Marginal Column

Using the power of the seas

July 2012

The oceans harbor massive energy potentials, just by virtue of their ceaseless motion. But how can this marine energy be put to use?

 
 

The concept

The tides, marine currents, waves, marine heat, osmosis – the term “marine energy” refers to forms of energy that are highly diverse and fully independent one of another. They have just one thing in common. In contrast to other renewable energies like the wind and the sun, they are available at all times of the day and night. Thus marine power plants are suitable for supplying the base load to power grids. The potentials for exploiting the various kinds of energy do vary, however. From today’s point of view, the most promising energy sources are the tides, currents and waves.

The technology

The so-called marine energy converters are to make this energy available for use. These converters comprise a mechanical structure which picks up the motion and a generator that converts mechanical to electrical energy and makes power available at the system’s output. The generator assembly may also include additional functions such as frequency matching, limiting, smoothing, storing, and regulating the energy absorbed. Marine use places severe demands on these components, which have to achieve:

  • Long service lives, as long as thirty years
  • Uninterrupted availability
  • Extended service intervals, of up to five years
  • Protection against ambient conditions (such as saltwater)
  • High degree of efficiency, even over widely varying load ranges

The concepts

A wide variety of energy converters may be employed to utilize the various energy sources. The most important are introduced below.

Tidal and marine current generators

 
llustration | design hoch drei GmbH & Co. KG

Marine current power plant

Since this technology is relatively new, there are now about fifty different mechanical concepts for this field. Most of them are based on a proven technology: wind turbines. The difference here is that constant marine currents or the tides drive the rotor. These units are, however, far smaller than their wind-driven predecessors.

This is due to water’s greater density, which can develop very large forces even at low current speeds. These forces are also used to drive the Morild II, a floating tidal water turbine in the Lofoten Archipelago, off the northwest coast of Norway. This facility, one of the largest of its kind, provides installed capacity of 1.5 megawatts. Its turbines are 23 meters in diameter.

Wave power plants

 
llustration | design hoch drei GmbH & Co. KG

Wave-driven power plant

These power plants utilize the vertical movements of the waves. Extracting electrical energy from waves is especially challenging because, as opposed to relatively uniform underwater currents, the energy potentials in waves fluctuate considerably. During storms, for instance, they can develop major destructive force. As a result there are many different development vectors being pursued to identify the perfect technology. In one version, bodies floating on the surface of the ocean – like buoys or the so-called “sea snakes” – are being tested. Here hydraulic cylinders capture the motion and convert it into a flow of fluid. In other designs, converters anchored on the seabed are being used.

One promising concept has been developed by Wave Star, a Danish company. The waves pass through the entire length of the system, where they raise and lower floats, and this oscillating motion is used to generate electricity. A third approach is found in power plants installed along the coastline. Pneumatic chambers are one example. The surging waves flow through pipes into a chamber. The column of water, as it rises and falls, passes air through a wind turbine mounted at the upper end, and it drives a generator to produce power.

Prospects

Many systems are currently in the test phase. Rexroth is a highly experienced partner that can provide exactly the right drive solutions for a wide variety of energy conversion units. In marine current power turbines, for instance, Rexroth hydraulic technology reduces the number of components used underwater to just a single rugged pump, which pumps fluid to a motor and generator set located above the waterline. This simplifies maintenance and lowers operating costs. Over the long term, only a few of today’s system concepts will prove to be marketable. They are those which score well in regard to the decisive success factor: the cost for the electricity. And that is determined by the investment costs, service life, efficiency, and availability.