Green Building Forum - Copper or plastic pipe for solar thermal

Copper or plastic pipe for solar thermal

JSHarris — Wed, 18 Jan 2012 09:47:37 +0000

Posted By: borpin
Posted By: JSHarrisI've just done some a quick and rough calculations for this.
Gets the mind going in the morning....

That's mainly why I did it! Now I'm retired I miss some of the mind-exercise I used to get at work, so I inflict it on members here instead................

Copper or plastic pipe for solar thermal

Ed Davies — Wed, 18 Jan 2012 09:52:18 +0000

Sorry, but what's the height of the system got to do with anything?

Copper or plastic pipe for solar thermal

JSHarris — Wed, 18 Jan 2012 09:55:55 +0000

Posted By: Ed DaviesSorry, but what's the height of the system got to do with anything?

I assumed that the pump has to be able to start against the static head of the system (the height difference between the top and the bottom with the pump at the bottom) when initialising from a drain-back condition. For a sealed (non-drain back) system this wouldn't apply as the pump would only be circulating but I was being a bit cautious and covering all options.

Copper or plastic pipe for solar thermal

joe90 — Wed, 18 Jan 2012 10:06:41 +0000

Posted By: wookey
If you do pick 10mm copper remember to buy inserts for all compression joinst otherwise you'll regret it...

posted by: Borfin
Will the insert (which will reduce the bore at the fittings), have any impact?

The inserts go into the compression fitting around the outside of the pipe so they dont reduce the effective bore of the pipe.

Where at all possible dont use compression 90'bends as this adds a lot to the resistance of the flow. You can get very cheap 10mm pipe benders that give a radius of about 50mm.

Copper or plastic pipe for solar thermal

borpin — Wed, 18 Jan 2012 10:14:34 +0000

Posted By: joe90The inserts go into the compression fitting around the outside of the pipe so they dont reduce the effective bore of the pipe.

Ah, Ok. All inserts I have ever used (including plastic 10mm) went inside the pipe (hence insert). Cheers.

Copper or plastic pipe for solar thermal

owlman — Wed, 18 Jan 2012 11:45:09 +0000

Posted By: borpin...............Ah, Ok. All inserts I have ever used (including plastic 10mm) went inside the pipe (hence insert). Cheers.

I guess OUTsert's then Brian.

Copper or plastic pipe for solar thermal

joe90 — Wed, 18 Jan 2012 12:08:50 +0000

sorry no! they are inserts for the fitting, you put them in the compression fitting then insert the pipe. They effectively make the OD of the pipe up to the ID of the fitting.

Inserts for plastic pipes are there to stop the pipe collapsing when compressed as plastic pipe is not a rigid as copper.

Copper or plastic pipe for solar thermal

borpin — Wed, 18 Jan 2012 12:51:03 +0000

Posted By: joe90sorry no! they are inserts for the fitting, you put them in the compression fitting then insert the pipe. They effectively make the OD of the pipe up to the ID of the fitting.

Ah OK I'm with you. I would call that a reducer as it reduces the dia of the fitting to suit the pipe. Used them years ago in my Dad's house that had a microbore system and some was 10mm and some 8mm IIRC. Most fittings were 10mm so we used these reducers for the 8mm pipe.

Copper or plastic pipe for solar thermal

borpin — Wed, 18 Jan 2012 12:57:05 +0000

Has anyone else come across Tectite fittings from Pegler. http://www.pegleryorkshire.co.uk/MEDIA/Downloads/88782503_PE993TectiteDatabook09.pdf. I used the classic fittings extensively in my last house and they are outstanding. Demounted and remounted some many times and never a leak (except one I had not pushed right home). They are expensive, but IMHO they are the best.

Anyway, what pressure / temperature is needed for Solar installations? These will take 114degC at 10Bar (p52 of link). If they can withstand the environment, I would use them without hesitation.

Copper or plastic pipe for solar thermal

JSHarris — Wed, 18 Jan 2012 13:18:00 +0000

Posted By: borpin
Anyway, what pressure / temperature is needed for Solar installations?

UK solar systems seem to run at around 1 to 2 bar from what I've read. The boiling point of 50% glycol solution is around 107 deg C at atmospheric pressure, but this would increase under pressure to be around 120 to 130 deg C at 2 bar. From what I've read, solar collectors tend to be self-limiting in temperature to some degree, because the losses increase as the temperature increases. Nevertheless, there is evidence that they can get up to around 200 deg C under extreme conditions.

I'd suggest that it might be risky to use pipe rated to only 114 deg C, as it would seem that there is a fair chance that this could be exceeded.

Copper or plastic pipe for solar thermal

Ed Davies — Wed, 18 Jan 2012 13:48:51 +0000

Friend had a flat panel installed by a professional plumber with plastic pipes. First time he had a stagnation (turned off electrics to work on something else) the pipes burst at 118 Â°C. 114 Â°C is definitely not enough.

Copper or plastic pipe for solar thermal

djh — Wed, 18 Jan 2012 14:39:08 +0000

Posted By: JSHarrisAs wookey rightly says, small bore pipe has a lower surface area, so lower heat loss. By the same token, smooth pipe will have a lower surface area than corrugated pipe, so will have lower heat loss.

It's not just the surface area; it's also the volume. Unless your pump is running continuously, there is a slug of hot water in the pipe that cools down when the pump stops. And a [the same] slug of cool water that runs through the tank heat exchanger when the pump starts. Both effects waste heat, so minimising the volume of water in the pipe is beneficial.

Copper or plastic pipe for solar thermal

owlman — Wed, 18 Jan 2012 14:41:04 +0000

I would guess most solar installs are done using off the peg pre-assembled pump stations. The big suppliers PAW, Resol, Meibes all seem to put DN16 fittings on their connecting unions. So I would guess the clue as to which dia pipe to use is there. Perhaps it's because they use standard components, like Grundfoss and Wilo modulating pumps in order to keep the costs of the whole assambly down. One assumes they know what they are doing and in any case I wouldn't want to see the Resol reps face when I told him I'd fitted 10mm copper then enlarged it to 3/4 BSP because some bloke on the internet said it would be better. You also have to think about the other side of the pumpstation thats trying to push/pull this same fluid at the same time into relatively large bore exchanger coils in the cylinder. I guess its a balance. I've no idea which is better 10mm copper or DN 12 or 16 suffice it to say I have enough slack in the system to take care of any percieved loss of efficiency on a few metres of pipe. Also the modulating pump speeds up and slows down adjusting the flow according to the controller and its manifold temp probe. It may be that in order for microbore to work well and to the calculated efficiencies it needs to be incorporated with a solar pumpstation with a low volume modulating pump, and I haven't seen any of those. If so then the percieved efficiencies maybe need to be balanced against the increased cost of having such a pump assembly specially made up. Do whatever you feel comfortable with just make sure you're not negating any warranties.

Copper or plastic pipe for solar thermal

djh — Wed, 18 Jan 2012 14:48:10 +0000

Posted By: owlmanThe big suppliers PAW, Resol, Meibes all seem to put DN16 fittings on their connecting unions. So I would guess the clue as to which dia pipe to use is there. Perhaps it's because they use standard components, like Grundfoss and Wilo modulating pumps in order to keep the costs of the whole assambly down.

Well they have to choose some size to fit - it isn't sensible to have different models just with different size unions. And for some installations (big ones) you do need bigger pipes, so you need to make sure the flow for those situations isn't restricted. And as has already been described, it's quite easy to reduce from a larger union down to a smaller pipe. So I don't think the union size says anything at all about the best pipe size.

Copper or plastic pipe for solar thermal

borpin — Wed, 18 Jan 2012 15:20:18 +0000

Posted By: JSHarrisI'd suggest that it might be risky to use pipe rated to only 114 deg C, as it would seem that there is a fair chance that this could be exceeded.

It was the tectite fitting rather than the pipe I was interested in but thanks. I sort of suspected as much but thought it was worth a punt.

Surprised in someways that the system does not require to be pressure tested if such extremes can occur.

Copper or plastic pipe for solar thermal

JSHarris — Wed, 18 Jan 2012 15:33:41 +0000

Posted By: borpin
Surprised in someways that the system does not require to be pressure tested if such extremes can occur.

I think it's normal to fit a pressure relief valve that lets the steam out if it boils, so making it no different to a pressurised hot water system in principle. Presumably this is the way they avoid the need for pressure testing.

Copper or plastic pipe for solar thermal

TimSmall — Wed, 18 Jan 2012 15:46:41 +0000

FWIW, I independently came to the conclusion that 10mm copper micro-bore was the answer (and is also conveniently easy to source and install - a 10mm external bending spring is very useful indeed, and easy to use)...

I have heard of people use a high temperature solder (e.g. 100% tin has a melting point of 232 Celsius) instead of compression fittings, but I've not tried this myself...

Tim.

Copper or plastic pipe for solar thermal

joe90 — Wed, 18 Jan 2012 15:50:37 +0000

Is high temp insulation available for 10mm microbore for this application?

Copper or plastic pipe for solar thermal

Ed Davies — Wed, 18 Jan 2012 17:29:32 +0000

Posted By: borpinSurprised in someways that the system does not require to be pressure tested if such extremes can occur.

It's more temperature testing that's required. I would suggest that it would be irresponsible to leave a solar thermal system in operation without having tested it for stagnation on a suitable sunny day early in its life while making sure there's nobody anywhere that they can get scalded with 100+ Â°C water if anything fails.

Copper or plastic pipe for solar thermal

owlman — Wed, 18 Jan 2012 18:20:29 +0000

Posted By: borpin..........................Surprised in someways that the system does not require to be pressure tested if such extremes can occur.

It is, as part of the filling procedure,

Copper or plastic pipe for solar thermal

owlman — Wed, 18 Jan 2012 20:37:50 +0000

Posted By: JSHarris..........I think it's normal to fit a pressure relief valve that lets the steam out if it boils,

Normally part of the pumpstation, it ejects the glycol into a collecting vessel.

Copper or plastic pipe for solar thermal

borpin — Wed, 18 Jan 2012 21:23:47 +0000

Posted By: owlman Normally part of the pumpstation, it ejects the glycol into a collecting vessel.

Any idea what pressure this is set to?

Copper or plastic pipe for solar thermal

wookey — Thu, 19 Jan 2012 00:05:45 +0000

owlman. I think the pumpstation fitting thing is because the flexi stainless pipe is easy to fit as you said. And for professionals pretty-much the only thing that matters is speed/ease. DIYers can afford to optimise even if it takes a smidge longer to fit. If you aren't in a hurry you can make a system that is both better and cheaper.

It's not a 'percieved loss of efficiency' in the pipes - it's real. Not a huge effect, but not tiny either. People who have plumbed their solar in 28mm (that used to be popular) are wasting a non-trivial amount of the collected heat. I don't know why you are so skeptical - team Navitron have spent a _long_ time looking at this stuff. And it doesn't require a modulating pump - almost any pump that won't melt will do. I'm a big fan of low-power pumps, because as pumps run for about 1500hrs/yr a big pump+controller can easily parasitise 7-10% of your annual energy gain, but bog-standard CH pumps are also reasonable (cheap, standard, just not particularly efficient). Solar system efficiency is actually very insensitve to pump speed.

The inserts for 10mm pipe I was on about are not the 'reducing sets' (which are also very handy as a lot of bits are 15mm) - they are these: http://www.bes.co.uk/products/139.asp (bottom of page). And they do go on the inside of the pipe, but really don't restrict it enough to matter.

Copper or plastic pipe for solar thermal

owlman — Thu, 19 Jan 2012 09:01:46 +0000

Posted By: borpin..............Any idea what pressure this is set to?

Operating pressure depends on the array manufacturers recommendation usuually 1.8 - 2.3 bar. Pumpstation PRVs normally "crack" at 3 bar. When the system is filled and tested it's quite mormal to raise pressure to 6 bar to check joints for weeping and leakage.

Copper or plastic pipe for solar thermal

Gotanewlife — Thu, 19 Jan 2012 09:09:09 +0000

Posted By: wookeyeven if it takes a smidge longer to fit

Not on my system it wouldn't have, 50cm thick solid walls (5 penetrations some not possible at 90 deg), 8m at above 3m height, 24m in total one way. There would have to have been a load of joints at plus 130 deg capable, just feeding on the insulation and taping it to the last bit would have taken longer than fitting the whole ss pipe! And 10mm pipe is so delicate I think it would have been a 2 person job (ie plus wife).

Copper or plastic pipe for solar thermal

owlman — Thu, 19 Jan 2012 10:08:03 +0000

I'm not anti microbore wookey I guess I don't really see the point of beating yourself up over only a few metres of pipework, in some cases. I don't know what intensive research has been done to validate the use of microbore in a "holistic" system sort of way. For instance is the research just all about pipework temp losses and fluid transmission, or are these gains perhaps negated by the venturi effect of "choking" down pipework and then opening it up again over and over again. After all the pump, the cylinder coil, the manifold, are all larger capacity/bigger bore. Perhaps PAW, Meibes etc have done their sums and come to the conclusion all in all it's better to use the slightly larger pipe, and keep the flow even. Having said that, "annular " construction flexible SS pipe does have a disadvantage in that it can create current eddies hence the use of slightly larger bore pipewark. The better, some say, is " flexible SS pipe with a " helical "construction but it's harder to find, and I guess it's more expensive. The not inconsiderable advantage as Gotanewlife says and you allude to, is in the ease of fitting. When you're balancing on a roof or awkwardly on your back in a loft trying to thread pipework, undamaged, through a convoluted space you'll quickly realise that pre-insulated, pre- wired pipe is a boon. Microbore soft copper gets it's flexibility from reduced wall thickness 0.7mm as opposed to 0.9mm in normal more rigid microbore, so again as Gotanewlife pointed out the risk of "kinking" is greater. then as joe90 points out there's the insulation, can you get say 20mm wall thickness neoprene for 10mm microbore and once it's on, will you see any kinks in the pipe. Given all that I'm happy to be steered by the pumpstation manufacturers, most new domestic pro installs are done with DN12 or DN16 and I'm happy with that and I've absolutely no complaints about my install.

Copper or plastic pipe for solar thermal

JSHarris — Thu, 19 Jan 2012 10:47:44 +0000

From what I can gather reading some of the posts here and elsewhere, it's the decrease in fluid volume that seems to play the most significant part in reducing losses.

A 15mm system (around 13 mm bore) that has 10m of pipe will hold around 1.1 l of fluid, the same length system using 10 mm pipe (around 8.6 mm bore) would hold about 0.58 l of fluid. If the temperature difference between the system being cold and hot is 60 deg C, then the volume difference means absorbing an extra 0.03 kWh int the working fluid each time the system starts from cold, or around 11 kWh through the whole year.

The surface area of each pipe would be 0.47 mÂ² and 0.31 mÂ² respectively, so in theory the pipe losses for the larger pipe would be around 50% greater, although if well insulated I doubt this is a big issue.

If you're a perfectionist then the smaller pipe is better for most systems I'd have thought, but the saving is so modest when compared to 15 mm pipe that I doubt it's worth worrying about unduly. After all, even a small 20 tube ET array is likely to give around 1500 kWh per year at around 40 deg pitch angle in southern England, so is an extra loss of maybe 1% worth losing sleep over?

Copper or plastic pipe for solar thermal

Ed Davies — Thu, 19 Jan 2012 11:26:00 +0000

Posted By: JSHarris...then the volume difference means absorbing an extra 0.03 kWh int the working fluid each time the system starts from cold, or around 11 kWh through the whole year.

That's assuming the system starts and stops once per day. On the more marginal days (which are the ones we care about) it'll probably stop and start many times per day as the cloud comes and goes.

Copper or plastic pipe for solar thermal

owlman — Thu, 19 Jan 2012 11:26:59 +0000

Posted By: JSHarris.................... so is an extra loss of maybe 1% worth losing sleep over?

IMHO no.

Copper or plastic pipe for solar thermal

JSHarris — Thu, 19 Jan 2012 11:56:33 +0000

Posted By: Ed Davies
Posted By: JSHarris...then the volume difference means absorbing an extra 0.03 kWh int the working fluid each time the system starts from cold, or around 11 kWh through the whole year.

That's assuming the system starts and stops once per day. On the more marginal days (which are the ones we care about) it'll probably stop and start many times per day as the cloud comes and goes.

But if the pipe insulation is good, would the temperature really change by 60 deg during cloudy periods during the day? TBH I don't know, but would guess that the fluid doesn't have time to change temperature by a lot during the day, particularly for the many times during the year when the air temperature is relatively high, reducing the temperature differential from the 60 deg C all year around simple approximation I used. If anything I suspect my 1% guesstimate is probably on the high side when the whole year is taken into account.

The losses would be a lot higher if bigger pipe was used, though, and I heard mention somewhere of systems being plumbed with 22 mm copper. That would make a much bigger difference, as 10 m would hold around 3.8 l, increasing the loss to maybe 6 or 7 % when compared to 10 mm pipe.

It seems that there's really not much to choose between 10 mm, 12 mm or 15 mm pipe work (accepting that smaller is generally slightly better in most cases), but it looks as if there's a potentially a much bigger loss if using 22 mm pipe.