WO2004011799-A1  E. DICK: ""Aerodynamic optimisation of a multirotor wind energy system with a large diameter tower"" WIND ENGINEERING, vol. 1, no. 4, 1987, pages 207-224, XP008022904;  G.P. GORTEN: "Thesis "Flow separation on wind turbine blades", ISBN 90-393-2592-0" , UNIVERSITY OF UTRECHT (THE NETHERLANDS) , 08-01-2001 XP002258620 Page 112 to 114, heading " The Terrain concentration Hypothesis";  H. WEIMERSKIRCH ET AL: "Energy saving in flight formation" NATURE, vol. 413, 18 October 2001 (2001-10-18), pages 697-698, XP002258619;  STEINBUCH M ET AL: "OPTIMAL CONTROL OF WIND POWER PLANTS" JOURNAL OF WIND ENGINEERING AND INDUSTRIAL AERODYNAMICS, XX, XX, vol. 27, 1988, pages 237-246, XP008023141 cited in the applicationUS7299627-B2  Dick, E., "Aerodynamic optimisation of a multirotor wind energy system with a large diameter tower", Wind Engineering, vol. 1, No. 4, 1987, pp. 207-224, XP008022904, "Conclusion", 2nd paragraph, p. 224.;  Gorten, G.P., Thesis "Flow separation on wind turbine blades", ISBN 90-393-2592-0, University of Utrecht (The Netherlands), Aug. 1, 2001 XP002258620, pp. 112-114, heading "The Terrain concentration Hypothesis".;  Steinbuch, M. et al., "Optimal Control of Wind Power Plants", Journal of Wind Engineering and Industrial Aerodynamics, vol. 27, 1988, pp. 237-246, XP008023141, p. 245, "Conclusions wind farm control".;  Weimerskirch, H. et al., "Energy saving in flight formation", Nature, vol. 413, Oct. 18, 2001, pp. 697-698, XP002258619.EP1534951-B1  G.P. GORTEN: "Thesis "Flow separation on wind turbine blades", ISBN 90-393-2592-0" , UNIVERSITY OF UTRECHT (THE NETHERLANDS) , 08-01-2001 XP002258620 Page 112 to 114, heading " The Terrain concentration Hypothesis";  E. DICK: ""Aerodynamic optimisation of a multirotor wind energy system with a large diameter tower"" WIND ENGINEERING, vol. 1, no. 4, 1987, pages 207-224, XP008022904;  STEINBUCH M ET AL: "OPTIMAL CONTROL OF WIND POWER PLANTS" JOURNAL OF WIND ENGINEERING AND INDUSTRIAL AERODYNAMICS, XX, XX, vol. 27, 1988, pages 237-246, XP008023141 cited in the application;  H. WEIMERSKIRCH ET AL: "Energy saving in flight formation" NATURE, vol. 413, 18 October 2001 (2001-10-18), pages 697-698, XP002258619