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ENDOW Project Summary |
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Europe has large offshore wind energy potential that is poised for exploitation to make a significant contribution to the objective of providing a clean, renewable and secure energy supply. Offshore wind energy developments are underway in many European countries with planned projects of several thousand megawatts to be installed in the first decade of the new millennium. While experience gained through the demonstration projects currently operating is valuable, a major uncertainty in estimating power production lies in the prediction of the dynamic links between the atmosphere and wind turbines. Due to lower turbulence offshore, wake effects (velocity decrease, turbulence increase downstream of a wind turbine rotor) will be propagated over larger distances downstream than is the case over land. The likely result is that in order to optimise power output offshore wind farms will require larger distances between rows than is common in design of onshore wind farms. This has a major economic disadvantage because undersea grid connections and connections between turbines are proportionally more expensive than their cost and installation at land sites. An additional uncertainty is introduced because coastal boundary-layers are not well characterised by current atmospheric models. Since most offshore areas are not routinely monitored, boundary-layer models are used to predict wind and turbulence regimes for prospective offshore wind farm sites. Large wind farms will cover distances of several to tens of kilometres over which changes in boundary-layer growth and feed-backs to/from the sea surface and to/from flow around and through individual turbines in the array are significant. Configuring offshore wind farms for optimal power output and minimum cabling costs is a large and complex operation. The objective of this proposal is to evaluate, enhance and interface wake and boundary-layer models for utilisation offshore. This will result in a significant advance in the state of the art in both wake and marine boundary layer models leading to improved prediction of wind speed and turbulence profiles within large offshore wind farms. Use of new databases from existing offshore wind farms will provide a unique opportunity to undertake the first comprehensive evaluation of wake model performances (this will be conducted for an offshore environment). The wake models to be evaluated vary in complexity from empirical solutions to the most advanced models based on solutions of the Navier-Stokes equations using eddy viscosity or k-epsilon turbulence closure. Results of wake model performance in different wind speed, stability and roughness conditions will provide criteria for their improvement focussed on near- and far- wake development in addition to the characterisation of multiple wakes. To improve predictions of boundary-layer characteristics offshore a local-scale model (CDM2) will be enhanced to account for stability and roughness variations and boundary-layer development within and over a large offshore wind farm. Coupling of the CDM2 with wake models will provide wind speed and turbulence profiles upstream and downstream of each wind turbine accounting for fetch variations and wake impacts. Case studies using a mesoscale model which can account for large-scale thermally driven flows will provide an uncertainty analysis for the CDM2. The model hierarchy will form the basis of design tools for use by wind energy developers and turbine manufacturers to optimise power output from offshore wind farms through minimised wake effects and optimal grid connections. Output from the design tool will also be suitable for input to aeroelastic models for load calculations. The design tools will be built onto existing regional scale models and wind farm design software which was developed with EU funding and is in use currently by wind energy developers. This will maximise the expected impact of this project through efficient use of existing resources and ease of upgrade for end-users. Part of the design tool evaluation will include the issue of computational feasibility and ease of use (in addition to the scientific and technical aspects usually considered). To form part of the evaluation process for both wake models and for the design tool, a large-scale experiment is proposed. For the first time, the wind turbines in an offshore wind farm will form the elements of an experimental array. Selective operation of turbines in carefully chosen conditions reflecting wind speed and direction (influencing the fetch variation), atmospheric stability, and wave/roughness conditions will allow the direct impact of turbine operation on wake effects to be measured. Use of a SODAR instrument will provide vertical wind speed profiles to hub and rotor heights of offshore wind turbines currently being developed by manufactures. This consortium brings together both research and industry from European countries which have experience in offshore wind farm development and those currently planning new projects. The combination of research and industry forms the basis for both the design tool development and its evaluation. Finally, the tool will be demonstrated by production of optimised designs of offshore wind farms in three locations in Europe, chosen for the differences in their physical characteristics.
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