EE535: Renewable Energy: Systems, Technology & Economics

1. EE535: Renewable Energy: Systems, Technology & Economics Session 7: Wave Energy (2) - Devices

2. Device Types • These devices need to convert the wave motion into electricity • Classifications: • Location • Shore and Bottom Mounted Near Shore Devices • Off-shore Devices • Geometry and Orientation • Terminators (principal axis parallel to the incident wavefront, and physically intercept the waves) • Attenuators (principal axis perpendicular to the wavefront, so wave energy drawn to the device as the wave moves past it) • Point Absorbers

3. Fixed Device: Oscillating Water Column Air out Air in Water rising Water Falling

4. Oscillating Water Column • Partly submerged structure with a chamber having 2 openings • Air trapped in the chamber above the water surface is alternately pushed out of and then drawn back into the chamber • The motion of the air drives a turbine (such as a Wells turbine), which generates electricity • A Wells turbine rotates in one direction, regardless of the direction of low of air

5. Floating Devices: Point Absorber or Buoy • The rising and falling of waves can move buoy-like structures, creating mechanical energy – which is then converted to electricity

6. A wave entering from the left sets the beak of the duck into oscillation The back of the duck is circular to ensure no wave is propagated to the right With very little energy transmitted or reflected there is a very high conversion rate – over a broad frequency band to match the conditions The Salter Duck Duck Motion in Waves Wave Motion

7. Originally envisaged as many cam shaped bodies linked together on a long flexible floating spine Spine oriented towards the principal wave, making the duck a terminator Scale of a duck is circa 0.1λ Benefits of duck design: Can flip over and recover again after an unusually large wave Mooring relatively straightforward due to flexible spine and number of ducks oriented along axis Disadvantages of the duck design: Slow oscillatory motion is difficult to couple to electrical generators Extracting energy from a ‘randomly’ rocking body The Salter Duck

8. The Circular Clam • 12 interconnect chambers arranged around the circumference of a toroid • The chambers are interconnected, separated by Wells turbines. • The chamber is sealed against the sea water by a flexible reinforced membrane • Movement of the sea against the membrane forces air to pass through the Wells turbine- generating electricity http://www.sealtd.co.uk/files/31seaclampart1of7.pdf

9. The Wave dragon Is an overtopping device, which elevates ocean waves to a reservoir above sea level Water is let out through a number of turbines and in this way transformed into electricity The prototype is deployed in Nissum Bredning, an inlet in the northern part of Denmark Offshore devices

10. Tethered Devices • Main body of device floating on the surface but moored to the seabed via a pump Buoy anchor

11. Wavebob • The Wavebob is an axi-symmetric, self-reacting point absorber, primarily operating in the heave mode.  It is specifically designed to recover useful power from ocean wave energy, and to be deployed in large arrays offshore http://www.wavebob.com/how_wavebob_works/

12. Wavebob Key Features • Survivability:  The Wavebob is an axi-symmetric buoy structure on slack moorings which makes it inherently sea-worthy.  Its ability to de-tune in seconds is vitally important in a resonating energy absorber.Response to long period and high waves: Unlike all other self-reacting heaving buoys, the Wavebob’s natural frequency may be set to match the typical ocean swell (Atlantic 10”, or Pacific 15”), facilitating good energy absorption. It can ride very large waves and still recover useful power.Tuning and control: The Wavebob has exceptional facilities for almost instantaneous tuning and longer period adjustment of natural frequencies and bandwidth. On-board autonomous control is a feature, and there is considerable scope for intelligent systems, for example individual units co-operating in arrays. These are highly significant attributes in changing wave climates, so typical of the North Atlantic.Accessibility: The outer torus has a diameter of the order of 20metres, and an overall height of 8 metres, allowing adequate space for the power train and control systems below decks.  As a large floating structure, Wavebob is relatively stable in all but the most severe storms. Low operating and maintenance costs, high availability:  O&M costs have a massive bearing on the costs / kWh delivered. Only well-proven and standard marine hydraulic components and generators are installed.  The Wavebob typically carries three or four motor-alternator sets, all or some of which may be entrained, depending on incident wave energy. In-built redundancy facilitates remote switching and high availability when weather conditions might preclude maintenance visits. The main device remains on site (for up to 25 years), with individual components being replaced and taken ashore for servicing as necessary.Low capital costs: The main hull structures will be assembled from smaller pre-cast and extruded concrete units manufactured using widely available and standard processes. There is no requirement for deep water facilities or dry docks. The main hull structures would be towed to site and attached to prepared moorings.High power output:  Average electrical power 500kW and greater is expected from North Atlantic sites.  Power output will be synchronous with low VAr http://www.wavebob.com/how_wavebob_works/

13. Wave Power Imperatives • Survival: Especially challenging for the North Atlantic. We design for the 100-year extremes, the greatest hazard being a freak ‘wall-of-water’ presently estimated to be ~24metres.  Certification by Det Norske Veritas or similar is necessary for marine insurance.  Fail-safe modes are essential during extreme events and breakdowns (eg failure within the device or of grid connection). • Deep water: Ocean waves lose energy and become steeper as the water shoals; losses become significant as the depth becomes less than half a wave-length.  The North Atlantic energy ‘hot spot’ West of Ireland is centred on ~178 metre wavelengths, ie longer wavelengths are important. The equivalent off West Coast USA is over 300 metres.  As might be expected, the bathymetry is ideal in the regions mentioned above, deep water is available within a few kilometres of the shore. • 25+ year life on site: The main hull structures should be capable of remaining on site for at least 25 years, and be readily decommissioned thereafter.  The costs of recovering and re-deploying a device at any intermediate stage should be avoided completely. • Self-reacting point absorbers: Oscillating systems capable of resonant energy absorption have been the subject of a great deal of attention since the 1970’s.  The theory is now well established but, until recently, a number of technical challenges limited the prospects of commercial success.   Self-reacting point absorbers have two advantages, - independence from the sea-bed (other than slack moorings and grid connection) thus minimising installation and maintenance costs and, secondly, if axi-symmetric, can respond to waves from any direction. • Arrays:  The energy density of ocean waves is considerably greater than wind and consequently closer spacing is possible.  Theoretically defined by each unit’s absorption or capture width, in practice an array layout will be dictated by moorings (slack, for self-reacting devices), the prevailing wave direction, and foreshore consents.   http://www.wavebob.com/wave_power/wave_power_imperatives.php

14. Wave Power Imperatives • Tuning and control: Ocean waves are typically a mix of wind-waves and swell. Most of the time the wave climate is far from regular, and varies very significantly.  North Atlantic wave periods and wave heights can more than double within 24 hours as depressions pass over.  It is essential that any commercial device will have autonomous control (on-board ‘intelligence’) allowing it to tune to changing conditions and to maximise useful power output.  An ability to vary bandwidth is desirable. • Significant installed capacity:  Installed capacities should be greater than 1MW, otherwise per unit costs of moorings, grid connection, operations and maintenance become excessive.   • Power capacity: The amount of wave energy that an oscillating system can in theory absorb is a function of the prevailing wavelength and the oscillating mode(s).  For a North Atlantic site the theoretical limits are well above 1MW, averaged across the expected distribution, ie there are many occasions when the theoretical limit is much higher. A good point absorber, if ‘run backwards’, becomes a good wave generator.  To do so requires that a suitably large volume of water is displaced each wave period, and that is a function of water-plane area and stroke length. • Fabrication:  Low cost / long life / low maintenance materials such as concrete are to be preferred over steel or polymers, other things being equal.  Similarly, any need for large dry docks, deep water harbours, jack-up barges, etc., will add to costs and limit the number of suitable facilities for the construction and deployment stages. • Cost / kWh: This is a matter of minimising costs (capital, opex) and maximising useful electrical power delivered to the grid.    • Health and safety:  Although not expected to carry permanent crew or volatile hydrocarbons, access for routine and un-planned on-board basic maintenance requires clear procedures.  The installed equipment should be safely housed and accessible above the water-line.  Boarding and dis-embarking via a rib, small service craft or helicopter should be well within acceptable standards up to at least Force 5.  The device must be capable of being switched remotely to a non-operational safe mode, and of failing safe. http://www.wavebob.com/wave_power/wave_power_imperatives.php

15. Offshore devices: The McCabe Wave Pump

16. Offshore devices • The McCabe Wave Pump • The device consists of three rectangular steel pontoons, which are hinged together across their beam • The MWP was primarily designed to produce potable water although it can also be used to produce electricity • A 40 m long prototype was deployed in 1996 off the coast of Kilbaha, County Clare, Ireland

17. Offshore devices • The Pelamis • Is a semi-submerged structure composed of cylindrical sections linked by hinged joints • The wave induced motion of these joints is resisted by hydraulic rams which pump high pressure oil through hydraulic motors via smoothing accumulators • The hydraulic motors drive electrical generators to produce electricity

18. Offshore devices • The Pelamis • Several devices can be connected together and linked to shore through a single seabed cable • A typical 30MW installation would occupy a square kilometre of ocean and provide sufficient electricity for 20,000 homes

19. Economics • Reducing operation and maintenance costs is key to successful economic implementation of wave energy stations • Capital costs per kW of wave energy estimated to be double that of fossil fuel installations • Load Factor (average power divided by peak power) lower than conventional due to variability of the wave climate • Wave energy costs can only be competitive if running costs are significantly lower than for a conventional station • Fuel costs are zero, therefore operation and maintenance costs are determining factors

20. Question • A typical efficiency of a wave energy device is 30% • If the average annual electricity consumption in Ireland is 26000 GWh, what is the required scale of a wave energy device to meet this demand? (Giga = 109) • Assume storage capability etc are already in place

21. http://www.emec.org.uk/