toiture solaire

[olivir]

Bonjour,

Je travaille sur un projet de toiture solaire (photovoltaïque) pour lequel nous essayons de mettre en place une composition perspirante.

Je traite la couverture photovoltaïque ventilée en supposant une source d'humidité sur le panneau bois extérieur (infiltration d'air), une épaisseur d'air équivalente pour la résistance à la vapeur et un état de surface moyen (absorptivité du panneau solaire/2 x absorptivité de la face ext. de la composition) rendant compte du comportement des dernières couches.

toit solaire perspirant.png (15.99 KiB) Viewed 17406 times

La valeur du Sd ext. ne pose pas vraiment de problème, en revanche, l'absorptivité moyenne, en jouant fortement sur l'échauffement de la couche la plus extérieure, modifie la teneur en eau de la composition de façon nette.

Prenons deux cas :
- Absorptivité ext. = 0,44
Cela pourrait correspondre à un panneau solaire noir devant un pare-pluie noir (0.94/2*0.94, 0.94 étant l'absorptivité de la couleur noire). Dans ce cas, le panneau OSB ext. sèche...

absor 044.png (57.59 KiB) Viewed 17406 times

- Absorptivité ext. = 0,12
Cela pourrait correspondre à un panneau solaire noir devant un pare-pluie blanc (0.94/2*0.25, 0.27 étant l'absorptivité de la couleur blanche). Dans ce cas, le panneau OSB ext. s'humidifie...

absor 012.png (57.02 KiB) Viewed 17406 times

Est-ce que mes modèles vous semblent corrects ? Comment procédez-vous de votre côté ?

[Christian Bludau]

Hello Olivir,

Your thoughts are on the right way. The surface temperature on the roof surface below solar panels is close to the temperatures you get on very bright surfaces. I worked much about that topic.

Please see Figure 4 in this paper: You can see the temperature of a dark (redbrown) surface, a white surface and the temperatures below the panels on the red surface (pv_in and pv_out):
---
Bludau, Christian; Künzel, Hartwig M.; Zirkelbach, Daniel: Hygrothermal performance of flat roofs with construction moisture.
(Buildings Conference <11, 2010, Clearwater Beach/Fla.>)In: American Society of Heating, Refrigerating and Air-Conditioning Engineers -ASHRAE-, Atlanta/Ga.: Buildings XI, Thermal Performance of the Exterior Envelopes of Whole Buildings XI 2010. Proceedings. CD-ROM: December 5-9, 2010, Clearwater Beach, Florida. Atlanta, GA: ASHRAE, 2010, 7 pp.
http://www.ibp.fraunhofer.de/content/da ... -86499.pdf
---

I also work on a model about simulating solar panels in hygrothermal building software. A first idea, which is also using a reduction of the absorptivity and emissivity is published here, but at the moment thats only available in German:
---
Bludau, Ch., Kölsch, Ph.: Verschattung von Holzflachdächern - Holz-Flachdächer: Neue Forschungsergebnisse zu Dachterrassen und Verschattung durch PV-Module. 5. Internationaler Holz[Bau]Physik-Kongress 20.–21.03.2014, Leipzig
http://www.hoki.ibp.fraunhofer.de/ibp/p ... Bludau.pdf
---

For the model I reduced the existing absorption and emission of the surface the following way:
shaded absorption = absorption of roofing felt * 0,3
and
shaded emission = emission of roofing felt * 0,5

So for a dark felt that would be:
abs_shaded = 0,8 * 0,3 = 0,24
em_shaded = 0,9 * 0,5 = 0,45

and for a bright surface:
abs_shaded = 0,4 * 0,3 = 0,12
em_shaded = 0,9 * 0,5 = 0,45

This models are only thought to be used with naked roofing felts, they are not tested for the combination of green roofs or gravel roofs.

Maybe thats also from interest for you:

Bludau, Christian; Schmidt, Thomas; Künzel, Hartwig M.: Hygrothermal effects in lightweight roofs shaded by PV-elements.
(Conference ""Thermophysics"" <12, 2010, Valtice>)In: Zmeskal, O.: Thermophysics 2010. Proceedings: 3rd to 5th November 2010, Valtice, Czech Republic, Brno University of Technology. Brno: Brno University of Technology, 2010, pp. 56-62
http://www.ibp.fraunhofer.de/content/da ... -86508.pdf

More literature you might find here:
http://www.wufi.de/frame_en_literatur.html

Greetings,
Christian

[olivir]

Thanks a lot, Christian, that's a clear, concrete and documented answer !

I have just one small question. Is your model of reduction an empiric one, that's what the second paper say ?! Valid for any climate, you think ?

By the way, I note that, even with a dark felt, shaded absorptivity is much lower than my estimate, so not sure we could make a breathable roof. Did you install successfully a breathable PV roof yet ?

Oh we can't comment the 1st pdf Isn't it possible to make a document with allowed comments, there so much things to highlight !

[Christian Bludau]

Hello Olivir,

the model is a empiric one and should work at least in our degrees of latitudes. At the moment I see no reason, why it should not work with any climate.

I do not really like the word breathable, but I think you are talking about roofs with a moderate sd-value at the inside. In most cases you can not build them if they are shaded. The solution for that is to adapt an additional layer of insulation on the top. For example you put a vapor retarder on the wooden sheathing and then something like 6 to 10 cm of insulation (EPS for example) and then the roofing felt.

Christian

[olivir]

Thank you, Christian, it's a pleasure to frequent this forum

Sorry for breathable, my English is not good enough to choose a better word. What is good with this word is that people imagine easily that vapor can migrate in and out.

In that project, we try not only to use a variable (humidity controlled) vapour retarder but also just a low Sd rain screen on the outer face under the PV panels, if possible.

[Christian Bludau]

Hello Olivir,

my papers are about inclinated panels. In your case i would guess that it is not really a kind of shading. More a question of the backside-temperature of the panels. I think if you reduce the absorption the high I wrote before, you will be far on the save side.

You more should find out, what temperatures can form on the back side of the panels. I would guess it is something like 40 to 50 degree in summer times. More something like absorption of eg. o.5, depending on the insulation of this panels.

Unfortunately I have no experience with this kind of panels. If you can get some measured values, I would be interested about that.

Regards,
Chirstian

[olivir]

In fact, there's not a lot of differences between my model and ours :



I have no measured values, so you advise me to keep my first model ?!

For me, the absorptivity of my model seems correct enough (upper than your model is normal because panels create an air gap less ventilated) but I have no idea for the equivalent emissivity :/

About measuring values, we may invite you to join us on the project. It is a somewhat innovative project that will host you without problem !

[olivir]

One more question : I added a PVC film on the OSB and changed nothing more. I obtain exactly the same behaviour !

toit solaire non perspirant.png (16.5 KiB) Viewed 17268 times

I am not totally surprised but I want to check I didn't made an error. If the behavior is exactly the same, it means that the roof can only dry by the inside, for the 2 cases, isn't it ?

[Christian Bludau]

Hello Olivir,

there is only a little difference, as you calculated with an sd-value on the outside in both cases.
Your first case contains the sd-value in the surface transfer conditions with sd=50m, the second case has both: the 50m again and the sd of the PVC. Thats 50m + sd_PVC. Both values are quite high. Further you have to add the sd-value of the OSB.
Additional you are right with your thoughts about reduced drying effect to the outside. The drying is directed by the vapor pressure and so during sunny times the drying process is mainly directed to the inside direction. A drying to the outside can work best, if you have a diffusion open insulation and a weather barrier (eg sd=0,1m) to the outside.

Christian

[olivir]

Ok, thank you Christian.
Here with the PV panels, I imagine there's an exterior Sd high enough so no really possibility to dry by the outside.