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How much does convection slow heat flow downwards in still water?

06/08/2022 5:08 PM

I made a small 4 seasons greenhouse 8ft by 12ft (max I am allowed) but I didn't know to insulate the side walls. Then this year, I found videos of researchers on 4 seasons greenhouses at universities in the north central USA. Their greenhouse are at least twice the size and usually much bigger than that. They insulate their greenhouse sidewalls down 2 to 4 ft below the surface. They generally fill under the greenhouse with gravel or clay and then they blow air from the greenhouse into the gravel during the day to store its heat (and condense some of its water) . Heat still escapes downwards through the gravel by conduction. Why not use water as your fill material? Water has a higher heat capacity than gravel. And rainwater is free while gravel here is $70 or $80 per yard plus delivery. And if you had a meter or 4 ft deep water tank under the greenhouse, wouldn't the conduction of heat down be slowed down by convection moving heat up? I realize it's a bit more complicated to have a radiator and fan to cool greenhouse air rather than just a fan blowing the air under the soil but maybe it can be worth it? (I have a water tank under my greenhouse because we get drought in summer, so it was practical to store water in it). Is the risk of it freezing too great in a 4 seasons greenhouse in the central states? Anyway, I'd love to know how much convection slows heat flow down in still water compared to in soil or gravel. I looked online but found nothing then I asked some lake researchers and lecturers in emails a few months ago and I never got a reply. (Thermoclines are aparently a big deal in many lakes and these people would know). Any of you have the figueres or a formula? Thanks Brian

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#1

Re: How much does convection slow heat flow downwards in still water?

06/08/2022 9:37 PM

I see a problem with moist warm air flowing into a greenhouse on a cool night, that being condensation....now I don't know what you're growing and if it can handle wet leaves, which it seems would be a possibility...but I guess you just have to keep trying different methods until you find something that works for you....What you would do is called forced induction, where the pit is sealed with a fan blowing the air in and a similar sized vent at the bottom on the opposite end, which closes at night.. that would have another vent that then opened at night with another fan mounted that blows through the pit the opposite way.....or you could have a closed loop water system that heated the water from an external solar panel, like they use for pools, that had a converted steam radiator beneath whatever cover you use, this would give you the heat without the high humidity....probably could use an old water heater for hot water storage buried nearby...

https://www.youtube.com/watch?v=MvEzmgJ5nDM&ab_channel=kukomio

https://www.youtube.com/watch?v=fNLpeN34efU&ab_channel=J0Boa

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#2

Re: How much does convection slow heat flow downwards in still water?

06/08/2022 10:10 PM

It seems you could insulate the side walls the way it's done with windows - two panes of glass (or plastic) with an air space within.

And if you had a meter or 4 ft deep water tank under the greenhouse, wouldn't the conduction of heat down be slowed down by convection moving heat up?

Convection only operates when the bottom of the fluid is warmer (lower density) than the top (higher density), causing circulation. When the water gets warmer with elevation, convection is turned off.

Conduction transports heat from higher temperature to lower temperature. If the ground temperature is lower than the greenhouse air temperature, heat will be conducted into the ground.

So it boils down to which is the better heat insulator, liquid water or gravel. Solid rock is a somewhat better heat conductor than water, but gravel has air spaces and limited contact between gravel stones which increase its thermal resistance. I'm betting that the gravel with trapped air would be the better insulator.

Here is a chart with the thermal conductivity of various materials.

https://material-properties.org/thermal-conductivity-of-materials/

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#10
In reply to #2

Re: How much does convection slow heat flow downwards in still water?

06/13/2022 4:42 AM

Water has a specific heat of 4187 J/kg.K ,which is not exceeded by many materials, except Hydrogen and Helium,but gravel (which is approximately 504-520 J/kg.K) has a higher density,so for the same space,gravel will hold more heat,on a weight basis,water will hold more heat.

A cubic yard of gravel weighs approximately 2800 lb.A cubic yard of water weighs about 1682 pounds,and when saturated in water,the water will fill in the voids in the gravel,adding even more heat capacity to the gravel.

There are certain salt lakes that have such a high,saturated salinity,the normal convection currents are reversed.Heated water from the surface absorbs more salt,which makes it heavier,so it sinks to the bottom,and the colder water rises,which is heated,etc.which traps heat at the bottom of instead of the top.

A saturated saline solution is approximately 25% salt per pound,depending on temperature of the water,so adding more than this will give a buffer to allow for temperature changes.A gallon of water weighs about 8.3 pounds,so approximately 2 pounds of salt per gallon,minimum.Do you have an in ground swimming pool?

Solar energy on land is approximately 1000 watts per square yard,but nothing is 100% efficient at collecting it.Solar cells are approaching 25%.

A 20x40 ft pool might collect 10-15 %,so 800sq ft might capture around 13000 watts based on a SWAG of mine,no data to back it up,just a guess.

I may be way off;Just a back of the napkin estimate,so take it with a grain of salt(pardon the pun).

If the water freezes,(not likely with saturated saline solution) the latent heat could be captured by a properly designed heat pump.

I am surprised this has not been used to store solar energy;sounds like a cheap way to store solar energy to me,especially if you color the water a dark color to absorb more heat and perhaps a bubble wrap type cover to insulate it and prevent evaporation cooling by wind,etc.-----I am sure this method has been explored,and there must be a reason it is not used.

Hmmm----Jus' thinkin'$$$$

Constructive feedback is always welcome.

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#11
In reply to #10

Re: How much does convection slow heat flow downwards in still water?

06/13/2022 5:05 AM

"Water has a specific heat of 4187 J/kg.K ,which is not exceeded by many materials, except Hydrogen and Helium,but gravel (which is approximately 504-520 J/kg.K) has a higher density,so for the same space,gravel will hold more heat,on a weight basis,water will hold more heat.

A cubic yard of gravel weighs approximately 2800 lb.A cubic yard of water weighs about 1682 pounds,and when saturated in water,the water will fill in the voids in the gravel,adding even more heat capacity to the gravel."

4187/520 x 1682/2800 ≈ 4.8

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#12
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Re: How much does convection slow heat flow downwards in still water?

06/13/2022 5:22 AM

I will leave the heavy math to you.

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#3

Re: How much does convection slow heat flow downwards in still water?

06/09/2022 2:46 AM

To determine how much heat you need you need to run a heat load calculation on the greenhouse, get the cubic feet, then the maximum temperature differential under which it will be operating, then you determine the amount of BTU's that will be required, then you can calculate the amount of water that needs to be stored and heated, and the size solar heater required....then you need to figure out the cubic feet per minute of air required to maintain temperature, that then you determine the fan and power required to pump the water and blow the air... for instance say your greenhouse is 8x8x12 = 768 cu ft max temp differential 60°- 10°, so when it's 10° outside your system will be able to maintain 60° inside ...so glass has an r value of about 5, and each material that is used in the building has an r value, you add up all the values by square footage....let's say your total heat load for the required temp differential is 10,000 btu's and your duty cycle is 8 hrs on....so you have to produce 10k btu's for 8 hrs a day....1 BTU is enough heat to raise the temperature of 1 pound of water by 1°F. a gallon of water weighs about 8.3 pounds. say you want to heat the water from 70 to 110 degrees, that's 40 degrees, say you've got 60 gallons or 500 lbs of water, to raise it 40 degrees = about 20k btu's

Probably need about 2 ea 4' x 8' collectors...

https://www.builditsolar.com/Projects/WaterHeating/CPVCCollector/CPVCCollectorTest.htm

Lot cheaper to just put a little space heater in there...or some high powered grow lights...

https://greenhouseemporium.com/blogs/greenhouse-gardening/grow-lights-for-your-greenhouse/

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#4

Re: How much does convection slow heat flow downwards in still water?

06/09/2022 8:29 AM

Well, whatever happens do please allow for dealing with the build-up of unwanted biological material in the stored, warm water in this proposal. Legionella pneumophila is the principal problem, as many have found to their cost.

And, er, <...gravel...per yard...> costs nothing either, if one has access to it. It's the access and delivery that costs.

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#5

Re: How much does convection slow heat flow downwards in still water?

06/10/2022 5:35 AM

This is a bit off topic. Have you considered mounting a simple wind turbine above the greenhouse with the bottom of the shaft driving a rotor in the water under the green house.

There are many different types of Vertical Axis Wind Turbines: you can make a Savonius one from an oil drum cut down the middle.

The rotor in the water could be as simple as the rectangle shown above. I know that it seems counter intuitive, but, as long as the rotor allows the VAWT to turn but not too rapidly, then, all the energy will be transferred to the water as heat. Think about it: where else can the energy go?

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#6

Re: How much does convection slow heat flow downwards in still water?

06/10/2022 7:03 AM

It depends upon temperature, like most things. Water has a peculiar characteristic that it reaches its maximum density at around 4degC, which is why only the top of the pond freezes in winter, and not all of it, and marine organisms continue to thrive through it. Which is a Good Thing, really.

What is sought is a relationship between heat transfer by convection and heat transfer by conduction. An internet search might just find it.

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#7
In reply to #6

Re: How much does convection slow heat flow downwards in still water?

06/10/2022 7:17 AM

Just checked. Yep. There's loads of it.

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#8
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Re: How much does convection slow heat flow downwards in still water?

06/10/2022 12:40 PM

Ok, Imagine in a room at 10 C heating the top inch of a 3 ft long 1 inch diameter vertical steel bar to 95 C. Its on a thermostat. Heats to 95 then cuts out till it drops to 90 then back on again. How long before the bottom inch gets to 15 C? Then we put the bar in a 2 ft high 6 inch wide vase of water at 10 C and re-do the experiment. 3rd experiment is with the steel through the bottom of the vase through a cork ring so it doesn't directly heat the glass, heat the steel at the bottom to 95C and see how long it takes before the top of the bar gets to 15 C. I am pretty sure that the bar in water heated at the top will take far longer to reach 15C at the other end, not just because of the thermal capacity of the water but because convection is acting in direct opposition to conduction in that particular case. I haven't found anything specific about how much heat you need to "beat" the convection effect and have convection creating enough of a current in the water to get heat down through the water faster. In lakes, it seems to need wind over the lake to move the water and break a termocline and get heat to reach the bottom. In my case, I store water in a tank under the soil because it's a good way of storing rainwater and because I use an airlift pump to circulate the water and because I get charged for water use in my municipality while rainwater is free. I don't get why researcher in universities are adverse to using water under the greenhouses as thermal buffer. I guess they don't pay for their water. I will have to check what creatures live in the water tanks. (It's been about 18 months so it should have a mature community. I expect ostrocods, copepods etc. (remember that water drips down every day through the soil, then through a couple of inches of an air gap as the water circulates to the plants) so it's likely to be decently oxygenated water and no more dangerous than pond water. Anyways, thanks for the answers. We finally had an unusually cold spring in Victoria and the greenhouse produced way more than I expected while outside, even the kale did poorly.

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#9
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Re: How much does convection slow heat flow downwards in still water?

06/12/2022 4:13 PM

<...Imagine...>

It's difficult, because in this universe, the <..steel bar...95degC...> wouild heat the <...room...> instead.

Perhaps the <...we...> will put a video of an experiment on Youtube for peer review, and copy the link into this thread.

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#13
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Re: How much does convection slow heat flow downwards in still water?

06/15/2022 2:40 PM

Ok, Imagine in a room at 10 C heating the top inch of a 3 ft long 1 inch diameter vertical steel bar to 95 C.

Water flows downhill and heat flows from hot to cold. To move water uphill requires a pump, and to move heat from cold to hot requires a heat pump, both of which require an energy input.

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#14
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Re: How much does convection slow heat flow downwards in still water?

06/16/2022 12:55 PM

Ok, Rixter, thank you. How does heat flow from hot at the top (15C to 20C air interphase) to cold at the bottom in an insulated (on the sides) pool of water that is 80 cm wide and 1 meter deep and is in direct connection to the earth (5C) at the bottom of the pool? All I am saying is that there will be a "fight" between convection bringing heat up and radiation bringing heat down towards the cold earth if the water is still. There will be tiny eddies at the top as the water warms that move heat back up but no massive convection that moves heat down. I have been unable to find a formula to characterize that "fight". Maybe it only slows radiation directly downwards (and heat loss) a little bit, maybe it slows it a lot. All I am asking is if any of you know the formula for that. It's ok if you don't.

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#15
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Re: How much does convection slow heat flow downwards in still water?

06/16/2022 3:14 PM

Heat flows by radiation, convection, and conduction, from a higher temperature to a lower temperature.

Water is opaque to thermal radiation (wavelength ~10 microns), so that's out.

Heat flow by convection requires the fluid to circulate. Unless it is forced to circulate with a pump, this requires the density to decrease from top to bottom. Most of the time density increases with decreasing temperature, which would limit the direction of heat flow to be in the upward direction for convection.

However, below 4o C, the density of water decreases with decreasing temperature as shown in the diagram below. So if the temperature is between 0o and 4o Centigrade, heat can flow downward by convection as the warmer water is slightly heaver than the colder water.

Outside of the 0o to 4o C range, heat transfer downward is only by conduction. The thermal conductivity of water is 0.598 W/mCo.

How does heat flow from hot at the top (15C to 20C air interphase) to cold at the bottom in an insulated (on the sides) pool of water that is 80 cm wide and 1 meter deep and is in direct connection to the earth (5C) at the bottom of the pool?

Think of it like an electrical circuit, where temperature is analogous to voltage potential and heat flow is analogous to current.

Thermal conductivity of water K=0.598 W/ (m Co)

For temp = 20 Co

Heat Flow = ΔT * K * Area / Depth = (20 Co - 5Co) * (0.598 W/mCo) * (.8m * .8m) / 1m.

Heat Flow = 5.74 W = 4935.51 calories/hr

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#16
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Re: How much does convection slow heat flow downwards in still water?

06/17/2022 3:29 AM

Good answer.

Just to keep it simple: there is no "fight" between conduction and anything else because there is no radiation and no convection will occur in the stratified water.

(Assumes everything above four degrees C)

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