Tapping the ocean's inexhaustible store of solar and lunar energy
Earth's enormous bodies of water can provide mankind with an inexhaustible supply of clean energy from the sun and moon
Last month, I saw one of the world's greatest natural phenomena while lingering over dinner at a cafe beside the Bay of Fundy in Nova Scotia, Canada. The sun was setting over the wide marshes and tiny islands in the bay and the tide was rising, fast. Between the appetiser and dessert, high tide had all but submerged the marshland and islets. Only six hours earlier, driving along the coast during low tide, I had seen boats lying on the dry sea floor. Now they were all afloat on water that had risen up to 16 metres. (The average tidal range of the world's oceans is one metre.)
Due to its unique funnel shape, size and depth, the Bay of Fundy has the highest tides in the world: twice a day the bay fills and drains a billion tonnes of water - more than the flow of all the world's freshwater rivers combined. And just as rivers are dammed to generate hydroelectricity, the ebb and flow of the tides can be harnessed to produce clean, renewable energy. A 20-megawatt-capacity tidal power plant on the Bay of Fundy already generates electricity for 4,000 homes.
Using our seas to generate power gets less publicity than wind and solar energy, but marine energy has immense potential. The ocean covers 71 per cent of the earth's surface and is the biggest absorber of the sun's energy. If the kinetic energy of ocean movement and the thermal energy of the absorbed sunshine can be unleashed, we could tap into an unlimited energy supply.
That's a big "if" of course, but technologies are being developed to harness the ocean's energy three ways: from tides, waves and thermal energy within the ocean.
Tides are the periodic rise and fall in sea levels caused by the gravitational attraction exerted mainly by the moon as it revolves around Earth, pulling tonnes of water into, say, the Bay of Fundy. As the waters flow into the bay and back out to sea, turbines positioned in the estuary convert the force of the tides into electricity.
Due to the consistency of the moon's orbit around the earth, tides rise and fall constantly and predictably, making tidal power an inexhaustible source of energy. At present Canada, China, France, South Korea, Britain and Russia have tidal stations generating hydroelectric power. South Korea has the largest capacity at 254MW. China's tidal power station in Zhejiang province has 3.2MW of capacity and transmits electricity to local villages.
Unlike wind and solar energy, tidal energy is not weather-dependent. It is more efficient than wind because of the much greater density of water. However, tidal power plants are costlier to build than hydroelectric dams in rivers. They generate power for only about 10 hours a day as the tides move in and out. And their capacity is small compared to a typical hydroelectric plant's
2 gigawatts. So, as the technology stands, tidal power can only play a complementary role, providing energy regionally where topography and tidal conditions allow.
While tidal energy is already in use, scientists and engineers are still experimenting with how to capture the energy of the up-and-down, back-and-forth motion of waves in the ocean. Winds blowing over the ocean transfer their energy to the surface of the water, creating waves.
In many large areas of ocean, winds blow with enough consistency and force to make continuous waves that can be used to generate energy. Wave energy devices generate electricity through the movement of the floating part of the device driven by the rise and fall of the waves.
In the past decade, wave energy converters have been tested in 10 countries, ranging from Portugal to India. The most ambitious test starts next month in the northern Pacific, where wind and wave conditions are consistent across a huge area. Off the coast of the US state of Oregon, a 250-tonne buoy, which is essentially a floating turbine, will be anchored in the ocean, allowing it to bob up and down generating electricity.
A computer in the floating turbine will communicate with an array of sensors nearby that detect ocean wave characteristics and adjust the turbine's giant internal shaft as it moves up and down on the waves passing through it. The up-and-down motion of the shaft creates electricity, which goes to shore through a seabed cable.
The third method of harnessing marine energy is based on the enormous amounts of energy the ocean absorbs from the sun every day. Consider this: If less than one-tenth of 1 per cent of all the solar energy trapped in the oceans were converted into electricity, it would supply more than 16 times China's daily consumption. But that's another big "if"; the concept is untested on any significant scale.
An ocean thermal energy conversion plant works like this: water near the ocean's surface is heated by sunlight while water deep below is much colder. The plant uses warm surface water to heat ammonia or some other fluid that boils at a low temperature. The resulting gas drives turbines to produce electricity. The gas is then cooled by cold water pumped up from the ocean depths.
Such a plant, built in Cuba in 1930, produced 22 kilowatts, enough to supply roughly two households. A handful of similar plants were built elsewhere following the oil crisis of the 1970s, but funding dwindled after oil prices dropped, and none are now operating. Both ocean wave and ocean thermal energy devices must be placed in the open sea, where they are vulnerable to extreme weather. The need to anchor the devices securely to the ocean floor adds to their risk, capital and operating costs.
But it's only a matter of time before technology enables more cost-effective ways to extract energy from the sea. We will be swimming in a sea of energy and it won't be just surfer dudes who will be riding the waves.
Tom Yam is a Hong Kong-based management consultant. He holds a doctorate in electrical engineering and an MBA from the Wharton School of the University of Pennsylvania