The feed's dissolved salt creates an osmotic pressure that a high-pressure pump must overpower before any fresh water crosses the membrane. Renewable power availability caps how much pressure the pump can actually deliver — undersized solar/wind means the membrane may stall even with plenty of salt water on hand.
Δπ = k · S (van't Hoff osmotic pressure)
Jw = A · max(0, ΔP − Δπ) (solution-diffusion water flux)
R = Jw / (Jw + B) (salt rejection, low flux leaks salt)
ΔP = min(P_set, 0.9 · availability · P_max)
When the applied pressure can't clear the osmotic pressure, flux drops to zero — the pump is "power-limited" rather than a genuine membrane failure.