[Bldg-sim] Cooling Design Calc Methods

[Hall, Brendan]

Thanks for all the info, the OK State papers address a lot of what I had been looking for. I’m going to try and track down that presentation sometime today. I know what I was doing was about equivalent to back of the envelope but I was looking to understand where the differences were as opposed to doing a thorough validation. My understanding of the design calc that E+ does was that it used the heat balance method but made certain assumptions that were typical of a load calc (ex no sun or interior loads in winter). The point that was made of being careful not to use loads and gains interchangeably was a very good one. I think some of my confusion was stemming from that. It appears that HAP reports component loads while E+ was reporting component gains, and would therefore look much different. I re-ran my E+ run with a adiabatic floor and it corrected much of the overnight heating/cooling disparity that I was seeing. Apparently, ASHRAE recommends excluding the floor loss in cooling design calculations, which is why it was correctly excluded in HAP. It looks like all of the difference in the peak cooling load is from the roof (yes they are defined the same), which could be explained by the difference between using HB and the sol-air temp method. Thanks everyone for all the comments, I think I’m slowly getting a more complete picture of this process.


Brendan Hall


From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Pegues, James F CCS
Sent: Thursday, July 05, 2012 5:20 PM
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Cooling Design Calc Methods

Brendan,

The ASHRAE Seminar was Seminar 21 presented on Monday morning (8:00 am session).  The presentation was by Steve Bruning – 2nd of the 3 speakers.

I think the differences you note in the 2nd paragraph may still involve comparing heat gains with loads.  Because of building heat storage, a heat gain that occurs during one hour will result in loads spread over a series of following hours.  Late afternoon solar gains, for example, will spread their loads over a series of hours which can run into the evening, after the sun has set.  If you turn the equipment off at, say 6pm, the residual heat from those occupied operating period serve to raise the air temperature in the space overnight.

To Mr. Dirkes’ point that there should not be solar loads at night.  I’d agree there should not be solar heat gains at night.  But there usually are solar loads after sunset because heat gain from the daylight hours is still going through the process of storage and heat release after the sun sets.  In a room with significant mass this can continue thru the nighttime hours.  As a qualitative check, Table 20 in Chapter 18 of the 2009 ASHRAE Handbook (Solar RTS factors) shows the decay period for a solar heat gain in a medium weight room varies from 19 to 21 hours in length.  A solar heat gain at 6pm would therefore still be converting to load at 1pm the next day.  Any heat gain with a radiative component has this type of behavior to one extent or another.

Jim Pegues

From: bldg-sim-bounces at lists.onebuilding.org<mailto:bldg-sim-bounces at lists.onebuilding.org> [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Hall, Brendan
Sent: Thursday, July 05, 2012 4:50 PM
To: David Eldridge
Cc: bldg-sim at lists.onebuilding.org<mailto:bldg-sim at lists.onebuilding.org>
Subject: Re: [Bldg-sim] Cooling Design Calc Methods

Thanks for the response, do you happen to know what the name of the presentation was or who it was that did it? It sounds like just what I’m looking for.

To address the couple points you brought up, someone else had noticed that the HAP run was running at the high end of the throttle range (75 + 1.5°F default). I think since the space I used is very small , the floor slab has a larger than usual influence (high P/A). It seemed like in the unoccupied time HAP was still applying a small load through the transfer functions but ignoring the slab loss, hence why HAP goes up but E+ goes down. The schedules are the same, I had noticed that also and I think it is because the 75° setpoint is not hit until 7am even though cooling is available and the HAP run cooling is need as soon as it is available.  It just seemed odd that they could be so different for such a simple case.


Brendan


From: bldg-sim-bounces at lists.onebuilding.org<mailto:bldg-sim-bounces at lists.onebuilding.org> [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of David Eldridge
Sent: Thursday, July 05, 2012 4:35 PM
To: bldg-sim at lists.onebuilding.org<mailto:bldg-sim at lists.onebuilding.org>
Subject: Re: [Bldg-sim] Cooling Design Calc Methods

If someone from your company attended the recent ASHRAE conference there was a great presentation on using simulation software to calculate loads. Results using a software package that calculates loads and also performs simulations showed a pretty good match, assuming thte assumptions for the space were the same. (i.e. worst case gains/schedules vs typical daily gains/schedules)

As you mention, heat balance method is the most fundamental procedure to use. There is certainly the potential for the results to be smaller numbers than what people at your company are accostomed to seeing. Your results show a pretty big difference though, for a relatively simplified case.

One item in your pilot results, for your test case it doesn’t look like the same temperatures were maintained, to get a true apples-to-apples comparison of the two methods (and to determine the load in your space for which the VAV terminal would be sized) the temperature should be maintained at the same level. Syncing those temperatures is only going to enhance the difference though, as e+ is currently a lower zone temp.

Also at the end of the occupied period the HAP temperature goes way up, and the e+ temperature drifts down (you mention heat loss through the slab? Seems like a pretty large •T over the course of two hours from the slab), and it doesn’t look like the schedule inputs are an exact match. (e+ zone control starting at 7:00 a.m. and HAP starting at 5:00 a.m., etc., e+ run seems to have been pre-cooled while HAP was setback.)

Overall I think there might need to be a further review of what the e+ and HAP inputs/outputs are, and be sure that these are consistent with the load calculation assumptions in use at your company. As Jim mentioned, the space temperatures shouldn’t be diverging under a consistent set of assumptions.

David



David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP
Grumman/Butkus Associates



From: bldg-sim-bounces at lists.onebuilding.org<mailto:bldg-sim-bounces at lists.onebuilding.org> [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Jim Dirkes
Sent: Thursday, July 05, 2012 3:22 PM
To: 'Hall, Brendan'; bldg-sim at lists.onebuilding.org<mailto:bldg-sim at lists.onebuilding.org>
Subject: Re: [Bldg-sim] Cooling Design Calc Methods

Dear Brendan,
Caveat emptor: I have not attempted this type of comparison, since my “day job” is energy modeling with E+, not load calculations.
In no particular order:

1.       I suspect you are correct to assume that E+ has a more rigorous load calculation routine, but do not know much about HAP

2.       I applaud your effort to “right size” equipment.  There is a lot of anecdotal and some published information which suggests up to 50% oversizing for cooling systems, and everyone wins if less capacity can be installed and still work properly.

3.       I have the feeling that HAP and E+ define various load differently.

a.       There’s no other reason why HAP would show a solar load in the middle of the night; that’s plainly wrong if we’re talking about true solar impact.

b.      Similarly for roof conduction, ASHRAE and thermodynamics tell us that a roof will probably radiate energy to the night sky at least in the pre-dawn hours (and thus give a negative load to the zone).  HAP is showing low, declining, but still positive values at all hours.  Not knowing your roof construction, however, I am not sure what to expect.

c.       Lighting load for E+ reflects your lighting input energy, but HAP appears to be showing something like “net load to the cooling system”

d.      Ditto for Occupant loads

e.      If it is true that E+ and HAP define things differently, I don’t think you can directly compare loads as you are doing in the “E+ loads” and “HAP loads” charts.   HAP seems to miss the lunchtime load dip altogether, for example.  Did you check the Output:Variable, zonename, Zone/Sys Sensible Load Predicted, Timestep? Or some other variable? (or check with OpenStudio or DesignBuilder to see what their graph data represents?)

4.       Floor conduction was a discussion thread on the EnergyPlus Yahoo forum about a month ago and, apparently, is a big factor in some zones.  This is a two-edged sword; if you ignore it, a major load influence may be missing.  If you include it, better define it properly!

5.       With properly or autosized systems, E+ always displays the same exactly-at-setpoint as you are showing.  I am not sure why HAP shows that the room is 1-2F away from setpoint.

6.       In the E+ Loads chart, there appears to be NO load from “Computer + Equip” and “Lighting” appears only during mid-day.

7.       In the “HAP loads” chart, Window Transmission is always a positive load; I suspect that the outdoor temp falls below zone setpoint at some time during the night, so this should be a negative load.

Executive summary:  Load calculations are the stuff of which lawsuits are made if done poorly, so you want to be well informed about how your software is responding to your inputs.  HAP was, I think, primarily a load calc program that evolved to do energy calculations.  EnergyPlus was an energy calculation program that, with proper inputs can be used for load calculations.  You may want to nose around or ask a question on the E+ forum (http://tech.groups.yahoo.com/group/EnergyPlus_Support/ ) or on the E+ Help Desk site (http://energyplus.helpserve.com/ ) for more on this topic.

From: bldg-sim-bounces at lists.onebuilding.org<mailto:bldg-sim-bounces at lists.onebuilding.org> [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Hall, Brendan
Sent: Thursday, July 05, 2012 2:43 PM
To: bldg-sim at lists.onebuilding.org<mailto:bldg-sim at lists.onebuilding.org>
Subject: [Bldg-sim] Cooling Design Calc Methods

Hey all,

I am looking at an energy plus based sim program and one of the major selling points would be doing the load calcs in the same program as the energy modeling. From what I have read, the heat balance method that E+ uses should be more accurate than the transfer function method that a widely used load calc program like HAP would use. However the results are different so I wanted to get people’s thoughts on the subject.

I did a test case, a 20 x 20 office space (384 ft conditioned area) with one southern facing window.

-          10 ft tall

-          4 People

-          1 W/ft2 Lighting

-          0.5 W/ft2 Equipment

-          ASHRAE Office default schedules

-          Basic Walls and Roof, ASHRAE minimum windows (U=.55,SHGC=.4)

-          Design Weather – 95 db / 75 wb

I’m attaching some of the results, but overall the E+ calc has a lower peak load. Occupant, lighting and plug loads seem to follow their schedules. HAP looks like it very heavily weights the delayed load effect (TFM) even though I used a medium weight (70lb/ft2) wall (fyi - changing it to a lightweight wall has some but not a huge effect). HAP also ignores the slab heat loss in its cooling calculations. This shows up most clearly in the unoccupied zone temperature. In HAP the residual loads drive the temperature up to 82 where in the E+ calc the slab loss dominates, driving the temperature into the 60s. I could see leaving it out of the peak to be conservative but then allowing the unocc temperature to jump up overnight and having to deal with that load seems a bit too unrealistic. HAP also seems to over predict the peak roof conduction gain, probably due to the use of the sol-air temperatures, which I have read can over predict gains.

I tend to want to believe the E+ analysis because I know that it’s calculation methods are in general more rigorous but I am very interested what others may think, sim engines are known for being poor design load predictors (I’m looking at you eQuest) and HAP is a very established and trusted program.

Thanks in advance for anyone that feels like diving into this with me.

Brendan Hall
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.onebuilding.org/pipermail/bldg-sim-onebuilding.org/attachments/20120706/07ea5f85/attachment-0002.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Load Calc Comparison - Adiabatic Floor.pdf
Type: application/pdf
Size: 94263 bytes
Desc: Load Calc Comparison - Adiabatic Floor.pdf
URL: <http://lists.onebuilding.org/pipermail/bldg-sim-onebuilding.org/attachments/20120706/07ea5f85/attachment-0002.pdf>