Passive Solar heater for my pool

[Gil Martino]

Here you go!
We live in NY, so we don't typically have very hot weather in the summer months. SO I wanted to extend the life of my pool, in order to swim a bit longer at an acceptable temperature.

pool is 21ft x 4ft = 41,455 gallons
area = 1303 ft squared
8.33 lbs/gallon
takes 1 btu to raise 1 pound of water 1 degree Fahrenheit

125ft of hose is .5 inches of diameter = 295 cubic inches of water.
temperature of water is going from 77 degrees to 84 degrees (+7 degrees)

1 US Gallon is 231 cubic inches.
So the hose holds 1.2 Gallons of water.
9.996 lbs of water in the hose. or we say 10 lbs of water in hose.

Note from my friend Joe, who analyzed my data.

Gil,

Heat transferred is what we want to maximize here and heat, Q(dot), is equal to: Q(dot) = m(dot) * Cp * delta T, where m(dot) is mass flow rate, Cp is the specific heat of water (39 btu/lb degF), and delta T is the temperature change.

I did an analysis on your data. We know that at 0.85 gpm volumetric flow the temperature change (delta T) is 7 deg F and that at 0 gpm the temperature change is 13 deg F (the water just sits there and heats up to 90 degrees). Knowing that the curve is going to be a first order differential equation we can do a regression analysis and fit the data to a curve, in this case Temp change (in degrees F) as a function of volumetric flow = about 6.5x^(-0.5). The mass flow rate is linear (8.34 lb / gallon). The energy recovered is maximized at the highest flow rate as shown below.

Attached spreadsheet with data....


Cheers,
Joe Weiss
Engineer

Attached some pictures of the system.
Regards,
Gil

[Gil Martino]

As an update to this, it did effectively add a few weeks to this summers pool time. We just closed the pool last weekend, but reports from my friends indicate that their pools were in the low 60's(they have expensive gas heaters) and mine was at 70 degrees when we closed it up.

Now 70 is a bit cool, but I braved it and went in on a pretty nice sunny day.
Regards.
Gil

[Joe Blake]

Gil,

Have you considered, with such a small pool, a sheet of bubble wrap to float on top of the water to reduce heat loss, or perhaps even act as a "greenhouse" to trap incoming solar heat?

This would also reduce evaporative loss, and could be just rolled up and put to one side when you want to swim? Might also reduce your cleaning by helping to trap wind blown debris?


Joe

[Rob Beckers]

To add to what Joe wrote: My understanding is that evaporative heat loss is by far the largest source of heat loss for a pool (it takes lots of BTUs to evaporate water). So a pool cover for when not in use will really help extend the swimming season.

Neat pool heater by the way! :cool:

-RoB-

[Joe Blake]

Gil,

Something else occurs to me.

http://www.publish.csiro.au/?act=view_file&file_id=EC117p8.pdf

This article from CSIRO shows that a gradient of salinity in still water will more effectively trap heat because the different densities prevent convection currents, so if you were able to deepen your "trapping pond" so that it was (I'd guess) maybe twice the depth of the diameter of the black hose, then add salt to it, the heat would be more effectively retained in the pond to be removed by circulating fluid.

How would I do it? I'd partially fill the pond with water to cover the pipe, add the salt to it, then put a layer of some thin plastic like cling wrap or similar over the brine then GENTLY add fresh (distilled if possible) water on top. The cling wrap would help to separate the brine from the fresh, but without impeding the passage of light to the bottom of the pond where it strikes the dark surface and converts to heat. This warms the brine, but the wrap/ water density minimises convection currents, so retains the heat in the brine layer.

Then this led me to wonder whether you might have a more efficient heat transfer if, instead of using one thick hose pipe you used several smaller diameter pipes, because if I remember my high school geometry, you would have a greater surface area per unit volume of circulating fluid.

And lastly, I'm not sure of this, since the circulating fluid is in motion, but I wonder does brine per se have a higher "specific heat" because of its greater density? In other words, would you be better off using brine to circulate from the pond to the pool?

I guess this is an open question for anybody with greater physics and maths than mine.

I couldn't see from the photos but have you insulated the heat transfer pipe until it reaches the bottom of the pool? My reasoning is that if you have an uninsulated pipe then it will start heating the top of the water column in the pool, so you'll wind up with a warmer, less dense layer on top, and by the time the circulating fluid reaches the coolest (deepest) water, there will be less heat to warm the water, whereas with an insulated pipe the circulating fluid will have the maximum heat for shedding in the coldest part of the pool, and the warm water will be more evenly distributed. Thus there will also be less heat loss at the surface because it will be cooler.

Joe

On Edit: I did a very quick Google on brine specific heat.

http://answers.yahoo.com/question/index?qid=20060926005806AA9feUJ

Has got some answers, but also links that may prove useful down at the end.

Joe

[Gil Martino]

Joe,
I wanted to thank you for those responses.
Sadly, I must admit to everyone that my temperature reading was on the top of the pool and not on the bottom. When I returned my heated water to the pool, I returned it on the top, not on the bottom as was suggested. So my realized temperature change for the whole pool was not completely correct.

Thanks for your comments,
Gil

[Alex Bennett]

Very cool, thanks for sharing this.

I have a few questions if you don't mind.
Is that a black hose inside of your collector? Did you buy it that way, did you paint it?
What's the blue thing in your photograph?

Now that you have first hand experience with passive solar water heating in an outdoor setting, what do you think about its potential? Do you think a larger solar collector could heat your pool year-round?

Sorry for all the questions :D

[Gil Martino]

Alex,
Yes that's a black hose, I bought it that way from Sears because of the lifetime warranty.

If you mean that thing on the hose? I'm not sure what it is, but you unscrew the whole thing and replace it annually. I can try to find out what it is.
Gil

[Gil Martino]

Alex,
As far as heating year round, I don't think so. I'm too far north without enough sun.

I do believe a larger solution would be better, and I'd like to generate enough power to run my pump throughout the day.

Gil

[Ronald Williams]

Hello
I really thanks to you for sharing your views on solar heating. Really its a great technique.
I also want to share my views. According to me Solar water heating (SWH) or solar hot water (SHW) systems comprise several innovations and many mature renewable energy technologies that have been well established for many years. It has been widely used in Greece, Turkey, Israel, Australia, Japan, Austria and China.

In a "close-coupled" SWH system the storage tank is horizontally mounted immediately above the solar collectors on the roof. No pumping is required as the hot water naturally rises into the tank through thermosiphon flow. In a "pump-circulated" system the storage tank is ground or floor mounted and is below the level of the collectors; a circulating pump moves water or heat transfer fluid between the tank and the collectors.

This systems are designed to deliver hot water for most of the year. However, in winter there sometimes may not be sufficient solar heat gain to deliver sufficient hot water. In this case a gas or electric booster is normally used to heat the water.


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[Penny Walters]

Thanks for sharing this, it's pretty cool! Now I only need the pool :(