Cross-flow turbines convert kinetic power in wind or water currents to mechanical power. Unlike axial-flow turbines, the influence of geometric parameters on turbine performance is not well-understood, in part because there are neither generalized analytical formulations nor inexpensive, accurate numerical models that describe their fluid dynamics. Here, we experimentally investigate the effect of aspect ratiothe ratio of the blade span to rotor diameter -on the performance of a straight-bladed cross-flow turbine in a water channel. To isolate the effect of aspect ratio, all other non-dimensional parameters are held constant, including the relative confinement, Froude number, and Reynolds number. The coefficient of performance is found to be invariant for the range of aspect ratios tested (0.95 -1.63), which we ascribe to minimal blade-support interactions for this turbine design. Finally, a subset of experiments is repeated without controlling for the Froude number and the coefficient of performance is found to increase, a consequence of Froude number variation that could mistakenly be ascribed to aspect ratio. This highlights the importance of rigorous experimental design when exploring the effect of geometric parameters on cross-flow turbine performance.

While biofouling is known to degrade the performance of marine energy conversion systems, prior experimental work has not explored this topic for cross-flow turbines. Here, we present experiments that investigate the impact of biofouling on turbine power output and structural loads. Using additive manufacturing, a three-dimensional scan of a barnacle was patterned onto the surface of turbine blades at three sizes and number densities, representing the progression from initial colonization to maturity. The impact of barnacles on turbine power output was found to be substantial and, for the most severe cases of fouling, the turbine does not produce power at any rotation rate. Conversely, barnacle fouling was found to have minimal impact on structural loading. To maintain generation capacity over extended periods, these results highlight the importance of antifouling coatings and proactive blade cleaning.