A time-dependent 3D numerical model of the solidification process of large size photovoltaic Si ingots is presented. The difficulty of the model is related to the relative movement of various parts of the furnace that we solve by using a dynamic layering mesh approach. This permits to calculate the thermal gradient, solidification rate and hydrodynamics of the silicon, which are important in order to control and optimise the grain structure of the ingot. The comparison between the numerical predictions and the experimental measurement shows a reasonable agreement. The effect of some geometrical modifications of the equipment on the thermal field is studied in order to improve the solidification process and the structure of the ingot. (c) 2007 Elsevier B.V. All rights reserved.