Data for comparing solar pool heating performance

[JohnRoss]

It would be very helpful to me if some of you good folk who use solar heating for the pool could post your heater panels performance figures. i.e. flow rate and temperature rise in full sun and little or no wind. I need the information to see just how effective, or otherwise, my humble efforts have been.

A heater goodness factor could be obtained by multiplying the flow rate through the solar heater in US gallons per minute by the difference in temperature between the pool and the water returning to the pool having passed through the heater.

My figures for my DIY system with flow rates of 14.04, 9.36 and 5.46 gallons/minute and temperature rise of 9, 12.6 and 16.2 degrees Fahrenheit giving a goodness factor of 126.36, 117.936 and 88.452. This indicates to me that the higher the flow rate the better and that one could assume that high flow rates rather than high temperature rise suggests a more efficient system.

In case you prefer metric here are the figures in those units: Flow rate through the solar heated pipes 54 litres/minute 5 degrees rise (Celsius) on the return to the pool, 36 litres/minute 7 degrees rise, 21 litres/minute 9 degree rise. Pool is 75 000 litres and solar heated pipes are some 260 metres (853 feet) of 100mm (4 inch) plastic pipe painted black with non-reflective blackboard paint................JR.

 

[mas985]

Heat transfer in solar panels is proportional to the temperature difference between the water and the incident heat from the sun. So this why heat transfer increases with increasing flow rates. However, the effect does taper off at a certain level depending on the solar panel used. Most panels reach about 80% efficiency after which the heat loss in the panels takes over and you really can't achieve much more than that. For Fafco panels, this about 4 GPM per panel.

If you are interested in the relative efficiency of panels, this link contains data for many manufactures. Notice that most panels will produce about 1000 BTU/Sq-ft/Day.

 

[chem geek]

Also, as I show in the tables in this post, for flat black unglazed (that is, not glass enclosed) panels of the type you describe, they are very sensitive to the difference in temperature between the water in the panels and the air and are also very sensitive to wind if there is such a temperature difference.

As Mark noted, higher flow rates reach diminishing returns, especially since they require far higher energy costs for the higher pump speeds and greater flow resistance.

Assuming 80% panel efficiency so no or minimal temperature difference with little or no wind, then if one had panels of the same area as a pool with an average water depth of 4.5 feet, this would result in a temperature increase of around 0.9ºF per hour at peak solar noon. For comparison, a white plaster pool absorbing the sun's energy via the water would have the water temperature rise by about 0.7ºF per hour at peak (a black bottom pool could gain as much as 1ºF per hour at peak) as described in this link. Of course, there is heat loss from evaporation, but this is usually less than the heat gain from the sun during the day which is why it makes sense to cover the pool at night (at least) since evaporation and heat loss still occurs.

The heating rates for my pool with Fafco panels pretty much tracks what is predicted from the panels alone as my pool is usually covered with an opaque electric safety cover.

 

[JohnRoss]

Thanks for coming back on this folks and Hi again Chem Geek you are a long way from the Complete France forum just now, I don't think Pool Guy visits this site!

My problem is that all I have is my experimental results obtained simply by measuring the flow rate with a bucket of known capacity and a stop watch and a thermometer. I have guessed that about a little less than a third of the circumference of my 4 inch diameter pipe picks up significant energy from the sun and I have taken readings at the solar peak when the pipe is broadside on to the sun which is when, I assume, the energy picked up should be at a maximum. From this I deduce that the area equivalent of my DIY system is about 26 square meters (280 sq feet), i.e. a tad more than half the surface area of the pool.

Flat plate manufactures quote performance figures typically in term of BTUs/square foot and whilst it might be possible to convert my figures to such I must confess I am at a bit of a loss to know how to do that, just getting old and silly I guess, so I am finding it difficult to make comparisons. Any suggestions?.................................JR

PS If I take 1000 BTUs/Sq foot/day and assuming a day when there is a reasonable amount of sunshine is 8 hours then this equates to 2.08 BTUs/sq foot/minute. Taking my own reading of 14.04 gallons/minute or 112.32 pints/minute and a temperature rise of 9 degrees F gives me 1010 BTUs/minute over 280 square feet gives me 3.61 BTUs/sq foot/minute. However my figures were taken at the solar peak and near the beginning and end of an 8 hour period the temperature rise obtained would be less so an average BTU input would be less than the 3.61 BTUs calculated. So my system will not be as good as it appears to be at first sight. A comparison therefore is still not really possible or have I got my math wrong which would not surprise me in the least?

 

[mas985]

I have measured about 3-4 degree rise on my panels but the controller only reports to the nearest degree. If I use 3.5 degrees, 65 GPM and 480 sq-ft, I get about 237 BTU/Hr/Sq-ft or 3.95 BTU/min/sq-ft.

Also, you can get a better estimate of flow rates with a flow meter or measuring it indirectly using the pump's head curve, pressure measurements and suction measurements. If you use a bucket to measure flow rate, it changes the dynamic head of the plumbing and thus the flow rate will change so the measurement will have some error.

 

[JohnRoss]

Thanks Mark allowing for the fact that your pick up area is significantly greater than mine my efforts don't look so bad. I don't know what time of day you took your readings but I took some today towards the end of the 8 hour period and I was getting a 5.4 degree rise at 14.04 gallons a minute which I calculate to be 2.2 BTUs/sq foot/minute. Tree shadows were already falling on some of the pipe. On balance it looks as though my system is not performing too badly but I think I need a bigger collection area for late Spring and late Summer. The return of the heated water to the pool is about a 18 inches above the water level so I can stick my bucket under without affecting the flow at all. 14.04 galls/minute is only a fraction of the main output of the pump which returns to the pool after passing through an electrolytic cell to produce chlorine. I don't think the pump would like it if I tried to put all of its output through the solar heated pipes.........JR

PS Just for reference I am at Latitude 46.29 and Longitude -0.132 so maybe a good bit further North than you.

 

[mas985]

I don't think the pump would like it if I treid to put all of its output through the solar heated pipes.........JR

Why not? Pumps will work just fine with higher head loss. Less energy efficient but there really is no issue with doing so. But I suspect that with 4" pipe, you have very little additional head loss anyway so you should be able to divert most if not all of the water through the panels. 853' of 4" pipe will only produce 2' of additional head at 50 GPM. This is less than 1 PSI rise in filter pressure. So there should be no fundamental change in the pump performance if you send all the water through the pipe.

Can you post a picture of your panels? Just curious.

 

[JohnRoss]

Ok I will try and take a picture tomorrow, it is dark here now, and then I will have to find out how to post it on this site. The solar heated pipes are 4 inch but there are a few yards of undergound but insulated 2 inch pipe feeding them. The 4 inch pipes are layed out in my field at a level lower than the bottom of the pool. The feed to the 2 inch pipes comes from the output of the pump through a 40mm (1.6 inch) fitting. The main output from the pump which just goes back into the pool is a 2 inch pipe. Cheers.............................JR

PS Took this pic a couple of years ago and have extended the pipes by about another 80 metres (262 feet) since then.
PPS I have to spray the weeds with Glyphosphate three times a year to keep them under control.
[attachment=0:3jizin57]Pipes.jpg[/attachment:3jizin57]

 

[mas985]

Even with the 2" pipe, pressure loss should still be below 2 PSI. I would send all of the water through the pipes. You will find that the pool will heat up faster.

 

[JohnRoss]

Ok well I can choke off the main output port a bit and thereby increase the flow through the pipes and see if the resulting flow temperature rise product is greater so thanks for the suggestion will give it a go.......................JR

PS Still getting a 9 F rise in temperature input to output at 14.04 US gallons/minute this pm at 4 local time (2 hrs after the solar peak) so the curve of time of day against temperature rise must be pretty flat.

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Desjoyaux 5 x 10 metre saltwater pool 75,000 litres, plastic liner, electrolytic cell chlorine production and 6 micron filter bag