Solar Flat Panel : In Roof v On Roof

youtheman

I'm about to fit solar panels on my South facing roof. I'm going for flat plate (instead of tubes), purely for the asthetic value.

I can go for In-roof (integrated) or On-roof, the latter being less expensive. But there will be a certain amount of heat loss from the back of the on-roof panel due to the wind etc.

Does anyone know of a rule of thumb, or published data, for the actual efficiency of an integrated panel versus an identical on-roof panel.

quentingargan

You can look up this information on the EN12975 certification for the panel. There are two figures to watch -

zero loss efficiency, which should be about the same and is the efficiency assuming the air temperature is the same as the fluid temparature. , and

heat loss coefficient, which is the heat loss from the panel (which should be better for in-roof.

I personally would always go for in-roof, especially if aesthetics are what is keeping you away from tubes, but mainly because I think the wind loading for on-roof types are a complete unknown. The lift and buffeting forces are much greater, whereas an in-roof panel is the same as a roof window (and is fitted and flashed the same way).

fergus.wheatley

Agree with Quentin here. Wind loads are a big deal for on-roof panels. If you do use them a good tip is to keep panels a close to roof as possible to reduce suction forces caused by the different speed of wind blowing under and over the panel. The thermal efficiencies calculations used by the solar keymark only take account of the temperature differences between the panel and ambient, so underestimate the extra losses caused by wind chill. Disadvantage with the in-roof panel is stripping the roof and fitting the flashing can add an extra day to the install.

If you're building a new house watch out for the dreaded Agrement certs or you might get into trouble when it comes time to get your floor area cert.

PeteHeat

Hi,

"In Roof" makes for clean lines therefore a much neater finish, also the pipe work is both hidden and protected by a good flashing system, some systems do not have the correct flashing for slates and try to adapt tile flashings to work with slates so it is important that your installer knows the difference.

"On Roof" systems still need a good mounting system, I am not impressed by the so called solar bolts as there tends to be too much expected of silicone to maintain an effective seal.

"On Roof" panels are usually held at the top and bottom so the panel itself has to take a lot more stress from wind loads as they are expected to span up to 2m between the supports, also the pipe work must be flashed seperately and needs to be a neat finish or it can become a focal point on the roof.

My preferance is "In Roof" for both slate and tile roofs, as the panels are fully supported so there is no future stress on the panel, the glass or the plumbing.

.

heinbloed

If the roof has to be used as a habitable area it is nice to have it warm but not overheated. ST flat panel collectors are designed to catch the sun, therefore they are heating up.
Touch the backside of a standard flat panel collector when exposed to a few hours of sunshine... better not.
Make sure the roof membrane is suitable to withstand these tempereatures for decades to come, check the manufacturer's home pages, certificates.

The ventilation of a standard tiled roof is a must anyhow, so the energetic efficiency of a flat panel collector installed for summer usage (30-40 degrees angle) is hardly effected by summer weather's air temperature striking along it's backside in Ireland.
But the radiation heat emitted from the back of a flat panel ST collector can be enormous. Read the manufacturer's information as well as the ESTIF report on the issue! Include stagnation times in your investigation.

Make sure that the roof membrane as well as the roof insulant can cope with the heat. If the roof is insulated and/or fitted with a membrane or felt that is. Bitumen felt will almost certainly give in, PE very likely (less so if reinforced), PP felt would cope better with heat than the two.
The higher the slope the better the ventilation, usually.....
The more shallow the installation the hotter the background will get.

For the potential summer thermal harvest it matters very little how the flat panel collector is installed, if integrated or on-roof.
But for winter harvests it does. Here the cold air behind the tiles hardly moves, at least not as strong as in summer. Due to the temperature difference between inside(under-tile) temperature and outside temperature. The stack effect.
The developed thermal energy will be transported away faster than it will be replaced by the sun, a stagnation of the shallow fixed thermal collector can hardly be expected during wintertime.

PeteHeat

heinbloed wrote: »

If the roof has to be used as a habitable area it is nice to have it warm but not overheated. ST flat panel collectors are designed to catch the sun, therefore they are heating up.
Touch the backside of a standard flat panel collector when exposed to a few hours of sunshine... better not.
Make sure the roof membrane is suitable to withstand these tempereatures for decades to come, check the manufacturer's home pages, certificates.

The ventilation of a standard tiled roof is a must anyhow, so the energetic efficiency of a flat panel collector installed for summer usage (30-40 degrees angle) is hardly effected by summer weather's air temperature striking along it's backside in Ireland.
But the radiation heat emitted from the back of a flat panel ST collector can be enormous. Read the manufacturer's information as well as the ESTIF report on the issue! Include stagnation times in your investigation.

Make sure that the roof membrane as well as the roof insulant can cope with the heat. If the roof is insulated and/or fitted with a membrane or felt that is. Bitumen felt will almost certainly give in, PE very likely (less so if reinforced), PP felt would cope better with heat than the two.
The higher the slope the better the ventilation, usually.....
The more shallow the installation the hotter the background will get.

For the potential summer thermal harvest it matters very little how the flat panel collector is installed, if integrated or on-roof.
But for winter harvests it does. Here the cold air behind the tiles hardly moves, at least not as strong as in summer. Due to the temperature difference between inside(under-tile) temperature and outside temperature. The stack effect.
The developed thermal energy will be transported away faster than it will be replaced by the sun, a stagnation of the shallow fixed thermal collector can hardly be expected during wintertime.

Dear Sir,

I am havng a problem trying to figure out how the back of a flat plate collector could get hotter than the traditional slate or newer man made roof coverings ?

Having spent many years on all types of roofs I would agree do not touch the average blue / black asbestos or fibre cement slate on a hot sunny day as they do get very hot, the same applies to the natural slate.

Assuming the roof is south facing and we have one of our sun splitting the stones summer days your post suggests the solar gain of the blue black slate is less than that of the "in roof" solar panel ?

The reason I have chosen slate as the example is it's a flat fairly uniform roof covering unlike the thick concrete roof tile which is usually profiled causing up to 50% of the tile to up to 30mm further away from the underfelt.

The solar panel absorbs the UV (or heat) and transfers it to the manifold which in turn transfers it to the cylinder, any excess has to pass through Rockwool insulation, some a reflective panel others nothing more than a sheet of hard board on the back.

The panel sits on the roofing laths at approximately the same distance from the underfelt / membrane as the slate roof covering, I have never seen the cheapest of the old type hessian based bitumen felts melt from the summer sun.

Many of the old roofs used slating laths as small as 18 > 20mm in thickness, the more modern roof has laths 44 > 50mm in thickness which means both the slate covering and the solar panel are further away from the underfelt / membrane than in the past.

Based on my experience and knowledge of roofs I have no problem recommending an "In Roof" solar installation as my preferance over the "On Roof" solar system for the reasons posted earlier.

.

heinbloed

@ PeteHeat :

The temperature of ST collectors will get higher than roof tiles.
Most ST manufacturers are quoting the stagnation temperature of their collectors meassured at the back of them, the roof facing side.
I think - but check this out - this meassurement is part of the solar keymark certification process.
Some collectors come with better insulation, some with less. Usually the better insulating ones are higher, thicker.
The reason for this high temperature is that the collectors are not exposed to cooling air at their front, the glazing keeps the heat in. And of course of the the selective coating, a special black paint which takes up up to 95 % (and more!) of the ST energy.

Flat ST collectors which are used for roof integration as an architectural feature are usually shallow, so they don't stick out. This means there is only little insulation build-in. These are getting hotter than the thick ones.
For roof integration , to make them flush with the roof tiles, frequently the battens are cut off and the collectors placed straight onto the loadbearing beams. But on the loadbearing beams the roofing membrane sits as well, being in direct contact now with the back of the panel.

This can be done technically but needs carefull consideration of a.) the thermal impact on the background (the heat) and b.) of the lost 'brethability' of the roof membrane at this spot and c.) the influence of water which might get into the roof but who's free drainage down the membrane/felt is now hampered with.

The more shallow a flat plate collector is fixed, the more 'star facing' it is the colder it will get during clear nights. There is a reason why antifreeze is used with installations. Ice could form at the back of them, condensing water coming from the breathable felt.

As said before: these technical problems can be overcome by the right workmanship and the correct material choice.

About the roof insulation: standard PU/PIR foams can give in at these temperatures as well, depending on their long-term thermal stability. So can EPS foams.

The temperature sensor cables used for the ST collectors are not insulated with PVC as standard electric cables are. The electric insulation of these sensor cables is guaranteed by the usage of silocone. Which has a much higher thermal stability than most other plastics. Baking trays and molds can be made from silicone.This is necessary because the surface of the collectors get that hot that most plastics melt, or become brittle losing their softening agent. And these cable are not running within the collectors but outside.

Check the max. possible temperature of the choosen collectors by looking at their certificates, most manufacturers will have the data available. As said, it is part of the collector certification process to meassure the temperature of them at the back during stagnation and full sun exposure.

fergus.wheatley

heinbloed wrote: »

@ PeteHeat :

The temperature of ST collectors will get higher than roof tiles.
Most ST manufacturers are quoting the stagnation temperature of their collectors meassured at the back of them, the roof facing side.
I think - but check this out - this meassurement is part of the solar keymark certification process.
Some collectors come with better insulation, some with less. Usually the better insulating ones are higher, thicker.
The reason for this high temperature is that the collectors are not exposed to cooling air at their front, the glazing keeps the heat in. And of course of the the selective coating, a special black paint which takes up up to 95 % (and more!) of the ST energy.

Inside the panel gets very very hot, also the pipework does, but at the back of the in-rrof panels there is at least 30mm of high density rockwool and a sheet of plywood. When i have installed these in full sunshine, we are very careful to avoid the copper pipes and we normally screw wooden battons onto the panel itself to help with lifting and fixing. I have never noticed the temperature at back of the panel anything more than ambient.

fergus.wheatley

heinbloed wrote: »

@ PeteHeat :

The temperature sensor cables used for the ST collectors are not insulated with PVC as standard electric cables are. The electric insulation of these sensor cables is guaranteed by the usage of silocone. Which has a much higher thermal stability than most other plastics. Baking trays and molds can be made from silicone.This is necessary because the surface of the collectors get that hot that most plastics melt, or become brittle losing their softening agent. And these cable are not running within the collectors but outside.

The PT1000 sensors used for Solar are rated to either 180C or 300C. RESOL controllers normally supply 2 sensors rated to 70C (Quentin correct me here if I'm wrong), for the cylinder and one rated to 180C for the panel. Just as an aside, I was on a job today and I let the controller in manual and the vacuum tube panel was at 163C at 6pm.

PeteHeat

heinbloed wrote: »

@ PeteHeat :

The temperature of ST collectors will get higher than roof tiles.
Most ST manufacturers are quoting the stagnation temperature of their collectors meassured at the back of them, the roof facing side.
I think - but check this out - this meassurement is part of the solar keymark certification process.
Some collectors come with better insulation, some with less. Usually the better insulating ones are higher, thicker.
The reason for this high temperature is that the collectors are not exposed to cooling air at their front, the glazing keeps the heat in. And of course of the the selective coating, a special black paint which takes up up to 95 % (and more!) of the ST energy.

The reason I was using Blue / Black slate as the primary example is the colour match with the collector, some of the experiments with collecting solar energy for hot water were painting a radiator black and leaving it in the sun.

The collectors are not double glazed therefore they are about as effective at keeping in the heat as a single glazed window, the ambient air temperature across the panel will actually cool the panel down.

The space between the back of the panel and the underfelt / membrane will be subject to air circulating the same way that it circulates under the slates or tiles.

Prior to underfelt the back of the slates were rendered one of the reasons being to try and keep out the draughts, we could call it the first example of insulating our homes.

The arrival of felts / membranes means the joints can no longer be rendered (plastered) so the air flow between the slates / tiles is no longer restricted in fact it is encouraged.

heinbloed wrote: »

@ PeteHeat :

Flat ST collectors which are used for roof integration as an architectural feature are usually shallow, so they don't stick out. This means there is only little insulation build-in. These are getting hotter than the thick ones.
For roof integration , to make them flush with the roof tiles, frequently the battens are cut off and the collectors placed straight onto the loadbearing beams. But on the loadbearing beams the roofing membrane sits as well, being in direct contact now with the back of the panel.
This can be done technically but needs carefull consideration of a.) the thermal impact on the background (the heat) and b.) of the lost 'brethability' of the roof membrane at this spot and c.) the influence of water which might get into the roof but who's free drainage down the membrane/felt is now hampered with.

I have yet to see the installation manual that suggests interfering with the structure of the roof, although the laths can be small they actually form part of the roof structure.

Anyone who removes the laths is making a very big job for themselves because the "In Roof" systems tend to follow the design / height of the average roof window.

For Example: Where the roof is a prefabricated truss, the amount of cross bracing needed would amount to at least a days work for a carpenter working to a structural engineers specification.

Also siting the panel would be restricted to meet up with the truss and the installer would have to be sure the second or subsequent panels all landed full on the truss or rafter some of which are at 600 mm centres.

I believe the "In Roof" systems are better because when fitted on the laths the weight is equally distributed across the structure, also they are not subject to the wind loads without support.

heinbloed wrote: »

@ PeteHeat :

The more shallow a flat plate collector is fixed, the more 'star facing' it is the colder it will get during clear nights. There is a reason why antifreeze is used with installations. Ice could form at the back of them, condensing water coming from the breathable felt.

Glycol is used to prevent the "Content" of the pipes from freezing and expanding causing them to burst, it does not prevent the surface of the pipe from reaching freezing temperatures because the Glycol is inside the pipes.

heinbloed wrote: »

@ PeteHeat :

As said before: these technical problems can be overcome by the right workmanship and the correct material choice.

With respect I suggest every problem referred to does not actually apply to the vast majority of installations in this country.

heinbloed wrote: »

@ PeteHeat :

About the roof insulation: standard PU/PIR foams can give in at these temperatures as well, depending on their long-term thermal stability. So can EPS foams.

The insulation should not be even close to the back of the panel as there is height of the roofing laths and the building regulations regarding the space between the insulation and the waterproofing (felt is the second line of waterproofing).

Insulation between rafters must be kept at least 50mm from the waterproofing and there must be a through flow of air circulating within the space to prevent interstitial condensation within the insulation and of course rot in the timbers which would be caused by condensation on the back of traditiona underfelts.

heinbloed wrote: »

@ PeteHeat :

The temperature sensor cables used for the ST collectors are not insulated with PVC as standard electric cables are. The electric insulation of these sensor cables is guaranteed by the usage of silocone. Which has a much higher thermal stability than most other plastics. Baking trays and molds can be made from silicone.This is necessary because the surface of the collectors get that hot that most plastics melt, or become brittle losing their softening agent. And these cable are not running within the collectors but outside.

Check the max. possible temperature of the choosen collectors by looking at their certificates, most manufacturers will have the data available. As said, it is part of the collector certification process to meassure the temperature of them at the back during stagnation and full sun exposure.

I think Fergus has answered the above.

I hope my answers to the above points are helpful.

.

heinbloed

@ PeteHeat:

Refering to your last post:

"I hope my answers to the above points are helpful."

The OP was asking for the difference in effeciency between the two options had was looking at....

The most efficient way to save on energy (to make investments in energy systems efficient that is) is to reduce the energy demand.

When you say a 50 mm gap should be kept between roof insulation and felt than this way of building would waste a lot of energy. Incompetent builders trying to save pennys still go for this method...
Modern insulation technics make use of every available space, a 50 mm gap between roof felt and insulant would be a huge waste of energy saving potential.
'Breathable' felts are used nowadays at the outside combined with a vapour barrier at the inside. And no gap left CAUSING condensation.
Condensation droplets running down on the felt need an outlet. The provision of this outlet will allow plenty of air movement/wind to blow into the roof's insulation. What a waste of energy!

Airtight building technologies are a must nowadays.

I would suggest you actually read the building regulations, the statement you made

( " The insulation should not be even close to the back of the panel as there is height of the roofing laths and the building regulations regarding the space between the insulation and the waterproofing (felt is the second line of waterproofing).

Insulation between rafters must be kept at least 50mm from the waterproofing and there must be a through flow of air circulating within the space to prevent interstitial condensation within the insulation and of course rot in the timbers which would be caused by condensation on the back of traditiona underfelts.")

is not correct.
The building regulations demand a propper building method, the GUIDELINES you propably refer to coming with them are for the grandad builder who doesn't understand the laws of physics. Like road signs for those who can't read the map.



@ the OP:
Go to the Estif home page, there you'll find a calculator for the various angles of a ST collector and their advantages and disatvantages concerning efficient harvest.
See also http://re.jrc.ec.europa.eu/pvgis/index.htm , at the left column you'll see the term " solar radiation ". As well as " interactive maps ". Make use of these.

As long as the backside of the flat panel collector is exposed to outside temperatures, be it installed roof integrated or on-top of the roof tiles the energy loss is the same. The surrounding air temperature determines the energy loss, mainly.
The financial amortisation of an installation however has to take into account the deterioration of surounding materials.

Relying at the collector manufacturer installer manual to determine this risk of causing deterioration would be foolish. The manufacturer of the ST collector simply doesn't know what situation will be met at the site, what meterials are in the vicinity at the point of fixing.
All that the manufacturer can give are guidelines, but what needs actually to be done has to be determined at the site.
Therefore the collector manufacturer must state the stagnation temperature of his product.
Check the manufacturer's manual for the heat emissions of the collector when in stagnation and compare this data with the limits of the surrounding materials. Like felt, cables, insulation material and so on.
This is plain logic, as any building job.

PeteHeat

heinbloed wrote: »

@ PeteHeat:

Refering to your last post:



The OP was asking for the difference in effeciency between the two options had was looking at....

The most efficient way to save on energy (to make investments in energy systems efficient that is) is to reduce the energy demand.

When you say a 50 mm gap should be kept between roof insulation and felt than this way of building would waste a lot of energy. Incompetent builders trying to save pennys still go for this method...
Modern insulation technics make use of every available space, a 50 mm gap between roof felt and insulant would be a huge waste of energy saving potential.
'Breathable' felts are used nowadays at the outside combined with a vapour barrier at the inside. And no gap left CAUSING condensation.
Condensation droplets running down on the felt need an outlet. The provision of this outlet will allow plenty of air movement/wind to blow into the roof's insulation. What a waste of energy!

Airtight building technologies are a must nowadays.

I would suggest you actually read the building regulations, the statement you made


is not correct.
The building regulations demand a propper building method, the GUIDELINES you propably refer to coming with them are for the grandad builder who doesn't understand the laws of physics. Like road signs for those who can't read the map.



@ the OP:
Go to the Estif home page, there you'll find a calculator for the various angles of a ST collector and their advantages and disatvantages concerning efficient harvest.
See also http://re.jrc.ec.europa.eu/pvgis/index.htm , at the left column you'll see the term " solar radiation ". As well as " interactive maps ". Make use of these.

As long as the backside of the flat panel collector is exposed to outside temperatures, be it installed roof integrated or on-top of the roof tiles the energy loss is the same. The surrounding air temperature determines the energy loss, mainly.
The financial amortisation of an installation however has to take into account the deterioration of surounding materials.

Relying at the collector manufacturer installer manual to determine this risk of causing deterioration would be foolish. The manufacturer of the ST collector simply doesn't know what situation will be met at the site, what meterials are in the vicinity at the point of fixing.
All that the manufacturer can give are guidelines, but what needs actually to be done has to be determined at the site.
Therefore the collector manufacturer must state the stagnation temperature of his product.
Check the manufacturer's manual for the heat emissions of the collector when in stagnation and compare this data with the limits of the surrounding materials. Like felt, cables, insulation material and so on.
This is plain logic, as any building job.

Dear Sir,

I respectfully request you leave my Grand Father out of this discussion he is Dead a long time and cannot defend himself.

Also go and do a little research on insulation in roof spaces.

For your Amusement:

<SNIP>

Kind Regards,

Peter Crawley, M.I.o.R.

.

muffler

Im locking this as its getting ot of hand and its a Saturday night and I have better things to be doing than baby sitting people here. Honestly you lot are behaving like spoiled brats.

I'll look at this again on Monday after I discuss it with the other mods.

Oh and be warned - bannings will be issued.