Analyzing the Near-Field Effects and the Power Production of an Array of Heaving Cylindrical WECs and OSWECs Using a Coupled Hydrodynamic-PTO Model

Abstract

The Power Take-Off (PTO) system is the key component of a Wave Energy Converter (WEC) that distinguishes it from a simple floating body because the uptake of the energy by the PTO system modifies the wave field surrounding the WEC. Consequently, the choice of a proper PTO model of a WEC is a key factor in the accuracy of a numerical model that serves to validate the economic impact of a wave energy project. Simultaneously, the given numerical model needs to simulate many WEC units operating in close proximity in a WEC farm, as such conglomerations are seen by the wave energy industry as the path to economic viability. A balance must therefore be struck between an accurate PTO model and the numerical cost of running it for various WEC farm configurations to test the viability of any given WEC farm project. Because hydrodynamic interaction between the WECs in a farm modifies the incoming wave field, both the power output of a WEC farm and the surface elevations in the ‘near field’ area will be affected. For certain types of WECs, namely heaving cylindrical WECs, the PTO system strongly modifies the motion of the WECs. Consequently, the choice of a PTO system affects both the power production and the surface elevations in the ‘near field’ of a WEC farm. In this paper, we investigate the effect of a PTO system for a small wave farm that we term ‘WEC array’ of 5 WECs of two types: a heaving cylindrical WEC and an Oscillating Surge Wave Energy Converter (OSWEC). These WECs are positioned in a staggered array configuration designed to extract the maximum power from the incident waves. The PTO system is modelled in WEC-Sim, a purpose-built WEC dynamics simulator. The PTO system is coupled to the open-source wave structure interaction solver NEMOH to calculate the average wave field η in the ‘near-field’. Using a WEC-specific novel PTO system model, the effect of a hydraulic PTO system on the WEC array power production and the near-field is compared to that of a linear PTO system. Results are given for a series of regular wave conditions for a single WEC and subsequently extended to a 5-WEC array. We demonstrate the quantitative and qualitative differences in the power and the ‘near-field’ effects between a 5-heaving cylindrical WEC array and a 5-OSWEC array. Furthermore, we show that modeling a hydraulic PTO system as a linear PTO system in the case of a heaving cylindrical WEC leads to considerable inaccuracies in the calculation of average absorbed power, but not in the near-field surface elevations. Yet, in the case of an OSWEC, a hydraulic PTO system cannot be reduced to a linear PTO coefficient without introducing substantial inaccuracies into both the array power output and the near-field effects. We discuss the implications of our results compared to previous research on WEC arrays which used simplified linear coefficients as a proxy for PTO systems.

The Expected Shoreline Effect of a Marine Energy Farm Operating Close to Sardinia Island

“…By extracting the energy from waves through a marine farm, it possible to attenuate their action at the shoreline level, an aspect that is directly related to coastal erosion [46][47][48][49]. In order to develop such complex projects, one way will be to use the infrastructure of offshore wind farms located in coastal areas, affected by the erosion problems associated to the wave action.…”

Analysing the Near-Field Effects and the Power Production of Near-Shore WEC Array Using a New Wave-to-Wire Model

“…The site bathymetry, based on surveys carried out for the WATTMOR project, as shown in Figure 3, is approximated in this study by a slope of 1/200 which easily fits within the criteria for the application of MILDwave as detailed in [9,10]. The modelled OSWEC has dimensions equal to those of the OSWEC investigated in [11] and is based on the publicly available information on the WaveRoller R technology. The PTO system is reproduced in WEC-Sim [12] as a simplified, yet accurate, hydraulic time-domain simulation which has been introduced in [1] and detailed in [11].…”

“…This means that the WEC arrays are located 24 • off the average incidence wave direction and 30 • off the summer wave incidence. To simulate a realistic array of WECs, it was chosen a staggered configuration as has been previously utilized in both [1] and [11]. The configuration, along with the dimensions of the WEC arrays, is displayed in Figure 5.…”

“…The PTO of each modelled WEC is a hydraulic system described in detail in [11]. The PTO is constructed in the Matlab Simulink R environment and then simulated in WEC-Sim, an open source purpose-built WEC dynamics simulator developed jointly by Sandia Laboratories and the National Renewable Energy Laboratory in the USA [12,34].…”

“…The modelled OSWEC has dimensions equal to those of the OSWEC investigated in [11] and is based on the publicly available information on the WaveRoller R technology. The PTO system is reproduced in WEC-Sim [12] as a simplified, yet accurate, hydraulic time-domain simulation which has been introduced in [1] and detailed in [11]. The WEC farm absolute value of the total free surface elevation |η| is determined via an iterative the method first developed in [1] and detailed in Section 2.8.…”