One of the world greatest priorities is to move to a sustainable world, while the global energy demand is growing. Ocean Thermal Energy Conversion (OTEC) is a base-load renewable electricity technology that has the potential to contribute to a sustainable power supply on a world-wide scale. OTEC creates sustainable power utilizing the temperature difference in different depths in the ocean. The relative small temperature difference between the heat source (upper layer of the ocean) and cold sink (lower layer of the ocean) is the power resource. To obtain cost effective electricity from this small temperature difference is one of the major challenges of this technology. In order to determine an optimum power plant plant under specific environmental conditions, a techno-economic optimization model is developed, where the geometries of the components is input in the off-design model. The off-design model determines the operating conditions of the OTEC cycle with respect both the working fluid, the geometry of the components and the mass flows of the system. Nowadays, a simulation model predicts the steady state point of a test set-up at the TU Delft, despite of a lack of knowledge in the off-design performance of the OTEC cycle. Especially in the off-design performance of the turbine. To investigate the influence of the turbine performance on a 10 MW OTEC power plant, the off-design model is now equipped with an axial turbine and applicable for larger scales. As a result, the model determines the pressure drop over the turbine and the performance of the turbine. The model uses the currently best correlations for the heat transfer and the pressure drop prediction of the components to predict the influence of the off-design turbine performance on the overall performance of the system. Ultimately, the cost per produced $kW$ as optimization criterion is calculated and compared for the different power plants during warm seawater temperature fluctuations over a year or seawater mass flow fluctuations. As a final conclusion, the influence of the turbine is calculated to the overall power performance. The results of varying the warm seawater temperature results in a specific power curve, which shows proportional behavior to the temperature difference. The turbine influences the pressure drop in the system and therefore it is recommended to optimize the position of the vanes, especially when the vapor flow is higher than its design value. Ultimately, the different power curves are derived for every specific power plant. Within the relatively small range of temperature differences, the non-linear effect of the off-design performance of the turbine is too small. This leads to the conclusion, the seasonal fluctuation of sea water temperature difference have a significant impact on the net power production, while the performance of the turbine is rather constant. Variations in seawater mass flows show non-linear characteristics. The turbine performance influences the cycle performance, especially if the seawater mass flows drop significantly. This results that the cycle efficiency drops non-linearly downwards as the mass flows decrease. Finally, the turbine performance remains constant during off-design conditions. Therefore, the accessibility of a thermal energy from the ocean is one step closer. This is an important step in creating a renewable energy powered world.