The combination of processing technologies, low power circuits and new materials integration makes it conceivable to build autonomous integrated systems, which would harvest their energy from the environment. In this paper, we focus on mechanical energy harvesting using piezoelectric materials integrated into flexible substrates. Modeling of these devices is important for the evaluation of their transduction efficiency. Other than classical thin piezoelectric layers, a composite layer using piezoelectric nanostructures is considered in order to evaluate its performance in a bending configuration.