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Solar heating hottub (assisting heating)

pops106

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Aug 24, 2020
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141
Hello,

I have a 900L inflatable HotTub which costs a small fortune to heat, I have recently lifted it off the floor onto some pallets and rubber matting on top of the pallets to reduce the heat lost through the floor.

I then built a small solar thermal box with around 30 metres of 1/2in pipe, sprayed black and plexi cover. I use a 100w panel, PWM controller and 12Ah battery to power a small 18w submersible pump which I dropped straight in the tub. I have left a hose end on the return so I can control the flow, the idea been when the sun goes down the battery runs out of power and it stops pumping during the night.

I am using "shade" to allow the thermal box to get some sun before the solar panel which has to bring the battery up to charge before the pump kicks in and we are off again.

With lots of sun it actually works surprisingly well, it managed to bring the water up to 32C after several days of sun but next to nothing with little sun obviously. But I was thinking I have some panel watts and battery capacity spare so can I improve it.

I have found a couple of small heating elements around 12v dc 20w each which I could build into a insulated box and join that into the mix.

Cold water out of the tub > thermal box > heating element box > warm water back to the tub.

My concern is the panel, is their enough power

Panel is 5.55a at 18v 100% sun
Pump is 1.5a
Elements are 3.22a

PWM controller can only output 5a just read it might be 10a so this is just under the limit and I would be pretty much maxing the panel out even at 100% so very little charge going to the battery.

I just ran the numbers through a calculator I found online and it says 900L of water from 20C to 30C over 2,100mins (5 hours sun for a week) would need 359wh of heating and that's not accounting for it dropping temp during the night.

Do the heating elements output at a reduced power if they cant get all the power they need, or do they just not work ?
Is there a way to calculate the wh for the thermal box ?
Are these small elements going to add even 1C of heat to the water ?

Cheers

thermal.jpgboth.jpgwiring.jpg
 
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Generally, yes, but their resistance varies with temperature, so there is a point where low voltage will result in negligible heating.
1m^2 of Earth gets 1000W of solar energy.
Unlikely. They are a monumental waste of time and energy.

100W panel can produce 400-600Wh per day in optimal conditions.
Your pump uses 18W.

Your panels should be able to gather enough energy to run your pump for 24 hours.

Increase your thermal collectors and insulate tub at night to increase your temps. Abandon all hope of PV providing meaningful heating.
 
I disagree with @snoobler which is going to be a common occurrence I feel. You have an 18 watt pump and a 100 watt panel that leaves 60 watts to play with after inneficiebcies. while this is not a lot of energy but 60 watts could power 3 of those elements. If they are relatively cheap you have nothing to lose. obviously the traditional pool heating system you have will work best but every little bit counts. If you are not someone as experienced as @snoobler just the learning process alone could be worth a lot as well
 
Craig, you can disagree with me as often as you like, and we can address each of those as they arise. However, this is where physics wins. I always try to be on the side of physics and math, but I definitely don't know everything and occasionally have my head unknowingly embedded deep in my ass, but this I know:

We have:
900L of water
60W of heating assuming 100% efficient.
5 hours of irradiance
Thus, 300Wh of solar energy per day from heating elements

Definitions:
1W = 1J/s
1cal = energy required to raise 1mL of water 1°C = 4.1868J or .001163Wh
900L = 900,000mL

900,000mL requires 900,000 calories to raise the temp 1°C

300Wh/(.001163Wh/cal) = 257,954 calories.

257,954 / 900,000 = 0.29°C/day

The lesson to be learned here is that things can be calculated before time, money and effort is wasted. Now the OP can decide if it's worth 0.29°C per day.
 
I disagree with @snoobler which is going to be a common occurrence I feel. You have an 18 watt pump and a 100 watt panel that leaves 60 watts to play with after inneficiebcies. while this is not a lot of energy but 60 watts could power 3 of those elements. If they are relatively cheap you have nothing to lose. obviously the traditional pool heating system you have will work best but every little bit counts. If you are not someone as experienced as @snoobler just the learning process alone could be worth a lot as well

It is very much a "why not do something" thought I had, I was also thinking every little bit would count.

Craig, you can disagree with me as often as you like, and we can address each of those as they arise. However, this is where physics wins. I always try to be on the side of physics and math, but I definitely don't know everything and occasionally have my head unknowingly embedded deep in my ass, but this I know:

We have:
900L of water
60W of heating assuming 100% efficient.
5 hours of irradiance
Thus, 300Wh of solar energy per day from heating elements

Definitions:
1W = 1J/s
1cal = energy required to raise 1mL of water 1°C = 4.1868J or .001163Wh
900L = 900,000mL

900,000mL requires 900,000 calories to raise the temp 1°C

300Wh/(.001163Wh/cal) = 257,954 calories.

257,954 / 900,000 = 0.29°C/day

The lesson to be learned here is that things can be calculated before time, money and effort is wasted. Now the OP can decide if it's worth 0.29°C per day.

Thank you for breaking that down, I will pretend I wouldn't need an hour to understand it all.

I guess that is why even with the tubs own heater around 1.2kw still takes about an hour per 1C to come up to temp.

Found a good calc that shows how daunting it is heating water with electric.

 
Thanks for the calculator. Sanity check of my numbers:

1598547845791.png

Couldn't specify 0.29 degrees, but...

1/0.29 = 3.448

The computation of 60W/5hr per day to get 0.29°C would take 3.448 days for 1°C or 3.448 * 5 = 17 hours and 14 minutes vs. 17 hours and 27 minutes

Not sure where the 13 minute discrepancy comes in, but there was a lot of rounding of a lot of digits.

I'm saving a link to that calculator. That would have taken me a lot less time... :)

This section would have answered your question, 1°C:

1598548228088.png

Assuming 5 hr of power available, you'd need 209W over those 5 hours to heat it 1°C.
 
Thanks for the calculator. Sanity check of my numbers:

View attachment 21090

Couldn't specify 0.29 degrees, but...

1/0.29 = 3.448

The computation of 60W/5hr per day to get 0.29°C would take 3.448 days for 1°C or 3.448 * 5 = 17 hours and 14 minutes vs. 17 hours and 27 minutes

Not sure where the 13 minute discrepancy comes in, but there was a lot of rounding of a lot of digits.

I'm saving a link to that calculator. That would have taken me a lot less time... :)

Oh come on... 13 minutes, I would of been 13 hours out lol

I played around with the figures a bit roughly working out how long it took my collector to bring the water temp up and I am guessing my thermal panel is around 400w.

It took several days, around 5 or 6 to bring it up about 10C, I guess maybe a little more as the temp would drop a couple of C on the night.

It is about 1.5m2 roughly then with all the losses it could be around there.
 
You may find you get different results with different flow rates. This is a complex fluids and heat transfer problem that I forgot how to work 25 years ago... :)

More/bigger units should get you some boost. A sheet of lexan/plexiglass/glass over the top side will trap more heat from the greenhouse effect. Also, if that's a regular rubber garden hose painted black, your heat transfer isn't very good. The polyethylene irrigation tubing is thinner wall and will transfer more heat.
 
You may find you get different results with different flow rates. This is a complex fluids and heat transfer problem that I forgot how to work 25 years ago... :)

More/bigger units should get you some boost. A sheet of lexan/plexiglass/glass over the top side will trap more heat from the greenhouse effect. Also, if that's a regular rubber garden hose painted black, your heat transfer isn't very good. The polyethylene irrigation tubing is thinner wall and will transfer more heat.

I actually started off with irrigation pipe but the flow was crazy slow, almost dripping out but wow did it create heat. I had it connected to an outside tap testing it, I left it off for about an hour and half in full sun and made a cup of tea out of the steaming water that came out. The thermometer I had only went up to 60C and it flew to the top.

I then went to a "cheap" hose which kinks if you look at it, thinking it might be a bit thinner and still work ok, it seems ok and I have adjusted the flow to be a bit more than a trickle.

Maybe I could make a second box up with the irrigation pipe again and grab another bigger pump.

Thanks for all your help really appreciated.
 
ID of the pipe matters. If it's too small, the friction losses are massive, and you get very little flow. What size pipe did you use? I think most use 1/2 or 3/4.
 
@snoobler Sorry im not saying you are the physics are wrong but sometimes doing things even if they are not the most efficient or are best practices can lead to learning other things and a better understanding of how things work. Even a small amount of heat could be worth it especially if you only spend a few bucks on an element.

Bottom line is I want to know and experiment I hate hearing how this or that wont work, It really takes away the fun. I would much rather try and waste a few bucks even because everything one does teaches us something.
 
you need to forget DYI thermal solar if you use this kind of equipement.
the only way to go is to use thermal solar tube (that are not so expensive).
The yield can be as high as 70%, so you would get 700W form the theorical 1000w*square meter.
you can also cycle hot water into the solar tubes from an insulated tank, so you can raise temperature to very high values.
then you need to use less water to heat the HotTube.
 
ID of the pipe matters. If it's too small, the friction losses are massive, and you get very little flow. What size pipe did you use? I think most use 1/2 or 3/4.

I think it was 1/4in, you couldn't get a pencil inside it or maybe with a hammer.

you need to forget DYI thermal solar if you use this kind of equipement.
the only way to go is to use thermal solar tube (that are not so expensive).
The yield can be as high as 70%, so you would get 700W form the theorical 1000w*square meter.
you can also cycle hot water into the solar tubes from an insulated tank, so you can raise temperature to very high values.
then you need to use less water to heat the HotTube.

Is that the evacuated tubes ?

I watched a few videos on how to DIY building the tubes but way out of my skillset.

There not exactly cheap but yes they would be much much better.

@snoobler Sorry im not saying you are the physics are wrong but sometimes doing things even if they are not the most efficient or are best practices can lead to learning other things and a better understanding of how things work. Even a small amount of heat could be worth it especially if you only spend a few bucks on an element.

Bottom line is I want to know and experiment I hate hearing how this or that wont work, It really takes away the fun. I would much rather try and waste a few bucks even because everything one does teaches us something.

I think I am similar in thinking to Craig in that this is part for fun and part looking to reduce electricity bill but for as cheap as possible.
 
I have a solution that works for me.

For starters, My tub is covered with a semi-clear plastic when I want more heat and I cover that with an opaque cover when I am getting too much heat.

I also have a tiny pump controlled by 2 temperature control relay boards ($4 each). The water is pumped through 100feet of 3/4" black pipe, in a coil similar to yours. This pipe also has a semi-transparent plastic cover to help trap the heat. A sensor in the tub detects when heat is needed and turns on the relay. A sensor near the outlet end of the heat pipe detects when the pipe water is hot enough to bother turning on the pump. When both relays are on, the pump runs. When the hot water in the pipe is replaced by cooler water from the tub, the pump turns off and waits for the pipe water to get hot again. This keeps the pump from running when it will not help. The pump runs off a 12v battery with a 10watt panel. The temp can easily go up from 30c in the morning to 36c in the late afternoon, even with the opaque cover on...on a hot sunny day.
 
yes they are evacuated tubes.
basically is it a glasse tube with double wall, like a thermos bottle.
inside the tube there is a metallic plate to capture heat and a copper tube that goes along the tube to a "bulb" at the top of tube.
the copper tube and the bulb are filled with a very volatil liquid.
the liquid is evaporating inthe copper tube under effect of heat, so cooling it. it goes up into the bulb and condensate
because the bulb is in contact with (cold) water).
That is why the water tank is on top of the tubes.
1598557251941.png
On simple system , the water tank is used directly for consumption, but its size must be important to get a usueful amount of water..
In more complex system (see picture) , the water tank is minimal (it could be oil too) , and there is an exchanger that can be remotely installed.
the condensated liquid goes down to the copper tube and the cycle start again.
you find the same principle in headsink for computer. it is called heatpipe.

1598557056956.png1598557306586.png
 
I have a solution that works for me.

For starters, My tub is covered with a semi-clear plastic when I want more heat and I cover that with an opaque cover when I am getting too much heat.

I also have a tiny pump controlled by 2 temperature control relay boards ($4 each). The water is pumped through 100feet of 3/4" black pipe, in a coil similar to yours. This pipe also has a semi-transparent plastic cover to help trap the heat. A sensor in the tub detects when heat is needed and turns on the relay. A sensor near the outlet end of the heat pipe detects when the pipe water is hot enough to bother turning on the pump. When both relays are on, the pump runs. When the hot water in the pipe is replaced by cooler water from the tub, the pump turns off and waits for the pipe water to get hot again. This keeps the pump from running when it will not help. The pump runs off a 12v battery with a 10watt panel. The temp can easily go up from 30c in the morning to 36c in the late afternoon, even with the opaque cover on...on a hot sunny day.

Oh I think I got wrong end of the stick when this was mentioned earlier. When you say semi clear plastic is that instead of the hottub cover so instead of trying to stop heat getting out you are trapping heat in as green house effect.

I like the idea of controlling the water the same way instead of trying to reduce flow depending how warm it is.

So basically let the water get nice and hot then dump that in the tub.

I can do this pretty easy in nodered with a couple of relays and sensors, I could probably control it all actually with a adrunio pretty cheap.

Then bring it into my home assistant as well

Screenshot_20200827-211331_Home Assistant.jpg
 
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yes they are evacuated tubes.
basically is it a glasse tube with double wall, like a thermos bottle.
inside the tube there is a metallic plate to capture heat and a copper tube that goes along the tube to a "bulb" at the top of tube.
the copper tube and the bulb are filled with a very volatil liquid.
the liquid is evaporating inthe copper tube under effect of heat, so cooling it. it goes up into the bulb and condensate
because the bulb is in contact with (cold) water).
That is why the water tank is on top of the tubes.
View attachment 21106
On simple system , the water tank is used directly for consumption, but its size must be important to get a usueful amount of water..
In more complex system (see picture) , the water tank is minimal (it could be oil too) , and there is an exchanger that can be remotely installed.
the condensated liquid goes down to the copper tube and the cycle start again.
you find the same principle in headsink for computer. it is called heatpipe.

View attachment 21105View attachment 21107

Yes when I first read about them I thought ah just like a heat pipe.

I found a guy selling 15 tubes I think it was second hand, just the tubes for £15 if I remember right was about to snap them up when realised £15 each, to be fair that probably still wasn't bad.
 
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