Cable optimization in a specific Wind Farm
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Most commonly used wake models were developed in the 80`s, and gave satisfactory results for small WF`s or single turbines. Amongst these models are Ainslie, J.F, Jensen, N.O, and Crespo, A. It has however been agreed that more complex methods should be developed in order to fully describe wake effects in large wind farms, and that modeling can no longer be solved with engineering approximation methods. Being able to estimate wake losses is particularly relevant for offshore WF`s, as current models performs poorly in the wake near the turbines, and because of the extended wake region in the mean boundary layer, because of less intense turbulence in these environments. The development of more powerful and less costly computer technology makes computer modeling based on iterative solutions of the Navier-Stokes equations an attractive approach. It is believed that these models to a larger extent can describe the wind variation within a large WF by combining the development, generation and interaction of wakes with the atmospheric boundary layer using computational fluid dynamics (CFD). Commercially available CFD-methods under development has been addressed, where WindSim, FLACS-Wind and ANSYS -Windmodeller has been discussed in detail. Validation cases for both WindSim and Windmodeller proves that these are promising methods, with the scope to describe AEP and the interaction of wakes to a high level of accuracy for large OWF`s. More validation cases and further optimization of the CFD-code are however needed in order to meet the criteria for less than 3 % prediction deviation, set by The European technology platform for wind energy. FLACS-Wind is in the early phase of development, but results, progress and validation cases are expected to be submitted through the NORCOWE-project, which has an 8 year timeframe, and started in 2009. WindSim is considered to be the presently most reliable wind field modeler, because of their extensive experience within this field, and the release of their new software WindSim 4.9.2 in the spring of 2010. Three different infield cabling layouts for an existing WF with 288 MW installed capacity were investigated using the demo-version of Power factory, and technical and economical information provided by TWP. Case A addresses the single radial design, where cable sizes between 95m² and 300m² are used, and the cable distance is relatively short. Case B investigates the single-sided ring design. This design demands twice the cable length and a cross-section of 300 m² for all cables in the system. Case C looks into the double-sided ring design, where two adjacent radials are coupled with an interconnected cable. In order to carry the full power flow of the two radials, the cable size was increased to 800 m². Simulation shows that case A experiences total power losses of 1,076 %, when the turbines are producing on maximum capacity. Power losses for case B and C are 0,510% and 0,243% in the same comparison. It was found that longer cable distance increased power losses, and larger cross-sections decreased power losses, in accordance with theory. With results from Power factory, economical considerations were done and the three cases were compared against each other in terms of initial expenses for cables and lost income from power losses over the lifetime of the WF. Costs of cables for the three different cases in this comparison are 14.62 M , 38.82 M and 44.69 M . With a WF- lifetime of 25 years, discount factor of 7% and electricity-price of 150 /MWh, NPV-calculations showed that case A should be the preferred choice of cable pattern.