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Author Topic: A better way for retro fits than common flat panel installs  (Read 18697 times)
stuartiannaylor
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« on: November 19, 2010, 02:45:10 AM »

Hi All,

I have an idea and I am pretty sure its an hairbrained one at that but if you would do me the favor of pointing out the pro's and con's I would much appreciate it.

Firstly I am going to have a rant and rave at some of the stuff I have seen.

Some of what I think are the worst installs have been done under government warm front grants.
The style seems to be a singular in-roof flat plate collector with a small storage tank.
(I have seen these with shade and houses that are far from pointing south but enough of that)

To be honest I just don't get it as there are so many questions that I can't really make any sense of.

Firstly retro fitting in roof, why rip up a perfectly good roof for a solar collector?
Then because its in-roof for a flat panel type the pitch is really wrong for the UK especially in winter months?
The storage tanks are tiny especially if your a family as it is common to have a peak binge on hot water usuage?
I really don't understand these systems apart from they are offered under a grant and the small top up from the user seems to make them viable?

OK my simplistic point is that if you are going to the expense of retro fitting a solar collector then why not do the following?
Firstly instead of flat plate collectors get direct flow evacuated tubes then at least you can maximise the collector pitch to gain as much winter sun as you can get. They will still roast in summer even with elevated pitches.
Then instead of in-roof and ripping the existing place on tile brackets and have the hot feed go to an external heat exchanger. That way you have no problems with stagnation temperatures or complex water tight seals.
I did say a heat exchanger but simplistically a big lump of concrete with a feed loop and DHW loop that is in an insulated hole in the garden. I know the lump would need to be approx four times the size of an equivalent water body but its just a bigger hole.
I did say concrete but I guess you could bury a water tank and feed from there as I presume so many storage tanks are specified due to lack of space.
Say if you went with concrete and a evacuated direct system then they have a potential to reach much higher temps than 100 C so why not utilise this and use a thermo oil heat carrier.
If you really maximised your collector coverage then you could start thinking about providing heating and not just summer hot water.
But going back to the huge lump of concrete as it could retain heat maybe supply a couple of refills and its higher temp would speed reheat.
You could even use the remaining to recapture some of that energy in a ground source heat pump.
Lol I have this feeling I am going to get ripped asunder.

But go on why not ?
Stuart 
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Greenbeast
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« Reply #1 on: November 19, 2010, 08:27:25 AM »

This sounds overly complicated if someone just wants free hot water.

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stuartiannaylor
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« Reply #2 on: November 20, 2010, 04:39:40 PM »

Erm, I prob just went on for to long ... Smiley

Instead of a in-roof flat plate collector and having to remove slates .... (all that work)

Install  on-roof to have external piping to a concrete heat store. Advantages higher heat capacity due to more available space and also higher temperature capacity.

Thats it really.

But when you start looking at the possibillities and diiferent usuages it does throw up a lot of alternatives.

Stuart
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Baz
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« Reply #3 on: November 20, 2010, 05:01:14 PM »

Even on-roof has to follow the roof pitch because you're not allowed to rise much above the roof surface (200mm).
I'm sure in 20 years those of you still alive will have got used to them but I find most above roof systems very ugly except on modern buildings.
Water cylinders are ridiculously expensive, so driving size down.
Size of store is a point of controversy on the forum.
Obviously the bigger the better  stir
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martin
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« Reply #4 on: November 20, 2010, 05:09:32 PM »

-if you want hundreds of litres of tepid (useless) water........ Roll Eyes
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desperate
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« Reply #5 on: November 20, 2010, 07:10:05 PM »

Evenin

One big advantage of stripping off the tiles and installing a flat plate panel is that you would be reducing the total loading on the structure, also if the connections are positioned to come out the back of the casing then there is no need for extra holes and flashing to the roof covering. I have done quite a few flatties over the years and overall I would say it is about the same amount of work either way.


Desperate
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stuartiannaylor
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« Reply #6 on: November 21, 2010, 12:41:01 AM »

-if you want hundreds of litres of tepid (useless) water........ Roll Eyes

Oh come on please at least say why? Is there a reason?

Also I get the 200mm scenario thats why direct flow are much superior to flat plate.

"This type of construction means that each single pipe can be easily rotated to allow the absorber fin to be at the desired tilt angle even if the collector is mounted horizontally"

In the UK I can't remember the latitude but with a flat plate its optimum angle of a common roof pitch is far from ideal especially for winter conditions. This is not true of direct flow evacuated tube designs. I don't understand the tepid water comment at all as the down side of evacuated tubes is the hardware cost.
The majority of cost in any solar collector is installation though ...
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stuartiannaylor
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« Reply #7 on: November 21, 2010, 12:45:51 AM »

Evenin

One big advantage of stripping off the tiles and installing a flat plate panel is that you would be reducing the total loading on the structure, also if the connections are positioned to come out the back of the casing then there is no need for extra holes and flashing to the roof covering. I have done quite a few flatties over the years and overall I would say it is about the same amount of work either way.


Desperate

If you have on-roof that feeds an external store there is no going through the roof. No extra holes or flashing needed
Stuart
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martin
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« Reply #8 on: November 21, 2010, 12:54:16 AM »

"Oh come on please at least say why? Is there a reason?" - physics! ralph

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Baz
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« Reply #9 on: November 21, 2010, 01:09:09 AM »

The panels produce an amount of heat during the day which if spread out over a large tank will result in a lower temperature than in a small tank. However there are solar tanks which use baffles etc to concentarate the heat and provide a top down heating effect. Alternatively you heat up your luke warm water easily overnight with E7, or just wait for the next day to heat your tpid water to a big tank all full of hot water. either way the amount of heat is the same.
A big tank has several advantages. Once warmed up it provides a reserve for dull days, and in summer means there is less need to dump heat.
A bigger consideration is the capacity of the collectors to the amount of water used to get as much as possible cost effectively.
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stuartiannaylor
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« Reply #10 on: November 21, 2010, 01:38:24 AM »

"Oh come on please at least say why? Is there a reason?" - physics! ralph



Sorry I think I haven't explained it all that well. The problem with small tanks is quite commonly expense and space in a dwelling. As I said said before the majority of cost is in installation and I do believe in matching storage capacity with collection.
Its just a personal annoyance that I see a lot of companies pushing storage capacities that are seriously small as a solution.

The idea is to minimise cost and maximise collection you cover as much roof space with collectors as possible without any stagnation worries. Yeah more, costs more but it does spread the installalation costs thinner.
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Ivan
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« Reply #11 on: November 21, 2010, 02:17:17 AM »

wow, that's a lot of misleading info in such a small number of posts. I won't go into explaining everything, as it's all be discussed before, but I'll mention a few key facts, which should make things self-explanatory:

1)85% of the solar energy falling on the UK occurs in just 7months of the year. The remaining 5months therefore only receive 15%. Optimising for winter (when there's b*****r all to start with, and the sun is usually behind cloud, in which case optimised elevation angle is of no benefit) will reduce output in the summer.

2)Evacuated tube collectors are available at a lower price than flat plates (This is the concept that Navitron was founded on!!)

3)Yes 100s of litres of tepid water may well be the same energy as a fewer number of litres of hot water. But if you don't need 100s of litres of water, then you're going to need to top the heat up every day if you want to use it.

4)Big tanks = big losses through larger surface areas

5)The myth that you should fit as large a tank as possible has long been discredited. If you fit a huge tank, you'll need to fit extra panels to compensate for the extra losses. The only time it makes sense to fit an oversized tank is if you have a poor quality flat plate which becomes inefficient as the collector temperature increases
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stuartiannaylor
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« Reply #12 on: November 21, 2010, 03:09:17 AM »

lol sorry to post again but honnestly I never said fit an oversized tank. Infact I never mentioned water, so far I only got to a situation in the uk where a large solar array could produce and store heat that exceeds 100C.

Yeah I am glad Navitron make such a product as I definately wouldn't buy a flat plate that costs more than a evacuated tube model.

I am not at all saying anything contrary to Navitrons products but I am sure your aware of certain suppliers who provide kits that are just woefull.

One thing I will say is an oversized collector array could extend the usefull collection period and with fairly simple plumbing the size of your heat store could be selective.

Who says we need to store the heat in water ?  Grin
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stuartiannaylor
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« Reply #13 on: November 21, 2010, 03:40:30 AM »

There is one question I would like to ask as there seems to be some very knowledgable people on here.

From a UK perspective whats the maximum temp you could heat say for example some form of thermo mineral oil, with an on-roof evacuated tube solar array?

Also going back slightly but evacuated tubes can still capture heat on cloudy days and again its all dependent on your size of array. If you have a big enough array you could make hot water in winter. You could also accept winter pitch angles in summer purely because you have a large enough collector area.

Stuart

Thanks for your posts as its all interesting.

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martin
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« Reply #14 on: November 21, 2010, 09:33:56 AM »

It's physics again! - it's that brick wall against which many bang their heads in vain, some "inventors and scam merchants" even go so far as to make claims that fly in the face of several immutable laws of physics, and do such with such credibility that the government is taken in and awards grants - witness the Swindlesave roof-mounted chocolate teapot scandal, whereby they overlooked the fact that if you're going to extract power from the wind, it has to be there in the first place (which it just isn't over a roof!)

Taking this "let's heat the house using solar" thing - as well as physics, there is practicality and cost to take into account - as Ivan has pointed out, the amount of energy obtainable from the sun in winter (when you most need heat) is miniscule - simple sums will show that just to provide domestic hot water in midwinter would require a solar panellage of several times that which you need April-October - say 200 tubes - 10x20 tube panels, bang went all your roof space.....(and all the expense of acquiring/fitting them) - then to give you space heating as well will require further multiplication of tubes/panels. Then we have to cost the enormous tanks you seem to think are a good idea, and the huge amount of insulation necessary to enable them to be worthwhile (have you seen the cost of Akvaterms?)

Then you used the "C" word - concrete! - as any good greenie knows, the manufacture of concrete is an eco-disaster - the embodied energy involved in it's manufacture means the damn stuff is best avoided

The sums have been done (both on the systems and costs involved), by a great many people, all round the world for many years and several very bright people are working constantly trying to improve renewables, sadly there are no short cuts, no magic bullets, just those beloved and unbreakable laws of physics......... Smiley
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