Abstract
Among many sources of renewable energy available, tidal energy, has many attractive features as a clean energy resource. It is a sizable resource, distributed along coastlines and is considered to be one of the more promising renewable energy sources. However, a key concern associated with tidal turbines is their long-term reliability when operating in the hostile marine environment. Biofouling changes the physical shape and roughness of tidal turbine components, hence altering turbine performance. This represents a large stumbling block for adoption of the technology. Among the various types of fouling on man-made structures, barnacles are considered to be one of the most problematic organisms. Therefore, the main objective of this study was to determine the effect of barnacles on the performance of a twin-bladed horizontal axis tidal turbine. Three research questions were investigated: how barnacle roughness alters the performance of aerofoil sections of which tidal turbines are based; how barnacle fouling changes the long-term turbine performance of tidal turbines; and how the presence of barnacles affects drag on a flat plate.
The first two questions were investigated using Computational Fluid Dynamics (CFD). The geometry and density of the conical shaped barnacle elements for the adult sized Amphibalanus Amphitrite barnacle, were estimated to determine an equivalent sand-grain roughness. A commercial Reynolds Averaged Navier-Stokes (RANS) solver with Shear-Stress Transport (SST) turbulence model was used to simulate the flow around a two-dimensional NACA63-618 aerofoil with and without surface roughness. The model was validated against published experimental data for a smooth case. The results showed the presence of the adult barnacle fouling decreased the maximum lift coefficient by an average of 21% and lift-to-drag force ratio by an average of 60%. The performance of a twin-bladed horizontal axis turbine with rotors of the same aerofoil section was also studied. The barnacle roughness decreased the peak power coefficient from 0.42 to 0.37 at the design tip-speed ratio of 6. This represents a decrease in turbine output power of 12%. The approximate time taken to reach adult size and establishment of this barnacle fouling community is approximately 8-12 months.
The effect of barnacle roughness on total drag force and the turbulent boundary layer on a test plate covered with artificial barnacles was studied experimentally in a water tunnel using a floating element force balance. The artificial barnacle models tested were obtained using a novel method of scanning real barnacles, 3D printing and then moulding them using an epozy resin. Three fouled plates were tested with low, medium and high barnacle fouling density. Based on roughness equations for cone shaped barnacle, a reduction in barnacle spacing (an increase in barnacle density) caused an increase in equivalent sand grain roughness. The results of roughness correlations indicated the equivalent barnacle roughness was 34.78 mm, 78.47 mm, and 112.9 mm for low, medium and high barnacle fouling density respectively. The testing showed the presence of artificial barnacles produced a maximum increase in the drag coefficient of 429% for the low density case.
Based on governing equations (geometrical formulations)for tidal turbine blade and single roughness elements, an increase in sand grain roughness causes a reduction in power coefficient results. According to theoretical hydrodynamic power of turbine, the smooth case gives the highest power coefficient. It is around 0.42. As shown and discussed, the lowest barnacle density produced an average 5% reduction in power coefficient over the clean case. For low density status, the power coefficient is around 0.4 and there is a slight difference between low density and smooth result with the same tip speed ratio. For a medium case, the percentage is over a 10% decrease in the power coefficient over the smooth case. An increase in sand grain roughness value causes a reduction in power generation from 0.38 to 0.34. For high-density status, the percentage of decreasing of power generation is 22%.