ocean energy

Hydrocarbons: oil, gas and methane hydrate

Hydrocarbons currently provide 84% of the world’s energy, with about 30% of oil and gas coming from almost 4,000 production facilities located on the continental shelves of just under 100 States. Offshore drilling technology is already well-established: since 1938 when the first well was set up in the Gulf of Mexico to drill for oil at a depth of just 4 metres, offshore rigs can now reach oil or gas deposits at depths of around 7,000 metres. The costs involved are still much higher than for onshore or shallow water production but as land-based reserves of oil and gas are depleted and as prices increase, resources in less accessible locations become more economic to exploit. Offshore fields are still expected to contribute significantly to meeting the future energy needs of society until a serious commitment to avoiding fossil fuel extraction is made.

Nobody knows for certain how long global resources of oil and gas will last – particularly as it is difficult to predict future consumption trends. From today’s perspective, oil reserves equivalent to about another 50 years at current consumption levels exist while resources of natural gas will probably suffice to ensure supply well into the second half of the 21st century.

One resource yet to be exploited is methane hydrate, sometimes known as fire ice, which some believe will play an important role in our future energy supply. Deposits of methane hydrate are found in places that are gassy, wet, cold, and under pressure, such as in permafrosts or at the bottom of the sea. The first sample was found at the seafloor in 1979 and extraction methods have been researched ever since. Although methane hydrate accumulations are located in difficult environments and present numerous technical challenges, they are widely distributed: there is thought to be between 1,500 and 15,000 billion tonnes of carbon locked up in methane hydrates around the world — comparable to the 5,000 billion tonnes of carbon in all the planet’s oil, gas, and coal. The extraction of natural gas from methane hydrates onshore was successfully tested for the first time in 2008 by Japanese and Canadian scientists but it is still a long way from being mined on an industrial scale. Retrieving methane by replacing it with CO2 is now being tested, research into extraction methods is ongoing in the United States, Canada, Japan, and India.

Renewable energy

A viable alternative to burning fossil fuels lies in the ocean: wind, wave, tidal and thermal energy, as well as energy harnessed from changes in salinity, could transform our current energy sector, reducing our reliance on fossil fuels and thus helping in the fight against climate change. Estimates put the potential for ocean energy at 300 times that currently consumed by humankind.

• Wind energy: according to the International Renewable Energy Agency, shore and offshore wind could generate about 5,000 terawatt-hours (TWh) of electricity a year worldwide (more than 35% of total electricity needs), becoming the prominent generation source by 2050. Wind turbine towers can be anchored to the ocean floor at depths of up to 50 metres, floating offshore concepts are being developed for deeper waters.

• Wave energy: generated by converting the energy within ocean waves (swells) into electricity, its sustainable generating potential of 1,700 TWh per year would cover about 10% of global energy needs.

• Tidal energy: tidal range technologies can harvest the potential energy created by the height difference between high and low tides while tidal current technologies capture the energy of currents flowing in and out of tidal areas. Current energy can also be harnessed using submerged rotors which are driven by the motion of the water. It has been estimated that ocean current power stations and tidal power plants together could harness several 100 TWh of electricity per year worldwide.

• Thermal energy: land-based or floating ocean thermal energy conversion (OTEC) plants can convert the temperature difference between the warmer ocean’s surface water and colder deeper water into energy. In order to drive the steam cycle in an OTEC power station, the temperature difference must be at least 20°C, making this technology more suited to warmer regions. The warm water is used to evaporate a liquid which boils at low temperatures, producing steam which drives a turbine. Cold seawater (4 to 6 degrees) is then pumped up from a depth of several 100 metres and used to cool and condense the steam back to liquid form.

• Salinity: osmotic power plants can generate energy by exploiting the osmotic pressure which builds up between freshwater and saltwater when they are pumped into a double chamber and separated by a special semi-permeable membrane. Still very much in the research phase, it is however thought that the sustainable global production capacity of osmotic power could in future amount to 2,000 TWh annually.

Ocean energy together with solar and wind energy can lead the way for the transformation of the global electricity sector, green power solutions to our current fossil fuel reliance will most certainly be blue!