This picture shows about the structure which produces electricity using tidal power.
Generally in the both images the turbain is constructed on the cliff of a sea , it consists of two components one is round part which is open to the sea side , another is vertical open to atmosphere . In between the vertical turbain one small wheel is present which generates electric power.
Primarily, in the picture a it shows that wave approaching to the sea creates a air pressure by the action of the waves through which the produced air tends to rotate the wheel by the air that exerts from the vertical turbain that produces electricity.
Moreover ,in the picture b it shows that the receding tide which creates a pressure due to the tidal flux that leads to the entry of the atmospheric air into the turbain whereas the wheel rotates by the outside air produces electricity .