Wave energy converters

Introduction

Vast and reliable, wave power has long been considered as one of the most promising renewable energy sources. Wave Energy Converters (WECs) convert wave power into electricity. Although attempts to utilize this resource date back to at least 1890, wave power is currently not widely employed (Miller, 2004). The plethora of innovational ideas for wave power conversion have been invented in the last three decades, resulting in thousands of patents over recent years. At present, a number of different wave energy concepts are being investigated by companies and academic research groups around the world. Although many working designs have been developed and tested through modelling and wave tank-tests, only a few concepts have progressed to sea testing. Rapidly decreasing costs however, should enable wave plants to compete favorably with conventional power plants in the near future (Pelc and Fujita, 2002).

Classifications

Figure 1: DEXA concept

Wave Activated Bodies

Wave activated bodies (WABs) are devices with moving elements that are directly activated by the cyclic oscillation of the waves. Power is extracted by converting the kinetic energy of these displacing parts into electric current. One example of such a WAB, is made by a single floater connected to a linear magnetic generator fixed to the seafloor. In other cases, only parts of the body are fully immersed and dragged by the orbital movements of the water. In order to maximally exploit this resource, the moving compounds need to be small in comparison to the wave length and preferably they are placed half a wavelength apart. For these reasons, wave activated bodies are usually very compact and light. The main disadvantage of this type of wave energy converters is the high cost of the power generator needed to convert the irregular oscillatory flux into electricity.

The "DEXA", developed and patented by DEXA Wave Energy ApS , is an illustrative example of a WAB. The device consists of two hinged catamarans that pivot relative to the other (Fig. 1). The resulting oscillatory flux at the hinge, is harnessed by means of a water-based low pressure power transmission that restrains angular oscillations. Flux generation is optimized by placing the floaters of each catamaran half a wavelength apart. A scaled prototype (dimensions 44x16.2m). placed in the Danish part of the North Sea should generate 160 kW (Martinelli et al., 2009). Full-scale models are thought to be able to generate up to 250 kW.

Figure 2: Different concepts of oscillating water columns

Oscillating Water Columns

The function of the oscillating water columns (OWCs) is very similar to that of a wind turbine, being based on the principle of wave induced air pressurization. The device is set upon a closed air chamber, which is placed above the water. The passage of waves changes the water level within the closed housing and the rising and falling water level increases and decreases the air pressure within the housing - introducing a bidirectional air flow. By placing a turbine on top of this chamber air will pass in and out of it with the changing air pressure levels. There are two options to separate the bi-directional flow: a Wells turbine to create suction or alternatively, pressure generating valves (Kofoed and Frigaard, 2008). OWC devices can be moored offshore or be placed on the shoreline where waves break.

An example of an offshore OWC is the "Sperboy", developed and patented by Embley Energy LTD . It is circular in plane and therefore invariant to wave direction (Fig. 2). Its size varies according to the target sea conditions at the deployment site but maximum dimensions are set at 30m diameter, 50m height and 35m draft. Up to 450 kW mean annual output can be obtained from this concept. An inshore example is the resonant wave energy converter REWEC-3, created by the Università degli Studi "Mediterranea" di Reggio Calabria (Fig. 2). It operates much like conventional concrete caisson breakwaters but here, each caisson is fitted with a Wells turbine. Efficiency of these devices is generally considered to be high (Boccotti, 2003).

Figure 3: Different overtopping devices - Wave Dragon (Above) and SSG (below)

Overtopping Devices

Another type of Wave energy converter is the overtopping device, which works much like a hydroelectric dam. The Wave Dragon created by Wave Dragon ApS is an example of an offshore overtopping device (Fig. 3). Its floating arms focus waves onto a slope from which the wave overtops into a reservoir. The resulting difference in water elevation between the reservoir and the mean sea level then drives low-head hydro turbines. Proposed optimal size design of 260m width and 150m length will produce 4 MW. In wave climates above 33 kW/m, this technology is expected to be economically competitive with offshore wind power in the near future. After a combined cost saving and power efficiency increase, the power price will eventually be in line with costs of fossil fuel generation (Christensen et al., 2005).

Near shore, OVTs can be installed in front of or as part of caisson breakwaters. The Norwegian company WAVEnergy is developing an integrated multi level overtopping device named the SeaWave Slot-Cone Generator (SSG) (Fig. 3). The SSG has the advantage of harvesting wave energy in several reservoirs placed above eachother, resulting in high hydraulic efficiency. The reservoir capacity smooths out the irregularity of incoming waves, providing a regular electricity output to the grid. Additionally, with the turbine shaft and the gates controlling the water flow, SSG is built as a robust concrete structure with few moving parts in the mechanical system. This most likely makes it a low maintenance, durable system. Other SSG designs can be deployed onshore or offshore.

Figure 4: The FO3 point absorber (above) and the Wave Star attenuator

Point absorbers and Attenuators

Point absorber are buoy-type WECs that harvest incoming wave-energy from all directions. They're placed offshore at or near the ocean surface. A vertically submerged floater absorbs wave energy which is converted by a piston or linear generator into electricity. One such a point absorber WEC is the FO3 concept developed by Norwegian entrepreneur Fred Olsen. It consists of several (12 or 21) heaving floaters attached to a 36 by 36 meter rig (Fig. 4). By means of a hydraulic system, the vertical motion is converted into a rotational movement that drives the hydraulic motor. This motor in turn powers the generator that can produce up to 2,52 MW (Leirbukt and Tubaas, 2006).

Comparable, the attenuator type WEC Wave Star, developed by Wave Star ApS, has a number of floaters on movable arms (Fig. 4). The energy of the motion of the arms is again captured in a common hydraulic line and converted into electric current. Most noticeably, being able to raise the entire installation along its pillars, this system has a high endurance for rough storm conditions. So far, this method has not been deployed at full scale. A 1:2 scaled installation has been built at Hanstholm which turns out 600 kW. However, production is thought to be scale-able up to 6 MW (Bjerrum, 2008). A major benefit of these types of exploitation is the minimal contact with water, placing any delicate machinery and electrics out of reach of any corrosion or physical forcing of the waves.

Challenges

Designs are quite different from WEC to WEC, mainly due to differences in energy harvesting and subsequent conversion (Power Take-Off). Nevertheless, each design faces the same challenges. They should be optimized to effectively extract wave energy under most wave conditions while used materials are to withstand the classical problems of marine technologies i.e. corrosion, fatigue, biofouling, impact loading and fractures. The classical protection measure against fouling and corrosion of steel structures is regular maintenance and repainting. But this is time-consuming and costly. In addition, the use of antifouling paints may be detrimental to the marine environment (e.g. Tributyltin paints). Fullscale devices out of concrete (e.g. Dexa, WaveStar, SSG, REWEC3, Wave Dragon) may provide a valuable alternative since concrete is long-lived if properly mixed (Metha, 2001).

Crucial for any design is the mooring which ensures a maintained position under both normal operating loads as well as extreme storm load conditions. It shouldn't exert excess tension loads on the electrical transmission cables and ensure the suitable safety distances between devices in multiple installations. Most commonly, a free hanging catenary configuration is used for mooring but multi-catenary systems and flexible risers are not infrequent as well. Every configuration should be sufficiently compliant to accommodate tidal variations and environmental loading while remaining sufficiently stiff to allow berthing for inspections and maintenance. Finally, the system should be capable of lasting 30 years or more. A tall order as demonstrated by the mooring of the Wave Dragon that failed during a severe storm on the 8^th of january 2004.

Wave energy converters as a coastal defense technique

The design of coastal defending WECs has to be ideally optimized to reflect and/or absorb a significant part of incident wave energy under all wave conditions, especially when presented with rough conditions. Unfortunately, the geometry of the layout which maximizes wave attenuation is yet to be determined. Existing WECs such as the Wave Dragon and the DEXA devices are being investigated as coastal protection measures. The DEXA devices are small and therefore might be cost effective. Placement in shallow waters may reduce the transmitted energy in a differential way which could alter the coastal morphology (Ruol, 2010; Zanuttigh, 2010)

References

The main author of this article is De Rijcke, Maarten Please note that others may also have edited the contents of this article. Citation: De Rijcke, Maarten (2011): Wave energy converters. Available from http://www.coastalwiki.org/wiki/Wave_energy_converters [accessed on 25-11-2024] For other articles by this author see Category:Articles by De Rijcke, Maarten For an overview of contributions by this author see Special:Contributions/MaartenDeRijcke