Boundary conditions

Amine

Hey everybody, I work on a model of chimney solar. In brief description, it's a plant which produces electrical energy, it's constitute of: collector, Chimney and turbine; the sun radiation heats the air in the collector and the movement of the air throw the chimney turns the turbine. The problem is that, at the inlet of the collector I put pressure outlet for the boundary condition and set the value to 0, but it's a rude assumption because the air is in movement at the inlet of the collector so what condition I can put on this boundary? Thank you.