The fundamental equations about wind energy extraction date from the year 1915 when Lanchester published the basic momentum and energy balance for a device which extracts energy from wind.  In most cases this theory is attributed to A. Betz who published the same in 1920.  The theory proofs that a wind turbine maximally extracts 16/27 (=59%) of the energy from the flow. Corten re-analysed this theory and added the a wind turbine when extracting 16/27 of the energy from the flow, in fact has an inherent extra extraction of 8/27 which becomes heat. So according to Corten the extraction of kinetic energy from the flow is not 16/27, but 24/27. 

The maximum extracted useful energy by a wind turbine in Corten's model is the same (16/27), however the total reduction of the kinetic energy is 24/27 in Corten's model. That means that less energy remains for other wind turbine downwind. So it is relevant for wind farms and Corten concluded that the optimum energy extraction of wind energy in a wind farm is different from the classic insights. Ref. PhD thesis of Corten, chapter 2.