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Author Topic: Problem with system performance degrading over several years  (Read 1263 times)
guyrhall
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« on: March 28, 2020, 06:44:10 PM »

Apologies in advance for the large post.

• Type of panel: Evacuated tube
• Number of panels and tubes: 30 x 58mm tube solar collector on a south facing roof (Navitron 2010).
• Pipe diameter and approximate length of pipe runs in flow and return connections to panel: 10mm, ~14m to flow and approximately the same for return.
• Cylinder type: Navitron's 255L - UNVENTED STAINLESS STEEL CYLINDER (2010)
• Top and bottom cylinder/store temperatures and panel/manifold temperatures: I'm hoping my analysis table below is better than one-off measurements.
• Water consumption through the day (approx.): Water meter suggests ~204L per day average, I have rain water harvesting, therefore 50% of that is hot: ~100L?
• Outside temp (mainly appropriate to winter issues): Southern UK
• Antifreeze percentage if used: Don't know. TDC2 settings suggest 40%.
• Pressurised system components used e.g.
   ○ Expansion Vessel make: VAREM SOLARVAREM 18L, code R8018241 (Navitron 2010). Service engineer wrote the following on the tank in May 2015: 35.5psi -> 25.5psi. Reading on gauge (running) at the time I write this: 2.25 bar.
   ○ Non-return valve type: Not sure, is that the value the pressure gauge is attached to? It has "DM15" on it?
• Location of air vents, vent type i.e. manual or automatic: No vents
• And if air is still being collected/vented/circulating: N/A
• Solar controller manufacturer/model and settings: TDC2 (2010), Tmin S1: 29c, Tmax S2: 65c, Delta TR1 6c/4c
• Pumped or thermosyphon system: Pumped.
• Pump running times: Not sure how to describe?


I've been suspicious over the last 2 summers (2018 and 2019) that my solar thermal system is getting less efficient each year. The last few days in glorious sunshine suggests it's degraded further this year.

I have a temperature monitoring system that measures the solar heated part of the tank and the top, gas heated section and takes a reading every minute. Unfortunately I don't have readings for the panel output (a work in progress) but it typically is 4-6c above the other end, the solar tank input.

Before May 2015 the system seemed to have trapped air as the flow rate was unstable, the float measure jumped up and down in the glass, and you could hear air bubbling on pump start up but as you can see from the table below it was providing lots of hot water. It was serviced May 2015 with new glycol/water fluid in the heat transfer circuit and the engineer said the pump rate was too high (3) and set it to (1) which provides a flow rate of between 2 and 3 on the scale in the window. My reading on the subject suggests this is correct for a 2.8m2 set of tubes. From that point on the float appeared stable and the sound of trapped air stopped and 2015 continued as a well performing year.

This stable behaviour without trapped air has been the same since.

To compare how it is performing I analysed data between the 1st June and 31st August in 2014, 2015, 2016, 2017, 2018 and 2019. I decided to capture the number of data points (minutes) readings were above a certain level and the maximum temperature achieved that year. I have a 2kw PV system next to the solar thermal and have included how much output it generated over the same period as an indicator of the relative sunshine in that summer window over the 6 years.



Here's my observations and thought process:
• Hot water usage was consistent in all 6 years, e.g. number of people in household.
• For an anecdotal view, in the early years of the system we could easily go several days without gas and on a couple of occasions several weeks. This has not been the case in the last 2-3 years.
• There's no sudden change in performance it has been gradual.
• The tubes look clean. The underside appears to have a build up of dust but I assume this has no effect?
• If the flow rate was wrong, either too high or too low: heat would build up in the circuit, top sensor would be very high?
• If heat was not transferring from the coil to the water (due to scale, we live in a hard water area?): As above, heat in the circuit would increase to very high levels?
• Heat loss in pipes: difference between panel sensor and tank is always around 4-6c so there is loss but not significant. The pipe length, insulation, etc. has not changed over the period so any loss should be consistent?
• Therefore is the problem heat transfer of the tubes themselves? Do they degrade over time? Could there be something in the fluid mix reducing heat transfer or could the fluid be depositing something in the tubes' surfaces blocking heat absorption?

Any advice on further diagnosis gratefully received.

Thank you, Guy.

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AndrewE
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« Reply #1 on: March 28, 2020, 07:00:32 PM »

My system isn't quite the same (the tubes feeding a lower single coil in a twin-coil cylinder) but I wonder whether the heat transfer tubes in the cylinder could be getting furred up?
I have emptied my roof header tank and the copper cylinder fed from it and in both cases I was astonished by the amount of sludge in them, which could only have come from the mains water! 
I don't think the performance of my set-up is degrading, but it doesn't seem to give us a tank full of scalding water as frequently as it used to...  I am surprised at yourPV figures as I thought we had a poor summer last year (in the Midlands.)
If you have got hard water there might be limescale on the outside of your solar coil too...
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guyrhall
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« Reply #2 on: March 28, 2020, 09:26:35 PM »

Thank you for your feedback. Sludged up tank or furred up solar coil is one of my theories but if that was the case and the heat wasn’t transferring to the water the circuit would just get hotter and hotter but it doesn’t? I’m not sure.

Re. PV in 2019 each summers been fairly consistent. Heat waves just make the panels hot and less efficient. Cooler less sun but more efficient. I think.
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rogeriko
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« Reply #3 on: March 28, 2020, 09:38:15 PM »

Put the pump back on 3 and see if you can hear air in the system. Solar thermal tubes do not degrade with age unless the glass is broken. The speed of the pump is irrelevent, heat is heat, its not going to change just by pumping fast or slow.
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brackwell
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« Reply #4 on: March 29, 2020, 07:51:01 AM »

The tank coil must be furring up if you live in a hard water area and do not have water softening.  However that would manefest itself as rising flow temps as you say.

Back along i had solar thermal and one yr when i came back from holidays my cheap insulation had melted on the pipes although the proper stuff was OK.  What had happened was the tank temp and flow temps had increased because of lack of draw off. What i found was that the controller had a factory setting (which can be changed) which switched the pump off above a certain temp (60+ something) and stopped the water circulating and hence the panels stagnating which in summer could be c150C i believe and the conducting heat down the pipes had melted the insulation and water had evaporated.

I believe you have had something similiar happen. The temp drop along the pipe length from panels to tank i would have thought would be very small once steady state was reached.

I suggest you check that the pump is actually pumping water and not fresh air in the main ie you have lost your water.  Dont realy on the gauges go and put hands on pipes particularly on the outlet side of the panels.

I do have the Navitron system and therefore i suspect the same controller and therefore the hand book if you cannot find yours.

Ken
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Tinbum
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« Reply #5 on: March 29, 2020, 10:15:50 AM »

  Dont realy on the gauges go and put hands on pipes particularly on the outlet side of the panels.
Ken

Warning- don't burn your hands though!!
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85no 58mm solar thermal tubes, 28.5Kw PV, 3 x Sunny Backup 5048, 3x Sunny Island 5048, 2795 Ah (135kWh) (c20) Rolls batteries 48v, Atmos wood gasification boiler, Brosley wood burner, 2000lt buffer tank and 250lt DHW
Philip R
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« Reply #6 on: March 29, 2020, 01:28:29 PM »

Hello Guy,
Welcome to the forum.

Regarding your observed system degradation. I have to following to add:

Are all the solar tubes intact regarding the glass jacket vacuum? Is the internal silverring intact (Barium Getter, it maintains  a good vacuum after the manufacturing seal is applied.) Or do you see a whitish deposit instead where the silverring was. The latter indicates loss of vacuum and will reduce the heat input from the relavent tube.
Tubes can be replaced as the spares should be availible.

Hard water supply can seriously degrade any heat exchanger, ie the tank heat exchange coil.
The changing of the coolant was mentioned too. If it has overheated in the past, it can certainly breakdown and form a sludge like goo,(Discussed in other much earlier threads on this forum). If it is sludgy in the pipes, then it must be flushed out.
I have certainly found similar instances of sludge in open vented central heating systems ( which although different in there chemical conditions) should not be dismissed in a closed solar system. Was the pump inspected during its inspection? (Pump head removal to see if the impellor was blocked or indeed that the shaft to the pump impeller not fractured. Unlikely, but I have seen it in CH systems.

Finally the insulation. There have been stories about that foam type insulation applied to cylinders, degrades over time and its heat loss increases. Does your cylinder cupboard seem warmer than it used to be, i.e. increased losses from the cylinder.
Also you mention the central heating coil, is the motorised valve feeding the cylinder coil passing, and removing  heat from the cylinder to the central heating system.

I have just re-read your post again. 28 Metre loop of 10mm pipe. The flow rate down this is well restricted, especially with some dirt in the system. For a system of that length you need a larger pipe size like 15mm.
Philip R
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guyrhall
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« Reply #7 on: March 29, 2020, 02:40:07 PM »

Thank you all, this is great feedback.

Flow in the circuit looks good. It's a patchy sun day today so I can see/hear the pump switching on and off regularly. The float flow rate meter follows the pump's action. Fluid appears clear in the flow meter window. The pipe just before entry to the tank does get warm (not hot today). Taking these observations and that the figures have not crashed but declined over several years, I don't think I've lost any fluid or that the fluid has sludged/degraded.

In answer to brackwell's question Tmax S2 is set to 65c.

Philip R. The pump was inspected and I can see/hear it working as I've tried switching the speed. I'm currently getting some data with it set to (2) and will switch to (3) tomorrow based on rogeriko's suggestion.

My tank insulation is good. On gas is heats quickly and retains heat, the area it's in is not noticeably warmer and from some data today over 4 hours it dropped from 51.8c to 50.8c (top of the tank, gas coil). The central heating system is not removing the heat. The circuit's pipes run through the house so I'm stuck with 10mm for now.

I have measurement points as the circuit goes into the tank and as it exits on its return. The data here is what I would expect, whatever heat is able to reach the coil is being transferred to the water.

I think the evidence is heading toward the tubes.

I tried to get some video of the tubes from my loft window but there's too much reflection and you can't see anything so I'm going to try again tonight using the phone's LED.

Thank you all again.
« Last Edit: March 29, 2020, 02:43:19 PM by guyrhall » Logged
brackwell
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« Reply #8 on: March 29, 2020, 04:43:58 PM »

I dont buy the tubes. Your figs would suggest that your output is now of the order of 10% of new ie 90% of your tubes are down - very much doubt it when people have had their tubes for decades without reporting this prob.

If you are convinced that the pump is working and not pumping air then i think it must be the coil. The efficiency of solar thermal is linked to the temp of the circulating water.  If the heat is not deposited in the tank because of the fured up coil then the circulation temp will build up but the tank temp will not. As the circulation temp increases then the efficiency goes down and if it gets above 65C which is most likely in summer your pump will switch off.

A lot of the coils in tanks were on the small side in the first place.  I trust you are using a tank with a special solar coil and not using one with a CH coil as these are 2 different animals.
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guyrhall
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« Reply #9 on: March 29, 2020, 05:52:17 PM »

The system is all Navitron the tank is one of these:

https://www.navitron.org.uk/255l-unvented-stainless-steel-cylinder
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dimengineer
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« Reply #10 on: April 02, 2020, 11:37:58 AM »

I dont buy the tubes. Your figs would suggest that your output is now of the order of 10% of new ie 90% of your tubes are down - very much doubt it when people have had their tubes for decades without reporting this prob.

If you are convinced that the pump is working and not pumping air then i think it must be the coil. The efficiency of solar thermal is linked to the temp of the circulating water.  If the heat is not deposited in the tank because of the fured up coil then the circulation temp will build up but the tank temp will not. As the circulation temp increases then the efficiency goes down and if it gets above 65C which is most likely in summer your pump will switch off.

A lot of the coils in tanks were on the small side in the first place.  I trust you are using a tank with a special solar coil and not using one with a CH coil as these are 2 different animals.

I'd add to that, for info. When my system was installed, the installers knocked the nipple ends off most of the tubes - 14 out of 21, losing vacuum in them. It reduced the performance by about 50% I'd say, overall. So a broken tube only gives about 35% the performance of a good tube.
So if you lost all your tubes - you'd be down to 35% (ish) of normal performance.
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21 tube Kloben Panel/250L Megaflow, 1.68kWp Solar PV - 7 x 240W Sanyo Panels. Morso Squirrel WBS
guyrhall
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« Reply #11 on: April 02, 2020, 04:45:21 PM »

Thanks dimengineer, I'm struggling to get a good view of my tubes. Filming at night didn't help as the phone LED was as bad as the sun for reflections obscuring a good look at the tubes! I'm expecting to get a good look this weekend and also I have a couple of spares in the shed that I can compare.
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guyrhall
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« Reply #12 on: April 05, 2020, 06:25:21 PM »

Apologies for the radio silence I've been trying a few more checks and getting some pictures. In the link below I've put some high resolution pictures of my tubes and I'd be grateful if anyone could take a look and feedback if they look normal or should I should worry that they appear to have some form of residue on the inside of the glass wall?

https://drive.google.com/drive/folders/1nK8IpPRIMa48qQ0pXNIy3NkGjnOnOSRE?usp=sharing

Thanks again. Guy.
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