WAVE ENERGY

WAVE ENERGY

Introduction

A well-known beach in Kerala is at Kovalam. Not much is, however, known of the fishing village at Vizhinjam, which is about a kilometer away. This fishing harbor is the site of a unique demonstrations plant that converts wave energy into electrical energy that can be exported via the local electricity grid. The plant works on what is known as the OWC principle. Energy is extracted from the system and used to generate electricity by allowing the trapped air to flow via a turbine. The wave power plant at Vizhinjam (Figure 1) has demonstrated that this is possible. The technology of converting wave energy into electrical energy thus exists. In order to appreciate the tremendous progress that has been achieved over the past decade, it would be appropriate to start from the beginning.

Principle

A considerable amount of energy is present in the ocean waves pounding against a breakwater. However, it is not from the breaking waves that the energy can best be extracted. The force of the wind blowing over the ocean’s surface generates waves. The regular breakers seen on most beaches originate at sea and can come from a variety of storms. The water’s surface acts like a great conveyor belt, delivering power from great distances. Waves vary in several ways. They vary from location to location and also from season to season. Where the winds are steady the waves persist for long periods of time. Waves at deep sea can have a shape that is close to a sinusoidal wave. As a first approximation, the most important characteristics of wave are its height H, its period T, and its wavelength L (Figure 2, [Chakrabarti 1987]). The wave height is the vertical distance between a wave crest and an adjacent trough; the wave period is the time it takes to successive crests to pass a fixed point; and the wavelength is the horizontal distance between two crests. The wave shape propagates unchanged in a regular manner with a phase speed c; however, the water particles do not propagate horizontally within wave shape, instead, they move in a circular motion. At the surface, the circular motion has a diameter equal to wave height, but this decreases away from the free surface as shown in Figure 3. An object floating on the ocean’s surface will not be transported very much by the waves. Rather, it will bob up and down and drift back and forth as the waves pass by. Transportation of water from one location to another is primarily due to ocean currents not ocean waves. It is the power of the circular motion that the wave energy device must convert to useful energy.