Energy from the Oceans
The ocean has long been a source of bounty, supplying mankind with basic needs like food and transportation, and delighting us with its beauty and wonder. Man is now turning to the seas once again, seeking another vital resource – energy. Within and beneath the waves lie proven reserves of conventional, non-renewable energy stores, as well as the promise of clean, renewable power.
Renewable power can be generated by the ocean’s mechanical energy – the physical movement of water in waves and tides, and by its thermal energy – the heat absorbed from sunlight shining on the sea. Waves started by winds transmit energy across the surface of the sea. The bigger the wave, the more energy it contains. Wave power devices tap into this energy in two ways. The up and down motion of water can be used directly to crank a generator, or pressure changes caused by wave motion can force air though turbines to make electricity.
Unfortunately, waves are not as reliable as many other energy sources – their size, speed, and direction vary considerably over time. Only a few areas – mainly western coastlines in mid-latitudes - get large waves consistently enough to make commercial wave power production feasible.
Tides are another source of renewable energy. Caused by the relentless gravitational pull of the sun and the moon, tides are powerful enough to lift and drop the world’s oceans twice daily. Electricity can be generated by both the up and down motion of the tides, and by the lateral flow of tidal currents. Most of the few commercial tidal power plants in operation consist of dam-like structures, called barrages, built across the mouth of bays. Water is trapped behind the barrage at high tide, and then released through turbines during low tide to make electricity. Other tidal power strategies use devices much like underwater windmills to tap the energy of tidal currents.
Tidal power is more dependable and predictable than wave power, because the rise and fall of tides is due to the regular, reliable movements of the earth and the moon. However, tidal flows, and thus energy production, are intermittent, stopping, starting, and changing direction once or twice a day. And current technology makes tidal power practical only where there is a large difference between high and low tides. Only a few coastal areas experience the required tidal range of 5 meters or more.
Another source of renewable energy is ocean thermal energy conversion, or OTEC, which uses seawater to turn solar energy into electricity. Every day, the sun shines on the sea, heating up surface waters. At the same time, icy currents flowing from the poles chill the ocean’s deep waters. This creates a thermal gradient - a temperature change from hot surface waters to cold deep waters – that can be tapped to create power.
OTEC plants pipe in hot surface water and cold deep water and use them to first vaporize and then condense either seawater or another fluid, in the process spinning turbines that generate electricity. Ocean thermal energy conversion has a number of potential benefits aside from energy production. The deep ocean water discharged from the plants is cold and nutrient rich. It can be used for agriculture and aquaculture, and for air-conditioning and refrigeration. Even more importantly, when salt water vaporizes then condenses, it is desalinated – the salts are removed. This fresh water is desperately needed in many coastal areas for drinking and irrigation.
Ocean
thermal energy conversion can only be done effectively where the thermal
gradient exceeds 20° Celsius within the upper 1,000
meters of the ocean. These conditions occur in most of Earth’s tropical waters.
Nearly 100 countries, including the
All three sources of renewable energy from the sea – wave and tidal power and ocean thermal energy conversion – hold the promise of clean, nearly limitless power. But widespread use of all of them also has significant drawbacks. All are currently more expensive and less efficient than conventional methods of generating electricity. Offshore and shoreline facilities must endure harsh conditions, and can disrupt coastal processes, interfere with wildlife, and degrade scenic views.
Other important energy resources are found not within the oceans, but beneath them. Oil, natural gas, and methane hydrates are non-renewable but potent fossil fuels that lie buried in sea-floor sediments. Oil and gas come from the remains of countless microscopic plants that fell to the seafloor, and were buried, liquefied, and vaporized by the weight of overlying sediments. Offshore petroleum deposits, found on many continental shelves, are tapped and drained from fixed or floating platforms, and the raw product shipped ashore for processing.
Marine drilling operations are more expensive and difficult than those on land, but the fuels they produce are still cheaper and more efficient than alternative energy sources such as wave and tidal power and ocean thermal energy conversion. Offshore oil and gas recovery does carry a large environmental cost, however. Approximately 11 million gallons of petroleum are spilled into the sea each year during exploration and production activities.
Seafloor petroleum deposits have been mined for more than 100 years. More recently, a potential new energy source has been under study. Methane hydrates are frozen cages made of water and filled with methane-- a type of natural gas. On the deep sea floor, methane forms when bacteria break down organic debris. High pressure and low temperature combine to freeze the gas and seawater together into crystals that look like ice but that will burn when exposed to a flame.
Global reserves of methane hydrates, which form in permafrost on land as well as in the sea, are enormous. They have been detected along most continental shelves, sometimes in thick layers, sometimes scattered throughout seafloor sediments. However, extracting methane hydrates from the ocean bottom is difficult and dangerous, and reliable methods have not yet been developed. Hydrate crystals are stable only under certain conditions and can explode when disturbed. They can also disintegrate, causing the seafloor sediments around them to collapse under drilling rigs or cascade into landslides.
For the near future at least, petroleum will continue to be the greatest source of energy from the sea. However, if technologic and economic barriers can be overcome, methane hydrates offer many nations hope of long-term energy independence, while tide, wave, and ocean thermal energy conversion could one day reduce our reliance on polluting fossil fuels.
Global Impact
Global energy use is exploding as the population rises and countries become more developed. Energy demands are projected to rise by 60% in the next 20 years, and more than double by 2050. These needs will require an ever-increasing exploitation of oceanic resources.
Some marine energy sources are already in use. Offshore deposits account for roughly 30% of world oil and 50% of global natural gas production. A few commercial tidal power plants are on line, and several demonstration wave and ocean thermal energy conversion plants are in operation.
But the potential for oceanic energy far exceeds its current output. The oceans are a storehouse of oil and natural gas, in both familiar and poorly known forms. Geologists estimate that 40% of all new petroleum discoveries will be found offshore. Methane hydrates are thought to contain twice the energy of the combined global petroleum and coal reserves. If even a fraction of these marine deposits can be exploited, they can extend the world’s supply of fossil fuels for many tens, perhaps even hundreds, of years. Of course, the use of oil and natural gas contributes to serious environmental problems, including water and air pollution and global warming.
Other forms of oceanic energy hold out the promise of both abundant and clean power. The three main contenders for renewable, non-polluting energy are wave and tidal power and ocean thermal energy conversion. They are for all practical purposes limitless – waves and tides will roll across the sea and the sun will shine down upon it day after day far into the future.
The mechanical and thermal energy held within the seas dwarfs that of marine fossil fuels. The amount of energy that can potentially be harvested from the world’s waves and tides is estimated at more than twice the current global electricity production. Even greater energy is available for OTEC - every day, the ocean’s surface absorbs the solar heat equivalent of more than 250 billion barrels of oil.
These energy sources are not without serious drawbacks of their own, even if the technology to exploit them is perfected. They are geographically limited - electricity can only be generated in those areas with sufficient tidal range, wave strength, or thermal gradient. And unlike fossil fuels, electricity cannot be transmitted to distant areas with efficiency. Although these technologies don’t produce harmful wastes or emissions, they carry their own set of environmental burdens. Tidal barrages, for example, must be built across estuaries, which are some of the most productive and sensitive habitats in the oceans.