The performance of a new type of flow energy harvester based on oscillating foils is investigated through numerical modeling by using two methods, a 2D thin-plate model and a 3D nonlinear boundary-element model. The fluid–structure interaction problem involved in the dynamics of a heaving/pitching foil coupled with an actuation/energy harvesting system in this device is examined. The 2D analysis allows us to simulate dynamics of the flapping-foil system over a large range of parameters and to identify areas of special interests (e.g., high energy output or high efficiency). In the vicinity of these areas the 3D model can accurately predict the performance of the system. By examining the power extraction capacity and efficiency of the system at various geometric, mechanical, and kinematic parameters, the optimal performance of the system is determined. In addition, the performance is found to be enhanced by the presence of a solid ground, as well as the thickness of the foil (at certain frequencies).