Cross-flow turbines (CFTs) are an attractive option for marine energy due to their ability to rotate in the same direction independent of the incident velocity. However, one of their main barriers to market entry is their levelized cost of energy (LCOE). Selecting non-traditional manufacturing processes for components of the turbine can play a role in making them a more competitive technology by lowering their LCOE. In particular, additive manufacturing can provide multiple benefits including lowering the cost of production through a reduction in material waste, a quick turnaround in comparison to conventional techniques, and the ability to print complex geometries. With exploring the application of additive manufacturing comes the need to look at the resulting physical properties of the product, specifically surface roughness which can impact turbine performance negatively, as seen by studies investigating marine biofouling.
A 1-meter diameter vertical-axis CFT consisting of three helically twisted NACA 0018 blades with two support struts was tested in a towing tank to investigate the impact of surface roughness on performance. For these tests, two support struts were fixed at a separation distance of z/H = 1, where z is the upper strut location with respect to the bottom strut, and H is the overall blade span length. The support struts were fabricated to allow for easy installation and removal of turbine blades. Two blade materials were used, e-glass composite blades made from glass fiber prepreg and electron-beam melting (EBM) 3D-printed titanium blades. With these two blade material types, turbine performance was compared across a total of three surfaces with varied roughness, i) e-glass fiber blades coated with epoxy, ii) titanium blades with the as-printed rough surface, and iii) titanium blades after smoothing the blade surface. Roughness of each of the three surfaces was quantified through use of a laser scanning confocal microscope. A 3D coordinate measuring machine (CMM) was used to quantify blade shape of the two material types. Turbine performance was measured by varying Reynolds numbers and tip speed ratios. Turbine performance was evaluated using a towing speed that was sufficiently high for it to be independent of Reynolds number.
Preliminary experimental results showed that increased surface roughness leads to an increase in drag, which, in extreme cases, can result in a negative performance curve. CFTs are inherently unsteady devices and an increase in drag can lead to lower tip speed ratios. Thus, resulting in a larger envelope of angles of attack and larger regions of dynamic stall, which is particularly detrimental to their performance. The results provide insight from a manufacturing perspective on how surface roughness can significantly impact turbine performance.