Presents from the Ocean

Chuankun+Wang

71% of the Earth surface is covered by ocean， a huge water body that is always vigorous and undulating. Ocean energy， also known as ocean renewable energy， is a unique natural resource that the ocean contains. Scientists estimate that the total storage amounts to around 76.6 billion KW s， with an accessible storage reaching 6.4 billion KW s， almost equivalent to the overall global installed power generation capacity. Our ocean contains a huge amount of energy， including tidal energy （tidal range energy and tidal current energy）， wave power， ocean current power， ocean thermal energy and osmotic power. Where do those energy come from？

Tidal range energy： energy of rising and falling tides

Coastal residents are familiar that sea water sometimes surges forward， rolling up waves； and sometimes ebbs back， revealing a beach. The phenomenon of sea water ebb and flow is called tide， which is caused by the gravitational force of the moon and the sun. Since the moon rotates around the Earth， the moon and the Earth rotate together around the Sun， both rhythmically and periodically， there-force the ebb and flow of Sea water is also periodical and rhythmical.

There are frequent tides in bell shaped gulfs， straits and estuaries regions along the coast due to geographic factor and others. Tidal range （difference between high tide and its next low tide） could reach 7 to 8 meters， or even more than 10 meters. The maximum difference near the coast of Hangzhou Gulf of the well known Qiangtang River Tide could reach 8.9 meters. While at Monckton Port， the figure is 19. When tides rise in these regions， the crests look like cliffs； the surging tides look like thousands of galloping steeds， creating thunder-like noises which could be heard even miles away and presenting a grand scene of “Waves dashing high up into the sky， with an overwhelming force that could destroy a mountain.” To conclude， tide contains enormous energy – tidal energy （tidal range energy and tidal current energy）. Scientists estimate that the overall storage of tidal energy across the world is around 3 billion KW s， with 100 million KW s accessible technically.

Human beings development and utilization of marine energy resources began with tidal range energy. As early as over 1000 years ago in Tang Dynasty， coastal residents of China started grinding grains by using tidal force， which is the same in Europe. Scientifically advanced countries have started research of using tidal range energy to generate electricity in the modern era.

The theory behind power generation with tidal range energy， commonly known as tidal power generation， is to build tidal power station， which is to construct a sea dam near mouth of bell-shaped gulfs or rivers， cutting the upstream of gulf or river mouth from the open seas， and to construct floodgates and power station on the other side of the dam， equipped with hydro-turbines and generators， the runner of each hydro-turbine being connected with the reservoir and the open sea with channels. When tide rises， the water level outside the reservoir is higher than inside； on the contrary， when tide ebbs， the water level inside the reservoir is lower. Hydro-turbines are pushed to rotate by force of tidal range and thus generating power. Tidal power station have been put into use in 1960s both in and out of China， being the only marine energy thats commercialized to generate electricity.

Tidal current power： an alternative method to use tides

The theory of tidal current power generation is similar to that of the wind power. For example， there is a power generation device called “underwater windmill”， which uses currents to push the impeller so that the generator can generate electricity. There are propeller-type impellers with horizontal axis and wheel-type impellers with vertical axis. However， impellers of tidal power device are much smaller than that of the wind power because density of sea water is much bigger than that of the air.

Both China and foreign countries started research of tidal current power generation since 1980s， and a research fever came around 1950s. Many countries are carrying out researches in this regard， with Britain leading and China being a member of the advanced group. International research of such devices have entered demo and trial period， and can be put into use in predictable future.

Wave power：

a double edge sword given by the ocean

Everyone will be impressed with the breathtaking waves of the sea if he/she has ever been near the shore. When heavy winds blow， huge waves are turned into untamed wide horses， surging and galloping on the surface of the sea. Sea waves led to many marine disasters， but they are also invaluable resources. Scientists estimate that the global storage of wave power amounts to 3 billion KW s， with 1 billion KW s accessible technically.

There are many kinds of useful wave power， the force of up-and-down and rocking movement， pressures， etc. A common and easy example is an upside-down pump device which could use the up-and-down movement. The vertical motion of waves pushes pistons installed in the floating devices， just like an upside-down pump. Relative movement of pistons and devices compresses the air and thus push the turbine to rotate， and generate electricity. This simple power generation device has been powering fairway buoys and lighthouses since long time ago.

Many countries are researching how to generate electricity with wave power， with Britian leading and China being a member of the advanced group. However， study of devices using wave power to generate electricity is now at demo and trial stage both in and outside China， therefore it will still take some time to be commercialized.

Osmotic power of sea water： the saltier， the better

Average salt concentration of global sea water is measured to be 35% （also say salinity of sea water is 35）. In the estuary area or where river meets with sea water， salt concentration of water bodies are different， also known as salt concentration difference. Power stored in such difference is called ocean osmotic power， also termed as salinity gradients energy.

Scientists estimate that the overall ocean osmotic power in store is around 30 billion KW s， with 3 billion KW s accessible technically. Energy density of this ocean power surpasses that of the rest. Osmotic power can be used in many ways， among which osmotic pressure method attracts the most attention. We can divide a pool into half with a piece of semipermeable membrane （only allowing solvent to pass through） and pour sea water into one side and fresh water another till water levels are the same， and sometime later， we could find that： since fresh water passes through the membrane and permeates to the sea water half， water level of the fresh water half will steadily goes down， and sea water side rises up. After pouring the 35 salinity sea water and fresh water respectively into each half of the pool， we will end up with 248 meters of water level difference caused by osmotic pressure， which equals to 24 times of the barometric pressure.

Water level difference contains potential energy that could be utilized to push hydro-turbines to rotate and drive the generators to produce electricity. As long as high salinity sea water keeps pouring into the sea water side to maintain the salt concentration， fresh water will keep permeating， thus keeping the water level difference and electricity generation. However， to use osmotic power is technologically demanding， difficult and costly. Whats more， most scientists think the obstacles hard to overcome in the near future， therefore， there are currently only a limited number of researches into osmotic power. Foreign reports say that Norway， U.S.， Holland and others have conducted researches in this field in recent years.

Ocean thermal energy：

heat transfer of sea water

In tropical sea areas located on both sides of the equator， surface and deep sea water has a temperature difference of 20-24 degree Celsius. This stores huge thermal energy. Scientists estimate that the overall thermal energy in store is around 40 billion KW s， with 2 billion KW s accessible technically.

Thermal difference of sea water could help generate electricity. The basic principle is： to absorb heat from the ocean through working medium of the heat transformation system to produce steam， steams push turbines to rotate and drive the generators to produce electricity. After one-century of research， American scientists finally set up an experimental power station that uses thermal difference in waters near the Hawaii Island. This is the first thermal difference power station that generates more electricity than it costs， before which electricity generated during experiments were even less than the electricity used to extract cold deep sea water and surface warm water.

Ocean current power： migration of sea water

There are large scales of ocean water movement globally， forming “rivers in the sea”， also known as ocean currents. Ocean currents are caused by uneven density distribution of the sea water， which depends on wind， water temperature or salt content. Ocean currents continuously move along their own routes， with relatively stable direction and speed. They are different in length， some spreading several hundred kilometers， some even as long as several thousand or ten thousand kilometers. They are also different in width and depth. The speed of ocean currents is generally 0.5～1 m/s， some could reach 2 m/s. Scientists estimate that the overall ocean current power is around 600 million KW s， with 300 million KW s accessible technically. However， since ocean currents are normally away from the continent， therefore it is difficult to develop this kind of energy， and few countries are conducting research in this regard.

The ocean brings human beings endless treasures， and humans exploration of the ocean never stops. Though there are many categories of ocean energy， we can only extract limited values from them. Human science is still progressing， and we can surely transform those presents from the ocean into forces that advance human society in foreseeable future.