Indoor Solar Heated Swimming Pool

[mcfridge]

Hey guys,

I did a search on here, but I still need some help so I thought I'd post. I was thinking of putting an indoor swimming pool in my new home, and I want to compare solar heating it with natural gas. The pool would probably be 10m long x 7m wide, with a depth varying from 1m to 3m. This pool will be used all year round, and it gets damn cold up here, so I'm guessing I'll need to run some sort of antifreeze solution in the piping (propylene glycol maybe?). I've done a bit of reading on solar collectors, and I'm gonna probably use glazed flat-plate collectors, and they have to be a certain % of the size of the pool, etc. But right now I'm more concerned with the pump, piping system, and materials.

I was thinking that the antifreeze would circle through the collectors, then go to a tank where I used some sort of heat exchanger to transfer the heat to the water. Is this the best way to do this? I'm having trouble determining how much water I need to send out (i.e. the flowrate). How do I determine this? Do I need 2 pumps (one for the antifreeze, one for the water, etc)? I feel once I determine this I can find the pipe size, pump size, etc, or am I going about this all wrong?

Sorry for the long post. I know I could probably hire someone to figure this out, but what fun would that be? I'd like to do this myself (with help from you guys of course). Thanks alot.

 

[DRWeig]

mcfridge,

Back in the good old days of the early 1980's, I had to design a bunch of solar stuff. Memory is foggy, so I'll just try to give a pointer or two.

There is an optimum flow rate through your solar collectors, the manufacturer can tell you what it is. Too little flow, you risk boiling and/or pressurizing. Too much and you don't get warm enough water. They'll be able to tell you a GPM per square foot of collector in your latitude.

There are also alternatives to glycol. A drainback system would only pump when solar heat is available. When it's not, water drains back into the heated area so the collectors aren't at risk.

Your heat exchanger should be double-wall if you want to avoid swimming in glycol (if you use it). You can get by with a single-wall if it's a drainback.

Talk to the collector manufacturers. They have good data.

Let us know what you find out!

Old Dave

 

[mcfridge]

Thanks for the reply.

I live in Saskatchewan, so it normally gets down to about
-40C some days, and I need to run this in the winter too, so would a drainback system work with just water?

Thanks for your help.

 

[hydrae]

I worked on swimming pools while in high school and college.
The solar systems I worked on were for summer use only.
As for flow rate, the faster you can run the water through the collectors the more efficient they are because they lose less heat to the air above the collectors. If you run the water slowly, the panel feels warm you are losing that heat.
The medium was pool water directly again less heat loss through the heat exchanger and by keeping the panels cooler. Also allowed the use of the filter pump as the driver
The panels were not covered, more heat gained by the transmissivity of the glass but was for summer only.
Systems were auto drain back when available.
The area of the panels were sized between 50% to 100% of the surface area of the pool (45 degree lat.) south to southwest exposure.
My recomendation for winter heat of a swimming pool is a water to water heat pump if you can use the ground as your source of heat.
Hydrae

 

[DRWeig]

Hydrae's got good points. I was concerned about warmer water because my experience is in solar supply for domestic hot water or space heating. I agree that cooler water is better for efficiency.

If you do a drainback type system, it doesn't matter what the outdoor air temp is. It should be designed to all drain back into a heated space. Of course, if you're directly using pool water, the drainback is simple. It all comes back into the pool.

Heat pump's a good choice too.

Let us know how it works out!

Old Dave

 

[mcfridge]

Hey Old Dave,

I found this system on the this website

http://www.powermat.com/PG5ht.html

would this be the type of drainback system I would want to use? I'd prefer this over having to do the whole heat exchanger system shown at the bottom of this page

http://www.thermo-dynamics.com/solar_pool_heating.html

Also, this sounds like a stupid question, but the guy at the powermat site told me I needed 3-9GPM/collector, so to get my overall flowrate do I just multipy the # of collectors I need by this number?

 

[mcfridge]

So I've decided that the system won't work in the winter, but I was trying to figure out the max temp rise I could get in the winter, using either anti-freeze or water running through the collectors (I'm either going to supplement the system with a heater, or just bypass the solar all together in the winter). I know the flow rate and the initial temperature, is there a way to calculate the temperature rise? I was thinking q = mcp(deltaT), but I don't know one of the temps and the heat transfer. Can anyone help me out with this?

Chris

 

[DRWeig]

Chris, you'll have to get the heat transfer coefficient for both the collector heat exchanger (to working fluid) and also the heat transfer coefficient for the collector to the surrounding ambient (get these from the manufacturer). Then you need the entering water temp, flow rate, outdoor temp, and incident radiation for your latitude and the orientation of your collector. Most of the climatic data is available in ASHRAE handbooks.

It's a kinda complex heat-balance that you have to do. Perhaps the collector manufacturer can give you an approximation though.


Let's see if anybody else in the forum has a better answer.

Best regards,

Old Dave