Optimization of Windfarm Layout P M V Subbarao Professor Mechanical Engineering Department Maximization of Power per unit Site Area........

Wind Farm Layout Optimization Wind farm layout optimization is referred as the optimization task that chooses the best turbine positions. The windfarm optimization can be executed by applying specific objective function. The most extensive method is by maximizing the total produced annual energy by a windfarm. The annual energy produced is contingent upon the total number of wind turbines in a farm and their positions with respect to one another in order to reduce the wake effect.

Annual Energy Generated by A Windfarm A windfarm scenario s, is characterized by a wind direction and an ambient wind speed. L is the size of a Windfarm. The power produced by the wind farm can be obtained by computing the wind speed Vj at each turbine location j  L. Let P(V) be the known function that computes the power generated by one turbine if the wind speed is V at the turbine location. In general, several scenarios are possible for a windfarm. The expected total power is calculated by summing up the power produced in each scenario s weighted by the probability of its realization rs.

A Typical Scenario Function

Computation of Wind Velocity At Turbine Location Since many turbines are installed in a wind farm, wakes will intersect and affect upstream wind velocity of jth turbine. In the Jensen multi turbine model, the upstream wind velocity is obtained by estimating the velocity deficit due to ith turbine at jth location.

Influence of Multiple Wake Interactions The upstream velocity at any location j will be influenced by a set of turbine present at upstream locations. The value of upstream velocity also depends on the Wind Scenario. Let Ws(j)is the set of turbines affecting position j with a wake under scenario s. Ws(j) plays a role of decision variable about j.

Optimization Methods for Windfarms There are a number of optimization techniques which have successfully been used in solving wind farm layout problems during present and last decade. GeneticAlgorithms SimulatedAnnealing Differ- ential Evolution Simulated Evolution (SimE) Ant Colony Optimization (ACO) Particle Swarm Optimization (PSO) Stochastic Evolution Definite PointSelection (DPS) Bionic Optimization Gradient based optimization, Numerical added simulation Montecarlooptimization technique.

Formulation of Objective Function The main objective of the analysis is to get the best location for each turbine in a windfarm. This must result in the reduction of cost and an increase in annual energy output. Describe two fitness functions. One for the reduction of cost, and the other for maximizing the annual energy. Develop a single equation to aggregate the two fitness functions. Each fitness function may be assigned weights depending on the preferences given to each optimization factor.

Objective Function The final optimization function is described mathematically as: Where, 1 and  2 are randomly chosen weights and Cwindfarm is the cost per annum of the wind farm with N turbines. Mosetti et al. was the first attempt to use Jensen's wake model with genetic algorithm in solving windfarm optimization problem in 1994. No work has been done after Mosetti et al. with in the duration of 11years.

Wind farm design including energy yield predictions ‘WasP’, ‘Wind farmer’ and ‘WindPRO’ are well known computer models to calculate energy yield. The modelling takes into account not only wind speed and direction distributions but also the geography and terrain; for instance, a steep slope in the terrain will cause higher winds at the hill top. Such details in the modelling ensure that wind turbines are optimally sited. Generally the layout is optimised for exposure to the prevailing wind direction.

Mutual Distance The mutual distance between the wind turbines has to meet the requirements of the manufacturers. If the wind turbines are too close together output may be reduced. Another, more serious, consequence may be damage to primary structural parts caused by the wake of wind turbines sited upwind. The minimum distance depends on the siting with regard to the prevailing wind direction. For turbines sited perpendicular to the prevailing wind direction, the mutual separation distance has to be at least four and preferably five times the rotor diameter.

Availability of A Wind Farm The gross energy yield of the wind farm is determined by the local wind distribution and the siting of the wind turbines. To calculate the net energy yield the anticipated losses must be determined. These losses include: wake losses grid losses availability.

Development of Wind Farms : Case Study P M V Subbarao Professor Mechanical Engineering Department I I T Delhi “General Guide Lines…. Not Obviously valid for All…

Phases in Wind Farm Development Initiation and feasibility (concluded by go/no-go) Prebuilding (concluded by go/no-go) Building Operation and maintenance

Wind farm initiation and feasibility phase In this phase, the basic parameters of the project are determined and potential sites are assessed and compared. Considerations: The wind resource available. The proximity of suitable grid connections and the limitations of regional, national and local planning regulations. At the end of this phase the least suitable sites will have been eliminated and a decision made whether to proceed with further study of the remaining sites or to abandon the project.

Site selection and wind assessment Wind farms require large sites. Depending on the rotor diameter the required mutual separation is 300 to 500 metres. A further separation distance from dwellings and commercial buildings to limit noise nuisance and to provide a safety zone. Even for a medium size wind farm, such as the 5 wind turbines of 2 MW considered here, a substantial land area is required. Generally speaking, potential wind farm sites are preferably open areas of flat land or on top of hilly areas. Obviously, the sites should be known to be windy, with high and recurrent wind resources.

Having selected the site, the next step is to assess the local long-term wind climate by reference to existing data or by long term monitoring. The objective in this phase is to eliminate all sites that may be unsuitable (in other words, unprofitable) in the long term. If the site screening process does not identify any prohibitive limitations the feasibility study may proceed

Wind Resource Assessment The wind resource assessment and the consequent estimation of the yearly energy yield is of crucial importance since they determine the project yield. The energy available from the wind is proportional to the third power of the wind speed. Based on local wind speed data from meteorological stations a local wind atlas of the planned wind farm can be determined. It is necessary to use at least one full year of wind data to take into account variations in wind speed during the seasons.

Wind Energy Data

Power/wind speed curve of a 2 MW wind turbine

Wind speed distribution

Gross yearly energy yield of a 10 MW wind farm