Offshore energy storage in deep water for floating wind turbines

Introduction

Belgium has been considering offshore energy storage next to the Thorntonbank wind farm by building an artificial lake. The water in the lake is pumped up to a higher level than the sea level. That way potential energy is stored. In this abstract another type of potential energy storage is proposed: compressed air in deep water. The thermodynamics of air compression and expansion requires special attention. But it is a more dense energy storage than an artificial water lake, and probably also cheaper to build.

Approach

A standard onshore pressure vessel is often made of steel sheet metal with good tensile strength to hold the compressed air. It is almost impossible to build a reservoir of thousands of m³: the vessel wall simply won’t hold.
At 100 m below sea level a static pressure of about 10 baro is present. There are 2 ways of building a pressure vessel in deep water, avoiding the requirements of strong walls: either rigid or flexible. The rigid type looks like a bell-jar. It can for instance be made of a thin walled vertical tube of a large diameter. Compressed air is blown in from a pipe, pushing away the water at the open lower end of the tube. The flexible type is a closed balloon: compare it with inflating a rubber boat at the bottom of the sea. In both cases, rigid or flexible, the pressure vessel does not need strong walls, as the compressed air is in isostatic balance with the deep water pressure. A compressor and an expander at sea level will take care of the energy conversion.


Main body of abstract

There are buoyancy forces pushing the pressure vessel up. Archimedes taught us this force is proportional to the volume of the vessel. Ballast is needed to keep it down. The pressure vessel has to be enclosed by a ballast frame, or a frame attached to ballast. And this is the only reason why the vessel needs some strength. Fortunately, the volume of air at 10 baro (11bara) is 11 times smaller than atmospheric air. The deeper under water the vessel, the smaller the buoyancy forces.
It can be calculated that it takes about 242 kiloJoule for isothermal compression of one atmospheric m³ of air to 10 baro (11 bara). And as it is compressed it requires a space of about 0,09 m³ only. Compared to the artificial lake, assuming the level difference between the lake and the sea is 10 meter, 1 m³ of water in the lake has a potential energy of only 100 kiloJoule. Hence the energy density per m³ is about a factor 27 better for the air.
Isothermal compression/expansion requires excellent heat exchange. Several techniques are possible. They can all rely on the enormous thermal inertia of the sea water. The sea won’t heat: the specific heat of air is several orders of magnitudes smaller than the specific heat of water.


Conclusion

If an artificial lake for potential energy storage is worth considering, then compressed air storage may be a solution for deep water. It is a denser way of storing energy, and the cost of the infrastructure will probably turn out lower. The energy conversion requires special attention: isothermal state of change of the air is favorable.


Learning objectives
Delegates get a new pespective on energy storage, using not so complex techniques or materials. A pilot project for floating wind mills would be interesting.