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^[1]). 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^[2]).

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 ^[3], 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^[4]). placed in the Danish part of the North Sea should generate 160 kW (Martinelli et al., 2009^[5]). Full-scale models are thought to be able to generate up to 250 kW. ^[3]

Oscillating Water Columns

Figure 2: Different concepts of 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^[6]). 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 ^[7]. 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^[8]^[9]).

Overtopping Devices

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

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^[10] 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^[11]).
Near shore, OVTs can be installed in front of or as part of caisson breakwaters. The Norwegian company WAVEnergy^[12] 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.

Point absorbers and Attenuators

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

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. 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^[13]). Comparable, the attenuator type WEC Wave Star, developed by Wave Star ApS^[14], has a number of floaters on movable arms. 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.

Mooring