If you’ve ever been swimming in the ocean, you have some idea of the power of wave energy. Surfers have an even better idea: efforts to catch that perfect wave often result in wipeouts, when the ocean asserts its dominance over the tiny human rider. The enormous size and power of the ocean has given some people hope that wave energy will be the answer to the renewable energy problem.
Wave energy refers to energy obtained from wave power. To understand how the force of the waves can be used as an energy source, we must first understand how waves work. Waves are created by the force of the wind blowing across the surface of a body of water. Wind, in turn, is caused by the uneven heating of air by solar radiation. The warmth of the sun creates warm air that expands and rises, allowing cooler air to rush in and take its place, creating winds. So, indirectly, wave power is another form of solar energy.
There are two different parts of a wave: the highest point is the crest, and the lowest point is the trough. One thing to keep in mind is that waves don’t actually move water, they move energy. Specifically, kinetic energy, which is the energy contained in an object in motion. The water itself moves in a circular fashion, so that if you were to follow a single water droplet through a wave, it would form a vertical circle, thereby moving anything on the water’s surface forward while remaining in the same place. This is the principle that explains why a buoy appears to bob up and down instead of moving directionally.
So how do we convert wave energy into usable form? There are several machines, generally called wave energy converters (WECs) that are used to collect the energy that waves produce. The four main types of WECs are: oscillating water columns, overtopping devices, attenuators, and point absorbers. The first two are what’s known as terminators, and contain a stationary component that is attached to the sea floor or shore. The first, an oscillating water column (OWC), contains air in a pressurized column with only a small passage for release. Waves enter the bottom of the device, pressurizing the air and forcing it into the small passage, driving a turbine. As the wave retreats, the air is depressurized, driving the turbine again. An overtopping device is also classified as a terminator, but works differently from an OWC. In this device, waves wash over the top of a structure and into a reservoir. Eventually, the reservoir fills to a height higher than the ocean, and the potential energy stored in the reservoir drives a turbine, generating electricity.
The next type of WEC is called an attenuator. This is a floating structure that is oriented parallel to the direction of the waves. The floating components are on movable arms, which contain hydraulic pumps. When wave energy moves the floating structure, it drives the hydraulic pumps, which generate electricity. Another floating type of WEC is a point absorber, which can absorb the energy of waves from any direction and does not need to be oriented a particular way. In point absorbers, movement of a floating component drives a piston to generate electricity.
The big question with wave energy is how it compares to other energy sources. As far as renewable technologies go, wave energy has the advantage of reliability, since it can be gathered around the clock, rather than only being available during strong winds or when the sun is out. In addition, there’s a lot of potential out there: the World Energy Council has estimated that if all the ocean’s wave resources were used, we could be producing about two terawatts of electricity a year; nearly double current world energy production. Unfortunately, at this early stage in development of wave energy, costs are high. If the costs of wave energy remain above those of fossil fuels, it may not ever become a contender in the arena of renewable energy. A possible solution is development of efficient WECs that can withstand the harsh environment of the open ocean, but it’s a solution we’re still waiting for.