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Wind energy is an alternative source of energy that is becoming more popular as fossil fuels are becoming more expensive.
To profitably harness this power of nature, a multiple wind turbines have to be erected in favorable sites. Whenever more than three wind turbines are erected in a space, it is called a wind farm. If the wind turbines are located so that the direction of wind flow is perpendicular to the row of the turbines, then all the turbines will run at optimum efficiency.
However, if the wind direction or layout of the wind farm is such that the wind blows parallel to the rows of the turbines, then turbines following other turbines in the rows will have higher turbulence as well lower wind speed. The effect of the turbulence and fluctuating wind speed is not only loss in the production of electricity, but also the reduced life of the wind turbines due to fatigue failure.
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Optimum Wind Turbine Spacing
Many technical problems arise when we need to put multiple wind turbines close together in a wind farm. The most important questions are where to place the turbines, how closely to space them, and what the optimum arrangement for them is.
The wind flow and profile may vary across the wind farm due to the terrain, and also the wind direction may change due to time of year and as per the weather. Generally wind turbines are located at sites where there are no obstructions like trees, houses, etc. to the incoming wind. Locations at the sea coast, the top of the hills, and open grass lands are favored by wind farm designers.
The convention when installing a wind turbine is to place it at least twice the height of any obstacle in front and at least twenty times far away to avoid any turbulence effects. However, this is the case only for single turbines. In a wind farm, the turbines usually have to be put closer due to space and cost constraints.
Wind turbines arranged in a wind farm form an array. The spacing in an array is of two types: downwind spacing and crosswind spacing. A wind turbine in an array will never get 100 % of the energy that an isolated turbine will get. This wind energy loss is called "array loss." The extraction of energy from the incoming wind causes a loss in the kinetic energy as well as the velocity of the wind in the wake of the turbine. This loss is made up after a certain distance (after it exchanges energy with the surrounding wind).
When turbines are placed one behind the other, turbulence and the variation in wind speed make the following turbines more prone to wear and tear. Also as the wind there has less energy, it makes running turbines behind other turbines economically less appealing. Sufficient distance and spacing must be provided so that wake and turbulence effects are minimized and all the turbines work efficiently.
Wake effects reduce energy production, as well as causing fluctuations in the overall power output. Turbulence behind any obstacle like a tree or a house slows down the wind for a distance about three times the height of the obstacle. Known as "obstacle shade," this effect reduces wind velocity and energy production due to eddies and turbulence.
Array losses can be reduced by efficient calculation of the downwind spacing and the cross wind spacing. Wind turbine studies have shown that turbines spaced eight to ten times the rotor diameter in the downwind direction and five times the rotor diameter in the crosswind direction have very little turbulence- as little as 10%.
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Wind Turbines: by Mohit Sanguri, Chief Engineer