The present study illustrates the development of a detailed model to estimate the part-load performance of an ammonia closed OTEC system for on-shore installations. A previously published Matlab® suite is extended by accounting for off-design conditions in terms of variable seawater temperature and mass flow on the cycle performance. The off-design behavior of each component is thoroughly discussed, with particular attention devoted to the single-stage axial-flow turbine, whose performance maps are obtained by means of three-dimensional CFD simulations. Assuming a representative plant sized for warm seawater temperature of 28 °C and cold seawater temperature of 4 °C (8500 kg/s taken from 1000 m depth), the model predicts an annual electricity yield of 15.963 GWhe and LCOE of 316 €/MWhe when including seawater measured data of a simile-Hawaiian site. Moreover, a sensitivity analysis is assessed in order to identify the best design parameters (i.e. warm seawater temperature and cold seawater mass flow rate) that minimize the LCOE for the given location. The new design guarantees a reduction of approximately 11% of the LCOE (284 €/MWhe). The simulation capabilities of the developed model prove it as valuable tool to estimate the OTEC competitiveness in different scenarios.