Green Building Forum - Roof angle and latitude

Roof angle and latitude

gyrogear — Thu, 23 Mar 2017 11:19:06 +0000

This just out of pure curiosity !

I always had suspicions, but painting the mezzanine, it is not a right-angle at the ridge, but 93Â°

Did some measuring, drew a triangle and resolved it on online calculator:
the south slope has a ridge angle of 48Â° and the north slope is 45Â°

Still curious, I notice that 48Â° corresponds to my latitude: did some more webbing and discovered the notion of "optimizing for winter or summer".

So what was the Architect's intention (1983) : clearly he did not optimize for winter warming with a steep pitch, nor for summer overheating () with a shallow one, so was he being Climate Neutral ?

Curious,
N. Brittany

P.S. you can actually get a butchers on the item in question, as I used it elsewhere on here for other reasons...

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14993&page=1#Item_23

Roof angle and latitude

skyewright — Thu, 23 Mar 2017 12:05:48 +0000

What is the local tradition? Could that have been the main factor, even in 1983?

I'm only an amateur, but my impression is that local roof angle tradition had a lot to do with traditional local construction materials - if the local traditional roof covering is on the heavy side that might tend towards a lower slope to reduce the total area and thus total weight?

e.g. the 'flag' roofs in the Pennines which have a low pitch.

When we lived in the English Peak District the house roof had been replaced by a previous owner. The old flags had been taken off and a new slate roof fitted. The pitch of the new roof was sufficiently different that they'd left the old (round) timbers in place rather than disturb them (the gables had been built up to the new angle)!

Roof angle and latitude

owlman — Thu, 23 Mar 2017 12:08:45 +0000

3 degrees,- builders/roof joiners, error?

Roof angle and latitude

djh — Thu, 23 Mar 2017 12:33:36 +0000

I think the angle of the roof traditionally has more to do with shedding precipitation than optimising solar gain. Or rather a balance between the added cost of a steeper roof, because of additional materials, and improved water, ice and snow shedding.

The angle shown in the drawing is clearly not a right angle, but it's greater than 90Â° which isn't consistent with slopes of 45Â° and 48Â°. What are the walls made of? Maybe the difference is due to the limitations of whole courses of whatever it is.

Roof angle and latitude

gyrogear — Thu, 23 Mar 2017 14:50:58 +0000

Posted By: djhWhat are the walls made of?

The gable walls are concrete units, 50 cm long x 20 high, by 22 cm thick (according to plans)

I don't know if they contain cavities...(the crosswall ones are the same size, and they *do*).

Posted By: djhit's greater than 90Â° which isn't consistent with slopes of 45Â° and 48Â°.

I understand what you mean, I was just reporting the apex angle (sorry, I don't know enough about roofs to know whether it is *this* angle or the lower one that counts !).

gg

Roof angle and latitude

djh — Thu, 23 Mar 2017 17:36:21 +0000

Posted By: gyrogear
Posted By: djhit's greater than 90Â° which isn't consistent with slopes of 45Â° and 48Â°.

I understand what you mean, I was just reporting the apex angle (sorry, I don't know enough about roofs to know whether it is *this* angle or the lower one that counts !).

Are you measuring the angle from the roof slope to the vertical or to the horizontal?

Roof angle and latitude

Ed Davies — Thu, 23 Mar 2017 17:48:01 +0000

A few degrees one way or the other won't make much difference to solar gain, anyway.

Still, 45Â° gives better annual gain than 48Â° but 48Â° gives better December/January/February (DJF) gain but if you were optimizing for DJF you'd want a steeper angle still. I suppose 48Â° could be optimum for a longer heating season - can't be bothered to calculate as it'll likely be so close anyway.

That's with PVGIS for an arbitrary spot just north of Pontrieux. http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php

Roof angle and latitude

John Walsh — Thu, 23 Mar 2017 19:09:03 +0000

Looked at slightly differently - there's a rough rule-of-thumb that, at a given latitude, in the winter period when the sun doesn't get above 20deg max altitude don't expect to get much usable solar gain (if you're not in a PH standard house). Where I am (53N) the period is around Nov 10th to Feb 1st. What's surprising is that for Pontrieux (48.6N) the period is around Nov29th to Jan 13th, i.e. not that different. Does this fit with your experience gg?

Max sun altitude data from https://www.timeanddate.com/sun/@6429101?month=1

Edit: doing the sums it's 45 days vs. 83 days, so is quite different. My original thought probably still holds though - December looks to be off limits for solar gain so the roof design isn't likely to be oriented to mid-winter solar gain.

Roof angle and latitude

gyrogear — Thu, 23 Mar 2017 20:24:04 +0000

Posted By: djhAre you measuring the angle from the roof slope to the vertical or to the horizontal

The former : the roof makes an angle of 42Â° to the horizontal...

Posted By: Ed Daviesbut 48Â° gives better December/January/February (DJF) gain

AHA - thanks, Ed, this might be the explanation, then ! (coldest part of the year...)

Posted By: John WalshWhat's surprising is that for Pontrieux (48.6N) the period is around Nov29th to Jan 13th, i.e. not that different. Does this fit with your experience gg?

Thanks, JW - I dug out my archives:
I built a table of
IRRADIANCE LEVELS
using tables at http://www.builditsolar.com/References/SolRad/Lat48.htm
OCT = 10.1% of year and 14.24% of winter radiation
NOV = 8.8 % of year and 12.63% of winter radiation
DEC = 8.1 % of year and 11.42% of winter radiation
JAN = 9.2 % of year and 12.94% of winter radiation
FEB = 12.7 % of year and 15.06% of winter radiation
MAR = 10.1 % of year and 14.29% of winter radiation
APR = 7.7 % of year and 10.79% of winter radiation
MAY = 6.1% of year and 8.6% of winter radiation

So OCT and FEB are my best months, so it looks like ED's theory is good...

Thanks you all for helping me out (yet again!)

gg

Roof angle and latitude

gyrogear — Thu, 23 Mar 2017 21:57:08 +0000

FWIW, I tabulated the data to get a better handle on things...

=================

Tentative conclusions:

Over the year, DN receives basically twice the radiation of a vertical south-facing collector.

Over winter (8 months), the DN radiation is 61% of the year (â€œstatisticallyâ€ one would expect 66,66%...)
Whereas over winter, a VSF receives 71% of its year's-worth of radiation (= +5%.)
VSF is therefore optimised for winter.

Per ocular re-examination of the tables, a 48Â° roof slope is optimized for Nov, Dec, Jan and Feb

A 58Â° slope would be better than 48Â° in Jan and Feb but not in March or April, and slightly better in Oct, Nov and Dec.

After JWâ€™s comments, I took a look at local roofs, and they are just about ALL the same as mine !

Conclusion: roof angles must be a compromise between weather conditions and local tradition

gg

Roof angle and latitude

John Walsh — Fri, 24 Mar 2017 08:19:35 +0000

Just a little add-on to the idea (above) that the mid-winter period when solar gain isn't that useful is longer the further north you are (45 days at 48N, 83 days at 53N). For solar gain purposes, the advantage of being further south changes round by mid-spring. By late April, the further north you are the greater the effect of spring time 'seasonal lag' - i.e. average air temps lag behind rising insolation levels. Some data ...

On around April 24th each year 53N (where I am) sees the first day of the year that the sun reaches a max altitude of 50 deg - i.e. an altitude which for much of the day it would have to be very rainy to block out usable solar gain heat. By comparison, in Northern France (48.6N), gg is wallowing in a noon sun altitude of 54.3 deg while someone in Edinburgh sees 47.1 deg at noon - still a very usable amount of sun.

Now looking at 'heating degree days' (data from http://www.degreedays.net/ ), in the week from 24th to 30th April the average HDD (with a 15.5 deg base) for gg (weather station LFRO) in the last 3 years has been 41.6, whereas for me (EGNX) it's 50.2 and for Edinburgh (EGPH) it's 58.7 HDD. In short, by the end of April there's plenty of sun to harvest in places where heating is still required (unless you're in a PH standard house).

The wider point being that as the spring period progresses we've all got very usable sun and 'up north' it's needed and can (should) be made use of. This is, of course, counter intuitive (and hasn't been well received by, for example, the Cornwall contingent on here in the past*) which tends to hold back the development of solar gain technologies, which in turn ... well, we all know the effect of using more fossil fuels.

* http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14558&page=3

Roof angle and latitude

SteamyTea — Fri, 24 Mar 2017 08:29:47 +0000

Don't confuse the potential and the economic cases, they are different things.

And sea surface temperature of the Atlantic, which affects the UK's weather more than anything else.

Roof angle and latitude

Peter_in_Hungary — Fri, 24 Mar 2017 09:43:33 +0000

Posted By: owlman3 degrees,- builders/roof joiners, error?

+1
possibly aided and abetted by the local favorite brew !!

Roof angle and latitude

Ed Davies — Fri, 24 Mar 2017 09:46:39 +0000

Posted By: gyrogearI built a table of
IRRADIANCE LEVELS
using tables at http://www.builditsolar.com/References/SolRad/Lat48.htm

From a quick glance it looks like those are generic tables for that latitude and therefore presumably don't take cloud into account. PVGIS gives different results for places at the same latitude as they have a model of how cloudy it is in different locations at different times of the year.

Roof angle and latitude

Viking House — Fri, 24 Mar 2017 11:45:12 +0000

For maximum solar PV yield its 48Â° minus 15Â° = 33Â° roof pitch.
For maximum solar heating yield its 48Â° plus 15Â° = 63Â° roof pitch.

Roof angle and latitude

gyrogear — Fri, 24 Mar 2017 14:10:57 +0000

Posted By: Ed DaviesFrom a quick glance it looks like those are generic tables for that latitude and therefore presumably don't take cloud into account.

Good observation, Ed, ! I am kicking myself !
In effect, I need another column in that file, to give *my* weather (which gives only 70% of the theoretical values)

Will try & get round to updating the file, for interest's sake !

Posted By: Viking HouseFor maximum solar PV yield its 48Â° minus 15Â° = 33Â° roof pitch.
For maximum solar heating yield its 48Â° plus 15Â° = 63Â° roof pitch.

Thanks VH, that is interesting also (I am conteplating some garden-mounted PV, as a matter of fact...).

Cheers, everybody !

gg

Roof angle and latitude

John Walsh — Fri, 24 Mar 2017 15:00:44 +0000

gg, maybe best to go easy on the 'kicking myself' - I take it that what Viking House is pointing to is that solar PV and solar gain heating aren't the same thing, that they have different characteristics and that, by extension, be careful when extrapolating from PV data.

To take just one example, when taking warm air from a solar gain space, providing you're not exceeding the ACHs the space/sun strength can handle, occasional cloud cover on an otherwise sunny day will make very little difference to the heat you can harvest. Whereas, with PV of course, there isn't any 'store' of energy, there's a direct relationship between cloudiness and PV yield.

Roof angle and latitude

fostertom — Fri, 24 Mar 2017 15:38:19 +0000

There may be more of a heat-store buffer in a solar gain space, or a wet solar panel, than in a PV panel, but the input is interrupted just the same. Why should the total harvested be different?
Indeed a heat-storing solar space or wet panel will be much more liable to nett loss (radiation to cold scenery/sky) than a PV, when the sun goes behind cloud.

Roof angle and latitude

gyrogear — Fri, 24 Mar 2017 16:41:20 +0000

All these are interesting arguments !

I was "kicking myself" because I already had a table made up, adjusted for my local weather (which is "not brilliant") but must not complain !

So I have modified the PDF in question, FWIW.

Thanks to JW for the HDD site, very nice !
In effect, APRIL can be a very cold month here -- the sting in the tail of winter...
We feel the cold because we are old () but mainly because it is very humid !

I have no particular love for the electric company, so all of my efforts & energies are aimed at trying to grab as much solar thermal as possible, and store it "for a rainy day" - and I'm still trying to understand how my house (not to mention the sun...) works !

Good news is, I am slowly but surely working out my "heat-loss coefficient" - will post when done !

Thanks to all, your ideas and input help *A LOT* !

gg

Roof angle and latitude

skyewright — Fri, 24 Mar 2017 16:53:57 +0000

Posted By: fostertomThere may be more of a heat-store buffer in a solar gain space, or a wet solar panel, than in a PV panel, but the input is interrupted just the same. Why should the total harvested be different?

Perhaps the difference is potential use for the energy?

If the PV is grid connected then you can harvest as much as technically possible, and most of that will be in summer, so a low angle (i.e. closer to horizontal) is advantageous (assuming maximisation of annual production is the aim).

With a wet system, unless you have a small system or a very large heat store (e,g, something like 'charging the earth'), there is probably more potential capacity in the summer than you can actually use (or at least store), so better to angle the panels steeper for greater advantage in the 'shoulder' months.

Roof angle and latitude

fostertom — Fri, 24 Mar 2017 17:44:34 +0000

Posted By: Viking HouseFor maximum solar PV yield its 48Â° minus 15Â° = 33Â° roof pitch.
For maximum solar heating yield its 48Â° plus 15Â° = 63Â° roof pitch.

I'd love to hear what VH means, in this typically radical (and often correct) statement.

Does 'maximum yield' mean 'year-round useful/useable yield' i.e. discounting unuseable summer 'wet' heat (or unwanted solar space heat) excess, as skyewright is suggesting I think? If so, it's a rule of thumb that contains big assumptions as to how much 'wet' heat can be utilised in summer - 24/7 outdoor hot tub, hydroponics, anyone?

'More vertical' could also reflect, with wet panels, that nett loss (radiation to cold scenery/sky), when the sun goes behind cloud. The sky is often at lower radiant temp than the scenery, so tilting a bit more upright, to 'see' more scenery than sky, may reduce those losses even more than it reduces gain while the sun is shining.

Personally, I'm highly interested in maximising the Dec/Jan harvest, whether PV or 'wet', to supply that last bit of 'supplemental' (cold snap) space-heating demand, in mass-store (thick walls) form, in near-PH buildings - especially not-so-near-PH Enerphit (refurb) buildings which need something more (and much more often) than an occasional fan heater blast.

Roof angle and latitude

John Walsh — Fri, 24 Mar 2017 18:52:07 +0000

fostertom, I think I understand your position a little better now, your key phrase being "Personally, I'm highly interested in maximising the Dec/Jan harvest". Also, I think skyewright has helped somewhat with the idea of a system producing more than you can actually use.

Where I am (53N) there doesn't seem much point in maximising the Dec/Jan harvest as there just isn't enough sun at that time. Instead, the strategy we chose is for solar gain spaces which will easily produce enough heat in the shoulder months and to burn coppiced willow in mid-winter. Clearly, that's not for everyone but the experience we gain from using solar warmed air could be sharable in the interests of promoting wider usage.

I can see, however, why you need to have a measurable, full-use system. Whereas, what I've chosen isn't full-use and defies careful measuring. As I type this at around 7pm, it's 8 deg outside (and falling) and 24 to 26 in the solar gain spaces. The MVHR switched to 'summer bypass' at around 10 this morning and will switch to 'normal' at around 7.30-8pm. Clearly, it would be a challenge to predict exactly when it will switch to normal and to what end?

The bottom line, though, is that what you seem to be doing and what I'm doing is developing low-carbon strategies which suit our respective locations. These strategies involve differing philosophies which have different emphasis and measures. They're surely both of equal merit: to use what seems to be a fitting acronym - YMMV

Roof angle and latitude

fostertom — Fri, 24 Mar 2017 19:48:47 +0000

Yes, it's a waste of time hoping to collect solar as heat in those deep winter months, if the collection system is subject to nett-loss re-radiation when the sun goes behind cloud. There has to be a way of moving the collected heat away, into safe storage, as it is collected. Then when the sun goes behind the cloud and all the collector can 'see' is cold sky and scenery, there's no heat in the collector, to re-radiate.

A solar space is totally liable to such re-radiation loss, so won;t do anything useful in deep winter.

However a 'wet' panel collector that's only kept filled by a PV-powered pump will collect while the sun shines, but as soon as the PV stops powering the pump, the thermofluid drains-back out of the panel, taking all residual heat away with it so it can't be lost by re-radiation.

On that basis, deep-winter 'wet' heat colection can be useful - especially if the thermofluid flow temp is kept really low - like 21C - which again greatly reduces re-radiation loss. 21C is just right for mass-storing into thick walls aimed at keeping the interior at a high-radiant-component comfort temp of 17-20C.

PV collection suffers none of that re-radiation loss - but on the other hand is several times less efficient, area-for-area, as optimised 'wet' collection.

Roof angle and latitude

djh — Fri, 24 Mar 2017 20:21:01 +0000

Posted By: fostertomYes, it's a waste of time hoping to collect solar as heat in those deep winter months, if the collection system is subject to nett-loss re-radiation when the sun goes behind cloud. There has to be a way of moving the collected heat away, into safe storage, as it is collected. Then when the sun goes behind the cloud and all the collector can 'see' is cold sky and scenery, there's no heat in the collector, to re-radiate.

Good, agree so far.

A solar space is totally liable to such re-radiation loss, so won;t do anything useful in deep winter.

Well, not quite so convinced since the incident radiation is short wavelength, which passes through glass, whilst the heat trying to escape is long wavelength, which doesn't so much. Still not as efficient as PV or 'thermal diode' solar thermal tubes, of course, but still better than a kick in the teeth.

However a 'wet' panel collector that's only kept filled by a PV-powered pump will collect while the sun shines, but as soon as the PV stops powering the pump, the thermofluid drains-back out of the panel, taking all residual heat away with it so it can't be lost by re-radiation.

Right, but solar thermal tubes do this automatically via their 'thermal diode' effect.

On that basis, deep-winter 'wet' heat colection can be useful - especially if the thermofluid flow temp is kept really low - like 21C - which again greatly reduces re-radiation loss. 21C is just right for mass-storing into thick walls aimed at keeping the interior at a high-radiant-component comfort temp of 17-20C.

True.

PV collection suffers none of that re-radiation loss - but on the other hand is several times less efficient, area-for-area, as optimised 'wet' collection.

Also very true. The real problem is the lack of solar energy at the time of year when it is needed for space heating. Indeed the lack of solar power is the fundamental reason why we need heating! Hence the mania around the holy grail of seasonal storage.

Roof angle and latitude

gyrogear — Fri, 24 Mar 2017 20:43:43 +0000

Posted By: fostertomespecially if the thermofluid flow temp is kept really low - like 21C - which again greatly reduces re-radiation loss.

Would *air* qualify as the thermofluid ?

I am thinking of grabbing air from behind slates, and shoving it through an open-ended system (= crawlspace) - the air would be directly exhausted back to the garden. Something like 500 CFM.
Dreadfully draughty and noisy, but all of that happens in the crawlspace, where we do not go !
(well, *I* do, most weekends actually, for an hr or 2's digging...) (afraid it's becoming a hobby...)

In event of cloud, per your scheme, a small PV panel (losing the light...) would cut the fan ; however the slates have inertia, so would not "react" to that cloud for, say 12 or 15 minutes (I have tested this...)

There would also be PEX behind the slates, with a circulator, to a storage.
So the slates are a hybrid "collector" in that sense (air and water).

I would need a smart control system, sensors and stuff, but that is all planned.

I think I have enormous solar-gain potential, that I want to use, especially during summer: the PEX heat would be sent to earth...

gg

Roof angle and latitude

SteamyTea — Fri, 24 Mar 2017 21:52:17 +0000

Posted By: fostertomI'd love to hear what VH means, in this typically radical (and often correct) statement.

Does 'maximum yield' mean 'year-round useful/useable yield'

I think he means total yield i.e. the kWhs/time period.

Roof angle and latitude

fostertom — Fri, 24 Mar 2017 22:02:48 +0000

Posted By: gyrogearWould *air* qualify as the thermofluid ?

Can do - but only if circulated on a buoyancy/thermosyphon basis - no question of fans forcing 500cfm! And needs to be a closed-loop recirculator.

Imagine a south facing massive external wall with corrugated (max surface area) face. Spaced off it by 200mm, insulation, then another 100mm airspace then thin corrugated aluminium sheeting optical-black selective-coated both faces. Spaced off that by 100mm, selective-coated glass - whether single or multi-pane is a balance of transmitance vs insulation effects.

Solar strikes thro glass, heats the corrugated, which heats the air in the two 100mm airspaces each side of it, which rises over top edge of and back down the back of the insulation, where it heats the wall and cools the air, which re-enters at the bottom and so on.

With v large cross-sectional airways (no question of ducting) all the way, and minimised path length, thermosyphon rate might be high enough to move significant heat. When the sun goes in, there's minimum thermal mass (air plus corrugated) therefore minimum heat to be radiated away to cold sky and scenery. Then have to guard somehow against reverse circulation, as the wall becomes the relatively hot source.

Snag - can't afford windows in this sunny face of the building! There's prob easier ways to do capture and store solar.

I'd say glass fronting, not slate, would be vital to capture anything useful during heating season - the fact that slates get red-hot in summer won't help in winter.

Unless you're sending it underground, as you hint, for winter recovery
http://www.norishouse.com/PAHS/UmbrellaHouse.html
http://diygreenbuildingwithjerry.blogspot.co.uk/2014/07/timeline-annualized-geosolar.html (sadly this article says "Stephens' original paper -- http://www.greenershelter.org/TokyoPaper.pdf -- is no longer available on the web. However, it can be accessed in a round-about way via Stephens' original detailed paper (link provided)." I think I've got the TokyoPaper saved somewhere, if interested.

Roof angle and latitude

fostertom — Fri, 24 Mar 2017 22:42:45 +0000

Posted By: djhsolar thermal tubes do this automatically via their 'thermal diode' effect

as does collection by PV. And similarly, glass esp if selective-coated has semi-diode (one-way) effect. Pros and cons in all these alternatives.

But some years back my boffin pal modelled extraordinarily high collection efficiencies, like 60%, for basically home-made build-in-place (serving as whole-slope roof covering) ultra-lo-temp wet collectors storing into masonry plus buffer tank, with quite a lot of continuous intelligent control (self-learning AI now seems ever more accessible even on a DIY basis) to optimise for lowest poss feed temp.

The plumbing is in place on the built project, for time being taking ultra-lo-temp mixed feed from the client's biomass boiler - all extremely comfortable high-radiant-component 'cool' heating from whole wall area. One day it's still in the plan to feed same with solar as described - or a PV source.

Roof angle and latitude

fostertom — Fri, 24 Mar 2017 22:44:18 +0000

12mm PEX pipes thermally bedded to wall, covered with EWI

Roof angle and latitude

Viking House — Fri, 24 Mar 2017 23:01:26 +0000

For maximum solar PV yield its 48Â° minus 15Â° = 33Â° roof pitch.
This is the optimum pitch for solar PV array when selling electricity to the grid.

For maximum solar heating yield its 48Â° plus 15Â° = 63Â° roof pitch.
In a Passive House with a low heating demand a large Solar Roof (20% of floor area) is sufficient to heat it with back up electric fan heating (1kWh/m2.annum). Using drain-back solar techniques the Solar heated water is pumped directly into the slab, the harvest temperature from the Solar Roof is changed from 70Â° in summer to 30Â°, increasing the December yield by 60%.
So if you're building a 200m2 Passive House and want to Solar heat it, fit a 40m2 Solar Roof angled at 65Â°.