Solar cylinder temperatures

[mick]

How's this for a freezing but bright day -

20 tube standard panel,
172 lt cylinder - 80 lt of which dedicated to solar, boiler on all the time (we have unpredictable and varying HW demands) so top 92 lt doesn't count. I don't do a great deal of logging

9.00 am  - TST (lower cylinder)  24⁰C
12.30 pm - TST                        39.8 ⁰C
1.30 pm - TST                         22 ⁰C (son had had usual deep bath in between!)
3.44 pm - TST                         34.4 ⁰C

Yesterday - without son! - TST got up to 45 ish from a standing start of 16 ⁰C before washing machine (old hot fill) used some (about 3.30 pm I think).

If weather is going to be like this in future we may all have to revise our performance projections - UPWARDS!!! 

Mick.

[acmetowers]

Had to put this temperature in the Navitron forum as well as the "original" O MidKnight link in INEBG forum as I seriously impressed. Today with an outside temp of 0 deg with a blue sky my 30 tube system got the water up to 64.7 deg! The system is the 30 tube 58mm panel with a 210ltr tank Resol 4 controller with an on temp 12 and an off temp of 10, pump set to pos 1.

[Paulh_Boats]

Had to put this temperature in the Navitron forum as well as the "original" O MidKnight link in INEBG forum as I seriously impressed. Today with an outside temp of 0 deg with a blue sky my 30 tube system got the water up to 64.7 deg! The system is the 30 tube 58mm panel with a 210ltr tank Resol 4 controller with an on temp 12 and an off temp of 10, pump set to pos 1.

That's an amazing result. There will be some serious heat to dump in the summer - you will need a swimming pool or solar hot tub. 


BTW: Is 12/10 the usual on/off temps?

-Paul

[acmetowers]

Yes there is a heat dump, my sort of explanation is on the other thread "Novel solutions to prevent overheating ".

For the on off values they are of are of my own (can not remember the default values). I find that the higher the start and lower difference results in a higher tank temp.

[Ivan]

My 260litre tank was up to 43C yesterday. The previous day, which seemed to be similar in terms of sunshine I only got 34C. The difference was made by changing the DTO temp 10 to 12.5C and the DTF temp from 8C to 10C. Incidentally, from summer experiments, 10C/8C worked best - ideal temperature settings seem to vary with time of year.

Before you all rush off to increase your temperature settings, I should explain that the benefit derives from the long pipe runs I have (around 16m each way) - at this time of year, there is a large temperature drop in the piperun, so by increasing the differential temperature, although the losses are also increased slightly, there is a greater temperature difference remaining by the time the water reaches the cylinder (don't forget that you will experience temperature losses on the return pipe, even if the solar loop water is cooled right down to cylinder temperature.

Temperature settings seem to rely on a very empirical process, and it would be nice to discover a scientific method for setting them...based on thermal characteristics of pipework, length of piperun and temperature of cylinder. I have yet to find any published methods.....


Ivan

[O MidKnight]

48 x 18 twin coil cylinder

26th Feb Good sun (11.00 on)

Start of day temperature 25C

End of day temperature 59C (no boiler in operation)

Estimated water drawn off in between these times in litres = 10

[shambles]

Can anyone suggest any general principles? Starting from the standard settings of 6 and 4...

Should DTO/DTF be higher or lower if the store temperature is low (say 20C)? Should they be closer together (say 2C) or further apart (say 10C)?

What if the tank is hot (say 45C)? Generally, how would this affect the settings?

What about the general difference between a bright day and a dull one?

My (limited) experience so far suggests that if the tank is starting off at 45C, then DTO and DTF need to be high and wider apart to stop the pump running continually - I have settings of 10 and 6 at the moment because I have a larger tank that does heating and water (at this time of the year) so it rarely gets colder than 40C... The higher settings ensure the water going around the coil is hot enough to transfer some heat. With the lower settings, I couldn't be sure if I was just shifting water around the coil to no effect...

[Antman]

Shambles

There have been numerous debates on the preferred settings of DT0 and DTF. We all have our preference based on individual systems.
In general, the longer the pipe runs to the panel, the higher the differential between DTO and DTF needs to be. This is to overcome the increased pipework thermal losses.

The settings are relative to the cylinder temp so I am not convinced there is any gain in changing the ratio with rising cylinder temp.

For short pipe runs, it is more a trade-off between pump run time and maximising the thermal energy delivered to the cylinder.

Regards

Antman

[manwithtool]

Well I was quite happy with my system until I read about other peoples systems !!
Although my panel faces South West so I guess not ideal.  But my 20 x 70mm tubes get my 117 litre standalone tank up from a nromal 15 deg up to 37 deg on a bright day.....I haven't seen it get any hotter since I installed at the beginning of Jan.  I have kept all the standard settings on the Resol BS4.  Pipe runs must be about 25m in total at a guess, all in 15mm and all insulated.

As my tank is a standalone tank I know that none of the heat comes from anyting other than the solar panel.....anyone think this sounds like it's not working properly ?  We don't seem to have had much sun, so I think the system IS ok, but facing SW I miss some of the early morning sun.   But it is only Feb !!

[O MidKnight]

It is only Feb you are right.  You know that I like to use smaller 10mm pipes for my flow and returns. More friction yes but a lot less wasted heat. It is not always just once a day that the solar has to heat the pipes from cold (ish) Remember that in the winter performance it is not quite so important as you have your boiler on anyway. Could you later fit tubes south east if you like the morning sun ( afternoon sun always hotter ). Be happy its great.

[Ivan]

Long piperuns (over 10m) will definitely reduce the performance, as heat loss in pipework is significant at all times of year, but particularly in the winter.

Also, it depends on the amount of water you draw off. If you are looking at bottom-of-tank temperatures, the temperature will reduce even if you only draw a very small amount of water off.

I would say your panels are working fine. Solar performance seems to kick off about the middle of march, the rate of increase of solar irradiation is very fast from this point onwards. Remember, 85% of annual solar energy reaching the UK is received during March-September, the remaining 15% is spread from October to February, so performance is much worse throughout the winter period.


Ivan

[manwithtool]

Cheers Ivan & OMidnight,  I am really happy with the performance.  All my temps are taken from the tank at about third of the way up, so temps are conservative, as I assume the temp at the top will be hotter.  We don;t normally draw much water off during the day...It all goes in the evening, whcih I think is perfect for Solar heating.

Thanks again....Forum still proving useful even after finishing the project !

[O MidKnight]

manwithtool

Please fit another thermometer at the top of the cylinder - even if it is a clamp on cheapie. I hate to see you unhappy.

[O MidKnight]

48 x 18 twin coil cylinder heated by 17 square metre 26 year old matt black painted aluminium roll bond flat plate collectors plus 20 tube vacuum panel.

1st March

End of day temperature 62C

[shambles]

The settings are relative to the cylinder temp so I am not convinced there is any gain in changing the ratio with rising cylinder temp.

Hi again...

I suppose what I was getting at (remember I am a newcomer to this) is this:

When you cool a liquid (a cup of tea?) the first 50% of the temperature drop happens much more quickly than the second 50% - because of the temperature differential, I guess.

Sometimes, you can get into a situation where the solar pump is running almost continually.

I was thinking that the best efficiency is gained by reducing the amount of time the pump runs to the lowest possible, whilst maximising the transfer of solar energy from the panel to the tank. You can do this by raising the DTO, so the pump only kicks in when you get a decent head of heat in the panel header. But then I am left wondering about the DTF. If I leave it alone, then the situation where the pump doesn't stop can arise (because the energy going into the panel is more or less matched by the energy exiting at the coil). So I need to raise the DTF as well - this means the pump will stop sooner and has the advantage that I am putting that first fraction of solar energy into the tank efficiently and not running the pump on to transfer the last more time-consuming fraction. (Is this right so far?).

My theory is that DTF has to be a percentage of the DTO rather than a fixed number of degrees below it. So there is some merit in looking at DTO and DTF. If the starting temperature of the store is high, then DTO needs to be high and DTF needs to be further from it than it would be if the DTO was lower, to maximise the heat transfer, but not too low... The question in my mind is about the percentage of DTO that might be used to set DTF...

The more I think about this, the more my head hurts. Maybe I should just pick a couple of figures that more-or less work and leave it at that. But I can't help thinking that it must be possible to devise an easy to use graph that might help someone find the most efficient settings... I haven't even thought about piperun length yet!