Solar heater performance figures

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JohnRoss
Pool Newbie
Pool Newbie
Posts: 2
Joined: Mon 24 Aug, 2009 02:30
My Pool: Desjoyaux 5 by 10 metres (16 x 33 feet approx), 75 000 cubic metres (19800 US gallons)
6 micron bag filter. Saltwater pool with an electrolytic cell producing chlorine
Location: France

Solar heater performance figures

Postby JohnRoss » Tue 25 Aug, 2009 14:07

It would be very helpful to me if some of you good folk who use solar heating for the pool could post your heater performance figures. i.e. flow rate and temperature rise.

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. Details of the system I am using in the post Time It Takes For DIY Solar Heater To Raise Water Temp ................................JR


JohnRoss
Pool Newbie
Pool Newbie
Posts: 2
Joined: Mon 24 Aug, 2009 02:30
My Pool: Desjoyaux 5 by 10 metres (16 x 33 feet approx), 75 000 cubic metres (19800 US gallons)
6 micron bag filter. Saltwater pool with an electrolytic cell producing chlorine
Location: France

Solar heater performance figures

Postby JohnRoss » Mon 07 Sep, 2009 08:19

Having looked at American pool sites I have done the following calculation.
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 (100mm) 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 280 sq feet (26 square meters), i.e. a tad more than half the surface area of the pool.

American flat plate manufactures quote performance figures typically in term of BTUs/square foot/day 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 as a typical flat plate collector figure 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 maths wrong which would not surprise me in the least?
Only Pool Heating

Solar heater performance figures

Postby Only Pool Heating » Wed 11 Nov, 2009 17:08

All solar panels work very well, in fact there is very little difference in the end result temperature to your pool between the best overall " btu " producing panel and the least btu producing panel assuming you have the same square footage of surface collector area.

I realize this may sound complicated but it is important to not compare exterior dimensions of panels.

Some manufacturers have panel dimensions that have larger exterior dimensions that are 5 to 12 % smaller when you compare the actual collector ( the area that absorbs the suns heat ) surface area. A 4 x 12 panel may appear to be 48 sq ft but actually only have 45 sq ft of collector surface area. This times 8 or 10 panels can equal or exceed an entire panel, 10 to 15 % or more overall performance.

The greatest differences are truly in three different categories.

1) The manufacturer and the GENUINE warranty. Get the actual warranty from the manufacturer and review it for exclusions and exceptions.

2) The proven track record and lifespan. How long has it been around and can it withstand your weather climate year after year.

3 ) Who are you actually doing business with. How long is the company in business and are they licensed and insured.

HELIOCOL is the brand I and my 21 years of poolheating experience recommend. Fafco among others are also great panels regarding performance, lifespan but not for wind resistance or roof friendliness.

Nationwidepoolheating ( dot ) com
chem geek
Pool Industry Leader
Pool Industry Leader
Posts: 2382
Joined: Thu 21 Jun, 2007 21:27
Location: San Rafael, California

Solar heater performance figures

Postby chem geek » Thu 12 Nov, 2009 00:22

JR,

It's not just with solar panels, but with heat transfer (exchange) in general that it is more efficient from a thermal perspective to have higher flow rates, though as you get to higher flow rates you get diminishing returns. Look at the "EFFICIENCY vs. FLOW" graph in this file where you can see what I am talking about. The reason for the lower efficiency at lower flow rates is that the temperature rise is greater so there is greater loss from the panel that now becomes much hotter than the air around it.

On the other hand, as you can see from the "PRESSURE vs. FLOW" graph, the higher flow rates will result in greater pressure loss (head loss) and this results in needing a much higher RPM at the pump to compensate. The net result is that the electricity cost to run the pump increases rapidly at higher flow rates. So there is a sweet spot of greater thermal efficiency vs. not having too high a pump electricity cost. The recommended flow rate for the panel is usually near this sweet spot for a typical installation and is what I do with my own Fafco panels that I run at 4 GPM per panel with 12 panels (in parallel) for 48 GPM total. I have a variable speed (flow) pump so am able to tune this easily.

During peak summer when the panels are most efficient, I get roughly the 80% theoretical efficiency for the recommended flow rate. I have 12 panels with around 375 square feet of effective area, though 75 square feet of this is usually in partial shade. So 80% at 1000 Watts/square meter ideal is around 95,000 BTU/hr (assuming 300 effective square feet). At 48 GPM this translates to a temperature rise of nearly 4ºF. The turnover rate for my 16,000 gallon pool is 5.55 hours so this translates to around 3/4 of a degree per hour rise and that's in fact about what I see as the peak heating rate using solar with my pool -- about 3 degrees in 4 hours in the hours around peak solar noon (11-3). My solar cover is mostly opaque and light tan so there isn't much additional heating there, but if the cover is open during the day with sun hitting the pool, then there is substantial additional heating of up to 0.9ºF per hour (a white plaster pool's water absorbs about 60% of the sun's energy) with some of this lost through evaporation.

Richard

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