Looking for help calculating heat transfer for PEX ground loop project

[mcantrell71]

TL;DR Summary

hydronic heat transfer calcs ??

Looking to build a suitable closed loop water (probably glycol mix) circulation system to prevent livestock water troughs from freezing. I assume that I need to calculate how much heat loss is predicted from the water tank in ambient air (varies, but assume worst case), then what length of heat transfer pipe will be required to be buried in stable ground temp (55 degrees?) to supply the necessary Btu's. The plan is to have a solar panel with battery to power circulation pump.
1/2" PEX. 0.675" OD, 0.070" wall
ground temp - 55 F
ambient - 0F
100 gallon trough uninsulated (for calculations)
0.5 - 1.0 gallon/min pump

THANKS FOR ANY INSIGHTS!!

 

[jrmichler]

Start by finding the lowest air temperature at which it needs to not freeze. Then calculate the heat loss from the trough at that temperature, then recalculate assuming insulation around the sides and under the bottom. If you can teach the livestock to poke their noses down through floating balls, calculate again with floating balls. Calculate heat loss assuming 32 deg F water in the trough. That heat loss is how much heat your system needs to supply.

Find the ground temperature in your area. The water temperature in the circulating loop will be roughly halfway between the ground temperature and the 32 degree trough water temperature. From the heat loss calculated above, and the temperature difference between trough water and circulating loop water, calculate the minimum flow rate.

Then search ground source heat pump. Look at the various methods of thermally connecting to the ground. Choose one. The easiest way to size the ground loop is to find the sizes of the ground loops for different size heat pumps, then interpolate based on your calculated heat loss.

 

[Baluncore]

THANKS FOR ANY INSIGHTS!!

Mount the solar panels above the water trough, so that on clear nights, the loss of heat to the cold sky above, does not increase the rate the water in the trough will freeze.

Arrange the solar panels, so any rain will clean the panel surface, then run into the trough.

 

[mcantrell71]

Mount the solar panels above the water trough, so that on clear nights, the loss of heat to the cold sky above, does not increase the rate the water in the trough will freeze.

Arrange the solar panels, so any rain will clean the panel surface, then run into the trough.

I'm not sure I follow this. The solar panel is a PV panel to power the circulation pump for the ground loop. The trough has to remain open for animals to drink.

 

[mcantrell71]

Start by finding the lowest air temperature at which it needs to not freeze. Then calculate the heat loss from the trough at that temperature, then recalculate assuming insulation around the sides and under the bottom. If you can teach the livestock to poke their noses down through floating balls, calculate again with floating balls. Calculate heat loss assuming 32 deg F water in the trough. That heat loss is how much heat your system needs to supply.

Find the ground temperature in your area. The water temperature in the circulating loop will be roughly halfway between the ground temperature and the 32 degree trough water temperature. From the heat loss calculated above, and the temperature difference between trough water and circulating loop water, calculate the minimum flow rate.

Then search ground source heat pump. Look at the various methods of thermally connecting to the ground. Choose one. The easiest way to size the ground loop is to find the sizes of the ground loops for different size heat pumps, then interpolate based on your calculated heat loss.

Thank you for the explanation. What I don't know how to do is run the calculations. I know it is not simple

 

[erobz]

Probably don't run the coil in the drinking water unless you want to poison them when it springs a leak? Unless perhaps the coil is a continuous loop of some plastic tube and the fittings are all external to the tank.

 

[erobz]

I started thinking about all the computational time, labor, material, and I suspect the initial cost of this system might have a 20 yr payback. Not to mention it's a potential maintenance nightmare. It seems like free energy but at what initial cost and annualized cost?

A friend had a geothermal system for their home, relatively new build. It sprung a leak somewhere in the ground coil. The general consensus among contractors...the system is a complete loss...get yourself a gas furnace.

Have you considered an electric emersion heater with a small circulation pump?

 

[Baluncore]

I'm not sure I follow this. The solar panel is a PV panel to power the circulation pump for the ground loop. The trough has to remain open for animals to drink.

I did NOT suggest you put a lid made of PV panels on the tank.

On clear nights, the temperature of the sky above can be between -40 °C and -60 °C. That is why the water surface freezes quickly with a clear sky above. You need a roof above the tank to act as an insulator at the local air temperature. Obviously, it must not obstruct the animals and the panels must also be out of reach of the animals that might obstruct or chew on them.

In post #1, you show the panels as being mounted up high, away from the trough. Once you think about it, you will realise that if the panels are mounted above the trough, they can do the same thing as a roof.

Indeed, if you build a simple roof, you may find that the trough does not freeze, so you do not need the PV panels, pump or thermal transfer plumbing. Maybe all you need is an evergreen tree, protected from the animals, with foliage above the trough to hide it from the cold sky above.

 

[erobz]

In either case you will need (for the calculations):

1) The tank dimensions(length, width, height, and thickness), tank material, surrounding insulation R value.

2) Worst case ambient temperature for which you desire this to remain functional. The higher loss of either a calm,clear,cold night ( lower convection, higher radiation ), or a cold, windy, blizzard type conditions (lower radiation, higher convection)

 

[mcantrell71]

I started thinking about all the computational time, labor, material, and I suspect the initial cost of this system might have a 20 yr payback. Not to mention it's a potential maintenance nightmare. It seems like free energy but at what initial cost and annualized cost?

A friend had a geothermal system for their home, relatively new build. It sprung a leak somewhere in the ground coil. The general consensus among contractors...the system is a complete loss...get yourself a gas furnace.

Have you considered an electric emersion heater with a small circulation pump?

I understand your perspective here....which is why i have done exactly that up until this point. I have about half of the materials to construct the system on hand already, and I will be trenching new water lines to various locations on my 160 acres. Burying pipe is WAY cheaper than burying electric, and my plan was to utilize a solar panel and battery to run the pump. A freeze proof livestock waterer runs about $400-800/each, and each needs electric. I'm pretty confident that the economics works out, plus it has a net zero footprint moving forward (disregarding the manufacturing of the materials involved).

 

[erobz]

I understand your perspective here....which is why i have done exactly that up until this point. I have about half of the materials to construct the system on hand already, and I will be trenching new water lines to various locations on my 160 acres. Burying pipe is WAY cheaper than burying electric, and my plan was to utilize a solar panel and battery to run the pump. A freeze proof livestock waterer runs about $400-800/each, and each needs electric. I'm pretty confident that the economics works out, plus it has a net zero footprint moving forward (disregarding the manufacturing of the materials involved).

IMO, That all depends on how much pipe you need for your heat exchangers, and the HP of the required pump to produce the desired flow rate, which is tied to the size of the solar panels, batteries that will inevitable become weathered/damaged and need replaced on some regular interval. Oh, and digging up frozen ground to fix a leak may not be as easy as you expect.

Anyhow, you need the information in post #9 to get started computationally. Do you have an engineering degree, because you are going to need one to figure this system out? between heat transfer rates, and flow rates I can promise you this will not be a trivial problem to work out from scratch.

 

[Lnewqban]

Just another option:
https://www.bioenergyconsult.com/biogas-prospects-in-rural-areas/

 

[mcantrell71]

I did NOT suggest you put a lid made of PV panels on the tank.

On clear nights, the temperature of the sky above can be between -40 °C and -60 °C. That is why the water surface freezes quickly with a clear sky above. You need a roof above the tank to act as an insulator at the local air temperature. Obviously, it must not obstruct the animals and the panels must also be out of reach of the animals that might obstruct or chew on them.

In post #1, you show the panels as being mounted up high, away from the trough. Once you think about it, you will realise that if the panels are mounted above the trough, they can do the same thing as a roof.

Indeed, if you build a simple roof, you may find that the trough does not freeze, so you do not need the PV panels, pump or thermal transfer plumbing. Maybe all you need is an evergreen tree, protected from the animals, with foliage above the trough to hide it from the cold sky above.

I am still confused. When you say "roof", are you implying that it can be any height above the water surface (vs. a lid covering the tank)? A tank full of water in the barn (with a full roof overhead) freezes just as quickly as one outside.

 

[mcantrell71]

IMO, That all depends on how much pipe you need for your heat exchangers, and the HP of the required pump to produce the desired flow rate, which is tied to the size of the solar panels, batteries that will inevitable become weathered/damaged and need replaced on some regular interval. Oh, and digging up frozen ground to fix a leak may not be as easy as you expect.

Anyhow, you need the information in post #9 to get started computationally. Do you have an engineering degree, because you are going to need one to figure this system out? between heat transfer rates, and flow rates I can promise you this will not be a trivial problem to work out from scratch

 

[mcantrell71]

Despite you discouragement, I will just build the system and see what performance i can achieve. Cheers

 

[Baluncore]

A tank full of water in the barn (with a full roof overhead) freezes just as quickly as one outside.

Not on a clear night. The tank outside freezes first.

 

[erobz]

Despite you discouragement, I will just build the system and see what performance i can achieve. Cheers

I'm not trying to discourage you. Its what we do in engineering. You try to figure out whether or not its feasible before you build it. Changes to design on paper are relatively cheap, changes to implemented design are expensive.

 

[mcantrell71]

Not on a clear night. The tank outside freezes first.

Possibly, but they both DO FREEZE. Even tightly covered with a lid, I would assume they would freeze at 0 F

 

[mcantrell71]

I'm not trying to discourage you. Its what we do in engineering. You try to figure out whether or not its feasible before you build it. Changes to design on paper relatively cheap, changes to implemented design are expensive.

If they use geothermal ground loops to temper housing in the thousands of square ft., Im sure keeping a 100 gallons of water from freezing is relatively easy.

 

[mcantrell71]

I'm not trying to discourage you. Its what we do in engineering. You try to figure out whether or not its feasible before you build it. Changes to design on paper are relatively cheap, changes to implemented design are expensive.

Let me try another approach.
If i use a 1500w tank heater element to keep 100 gallons from freezing, and it is effective, how much heat transfer piping/flow is needed to achieve the equivalent of that 1500w?

 

[erobz]

If they use geothermal ground loops to temper housing in the thousands of square ft., Im sure keeping a 100 gallons of water from freezing is relatively easy.

Those systems cost $15,000 on average. That is after the design is fully functional. The actual engineering to design the systems probably took significantly more that the cost of a unit. This is in essence you are trying to do...design and build your own geothermal system.

 

[Lnewqban]

If they use geothermal ground loops to temper housing in the thousands of square ft., Im sure keeping a 100 gallons of water from freezing is relatively easy.

Please, avoid glycol, it is corrosive to the pump and very toxic if the drinking water gets contaminated by accident.

Besides recommending biogas above (perhaps as a back up for coldest nights), I have found these links with some ideas that may interest you:
https://www.oakhillhomestead.com/2020/11/keep-livestock-water-from-freezing.html

https://www.oakhillhomestead.com/2020/11/keep-livestock-water-from-freezing.html

 

[erobz]

Let me try another approach.
If i use a 1500w tank heater element to keep 100 gallons from freezing, and it is effective, how much heat transfer piping/flow is needed to achieve the equivalent of that 1500w?

That is a complicated question to answer. It's not a one-line computation...and then, its only half of the story because you need to get the heat into the system with another coil. A completely different set of computations.

The answer to your question is that depends on the working fluid, the thickness and material of the pipe, its length, diameter, to name just a few things.

 

[Baluncore]

Possibly, but they both DO FREEZE.

If you cover the trough with a roof, the heating system will not have to do so much work. Since the clear nights are the coldest, that means a smaller heating system is needed for the worst case.

Why are you so resistant to arranging the panels to gain an improved economic advantage, especially one that requires no energy input. You will have to raise the panels anyhow.

 

[mcantrell71]

Please, avoid glycol, it is corrosive to the pump and very toxic if the drinking water gets contaminated by accident.

Besides recommending biogas above (perhaps as a back up for coldest nights), I have found these links with some ideas that may interest you:
https://www.oakhillhomestead.com/2020/11/keep-livestock-water-from-freezing.html

https://www.oakhillhomestead.com/2020/11/keep-livestock-water-from-freezing.html

pretty sure propylene glycol is harmless, and there would be no leak points inside a tank.

I am trying to avoid electricity. My most expensive electric bill last year was January--even higher than running the air conditioner in July. All because of livestock water

 

[mcantrell71]

If you cover the trough with a roof, the heating system will not have to do so much work. Since the clear nights are the coldest, that means a smaller heating system is needed for the worst case.

Why are you so resistant to arranging the panels to gain an improved economic advantage, especially one that requires no energy input. You will have to raise the panels anyhow.

I am not resistant to anything. I do believe that you must be magnifying the perceived benefit of this "roof" quite dramatically. Besides, if you have worked with livestock (particularly goats), you soon learn that they will destroy anything they have the opportunity to do so.

 

[Baluncore]

I do believe that you must be magnifying the perceived benefit of this "roof" quite dramatically.

Then do the calculations for heat loss to the sky on a clear night. Having a roof makes a big difference.

Besides, if you have worked with livestock (particularly goats), you soon learn that they will destroy anything they have the opportunity to do so.

I have, but what has that got to do with the subject. I am not suggesting you lower the panels, so they can be damaged. I am simply suggesting you put them in a sensible place where they might do some good during the night.

 

[mcantrell71]

That is a complicated question to answer. It's not a one-line computation...and then, its only half of the story because you need to get the heat into the system with another coil. A completely different set of computations.

The answer to your question is that depends on the working fluid, the thickness and material of the pipe, its length, diameter, to name just a few things.

That is a complicated question to answer. It's not a one-line computation...and then, its only half of the story because you need to get the heat into the system with another coil. A completely different set of computations.

The answer to your question is that depends on the working fluid, the thickness and material of the pipe, its length, diameter, to name just a few things.

well, most of that info was in post #1.

 

[Lnewqban]

pretty sure propylene glycol is harmless, and there would be no leak points inside a tank.

That is correct.
Its toxic cousin, ethylene glycol, is commonly used in geothermal systems.

Other links, just in case you have not seen those before:
https://www.123zeroenergy.com/sizing-geothermal-system.html

https://en.wikipedia.org/wiki/Geothermal_heating

https://en.wikipedia.org/wiki/Passive_solar_building_design

 

[mcantrell71]

That is correct.
Its toxic cousin, ethylene glycol, is commonly used in geothermal systems.

Other links, just in case you have not seen those before:
https://www.123zeroenergy.com/sizing-geothermal-system.html

https://en.wikipedia.org/wiki/Geothermal_heating

https://en.wikipedia.org/wiki/Passive_solar_building_design

yeah, I really wanted to just go passive solar, but I just cannot figure out how to keep the structure intact (goats, pigs, cows)

 

[erobz]

well, most of that info was in post #1.

Well...saying by fiat the heat loss from the tank is 1500 W is a not a good way to start. It's a complete guess. Unless you want to iterate the design when its undersized, or waste money in excess materials I suggest you try to actually make some reasonable conditional assumptions and calculate the losses from the tank under said conditions.

This will be the is the easy part.

 

[Dullard]

The calcs for this aren't particularly difficult. The real problem is how to size the ground loop. Different soil and groundwater conditions make it impossible to be very precise - you'll need to miss on the high side. The suggestion to find out what commonly works in your area is a good one - it will give you a place to start.

You said that 1500W did the job. Assuming that:

Warning - Round numbers:
1500W is equivalent to about 5000 BTU/HR or about 0.4 Tons. Assuming a good enough heat exchanger in the trough, you'll need about 42F water at 1 GPM. If you can get the water warmer, you can use a smaller HeatX.

One of the issues with ground loops can be geology. In addition to cooling the ground immediately around your pipes, you may be depleting an isolated pool of heat (longer term) - the system performance can decay over years.

 

[erobz]

They are making their own heat exchanger out of run of PEX tubing from scratch. The performance characteristic need to be evaluated. Its not nearly as simple a problem as being implied by

$$q = \dot m c_p \Delta T $$

Dissapating 1500W If you wanted to run a 1 gpm pump bringing the flow in at 42 F it leaves at 32.8 F if the length of that PEX tube (of that spec) in the tank (32 F) is approximately 230 ft.

Polyethylene (PEX) is an abysmal thermal conductor, as are most plastics in general.

If you think that is bad the ground side loop to bring it back to 42 F from 32.8F would undoubtedly make that look like a baby coil...

 

[brainpushups]

What about installing an automatic waterer (or several)?

 

[Averagesupernova]

Well...saying by fiat the heat loss from the tank is 1500 W is a not a good way to start. It's a complete guess.

You don't really know that. It's obvious to me that the OP likely has more experience in knowing how much energy it takes to keep HIS tank from freezing in HIS environment based on HIS experience. What he lacks is understanding how much heat he can pull out of the ground with this method. Which in my opinion is not much.
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That being said, think of it this way: No matter what, the temperature of the working fluid will never be more than ambient ground temp of 55 degrees (what the OP said). A huge rate of the working fluid needs to be pumped to make this work. The temperature difference between the working fluid at 55° and the minimum water temp of 32° is not large enough. What you are attempting to do is not a heat pump and if it were, it would take more energy than you can utilize from a battery and solar.
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Right now I am battling similar problems keeping water thawed for large animals. I've used many different methods over the years and by far the best is an automatic waterer with an electric heater and thermostat. Bite the bullet and bury the electric wire when you bury the supply pipe. By something from these guys:
https://ritchiefount.com/
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Keeping large animals is not an endeavor that can be done with little capital to work with no matter how you look at it. I wouldn't skimp here. I know there are also gas heaters that can be used for this sort of thing but I don't have experience with them. A 20 lb propane bottle would do a fair amount.