Determining the Proper Flow Rate for Solar Hot Water Systems

“Dr. Ben, it was a pleasure meeting you and your team at Holocene last week. I know that SRCC has used for many years .0294 GPM per square footage of collector to determine proper flow rates. In your many years of experience have you found another formula that works best, especially for drainback systems?” – Loren Zucconi, UMA Solar

Loren,
I have settled on .025 GPM/ft2 as a good all round value. There were some studies done in the early ’80s about flow rate vs performance and this number represents the middle ground.

You bring up a good point, however. In a previous blog post I discussed the relationship between flow rate and delta T control settings. Glycol systems have a built-in temp differential between the collectors and the tank, because of the heat exchanger. The approach temp is never the tank temp. So the effect of differing flow rates through the collectors is reduced by that “offset”.

However, a drainback system with no heat exchanger between the tank and the collectors is much more sensitive to flow rate. There is no offset. Now the dT/on and dT/off are intimately tied to the flow rate. When I use 0.025gpm/ft2, 18F dT/on, and 5F dT/off, Tlim = 160, I get good results.

I would love to be able to do research on these settings, and as we get our automatic data retrieval system working I should be able to see about optimization. The University of Kentucky has just installed one of our Fluid Handling Systems and is going to use it for research. I hope they share the results with me.

Regards,
Dr. Ben