eQuest - Energy consumption increasing on reducing Glass U-Value

13 posts / 0 new
Last post

Hi There

I am facing an issue in eQuest in almost all simulation models for most hot
and composite indian climates. Whenever we reduce U-value of glass in
model, from Single Glazing U-Value of 1 Btu/hr Sqft F to 0.32 Btu/hr Sqft F
(or any other U-value lower than 1), we see an increase in energy
consumption.

Can someone point out principal mistake that may be making in the model.

Thanks in advance.

Regards

Akshay Gupta

akshay gupta's picture
Offline
Joined: 2011-10-02
Reputation: 0

Akshay,
While there are many things that could contribute to this situation,
consider:
A smaller U value means that there is *less *conduction through the
construction. With less conduction, a smaller cooling load will normally
decrease cooling energy.

An exception to this is when internal loads are higher than normal. In that
case, the building needs to get rid of internal heat (provide cooling) most
of the year. When the envelope has more insulation (lower U-value), it
inhibits heat removal and can increase cooling energy.

Thanks to my friend Javed Iqbal for reminding me of this recently. He is
preparing a paper which discusses this phenomenon for buildings in the
central European climate.

James V Dirkes II, PE's picture
Joined: 2011-10-02
Reputation: 203

Hi Jim

Thanks for the reply. Actually, the project we are modelling are designed
to be energy efficient commercial spaces. So for example, interior LPD is
not more than 0.5 W/sqft (LED lights), EPD is around 2.0 - 2.5 W/sqft,
Occupant Density is around 50 Sqft /person. So I dont see any area where
internal heat gain is unusually higher where the building needs to cool
itself off by conducting heat outside (inside hotter than outside).

I think there something else.

Regards

Akshay

On 3 November 2017 at 18:42, Jim Dirkes
wrote:

akshay gupta's picture
Offline
Joined: 2011-10-02
Reputation: 0

That's good to know and rules out my scenario.

I think I can't help more without more details, but I'm also unfamiliar
with eQuest.

If you were using EnergyPlus, I'd recommend adding a set of detailed output
variables to study the energy use pattern more closely. Perhaps you can do
something like that with eQuest....

James V Dirkes II, PE's picture
Joined: 2011-10-02
Reputation: 203

Hi Akshay,
If you care to share your model, eQUEST users can take a look at it.
You could email your .pd2, .inp and weather files to equest-users at lists.onebuilding.org.
Regards,
~Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer

[cid:image002.jpg at 01D35487.0E16F4E0] [cid:image004.jpg at 01D35487.0E16F4E0]

134 South Fitzhugh Street Rochester, NY 14608

T: (585) 698-1956 F: (585) 325-6005

bbishop at pathfinder-ea.com www.pathfinder-ea.com

[http://png-5.findicons.com/files/icons/977/rrze/720/globe.png]Carbon Fee and Dividend - simple, effective, and market-based.

Bill Bishop's picture
Offline
Joined: 2012-02-25
Reputation: 7

50 sqft/person (5m2 per p) seems like a very high occupant density for an office building (depending how you calculate it, if you include corridors)

??
Sent from a mobile device, please excuse the brevity.
Julien Marrec, EBCP, BPI MFBA
Owner

Direct: +33 6 95 14 42 13
Website: www.effibem.com

LinkedIn (en) | (fr)

jmarrec's picture
Offline
Joined: 2013-01-09
Reputation: 0

50 sqft/person (5m2 per p) seems like a very high occupant density for an office building (depending how you calculate it, if you include corridors)

??
Sent from a mobile device, please excuse the brevity.
Julien Marrec, EBCP, BPI MFBA
Owner

Direct: +33 6 95 14 42 13
Website: www.effibem.com

LinkedIn (en) | (fr)

jmarrec's picture
Offline
Joined: 2013-01-09
Reputation: 0

I?ve seen the kind of behavior Jim mentions in some of the DOE Office Prototype models in hot climate zones, like Phoenix, AZ. So, it might not need an unusually high internal load to trigger this kind of behavior.

Given that it?s a hot climate, the U-factor has almost no correlation to cooling, which I presume is your dominant end-use. Look at the SHGC of your 1 Btu/h-ft2-F window and that of your other window. SHGC correlates very well with cooling. Also, check which end-uses are changing as this might give you clues about the source of the changes.

If you are holding SHGC constant, and a file compare does not reveal any other input changes, it might be something with the eQuest internal model itself (I?m not very familiar with eQuest).

Athalye, Rahul A's picture
Joined: 2016-02-04
Reputation: 0

Jim?s suggestion is a classic case for why energy would increase due to higher performance change in the envelope ? my other best suggestion is to review the model outputs that restate the space loads and window performance and verify that the inputs are as intended. Side note that 2.0 to 2.5 W/sf for equipment might be a little on the higher side, depending on what the commercial spaces are and density of computers or other electronics.

I suggest also to look at electricity and/or natural gas monthly to see the pattern of the changes (or to use a report variable to see heating, cooling, and fan motor energy separate if all-electric) which may help determine if the result makes sense or if a modeling input needs to be changed.

Another thing to look at is what energy flows increased and which deceased ? it could be possible if you have high airflow minimums or constant volume that there is a lot of reheat in the spaces. When the window was upgraded the amount of ?freeheat? from the outside would be reduced. I don?t think this is likely from your equipment loads, but I use it as an example of examining the monthly data to determine possible causes for the net increase in energy use ? if you can identify specifically what equipment used more energy it will help to debug the model.
David

David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP
Associate

Direct: (847) 316-9224 | Mobile: (773) 490-5038

Grumman/Butkus Associates | 820 Davis Street, Suite 300 | Evanston, IL 60201
Energy Efficiency Consultants and Sustainable Design Engineers

grummanbutkus.com | Blog | Facebook | Twitter

David Eldridge's picture
Offline
Joined: 2012-05-08
Reputation: 1

In my past experiences, such problems were resolved by using detailed
spectral properties (LBNL window output) in eQuest for an Indian
composite climate case.

Sent from mobile, pls excuse typos

javed iqbal's picture
Offline
Joined: 2012-05-18
Reputation: 0

From my experience, such kinds of counter-intuitive results, e.g.,
cooling energies going up with lower U-values,? are due to overlooked
systems/loads interactions rather than arcane loads considerations such
as spectral properties of window glazings.

The three interactions that I would look are:

(1) does the system have an economizer that vents the cooling loads when
the indoor temperature or enthalpy exceed that outdoor ?

(2) is the system sizing the same for both runs? If not, fix the system
size to the larger from the two autosized runs.

(3) (this gets rather arcane but explained the anomaly for increased
heating in mild cooling climates like the Pacific Northwest) change
Cooling Control from CONSTANT to WARMEST, i.e., control cooling control
for the warmest zone rather than all zones.

Joe

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
http://www.whiteboxtechnologies.com
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"

Joe Huang's picture
Offline
Joined: 2011-09-30
Reputation: 406

The other thing that could be going on is that a drop in SHGC is driving up
heating loads. That could definitely happen in mild, heating-focused areas,
but does not seem to fit with your case. Are you changing the system sizing
to account for the reduced load or leaving sizing fixed? Do you have reheat
going up because you are pumping too much cold air?

My number one thing when you're seeing something funny is to get into the
hourly output and compare the two cases. Which end uses are going up?
Where? When? Are space temperatures staying in range when they are supposed
to be? Do you have heating and cooling fighting each other?

You might have something else in your model that's messed up that will be
apparent when you look at the hourly output. Plus, there's nothing more fun
in simulation than looking at hourly output, IMHO.

Justin Spencer's picture
Offline
Joined: 2011-10-02
Reputation: 1

Hi Attiq

I understand how the heat loss is reduced. But in real world, the space is conditioned to 24 deg c inside while outside is more than 40 deg c. Hence there would always be a heat gain from outside and if the glass- u value is reduced then cooling consumption should reduce. Otherwise with both wall and glass u-value reduction there should be increase in cooling consumption. Which means that envelope insulation is not good strategy.

Or is it that the simulation calculations are based in natural heat loss or gain first and based on the resultant load, the building cooling energy is calculated.

Your views are welcome on the above.

Regards

Akshay

Sent from my iPhone

akshay gupta's picture
Offline
Joined: 2011-10-02
Reputation: 0