Renewables

Wave, Tidal & Ocean thermal

The ocean contains two types of energy: mechanical energy from the waves and tides, and thermal energy from the Sun’s heat.

Wave

The total power of waves breaking on the world’s coastlines is estimated at 2-3 million MW. In favourable locations, wave energy density can average 65 MW per mile of coastline. Three approaches to capturing wave energy are:

• Floats or pitching devices - these devices generate electricity from the bobbing or pitching action of a floating object. The object can be mounted to a floating raft or to a device fixed on the ocean floor.
• Oscillating water columns - these generate electricity from the wave-driven rise and fall of water in a cylindrical shaft. The rising and falling water column drives air into and out of the top of the shaft, powering an air-driven turbine.
• Wave surge or focusing devices - these shoreline devices, also called ‘tapered channel’ or ‘tapchan’ systems, rely on a shore-mounted structure to channel and concentrate the waves, driving them into an elevated reservoir. Water flow out of this reservoir is used to generate electricity, using standard hydropower technologies.

Tidal

Tidal energy traditionally involves erecting a dam across the opening to a tidal basin. The dam includes a sluice that is opened to allow the tide to flow into the basin; the sluice is then closed, and as the sea level drops, traditional hydropower technologies can be used to generate electricity from the elevated water in the basin. Some researchers are also trying to extract energy directly from tidal flow streams.

The energy potential of tidal basins is large - the largest facility, the La Rance station in France, generates 240 MW of power.

Tidal energy systems can have environmental impacts on tidal basins because of reduced tidal flow and silt build up.

Ocean thermal

Oceans cover more than 70% of the Earth’s surface, making them the world’s largest solar collectors. Each day, the oceans absorb enough heat from the Sun to equal the thermal energy contained in 250 billion barrels of oil. Ocean Thermal Energy Conversion Systems (OTEC) convert this thermal energy into electricity - often while producing desalinated water.

Three types of OTEC systems can be used to generate electricity:

• Closed-cycle plants circulate a working fluid (such as ammonia which has a low boiling point) in a closed system, heating it with warm seawater, flashing it to vapour, routing the vapour through a turbine, and then condensing it with cold seawater.
• Open-cycle plants flash the warm seawater to steam and route the steam through a turbine.
• Hybrid plants flash the warm seawater to steam and use that steam to vapourise a working fluid in a closed system.

OTEC systems can be land-based, mounted on the ocean shelf or floating.