Question for Chemgeek or some similar soul RE: solar heaters

keithw

Well-known member
Mar 30, 2007
305
Virginia Beach
I have two solar panels for my pool. I have them piped to through one of the returns right now and I can control the flow rate to that return. My question is this: Is it more efficient to run a higher flow rate or a lower flow rate through the panels? I would assume that at a higher rate, the water coming out would be at a lower temp since it is not in the panel as long. And at a lower rate the water would be hotter since it is in longer. Would that mean it it doesn't really matter the flow rate?
 

JasonLion

TFP Expert
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LifeTime Supporter
May 7, 2007
37,879
Silver Spring, MD
The panels are most efficient at higher flow rates. The higher the temperature differential between the water and the panel, the more total heat that flows into the water. Since the water stays cooler at the higher flow rate the total amount of heat is higher. It's kind of counter intuitive, cooler water means more total heat.

The only issue is that most panels are designed for fairly low flow rates, typically 4 or 5 GPM per panel. Pushing the flow rate above the design rate will shorten the life of the panel. So you want the highest flow rate that doesn't exceed the design rate for the panel.
 

mas985

TFP Expert
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May 3, 2007
12,832
Pleasanton, CA
The rate of heat flow is directly proportional to the temp difference between the water and the panels (i.e. incident solar energy). As the water flows through the panels, it heats up and the temp difference decreases thereby decreasing the heat flow at the output of the panels. By slowing down the water flow through the panels, it will heat up more while in the panels and the heat flow rate will decrease even more as the water temp in the panels increases.

So slowing down the water can have a large effect or a small effect depending on the temperature differences. The closer the water is to the panel temp, the bigger the effect will be with water flow.

However, as long as the pool is reaching the desired temperature, it really doesn't matter.
 

chem geek

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Mar 28, 2007
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San Rafael, CA USA
As was said, higher flow rates are more efficient, but you probably want to know by how much so you can trade off pump electricity costs which go up at a much faster rate than the GPM -- a doubling of GPM makes the pump power usage go up by a factor of around 2.7 at lower flow rates and around 3.5 at higher flow rates (it's theoretically closer to a factor of 4, but fixed losses have this be lower at the lower flow rates and actual flow vs. energy also depends on the specific characteristics of the system curve). So you should look at the solar efficiency curve for your solar panel.

For example, the "EFFICIENCY vs. FLOW" graph for this FAFCO solar panel shows that at 1 GPM it's about 60% efficient, at 2 GPM it's about 70% efficient, and at 4 GPM it's about 80% efficient and my guess is that at 8 GPM it'll be just under 90% efficient. It's asymptotic so you get diminishing returns from higher flow rates, but it is technically more efficient at higher flows. For this panel, min/max flow rates for the panel are 3 to 8 GPM while the recommended flow rate is 4 GPM where you get around 80% efficiency. Note also that these panels have a normal operating pressure range of 0-30 PSI so you should not exceed that.

Since you only have two panels, you either have a bypass (such as other returns) to handle most of the flow or you are using a different kind of panel than the one described above (i.e. one that handles high flow rates). If you hooked these two panels in parallel rather than in series, then a flow rate of 4 GPM through each panel would result in 8 GPM through the return which is reasonable if there are other returns to handle more of the flow from the pump.

Richard