DIY Solar Heater with Supporting Data

[BeardofZeus]

After doing a bit of research at the start of last spring and seeing a fair amount of incorrect information online about DIY solar pool heaters I set out to make a cheap version and get some real-world data around it.

My pool gets very little sun throughout the day due to tall trees, so we use the solar cover all the time to reduce heat loss and I wanted to see if we’d get enough sun in a given summer to make up for the heat loss and reduce our natural gas usage.

TLDR 1: Although it absolutely would be effective for some applications, it wasn’t effective enough to make up for all the heat loss in my 70,000 litre pool due to the amount of rain and cloudy days where I live. It did cut-down on my gas usage, but I will just be getting a heat pump next year. There are certainly applications where a solar pool heater would be effective and inexpensive to DIY.

TLDR 2: I was able to get around 1700 W (5800 btu/hr) of heat transfer out of 400’ of coil, or about 600 W/m2 of tubing surface area exposed the sun. This heated my pool around 0.2C/day on sunny days which was more than my heat loss. However, rain and a cool August increased heat loss, so I was still needing to use the gas heater. Note that It takes 277,331 BTU to raise my 70,000 litre pool by 1C (1.8F).



A couple key points:

• Solar pool heaters may or may not be effective for your specific use case depending on many variables such as:
• Pool size
• Pool heat loss per day
• Size of solar heater (surface area of tubing)
• How sunny it typically is where you are
• Faster flowrate is better. Stop arguing about this.
  • A common misconception that it's more effective if you slow the water flow rate down. This is not true. Yes, the water coming out the end is hotter, but the flow rate is lower and total energy transferred is less. The total solar energy captured is lower since more of it is radiating out of the tubing before going into the pool and heat transfer is reduced due to lower temperature difference between the tubing and pool water as the water temp heats up instead of being captured by the cold water and put in your pool
  • 

My System:

• 70,000 litre (18,500 gal) pool
• Around 35% of it gets sun throughout the day due to trees *sigh*
• 400 feet of black 0.70” OD irrigation tubing, wound in four 100’ coils and connected in series
• I used the return jet of the pool to pump water through it to avoid needing another pump.
• I’d connect it in the morning, then disconnect it in the evening to prevent heat loss through the system at night. This was fairly annoying to do but really only took like 30 seconds.
• I added a bit of obstruction/restriction to the other return jet to ensure there was enough pressure to push water through the coils

Efficiency
Incoming solar radiation here during my tests was probably around 830 W/m2 and captured was around 600 W/m2, so around 65-75% captured efficiency (range due to assumptions around incoming solar energy and coil surface area exposed). Overall, not bad!

How did it work out?
My 400’ of tubing heated my pool around 0.2C/day on sunny days which was more than my heat loss of around 0.1C/day on those sunny warm days. Remember that my pool gets very little sun throughout the day, and we keep the solar cover on when not in use.

Unfortunately, rain, clouds, and a cold weather increase heat loss per day, so I was still needing to use the gas heater (We had an especially cool and rainy August this year in Ontario). In ideal weather, this would work, but in my area with the varying weather and heat loss, I was still needing to use the gas heater to top up the temperature for the kiddos.

A stated at the top, I think these DIY systems can have some utility if you go into it with realistic expectations and design it accordingly:

• Verify the typical heat loss in your pool
• Figure out how much heat you’re going to add based on the size of your coil system and amount of sun/day.
• Consider your weather and figure out if this makes sense!

In the end, I’ll very likely be getting a heat pump next season, but this was a pretty fun experiment to do!

You can check out my calculation spreadsheet here if interested - it's somewhat all over the place but if you don't mind poking around a bit in the formulas you can see the various assumptions and such and work out what might be good for you.

Happy to answer any questions about the system!

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Quick Note on Heat Pumps
For a 70,000 litre pool (18,500 gal), the typical recommendation is for 100,000+ btu/hr heat pump (~$4,000 CAD). I think the one thing this really showed me is that I can get away with a much smaller heat pump for the heat loss I have than is typically recommended for a pool my size. And as heat pump prices really increase massively as you scale up this provides huge savings.

For my situation with a conservative 0.2 C temp drop per day, I’d need 55,450 btu of energy added to make up for it. So, only 1-ish hours of run time with a 50,000 btu heat pump ($2,000 CAD). So I'll be able to easily utilize a smaller 50,000 btu heat pump for my situation. If I stopped using the solar blanket and really wanted to try and heat the pool in the shoulder seasons, then there's be issues, but for what I need it should be totally fine to heat up the pool and keep it warm for our June 15 - Aug 31 swimming season.

For my utility prices and pool size, electricity for the heat pump costs around $1.50 / degC increase for my entire 70,000 litre pool and about $6.00 / degC for natural gas with my gas heater.

 

[Newdude]

I think the one thing this really showed me is that I can get away with a much smaller heat pump for the heat loss I have than is typically recommended for a pool my size

You are making calculations based on optimal heat pump conditions. In reality, it's efficiency will be less at the times you are looking to heat the pool, requiring more BTUs. Especially with how far north you are.

Thanks for the write up.

 

[mas985]

After doing a bit of research at the start of last spring and seeing a fair amount of incorrect information online about DIY solar pool heaters I set out to make a cheap version and get some real-world data around it.

You are new to this site so you may not know this but we strive very hard to keep things factual and to dismiss myths.

Faster flowrate is better. Stop arguing about this.

• A common misconception that it's more effective if you slow the water flow rate down. This is not true. Yes, the water coming out the end is hotter, but the flow rate is lower and total energy transferred is less. The total solar energy captured is lower since more of it is radiating out of the tubing before going into the pool and heat transfer is reduced due to lower temperature difference between the tubing and pool water as the water temp heats up instead of being captured by the cold water and put in your pool

We have long understood this since the beginning of this forum in 2007 and for some of us well before that so I don't think you will get any argument on this point.

However, we have had some heated debate on pump efficiency vs panel efficiency. For VS pumps, the lower the flow rate the lower the energy costs and so this has to be weighed against the loss in solar efficiency. And if used with a HP or NG heater against the additional costs of those items as well. So it is not always a simple determination that one should always run at higher flow rates. It really depends on the objective.

• 400 feet of black 0.70” OD irrigation tubing, wound in four 100’ coils and connected in series

23 sq-ft of panel capture area is fairly small and much less than what we would normally suggest using for a pool of your size. More concerning is that you have plumbed the spirals in series rather than parallel. Parallel would get a bit more in efficiency as the average heat rise in the tubing would be less so less heat loss to the environment. Which means you can get the same heat gain at a lower pump RPM and save on energy costs.

Efficiency
Incoming solar radiation here during my tests was probably around 830 W/m2 and captured was around 600 W/m2, so around 65-75% captured efficiency (range due to assumptions around incoming solar energy and coil surface area exposed). Overall, not bad!

Fairly typical for actual operational systems. Commercial panel manufacture specifications are usually overstated in this regards.