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Solar Water Heating - Lessons Learnt to date


Wetkit
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Right,

 

So I have been playing around with my own solar water heating system now for a couple of years and this is what I have learnt to date:

 

1. Vacuum Tubes is WAY better than Flat Plate collectors.

2. VT orientation is not that important compared to FP.

3. Tilt angle IS important. The more square you can get the sun in cold months, the better. It will also prevent overheating in summer.

4. Thermo siphon works when you follow all recommendations.

5. Problem with thermo siphon is the location of the geyser and the frame to mount it on. Also the circulation effect only starts working at a high temperature.

6. With the geyser located outside, heat loss is a problem.

7. With the geyser mounted outside, hot water piping runs can be very long, resulting in long wait times for hot water.

8. Work on 300L hot water storage for 2 persons.

9. 24 Vacuum tubes is ok for 200L geyser.

10. 2,5m2 Flat Plate collector is not really enough for 200L geyser.

 

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SO, now you asking yourself why I posted the above???

 

Well, I installed a small wood stove in the house this winter. All I can say is WOW, should have done it years ago!!!

 

Problem is, now I am getting ideas about how to utilise some of the excess heat to heat some water and perhaps heat the rest of the house via under floor heating.

So while looking up and down the internet I fell over at the pricing on some of the EU and USA specked hot water storage tanks.

Some of these units is really amazing!

Anyway, while looking I fell over some shop soiled EU specked Flat Plate collectors.

2 of them at 2,5m2 each. These things is really a work of art.

So now this summer I will be re-doing my complete solar water heating system, yet again :(

 

The plans for now:

1. Move both geysers inside the roof to have more protection from the elements.

2. By moving the geysers inside the roof, they will now also be closer to the usage points, meaning shorter wait times for hot water.

3. Put the Vacuum tube and both new flat plate collectors in a single loop feeding both geysers at the same time.

4. Install all 3 solar collectors on a single frame, where I can adjust the tilt angle, say from 45 to 60 degrees.

5. Connect both geysers to work as a single 400L unit. Currently they working in series, one feeding the next one.

6. I would have to install a solar pump and controller now as thermo siphon will not work with the new configuration.

 

For now I need some help with the sourcing the following:

1. Low flow, high temp Solar pump.

2. Solar pump controller.

3. Steam release valve.

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For now I need some help with the sourcing the following:

1. Low flow, high temp Solar pump.

2. Solar pump controller.

3. Steam release valve.

 

I can recommend a Laing pump. My controller is a home-made solution (still on breadboard after more than 5 years :)). I have 3 air/steam release valves in my installation, one at each of the highest points in the piping to prevent airlocks. Remember to also tilt the FP collectors slightly (1 - 2 degrees off horizontal) to ensure that any air bubbles in the header pipes always move along with the water flow direction and make their way to the next air release valve. If the panel is installed perfectly level or tilted in the wrong direction the air bubble build-up might cause airlocks and prevent water from flowing through certain of the riser pipes and effectively lessen the effectiveness of the FP collector.

 

panel tilt

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Well, I must say I was utterly amazed by this new flat plate collector.

If it was not for the internal flow restriction, I would say it nearly works better than the vacuum tubes.

The flat plate have 22mm headers at the top and bottom, but there is only a single 8mm tube running between the bottom and top section.

 

While removing the old flat plat unit, the new unit was catching some sun. It was so good at collecting heat, the plastic end caps melted!!!

While installing the new panel, the old panel was also catching full sun, but it really did not even get hot :(

Granted, the unit is around 8 years old, so technology have advanced a bit.

 

Seeing this test working so well I think my plans can progress.

 

post-23-0-61942500-1438150179_thumb.jpg

 

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Hi Wetkit is that the Austrian Selective Coated Flat Plate Collector that you have there?

 

I have a similar flat plate collector panel (harp type collector) on a guesthouse and it has a stagnation temperature of 210 Degrees Celcius.  It is on a pumped system and have had some problems with it going into Stagnation during load shedding. Have had to replace the collector temperature probe and pump within the first 3 years of operation. The full brass cobra NRV valve broke / corroded off at the hinge and blocked the pump discharge line causing it to run continuously. So have had loads of trouble with this system.   

 

In contrast at my home I have a 8 year old flat plate thermosiphon system that hasn't given me any problems to date. I replace the sacrificial anode every 3 years and flush out the collector every year. I am really impressed with the Thermosiphon system. On a clear winter day like yesterday my tank reached 58 Degrees Celcius. 

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Hi Carl, yes, you are correct.

Unfortunately I bought the panels 2nd hand from a shipping company.

They damaged the rear of the panels during shipping.

They were looking for R4,5k each but I offered R3k, which I thing is fair.

The damadge was not too difficult to repair.

 

What did pi$$ me off was the local agent. The refuse blankly to sell me any parts, even mounting kits, as I did not buy the panels directly from them.

They even refusing to give me installers details :(

 

The problems regarding a pumped system VS thermo siphon is a good one.

I have been running thermo siphon for 8 years without problems, but I think it is now time to put the geysers inside the roof closer to where I will be using the hot water.

I have thought about running just a pump directly on a solar panel, but a decent controller will give me better control options.

To run the controller and pump from solar UPS power should help loads, I think :)

The element can be controller via a contactor from normal power.

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Makes a lot of sense having the geyser located near the draw off point.

 

Seems crazy that the local agents won't support you regarding parts and installation. I am sure that you will manage to do a great install without their help.

 

Regarding pump controllers have you considered the locally made Geyserwise?

 

Sunscan and ITS both sell the imported SR 868 controller.

 

I would look closely at the quality and max temp. specs of the temperature probes and especially the collector probe as it is more likely to be exposed to higher temperatures. Also look at replacement costs as the SR 868 collector probe (PT 1000) is about R280 while the Geyerwise unit is about R100.

 

I have a second hand Wilo pump (RS 15/6) available at R500 if you are interested.

 

Any plans for your old flat plate collector?

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Hi Carl, what voltage is that Wilo pump? Connector sizes?

Perhaps swap the pump for the collector?

 

I really like the control options on the SR868. In summer there is an option to enable the pump to run at night. That is to cool down the geyser if the temp is too high.

Not sure if the Geyserwise unit have the same options?

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  • 1 month later...

I finally got the SR868 controller and connected it up to my existing vacuum tube system.

It took me a week to figure out I was getting reverse flow at night on my thermoshypen system :(

Like 12:00 at night the header was at the same temp as the geyser.

Some nights I could lose as much as 6 to 8 degrees from the geyser.

 

This motivated me to get my new plans going. The frame took a week-end to build, but 2 weeks to get it to the galvanisers and a further 2 weeks to get back home :(

 

While the frame was out, I looked at finalising my design. This took many drafts to get right.

 

post-23-0-66615000-1442817702_thumb.jpg

 

 

Right, so let

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post-23-0-07962600-1442817843_thumb.jpg

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One problem I notice - the air release valve on the panels should be at the highest point, that would be the left most point in the photos - not in-between the FP and VT collectors.  :) Any air build-up in the piping inside the VT collector will move along with the water flow and accumulate at the left most (highest) point and might cause an air lock and prevent water from flowing.

 

Edit: I see you do have it in the correct place in the diagram though.

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Another thing, looking at the diagram, it might be a better idea to move the non-return valve from the line coming from the solar collectors to the line feeding the solar collectors - that way you wont have so much boiling hot water flowing through the valve and the valve might last longer. When those valves age, they either get stuck inside or does not seal off properly and thus not prevent reverse flow - by not exposing them to extreme heat on a daily basis they might last longer.  :)

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Well, the way my panels and frame worked out, left that as the highest point.

Yes, it is not at the end, but as the water is driven by a pump, it should not airlock?

 

What I picked up from my other unit that did have a problem with reverse flow, was that it was the hot water returning, not the cold water.

Will keep an eye on it, thanks.

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Well, the way my panels and frame worked out, left that as the highest point.

Yes, it is not at the end, but as the water is driven by a pump, it should not airlock?

 
Those little circulating pumps are not in any way able to draw air down the piping all the way to the next (if any) air release valve. The only way you are maybe going to get the air out, is by forcing water under municipal pressure through the piping.
 

What I picked up from my other unit that did have a problem with reverse flow, was that it was the hot water returning, not the cold water.

Will keep an eye on it, thanks.

 

Water going to the panels, must come back from them on the other pipe - it will not make a difference where you put the non-return valve. You can compare it to a reverse polarity diode in electronics - does not matter if the diode is in the positive or negative line - it will prevent current from flowing in the wrong direction either way.

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Ok, so the pump is the lowest part in the complete system.

The pump is also pushing the water up into the system, not sucking the water down, so I think I am pretty much coverd wrt the airlocks.

Even if air does collect, it will just go out via the hot water taps.

I will keep an eye out for that problem :)

 

I did have a look at putting the non return in the cold line, but could not find a good location to do so.

It is the flap type, so no spring return. This means it must be mounted vertically for the flap to operate correctly.

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Ok, so the pump is the lowest part in the complete system.

The pump is also pushing the water up into the system, not sucking the water down, so I think I am pretty much coverd wrt the airlocks.

Even if air does collect, it will just go out via the hot water taps.

I will keep an eye out for that problem :)

 

I do understand your theory about the pump pushing the water up, but what goes up, comes down as well. Air will move upwards and will not come down with the water, unless forced at a high pressure. Air will not get out at the hot water taps, because the little circulating pump is not strong enough to move (force) the air (with the water) all the way down to your geyser to escape on the piping feeding the taps.

 

I had a few bleeder valves (taps) in my initial installation (before I installed 3 air release valves). The only way I could get all the air out of the piping was to close off the solar panel return just before entering the geyser and to open a bleeder valve, right there, just before the water entered the geyser. I rephrase - that was the only way I could get the air out - by forcing water through the piping. Then a day or two later I would notice a drop in water temperature and had to bleed the air out again. Installing air release valves on ALL THE HIGHEST POINTS in the installation solved the problem completely. That was my first-hand experience and I'm only telling you this, because I've learnt the hard way.  :)

post-594-0-09355600-1442846230_thumb.jpg

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what temperature do you guys heat the geyser to with solar? I have a split tube system and stop at 65 degrees but I see the panel regularly sits at 120. I was considering installing one of those thermal valves that mix hot and cold by the geyser output and heating it to 80, is this possible? Does anybody make use of the controllers ability to switch a second pump to cool the panel?

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I agree with Superdiy about moving the NRV. I would place it after the pump discharge but before the isolation valve. 

Moving it to that location will also offer the pump some protection from collector stagnation and you will be able to isolate it for maintenance.

Those full brass non spring type non return valves should be ok installed in the horizontal position.

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Some additional comments and questions:

 

1) I like the way you have connected your 2 geysers together and that geyser 1 will see the full solar collector array heating potential in winter.

2) I see that some insulation is missing near the geysers connections. Recommend insulating all pipes and bends etc. You will be amazed how much energy you lose if you don't insulate properly.

3) Would suggest replacing that green kwikhot vacuum breaker with a better quality unit like the cobra double seal unit. My uncles house was flooded when 1 of those vacuum breakers failed. Or at least double check the temperature specifications on it.

4) What is the distance from the collectors to the geysers?

5) Would recommend an annual check on the pump and NRV as a minimum. The cast iron casing on the pump seems to attract magnetized particles in the water and can clog up over time.

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Many questions !!!

 

1. The controller does have an alarm if the panel water temp goes above 130C.

2. The controller does have 2 secondary outputs. One for another pump and one relay.

3. If the geyser reaches 90C, I think the circulation pump will run at night too to cool down the water again.

4. There is a 22mm Tempering valve installed on the hot water output. With this type of system it is important, as you can burn somebody very badly with the hot water.

5. I tried a spring loaded non-return, but it restricted the flow too much.

6. The existing non return valve is the flap type, but the flap is not weighted, so in the horizontal position, there is nothing to help it close.

In the vertical position, the weight of the flap will help it close.

7. Pipes have been isolated as best I could now. First had to make sure there were no slow leaks anywhere.

8. The pump is circulating the water from the panels through both geysers.

9. Distance between panels and geyser is about 15m.

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  • 2 weeks later...

Feedback....

 

Since the installation all been working extremely well.

I must say the controller works brilliant.

If the weather is not too good, it allows the water to heat up before starting the pump.

So, we still have hot water even after the last couple of days with bad weather.

Also no more heat loss during the night, but looks like the old system had issues with reverse flow.

 

Also changed the default temperature setting from 60 to 75 degrees.

Again the controller reacted perfectly.

 

Now, I need to figure out how to fix the frame to my roof....

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Right, some good news and some bad...

 

Recently we had some overcast weather and the system performed brilliant.

The element never came on even after 3 days of overcast conditions and the lowest temp I saw was 55 C..

 

Now for the bad.

I did the long piping to the collectors with this new white Cobra plastic pipe.

Well, they can MELT!!! The max rating is 80 C.

The last 2 days I had a pipe melting off every day due to heat.

I did change the max collector temp setting to 90, but seems that is not low enough.

 

Getting my plumber out now to quote on copper piping. Not sure how much that is going to cost :(

For now I have installed my 80% shadecloth again...

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