The exponential increase of the cooling demand in buildings obliges to find alternatives to the high electricity consumption with air conditioning systems. This work investigates a new cooling system based on evaporation. The key component of this system is a porous tank filled with water. Evaporation process occurs at the outside surface, which cools down water in the tank. Fresh water is then used to meet the cooling demand in the building. To model the impact of this new system, an accurate model of the porous tank is needed. A numerical model is developed, based on the heat and mass transfer balances and compared with experimental results, showing accurate results in terms of energy balance and prediction of water-cooling