A Solar Enhanced Natural Draft Dry Cooling Tower (SENDDCT) is an air-cooled hybrid cooling system that uses solar energy to enhance the cooling efficiency of natural draft dry cooling. In this paper, a three-dimensional numerical analysis was carried out using commercial software ANSYS FLUENT to investigate the effects of the introduction of a partial blockage on the cooling performance of the SENDDCT and to determine the upper limitation of increasing the solar collector size in the SENDDCT design with a fixed area of heat exchangers. In the simulations, the cooling capacities of SENDDCTs with the same geometric and operating parameters, such as the heat-exchanger areas and tower heights, except for different collector sizes and blockage ratios (the ratio of the blocked area to the area offered at the collector outer edge), were evaluated and a comparative analysis was performed. The results showed that for an SENDDCT with fixed heat-exchanger area, the upper limitation of increasing the solar collector size is that corresponding to a blockage ratio reaching 50%, above which the heat dump capacity of the SENDDCT decreases with a further increase in the solar collector size owing to vortices generated at the back of the heat exchangers reducing the effective heat transfer area. In addition, with a distribution of small-unit heat exchangers, this effect can be significantly reduced. (C) 2013 Elsevier Ltd. All rights reserved.