Tube 'vs' Flat Plate

pete6296 · 27-03-2008 11:55pm #1

Hi,
I have room for 4 panels south facing and i was convinced on the idea of evacuated tubes until i was getting a prce for plumbing and plumber said he installs flat plate which are slightly cheaper but are they essentially as efficient. Everyone has different opinions but is there proof of which is the most efficient. I would really appreciate any opinions or suggestions.
Thanks,
Pete

CJhaughey · 28-03-2008 6:47am

There was a lot written about this very recently, most of which came from an article from the swiss solar energy institute.
Basically the gist of the article was that they had tested 140 flat panels and 40 or so Evac. tubes and the outcome was that the flat panels were actually better than the tubes when the total efficiency was concerned.
The best tubes were Swiss and from N.I.
Think about a tube for a minute, how much surface at any one time is exposed to direct solar radiation? now imagine a flat panel how much of it's surface is exposed?
There is a lot of info on the swiss site with a lot of tests, make sure that the panels/tubes that you buy are a recognised brand.
I see a lot of people buying cheap chinese tubes and I would suspect that the lifespan of the tubes won't be all that long, not to mention the efficiency.

pete6296 · 28-03-2008 9:19am

Thanks CJ,
The tubes are a recognised brand made in Germany, i would imagine more of the tubes would be exposed. It is elevated with a reflective surface behind it so in best case majority of 360 degrees of surface area is recieving light.
The flat plate sacrificies this as it only exposes the top surface. Does this make sense!!
Pete

CJhaughey · 28-03-2008 9:54am

I understand what you are saying but the tubes will never be 360 deg efficient, a reflective background will reflect some of the solar radiation but not all.
It's your choice, have a look at the tests in the link in my last post and see what the gross efficiency is, it will be on the test page under collector fact sheet.
My flat panels are 0.694 gross efficiency, which I think is pretty good compared with most of the tested tube collectors.
In the end it is your choice but good flat panels are as good as tubes IMO.

pete6296 · 28-03-2008 10:34am

Hey CJ,
I sent you a pm asking you about supplier, tubes, if you got a company to install. I would be grateful if you could let me know.
Thanks,
Pete

CJhaughey wrote: »

I understand what you are saying but the tubes will never be 360 deg efficient, a reflective background will reflect some of the solar radiation but not all.
It's your choice, have a look at the tests in the link in my last post and see what the gross efficiency is, it will be on the test page under collector fact sheet.
My flat panels are 0.694 gross efficiency, which I think is pretty good compared with most of the tested tube collectors.
In the end it is your choice but good flat panels are as good as tubes IMO.

AckwelFoley · 29-03-2008 6:42pm

Flat plate are working at 10% when the sun us sirectly over them. So mornings and evenings you do lose efficency. On top of that many ppl dont have a full south facing roof, and because the tubes are curved they pick up the light better morning and evening than the panel do,

Personally i think it depends on the situation.

pete6296 · 30-03-2008 12:50am

They will be SSE facing and i have room for 3 panels, what would u guys go for.
Thanks,
Pete

snyper wrote: »

Flat plate are working at 10% when the sun us sirectly over them. So mornings and evenings you do lose efficency. On top of that many ppl dont have a full south facing roof, and because the tubes are curved they pick up the light better morning and evening than the panel do,

Personally i think it depends on the situation.

AckwelFoley · 30-03-2008 3:30pm

pete6296 wrote: »

They will be SSE facing and i have room for 3 panels, what would u guys go for.
Thanks,
Pete

Is this the front or back of the house?

If its thefront i would def choose the flat plate. They look better imo

Mothman · 30-03-2008 10:09pm

One aspect I've never seen mentioned on any forum re solar panels is cleaning the panels.
I clean my flat plate panels once/twice a year and they are usually fairly dirty. Granted much of the dirt is from a nearby flue, but they'd still get dirty reducing efficiency. Flat plates are easliy cleaned once you can access them safely, but I can' imagine tubes and back reflector etc being anywhere near as easy to keep clean.

Just something I've wondered for a while.

Aside from that CJ gives good advice here.
I've been put off tubes by lying salesmen, both at the time of my research 10 years ago and more recently.
Bull**** like, even if 2 tubes fail, it'll still be 100%. My answer to that is why not only use 18 tubes instead of 20?
Also to tell me I was wrong in choosing 6sqm flat plate, 300l tank vs 3sqm tubes 150l tank for same money...............made my blood boil.

pete6296 · 30-03-2008 10:31pm

Its the gable side of the house which will not be seen. Good point in relation to cleaning by above post, something that one would never think about but thats very important.
Still undecided.
Pete

snyper wrote: »

Is this the front or back of the house?

If its thefront i would def choose the flat plate. They look better imo

bonkey · 31-03-2008 12:44pm

snyper wrote: »

Flat plate are working at 10% when the sun us sirectly over them.

I assume you mean 100% efficiency.

So mornings and evenings you do lose efficency.

By the same reasoning, tubes will never be at 100% efficiency. Given that they are tubular, and not planar, there will never be sunlight hitting the entire surface optimally.

While I'm no expert, the idea for a tube should be to balance out the loss caused by the angle of the sun to be more-or-less time-independant. At any given time, there will be parts of the tube perpendicular to the sun (which flat plates only have at optimal times), but there will equally be parts of the tube at every angle (up to parallel) to the suns rays.

To the OP, I would additionally note the following....

In your first post, you said: Everyone has different opinions but is there proof of which is the most efficient. You've since been handed a link to a study done whic conluded that flat-plates were actually the way to go. Since then, you've offered your own opinion that you would think tubes will be more efficient, and you've abandoned asking for proof, asked for more people's opinions, and generally seem to be looking for reassurance that tubes are a good choice / the right choice.

I know you say you're still undecided, but for someone who's undecided, you seem to be very keen on one side over the other. What would it take to convince you that flat-panels are the way to go?

pete6296 · 31-03-2008 3:11pm

Hi Bonkey,
Your points are very good and the reports show flat plate are as good, even better than tubes. I suppose i was convinced about tubes all along and now contemplating flat plate. As you say, the proof is in the reports which essentially answers the question i put to the board.
Thanks
Pete

bonkey wrote: »

I assume you mean 100% efficiency.

By the same reasoning, tubes will never be at 100% efficiency. Given that they are tubular, and not planar, there will never be sunlight hitting the entire surface optimally.

While I'm no expert, the idea for a tube should be to balance out the loss caused by the angle of the sun to be more-or-less time-independant. At any given time, there will be parts of the tube perpendicular to the sun (which flat plates only have at optimal times), but there will equally be parts of the tube at every angle (up to parallel) to the suns rays.

To the OP, I would additionally note the following....

In your first post, you said: Everyone has different opinions but is there proof of which is the most efficient. You've since been handed a link to a study done whic conluded that flat-plates were actually the way to go. Since then, you've offered your own opinion that you would think tubes will be more efficient, and you've abandoned asking for proof, asked for more people's opinions, and generally seem to be looking for reassurance that tubes are a good choice / the right choice.

I know you say you're still undecided, but for someone who's undecided, you seem to be very keen on one side over the other. What would it take to convince you that flat-panels are the way to go?

Gurgle · 31-03-2008 3:35pm

bonkey wrote: »

At any given time, there will be parts of the tube perpendicular to the sun (which flat plates only have at optimal time)

Only in a transverse direction, longitudinally they're at the same angle as flat-plates but with less surface area to compensate.

Very relevant here, being so far from the equator. In winter tubes will be almost completely useless as the sun is low and the incident angle is high.

pete6296 · 31-03-2008 5:06pm

So, in essense, would i be right in saying the only advantage is the tubes take up slightly less space!
Pete

Gurgle wrote: »

Only in a transverse direction, longitudinally they're at the same angle as flat-plates but with less surface area to compensate.

Very relevant here, being so far from the equator. In winter tubes will be almost completely useless as the sun is low and the incident angle is high.

AckwelFoley · 31-03-2008 8:18pm

I've been put off tubes by lying salesmen, both at the time of my research 10 years ago and more recently.
Bull**** like, even if 2 tubes fail, it'll still be 100%. My answer to that is why not only use 18 tubes instead of 20?
Also to tell me I was wrong in choosing 6sqm flat plate, 300l tank vs 3sqm tubes 150l tank for same money...............made my blood boil.

i think that sales man was not a teling lies - i just think hes a gobsh1te.

If you got 20 tubes and 2 fail, the system will operate but at a reduced efficency.. ie -2 tubes contribution.. if on panel fails the system fails, until u replace the panel.. due to the way they are plumbed in.

I dont understand how he can tell you, that you were worng with the 6 sq system, The appiture area is normally about less than 10% of the actual size of the unit.. and allowing any difference in efficency between panels and tubes.. i dont see how 6sq is worse than 3sq

Gurgle · 31-03-2008 8:36pm

pete6296 wrote: »

So, in essense, would i be right in saying the only advantage is the tubes take up slightly less space!
Pete

Yes, except they don't take up less space :b

The efficiency test is done by square meter. The tubes have less absorber area but the spaces between them means the array takes up as much space.

wazzoraybelle · 31-03-2008 9:44pm

Thanks for the great link above, I was always convinced by tubes but am not so sure after having a good look at the spf site.
I was fond of tubes because;
a. Easier for self installation.
b. Easy to swap out broken tubes and in most cases the panel will still work well with a couple of broken tubes.
c. Always thought tubes were more efficient, The spf site hasn't totally convinced me otherwise as I was aware that flat panels were better in direct sunlight (when both systems would produce sufficient or even surplus hot water for a household ) but tubes ruled when it came to overcast conditions (when it really counts). I didn't really see that aspect addressed properly in the test conditions but I'm open to being corrected.
Again, great link. essential reading for anyone considering solar.

pete6296 · 31-03-2008 10:06pm

I am the same way as yourself, convinced about tubes but when i seen website i am not sure. Very good point in relation to overcast days, diffuse solar radiation. Is this where the tubes may have a slight advantage?
Pete

wazzoraybelle wrote: »

Thanks for the great link above, I was always convinced by tubes but am not so sure after having a good look at the spf site.
I was fond of tubes because;
a. Easier for self installation.
b. Easy to swap out broken tubes and in most cases the panel will still work well with a couple of broken tubes.
c. Always thought tubes were more efficient, The spf site hasn't totally convinced me otherwise as I was aware that flat panels were better in direct sunlight (when both systems would produce sufficient or even surplus hot water for a household ) but tubes ruled when it came to overcast conditions (when it really counts). I didn't really see that aspect addressed properly in the test conditions but I'm open to being corrected.
Again, great link. essential reading for anyone considering solar.

wazzoraybelle · 31-03-2008 10:38pm

That was the impression I always had. Not so sure after seeing the site.

yellabelly · 08-04-2008 11:27pm

Good thread. I'm currently deciding between the 2 different systems. I have to choose between a WSW rear and SSE side roof. Installers seem to think tubes on WSW will work best but I'm not so sure. I was told less area of tubes is needed to produce the same output. I would have thought the vacuum tubes would have much less heat loss and greater efficiency.

09-04-2008 12:16am

It's worth considering the prevailing weather, do you get sunnier mornings or evenings where you live?

Best option for location is both (wsw & sse, half on each) then you'll get solar energy all the time the sun is above to horizon.

yellabelly · 09-04-2008 12:08pm

Deleted User wrote: »

It's worth considering the prevailing weather, do you get sunnier mornings or evenings where you live?

Best option for location is both (wsw & sse, half on each) then you'll get solar energy all the time the sun is above to horizon.

I thought both would be optimal. When I suggested this the installer said you would introduce a cooling affect from the side out of the sun. I think you could get around this by having 2 circuits but that would increase the cost.

Interesting to consider the prevailing weather. Here in Meath it will often start sunny but cloud up later in the day.

I suppose it also depends when you most need hot water. In our case this is normally the evening onwards.

smooth operater · 09-04-2008 2:13pm

yellabelly wrote: »

Good thread. I'm currently deciding between the 2 different systems. I have to choose between a WSW rear and SSE side roof. Installers seem to think tubes on WSW will work best but I'm not so sure. I was told less area of tubes is needed to produce the same output. I would have thought the vacuum tubes would have much less heat loss and greater efficiency.

Ok?! I cant seem to see why they would advise that? Peak gains is South, so obviously the more south the better, therefore.........SSE!?!

yellabelly · 09-04-2008 4:36pm

I came across a similar discussion where the tubes are coming out on top ...
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=1359&page=1#Item_0

LFCFan · 09-04-2008 5:01pm

We got 6sqM of flat plates installed on a new build. They were 'switched on' near Xmas and the day they were, the actual panels were at 91 degrees (even though the air temp was around 5 or 6 degrees). Since Paddy's day we've had a chance to see the panels do their work and on some days we've had 300 litres of piping hot water all day. I've even experienced the water getting 'hotter' as I was having a shower. All this and we're not even into the summer yet. Obviously I can't comment on the tubes but I am very happy with the panels and they fit in nicely with our black slate roof. Tubes would have stuck out like a sore thumb. It's nice to be able to have showers and have constant hot water without having to put the heating on

rosullivan · 10-04-2008 2:23pm

LFCFan, just out of curiosity, how much did the 6sqm of flat plates cost to buy and install (before you deducted the grant)? I am looking at getting much the same done in a new build extension.

LFCFan · 10-04-2008 2:58pm

rosullivan wrote: »

LFCFan, just out of curiosity, how much did the 6sqm of flat plates cost to buy and install (before you deducted the grant)? I am looking at getting much the same done in a new build extension.

It was around €6,500 and that included the panels, 300L tank, Immersion, installation and commissioning. Grant was €1,800 so final cost was €4,700.

Mothman · 10-04-2008 8:04pm

smooth operater wrote: »

Ok?! I cant seem to see why they would advise that? Peak gains is South, so obviously the more south the better, therefore.........SSE!?!

Agree fully, unless there are close obstructions to SE of site?
I would run a mile from a supplier saying WSW is better than SSE

Generally there is more sunshine in mornings as convection increases cloud during day.
When do most showers occur?
In the afternoon.

I have sunshine data going back 4 years here in coastal Wicklow.
Here are total sunshine hours for each hour. The hours are set for true time and not based on GMT
6-7 300h
7-8 418
8-9 532
9-10 620
10-11 646
11-12 639
12-13 606
13-14 578
14-15 536
15-16 447
16-17 345
17-18 240

The above is specific for my site, but I say the trend is same for most areas

steve82 · 10-04-2008 8:31pm

Panel will operate at about 90% if two tubes are broken. The two broken tubes will act almost like flat plates as they have lost their vacume. They are still black and they are still shaded from the wind by 1 layer of glass. Its best to replace them ASAP. Tubes from www.aei.ie cost €28 each and you can do it yourself with a ladder in most cases.

Gurgle · 11-04-2008 9:16am

steve82 wrote: »

Tubes from www.aei.ie cost €28 each and you can do it yourself with a ladder in most cases.

Climbing up on the roof of a 2-story house with a ladder is something I would not recommend to your average Joe Soap. It certainly shouldn't be neccessary every few months to maintain your solar installation.

steve82 · 11-04-2008 10:31am

Gurgle wrote: »

Yes, except they don't take up less space :b

The efficiency test is done by square meter. The tubes have less absorber area but the spaces between them means the array takes up as much space.

Thats not the only benefit. Tubes (generally) have lower thermal losses (due to the vacuum) that flat plates, therefor can reach higher temps than FP when there is a heat loss factor (winter/windy conditions). In summer there both equally good. In winter, tubes will out preform flat plates. In a test lab, under ideal conditions, they will both preform fairly closely. In reality, and done within a solar simulator, tubes will out preform the flat plates when taken over the year. Another benefit is there is a larger grant per m2 of tubes, when compared to FP. Aesthetically some people argue that tube aren't great. I agree, but personally don't care, as its saving money/environment. Installation of tubes is a simple job for any installer. FP requires hoist, scaffold or cherry picker. In most industrial solar applications, they also use tubes, due to high achievable temperatures.

Tube 'vs' Flat Plate

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