Device for converting ocean wave energy
by J Falnes, Department of Physics, NTNU, Norway

A new device, developed in Norway, harnesses energy from the ocean waves using a vertically oscillating float connected to a piston pump. Possible applications range from desalination of seawater and clean water supply to fish farms, to electricity production using a turbo-generator. This wave energy conversion unit is being developed by the Norwegian company ConWEC AS.

Background

Some of the solar energy received by the Earth is converted to wind energy, and some of this energy is converted to wave energy when the winds blow over the oceans. Each of these conversion steps results in a concentration of the energy flow. Averaged over the seasons and over nights and days, solar power/m2 of ocean surface is typically 100­250 W, depending on factors such as local climate and geographical latitude. The average flow of wind power/m2 of envisaged vertical area in the air is about 0.5 kW. Just below the ocean surface, the corresponding figure for wave power is typically 2­3 kW. This phenomenon is an important incentive for developing practical methods to utilise ocean-wave energy.

During the last 25 years, Norwegian specialists have made significant contributions to the science of wave energy. Research carried out at the Norwegian University of Science and Technology (NTNU) has focused on methods of controlling oscillatory motion for maximum capture of wave energy. The development of a particular system named "ConWEC" (Controlled Wave-Energy Converter) began in 1994 in co-operation with the Norwegian company Brødrene Langset AS. In 1998, a new company, ConWEC AS, was set up to pursue further technical development, demonstration and marketing.

The ConWEC device absorbs wave energy by means of a float (see Figure 1) which oscillates up and down inside a cylindrical structure with a submerged opening through which waves can act. The energy in the waves is converted to mechanical energy by a pump with its piston rigidly connected to the float. Seawater can be pumped into an energy storage in the form of a pressure tank (or possibly an elevated water reservoir) from which a turbine can be run to complete the conversion to electrical energy. Alternatively, a hydraulic fluid other than seawater may be used if the hydraulic system is a closed loop.

Figure 1: Operating principle.

Maximum energy capture is obtained in two ways: a control system latches the float during certain time intervals of the wave cycle to achieve optimum timing of the float motion; and control means are used to obtain optimum flow of hydraulic fluid into the energy storage reservoir.

The ConWEC device comprises a compact unit; the piston pump, valves and latching means are arranged inside the float, which is easy to remove for maintenance or replacement. During the design work, care has been taken to optimise the various components, and to minimise energy losses due to friction and viscosity. This concerns the hull and the float, as well as the pump, seals, valves and piping. Future maintenance requirements have also been taken into consideration in the design.

In Trondheim, experimental scaled-down models with float diameters of 0.14 m and 0.44 m have been tested inthe laboratory and in the sea, respectively. Full-scale ConWEC units are expected to have float diameters between 3 m and 8 m, corresponding to power levels from 10­300 kW, in a wave climate such as in the North Sea.

Applications and markets

The ConWEC converts wave energy to potential energy in a fluid under pressure. As mentioned earlier, electricity can be produced by draining the fluid through a turbine which drives an electricity generator. However, some potential applications can use the wave energy more directly, such as pumping of the fluid in a refrigeration plant, and pumping of clean seawater to fish farms or to polluted lagoons or other sea areas which are more or less isolated from the open ocean. Many coastal areas are in need of fresh water, which may be produced by desalination of pressurised seawater through the process of inverse osmosis.

In the short term, the potential market is in those coastal societies that can profit from utilising some of these direct applications of pressurised seawater (or other hydraulic fluid). Further markets are coastal societies which are not connected to a national electricity grid, but have to rely on expensive diesel-generation for their electricity supply. Globally this represents a substantial market.

For more information contact the CADDET Norwegian National Team at Rud.

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