Maria Kartezhnikov and Thomas M. Ravens – Renewable Energy, Volume 66, June 2014
A simple technique to estimate the far-field hydraulic impacts associated with the deployment of hydrokinetic devices is introduced. The technique involves representing hydrokinetic devices with an enhanced Manning (bottom) roughness coefficient. The enhanced Manning roughness is found to be a function of the Manning roughness, slope, and water depth of the natural channel as well as device efficiency, blockage ratio, and density of device deployment. The technique is developed assuming simple open channel flow geometry. However, once the effective bottom roughness is determined, it can be used to determine the hydraulic impact of arbitrary device configurations and arbitrary flow situations.
Ross Vennell and Thomas A. A. Adcock – Proceedings of the Royal Society, April 2014
While wind farms have no inherent storage to supply power in calm conditions, this paper demonstrates that large tidal turbine farms in channels have short-term energy storage. This storage lies in the inertia of the oscillating flow and can be used to exceed the previously published upper limit for power production by currents in a tidal channel, while simultaneously maintaining stronger currents. Inertial storage exploits the ability of large farms to manipulate the phase of the oscillating currents by varying the farm’s drag coefficient. This work shows that by optimizing how a large farm’s drag coefficient varies during the tidal cycle it is possible to have some flexibility about when power is produced. This flexibility can be used in many ways, e.g. producing more power, or to better meet short predictable peaks in demand. This flexibility also allows trading total power production off against meeting peak demand, or mitigating the flow speed reduction owing to power extraction. The effectiveness of inertial storage is governed by the frictional time scale relative to either the duration of a half tidal cycle or the duration of a peak in power demand, thus has greater benefits in larger channels.
C-H Jo, D-Y Kim, Y-H Rho, K-H Lee, Cameron Johnstone – Renewable Energy, June, 2013
Due to global warming, the need to secure an alternative clean energy resource has become an international issue. Tidal current power is now recognized as one of the clean power resources in Korea, where there are many strong current regions on the west and south coasts. Recently, large scale tidal devices have been deployed with a maximum rotor diameter of 18 m. These devices impose significant loading on supporting structures. In many cases, a pile fixed foundation is used to secure the structure. However, due to the high density of seawater, the drag and lift forces are much larger than in air, causing extensive stress and deflection to the pile tower structure. In this study, a numerical analysis of the hydro-forces from a rotating tidal current turbine to a tower was conducted to determine the deformation distribution along the pile tower.
I.A. Milne, A.H. Dayb, R.N. Sharmaa, R.G.J. Flaya – Ocean Engineering – March, 2013
Characterisation of the unsteady hydrodynamic loads is essential for accurate predictions of the fatigue life and ultimate loads of tidal turbine blades. This paper analyses a set of experimental tests of the hydrodynamic blade root out-of-plane bending moment response to planar oscillatory motion, chosen as an idealised representation of the unsteadiness imparted by waves and turbulence. Phenomena associated with dynamic stall are observed which are sensitive to the oscillatory frequency and velocity amplitude. Flow separation is shown to result in loads significantly greater in magnitude than that for steady flow. Following flow reattachment, the load cycles compare relatively well with Theodorsen’s theory for a two-dimensional foil oscillating in heave, suggesting that circulation due to the shed wake dominates the unsteadiness in phase with acceleration, over added mass effects. For attached flow, the effect of unsteadiness is comparatively much smaller. At low frequencies a phase lead over the velocity is observed, compared to a lag at higher frequencies. Multiple frequency oscillations are also briefly considered. Reconstruction of the multi-frequency response using both the steady flow measurements, and the single frequency measured response, is shown to offer a relatively good fit when the flow is attached, for lower frequency combinations.