Solar collectors - DIY glazing and temp control

[YonnyPiscinas]

Hi guys,

A couple of months ago I installed 12m2 of solar collectors on our pergola next to the pool.

After a week of testing the panels were not collecting much heat. They are of an open tube kind (gaps between the small black tubes). With more than 100% coverage the temp difference between the input and output was only about 0.5c even at the strongest time of the day.

We always seem to have a lot of wind (cool) blowing across and through the pergola from the coast (100-150 metres from the ocean). So with a lot of wind, evaporation and high flow through the panels we weren't getting any results.

Reading around the subject I understood the loose tube collectors can lose as much heat as they absorb in windy locations.
So I built my own glazing! Insulation layer below the panels and a 4mm double wall polycarbonate sheet above them boxed in.

Seems to be working well now. Output temp is higher on the windy days now. Pool temp between now 31-35c. The new cover has greatly improved maintaining this temp as we had so much evaporation every week.

Other observation with the pool cover is that the chlorine levels have risen significantly. I had to have the SWG on max before now it is on the min setting. This is good on my electricity bill also!

I'm going to install 2 temp sensors and wifi switches for the pump.

This way I can monitor the heat of the sun next to the panels and also the pool temp from my phone.

Once the temp of the sun in the morning is higher than the pool temp it can switch on the pump.

Also if we get a strong shower or cool day with heavy cloud it can prevent the pump from circulating the water through the panels.


Unfortunately in Uruguay I can't get hold of automatic 3 way valves to make this easier.


Most likely during my next trip to USA I'll bring one back.

Next job: I'll be moving the panels to another roof and possibly increasing the area to 20m2 or more to work well at the end of the season.

Thanks

 

[jseyfert3]

Good stuff!

One thing I want to point out is a common misconception people have though, which you seem to also have:

So with a lot of wind, evaporation and high flow through the panels we weren't getting any results.

High flow through a solar collector gives better heat gain to your pool than low flow! The temp difference between the inlet and outlet of your solar collector will be less, but your pool will gain more heat. This is because you always have a fixed amount of heat coming into the panels from the sun, but the panels themselves will typically loose heat (if the water is hotter than the ambient air temp). This heat loss is dependent on temp difference between the the water in the collector and ambient temp. The hotter the water in the collector, the more the heat loss from the collector and the less that gets into your pool. Flow more water and the water stays cooler in the collector because it's getting transferred into your pool quicker, and the cooler water looses less heat to the air.

The more flow, the better, until the increased power to run the pumps at higher flow cancels the gains from having more flow. Generally you want no more than a few °F temp rise at most from the inlet to the outlet of your solar collector. If you have more than that, you should probably have more flow.

 

[YonnyPiscinas]

Good stuff!

One thing I want to point out is a common misconception people have though, which you seem to also have:

High flow through a solar collector gives better heat gain to your pool than low flow! The temp difference between the inlet and outlet of your solar collector will be less, but your pool will gain more heat. This is because you always have a fixed amount of heat coming into the panels from the sun, but the panels themselves will typically loose heat (if the water is hotter than the ambient air temp). This heat loss is dependent on temp difference between the the water in the collector and ambient temp. The hotter the water in the collector, the more the heat loss from the collector and the less that gets into your pool. Flow more water and the water stays cooler in the collector because it's getting transferred into your pool quicker, and the cooler water looses less heat to the air.

The more flow, the better, until the increased power to run the pumps at higher flow cancels the gains from having more flow. Generally you want no more than a few °F temp rise at most from the inlet to the outlet of your solar collector. If you have more than that, you should probably have more flow.

Thanks for the information. Yes I have been reading around the subject - the T delta etc.
Interesting science.

btw - the manual of the panel manufacturer does state that an optimum flow is 250 l/hr per m2.
Mine is double the recommended flow.

But now as I have gotten the pool temp up above 30c then the faster flow does make more sense with the cooler days <30.

 

[YonnyPiscinas]

I have 2 temp sensors set up now (Sonoff TH16 with sensors). Playing around with the location of the one on the pergola where the solar collectors are. Currently under the glazing about 1cm next to the panel.
Set the pump to come on when the temp under the glazing reached 34c.
Temp dropped to 32c under the glazing once there was flow.
It has been steady at 32c for 45 mins now. 10am.

Pump will switch off if temp under glazing drops to 30c. Pool temp is at 30c right now.
Air temp just 17c!!

 

[jseyfert3]

That is awesome! I am bookmarking this thread for my future reference.

Are you watching the water temp coming out of the panels? It is possible for that to be less than the temp under the glazing, though probably unlikely. Obviously the solar panels are doing a good job if the pool is at 30 with an air temp of 17!

 

[YonnyPiscinas]

That is awesome! I am bookmarking this thread for my future reference.

Are you watching the water temp coming out of the panels? It is possible for that to be less than the temp under the glazing, though probably unlikely. Obviously the solar panels are doing a good job if the pool is at 30 with an air temp of 17!

Thanks!
I plan to add another sensor in the piping of the panel discharge next month so I can compare the pool temp and increase from the panels.

Currently, I manually measure with an infrared gun a few times a day.
34c under the glazing right now. Getting about 0.7c increase. 10:40am so still early.
Fresh wind and 20c air temp.

Pool was at 34.5c 3 days ago, but following day heavy cloud without sun and then yesterday heavy rain and thunder/lightning for 24 hours. So a 4c drop in 2 days not bad at all!

 

[YonnyPiscinas]

30 mins later temp under glazing is now 36c at 11:17am. rising every 10/15 mins or so as sun as getting higher in the sky.
Air temp still 20c.

 

[YonnyPiscinas]

11.40am - 39c under glazing.

 

[YonnyPiscinas]

11:53am = 41c!

 

[YonnyPiscinas]

13:02 = 51c!
Air temp 21c

 

[jseyfert3]

Do you have any pictures?

 

[YonnyPiscinas]

Sure I will take some shortly - it's pretty crude mind you.

13:11 = 53c!

 

[YonnyPiscinas]

I have 8 panels 3mx0.5m. 4mm double window polycarbonate. The largest size sheet didn't quite fit the entire length of the panel so I sealed the ends as you can see in the pics. Underneath I fitted impermeable insulation with a reflective sheet.

I do realise I will lose some of the solar energy using the polycarbonate but it appears we're gaining energy from the heat produced underneath the sheeting that would otherwise be cooled off with the coastal winds.
These kind of panels are very efficient at absorbing heat but also losing it if there is a cool wind blowing across them I read.

Before we had so much coastal wind blowing underneath through the 2x1's and from the sides.
I should have put a steeper incline on the pergola. Not all the water runs off after raining. Will have to periodically wipe it down with a cloth to keep it clean.

I may be just a temporary project as we're thinking of moving the panels to the top roof of the house and adding more. about 6 metres high though.

 

[YonnyPiscinas]

13:32 = 54c!
13:51 = 55c


Still I am surprised there is only about 1c difference between the in and out temps of the panels with the temp at 55c. I want to get a proper sensor in the pipe to measure the water temp and not just the plastic exterior of the pipe.

Update at 15:00hrs
temp under glazing = 52c beginning to fall a little
Temp of pool now 32c. Gained 2c since this morning.
Air temp still only 21c

 

[YonnyPiscinas]

The temp probe on the roof has been working well especially on days with wind and intermittent cloud.
Pool temp averages 33-34c.

We have cooler days 23-26c so as soon as the sun goes in the ambient temp drops quickly and I don't want my 34c pool water running through the panels.
I have the threshold set at 37c. Soon as the temp next to the panels reaches 37 the pump kicks in.
If it drops below 35c then switches off the pump otherwise it will start cooling the water.

Works great on those partly sunny days.

 

[YonnyPiscinas]

Ok now it is too hot!

reached 37c today. Day time air temp 31c today. (Had one night with 11c this week and only dropped 2c!)

Now with experience 33c is a nice temp. 36-37 is extreme and gives you a hot shock the same as getting into a hot bath!!
Never would I have imagined I could get a swimming pool as hot!

 

[jseyfert3]

Wow, that's crazy! We keep our spa at only 100 °F (37.8 °C). It's nuts you were able to solar heat a pool to almost that temp!

Oh, and I should mentioned, that 100 °F spa is outside, and lately it's been 30 to 40 °F (-1.1 to 4.4 °C) air temp when we are using the spa. So with the ~75 °F air temps you have right now (if I Googled correctly), that would be way hot!

 

[YonnyPiscinas]

Wow, that's crazy! We keep our spa at only 100 °F (37.8 °C). It's nuts you were able to solar heat a pool to almost that temp!

Oh, and I should mentioned, that 100 °F spa is outside, and lately it's been 30 to 40 °F (-1.1 to 4.4 °C) air temp when we are using the spa. So with the ~75 °F air temps you have right now (if I Googled correctly), that would be way hot!

Tell me about it. Been getting too hot this week. Had to adjust the bypass valve. Temp probe on the pergola reaches over 65c!!
Maybe a result of the glazing? Hard to tell.

Had an idea to build a hot tub next to the pool with the outlet of the panels going directly into that.

 

[jseyfert3]

Had an idea to build a hot tub next to the pool with the outlet of the panels going directly into that.

Do it!

I'm actually thinking of running a solar panel to heat our spa when we're not using it. Unfortunately up here that would mean a high quality commercial glazed panel, maybe even vacuum insulated, to get the required temps with our cold ambient air in winter, along with anti-freeze in the panel and a heat exchanger to the spa water, so it would be an expensive, ungainly mess of plumbing. This means it probably won't happen, though I need to measure the standby losses and do some calcs before I dismiss it out of hand.

 

[YonnyPiscinas]

We have hit mid-summer and it's getting very hot this week.

I have solar collectors and switched them off today as the pool was getting too hot.

Then the temperature on the roof has hit 70c (I have a probe) and was still rising. Soon as I saw it hit 70c I turned the collectors on again just to cool them down!!

I read somewhere that the max is 140f (60c).

What is the max temperature these blue pipes can withstand?

Thanks