Sweet Spot for Solar GPM

[RedHook]

For those of you who have solar panels, what's your ideal flow for max efficiency? Are you running your pump at a slow speed to maximize the time the water spends in the panels, or do you run it faster so the water isn't in the panels as long but is circulating more?

I don't have a controller on my pump, so I don't know exact GPM, but I'd happily hear your experiences with fast vs. slow.

Off to buy more lounge chairs- it's swim season in SoCal!

Thanks, all.

 

[JasonLion]

Higher flow rates are more efficient, but too high a flow rate can shorten the lifetime of the panels. Most panels are good around 4-6 GPM per panel (for 40 sqft panels).

 

[chem geek]

The 11 4' x 10' FAFCO SunSaver and 1 4' x 8' FAFCO Revolution panels I have give a recommended flow rate of 4 GPM with a minimum of 3 GPM and maximum of 8 GPM (see the SunSaver manual). Their efficiency vs. flow graph shows around 60% efficiency at 1 GPM, 72% at 2 GPM, 77% at 3 GPM, 80% at 4 GPM and would project to 84% at 6 GPM and 86% at 8 GPM. These numbers are with essentially no wind, however. For these panels,

Efficiency = 100% - 40%*SQRT(GPM)
(in practice, one doesn't get to 100% efficiency at high GPM because of other panel losses so one may peak near 85% or so at high flow rates)

However, as shown in this post, the Watts (power) varies somewhat higher than the square of the GPM. The required time for one turnover varies inversely with GPM so total energy cost for one turnover varies with the GPM to a power somewhat higher than 1.

So going from 4 GPM to 3 GPM would only lose 3% in heating efficiency, but would go from 1530 Watts to 895 Watts and for one turnover in my 16,000 gallon pool go from 5.6 hours to 7.4 hours and 8.5 kWh to 6.6 kWh, a 22% savings. Right now, I'm still at 4 GPM per panel, or 48 GPM total. The sun is only high enough and hitting enough panels for 6 hours or less during the day so I try to balance turnover rate and heating efficiency.

Another factor is that the panels are on the roof and need a minimum flow rate to create enough pressure to keep the vacuum relief valve closed. Nevertheless, I might try lowering the flow rate to 3 GPM per panel to see what happens.

 

[JamesW]

chem geek, how many kWh of heat gain do you lose by going from 4 GPM to 3 GPM? If it is more than you are saving (1.9 kWh), then it's not worth it.

Also, if you use solar at 3 gpm at 895 watts for 5.6 hours and then run an additional 1.8 hours without solar to get a complete turnover, then the energy usage would be less because the extra time would be at a lower power usage than 895 watts.

A few other things to consider:

1) Will operating at the lower efficiencies allow the pool owner to achieve the desired water temperature in the time that the sun is available? If the person wants the water to get to 86 F and it only gets to 84 F, then going to higher flow rates and more efficiency will probably be worthwhile.

2) If operating at the lower efficiencies will not allow the pool owner to achieve the desired water temperature in the time that the sun is available, and they have another heat source, such as a heat pump or gas heater, how much will it cost to run the other heat source to make up the difference?

3) What is the intensity of the sunlight? The efficiency is only part of the equation. If you have twice the sunlight, then that doubles the effect of the efficiency difference.

4) Will operating at higher flow rates shorten the life of the panels?

5) What pump does the person have and where does the flow rate put them on their pump's head curve?

The Heliocol 4 X 12 foot (HC-50) requires a minimum of 5 gpm per panel. (8) 4 X 12 panels will require a minimum of 40 gpm.

http://www.sunbelt-solar.com/documents/ ... Manual.pdf

 

[chem geek]

The loss in heat gain isn't very much in percentage terms. Instead of getting 80% of 1000 Watts per square meter at peak sun, you get 77% instead. Panel area is 466 square feet (43.3 square meters) though in practice not all 12 panels are in sun, but I'll ignore that. So that means going from 34.6 kW to 33.3 kW. Though over say 6 hours of solar on time that would be a loss of no more than 7.8 kWh (since sun is not at peak the entire time), this loss really isn't relevant to compare to pump electricity cost savings since the alternative is not heating with electricity, but with natural gas which in my case is much less expensive for the same amount of energy (electricity costs 8 times more for the same amount of energy as natural gas using marginal rates). Also, that 3% loss translates into a 3% lower temperature rise which really isn't very noticeable and one could just live with it. Of course, if one is struggling to get to a temperature each day and losing at night, then this 3% gets magnified, but if one gets to the desired temperature during the day, then it just takes a little longer to do so.

The Heliocol minimum actually varies by panel size, but translates to 0.1 gal/sq.ft. which for most of my FAFCO panels would be 3.95 GPM so FAFCO does apparently allow a lower minimum flow rate for their panels of 3 GPM. 2936 Watts for one 39.5 square foot panel is 10,018 BTU/hr or 167 BTU/min so for 3 gallons (about 25 pounds of water) at the minimum FAFCO panel flow rate this is a 6.7ºF temperature rise compared to a 5.0ºF temperature rise at 4 GPM. So we aren't talking about very high temperature rises here which is why things don't get terribly inefficient.

In my situation, I probably wouldn't run the pump any longer anyway. I pretty much run it mostly for solar time and have more flow than I need if the solar is on longer. I'll just have to make sure the lower flow rate doesn't cause problems with the pressure relief valve or with negative pressure on the hotter panels and piping.