1. Underwater Storage Technologies for Offshore Wind Energy
By-Prince Preet Saini

2. OUTLINE Objectives Offshore wind –current status Key challenges
Energy storage methods
Underwater Pumped Hydro Storage
Underwater Compressed Air Energy Storage
Discussion

3. Objectives
To inform about the underwater storage technologies which can be used to reduce the intermittent nature of offshore wind farms.
How energy storage can help offshore wind energy.

4. Offshore Wind Energy – Current Status
Source- US-DOE, NREL, GLOBAL WIND ENERGY COUNCIL(GWEC)

5. Key Challenges
Cost and Technological risks associated with Offshore wind energy
Intermittent nature of offshore wind energy causes supply-demand imbalance
Offshore wind electricity delivery and grid integration
Increasing understanding of the benefits and cost of offshore wind

6. ENERGY STORAGE METHODS
Underwater Pumped Hydro Storage
Underwater Compressed Air Energy storage

7. UNDERWATER PUMPED HYDRO STORAGE

8. Underwater Pumped Hydro Concrete Sphere

9. HOW THIS WORKS

10. Design concept CENTRALIZED CAPACITY
Several storage units are connected and the power is generated from single turbine & generator.
Additional pipes are needed to connect the units to turbine which leads to more pressure loss in the system.
Lower investment cost for power generating components but additional cost for extra pipelines
DECENTRALIZED CAPACITY
Each unit containing a separate turbine and generator
Higher investment cost for mechanical components
Pipe network containing several tanks not needed.

11. Figure- Construction of a scale model
Source- HOCHTIEF Solutions AG

14. Technical Feasibility Study
Operational depth is 300 m – 700 m which is suitable for present pumping technologies.
Design of the storage vessel is spherical due to optimal pressure distribution.
Turbine capacity (5MW) which is comparable to offshore wind turbine.
All mechanical components are inside the sphere which can be easily retrieved with a remotely operated vehicle.
Reduced corrosion because of low oxygen concentration and low temperature at higher depths.
Big scale spherical construction technologies already exist.
Source- StenSea Project Report

15. Comparison of Underwater PHS with Land based PHS
In underwater pumped hydro storage technology, there is no alteration of existing landscape
The target cost per KW of installed power(without considering the cable cost ) as given by StEnSea is 1238 €/KW as compared to 1300 €/KW for land based pumped hydro storage
UW-PHS can be integrated easily with offshore wind energy farms since it can be installed relatively closer to it as compared to land based pumped hydro storage.
Flora-Fauna-habitat compatibility and aquatic species protection may be an issue in underwater storage.

16. UNDERWATER CAES

18. METHODS OF INTEGRATION
A land based UWCAES system
A UWCAES system located on an offshore platform

19. ENERGY BAGS

20. Comparison of Underwater CAES and conventional CAES
The cost/meter cube of air storage is still high for underwater CAES as compared to conventional CAES.
Geological suitability for underwater CAES is more than 30% as compared to conventional CAES which is less than 5%.
In conventional CAES a portion of volume is unused since the pressure varies as air is supplied to turbine but in underwater CAES , the pressure remains constant due to variable volume energy bags .
Efficiency of mechanical components is higher in underwater CAES.
Underwater CAES requires minimal land footprint as compared to oversized cavern in conventional CAES and is ideal for island nations & coastal regions.

21. DISCUSSION
The demand for energy is going to increase significantly.
Energy production from offshore wind parks is going to increase significantly as we have seen from the offshore wind energy data of European and Asian countries.
The concept of underwater CAES and underwater pumped hydro storage is scalable and efficient and can be integrated with offshore wind to reduce its intermittent nature.
The technology needed to harness and store energy is present.
According to 2010, United Nation Environment Program ,close to 80% of the world population lives closer than 100 km to a sea or an ocean,
That means :
Energy storage close to the sea or an ocean along with offshore wind energy farm can be beneficial.

22. Thank You