April 25, 2011

Hurricane suppression system - Salter Sink

Energy from the sun heats up the surface of the ocean. As that heat irradiates up and fuels storms, they can become ever more dangerous hurricanes. Reducing their destructive potential is possible if we can just cool off the surface of the ocean. Even just one degree centigrade might be the difference between a category 4 or category 5 hurricane. This is a nearly ridiculous notion because of the scale involved. Thousands of square miles of ocean surface might need to be cooled off.

The Salter Sink is a simple idea, with massive potential. Two insights make it very compelling: Everywhere there is hot water on the surface of the ocean, there is cold water down below.

The Salter Sink works as a wave powered pump. Waves push hot water into the top of the cylinder, which pumps the water inside down. It comes out the bottom (around 200 meters below) and mixes with colder water. This brings the temperature on the surface down over time. A Salter Sink can move about a gigawatt of thermal energy! It may take thousands of these to protect America’s Gulf region (for example) but we estimate the cost would be much lower than the damage caused by one of these storms.



SALTER SINK harnesses waves and gravity to cool the surface of the ocean
enough to weaken a passing hurricane. (a) A ring 80–100 meters across floats
on an assembly of discarded tires, lashed together and filled with a bouyant
substance, such as foamed concrete. (b) The surface of the ring rises a meter
or so above the sea level. Ocean swells passing over the structure fill it with
warm surface water. (c) A skirtlike tube connected to the inside of the ring extends down 50–300 meters to the cooler water below (blue). Gradually the tube fills with warm water (orange), which drains out the bottom. The tube has a tapered profile to accelerate the velocity of the flow and to improve mixing of the warm exhaust water with the colder water at depth.


The invention, which we refer to as a Salter Sink, cools surface waters by mixing them with cooler layers of the ocean at depth. This general approach is not a new idea: a company named Atmocean (not affiliated with I.V.) has fielded prototype versions of a system that would use wave-driven pumps to lift deep, cool waters up to the surface, as a way to improve ocean fertility and CO₂ absorption.

A Salter Sink works by sinking hot water, rather than lifting cold water. Like a siphon, it exploits gravity to create a “drain” through which warm surface water flows. The figures on the next page illustrate the basic idea. A large, buoyant ring is attached to a long, skirtlike tube whose bottom opens in the cooler depths of the ocean. Ocean swells overtopping the ring will periodically fill it with warm surface water. Because the water in the ring is above sea level, the force of gravity will drive water out of the bottom of the tube. (The weight of the dammed water easily exceeds the buoyancy created by the temperature difference at the tube outlet.)

The Salter Sink has several advantages over previous ideas for suppressing hurricanes. It could work at a wide range of scales, from 10 meters in diameter to as large as 100 meters across. Salter Sinks of even moderate size appear, in principle, to be able to remove surface heat at gigawatt rates [Salter 2009b]. And because a Salter Sink contains no moving parts and is powered solely by gravity, it should be comparatively easy and inexpensive to fabricate, deploy, and maintain.

Low cost and ease of deployment and retrieval are crucial because of the unpredictability and tremendous power of cyclonic storms. Thousands of Salter Sinks might have to be deployed in a regular spacing, with perhaps three kilometers separating each sink, in order to cover the wide swathes of ocean necessary to protect major cities on the Gulf Coast of the U.S. against the worst effects of hurricanes. To be effective, the sinks would most likely need to be put out to sea months before hurricane season begins and retrieved shortly before major storms arrive. The hope is that by that time they will have accomplished their task.

A great many questions still remain to be answered about the practicality of this invention. One key issue has to do with subsurface currents, which in some areas of the ocean create strong shear forces that would deform the tube of a Salter Sink or even tear it apart. Another question we are exploring, through simulation and laboratory experiments, is just how efficiently Salter Sinks of various sizes would cool the surrounding warm layer of surface water.

COMPUTER MODEL of a Salter Sink has been constructed by I.V. and its collaborators.
Preliminary results indicate that the sink would cool the sea surface. Calculations suggest that a large Salter Sink 90 meters in diameter, covering 6,360 m² of ocean surface, appears capable of a thermal pumping rate on the order of 10 gigawatts for much of the year. This rate is roughly equivalent to the solar heating power received by 41 million square meters of ocean in the Gulf of Mexico, where insolation is typically on the order of 240 W/m².

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