Odd cooling loads

Replies to This Discussion

Permalink Reply by Octagon26 on December 16, 2015 at 10:43am

What is the outdoor temperature during the peak cooling condition?

If it is lower than the internal zone temperature, then the zone is shedding heat to the outdoors. In this case, lower wall thermal conductivity leads to lower peak cooling load.

Permalink Reply by Antonello Di Nunzio on December 16, 2015 at 11:57am

Hi Ducan,

The second major energy-performance feature of windows is the ability to control solar heat gain through the glazing. I have seen in your file two type of glazings:

a) ProposedGlazing

b) ExistingGlazing

I don't know the characteristics of your materials. Have you also changed the SHGC?

I think you have not considered the influence of the SHGC.

Bests

Antonello

Permalink Reply by Antonello Di Nunzio on December 16, 2015 at 1:11pm

But I think that the main problem for your simulation is this:
the WindowMaterial:SimpleGlazingSystem.
Is the method that you used for U-value (with frames) suitable for this type of model?
Look at the link below.
http://bigladdersoftware.com/epx/docs/8-0/engineering-reference/pag...

And "WindowProperty:FrameAndDivider" is not defined.

Permalink Reply by Chris Mackey on December 16, 2015 at 10:01pm

Duncan,

I see that the peak cooling in two cases of U-values are differing by only a very small amount (<1W out of over 300W) and this is more like a rounding error than an actual decrease in peak cooling.  This result makes a lot of sense to me when you consider all of the factors that contribute to your EnergyPlus peak cooling and is something that I have witnessed in some simulations of my own over the past couple of years.

The reason why the peak cooling is not deeply affected by the changing U-Value is that having a high U-Value can actually be a benefit in hot times of the year as it can assist in the shedding of heat in the night before a hot day and the cooling down of the buildings surfaces.  This is particularly important in your model where the nighttime setback is 30C, which is much warmer than the nighttime outdoor temperature and means that you certainly are getting more heat loss at night with a higher window U-value.  Contrarily, the day (especially in a south glazed building like the one you are testing) is driven mostly by solar load and (especially for retail) internal gain.  Whatever heat gain you may be getting through window conduction with a higher U-value is very small in comparison to these other heat sources and is understandably counter-acted by the coolness of building's thermal mass from the previous night (and it's ability to absorb the heat of these other intense sources).

I third the statement that, if you really want to see a major reduction in peak cooling, the SHGC is the term that you should be adjusting.

-Chris

Permalink Reply by Duncan Cox on December 17, 2015 at 2:44am

Thanks for everyone's time and insight. And yes we do recognise that the SHGC is the critical factor here but I just wanted to have some confidence that the materials were behaving properly in my script. 

Thanks for all your help as always.

Permalink Reply by Chris Mackey on December 17, 2015 at 5:46am

You're very welcome, Duncan.

For some extra confidence in your case, it may also be useful to look at the glazing conduction term of the energy balance in relation to the other terms for the peak cooling day using the following workflow:

http://hydrashare.github.io/hydra/viewer?owner=chriswmackey&for...

Visualizing the "Surface Energy Flow" with the "Color Zone Surfaces" may also be informative to understand the participation of the conduction in the overall energy flow across the window.

-Chris

Permalink Reply by Duncan Cox on December 17, 2015 at 9:04am

This energy balance output is great!