The electricity produced in tropical islands has to face the continuous growing demand of all the sectors and is mainly generated by fossil fuels with no possibility to connect the grid to other bordering countries. This is the case of the island of La Réunion, a French overseas territory situated in the Indian Ocean, off the east coast of Madagascar (Latitude 21S, Longitude 55.5E). La Réunion has launched a very ambitious energy program to reach the electricity autonomy by 2030. This context has contributed to the research of new ways of electricity generation by renewables. Among the various techniques to produce basic electricity, the one consisting in using Ocean Thermal Energy Conversion (OTEC) knows a very important growth worldwide. An onshore prototype of an Organic Rankine Cycle (ORC) driven by the temperature difference between seawater from the depth and from the surface has been designed and built and is under test in order to study the opportunity of producing electricity by this mean. This paper presents the whole installation and particularly focuses on heat exchangers that are key figures in such an ORC. Ammonia is the working fluid and the level of magnitude of exchanged heat in the condenser or the evaporator is about 500 kW for an aimed electricity production of about 15 kW. An experimental point is presented and analyzed in terms of temperature, pressure and of course energy balances. Results reveal quite close to those expected with an obtained efficiency of 2.5%, representing a thermodynamic efficiency (when compared to Carnot ideal cycle) of 35 %.