This thesis investigates the turbine performance of a particular tidal energy conversion device analysing the hydrodynamic variables based on computational fluid dynamics simulations. Renewable tidal energy technology is undergoing a rapid development due to its advantages and new and creative designs of marine current turbines which are constantly being studied and improved. As a result, incorporating diffusers to accelerate the flow is a method used to increase the efficiency of the turbines. However, this practice has to be evaluated and parameterised to ensure its viability. Therefore, an experimental test of a ducted turbine, carried out in 2010, was selected for analysis with the aim of improving diffuser efficiency without studying the diffuser design but other variables in the system. The advantage of this strategy applied to a particular case is that the knowledge achieved can be used to benefit any diffuser design on many tidal turbines. The diffuser that was used in the 2010 experiment exhibited a low increment in power extracted and the main objective of this thesis arises from the question of how it could be possible to extract much more power using such ducted turbine with the same rotor and diffuser because the interaction of device components and characteristics of water flow causes an impact on the flow field, and therefore in extraction of energy. A methodology based on two hypotheses was developed to understand the influence of diffuser over the flow. The hypotheses addressed two areas: effects produced by a strut characteristic of turbine design and influence of the free surface.