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tomhoffmo

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Posts: 4
Reply with quote  #1 
Glad I found this site, this is my first solar project (apart from those solar garden lights you stick in the ground). I want to see if I can reduce the electricity my hot tub uses by adding a small supplemental DIY system. I bought the following:
1. ECO-WORTHY Polycrystalline Solar Panels 5 Watt 12 Volt ($15)
2. YRD TECH DC12V 280L/H Submersible Pool Water Pump ($5)
3. 100 yard black flexible drip line (actually I had this already)
The plan is to coil up the drip line so it will absorb heat from the sun, and the PV panel will power the pump to circulate the water when the sun is out. 
I'd like to know if you think this will work, if others have tried this, if you have any suggestions, etc. 
I live in Southern California, so I think there should be plenty of sunshine available. The hot tub is 450 gallons, so I don't think it will overheat (go above 104 degrees, let's say). But hopefully it will keep the spa heater from turning on. I believe the spa circulates water through the filter continuously.
Thanks!
Tom

stmbtwle

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Reply with quote  #2 
I think it will work.
You'll probably need another solar panel or two.  Check the wattage on your pump.  
You MAY need a hi-limit switch.  The mechanical spa thermostats work pretty well, for the price, and they're adjustable. https://www.spadepot.com/Thermostat-14-Bulb-36-Capillary-P821C702.aspx

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Willie, Tampa Bay
tomhoffmo

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Reply with quote  #3 
Thank you Willie for your feedback! The specs for the pump are: Power consumption: 4.2W, Rated voltage: 12V DC, Working temperature: 0 ~ 60℃.
So, if the panel is rated 5 watts, would you expect that to be sufficient, or should I use two panels to have plenty of "headroom"? I confess I'm more of a software guy than hardware, so electrical circuits are not my area of expertise.
Would the panels be connected in parallel or series?
Thanks!!

stmbtwle

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Reply with quote  #4 
That 5w rating on your solar panel is under ideal, "standard" conditions (most of the time a lot less). Won't hurt to try it but I still think you'll need another panel.

Where is your collector going to be located? If on the roof you need enough power to lift the water to that height. If below the level of the tub, it may work as convection will help.

You'll want your panels in parallel. Series will probably destroy your pump. Check the "open circuit" voltage (Voc) of your panel. Many "12v" panels can output up to 22v (two in series 44v), more than enough to burn out some 12v pumps. Brushless pumps are more sensitive, been there.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
sundug

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Reply with quote  #5 
It may work in your climate, but if it gets cool where you live, you will need a glazed collector. Here's some of my efforts to heat my hot tub-

http://simplysolar.supporttopics.com/post/new-evacuated-tube-spa-swh-7477891?pid=1304990455

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stmbtwle

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Reply with quote  #6 
I would do it in stages. Build the collector, try it uncovered. As weather cools, cover it with "Suntuf" or similar, but leave the ends of the corrugations open for cooling. In winter you can cover them for maximum heating. In the spring, uncover them.

My experience is that once the tub reaches the desired temperature, you will have to shut off the pump and the collector WILL stagnate and overheat. A closed collector will easily reach boiling. An all-metal collector can take the heat, but others will need some provision for cooling.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
tomhoffmo

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Reply with quote  #7 
Thanks Willie and Sundug! I'm hoping this will be just a supplemental heating system, so I'm not really looking to get the full heat needed for a hot tub. Not sure how much the hot tub turns on during the day but if I can eliminate that, it's a start. I set the existing (built-in) heater at 90 degrees. I tend to get in the tub in the morning mostly, so if I can get the temp up to 102-110 during the day, that might keep the heater from kicking on for a while in the evening and night, so that would be a real gain. Once I get the pump (expected in a couple weeks as it is coming from China) I will update the post with some results. I got 2 pumps and 2 panels in case one fails.
Tom
sundug

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Reply with quote  #8 
Quote:
Originally Posted by tomhoffmo
Thanks Willie and Sundug! I'm hoping this will be just a supplemental heating system, so I'm not really looking to get the full heat needed for a hot tub. Not sure how much the hot tub turns on during the day but if I can eliminate that, it's a start. I set the existing (built-in) heater at 90 degrees. I tend to get in the tub in the morning mostly, so if I can get the temp up to 102-110 during the day, that might keep the heater from kicking on for a while in the evening and night, so that would be a real gain. Once I get the pump (expected in a couple weeks as it is coming from China) I will update the post with some results. I got 2 pumps and 2 panels in case one fails.
Tom

When I installed my tub, I wired in a 50 amp switch so I can turn it off completely and just let the solar keep it up to temp in the warm weather here in central TN.  I have a roll up shade exposing about a third of the collector right now, that's keeping temps about 100*F. When I got the tub, it had an extra foam insulation package, and I added a bunch more various types of insulation everywhere except around the motor. I insulated the short plumbing run between tub and collector. I have a differential thermostat wired in series with the pump, so the pump will not run unless the collector is hotter than the tub.  At first I had a high limit switch, but that stagnated and melted the poly tubing in the collector, even with water in it. I have twinwall for glazing.

http://www.builditsolarblog.com/2012_03_01_archive.html

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stmbtwle

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Reply with quote  #9 
That was my issue, too, stagnation.  While my Aretha collector was never damaged, it frequently hit temperatures of 160-180F in the afternoon and I worried about the fans.  I connected a second pump to 100' of poly pipe laid alongside the house, so that when the collector started to overheat the pump would pump water through this "radiator" instead of to the hot tub.  It worked pretty well, until the pump burned up, presumably from the heat. 

A second (manual) method was mounting the collector on a tilting mount, in the summer I would set it near vertical, in the winter or in rainy weather I'd set it for max harvest. That worked well but required me to adjust it more frequently than I'd like.  The PV panels driving the thing didn't work that well in the summer position, either. I'm still looking for a reliable, automatic system.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
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