Point absorbers

As the term suggests, these are systems which are focused on a nodal form of absorbing the energy of the waves to generate electricity. One technology which is on the verge of being market ready is the Interproject Service (IPS) Offshore Wave Energy Converter (OWEC) system. This uses only wave energy and is not affected by tidal or ocean currents (see Fig. 11.1).

The system consists of a floating buoy (A) attached to the sea bed by 'elastic' moorings enabling it to exploit the motion of the waves. The buoy height is 5-6 metres with a diameter of 5-10 metres and requires a water depth of 40-50 metres. Attached to the buoy is a vertical tube or 'acceleration tube' (B) of a length around three times the diameter of the buoy up to 25 m. Within the acceleration tube there is a free moving piston (C) which has a restricted stroke to prevent overloading of the system.

The principle is that the buoy moves vertically against the damping mass of the water in the acceleration tube under the buoy. The relative movement between the buoy and the water mass in the tube is transferred by the piston (D) into the energy conversion system consisting of a hydraulic pumping cylinder which, via a hydraulic motor, drives the generator (E). The OWEC converts 30-35% of the energy of the waves into electricity. It is suggested that there would be clusters of units making up a complete power plant which could generate over 200 kW.

Figure 11.1 Offshore Wave Energy Converter (OWEC) (courtesy of Interproject Services)

The Danish Maritime Institute has developed another form of point absorber which also employs a float connected by a polyester rope to a suction cup anchor. The rope is attached to a hydraulic actuator within the float which pumps fluid into a high pressure hydraulic accumulator. The return stroke is provided by a hydraulic fluid from a low-pressure accumulator. As waves create motion in the float, a pressure difference between the high and low pressure accumulators builds up. The pressure difference drives a hydraulic motor and generator. A system with a 5-6 m diameter would have a rated output of 20-30 kW The ultimate objective is to construct a 10 m buoy with an output of around 120 kW (see Fig. 11.2).

A variation on the theme has been developed by the Energy Centre of The Netherlands (ECN). It is called the Archimedes Wave Swing and consists of a number of air-filled chambers below the surface of the sea. Above these are movable floats in the form of hoods which oscillate vertically with the pressure created by the wave motion. These would be about 20 m in diameter and weigh roughly 1000 tonnes. The top of each float is shaped like a funnel which maximizes the point absorbing effect. As a wave crest moves over the hood, the internal pressure rises. The trapped air is pushed into another chamber and the hood begins its descent. The process is reversed in a wave trough. The vertical motion is converted to rotary action to drive a generator. The system is positioned about 20 m below the surface with the float designed to be in balance with that amount of water above it. At this depth the system is protected from damage from extreme storms (see Fig. 11.3).

The intention is that groups of three floats will be connected so that as a wave progresses it creates a succession of oscillations. It is reckoned that a three-chamber unit will produce about 2.7 MW

The first successful shore-based wave energy device in the UK was installed on the Isle of Islay in the north of Scotland. It was designed by The Queen's University, Belfast, and it works on the principle of the oscillating water column (OWC). Incoming waves enter a concrete chamber and force air into a turbine linked to a generator. The outgoing wave sucks air back through the turbine which is designed to rotate in the same direction under both circumstances. This patented system is called a Wells Turbine. This prototype generated

Figure 11.2 Danish Point Absorber (courtesy of Caddet)

75 kW which went directly to the Grid. Following the success of the prototype, a larger scale version called The Limpet was installed with a capacity of ~450 kW (see Fig. 11.4).

At Port Kembla in Australia a wave energy device which is a variation on the oscillating water column (OCW) principle is now operational. It was designed to be installed against the sea side of harbour walls or rocky peninsulas where there is deep water. A parabola-shaped wave concentrator extends into the sea, amplifying the waves by a factor of three by the time they reach the focal point. There they enter an air-filled chamber, forcing the air forwards and drawing it backwards through an aperture leading to a turbine as the waves arrive and then retreat. The angle of the turbine blades is adjusted via a sensor system so that they rotate in the same direction regardless of the direction of the air flow. This involves a pressure transducer which measures the pressure exerted on the ocean floor by each wave as it enters the chamber. A voltage signal proportional to the pressure is sent to a programmable logic controller (PLC) indicating the height and duration of each wave. The PLC adjusts the blade angle through a series of pinions and planet gears. A motion software program ensures that the information from the pressure transducer is translated into the optimal blade position at any given moment (see Fig. 11.5).

The Port Kembla installation has a peak capacity 500 kW and an output of over 1 GWh/year which is fed to the grid. The economics of the system compare favourably with solar energy and wind power. With refinements to the system the unit price is expected to outclass all competitors. A single installation of this kind has the potential to generate 1000 kW which would power 2000 homes (Caddet). Several sites in Australia such as the Bass Strait and Southern Australia coast have the wave potential to generate up to 1 MW per unit.

Figure 11.3 Archimedes hoods in the raised and lowered positions (courtesy of Caddet)

In the right location, wave energy is more consistent and thus less intermittent energy source than either wind or solar. It causes no pollution and by offsetting the fossil generation saved by the plant, the savings in CO2 are around 790 tonnes/year.

Currently under test in the Orkneys is a snake-like device called Pelamis, which consists of five flexibly linked floating cylinders, each of 3.5 m diameter (see Fig. 11.6). The joints between the cylinders contain pumps which force oil through hydraulic electricity generators in response to the rise and fall of the waves. It is estimated to produce 750 kW of electricity. The manufacturer, Ocean Power Devices (OPD), claims that a 30 MW wave farm covering a square kilometre of sea would provide power for 20,000 homes. Twenty such farms would provide enough electricity for a city the size of Edinburgh. The first commercial Pelamis wave farm is

Figure 11.5 Wave generator, Port Kembla installation (courtesy of Caddet)
Figure 11.6 The Pelamis demonstration machine (courtesy of Ocean Power Delivery Ltd)

being installed off the Portuguese northern coast. It consists of three P-750 units rated at 2.5 MW installed capacity. The Portuguese government has also issued a letter of intent to order a further 30 Pelamis machines subject to the satisfactory performance of the first project.

The UK and Portugal are currently the focus of wave power investment due both to their geography and to the subsidies available for demonstration projects.

Like Scotland, Norway enjoys an enormous potential for extracting energy from waves. As far back as 1986 a demonstration ocean wave power plant was built based on the 'Tapchan' concept (see Fig. 11.7). This consists of a 60-metre long tapering channel built within an inlet to the sea. The narrowing channel has the effect of amplifying the wave height. This lifts the sea water about 4 metres depositing it into a 7500 m2 reservoir. The head of water is sufficient to operate a conventional hydroelectric power plant with a capacity of 370 kW

A large-scale version of this concept is under construction on the south coast of Java in association with the Norwegians. The plant will have a capacity of 1.1 MW As a system this has numerous advantages:

• the conversion device is passive with no moving parts in the open sea

• the Tapchan plant is able to cope with extremes of weather

Figure 11.7 Wave elevator system, the Tapchan

• the main mechanical components are standard products of proven reliability

• maintenance costs are very low

• the plant is totally pollution free

• it is unobtrusive

• it will produce cheap electricity for remote islands.

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