How much energy can be extracted from moving water with a free surface: A question of importance in the field of tidal current energy?

This short technical note addresses the extraction of energy from a simplified channel in which flow is driven by a head difference between inlet and outlet. This model is used to indicate that there is a maximum rate at which energy can be artificially extracted from the flowing water and that this rate is related to the kinetic energy flux in the unexploited channel but with a multiplying factor which is related to the channel physical properties. Counter intuitively, this multiplier can exceed unity in some circumstances. The simple channel has some similarities to tidal channels but is here presented as an abstraction to allow appreciation of the relationships between energy extraction, flow speed and channel properties.