An improved heat dissipation system for buildings' air conditioning, based on a previous prototype, has been developed and tested. It acts as an extended solar driven cooling tower, reducing service water temperature. The paper presents the physical basis of the system, the geometrical modifications of the new prototype, the mathematical model and the performed tests. Experimental results obtained during summer 2000 in a full-scale prototype showed that the cooling power capacity of this model is about 50% greater than that of the preliminary model. For each square meter of Hydrosolar Roof, more than 400 W of heat released can be reached under common operating conditions. Correlations of the system thermal performance have been obtained as functions of solar radiation, wind velocity and ambient conditions, allowing the design of system for actual buildings' applications. (C) 2003 Elsevier Science B.V All rights reserved.