Solar Water Heating

[Ljn100]

Wasn't sure where to post this, could have gone for "In the garden", "Plumbing and Central Heating" or, if it existed, "Things People Try When Bored in Lockdown"...

We were given a 10ft paddling pool for the garden, which the kids love but is a bit cold at times. I saw some videos about making solar heaters by pumping the water through a panel of coiled pipe like this:
https://www.youtube.com/watch?v=Sc78nueDQ64

Using some scrap wood, pex pipe and a large picture frame I had lying around, I put together something similar to give it a go. I coiled about 30m of 15mm pipe around a frame approx 1m*0.75m, all painted matt black.

Results were... underwhelming! Produces very hot water initially but gradually cools so the output is 2-3deg c warmer than going in. Basically the water is keeping the box cool rather than the box heating the water.

Definitely not an expert in thermodynamics so was wondering whether it's worth trying to improve on it at all, or whether it's not worth it in the UK. Any opinions or advice would be appreciated. Please be kind of any of these are stupid questions .

Things I'd considered:

• The pipe is spiralled in there very tightly currently, would they absorb more heat if they weren't touching?
• Is there any consideration of ratios e.g. would a larger box area make a difference with same amount of pipes or it is the surface area of the pipes themselves that matters? Would there be more heat captured in the larger box?
  
• Similarly would the box height make a difference e.g. trap more heat so the cold water isn't chilling the entire box
• Would a better/thicker plastic cover be more effective at absorbing/trapping sunlight/heat?
• Does UK climate make it not worthwhile? Am on the south coast and get plenty of direct sun but it's presumably a factor still

 

[foxhole]

Choice of pipe is not ideal as it’s a fairly good thermal barrier.Heat collection dimensions are tiny.
Copper would be far more effective but also more expensive.
Flow rate needs to be very slow ( have a heater for pool which is two kilowatts, trickles thru and takes four Or five hours to have any noticeable effect.)
You would need it to run all daylight hours and be able to insulate the pool to prevent it cooling overnight.
None of which is very practical .

 

[bernardgreen]

Basically the water is keeping the box cool rather than the box heating the water.

Same thing. Either way heat is moving from the coil to the water and hence to the pool.

Did you put glass on the front ? If so is it the type of glass that blocks heat rays ( to keep rooms cool on a hot sunny day )

 

[Johnny Allround]

I simply cover the pool with black bin bags you’d be surprised how the sun heats the pool in the morning before I let the kids go into the water. Hope this helps.

 

[Ljn100]

Thanks guys

Foxhole - agree on the copper probably being better but was having a play and didn't want to buy a load at those prices (yet..). I have a small pump that i can adjust flow rate on, but I think there's a trade-off between a trickle of very hot water and a faster flow of warmer as I think how quickly you recirculate the existing water needs to be considered? e.g. if you can circulate the volume of the entire pool through it in a few hours (possible for me as it's not a proper pool) and raise it through that time by, say, 5 deg, is that more effective than adding a few litres or very hot water over the same period?

Bernard - appreciate it's the same thing, I had just hoped it might be stay warmer while circulating though (i.e. was collecting more heat), hence my question of ratios of pipe length and box size. On the front of it I've got a thin bit of clear plastic off an old large clip picture frame screwed on. I don't think it's blocking the heat is it feels very warm before you start the pump and then cools down (stays warm on the areas without the pipe directly underneath)

Johnny - good shout, had looked to try to buy a proper solar cover for it but seems to be sold out everywhere, might try the bin liner approach! Still interested in seeing what else can be done here though in addition.

 

[daviesm1976]

I read somwhere that max energy per metre squared is only 1kw per hour so you are straight away limited to 750w with the size of the collector.

You can estimate the temperature increase per hour by 750 x 3600 divided by (4200 x litres of water on the pool. ) This assumes no energy losses also! You might only be looking at a degree per hour!

It is usually quoted that the surface area of your collector should be least half that of your pool.

These designs are quite common and in fact it is a good thing that the water is only a few degrees hotter. If it was flowing slower the water re entering the pool would get hotter but more heat would be lost.

Another design ive seen is to use pex in a serpentine arrangement through aluminium heat spreader plates covered by twin wall polycarbonate. Pex would obviously not be the best thermal conductor but then again if you are adding chemicals to the pool it can react with the copper making your water turn a funny colour....

Depending on the voume of you pool it can take hours or days to significantly raise the temperature. You definetely need a pool cover to help absorb heat at the pool surface and minimise heat loss by evaporation at night.

 

[Johnny Allround]

I know tried the solar cover option together with recliner sun chairs not a cat in hells chance of getting one at a decent price for the moment anyway. Good luck

 

[daviesm1976]

Most of these solar covers are like bubble wrap. Could try some of that painted black or with bin bags sellotaped to the back of it!

 

[charliegolf]

Have a chat with the guys on the forums at navitron. A renewables site.

 

[StephenStephen]

I'd be wanting to get some metal in there - if not metal pipes, then a sheet of metal behind and in touch with the pipes
The other option that comes to mind is to get hold of an old radiator