Determine "Effective thickness of cavity behind exterior baffle"

asked 2024-10-17 03:26:16 -0500

MJ83
11 ●2

updated 2024-10-17 10:44:08 -0500

I appreciate your help in advance. I would like to know how to determine the field named "Effective thickness of cavity behind exterior baffle" which appears in the class "SurfaceProperty:ExteriorNaturalVentedCavity" from EnergyPlus. I have found the following definition:

"This field is used to enter a nominal gap thickness (m) for the collector. If the baffle is corrugated, use the average depth. This distance value is only used when the collector is near horizontal to determine a length scale in the vertical direction for buoyancy calculations. For example, if the collector is mounted on a flat roof, its tilt-adjusted height is zero and the program will use this gap thickness as a length scale rather than the height from the previous field."

Does this mean that if I am using "SurfaceProperty:ExteriorNaturalVentedCavity" to simulate a vertical ventilated façace, this field is not used by EnergyPlus? This way, I do not find any other field to indicate the thickness of the cavity to EnergyPlus.

I have also asked ChatGPT. I know it can make mistakes, but I have tried. This has been its answer:

----------Given the specific details of your vertical, ventilated cavity: - Cavity thickness: 0.3 meters (300 mm) - Emissivity: 0.85 (relatively high) - Orientation: Vertical - Ventilated: Airflow within the cavity Let's ensure that the effective thickness estimate is accurate, taking into account the vertical orientation and ventilation.

Step-by-Step Process:

Vertical Cavity and Convection:
- In a vertical cavity, convective currents are stronger due to the natural buoyancy of warm air rising. This creates more air movement within the cavity, which increases convective heat transfer and decreases the thermal resistance of the cavity.
- Since the cavity is also ventilated, external air is entering the cavity, which further reduces its thermal resistance by enhancing convection.
Emissivity Impact:
- The high emissivity (0.85) indicates significant radiative heat transfer between the surfaces of the cavity, which further reduces its thermal insulation performance. Radiative heat transfer between surfaces becomes a more dominant factor when emissivity is high, causing more heat to be transferred.
R-Value for a Vertical Ventilated Cavity:
- For a ventilated vertical air cavity with high emissivity and a thickness of 0.3 m, the ASHRAE Handbook and similar sources suggest an R-value (thermal resistance) in the range of 0.10 m²·K/W to 0.12 m²·K/W.
- This lower R-value reflects the fact that vertical ventilation significantly increases heat transfer through convection, reducing the overall insulation provided by the cavity.
Calculate Effective Thickness: To find the effective thickness in EnergyPlus, we convert the estimated R-value into an equivalent thickness of an insulating material with known thermal properties (e.g., a material with thermal conductivity λ = 0.04 W/m·K).

Using the formula:

Effective Thickness = Rvalue / Thermal conductivity

For an R-value of 0.10 m²·K/W (a reasonable estimate for your case):

Effective Thickness = 0.1/0.04 = 2.5 cm = 25 mm

For an R-value of 0.12 m²·K/W (upper estimate):

Effective Thickness = 0.12/0 ... (more)

answered 2024-10-17 07:26:45 -0500

Denis Bourgeois
2771 ●105 ●8 http://www.rd2.ca

updated 2024-10-17 15:16:23 -0500

Unable to load the ChaptGPT conversation. Maybe consider posting a print screen image? Curious to read its take on this. EDIT: Thanks for adding it.

My understanding is that the SurfaceProperty:ExteriorNaturalVentedCavity class is inspired (or derived) from the SolarCollector:UnglazedTranspired class (very similar inputs, terminology, etc.). Based on Line 745, EnergyPlus resets the user-defined height scale (or delta-Hnpl) based on the tilt of the surface. If the surface is ~horizontal, EnergyPlus picks either (i) the reset delta-Hnpl or (ii) the cavity thickness (i.e. the largest of the 2 values). In fact, the cavity thickness must be greater than zero (Line 702). So one is required to provide both height scale and cavity thickness inputs, even if the ventilated facade is vertical.

EDIT (in response to follow-up comments): As stated in the IO Reference, EnergyPlus does extract surface area from the linked BuildingSurface:Detailed when it comes to baffle/gap geometry. Aside from that, there's no way for EnergyPlus to guess the actual baffle details from BuildingSurface:Detailed geometry alone. So yes: one must provide the height scale (not the actual height of the BuildingSurface:Detailed) and the effective thickness of the cavity (not the thickness of any construction Material layer). The Engineering Reference section may help to better understand these differences. BTW, a recent UMH post on the question.

So in summary, EnergyPlus actually does need both the real height scale or the real cavity thickness for its calculations. It's just that it can't autoextract these from surface geometry or material layers. And as stated in the IO Reference excerpt you originally quoted, the cavity thickness value "is only used when the collector is near horizontal to determine a length scale in the vertical direction for buoyancy calculations".

I haven't checked, but I suspect the cavity thickness is likely used to output some related variables, like:

HVAC, Average, Surface Exterior Cavity Total Natural Ventilation Air Change Rate, ACH

This requires cavity volume (i.e. cavity thickness x height of the BuildingSurface:Detailed?). You could test this out by running 2 simulations, where you double the (vertical) cavity thickness for the 2nd simulation. I suspect the reported ACH would change, but not necessarily these other reported variables?

Not sure, but may help.

I think ChaptGPT ended up answering another question: how to determine an equivalent thickness of an insulating material (Construction Material), as a proxy for a vented/ventilated air gap. Not a bad answer BTW, but beside the point when it comes to EnergyPlus' SurfaceProperty:ExteriorNaturalVentedCavity. A bit of further question refinement may have yielded better results. Next time, maybe.

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Comments

Thank you very much, Denis. Please let me know if I am wrong. In conclusion, EnergyPlus does not take into account directly the value of the real thickness of the cavity. The software works by using the height of buoyancy and the effective thickness, and both values are the results of calculations the user has to develop by himself since EnergyPlus does not carry out them. This way, these two values give EnergyPlus the information it needs to carry out the calculations about the cavity, that is to say, EP does not need the real thickness of the cavity or its real height.

MJ83 ( 2024-10-17 10:45:41 -0500 )edit

Thank you very much for your kindness. I have added the "conversation" with ChatGPT to the first post. I do not use it very often, but it looks amazing and disquieting at the same time. I reiterate my thanks for your help and your time. Best regards, María.

MJ83 ( 2024-10-17 10:46:31 -0500 )edit

Hi Maria. I edited my original answer, in response to your follow-up comments. Hope this helps.