The hydrodynamics of air-filled bags for wave energy conversion

The search for an economic means to harness energy from ocean waves continues. This paper aims to summarise the findings from our recent studies of novel wave energy devices in which flexible, deformable structures are used in place of conventional rigid structures. The devices utilise a flexible air-filled bag to capture energy from the waves and three different configurations are compared. In each of the three device configurations, expansion and contraction of the bag in waves create a reciprocating air flow via a turbine between the bag and another volume. The bags are all in the form of a fabric encased within an array of longitudinal tendons. In the first configuration, the bag is floating and ballasted such that it pierces the free surface. In the second configuration, the bag is completely submerged and connected at its top to a rigid float and at its bottom to a weight. In the third configuration, the bag is fixed at its bottom and free at the top. A series of tests at approximately 1:20 scale in the laboratory was conducted to investigate the static behaviour of the bags in still water and their dynamic response in waves. Numerical models are developed for each configuration and the predictions agree closely with measurements. Both reveal some interesting properties that are distinct from one device configuration to another.