Ocean energy technology – a crowded field

Ocean renewable energy systems fall into one of three options:

• Wave energy conversion;
• Tidal energy conversion;
• Ocean thermal energy conversion (OTEC).

Each of these is vastly different from the others – and they are related only in that they all seek to extract energy from the oceans.

Wave energy systems

Wave energy systems, such as the Manchester Bobber pictured on page 34, are the most varied in their designs.

Fundamentally, they convert kinetic energy into electricity by capturing either the vertical oscillation of waves (as with point absorbers and attenuators) or the linear motion of waves (as with overtopping devices and the oscillating water column). These wave capture techniques can then be, to some extent, “mixed and matched” with other design elements, resulting in a large number of actual technology options (see table 1).
Individual devices range in size from less than 100 kW to about 2 MW. As with windpower, wave farms will need to be developed further to take advantage of economies of scale.

Tidal energy systems

Tidal energy systems are characterised differently, and have fewer fundamental design considerations (see table 2 ).

Tidal barrage systems are actually in commercial operation in a few locations, but most people consider their potential to be limited, not because the resource is scarce, but because of the environmental impacts of blocking off large estuaries.

Tidal and marine current technologies can best be described as underwater turbines or propellers that are driven by flowing water. These technologies are also sometimes called kinetic hydro or in-stream hydro, and can also be deployed in streams and rivers.

OTEC marine power

Ocean thermal energy conversion (OTEC) takes advantage of ocean temperature gradients greater than 20°C (36ºF). Where such gradients exist between surface waters and waters no more than 1 km deep, they can be used to extract thermal energy stored in the ocean, and convert it to electricity (and often, desalinated water).

Inherent simplicity of systems

Unlike wind or solar PV energy, which increasingly rely on sophisticated technologies and advanced materials, most ocean renewable energy systems are inherently simple. They are made mostly out of steel and concrete, and companies that develop the technologies can draw upon decades (if not centuries) of experience in building and placing man-made devices in the oceans, such as offshore oil & gas platforms and measurement and navigation buoys. True, ocean systems do require some unique electro-mechanical systems to convert the energy of the oceans into electricity, but most of these rely on well proven technologies, such as hydraulic rams, low-head hydropower turbines, and impellers.

Rather than focus on size, efficiency and complexity, ocean energy technologies must target reasonable efficiency and cost structures, while developing robust technology for the harsh marine environment.

The ocean energy resource potential is obviously large, and it is also relatively well understood. In one respect, ocean energy resource is superior to wind and solar energy. Why? The relationship between winds and ocean waves is well understood, and because ocean waves travelling in deep water maintain their characteristics over long distances, sea states can be predicted accurately more than 48 hours in advance. Thus, even though it is variable, wave energy resource is actually more predictable than wind energy. Similarly, tidal currents are created by the interaction of tides and bathymetry (the underwater topography of the ocean floor), and are thus very predictable and generally more constant than wind or solar energy. The high density of water also makes the resource concentrated – moving water carries with it a lot of energy.

The way ahead

The ultimate viability of ocean technologies depends on a number of factors; including environmental impact, permitting constraints (i.e. in marine sanctuaries) and long-term economics. But there are parallels to the early windpower development of the 1980s. In time, if ocean energy is given the support it needs, it could emerge as a significant renewable energy resource. And while it is unclear which technologies will come out on top, what is clear is that those companies pioneering this field need support now to develop to their full potential.