An Integrated Energy Storage System
Wind speed is unpredictable and variable such that the power output from wind turbines often does not coincide with demands from the national grid. In the UK, constraint payments are made to wind farm owners when the turbines are shut down because of lack of demand for their power. Clearly the wind farm owner would wish to sell any energy generated whatever the demand and also be able to deliver higher power if necessary on demand. Here a novel wind–tidal integrated storage power generation system is described that addresses these issues.
By Mike Lewis, RGL Associates, UK
Matching Output to Demand
There is increasing interest in storage devices that can buffer energy during times when the wind is blowing and there is little demand and allow that energy to be released when demand is present. By contrast, tidal energy is predictable throughout the year but still has the disadvantage that differential heads across water turbines that give the highest power outputs may not be in phase with demand. Storage devices are essentially energy supply shifters.
Energy Storage Devices
Many types of storage device for higher energies are under development or in use (see Figure 1), including compressed air energy storage (CAES), hydrogen storage, synthetic natural gas (SNG) and pumped hydro storage (PHS). Lithium-ion and sodium–sulphur batteries are currently limited to less than 0.1GWh energy capacity whereas capacity above 1GWh is ideally required for power generation plants. Conventional pumped hydro storage capacities of up to 30GWh are in operation that can provide 1GW of power over a period of one day.
Integrated Storage System Development
In developing the concept for a wind–tidal integrated storage power generation system, pumped hydro storage was selected because of its maturity and the availability of seawater in an offshore environment that combines energy input from wind and tide. Thus a three-tank system was selected in which a high tank is filled by the flood tide while a low tank remains at low level and is emptied by the ebb tide (Figure 2). The store tank is capable of storing seawater to a total depth of about 30 metres above low tide, being filled by the following means:
- Flow from the high tank to the low tank via bulb water turbines that generate electricity (typically 10 × 25MW)
- Flow from the high tank to the store tank via electric volute pumps powered by the water turbines in the high tank/low tank wall (typically 10 × 25MW)
- Flow from the high tank to the store tank via electric volute pumps powered by the wind turbines (typically 10 × 25MW)
Pumps, bulb turbines and Kaplan turbines are selected from existing applications that have proven reliability in order to reduce project risk.
System Arrangement
The performance of a system consisting of a 36km2 lagoon comprising high, low and storage tanks with 30 wind turbines of 8MW output (240MW total) was studied for sea locations with maximum tidal ranges of about 10 metres. The tank infrastructure is constructed using a new technology in which modular sections of seawall are fabricated at a coastal location, floated to the installation site, up-ended, connected to the previously installed section and then self-piled. This process allows large diameter tanks to be constructed offshore at a cost that allows electricity to be generated at a price that is competitive with alternative sources of low carbon power.
Tank Construction
The tank walls are periodically interrupted with foundations for the wind turbines that have an adjacent mounting for a crane that is used in installation and maintenance. This removes the necessity for specialist installation vessels apart from those delivering components that can be stored on site during construction. The tank walls also have a road for vehicular access.
Matching the Demand
Depending upon demand for electrical output, the wind turbines may be used to fill the store tank or provide power to the national grid. Electrical demand is supplemented by draining of the store tank through Kaplan water turbines to the low tank. The integrated storage system therefore acts to match power demand from the national grid to periods of high wind power generation when demand is low.
Energy Capacity
The energy capacity of the store tank is about 15TJ, equivalent to 4.2GWh, though larger and higher store tanks may be selected to give higher energy capacity but would take longer to fill. Typically 4 × 75MW high head Kaplan water turbines would drain the store tank, allowing the total power output to be tripled over some hours compared to the wind turbines alone by utilising the 10 × 25MW low head bulb water turbines and 4 × 75MW high head Kaplan water turbines. Annual energy generation is in excess of 1TWh for typical wind speeds encountered in the Irish Sea with approximately 50% being contributed by tidal range power.
Tank Levels
In Figures 3 and 4, tank levels and heads are illustrated over a week together with export power assuming that demand occurs from 06.30 to 20.30. At other times, wind and tidal energy is used to fill the store tank. Export power is maximised during the day when demand is present from the total of that from wind turbines and Kaplan water turbines.
Flexibility of Operation
Electrical interconnectivity of the various wind turbines, water turbines and volute pumps allows a strategy to be set up for filling of the store tank and/or power delivery to the grid depending upon the tidal conditions and anticipated wind speed. Thus the integrated storage system can be optimised on an hour-by-hour basis to maximise revenue and output to the grid.
Comparison with Other Tidal Systems
The proposed Swansea Bay (UK) tidal lagoon develops power four times per day with the bulb turbines isolated on the flood tide until there is sufficient head from sea to lagoon. Gates then open to flood the lagoon and power the bulb turbines. A similar situation arises on the ebb tide where flow from the lagoon is held until the sea level has fallen. Power is developed over about 14 hours per day. The Sihwa, South Korea, tidal lagoon develops power on the ebb tide only, with gates holding the level in the lagoon until sufficient head is developed for the bulb turbines to operate. It has sluices to fill the lagoon on the flood tide. Neither system therefore has the capability to store energy.
Addressing the Issues
This integrated energy storage system seems to address all the problems experienced by the current generation of renewable energy technology and provides a flexible source of renewable electricity when it is needed. The economic benefit of the system is also enhanced by the fact that the tank infrastructure has a service life of up to 120 years, while assets such as wind turbines may be upgraded when they become uneconomic to repair. The above benefits mean that this system offers a major advance in power generation from renewable sources.
Biography of the Author
Mike Lewis worked at the National Centre of Tribology for over 40 years before forming RGL Associates in 2015. He specialises in failure investigation of plant and machinery of all types, but more recently he has focused upon the performance, life and reliability of rolling element bearings in wind turbines. He has a particular interest in renewable energy. His qualifications are CEng, MIMechE, BSc (Mech Eng) and MSc (Tribology).
