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Best way to move air heat into a water tank?  RSS feed

 
pollinator
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I'm thinking about my future greenhouse again. It will have a large tank of water in it, probably a fish tank. What is the best way to move excess heat from the air into the tank? Bubbling lots of air through the water? Fountaining the water into the air? Running coolant through a radiator and back through a heat exchanger in the tank? Something else? An advantage of the first two is that it would aerate the fish tank. An advantage of the third is that the radiator fans would create air movement.

Which would be most efficient?
 
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Natural conduction is more than adequate for most greenhouses.

I would use the bubbler if you have fish.  The waterfall if you like to sit and relax in the greenhouse.

But zero input is most efficient.
 
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R Scott wrote:Natural conduction is more than adequate for most greenhouses.



Agreed. If you had a metal tank with a high ratio of surface area to volume then it would passively conduct the room temperature to the water at a faster rate. In other words there would be less of a temperature difference between the air and water. The opposite effect would also be true, if something happened where it got very cold then the water would cool off much faster too, so that is something to thing about. A passive system would be less likely to have a catastrophic failure.

A few ideas I can think of would be if you can get some or all of the water higher off the ground. The coolest air that sinks to the floor could be avoided by elevating the tank. If that isn't feasible, then perhaps if you elevate a filtration system with some sort of tank (with a high ratio of surface area, perhaps something short but wide) it could absorb heat before being put back into the tank.

I'm thinking there would have to be a significant amount of heat difference for bubbling air through the water to be effective. Even with an air stone, it may impart most of the heat into the water, but there would be an evaporative cooling effect, like an air conditioner, as the bubbles leave the water. If you are already doing it to increase oxygen in the water, then it would make sense to take air from the warmest spot, but adding more bubbling than needed to try to raise the water temperature might have diminishing gains.

One of the most effective things I have seen was to simply use a dark color on the tank so it absorbs light energy. Even if it doesn't have a ton of direct sunlight on the tank, it can absorb infrared heat energy from warm objects in the greenhouse, such as plants and their pots. Also, you could use smaller tanks or barrels as part of the filtration system that are dark and placed in the sun. You could even use masonry as a buffer to absorb heat in the day and release it at night to moderate changes to the water temperature. You could always cover it with a white cloth to stop that effect when things are getting too warm.
 
gardener
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I second the importance of keeping the evaporation factor in mind.  Circulating warmest air through ducts surrounding the water tank could be an option, depending on your other goals, what the tank is made of, where in the GH it is located, and countless other variables unique to your situation.  Good luck with it.   Planning ahead is surely one of the most important facets of any build.
 
Thekla McDaniels
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Another idea..... a heat diffuser, like a radiator in a car engine, or a heat diffuser for a wood stove.  It would involve many small diameter coppper or other highly conductive material tubing running through the water, where the water is the cooolest.
 
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Excess heat, which is accompanied by excess sun, can be moved quite efficiently from the ceiling to a deep bed of rocks underneath your water tank via a solar panel powered thermostat activated fan pumping air from the ceiling into the rock bed via a duct or pipe. The bed of rocks stores the heat and slowly releases it, in this case up into the water, which then slowly releases it back into the air.  While this will cost a little bit, with initial investment of cash and labor, the system basically runs itself and most of the stuff can be found at a scrap yard for next to nothing.  The main moving part-the fan- is a very common item and can be easily replaced if it burns out. 
 
Thekla McDaniels
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Roberto, I love the simplicity of your suggestion,  I may use it if/ when I have my own place again
 
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Gilbert Fritz wrote:I'm thinking about my future greenhouse again. It will have a large tank of water in it, probably a fish tank. What is the best way to move excess heat from the air into the tank? Bubbling lots of air through the water? Fountaining the water into the air? Running coolant through a radiator and back through a heat exchanger in the tank? Something else? An advantage of the first two is that it would aerate the fish tank. An advantage of the third is that the radiator fans would create air movement.

Which would be most efficient?



This is for summer time? First thing to consider is the fish's requirements of temperature for best growth I would think.
The second thing to consider would be plant requirements for humidity and then how to flush any excess heat.

Most fish don't do well in water above something in the 75 degree f range. Trout would not be liking any temperature even close to 75 degrees f.

Redhawk
 
Gilbert Fritz
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Thanks for all the suggestions!

Since the climate here has wild temperature fluctuations, any such system would mostly be working the Fall, Winter, and Spring, with excess heat being vented in the summer. For instance, an 80 degree February day can be followed by one only reaching 40 degrees for a high. Even more frequently, a 60 degree day can be followed by a 20 degree night. During the day, relatively high temperatures combined with strong solar radiation can quickly overheat any clear structure, even with significant thermal mass and passive ventilation, only to be followed by a freeze at night.

I'm hoping that this system can store surplus heat while providing a warm tank of water for Tilapia. (I'd have a backup tank heater for extended cold spells, and a vent fan that could be used during the summer or whenever the temperature in the tank started climbing too high.)
 
Bryant RedHawk
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Hau Gilbert, for such fluctuations in temperature, water tanks would indeed work quite well.
Black or black painted drums could be lined up or stacked for a heat sink effect, then you would be able to regulate the tilapia water quite easily without much worry of overheating the fish.
It would take some plumbing either with rigid pvc or pex piping and probably only one pump needed with a valve system.
For heating the water you have options such as those already mentioned or you could pipe water to the roof line and back to the holding tanks.
From there you could have a thermostat for the fish tank and rig it so when the temps there dropped to a set point you would get a signal to open that valve to send heated water to the fish tank and the cold water would flow to the holding system via the roof pipes so the cool water is heated prior to going to the holding tanks. Another idea would be to use the green house to set up an Aquaponics system set up.

You might want to do some drawings of different setups to help you select what will work best for you.

Redhawk
 
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I wonder if enclosing the tank in glazing would trap even more heat next to the tank?
So a mini tank greenhouse within your main greenhouse. Same principle as double glazing.
 
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Gilbert - Have you heard of Jerome Osentowski?  He's in your neck of the woods. He uses the concept of moving heated air from the peak of the greenhouse through ground tubes buried under the greenhouse...and the perimeter of the greenhouse is insulated to/below the frost line.  He has a book, you can get it on amazon... The Forest Garden Greenhouse.  There was an updated video from Justin Rhodes a couple months ago on youtube where he visits Jerome's place.  Justin Rhodes Colorado Bananas <--youtube search that and it'll come up.

I was going to use earthtubes in my greenhouse, but I'm unsure they're necessary in my area, the frost depth is only 18".  There are over 300 days of sunshine in my area, so I'm testing a double layer attached greenhouse with perimeter insulation without earthtubes.  I will decide then after this test setup if I will want earthtubes for my permanent greenhouse.  There is a general rule of thumb of how much air and how slowly to move it to make this system most effective.  I think Jerome talks about it in his book, yet I don't recall...I'll go thumb through it in a sec... but I learned this from someone else on the internet.  Not finding that info in his book.

There is a man in maybe Wisconsin or Minnesota doing the same concept.  He grows citrus in an attached greenhouse and he put his earth tubes outside of his structure wrapping around his home.  I may have seen video of it on youtube a few years ago....and/or read and saw pictures somewhere on the net.  I can't recall exactly how to find that one....I probably learned about it from someone here.

 
Roberto pokachinni
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Gilbert - Have you heard of Jerome Osentowski?  He's in your neck of the woods. He uses the concept of moving heated air from the peak of the greenhouse through ground tubes buried under the greenhouse.

  This is the basic concept of what I described above; and where the idea behind my post originated.  Geoff Lawton also did a video about a greenhouse project at a school in my province that uses a similar system to Osentowski's.  I don't know about the Colorado system but the B.C. system also have an insulated shutter that covers their curved sloped glazing at night.  The purpose of the earth tube capture of heat in both of these greenhouse projects is not to heat an aquaponic tank, but to moderate the winter cold from the ground upwards with a slow release.  In the case of heating the tank directly from the ground source, one can have all of the vent holes release upwards against the bottom of the tank.  A person could have a simple baffle at a manifold in the system (operated by the pull or push of a metal rod) that could direct the flow outside, into another structure, or against the bottom of a second tank.  This manifold baffle could also possibly be switched via a bimetalic thermostat, or another low tech thermostat when a certain temperature is reached in the tank.  
 
pioneer
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Joshua Parke wrote:There is a man in maybe Wisconsin or Minnesota doing the same concept.  He grows citrus in an attached greenhouse and he put his earth tubes outside of his structure wrapping around his home.  I may have seen video of it on youtube a few years ago....and/or read and saw pictures somewhere on the net.  I can't recall exactly how to find that one....I probably learned about it from someone here.



Are you thinking of the Citrus in the Snow gentleman from Nebraska?  If not and there is someone in WI/MN that is doing this I'd be extremely interested in hearing more about it!!!
 
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I have a aquaponics greenhouse with Tilapia in Michigan. In the winter no kind of daytime heat will heat the water in any way. While the sun (when we have it) will provide decent heat in the daytime I don't see any way to grab enough of that heat to get through the night. I have a wood heater with a thermostatically controlled propane heater as backup to maintain GH temps at 60F and a bucket heater in my 300 gallon IBC FT to maintain 70F. I have a design for a rocket heater setup where the IBC would sit on the heater mass and heat the water and GH while using the FT as an additional mass heat storage. I'd love to build it but would have to tear everything down and start again to do it. If I could figure out how to attach a pic I would add it here.
 
Joshua Parke
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Mike Jay wrote:Are you thinking of the Citrus in the Snow gentleman from Nebraska?



Yeah, that's the video I was thinking of.  I thought he was closer to the great lakes for some reason.
 
Thekla McDaniels
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Off-topic but not that far off there's a wood-burning stove that heats hot tubs I think it's called the snorkel

It might be of interest to those of you looking for ways to heat tanks of water
 
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For what it is worth, I have concrete figures, if it helps answer your question.  Simple passive radiative heat accumulation in large tanks is very inefficient - the surface area-to-volume ratio declines progressively as the tank volume is increased.  The temperature gain in my 450 Litre tanks with just radiation on the front surface, and conductive heat gain between the air through the tank walls, in the course of a day in March where the air temperature in the greenhouse reached 28 degrees, was about 2 degrees.  Adding active heat capture by pumping the water from the tank through a salvaged car radfiator, with the air in the peak blown through the rdaiator by computer fans, resulted in a 10 degree rise in temperature of the water (8 to 18 degrees) on a day when the air temperature hit 32 degrees.  (Outside temperature was -5 degrees).  Incidentally, the peak air temp was reached at about 3 pm, but the peak temperature in the water wasn't reached until 7:30 pm.  That is, there was a lag in the ability of the system to capture the heat available, but there was still useful heat to capture, stored in the air long after the sun set and heat gain had ceased. (The peaks of temperatures of air and water were offset by 4 1/2 hours.).  At the same time, the next day was heavily overcast, and very little heat was gained - the water temperature fell to 4 degrees (from 18) over the next 24 hours. (Soil temperature fell from 12 to 7, and remained around 7 over the succeeding several days of cloudy, snowy weather.  (In general, soil temperatures varied over 2 or 3 degrees, water temperatures varied over 5 to 10 degrees, and air temperatures varied over anywhere between 15 and 25 degrees over the course of a day.)
 
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Another physics based solution:
A water pump that produces enough pressure to run a mist nozzle above the water tank.
The water will evaporate in the air, removing heat from it.
The moist air then condenses at the surface of the water tank, releasing the heat into the water.
 
Thekla McDaniels
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I tried to use the quote, but no luck, it's my device I am sure....

About water vaporizing then condensing on the cooler surface of your he water, I am wondering, wouldn't the inside surface be cold enough to cause a lot more condensation, losing the heat to the outside?  I am thinking that but for the aeration needs, it would be more heat-saving to keep the surface of the water covered.  Am I missing something?
 
Sebastian Köln
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About water vaporizing then condensing on the cooler surface of your he water, I am wondering, wouldn't the inside surface be cold enough to cause a lot more condensation, losing the heat to the outside?  I am thinking that but for the aeration needs, it would be more heat-saving to keep the surface of the water covered.


If the water is warmer than the air, definitly!
The water at the surface will then evaporate and recondense everywhere (transfering heat from the water into the room). This may be good or bad.
 
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Gilbert Fritz wrote:What is the best way to move excess heat from the air into the tank? ...Running coolant through a radiator and back through a heat exchanger in the tank?...



Gilbert, (and all), it's an excellent question.  I think you've identified perhaps one of the most difficult engineering challenges in greenhouse design.  It's common for convection to be the bottleneck in heat transfer.  I think natural convection is nowhere near sufficient, especially for cooling during the daytime because "heat-rises".  I've been intensively researching and analyzing this engineering problem lately.

I think better technology can be developed (or exists somewhere and can be adapted), but right now I think the best, easiest, most commonly-available technology for this is a large aluminum-fin radiator (or three) with a fan blowing through it, like the type used in automotive radiators or air conditioner condensers.  It needs that huge amount of surface area.  While an evaporative "swamp cooler" and its variations could work, this dry setup also lets you avoid problems caused by high humidity. 

You can try to up-cycle a used car radiator if the price is right, but I'd only recommend that if it's a good value.  Be prepared for radiator leaks, and do what you can to prevent corrosion.  The “12 volt” fan is also about right to power with solar cells.  A radiator like this will reportedly flow heat at a rate of around 9,000 BTU/hr (daytime avg. 80,000 BTU over 9 hours of sun).  That's in this lower-temperature-difference application (the figure is more like 20-50 kBTU/hr in a car that's working with a higher temperature difference). 

Even if with this design, greenhouse can still have daily temperature swings of 40 deg.F!  In theory it could require *4* radiator-type heat exchangers to reduce that swing to 10 deg.F , plus the enormous amount of thermal storage to go with it, equivalent to a knee-high water tank.

It seems that usually what folks do is use one or more of the following techniques: 1) vent air in the summer, 2) put shades overhead to block summer-sun, 3) use the low solar heat-gain type of low-e glass, especially overhead.  4) use more insulation , triple-wall windows, etc. 5) heat in the winter.  6) don't use in the winter, or only keep cold-tolerant plants (like spinach, arugula, broccoli, carrots?) that were established in the fall and kept through the winter in a semi-dormant/slow-growing state. 

Also it may help to have all light-absorbing surfaces be thermally connected to the thermal mass, or else use light-colored materials for anything that isn't.  Once the heat goes into the air, it's surprisingly difficult to retrieve it and store it again,  (usually it's just vented). 

Even with the thermal storage and heat exchangers, the use of some of these techniques is probably needed just so that the water storage only has to be knee-high instead of waist high!  :-) 

Let me know if you think I should continue working on developing a better solution.  Any idea what a better solution might be worth?  I mean what would be a good target budget for this component?  I guess just something that's better than the existing way.  

edit: Oh by the way, here's a relevant thread that I think is worth a read:  http://permies.com/t/62395/Greenhouse-build-thermal-mass-storage

-Mike
 
Mike Jay
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Mike Phillipps wrote:Let me know if you think I should continue working on developing a better solution.  Any idea what a better solution might be worth? 



Yes, you should continue to work on this

My thoughts are coming together but I'm also looking for input.  Assuming you have a big thermal mass that can store a lot of heat in the 40-60 degree range, the challenge is definitely getting heat in and out effectively.  I can see at least three ways to get heat in:
  • Circulating water from a coil in the thermal mass to a radiator blowing warm ceiling air through it.  This would harvest warmth from the ceiling with a delta T of maybe 60 degrees?
  • Circulating water from a coil in the thermal mass to a lengthy black plastic pipe that runs back and forth in the ceiling and is exposed to the sun.  This would poorly harvest some warmth from the ceiling air but would also decently harvest direct radiant heat from the sun (when the sun is out).
  • Circulating water from a coil in the thermal mass to a proper solar thermal collector that is outside the greenhouse (or inside?).  It would effectively harvest additional solar energy and pump it into the greenhouse to add to the daily total.  But it wouldn't take any heat from the ceiling so it would remain hot up there.

  • And four to actively get it out:
  • Circulating water from the same coil in the thermal mass to a radiator blowing room air through it.  I'm assuming this would inefficiently move low grade "heat" with a delta T of maybe 10-20 degrees?
  • Circulating water from the same coil in the thermal mass to a long run of black irrigation pipe that is buried or laying on the surface of the soil.  It would heat the soil and roots, which is good, but the air temp could still get pretty low.
  • Pump the thermal mass liquid itself from the large tank to a series of smaller tanks or pipes that can radiate/conduct the heat better due to their much higher surface area.  I'm imagining a buried thermal mass tank and a series of 6" ABS pipes hovering 1' above the floor.  Once the warm thermal mass is pumped up into them, they give off the heat and then the fluid is allowed to drain back to the main tank.
  • Design a waterproof duct that travels within the thermal mass tank and blow room air through it.  You'd need a long enough duct run to heat up the air but then it could be ducted to various points in the greenhouse.

  • One big question regarding getting the heat into the thermal mass is: Do you want to collect hot air to avoid venting it or would you rather concentrate on collecting as much heat as possible regardless of if it comes from the ceiling air or from other sources (sun)?
     
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