Ocean Thermal Energy Conversion (OTEC) is considered as a non-intermittent base energy resource. It consists in using the difference of temperature between the hot surface seawater in tropical seas and the cold deep seawater to produce electricity by mean of a thermodynamic motor cycle. The Organic Rankine Cycle (ORC) is adapted for this kind of application. In the literature, performances of such a system are often studied under steady state conditions, mainly because the temperature of the sources do not present intermittency. However, the study of the transient behavior of an OTEC power plant could be of major interest for piloting purposes. With this aim, a dynamic model of heat exchanger is presented with the use of a Moving Boundary Model (MBM) in order to distinguish the monophasic and diphasic parts of the transfer. Moreover, experiments have been carried out on an onshore OTEC prototype located in Reunion Island and compared to simulations. The case being studied is an increase of 1 °C of the hot water during 3 minutes leading to a good agreement between simulation and measurement.