Japan’s extensive coastline, rivers could be perfect for clean new energy tech
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Scientists in Japan and Norway are actively pursuing practical ways of generating electricity from the difference in salinity between seawater and fresh water.
The process goes by several names, including salinity gradient power and osmotic power. But whatever the name, it is a dream type of renewable energy, inexhaustible and unaffected by the weather, unlike solar power and wind power.
Natural, renewable energies have a bad reputation for being both costly and inefficient, so there is a lot of hope riding on the practical development of osmotic power generation.
The technology is especially suitable for Japan, given the enormous length of coastline the nation boasts.
Just like making Japanese pickles. That is how Prof. Akihiko Tanioka of the Tokyo Institute of Technology likens the process of osmotic power generation.
Tanioka leads a project in Japan to develop a viable method of power generation based on the salinity difference between seawater and fresh water. The research is being carried out at a pilot plant in Fukuoka by the Tokyo Institute of Technology, Nagasaki University and water treatment equipment maker Kyowakiden Industry Co.
When seawater and fresh water are separated by a special membrane that is permeable to water molecules but blocks the passage of salts, water molecules will naturally migrate to the side where the salt concentration is higher due to the process of osmosis. That same process is used to draw the water out of vegetables like cucumbers and eggplants to make Japanese pickles.
In the process Tanioka and his colleagues are developing, the seawater and the fresh water are pumped through pipes that are divided down the middle by a permeable membrane. As the two separate streams of water flow down the pipe, fresh water migrates across the membrane to the seawater side, boosting the flow volume on that side and giving it the extra momentum to spin a turbine to generate electricity.
Conventional hydroelectric power systems also work to generate electricity by spinning turbines using the momentum of water. But whereas hydroelectric power exploits a difference in height to create a water force, osmotic power exploits a difference in salinity. The process thus also works on flat land, Tanioka explained.
At the Fukuoka pilot plant, the fresh water is pumped from a nearby wastewater treatment plant, and the seawater, which has double the normal ocean concentration of salt, is pumped from a nearby desalination facility where ocean water is used to make fresh water.
There are eight pipes in this pilot plant, each 30cm in diameter and 1.4 meters in length. When the two kinds of water move through these pipes, the osmotic effect boosts the water flow on the seawater side of the pipes by 50-80%. After passage through the pipes, the seawater is returned to the ocean and the fresh water is returned to a river.
“In one pipe alone the system generates a water force equivalent to a fall of 300m,” explained Hideyuki Sakai, president of Kyowakiden Industry.
Electricity must be consumed to drive the pumps, but more than that amount is generated, so the system functions as a generator, producing some 1-2kw of electricity.
Norway is another nation also making progress on the practical development of osmotic power. State-owned power company Statkraft AS is operating an experimental plant there, and it has also partnered here in Japan with Nitto Denko Corp. The Japanese company has developed an osmotic membrane that is only 0.1mm thick, and is investigating its use for osmotic power in cooperation with Statkraft at its plant in Shiga Prefecture overlooking Lake Biwa.
For osmotic power generation to work, the salinity differential between the seawater and fresh water must be maintained. “What you need is a technology that prevents the salt component of the seawater from migrating to the fresh water side, while enabling the fresh water to efficiently move to the seawater side,” noted Masahiko Hirose, head of the membrane division at Nitto Denko.
With that objective in mind the company has set out to develop a membrane for osmotic power, basing the work on its technologies for osmotic membranes for desalination of seawater. Hirose said the goal is to have a new membrane ready next year that can be tested at the facility in Norway.
One of the big draws about osmotic power as a renewable energy is that it is not affected by the weather. With an effective operating efficiency of 85% or more, it is more than four times as efficient as solar power and wind power.
According to estimates by Tanioka’s group, the power-generating cost of osmotic power is 9-26 yen (11-33 cents) per kilowatt-hour, which is cheaper than the 40 yen for solar power and competitive with the 14-24 yen for wind power.
As conceived, osmotic power plants would be situated near the mouths of rivers for ready access to seawater and fresh water supplies.
“Considering the flow volume of Japan’s rivers, there is a potential capacity for 6 million kw of power generation, the equivalent of five or six nuclear power reactors,” asserted Tanioka.
That said, many issues need resolving before osmotic power plants can be built.
One problem is the durability of the membranes, and another is the whole question of whether a large-scale power plant would actually work.
Tanioka’s group of the Tokyo Institute of Technology, Nagasaki University and Kyowakiden Industry aims to build a commercial-scale power plant in three years in cooperation with Toray Industries Inc., Toyobo Co. and Yamaguchi University.
Statkraft and Nitto Denko plan to build a 2,000kw experimental plant by 2015.
The earthquake and tsunami in the Tohoku region in March shattered the myth of the safety of nuclear power when they damaged the Fukushima Daiichi nuclear power plant.
The accident also attracted greater attention for solar and other alternative power sources.
When its territorial waters and its elusive economic zone are factored together, Japan has the sixth largest coastline among the nations of the world.
That fact alone explains the potential for Japan of osmotic power generation using ocean resources.