divided by the number of mills and divided by the output of a single turbine if it were on its own depends strongly on the direction of the wind the operating efficiency decreasing when the wind blows in parallel with the rows of turbines one example of this is the wind farm horns rev which has an overall operating efficiency of 90 5 but whose operating efficiency drops to 65 in case of given wind directions the natural thermal current vertically moving flows of air caused by differences in temperature between the air and the water may increase the farm operating efficiency by about 5 from 65 to 70 due to the wake of one wind turbine being destroyed since a larger degree of washing out of the vortices in the wake occurs simultaneously with an acceleration of the wind where the wind is conveyed upwards to the next rotor due to placement of one flow surface 201 behind one rotor 101 as illustrated in fig9 wind blowing underneath the rotor 101 is conveyed upwards into the wake of the wind turbine and an artificial thermal current is produced which thereby washes out the vortices guides the wind up to the next rotor 901 and increases its performance for instance the flow surface 201 can be mounted on the foundation or the lower most part of the tower and can be tilted horizontally in case of high wind speeds as shown by arrows 205 the largest effect of such placement of a flow surface at a wind turbine located in front is accomplished with offshore wind farms at sea where the wind flow is more laminar than is the case ashore it will be understood that the invention.