Solar panels with variable speed pump??

[mhoy]

I've got solar heater panels for the pool and a 1hp pump. There is also a controller for the panels that the previous owner disconnected (not sure why). Anyway.... if I'm pretty handy and am thinking a variable speed pump would cut the electrical use of the pool pump significantly. Now I also have solar electric panels, but as I've not lived here a year yet, I don't know how much power they actually generate in the summer.

If I put in a variable speed pump, will it be able to get water to the 2nd floor roof panels at a low speed or does it have to be programmed (or somehow connected for higher speed) when the sun is shinning.

Thanks in advance from a pool noobie.
Mark

 

[JasonLion]

You usually need at least medium speed to properly prime the panels. With solar and a variable speed pump it is generally worth getting an automation system that knows how to talk to the pump so it can raise the speed when in solar mode and lower it again when solar is off.

 

[mhoy]

Thanks. With VS pump what is considered medium? Perhaps just need to install and test it out. I'd guess the pump could be run slower for longer to get more heat from the solar panels as opposed to shorter run times at a higher speed (which of course does not work as well wrt solar heat).

Mark

 

[mas985]

You will probably need at least 50 GPM for priming and depending on your plumbing, that would be between 1500-2500 RPM.

I'd guess the pump could be run slower for longer to get more heat from the solar panels as opposed to shorter run times at a higher speed (which of course does not work as well wrt solar heat).

Actually, it is just the opposite. More heat is extracted from the solar panels at higher flow rates. Slower flow rates allows the water temperature to rise in the panels increasing the amount of heat lost to the environment and reducing the total heat transferred to the pool water. So to maximize heat gain (i.e. minimize heat loss), higher flow rates are better.

 

[mhoy]

While maximizing heat gain is good, reducing electricity bill due to pump run time is great (as you know PG&E in CA is $$$) . Just started reading "Energy Efficient Swimming Pools" from mas985's links, much to learn here. The FSEC Solar Panel info was a quicker read.

 

[JamesW]

With solar panels, it's important to stay within the manufacturer's recommended flow rates. As long as you're within the recommendations, then you should get reasonable performance. It might be worthwhile to get a flowmeter to tune the flow to meet the panels design flow rates.

 

[mhoy]

Next time I'm on the roof I'll take a picture of what I've got up there and perhaps someone can ID them. Unfortunately, a lot of the pool info isn't well known (yet) as I'm the new owner and the previous records are a bit lax in this regard. Still I've learned a lot in the last 3 months with plenty more to go...

 

[MattM]

A good solar company can do marvels by analyzing your current setup and tuning things. Unfortunately, from what I can tell, it appears that they are rather rare. We ended up in a dispute with the first company that did work and had a second company come in to fix what they did...and it was a learning experience all around.

General comments:
- The main idea with the panels is to get the minimal flow rate that both a) clears out all air from every panel in the series within a reasonable amount of time and b) keeps the difference between heated water returning to the pool and existing pool water within an acceptable threshold so that you are not losing too much efficiency due to airflow/wind/cooling before the water gets back and yet not having the pump run any faster than it needs to.
- The piping between panels and series of panels is critical, and depending on where panels are located you might want some to get a greater or lesser flow rate than the others (e.g. a north facing roof here in san diego is very inefficient so I send less water to those panels than to the south ones).
- Placement of panels and the roof temperature sensor are critical to having a good system
- Odds are that your pump will need to run at least 2100rpm, and perhaps up 2700 if you have a very tall house and bad pipes/panels. I run my variable speed at 2300. 2400rpm may heat more but it is noisier than I'm willing to tolerate and the pump uses markedly more power with each ~100pt increase in rpm (almost an exponential curve with ~2000rpm being where it really takes off).
- An automation system is almost necessary to ensure the system only turns on when needed and during the most optimal hours (nothing is worse than finding out that pool has been cooling because the water lost more heat going up to the roof and back than it gained from the panels, just a timer wont work as clouds passing over the house can bring temps down fast). The automation system should also prevent the system from continually cycling between solar and other forms of heating(you set a minimum threshold to activate, and a lower threshold to keep running). I have my system only allow solar to run from 9:15am-4:15pm daily. Before and after the pump still runs for filtration and chem mgmt reasons but at a much lower speed.

 

[chem geek]

All great points in the above posts. The only thing I'll add is that if you have long pipe runs to/from the solar, use the largest pipe you can -- at least 2.5" if not 3" -- do not use 2". Since the flow rate will be highest when using solar, you want the piping in that part of the system to offer the least resistance to flow.

For the panels on my one-story roof, I need to use 3000 RPM for 48 GPM feeding 12 panels in parallel, but the pipe runs to/from the solar are long and the run from the first panel to the last is also very long (the length of the house in a zig-zag around many roof hips). I'm at the recommended flow rate of 4 GPM per panel, but the minimum allowed is 3 GPM so if I wanted more savings I could lower the flow rate and it would make a noticeable difference.

My previous setup had a fixed sized pump and at 55 GPM through the panels it was more expensive to operate at around 1750 Watts compared to 1500 Watts (the variable speed pump isn't as efficient at the highest speeds, but at least I can tune it). It was way more expensive at 70 GPM with the solar off at the roughly same 1750 Watts compared to my now 26 GPM at 275 Watts.