Tapping the ocean's inexhaustible store of solar and lunar energy

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