A hybrid ocean thermal energy conversion (H-OTEC) system combines an ocean thermal energy conversion (OTEC) system, which generates electricity using the temperature difference between the surface and deep layers of the ocean, with a flash desalination system to produce freshwater from seawater. The H-OTEC generates steam by vacuuming the warm seawater and convey the steam to the power generation system as the heat source. The H-OTEC can prevent the performance degradation of the heat exchanger due to bio-fouling, improve the heat transfer coefficient by condensation heat transfer on the heat source side, and reduce the cost of the heat exchanger by using stainless steel or other low-grade materials instead of titanium. The basic thermodynamic characteristics of the H-OTEC have been clarified by parametric analysis, but the performance of the evaporation-condenser (Eva-Con), which condenses water vapor generated by flash evaporation and evaporates ammonia using the latent heat of condensation as a heat source, has not been experimentally clarified. In this study, the authors report the heat transfer performance test results of the Eva-Con.