Guidance with HEAT transfer to water

[rszkutak]

Hello all,
First I want to thank you for reading my posting here, hopefully you all will have some insight for my question.

ULTIMATE PROJECT GOAL:
To increase water tempature significantly using solar energy in the process. An increase of tempature of 20-30 degrees is desired from "origional" water tempature.

PROJECT OUTLINE:
What's this for, we'll this may sound cheesy and all but I am trying to create a simple solar heater for a pool that has great results as opposed to the other products available that really don't deliver good results.

As of now I am haivng individual holes punched in my pool surface for intake and outlet for the proposed heater. I am having my pool replastered, so their just doing it at that time for me... kinda nice how it worked out.

PLANNED MATERIAL:
I am planning on using a synthetic tubing type material, actually looking at polyethylene to be more exact (black). Water will be punped by an off the shelf pump into the pipe. Due to teh desire to maximize water tempature during the process I am going to use the tubing in length, specifically 250 - 500 per coil.

MY QUESTION:
First is polyethylene tubing < specific tubing in question > (

http://www.pepcoirrigation.com/technical_info/polyethylenetubing.htm

) one of the most cost effective and best to use for heat transfer.

Second, what would be the &quot;ideal&quot; configuration of the tubing to maximize thermal transfer to the water? Laid straight in a line with 4-6&quot; spacing between lines, or use a standard &quot;coil&quot; type design where i circle the tubing outside itself ultimatley making a large circle?

Third, obviously their are enviromental factors to consider such as outside tempature, days of sun and it's intensity. I live in Phoneix so sun's not a problem here.

Fourth and finally, I realize that inside diamater has a lot to do with the overall transfer rate as well, I am planning to use 1/2&quot; ID to minimize the amount of water per square inch that needs to be heated and hopefully allowing the water to have a signifiant increase in tempature over the course of 250-500&quot;.

Am I looking down the right road here, or have I lost sight of my ultimate goal.

Your information is appreciated, and I thank you in advance.

-Rob Szkutak

 

[pmover]

rob,

i'd almost hate to mention this, but surely there are off the shelf, ready made pool heaters to be used? how about using a pool cover to minimize evaporation, yet heat the water.

also, since the location is phoenix, i would THINK more of cooling the water than heating it, unless you are wanting to use the pool during the brief winter. u must understand that i am in alaska though.
good luck!
-pmover

 

[rszkutak]

pmover,
Ya know I knew someone was going to say EXACTLY those two things, i was just waiting for someone to say it though.

Pool cover, we'll they kinda don't work too great.

Off the shelf unit, yep there sure is... But the problem with those units is they tap into your existing plumbing system and go through some sort of copper metal grid at a high rate. Everyone I talk to that has them says they really hate it, it only extends their swimming season by a couple weeks tops.

Now for the LAST question, obviously living in phoneix you would THINK i want to COOL the water, we'll your somehwat right there. During the months of JULY & AUGUST the pool water is indeed like bath water mine reached a nice 92 degrees in mid august. However from the beginning of october until may the water is chilly to cold. <in 2 weeks my water temp dropped to 81, and right now is at 75> Although we are still in the triple digits here ( yes they are planning another triple digit day here today of about 101 ), the ground tempature has dropped drastically. Obviously the cooled ground tempature plays a large factor in the convection role.

The off the shelf units are about $800 or so, and in the scheme of things that's not all that bad, but they just don't work well. I want something that works solid, i don't even mind spending as much as a conventional solar heater i just want it to work.

All in all, allowing year round swimming.

thanks,
-rob

 

[IRstuff]

The reason these units suck is probably that you're attacking the wrong problem. You stated that while the air temp is triple digit, your pool is cooling nonetheless, and you even speculated that the ground temperature is affecting the pool.

This clearly suggests that your pool lining would need to be completely re-engineered to even come close to what you want to do. You would need to reduce the thermal loss through the pool lining by at least an order of magnitude to be able to maintain your pool at some arbitrary temperature.

Likewise, the reason pool covers don't work is the thermal loss elsewhere is completely swamping the equation.

some back of the envelope calculations with a 95F pool and 78F environment suggests a minimum heat flow of 10kW.

This would require a solar heater with a bare minimum of 30 sq meters exposed area to come lose to balancing the heat loss.

TTFN

 

[rszkutak]

Good point there on the heat loss thought, i would suspect that having the conventional white plaster and it's age of a nice 18 years old has something to do with the loss as well. The plaster is getting chipped out on monday and I am having a gray pebbletek installed.

I think drakening the color from white to a dark gray will help reduce the thermal loss, which is part of my goal, the other part is purely asthetic reasons.

Based on your estimate of 30Sq meters sounds like a good plan to me,

To combat that kind of loss sounds like I might want to invest in some seriously large heatsinks huh? lol

Obviously this is not going to be a single day heating process, i am planning on it going over the period of 5-7 days to get the temp up, and each day to maintin it more or less. I suspect the darker color will have some bearing on the pool itself in keeping the water warmer, but i want to try and combat the loss from the ground.

*****************************
I do appreciate your feedback so far, it has been very enlightening. Once i complete this project I will let you know the results I attain, and how it worked out.

Keep up the feedback!
thanks,
-Rob

 

[ClydeMule]

If you are still in triple digits AND the pool is colder than you want, why don't you get a water cooled condenser for your air conditioner. It will be pretty expensive and more work to maintin than a solar system, but will give you a lot of heat.

If you want you pool heated when it's cold outside you are out of luck though.

Clyde

 

[rszkutak]

Clyde,
Actually the solar system is only going to run during the daytime when the temp and sun are up.

Our winter daytime temps here seldom reach below the low 60's, although nighttime it can get into the upper 30's in late december or january. However our &quot;winters&quot; here last all of about 6 weeks, beyond that their's really 2 seasons here, HOT and NICE. Obviously i seriously don't want to heat the water during JULY, i would rahter consider putting a gun to my head as opposed to heating the water in JULY!

-rob

 

[chicopee]

About running the polyethylene tubing thru the focal points of parabolic reflectors in series. Guarantee the water will heat up and just make sure water circulation is maintained.

 

[GenB]

I think that you are in the right tract.
poly heating systems already exists. check with a wholesale plumbing house and ask for details. Goodyear, uniroyal and others make heating tubing,- plumbing and heating suppliers have all the controls as thermostats, bypass valves, manifolds, circulators, what else?
Are you embeding the tubes into the concrete around the pool and then into the plaster? i like to know.

Elmer.

 

[pennpoint]

rszkutak
If you are bent on doing this yourself then here's my input:
You can purchase 5/8&quot;OD (1/2&quot;ID) soft copper tube from many sources already spiral wound in a flat box. These would be approx. 24&quot; diameter. Buy a quantity of them, say 9 thats about 18 feet of lenial spacing when they are placed side by side (not allowing for space in between) (you pick the qquantity). Face them to the south 30-40 deg. relation to the sun.
The outer end of each spiral tube is connected to the supply flow and the inner end of the spiral is connected to the return flow. Paint them all flat black and do what you can to keep dust and dirt off of them (or off any glazing you place over them) and you'll have a pretty damn good solar heater. Spend a little more money and put simple temperature indicators on the inlet and outlets so you can brag a bit about the temperature increase.
Another thought is when the pool is heated to it's setpoint, add controls to divert the solar heated water (fluid) through the water heater or a preheat tank to reduce the required heating energy for domestic hot water use.
You can crunch the numbers all day and all night, in reality it's what you feel you acheaved that counts.
How much do you want to spend to feel good?

Good Luck
pennpoint