Nidal,
Is your collector an evacuated tube device in which water from the
tank circulates through the tubes or is it a heat pipe collector in
which a refrigerant in the evauated tubes boils in the tube and
condenses in the tank but does not come into contact with the tank
water? To my knowledge there aren't any proven models of either but we
have been working with a natural convection loop model that may be used
to estimate the performance of such collectors.
kind regards,
David