Solar Batch Heater using Rain Barrels

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planetgirth

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Dec 3, 2015
12
Raleigh, NC
I am kind of new to this site and apologize if this has been covered. I am planning on building a batch style solar heater using 5, 55-60 gallon rain barrels. One of my first questions that could kill the project before I start is whether the pool chemistry will have problems being heated long term in these plastic barrels? They are food grade and the water will be circulating, but I just want to know if anyone thinks that I may have adverse affects due to the heated plastic. If this has already been discussed please just point me in the right direction. I am definitely looking for any tips to improve the efficiency.

Thanks in advance and I hope I can add to this forum in the future.
 
Welcome to the forum. :wave:

I used to live in Raleigh (Creedmoor). About the only practical energy efficient way to heat a pool is with solar panels.

Tell us how your batch heater works....I don't think it has been discussed on this forum
 
Thanks, I just put a new roof on my house and was worried about the wear difference on the shingles from the additional weight and lack of sun in those areas. This got me looking in other directions. An efficiency expert at work thought that I should look into the batch heater and that is how I got here. I will definitely keep the board posted if I do not find a reason that it will not work, i.e. the plastic from the rain barrels seeping into the pool water and messing with the chemistry.
 
I think the plastic will be fine. Solar panels are made of plastic too and they don't cause any issues. Heating a pool effectively with the sun is all about surface area exposed to the sun, surface area exposed to the water, water flow and heat transfer. It doesn't seem like barrels will be very efficient. This is the first that I have heard of this idea. One time I drained my 100+ degree, 340 gallon hot tub into my 20k gallon pool to see if it would impact the temp at all. The answer was nope! :-D Somebody around here could have probably calculated the BTUs available and saved me the trouble.

Let us know how it goes and where your research leads. People are always looking for sources of cheap heat for their pools.
 
Let's do some quick math.

Theoretically, if you mix two volumes of water at different temperatures there will be a temperature rise. You can solve a simple energy balance that assumes a closed (adiabatic) system -

Qgain = Qlost

(mass) x (temperature delta) x Cp = (mass) x (temperature delta) x Cp

(Cp = 4.184 and is the heat capacity of water at constant pressure, mass will be in grams and Temps will be in Celsius)

So, let's assume you have 5 barrels at 50 gallons/barrel and you can heat the water in the barrels to 120F. You have no listed info so let's assume your pool is 10,000 gallons and it's sitting at 80F. If you plug the numbers in, you get -

(250 gal * 3780 gm/gal)(48.9-Tfinal)(4.184) = (10000 * 3780)(Tfinal-26.7)(4.184)

Solve for T (you can simplify it and get rid of the Cp if you want ;) ) and you get -

Tfinal = ~ 28.5C or 83.3F

So you will have raised the pool temperature by 3.3F assuming perfect insulation and heat transfer. My best guess for what that would look like in real life is probably a lot less than 1 degree F. On top of that, if the pool is not properly covered with a reflective and insulating blanket to keep in the heat, any temperature increase will be immediately lost to evaporation.
 
JoyfulNoise said:
Let's do some quick math.

Theoretically, if you mix two volumes of water at different temperatures there will be a temperature rise. You can solve a simple energy balance that assumes a closed (adiabatic) system -

Qgain = Qlost

(mass) x (temperature delta) x Cp = (mass) x (temperature delta) x Cp

(Cp = 4.184 and is the heat capacity of water at constant pressure, mass will be in grams and Temps will be in Celsius)

So, let's assume you have 5 barrels at 50 gallons/barrel and you can heat the water in the barrels to 120F. You have no listed info so let's assume your pool is 10,000 gallons and it's sitting at 80F. If you plug the numbers in, you get -

(250 gal * 3780 gm/gal)(48.9-Tfinal)(4.184) = (10000 * 3780)(Tfinal-26.7)(4.184)

Solve for T (you can simplify it and get rid of the Cp if you want ;) ) and you get -

Tfinal = ~ 28.5C or 83.3F

So you will have raised the pool temperature by 3.3F assuming perfect insulation and heat transfer. My best guess for what that would look like in real life is probably a lot less than 1 degree F. On top of that, if the pool is not properly covered with a reflective and insulating blanket to keep in the heat, any temperature increase will be immediately lost to evaporation.
Click to expand...
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So this just took some of the air out of my sails. But I'm curious, is this just getting the water in the barrels up to 120 degrees then pouring them in? How would you calculate it running through in a slow circulation if so?

Also I have seen people make batch heaters to preheat their water for their house and their temps reaching 165-170 degrees. If I can get the recirculating temp around 150 I would hope that would make more of a change than 3 degrees.

Thanks so much for your responses. I am so thankful to have this forum to bounce ideas off of.
 
Having a dynamic circulation makes your concept worse. Solar heating is all about contact area. Collecting water in drum means you have a high volume to surface area ratio. That's bad for solar heating because large volumes of water take a lot of time to heat up. So if you had a constant flow scenario, I doubt the water would have any time to properly heat up.

In the case of your friends with solar barrels feeding their water heater, that is a mostly static water volume. Hot water demand is intermittent and in small volume batches so the water in the barrel has lots of time to heat up.

Another downside to your proposal is this - a 50gal drum of water is going to weigh over 400lbs. You really want to be careful about loading your roof with 400lbs point loads or else you could have a water barrel come crashing through your living room.


Sent from my iPhone using Tapatalk,16k gal SWG pool (All Pentair), QuadDE100 Filter, Taylor K-2006
 
JoyfulNoise said:
Having a dynamic circulation makes your concept worse. Solar heating is all about contact area. Collecting water in drum means you have a high volume to surface area ratio. That's bad for solar heating because large volumes of water take a lot of time to heat up. So if you had a constant flow scenario, I doubt the water would have any time to properly hear up.

In the case of your friends with solar barrels feeding their water heater, that is a mostly static water volume. Hot water demand is intermittent and in small volume batches so the water in the barrel has lots of time to heat up.

Another downside to your proposal is this - a 50gal drum of water is going to weigh over 400lbs. You really want to be careful about loading your roof with 400lbs point loads or else you could have a water barrel come crashing through your living room.


Sent from my iPhone using Tapatalk,16k gal SWG pool (All Pentair), QuadDE100 Filter, Taylor K-2006
Click to expand...


______________________

Thanks. Absolutely would NOT put this on the roof! this is an alternative to putting solar collectors on the roof.

I understand about the circulating water not having as much time to heat. I guess this may not be the idea I thought it was. I am glad I haven't wasted any time on it yet.

I guess that I will try the collector route. Can anyone point me to plans for a collector that is proven? About how many feet of 1/2" tubing should I need? Would 400' be enough to make a difference in a 20k gal pool?
 
The more surface area the better. Practically if you can find a spot on your roof for a layout that has ~ 1.5X your pool's surface area that would be a good start. So a 400sq ft pool would need 600 sq ft of active collection area. The more the better. Check out Heliocol or Fafco for ideas.


Sent from my iPhone using Tapatalk,16k gal SWG pool (All Pentair), QuadDE100 Filter, Taylor K-2006
 

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Here in AZ we have lots of flat roofs so most people I know have Heliocol collectors. Worse thing I've heard of is water leaks which just rolls off the roof or down gutters. If you have panels, then they need professional installation as the panels will need to be properly pitched for your latitude and properly anchored to the roof with a frame.


Sent from my iPhone using Tapatalk,16k gal SWG pool (All Pentair), QuadDE100 Filter, Taylor K-2006
 
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