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walter matthews

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Posts: 17
Reply with quote  #1 
I'm not sure where to post but it lets me write here so I'll put my  opening here. Live in NCal, Sacramento area, and did two things three years ago that have paid off nicely. I purchased and had installed 19 photo voltaic solar panels that reduced my average monthly SMUD bill from $220 to about $80 a month. Also purchased a used Nissan Leaf that reduced my monthly gasoline cost by more than $200/month. The $80/month SMUD bill includes charging about 20KwHr every 1.5 days so the $80 to Smud also covers all my driving. ............. Roughly $140 saving in SMUD cost and $200 saving in Gasoline cost, not to mention zero maintenance on the car other than a new set of tires.                                                                                                                                          I want to put in a hot tub, that I will build from scratch using redwood lumber I have already purchased.  Current plan is to have a 6' diameter tub  at 4' deep with 3.5 ft of water. That translates into ~100 cubic feet of water or about 700 gallons.  I want to heat the tub with solar water heating and although I have looked over this site I can't find anything directly on the topic. So I have a few questions.                                                                                                                                 1. how much temperature rise can I expect to see in 700 gallons of water sitting at 70 degrees from a 8'x8' CPVC collector at 60 degrees facing due south on a clear day with the outside temperature at 40 degrees F. I know exact numbers are not going to be available but I can't even make an estimate from what I have been able to read so far.                                                                                   2. Has anyone installed a small fan to blow ambient air over the collector when the pump is turned off and the sun is still shining on the collector? It would prevent overheating.                                                                                                                                                                                    3. Would it be practical to build a water reservoir under a bench that rims the outside of the hot tub and would be allowed to get as hot as the system can get the water after the temperature of the hot tub water has been allowed to get to or slightly over 104 degrees. That way I believe I can get the hot tub up to 104 late in the evening by circulating hotter water from the under bench reservoir. 

stmbtwle

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Reply with quote  #2 
Hot tubs are tricky beasts.  It takes a lot of heat to get them up to temperature, but once there it doesn't take much to keep them there and stagnation of the collector IS a problem.  

I heated my 250 gal tub for several years with a 2'x4' ARETHA type collector. The first one with an aluminum heat exchanger failed after a couple years do to corrosion from the pool chemicals.  The second used an HVAC heat exchanger with copper tubes and it worked well.  The metal heat exchangers are more resistant to stagnation than a cpvc grid, and the computer fans I used seemed to have no problem with the heat, though I always mounted the fan in the bottom (cooler) part of the collector. 

I think your idea of a small fan to blow air through the collector box and carry away the heat is a good one.    It's been discussed a few times but as far as I know no one has actually tried it.  I'd power it with a PV panel and control it with a 110 or 120F snap switch. Most folks simply cover the collector in the summer, but of course this doesn't work with a hot tub. You'll probably have to cover part of the collector once your tub reaches operating temperature.

Calculating the heat rise:  Try here:  https://pvwatts.nrel.gov/
Enter your location in the first page and array tilt and azimuth in the second (you don't need the rest) go to the third page and look in the "solar radiation" column.  (1 kwh = 3412 btu) and multiply by the size of your proposed collector (8x8 is about 6 sq meters). guess at efficiency (50%).  You'll have a BTU number which you can apply to the mass of water 6260# and calculate your heat rise. 

Ex: If you get 3 kwh/sqm/day, x3412 x6 x.5  =30700btu. /6260= about 5F temp rise (per day)

The idea of heating additional water would probably work, but I'm skeptical of the economics.  It may cost you more in hardware than it's worth.

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walter matthews

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Posts: 17
Reply with quote  #3 
Thank you for the input and the observations. I plan to use a salt water purification and corrosion is certainly something that demands attention up front. Clearly thermal containment will be of paramount importance and I'm leaning toward building a cylindrical outer shell and filling it with foam. Having a larger collector in terms of space available is no problem-other than cost of course- It might be more practical to heat the tub to around 95 in the day and move it up when we want to use it.  At this stage I'm pretty preoccupied with the details of the woodworking aspect of building the tub, but conceptually it seems that if I put a standard pool solar collector that uses water, i.e. Alcor, and I encase it in 2x6's, with bottom insulation and glazing on top I should be able to get pretty good heating and of course add the cross collector air flow with a small fan  that blows from one side and opens a gravity louvre on the other side. Anyway, thanks again for the really good information.
SolarInterested

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Reply with quote  #4 
Quote:
Originally Posted by walter matthews
... conceptually it seems that if I put a standard pool solar collector that uses water, i.e. Alcor, and I encase it in 2x6's, with bottom insulation and glazing on top I should be able to get pretty good heating and of course add the cross collector air flow with a small fan  that blows from one side and opens a gravity louvre on the other side.


This might be of interest to you:
https://www.builditsolar.com/Experimental/OffShelfDHW/Glazing2/GlazingTest2.htm

"... The idea of the glazing is increase the efficiency of the pool heating collector in order to get it to perform better in low ambient temperatures and in partial sun. This is mainly of interest to people on cold climates.

The glazing is fitted with some intentional openings that allow a limited amount of air to circulate between the glazing and the collector. The idea is that this air circulation will keep the absorber temperature down to acceptable levels when the collector is stagnated (no water flow) during sunny periods. The collector is made from polypropylene which has maximum service temperature of around 250F (various sources quote max service temperature from 210F to 275F). The goal is to hit an airflow that significantly improves the collector efficiency under cold conditions but also keeps the collector surface temperature below 250F (preferably lower) during stagnation events."


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Both temperature rise and airflow are integral to comparing hot air collectors
jjackstone

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Posts: 77
Reply with quote  #5 
Walter,
Don't know how much room you have near the hot tub but you might consider getting a large (80-100 gallon_) hot water heater for extra hot water storage only. I also live in Sacramento and I see them on CL every couple months. You'd only want to heat the water for something like that using solar or you'd obviously be defeating the purpose. 

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myk3y

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Posts: 31
Reply with quote  #6 
Quote:
Originally Posted by walter matthews
Thank you for the input and the observations. I plan to use a salt water purification and corrosion is certainly something that demands attention up front. Clearly thermal containment will be of paramount importance and I'm leaning toward building a cylindrical outer shell and filling it with foam. Having a larger collector in terms of space available is no problem-other than cost of course- It might be more practical to heat the tub to around 95 in the day and move it up when we want to use it.  At this stage I'm pretty preoccupied with the details of the woodworking aspect of building the tub, but conceptually it seems that if I put a standard pool solar collector that uses water, i.e. Alcor, and I encase it in 2x6's, with bottom insulation and glazing on top I should be able to get pretty good heating and of course add the cross collector air flow with a small fan  that blows from one side and opens a gravity louvre on the other side. Anyway, thanks again for the really good information.


My friend in NZ owns a hot tub business and I’ve spent a bit of time with them. A cedar tub doesn’t need insulating - it loses more heat from the surface than through the timber, thanks to the coarse grain of cedar, it’s self-insulating.

It should be deeper than a fibreglass tub, about 1.5;1 depth to diameter.

I dropped him a line about solar heating - he sells solar hot water systems for pools, etc. - I’ll let you know what he comes back with,
walter matthews

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Posts: 17
Reply with quote  #7 
You may be correct and I'll experiment with both air across the collector, air inside the collector and even both at the same time. Either, or both ways it isn't a complicated or expensive thing to do.
walter matthews

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Posts: 17
Reply with quote  #8 
Quote:
Originally Posted by walter matthews
You may be correct and I'll experiment with both air across the collector, air inside the collector and even both at the same time. Either, or both ways it isn't a complicated or expensive thing to do.


After thinking about your comment I believe you are incorrect. No data yet but I can provide relatively high pressure air from a compressor that would allow relatively large volumes of ambient temperature air to pass through the plastic CPVC system that uses each tube in parallel with a plenum on both ends. Meaning that I am not limited to the amount of air that a small fan can provide. I have a 250 gallon pressurized tank that I use for air actuated tools and that source can easily run all day when the sun is shining. From what has been written that system would not even need the aluminum fins to reach relatively high water temperatures and I only need 104 degrees. 
stmbtwle

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Reply with quote  #9 
It might work, but air has nowhere near the heat capacity of water. It will take a lot of air, a big compressor, a lot of electricity ($$$) and the pressure might damage the tube grid.

Just blowing the water out of the collector will NOT work. The empty collector will continue to harvest heat from the sun at the same rate, and without the cooling water it will heat up even faster, and get even hotter.

A method you might try is diverting the hot water from the collector to a heat dump instead of the tub. I used 100' of irrigation tubing lying in the shade. It worked well for me until the pump failed. I eventually scrapped the whole system as more trouble (and expense) than it was worth.

Then there's this... 

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

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Posts: 17
Reply with quote  #10 
Update on solar heating of my imaginary, as yet not constructed, wooden hot tub. Tests and discussions show that the heat transfer capacity of air is pretty poor. So even if I run a high volume of air past the solar collector with the water pump off I am likely to get some uncomfortably hot temperatures and I will be asking for trouble.  Moving along, we here in Northern California have been through a few years of drought. To take some pressure off of my water requirements for plants around the house I put a 600 gallon polyethylene tank at a main down spout of the house and was able to fill it during the few days of rain we did have. The tanks are available for a really reasonable price, I think I paid $50 for mine.


New plan is to have the 600 gallon tank almost full of water and situated behind the solar collector assembly and sitting in the shade. I can put a pump and piping into the tank and when the pump for the hot tub switches off because the temperature in the tub has reached the set point, two things will happen.  First, a ball valve assembly that comes with the swimming pool solar heater unit will release the water in the solar collector back into the hot tub.  Then on a delay of say 3 minutes or so, a second pump will fill the collector with water from the polyethylene tank and that cool water will be pumped through the collector and back into the polyethylene the tank. Although the tank will initially heat up when the water first goes through the solar collector it will not raise the temperature much more than 15 to 20 degrees in the remaining sunshine of the day. Then as the sun goes down and the air cools (we get really nice 62 degrees nights here even when we have had 107 in the day) the water will continue to run through the collector, cooling the water temperature in the non insulated polyethylene tank, down to whatever the set point is. Not knowing what it really will do I am guessing 80 degrees in the cooling tank will be possible.  If it is done right there should be very little mixing of the water from the two systems and as long as the pump continues to work overheating from "Stagnation" should not be an issue.  
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