A novel buoy design for hybrid buoyancy-gravity energy system with integrated compressed air energy storage: Theoretical analysis of drag and efficiency

The current study investigates the performance of a buoyancy based-power generation system combined with compressed air energy storage, utilising a novel buoy frame. The novelty of the current study is the buoy's unique design and working mechanism, in which a flexible bladder filled with compressed air is enclosed within a rigid body frame. This integrated rigid–flexible configuration differs from previously reported buoyancy-based CAES devices. During the ascending phase, the inflated bladder displaces water from the frame and drives the buoy upward; upon reaching the upper level, the compressed air is released, causing the buoy to descend as water fills the space between the rigid frame and the flexible bladder structure. The system performance was evaluated based on theoretical calculations with key parameters such as buoy drag force and its velocity at the top and bottom of the water column determined using numerical simulations. Various frame designs were developed and tested to minimise the drag force. Drag evaluation showed that a curvature frame with the parabolic curve in the power of 0.75 resulted minimum drag force. The buoy's displacement in a viscous fluid (pure water) was simulated using the dynamic mesh method, and the unsteady characteristics of the flow field were analysed. Based on the numerical simulation, the drag force causes energy losses of 13.5 % during ascending motion and 8.1 % during descending motion. These losses are found to be more significant than the energy loss due to the residual kinetic energy and unreleased energy in compressed air. The proposed buoy design achieved an efficiency of 64.4 %, demonstrating promising potential to integrate buoy-based system with the compressed air energy storage system. However, further experimental investigations are necessary to validate and optimise the system for practical applications. • Proposed novel buoy design with flexible bladder for hybrid energy system. • Parabolic-shaped buoy (power 0.75) minimized drag force. • Energy loss analysis showed drag as main contributor (21.6 % total). • Achieved 64.4 % efficiency with reduced drag during ascending/descending phases.