Overview
In this group project at NTNU (Norwegian University of Science and Technology), our team of 6 modeled and simulated a gravity-buoy energy storage system in MATLAB/Simulink. The concept: store excess wind energy from turbines in the Flevopolder (Netherlands) by submerging buoyant spheres underwater, then release them to generate electricity for office buildings in Amsterdam.
Concept
The system works on a simple principle:
- Charging — When wind turbines produce excess energy, it powers winches that pull buoyant spheres down to the seabed (1 km depth), storing gravitational potential energy
- Discharging — When energy is needed, the spheres are released and their buoyancy drives generators as they rise, producing electricity
- Transmission — Power is transmitted to Amsterdam office buildings via cables, with Ohmic losses modeled
The net buoyancy force driving power generation:
Simulink Model
The model incorporates:
- Buoyancy physics — Archimedes' principle with seawater density
- Drag forces — Hydrodynamic resistance during ascent/descent
- Cable mechanics — Mass and elasticity of tethering cables
- Power transmission losses — Ohmic losses over the transmission distance
- Energy balancing — Monthly wind generation vs. building consumption profiles
Results
The simulation showed that approximately 40% of the building energy demand had to be bought externally — the gravity-buoy system could cover about 60% of needs. Key losses came from:
- Hydrodynamic drag during sphere ascent
- Cable weight reducing net buoyancy
- Transmission line losses
The system works in principle but has significant round-trip efficiency limitations compared to more established storage technologies.
Results & Discussion
This project was done during an exchange period at NTNU, which gave me exposure to a different academic environment and team dynamics. The Simulink modeling was the most technically valuable part — building a physical system model from first principles (buoyancy, drag, power transmission) and validating it against energy balance requirements. The honest result — that the system underperforms — was itself a useful engineering conclusion.
Technologies Used
MATLAB, Simulink, energy system modeling, buoyancy physics, LaTeX