[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [EnergyPlus_Support] Rooftop outdoor air temperatures



One thing to keep in mind might be that if the outside air duct is uninsulated and bound tightly to the roofing material, some significant heat transfer could occur through the duct if its surface area is large enough compared to the air velocity.  I would think this would be insignificant for a rooftop package unit though.




Robert Wichert P.Eng. LEED AP BD&C
HERS I/II     CEPE     CEA
BPI CERTIFIED SF/MF   GREEN POINT RATER
+1 916 966 9060
FAX +1 916 966 9068














===============================================
On 7/21/2014 6:53 AM, HJ wanghaojie630@xxxxxxxxx [EnergyPlus_Support] wrote:
 

I agree with Jean. Though surface temperature will much higher than outdoor air temperature, the air temperature might not be high as long as there is wind. The air could only be heated up through convection (almost no conduction and radiation). If you know fluid dyanmics, the height of thermal boundary layer is small (the part that has higher temperature in this case) and the free stream will have the same temperature as outdoor air. This might also be the reason that the papers Craig provided didnt see much difference with cooler roof or shading.


HJ

On 7/21/2014 7:22 AM, 'jeannieboef@xxxxxxxxx' jeannieboef@xxxxxxxxx [EnergyPlus_Support] wrote:
 
Sorry, no research I know of. 

But experience of rooftop ducting adding 2 K to the overburdened office supply air temperature  in summer is known of in my office and a factory project we had here in Berlin has similar issues.

Rooftop surface temperatures ranges definately go to 56 C (also measured by myself as licenced IR Thermographer) and seen in e+ simulations. 

Microclimate at the rooftop air intakes, i.e. air temperatures, I have not measured, but I can well imagine them being 2 K higher than weather air temps...similar to a parking lot heat island. It would be significantly wind dependant, I would think and only very near the surface (.5 m).

I have also never included this effect. I hope the number of hours that the effect is non-negligable is not many.  

Mit freundlichen Grüßen- Sent from my iPhone (excuse the brevity)

i. A.
Jean Marais
b.i.g. bechtold
Tel.   +49 30 6706662-23

On 19.07.2014, at 19:37, "Jim Dirkes jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

 

Dear Forum,

I have had clients in the past who refused to place HVAC equipment on the roof in Dallas, Texas because they knew the near-roof temperatures were much warmer than the temperature farther away.  The situations have been for large, black roofs which get very warm in the summer.

I have not thought about that much for energy models other than defining high SRI roofing materials.  Yesterday, however, I was talking to an infrared thermal imaging professional who says that he has measured roof temperatures that were 100F / 56C higher(!) than the nominal outdoor temperature.  If that is true, then all of my energy calculations for rooftop equipment are WRONG � not a happy thought for someone who wants results that are realistic.

The resulting questions to the most knowledgeable modeling community in the world (as far as I can tell) are:

·         Are you aware of research which has measured or characterized near-roof temperatures?

·         Do you have a modeling practice that accounts for the near-roof impact on outdoor air intakes or roof-mounted chillers, condensing units or cooling towers?

 

Thanks in advance for your insights!

 

James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653

 

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]
Sent: Friday, July 18, 2014 3:37 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] Re: Curve Fit Tool

 

 

Thank you very much for you help

 

Kind Regards,

 

Felipe Durán

Posted by: Jim Dirkes <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
Reply via web post � Reply to sender � Reply to group � Start a New Topic � Messages in this topic (1)
Yahoo Groups
Improved Group Homepage!
The About page of your Group now gives you a heads up display of recent activity, including the latest photos and files
Yahoo Groups
Control your view and sort preferences per Yahoo Group
You can now control your default Sort & View Preferences for Conversations, Photos and Files in the membership settings page.




__._,_.___
Posted by: RobertWichert <robert@xxxxxxxxxxx>


Primary EnergyPlus support is found at:
http://energyplus.helpserve.com or send a message to energyplus-support@xxxxxxxx

The primary EnergyPlus web site is found at:
http://www.energyplus.gov

The group web site is:
http://groups.yahoo.com/group/EnergyPlus_Support/

Attachments are currently allowed but be mindful that not everyone has a high speed connection.  Limit attachments to small files.

EnergyPlus Documentation is searchable.  Open EPlusMainMenu.pdf under the Documentation link and press the "search" button.




__,_._,___