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Let's talk about heat transfer

D

David G

Well-known Member
There was a lengthy discussion about using a cistern for cooling.

Heat is like water/electricity, you need potential (differential) to transfer it, so 54 degree water "might" cool air down to 70 degrees if you move enough water through the exchanger. Refrigeration takes weak energy (low differential) and moves it to where it can become usable by state changes thus creating enough differential to allow heat to transfer via an exchanger.
 
Water has one of the highest specific heats (mainly because of the hydrogen in it) thus a 1000 gallon cistern can contain solid calories of ability to absorb heat. If one considers the use of that absorption during part of a day, and the remaining time dissipating that heat into the soil under the house, I see solid cooling effect that is worth the effort. A truck radiator with squirrel cage fan and a circulation pump is a small investment. Jim
 
I would take an AC unit and put the condenser in the pond, must better results than a radiator flowing warmish air.

May take a window unit apart and make the condenser remote, or pump water over it from the pond.
 
While I had read the original post about using the cistern water to cool the home. My thought was it would work only until the cistern water becomes warm. I also understand the desire to have AC in many places the day time temperatures can become very uncomfortable. So I guess we in the frozen north are lucky, we get very few days with temperatures over 90F. And this year so far I don?t think we have had one day over 85F. AC isn?t even on my radar. Our big old farm house stays very comfortable, wife sometimes goes outside just to get warm.
 
My mother was a master of controlling heat in the house. ( I realize this won?t have anything to do with water, but none of it has to do with a tractor so what the hell.) We did not have AC or central air. The duct work snuck through the farm house was archaic at best. She would open and close windows and shades as the day and night progressed. The remodeling projects that Ma and Pa did put large windows on the south side and small ones on the north. This provide solar heating in the cold months much like you used to see on hog and chicken barns, but don?t see much anymore anywhere. It was not uncommon to open the basement door to vent some cool air up. Now the basement did have a canning room with a dirt floor and was sectioned off from the rest of the basement. That room was always the coolest. If you consider the liquid volume stored in the jellys, preserved meats, canned fruits, homemade wines, and stuff the regulators didn?t know about I?m sure there was a couple hundred gallons. That must count for something. That room was often open in the summer, but always closed in the winter. At any rate she kept the old farm house comfortable through the dog days of summer.
Now come winter she ran that old wood furnace like a boiler man. She also had a little wood stove next to the electric range in the kitchen and had a cast iron tea kettle chugging away to bring moisture into the house. I can?t tell you the math because I was picking rocks the day we had nuclear physics in school, but I know it works.
She also carried the wash water from the wash machine up the steps out the door and put that on the birch trees. And I?ll say we had some nice big birch trees.
 
David, wouldn't the end result be the same?

Using the pond for a source to cool the condenser will only work until the pond is saturated with heat. Plus it would be adding more heat from the compressor and motor.

Question is, will the surrounding dirt transfer the heat away fast enough to maintain the system? If it were dug down deep enough that there was water moving through, it would most likely be sustainable.
 
While, in theory, making the condenser remote from a window a/c, it is far from practical. First part is removing it from the window unit. This involves cutting, extending, and splicing refrigerant lines. That requires recovering, evacuating, and recharging the system. Note that window units are charged at the factory and the lines used for that are crimped and sealed. There are no service ports. They would have to be added. A lot of monkeying around for marginal gains. After all, a typical window unit is anywhere from 5000 BTUs for a room a/c to up to maybe just short of 30,000 for a "through the wall" unit. At the same time, a 3 to 4 ton unit that is more typical of a central a/c unit. that converts to 36,000 to 48,000 BTUs to cool an average house in a temperate climate. not a precise calculation, but simply demonstrates that it is probably more practical to either just go to a central unit or dive in with both feet and go geothermal all the way.
 
Not having to do with "heat transfer", specifically, but just an idea for anyone out there who might like to try this:

I saw a video some months back that I found intriguing, but only because of how well it worked. This guy in Nebraska added on a greenhouse to his home and is actually growing citrus trees in it. Here's a link to the video (it's quite interesting!):
https://www.youtube.com/watch?v=ZD_3_gsgsnk

If someone wanted to, they could get a 100' section of non-perforated drain tile, bury it in a loop from the home going down 4' to 6', and use just a fan to help regulate the temp inside the home (or shop!) year-round. It has an added benefit of taking moisture from humidity out of the building in Summer and leaving it underground.

I would think that might create air-quality problems, so I'd likely have some sort of condensate sump for that moisture to collect in and get pumped away.

Anyway, this is a low-cost method that could much better utilize the heat transmission capacity of the Earth to a building.
 
I don't think it will work period, as I believe it will saturate with heat, but if wanted to try I would do it as a heat pump (AC).
 
I ran the numbers on the other post.

You will need to flow 5 GPM, and assume a 20 degree heat rise on the water @ 3 ton of cooling.
 
I think it would work if the cistern was larger. I have a geothermal furnace and it has 1200 feet of pipe buried in the ground. It is a factor engineered system. They also have some with the pipe in deep wells or in lakes, but there is far more water than there would be in the cistern. He is doing some good thinking but needs to do some engineering work.
 
It depends on surface area and circulation of the water, I did not run those numbers, but would welcome someone too.

It is interesting concept.
 
Like a swimming pool.
Water takes a while to heat up but once it does it takes a lot to cool it off.

Big reason for our climate here in SE La.
Once the gulf warms up we seldom see temps above 100 but seldom see temps below 70.
The water keeps us cooler in the day and warm at night.

So it is not the amount of water or the size of the cistern.
It is the surface area of the water against the cooler earth.
The center of the water mass is not giving off heat because it is only next to warm water and not the cooler earth.
He would do better having 2 inches of water over the entire basement than a pool of water in a cistern.

Then there is the humidity factor of having water in the basement to consider.
Humid air always feels cooler or warmer than it really is.

So some of us understand your calculations and can follow your reasoning.
And others no amount of math will convince them or prove them wrong.
 
The tank also needs good circulation so the water in the center gets to the outside and vice versa, as only a molecular thin layer on the outside does the transfer.
 
I believe it would work, and if it becomes too heated just pump some water out so new cool water comes in. You could even dump the circulated water outside so you were always drawing fresh cool water from the sump. That would depend on how much water flows into the cistern of course. But it would also allow for lower water volume since the water would be cooler. They used to make what we called "swamp coolers" for tractor and combine cabs, they just used a small reservoir of water and I hear they worked good. They still make all kinds of them, so I don't see why it won't work.
 
Swamp Coolers worked great in Western Oklahoma but didn't in Mid-Iowa. Turned our 'swamp cooler' into a window fan. Hard to find 'excelsior' here, too.
 
[b:654c4848f0]depend on how much water flows into the cistern[/b:654c4848f0]

Your definition of a cistern must be different than mine.
For me a cistern is a tank to collect rain water so water only enters when it is raining.
A hole that has a constant influx of water from the ground is a well.

It was my understanding we were talking about running water threw a radiator and blowing a fan across it.
Much like a radiator cools a tractor motor except we are running cold water threw the radiator.
No water is used it is just circulated threw the system.

A swamp cooler uses water.
Just like a window a/c makes the air feel cooler by removing humidity
A swamp cooler works by adding water to the air and cools threw evaporation.
Like a wet rag on your head it removes heat threw evaporation of the water.
Problem is it works in dry climates.
In humid climates east of the Mississippi it does not work as well because the air is already saturated with water.
 
After reading this thread, and the majority of the original one, it sounds like a hybrid system is in order.


In other words, use the cistern as a source of water, and as a heat sink. Add on whatever length of underground lines needed to give any additional heat transfer needed, and you'd be good.


For winter, run additional lines through a black box sitting in the sun, and heat the water in the cistern. Circulate the hot water through the radiator just as you would the cool water.

If you used solar panels to keep a battery/s charged, and use that to run a DC circulation pump and the blower fan, you'd essentially have a source of free heating and cooling beyond the initial cost for the system......As handy as most of us are at scrounging and repurposing, I don't see that being too much either..LOL...
 
David,

Before my wife passed in 2000, she was in poor health. On O2 24/7. It was middle of August. I had to go back to work and had no time to install a new central AC. So I contracted out the job. Which took about 10 days contractor to do. He was extremely busy.

So like every redneck, I had a used 3 ton A coil. I used garden hoses and clamped them to both lines on the A coil. Slipped out my old 2 ton A coil and used the 3 ton. I ran my blower fan constantly. Ran the water from my well constantly.

It didn't produce the temp differential a conventional AC
It didn't remove the humidity either.
It kept my house where my wife could stand it.
New AC had to work hard to remove the moisture from the house.

IMHO, using a cistern for cooling is a pipe dream.

I think Monticello, Thomas Jefferson's estate, used water from a stream, circulated the water through large pipe, used bellows that were man powered, may have been the first Geo AC.
 
We had an old well by the barn, it was 6' square and 15' deep and had water in it. Dad called it a cistern. I thought that was what he had.
 
The first point I am trying to make, and failing at is the water is no where near cold enough to provide direct cooling, it needs to have a heat pump in the middle to change the temperatures. The condenser on the heat pump would be cooled by the water from the well. The second point is the volume of water in the well just postpones the inevitable of warming it to where it is unusable even to a heat pump. The ONLY thing that matters is a flow of fresh cold water through the condenser on the heat pump.

Bottom line the cistern/well, has no value unless it has a fresh cold supply of water coming in, and it could be 50 gallons reserve, not the 1885 that it is.
 
One of our relatives up north has a flowing well (artesian) and the water is about 50 degrees. He runs that through a coil that he make out of copper pipe and placed in his duct, it does cool his house, and it does remove humidity, condensate runs out of the drip pan.
 
I agree that a heat pump used as an intermediate step between the cistern water at 55 degrees, and cool house temps is solid technology, and not in dispute. 55 degree water will chill 70 degree air. Will it make it cold? no, but it will make a difference, and it is cheap. Warming the cistern water to 70 degrees from 55 is a massive sink. Will it recover over night? Maybe.
In 1978, I was living on a farm in northern Ohio. It had an abandoned 4 inch 60 foot deep well, with water 30 feet deep in it. I made a shelf in a porch to kitchen window, and dismembered a 5 ton air conditioner, leaving the exhaust side (cold) intact with its recirc system Hay wired on (literally). I cut and unfolded a truck condenser core so it would be about 20' long and 3 inches across. Soft aluminum. I adapted the inlet and outlet fittings to accept 3/4" automotive heater hose. I created an aluminum tub container into which I placed the condenser of the AC unit. I used a 1/5th hp circulation pump to push second circuit water through the well exchanger, and into the tub. Suction was from the bottom of the tub. the temp of the return water from the well raised over the month I used it only 1.5 degrees. the cooling was solid and cycled to keep it at 68 degrees in the old farm house. Fun stuff. Jim
 
Where I think most of you are getting lost in the weeds is confusing a cistern and a well.

A well with a renewable source of cold water may work with a heat pump in line.

A cistern using the same water over and over is going to heat up to where the water is hot and useless with no way to dissipate the heat away from the water and into the ground.

To use a closed system where you are using the same water over and over you need a lot more surface area in the cistern to dissipate the heat away.
 
The old houses and buildings were designed to work with natural events to keep cool in summer.

That's why they had high ceilings, double hung windows that actually opened top and bottom, transom windows above the doors, attic fans, porches around the house to shade the walls...

If only they had thought of insulation!

Ever wonder why so many old buildings had clock towers?

Two reasons, the clocks ran off weights. The weights hung down inside the building. The longer the weight chain, the less often it had to be wound.

But the real purpose of the high tower, it was a convection cooling system! It was open around the top, as the sun heated the sides of the tower, the heated air would rise up and out, drawing in cooler air from below!

Pretty smart!
 
I understand and agree with what you are saying. The area I live in has a high water table and lots of homes use ground water to act as the condenser. I have my heat pump cooled using a loop underground. I have seen some that have a coil in a pond but all those use geothermal to cool the heat pump not to cool the house.
If you want to talk about heat rejection you need the see the large plate and frame heat exchangers that we use at work to cool 95liter engines!
 
yes that will work from a flowing well , but this situation is a closed loop system and the water in the cistern will warm up and not cool back down fast enough. the water circulating in warm pipes and depends how long of pipes has no choice but to warm up. it will cool for the first bit though. plus look at it this way that water will be warming the basement also once it warms up.
 
Just run a garden hose on the condenser! Put some gauges on it and watch the head pressure drop.i have seen large building have sprinklers running on the condenser units. Pond will work for a form of geothermal, I know of several people that have them. The volume of the pond is more than unit will reject. The pond will also turn over in spring and fall placing cold water on bottom in summer and warm water in wi ter.
 

Since the cistern is an isolated source without regular makeup water, you have to think of the problem as a shell and tube heat exchanger

q= M Cp Dt Where

Q = specific heat of water 4.18
M = mass flow rate pumped water in lbs mass per hour in this case 5 gpm x 60 minutes =300 gph x 8.34 lbs/gal=2502 lbs mass/hour or 60048 lbs mass per day
Cp= specific heat 4.18 divided by Delta temp Dt inlet and outlet of heat exchanger
Dt= T2 - T1 outlet - inlet of heat exchanger

1 BTU is the energy to raise 1 lb mass of water 1 degreee


Its easier to convert to lb/mass per hour simply because thats the way I learned it many years ago. You need to make some assumptions too. Without the heat being added to the cistern water being removed the differential temps are a moving target and the efficiency of the heat exchanger is not 100% due to tube fouling and other stuff.
Further as the concrete container acts as an insulator temp in the heat sink cistern will gradually increase.
After 1 day or 7200 gallons pump throughput you should begin to see a temp increase depending on system efficicency.
7200 gallons is a pretty big cistern and thats just 1 day at 5 gallons a minute.

Also you're going to see a higher dewpoint as temperatures rise under the house in that space leading to all sorts of nasty molds and maybe rot.
Cooling water requires treatment. Bleach maybe? I don't know, I didn't work in chemistry. we used sulphuric acid and other stuff location dependent.

Bottom line line is it would work at least for while until your heat sink became elevated in temperature or what I would call saturated. Then use it in the winter for heat I guess. LOL
I just don't think your heat sink is big enough for it to last a long time.
 
I know the heat transferred from the uninsulated plug on the back of my welder to my hand quire Nicely made picking up bales a good time and pretty sure I burned the finger prints off at least two fingers
 
(quoted from post at 17:22:12 07/16/19) Where I think most of you are getting lost in the weeds is confusing a cistern and a well.

A well with a renewable source of cold water may work with a heat pump in line.

A cistern using the same water over and over is going to heat up to where the water is hot and useless with no way to dissipate the heat away from the water and into the ground.

To use a closed system where you are using the same water over and over you need a lot more surface area in the cistern to dissipate the heat away.
Click to expand...

^^ Whut he saided. I have used well water to cool my cabin. It's off grid. amazing what goofy things we can do when there is no internet, or cell service. I had an old radiator out of a class C motor home, a well with about 45F water(plenty of it), 24VDC regulated power and plenty of 1" sprinkler pipe. In a few hours I had a recirc setup in the cabin. Used a 12V blower fan out of the dead motorhome from the heater, and blew air through a homemade duct to the radiator. Used the feedwater pump to a valve and diverted it to the sprinkler line, through the MH radiator, and dumped back into the well.

It was not efficient, it was not clean, fancy, or quiet, but it cooled rather nicely from 1-6PM. The keys are 1) having a large differential between the transfer media(water) and the surrounding air. 2) using the largest heat exhanger you can find. I lucked out finding this big V8 3 row radiator, which had to dissipate about 150HP at 60MPH.

BTW, I did not do the math, I just started tinkering cause my wife complained it was so hot. We have a propane fridge, and it barely kept the freezer cold, but with the afternoon 'AC' unit, it did much better.
 
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