Dr. Li
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: ecoeficiente@xxxxxxxxx
Date: Sat, 8 Sep 2012 09:23:43 +0000
Subject: [EnergyPlus_Support] Re: Problem with Peak results in Summary Reports: a suggestion
Dr. Li,
how do you get peak values in a simulation?
The reson why I asked, because I was not sure which peak value you wanted to use for equipment sizing.
I have seen peak values being extracted by users from hourly simulation (which is an hourly averaged value) or in E+, from zone timestep results, which can typically be 10 minutes.
The fact is that if you use system timestep ("detailed" option in results), peak values can reach considerably higher values (several times hourly values), specially during startups.
These start-up trantients are not real sometimes. I do not know what is the equivalent of electrical inductance in thermal conponents. Thermal mass is equivalent to electrical capacitance. PCM can be simulated as a large capacitance at the critail temperature. The actual start up protectin of the HVAC equpment has nothing to do with zone demand change.s
In a real system, when the thermostat is turned on to start a system, there are a number of steps to be initiated before the source power is activated. These are not in the simulation.
The important point to note is that the transient peaks are never used for equipment sizing. You can include components to reduce the peaks. In digital mathematics, some of these peaks are not real. They are produced by the algorithm used in the sampling and interpolation process.
I think that using peak values from system timestep (which can go down to 1 minute) for system sizing you can be clearly oversizing the equipment. You could average a one minute peak value with the following minutes to a smaller equipment size without hardly affecting building response or comfort.
Transient peaks may not general hourly averaged c"omfort not met" warning.
Obviously, you can analyze the transient curve from the results in order to optimize the sizing but designers with a 400 zones building, cannot go zone by zone in such detail. If they look at the summary report, they get the peak in each zone. What I mean is that this peak can be overestimated if taken from system timestep, which is the case in Summary Reports.
If you have a 400 zone building, it is very unlikely that all the thermostat will be synconized to operate at the same time. The morning start-up of a real system is from the plant side, and it is not what you will see in the Eplus simulation. Therefore, do not worry about the transients in simulation..
Regards,
Germán.
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, YuanLu Li <yli006@...> wrote:
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> Why do you want to average the peak values after simulation. You already know that the statistical property of TMY in DesignDay is 0.4% 1%, etc. How do you define the average value of peak values? What is the "problem of simulation peak values in Summary Reports"? You can increase the size of your system or plant by using the multipler, if you wish. TMY peak is not the maximum peak of your local real weather condition. There may be larger peak values in the real weather condition. So you should design in some overload protection for your real system. It is not a simulation problem. For a large real system, the start up procedure is not by simply turn every power swithc on at the same time. Eplus provides you with the peak start up requirement, so that you can design the real system to minimize the transient effect. Dr. Li
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> From: ecoeficiente@...
> Date: Fri, 7 Sep 2012 12:32:21 +0000
> Subject: [EnergyPlus_Support] Re: Problem with Peak results in Summary Reports: a suggestion
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> Sorry Jean but peak values in summary reports come from Simulation results, not sizing results. I know the option for load averaging but it is only available for sizing calculation so it does not solve the problem of simulation peak values in Summary Reports.
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> Germán
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> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Jean marais" <jeannieboef@> wrote:
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> > This is implimented in the sizing:parameters object. It is the last field, Load Averaging Window Time. See the io ref as it is time step length dependant, not time dependant directly.
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> > Jean
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> > --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "ecoeficientees" <ecoeficiente@> wrote:
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> > > I have found that peak values in Summary Reports, which could be useful for HVAC design can lead to extremely high values usually due to system startups (e.g.after an unoccupied weekend) and the fact that the results come from system timestep.
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> > > I think it could be useful to be able to setup a time window for the peak values (to average them) so that Summary Reports could be more useful for HVAC design (when the designer is interested in peak values from simulation, not from designday sizing).
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> > > Just an idea for E+ developers...
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> > > Regards,
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> > > Germán.
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