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Abstract This paper evaluates the potential for co-location of offshore wind turbines at sites being developed for tidal stream arrays as a method for reducing the cost of electricity. It is shown that for a typical tidal site, MeyGen in the Pentland Firth, UK, increasing the wind turbine capacity reduces the cost of electricity compared to operating tidal stream arrays alone. This is due to increased energy yield combined with reduction of capital expenditure based on the use of common grid connection and shared support structures. Assessment is made using tidal, wave and wind resource data for a three year period. The overturning moment about the base of a monopile supporting a wind turbine with two tidal turbines is only 8% larger than for a wind turbine alone in a strong current typical of tidal farms. The increased cost of infrastructure is small relative to the increased energy yield and for all array configurations of practical interest, the levelised capital cost of energy is estimated to be 10–12% less from a co-located farm than from a tidal turbine farm alone. Highlights • Cost of electricity by co-locating wind turbines with tidal stream arrays is evaluated. • Energy yield and peak loads are modelled for array layouts in the Pentland Firth. • Peak loads for a shared support are 60% greater than typical offshore wind turbines. • For the same cost, co-location generates 20% more yield than tidal-only arrays. • Co-location offers 10–12% lower levelised cost compared to a comparable tidal array. [ABSTRACT FROM AUTHOR]

Co-location of wind and tidal stream turbines provides opportunity for improved economic viability of electricity generation from these resources relative to projects exploiting each resource separately. Here co-deployment is assessed in terms of energy generation and loading of support structures. Energy yield is modelled using an eddy viscosity wake model for wind turbines and superposition of self-similar wakes for tidal turbines. A case-study of the Inner Sound of the Pentland Firth is considered. For 3.5 years of coincident resource data, 12 MW wind capacity co-located with a 20 MW tidal array results in a 70% increase in energy yield, compared to operating the tidal turbines alone. Environmental loads are modelled for a braced monopile structure supporting both a wind and tidal turbine, as well as for each system in isolation. Peak loading of the combined system is found to be driven by wind loads with greatest overturning moment occurring with the wind turbine operating at close to rated-speed and the tidal turbine close to its shutdown speed. Mean loads vary across the tidal array by 6% indicating no significant shielding effects are gained by co-locating in more sheltered regions of the array. [ABSTRACT FROM AUTHOR]

A line absorber consisting of three cylindrical floats is shown to have high crest capture widths for wave energy conversion across a broad band of frequencies. The bow, mid and stern floats are small, medium and large respectively; the floats are spaced about half a wavelength apart so that forces and motion of adjacent floats are substantially in anti-phase. The bow and mid float are rigidly connected by a beam and a beam from the stern float is connected to a hinge above the mid float for power take off. The draft of the stern float enables heave resonance at a prominent wave frequency and the smaller draft of the mid float provides resonance at a somewhat lower frequency. Experimental results at about 1:8 scale show capture widths greater than 25% of a wavelength in regular waves and greater than 20% of a wavelength in irregular waves across a broad range of wave periods. A time-stepping model for regular waves with coefficients from linear diffraction theory showed similar power prediction with a generic drag coefficient of 1.8. The model shows the importance of surge forcing and heave resonance. The model also shows that reducing drag coefficient will increase capture width. [ABSTRACT FROM AUTHOR]

Abstract: The paper aims at introducing practical methods for power capture performance enhancement of a heaving wave energy converter in irregular seas. The optimum control solution requires tuning to wave frequency based on wave force information. However, identification of the wave frequency in irregular seas is considered to be a complex and difficult task. This is partly due to technical difficulties in determination of the wave force. Besides, there are no clear guidelines for identification of wave frequency from an irregular sea state based wave force information. In a typical application, one of the available sources of information about the wave properties is the wave elevation record. The proposed approach presents a method for estimation of the wave frequency information from the wave elevation data by using signal processing and filtering techniques. The proposed method uses filters to generate an estimation of wave force information, which is used to identify the local wave frequency by method of a time-series analysis of the data. This wave frequency information is then used in tuning the device. The details of the proposed techniques, the model of the wave energy converter, the simulated sea states and the related simulation results are also presented. [Copyright &y& Elsevier]

Fluctuating loads on tidal turbines are important for fatigue analysis and there is limited information or simulation available for full-scale conditions. Here, CFD simulations have been performed for a geometry-resolved full-scale tidal-stream turbine and compared with experimental data from a 1 MW machine deployed at the EMEC test site. Initially, Reynolds-averaged Navier-Stokes (RANS) and large-eddy simulations (LES) were performed using an inflow mean velocity profile representative of the site but low inflow turbulence. Mean blade pressures were similar for the two types of turbulence closure and yielded mean power coefficients comparable with measurements. Then, to simulate the effect of turbulence on loads, LES with synthetic turbulence prescribed at inlet was employed. For these simulations, inflow profiles of mean velocity, Reynolds stresses and length scales were determined from a precursor channel-flow simulation, with additional factoring of stresses and length scales to match hub-height conditions measured on site. Fluctuations in thrust, power and blade bending moment arise cyclically from onset mean velocity shear and the blocking effect of the support tower and over continuous spectral ranges from blade-generated turbulence, approach-flow turbulence and waves. LES simulations with realistic inflow turbulence satisfactorily reproduced the relative spectral distribution of blade bending moments in low-wave conditions. [ABSTRACT FROM AUTHOR]

Highlights: [•] Implementation of a new sliding mesh technique for an open source CFD solver using RANS & LES for a tidal stream turbine. [•] LES resolves blade tip vortices and their interaction with the supporting tower, neither of which are captured by RANS. [•] The nacelle and the mast strongly effect both the wake behavior and the underlying flow physics. [•] Varying upstream turbulence levels does not have a significant effect on the mean blade loadings. [•] The turbine wake and the rms of fluctuating forces, however, show a strong sensitivity to the upstream turbulence. [Copyright &y& Elsevier]