Can a Stirling power a 'Hurricane Tower" fresh water producer?

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nospamjock
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Can a Stirling power a 'Hurricane Tower" fresh water producer?

Post by nospamjock »

In Hawaii a professor and his partners including a big engineering firm are working on wringing out fresh water from salt water by mimicing the natural convection inside a hurricane cloud.

Their "Hurricane Tower" looks like an inverted mushroom. Saltwater in a pool at the bottom is heated by solar power. A fan inside circulates the heated water vapor. The vapor condenses as fresh water on the insides of the tower (which is cooled on the outside by cold ocean water). It runs down the sides and into a collecting trough, apparently in plentiful quantities. Of course pumping the water and keeping the fan going, take energy.

So the question is: would a Stirling engine driven by the temperature difference between cold ocean water (about 50 degrees) and the heat given off by condensing water vapor (say, 80 degrees) be enough to power at least the fan? Doubtless pumping the water also would be too much to expect.
rsteinke
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Response to Can a Stirling power a 'Hurricane Tower" fresh water producer?

Post by rsteinke »

Maybe you could power a Stirling engine from this temperature difference, but you wouldn't want to because you are already using it to run another heat engine. The process of evaporation and condensation that is purifying the water is effectively a heat engine running off the temperature difference between the sea water and the mushroom shell. The rate at which you will get fresh water is proportional to the rate at which heat flows through this system. If you divert some of that heat through a Stirling engine you will get less fresh water.An alternative would be to add a separate solar collector for the Stirling engine. You could focus the light with a lens or mirror to produce a higher temperature difference more suitable for a Stirling engine. Of course, you could take the money you would spend on a Stirling engine and build a bigger mushroom to get more fresh water, and just run your fan from the electric grid. To really decide what's best you need to do an engineering trade study relating your figures of merit (money, galons of water per day) and your requirements (desire to be 100% solar for example) to your options (Stirling engine vs. bigger mushroom w/ electric fan)
nospamjock
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'Hurricane Tower"

Post by nospamjock »

Thanks Bob for your thoughtful response. I am hesitant to agree though. The problem with the Hurricane Tower is excess heat. Condensing water vapor gives off prodigious heat. Pumping cold ocean water as a refrigerant to counter it takes a lot of energy. If a Stirling could take and use the heat instead, that would reduce the cold water having to be sucked up from about a mile deep in the ocean. Tremendous energy savings. If, that is, a Stirling can run on a 30 degree heat difference.
rsteinke
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Response to Can a Stirling power a 'Hurricane Tower" fresh water producer?

Post by rsteinke »

Ah, I misunderstood. I thought that you were going to run the Stirling engine between the hot sea water in the hurricane tower and the cold mushroom shell. Instead if I understand correctly, you are going to use the mushroom shell as the hot side of the Stirling engine and dump heat into cold sea water.It seems like you are more interested in removing heat from the mushroom shell than in producing power. Currently, to remove this heat you run a pump to pour large amounts of cold sea water over the shell. You figure, if I can run a Stirling engine I can remove the heat, save electricity by not pumping the water, and maybe produce some power as well.Well, as the cold sea water is a mile down you will certainly have to pump something whether it's the sea water itself or a working fluid in a Stirling engine. Friction due to the viscosity of whatever you are pumping will inevitably lose energy, but you might be able to lose less by using a better working fluid than sea water.However, trying to remove the heat through a Stirling engine is probably not the best idea. For the sake of thermodynamic efficiency Stirling engines try to use isothermal processes. These get the most mechanical work out of a fixed amount of heat, but the rate at which this heat passes through the system is slow. You are more concerned about fast heat transfer than getting out work.It seems silly that you should have to spend energy to push heat "downhill" to a lower temperature cold sink, but when that sink is a mile away and you want to move lots of heat quickly sometimes you have to push. You could look at some kind of fluid in a closed loop to take the heat down to the cold sea water rather than pumping the sea water up. This fluid could have better thermal conductivity, viscosity, etc. You could also look at the option of using some kind of refrigeration cycle to remove more heat per liter of fluid rather than pumping more fluid and see which option moves more heat per watt of input. You could use a Stirling refrigerator for this, but as I said stirlings are built for high efficiency, not high throughput.
stan.hornbaker
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Response to Can a Stirling power a 'Hurricane Tower" fresh water producer?

Post by stan.hornbaker »

The CHC,(Clean Hawaii Center) web site at has a schematic of the proposed tower. Construction of a prototype is in progress.
A commercial web page is at: http://www.oceanit.com/environ_industri ... salina.htm
It appears that the major power requirements are for pumping the cold deep ocean water AND rotating the "drag propeller" in the center of the tower.
erdoc
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Response to Can a Stirling power a 'Hurricane Tower" fresh water producer?

Post by erdoc »

You don't exactly need to bring up new water from a mile down. You can have a closed system circulating water with a heat exchanger a mile down to cool the return water in your closed system back down to the 50 degrees F ambient temperature. The pipe for the upflow in the closed system would be insulated to maintain the lower temperature of the cooled water until it was used on the cool side of the Stirling engine. The downflow side of the closed system would not need to be insulated. The warm side of the Stirling engine would be warmed by the warmer surface water. Once the flow of water got started, the power from the Stirling engine would be essentially inexhaustible as long as the surface water stayed warmer than the deep water. In Hawaii, that would be as long as the sun keeps shining.

Clearly, you'd have to think really big to make a project like this produce a meaningful amount of energy, but there's no theoretical maximum to the size of a Stirling engine.
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