Impact of blade mounting structures on cross-flow turbine performance

Abstract

Cross-flow or vertical-axis turbines are flow energy conversion devices in which lift forces cause blades to rotate around an axis perpendicular to the flow. In marine currents, rivers, and some wind energy applications, cross-flow turbines are a promising alternative to more conventional axial-flow turbines. The performance implications of the choice of structure used to mount turbine blades to the central shaft is examined experimentally in a recirculating water flume. Turbine performance is found to be strongly dependent on choice of mounting structure. Power loss due to rotational drag on these structures is estimated experimentally by rotating the mounting structure without blades. Through a perturbation-theory approach, interactions between turbine blades and mounting structures are examined. Analytical models for the power loss due to mounting structure drag are introduced and shown to be consistent with experiments. To provide guidance for cross-flow turbine design, the models are re-formulated in terms of non-dimensional turbine design and operational parameters. Mounting blades solely at their mid-span is shown to decrease performance through multiple fluid effects. Using foil cross-section struts located at the turbine blade tips is found to result in the highest turbine performance.

An experimental assessment of analytical blockage corrections for turbines

“…A schematic of the experimental set-up is given in Fig. 1(a), and additional details about the data acquisition system are given by Strom et al [35].…”

Implications of biofouling on cross-flow turbine performance

“…To estimate the relative contribution to performance loss caused by fouling of the blades versus fouling of the mounting structure, measured turbine performance was decomposed as for Strom et al [44] as where C P,blades represents the net non-dimensional power produced by the blades, C P,mount represents the nondimensional performance loss from the struts structure, and C P,secondary encompasses the effect of the mounting structure on the blades (e.g., reduction in tip loss, spanwise flow), as well as the effect of the blades on the mounting structure (e.g., induction). Based on Strom et al [44], the secondary effects are likely negligible, such that the blade performance can be estimated from measurements (5) C P = C P,blades + C P,mount + C P,secondary of C P and measurements of C P,mount (taken by rotating the support structure without blades).…”

Effect of aspect ratio on cross-flow turbine performance

“…These parameters include the dimensions of the rotor (e.g., aspect ratio 3,4 , helix angle 5 , and number of blades 6,7 ), properties of the blades (e.g., foil profile 8,9 , preset-pitch angle 10,11 , and chord-to-radius ratio 12,13 ), and the type of support members used to attach the blades to the central shaft (e.g., struts or end-plates 14,15 ). The breadth of this parameter space suggests optimization by reduced-order models, but this is complicated by the unsteady nature of cross-flow turbine fluid dynamics.…”

“…Here, computational shortcomings are exchanged for limits on turbine size and flow conditions imposed by the test facility (e.g., maximum Reynolds number 26 ). Several experimental studies have investigated the effects of a variety of geometric parameters on cross-flow turbine performance 5,7,10,11,14,[27][28][29] . However, experiments must be structured such that the geometric parameter of interest is fully isolated (i.e., all other non-dimensional parameters that affect turbine performance must be held constant).…”

“…Second, as aspect ratio increases, intermediate support structures may be required between the blades and drive shaft to limit mechanical stress. These intermediate structures can negatively affect power generation 14 .…”