The effect of windbreak walls on the thermal performance of natural draft wet cooling towers (NDWCT) under crosswind has been investigated numerically. The three dimensional CFD model has utilised the standard k-epsilon turbulence model as the turbulence closure to quantify the effects of the locations and porosities of the wall on the NDWCT thermal performance. Moreover, the improvement in the NDWCT thermal performance due to windbreak walls has been examined at different crosswind directions. Results from the current investigation, have demonstrated that installing solid impermeable walls in the rain zone results in degrading the performance of the NDWCT. However, installing solid walls at the inlet of the NDWCT has enhanced the NDWCT performance at all of the investigated crosswind velocities. Similarly, installing walls with low porosity has shown improvement in the performance of the NDWCT. A reduction of 0.5-1 K in the temperature of the cooling water coming from the tower to the condenser has been achieved at all of the investigated crosswind velocities by installing porous walls both inside and outside the rain zone. (c) 2007 Elsevier Ltd. All rights reserved.