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

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

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/

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.

"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

I will leave the heavy math to you.

"... 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 %..."

Solar cells are approaching converting 25% of the incident solar energy to electricity. Efficiency for merely collecting solar energy as heat can be much higher even for simple flat plate collectors.

I suspect efficiency for converting solar energy to heat in a pool that isn't too much above ambient can likely be very high.

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/

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?

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.

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

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.

<...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.

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.

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.

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)

I moved to the San Juan Mountains in SW Colorado and became a Plumber in 1972 and have been on thousands of "construction" jobs so I pass this on as a Journeyman and I am not a math guy other than reading the numbers on my tape measure.

We were the first trade on a job site and usually the last to leave, so I gained knowledge on "how" it's done. Working above 9,000' everyday was most invigorating, and you had to pay serious attention, because construction sites were the most dangerous places on planet earth.

We started to do, "In-Floor" Heating in 1977 and found that, heating a "mass" instead of the "air" was the way to go because it was not convection, and the slabs and floors just radiated. It took a long time to get them to temperature, but you had that same long time locked in when the power went out, and it does. The old timers used fire and mass to exchange to the air and to water. You could get a pot belly to glow red hot in a half hour, but when the fire went out it was back to cold in a half hour, Physics.

Convection heating and air conditioning in most residential buildings, or greenhouses means the air is moving from top to bottom, ceiling fans help this process, but you don't need heat above your head so why waste energy moving air when good "cross ventilation" at the right time of the day, does the trick. A "massive" floor only heats what's touching it, not the air to speak of.

A story of "what's touching it".

I plumbed an animal shelter once, and put warm floors in it for them, them being the animals. I made everybody happy.

The trouble with Loving is that pets don't last long enough and people last too long.

Back to the question of "insulating" a foundation from cold temperatures. The frost level up here is deep, and we have to bury water lines 6'! In your case not so hard. Water proofing, Radon proofing and good insulation are worth their weight in gold.

As far as moving heated air to gravels and water under the building, I wouldn't do it. The only place for "washed" gravels is "around" the building for good drainage, especially on sloped sites. You always want to "show" water where to go because if it "sits", it creates a "sump", and is almost impossible to fix. Older houses in flood plains and wet climates are very suspect, mold is bad.

Here is what we used to do first. on some of our building projects that used In-Floor Heating. When the concrete was finished and stripped, we would finish the mechanical room, get the boiler running, and heat the slab which had, "daylighted" drains installed, and it was constantly "snow melting" without salamanders, (we just paid the boilers), and plastic tents. Even on a small project there is nothing better than having a simple 4" concrete floor with tubes in it. It will always be there for your house, with no moving parts. It is money well spent.

A concrete floor also absorbs the ground temperature below frost line, which is an average of 55 degrees winter and summer, so it can actually radiate "cool" if allowed.

You can "heat" your green house, or a 30' x 50' pad, when needed, with a 50-gallon gas water heater, one circulator, and tubes in the floor serpentine at 9 inches on center, and one timer or t-stat.

Solar hot water is the same (the slab doesn't care) but more complex and not so user friendly, they can make for high pressures, in a closed system. I've seen gaping openings in a closed system. NOT homeowner friendly by any stretch. Here is an "open" system of heating a swimming pool with a copper roof. and piping.

Expensive as it is, it heats water simply by pumping the pool water to manifold the top of a copper shed roof and letting it drip back down and go into the pool. You get what you pay for but in this case the owners got a copper roof that could make a ton of hot water, collect rainwater, protect the house from the element, and be there untouched for the life of the house.

Solid copper, one pump, a little bit of electricity and gravity, simple. The trick was to put two coper roofs on, separated by 1/16th or so, spacers. The beauty of this system was that it was a one-time cost for the owner, and he was well skolled on how to use this simple machine, costing pennies on the dollar to run on demand.

A pilot light, a burner, a pump and a timer, is a perfect "owner friendly", inexpensive, system. and just cool it with good cross vents. Don't try to store hot air, just let it vent (with adjustable vents). A good rule of thumb is to close up the house in the morning, when the sun starts to shine hot, and then open up the north cooler air in the evening.

Most of the homes that I've worked in, look great on the inside and have "curb" appeal, but pay no attention the earth they are built on (a two and half foot crawler, or the southern sun, and the rain is rarely captured. "General Contracting" has absolutely squashed the "passive building" industry because they can't make any money off them.

Solar PV cells are great, but they are starting to accumulate in the garbage, which is a huge problem, they can't be recycled, not to say the CO2 that was emitted to get these collectors on your roof. Hot water and steam (which powers the generators) are tried and proven and made out of the best materials. Electric collection and storage are a completely different story and sometimes can't be fixed cheaply, and you are tied to the "Grid", but the marketing has sky rocked peoples demand for these foreign made collectors and running their meter backwards.

I ramble,

Once a concrete slab is poured properly and finished and cured of moisture, this new system of air pipes will carry heat into the floor very evenly and can be installed by the homeowner. The slab doesn't care how it gets its heat, it just absorbs and radiates. You have to be patient though because it takes time to get to temperature but once there it doesn't take much energy to keep it there, and a much less complicated control system. efficient and user friendly, and simple attic fans will vacate excess heat from above. No matter what, this slab on grade system needs proper placement of dense foam board insulation, AND proper washed gravels as backfills. Mechanical rooms and "wet" areas require "day-lighted flood drains, Not tied into the sanitary system, unless you like sewer smells when the traps run dry. Heated concrete is a great, "snow-melt" system also, even for flat roofs. Also, cool air can be pumped into this system in the summer months. Slab don't care.