Naked Science Forum
Non Life Sciences => Technology => Topic started by: peppercorn on 07/11/2013 17:09:37
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I came across a TNS Science Interview with Charlie Paton (http://www.thenakedscientists.com/HTML/content/interviews/interview/1157) from way back in 2009.
Here (http://www.seawatergreenhouse.com/process.html) is a simplified illustration of the principles - which I've been aware of for some time, but only just realised TNS covered it.
The potential of this simple concept seems very promising - Why, two decades after its inception, has such a powerful idea not spawned greenhouses across Africa and the Middle East?
Enthusiasm aside, I have to admit to still being a little unclear on the cycles (heat and water) involved. For instance, Charlie says "At the moment, we [send a strong brine back to the sea], but our intention is to separate out the various minerals and indeed use them for the plants themselves". - But I get the impression that the returning (highly saline) seawater is also carrying away excess heat.
...so what's actually going on?
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I think you have put your finger on one of several difficulties.
Plants need sunlight as well as water, but if you cover an area of land with a transparent roof, the inside gets hot (I think climate alarmists call this the Greenhouse Effect) rather than cold.
Seaater trickling down a cadboard filter sounds very green. And that's the problem. The damn stuff isn't sterile saline but about the most biologically active soup you can imagine, laden with minerals that crystallise out and block any holes that haven't been clogged with seaweed and crabs. The cost of maintaining anything cooled by seawater can be enormous unless it is made of copper.
You could indeed separate out the minerals and spread them on the land. That's how the Romans used to punish their enemies.
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Yeah, I didn't much like the sounds of a 'cardboard filter' either; problems, problems.
Though, so the argument goes, the salt should kill off many of the nasty bugs coming from the outside.
But that aside I think I'm getting a handle on the process a bit more now...
If I've got this right, there's basically just two steps which both rely on the relative humidity effect. The first is turning hot dry air into cooled wetter air - saturating it. Then as the air moves through the 'greenhouse' it absorbs the suns heat coming in through the roof - meaning by the end of its 'indoor' journey its R.H. has dropped again, so then a second lot of seawater is added and then immediately exposed to cool surfaces of a heat exchanger that condenses out much of the fresh water held in the 'wet' air.
You still have a lot of very salty water at the end of it all though.
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This is similar to what has been used in some commercial greenhouses called a fan and pad evaporative cooling system, but of course, with the salt water.
I think the pads can be made out of a variety of materials. Perhaps they could be cleaned if made with stainless steel or other salt resistant metal. Could you design a sturdy pad that could be dried, then have the salt shaken, punched, or blown out?
I could imagine drying the cardboard filters, and perhaps burning them which should allow some energy recovery, but the resulting ash would be a mix of good minerals, and salt.
If there is insufficient shallow well water, or one wishes to protect the shallow water table, perhaps this could also be done with briny water from deep wells. Or, even briny water from oil wells.
If the recovered water could be purified with algae and hydroponics, it may be able to be sold for human or livestock consumption at less of a cost than normal desalination.
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See later notes. The briny water from deep wells may be sufficient for an evaporator, however, it is typically warm, so it could not be used to passively cool the condenser.
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Though, so the argument goes, the salt should kill off many of the nasty bugs coming from the outside.
Hold on! Those "nasty bugs" live in salt water! They will colonise the filters until the salt concentration kills them and the filters are clogged with dead bugs.
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There are some bugs such as cholera that can survive in salt water.
However, most of the ocean bugs shouldn't harm your plants in the greenhouse or the people working in the greenhouse.
Of course, one may not desire the salt and caustic buildup on EVERYTHING.
And how is the heat rejected?
Ok, evaporation would cool the evaporator, and all the air flowing through the evaporator.
At the exit, however, all the moist air flows through the condenser. This should warm up the condenser. And, of course, a hot condenser would be ineffective.
Based on the diagram that you linked to above.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.seawatergreenhouse.com%2Fimages%2FSGdiagram.jpg&hash=a28ef28449134b00a15514ce0cc2a296)
Cool seawater is pumped into the condenser. Design it right, and any seawater that is pumped up from a mile deep will be somewhere near 4°C. And, other than requiring a long pipe, the pressure should more or less equalize with what it would be to pump surface water.
Anyway, so the condenser can be run at about 4°C to be effective.
Presumably the brine return will also provide some of the energy for pumping the sea water.
Thinking about this, one may have problems using deep briny well water as mentioned earlier as it is likely warm rather than cool as the deep ocean water is. So, one couldn't directly use the deep briny well water to run a condenser.
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On the evaporator side, why not build a tank and a bubbler rather than the cardboard/fiber evaporator.
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It would likely be much easier to maintain the bubblier than the "cardboard" evaporator or the fan & pad system. It should be cleanable.
I'd anticipate that some salt would still get atomized with either system, so the whole greenhouse would have to be designed to be salt-resistant, and also one should attempt to seal it so that the salt wouldn't leach into the soil (assuming not right on the coast line).
Thinking about this a bit more.
The goal is to cool the greenhouse.
Thus, sending the super-cold water to the bubbler is probably best.
However, one would get better evaporation by using warmer water left over from the condenser (and less water pumping/volume/consumption).
If one brings the temp up from say 4°C to somewhere between 10 and 15°C, one would still have it cooler than outside summer temp. Evaporation would also help keep a bit cooler.
Would one get cool, dry air out of the condenser? Perhaps some of the greenhouses would work best with lower humidity, so perhaps the cool could be reused. If one had lots of cool seawater, one could also combine the evaporator/condenser into a single unit to make a really complicated air cooler, but also get the distillation system for cheap water.
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I think the consensus here is that the cardboard inlet pad is perhaps (tbh its pretty definate) not the best material to use.. There is pretty much nothing you could use to handle the salt water for any protracted length of time (maybe on a rotary filter system with a scraper removing dried salt). A PTFE fibre nest may be a solution.
The bubbler idea is good, but due to the reduced surface area, cooling will be drastically reduced.. will it be reduced below what is required? Dunno. Practical experiments would determine.
Fresh water would be better across the board for this type of system, and fresh water generators are pretty simple to build just using the sun's energy, Deep sea water would still be able to be used for the cooling which is where the beauty of this would be..
It look green on the surface, but if we worked out the energy required to make and run it, it wouldnt seem so green. BUT still would work wonders in the hottest parts of the world.
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A lot of places have a greenhouse design in which the greenhouse can be mostly uncovered in the summer, thus better cooling. However, one would lack the condensation benefits of the above system. And, while transpiration is important for plants, that water would also be lost.
Shade cloths can be used, but reduce the sunlight.
The high humidity might be good for some tropical plants, but may reduce the transpiration and nutrient flow for other plants.
I think much of the cooling may be from the cold seawater necessary for the condensation, and the system may well put out cold air which might be able to be used in a drier greenhouse, or perhaps for other uses.
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Fresh water would be better across the board for this type of system, and fresh water generators are pretty simple to build just using the sun's energy, Deep sea water would still be able to be used for the cooling which is where the beauty of this would be.
But fresh water is what is at a premium in the regions his greenhouse is designed for. Surely this concept includes a 'fresh water generator', it's just that the water is consumed within the system.... it could even be said that the water is transported away in the veg grown.
It look green on the surface, but if we worked out the energy required to make and run it, it wouldnt seem so green. BUT still would work wonders in the hottest parts of the world.
Charlie states in the interview: "typically, we use around two kilowatts of electricity to remove about a megawatt of heat". And he accepts that this design really only works in arid locations.
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I could see a problem also with needing to insulate the pipes carrying the deep seawater in land. It would seem pointless to draw up cold seawater from the deep, only to end up loosing the differential in transit.
For the 'greenhouse', as a system, I would have thought that, in an ideal design (ie. one that is probably impossible in reality), the air leaving the condenser at the back of the greenhouse should be indistinguishable from the ambient air - ie. same hot, dry conditions as is drawn in to the greenhouse.
Looking back at the diagram, it would preferable if the temperatures were stratified with the hottest air near the ceiling - By switching the air flow back on itself with outside air entering lower down and then sent back round at the top it may be possible to create a low-level mist whilst the the higher warmer air does the job of stripping most of the sun's I.R. out.
Which makes me ask, what materials let through U.V.? Glass blocks it, but I think in (all?) greenhouses that would be a bad thing.
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I think there have been studies indicating that the growth of many plant species is actually inhibited by UV light, although I'm not sure if it depends on the amount of sunlight.
Plastic greenhouse materials typically have UV coatings to help prevent damage to the materials from UV light. Glass is at least transmits part of the UVA, unless it has specific UV coatings.
It may take some time for water to be pumped up a mile, or perhaps a couple of miles of ocean depth depending on the pipe size, and volume being extracted, so insulation would be important.
Double or triple walled pipe might help. One could put high pressure air in the outer pipe. However, the air pressure would be close to constant bottom to top, while the water pressure would not be.
Perhaps one could use pressurized insulated modules, each module say 100 feet each. Step-wise increasing internal air pressure with increasing depth.
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For pumping water up from a mile down, this is in fact NOT what will happen, the level in the pipe will be the same as the level of the sea as atmospheric pressure will do all that work for you. Its the final bit to the greenhouse that will use the energy.
A single wall pipe will be fine (as the pressure inside will be the same as the pressure outside) you wouldnt want to go too close to the sea floor as the crud it will 'suck' up will clog bits and pieces.. and a fish filter would be advisable if you dont want to give a whale a hickey and have to empty tuna out of the pump casing every time a shoal goes past.