift coefficient distributions which were then used as inputs for an inverse rotor design tool. The resulting rotors were then simulated to compare performance and subsequently optimized for minimum rotor mass. To achieve these goals, a high Reynolds number design space was developed using computational predictions as well as new empirical correlations for flatback airfoil drag and maximum lift. Within this design space, three rotors of small, medium and large chords were considered for clean airfoil conditions (effects of premature transition were also considered but did not significantly modify the design space). The results indicated that the medium chord design provided the best performance, producing the highest power in Region 2 from simulations while resulting in the lowest rotor mass, both of which support minimum LCOE. The methodology developed herein can be used for the design of other extreme-scale (upwind and downwind) turbines. 

Compressed air energy storage (CAES) is a low-cost, long-duration storage option under research development. Several studies suggest that near-isothermal compression may be achieved by injecting water droplets into the air during the process to increase the overall efficiency. However, little is known about the thermal-fluid mechanisms and the controlling nondimensional parameters of the expansion process, which has previously been assumed to mirror the compression process. Furthermore, the isothermal round-trip efficiency and the impact of spray-based CAES have not been investigated. This study uses a validated 1-D model for compression and expansion with spray injection to complete a parametric analysis to analyze the thermal-fluid time-dependent physics and resulting roundtrip isothermal efficiency of a CAES system. Comparing the results for compression and expansion simulations, compression is found to have a higher isothermal efficiency than expansion for the same set up. The polytropic index for both compression and expansion tends to decrease and approach the ideal isothermal limit as nondimensional mass loading increases and as nondimensional Crowe number (ratio of thermal response time to domain time) decreases. As such, the highest efficiency designs are those with slow compression speeds and high spray flow rates to achieve high mass loading and those with small droplets to achieve low Crowe numbers—as long as spray work is neglected. If spray work is included, the optimum spray conditions shift to those with lager drop sizes. For example, roundtrip isothermal efficiency peaks around 95 % at a mass loading of 14 and at Crowe numbers <0.1 with a pressure ratio of ten. The results indicate that near-isothermal CAES compression and expansion is possible but that spray work should be included for significant mass loadings (e.g. greater than unity). Further investigation is recommended to consider effects of multi-dimensionality, turbulence, wall-interactions, and droplet dynamics.