[EnergyPlus_Support] Re: Peak cooling loads

["omarvelling" <laxoayi@xxxxxxxxxxxxxxxx>]

Thanks for refering me to the ASHRAE 2009 HOF. It decsribes the heat balance method (HB) used in EnergyPlus for calculating space loads. This is stated in the EP engineering reference on page 340 under "Zone Design Loads and Air Flow Rates" but with no further descritions. I can now understand why results of sizing runs and simulation runs do not match even if the same weather data and schedules are used. If i got it right, the HB method contains some assumptions that do not exist in the procedures used in EP simulation runs, hence the discrepancies.


--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Dru Crawley <dbcrawley@...> wrote:
>
> Unless you've specifically made the schedules for design days the same as
> the days that they fall on, there can  be differences there (search for Day
> Type, about pg 110 of the I/O Ref).  In addition, the calculations are
> different (but from what I found, that isn't in the documentation...):  it's
> described as the ASHRAE Design Day method.
> 
> If you go to the ASHRAE 2009 Fundamentals... it describes the process.
> Chapter 18, page 28 talks about the assumptions in the heating load
> calculations:
> 
> - design interior and exterior conditions
> - including infiltration and ventilation
> - credits (heat gains) for solar or internal heat gains (people, lights,
> equipment) are not included
> - thermal storage effects of building structure and contents are ignored --
> heat transfer through the envelope is instantaneous.
> 
> 
> 
> 
> 
> On Thu, Sep 22, 2011 at 10:14 AM, omarvelling <laxoayi@...>wrote:
> 
> > **
> >
> >
> > Dr. Li and Dru, thanks for your comments.
> > I can appreciate that the temperature profiles for outdoor dry-bulb and
> > solar radiation pattern are not the same for the designDay and the weather
> > file data and so the results will not match. However, I'm not using a
> > designday for sizing, i'm actually using sizingperiod:weatherfiledays to
> > perform sizing and so would expect the results to match for the same
> > date/time. Any reasons why this is not the case? Thanks
> > Omar
> >
> > --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Dru Crawley <dbcrawley@>
> > wrote:
> > >
> > > Dr Li is correct -- in the schedules you can have different conditions
> > for
> > > the design days and the design day calculations can omit some of the
> > loads.
> > > In addition, the climatic condtions are based on the weather parameters
> > that
> > > you enter -- not the weather file (unless you use the weather file option
> > > for design conditions). And the solar radiation is calculated as clear
> > sky
> > > (default).
> > >
> > > To see what is going on, include the design days in the simulation output
> > --
> > > and select environment conditions such as temperature, wind speed, solar
> > > radiation for the design days -- and for the same date in the weather
> > files.
> > > You'll see big differences.
> > >
> > > On Wed, Sep 21, 2011 at 4:35 PM, YuanLu Li <yli006@> wrote:
> > >
> > > > **
> > > >
> > > >
> > > > I do not think the results would be the same, as they are averaged
> > values
> > > > for different input data..
> > > >
> > > > The peak load conditions for the building may be different for the same
> > > > day, because the temperature profiles for outdoor dry-bulb and solar
> > > > radiation pattern are not the same for the designDay and the weather
> > file
> > > > data.
> > > >
> > > > Compare the input data as well, to see whther they are the same. If you
> > > > made the designDay profile to track the extreme day weather file ODT,
> > it may
> > > > give you a better match.
> > > >
> > > > Dr. Li
> > > >
> > > > ------------------------------
> > > > To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> > > > From: laxoayi@
> > > > Date: Wed, 21 Sep 2011 19:33:27 +0000
> > > > Subject: [EnergyPlus_Support] Re: Peak cooling loads
> > > >
> > > >
> > > >
> > > > Thanks Jean, I've reached the same conclusion as yours so I tried to
> > > > perform one more run with both the sizingperiod:weatherfiledays and the
> > > > runperiod limited to the same single day. I then changed the "Number of
> > > > Times Runperiod to be Repeated" to 20 (the results actually converged
> > after
> > > > 15 repetitions). Despite this, the value of the sizing peak cooling
> > load did
> > > > not match that of the simulation although peak loads occurred at the
> > same
> > > > date and time. Anyways I now think I have a better understanding of
> > what is
> > > > going on thanks to you.
> > > >
> > > > --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Jean marais" <jeannieboef@
> > >
> > > > wrote:
> > > > >
> > > > >
> > > > > Okay, sorry. As far as I know the .eio file shows the design loads
> > used
> > > > for sizing your equipment, i.e. from your sizing run. I'm not sure what
> > > > sizing parameters sizingperiod:weatherfiledays uses (it may use the
> > worst
> > > > case day from the weather file and simulate it over and over during
> > > > sizing...there are also summer weeks data where the week is possibly
> > > > repeated, however it surely doesn't repeat years untill the start of
> > the
> > > > year converges with the end of the year which is what you'd have to do
> > to
> > > > get the sizing max load values the same as the max loads in the year.
> > as the
> > > > statistical weather year does not include the 1/100 year hot day,
> > unless you
> > > > tell it to...there is a way I think, most systems are not sized this
> > way.)
> > > > >
> > > > >
> > > > >
> > > > > --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "omarvelling" <laxoayi@>
> > > > wrote:
> > > > > >
> > > > > > Thanks Jean, I appreciate what you mentioned in your reply but I'm
> > not
> > > > trying to size any system here, I'm only looking for peak cooling loads
> > and
> > > > that is why I used sizingperiod:weatherfiledays instead of
> > > > sizingperiod:designday to determine the date/time that the peak load
> > occurs
> > > > on and its magnitude. Bearing that in mind, should'nt I expect to find
> > the
> > > > value of the peak load in the .eio file to match the value of the ideal
> > > > loads air system sensible cooling rate in the output variable file at
> > the
> > > > same timestep?
> > > > > >
> > > > > > Thanks a lot, your help is very much appreciated,
> > > > > > Omar
> > > > > >
> > > > > > --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Jean marais"
> > <jeannieboef@>
> > > > wrote:
> > > > > > >
> > > > > > > The peak cooling load during your statistical weather year is
> > "most
> > > > probable". The design day is used to size your systems for that once in
> > a
> > > > hundred years hot day (the same day occuring over and over again untill
> > > > building temperatures at the beginning and end of the day equal/match
> > those
> > > > of the day before and after) x sizing:parameter factors. the design day
> > > > cooling loads are therefore much more than the statistical year's worst
> > day.
> > > >
> > > > > > >
> > > > > > > --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "omarvelling"
> > <laxoayi@>
> > > > wrote:
> > > > > > > >
> > > > > > > > Hi all,
> > > > > > > >
> > > > > > > > I'm trying to determine the peak cooling loads for my model. I
> > used
> > > > weather file days as my sizing period and also performed an annual
> > > > simulation using an ideal loads air system to control the zones. I
> > checked
> > > > the time of the peak design sensible cooling load from the .eio output
> > file
> > > > and then checked the output variable file but found the value of the
> > ideal
> > > > loads air sensible cooling rate to be different from that of the .eio
> > file
> > > > at the same timestep. Is this normal? Am I missing something here? Why
> > do
> > > > the values of the sizing differ from the annual simulation although I'm
> > > > using weather file days to perform sizing?
> > > > > > > >
> > > > > > > > Thanks
> > > > > > > >
> > > > > > >
> > > > > >
> > > > >
> > > >
> > > >
> > > >
> > > >
> > >
> >
> >  
> >
>




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