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Re: [EnergyPlus_Support] Re: Unitary DX for cooling - not sizing well
Inserted answers......
On 6/13/2012 9:13 AM, Jean marais wrote:
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> 1 minute pulldown time is unrealistic, i aggree. As I understand it, this influences only the Air Volume Flow Calculation, right. Meaning the air volume flow rate capacities of the equipment will be too large.
Correct. For DX, air volume is used to set capacity based on delta
enthalpy across the coil at peak load time.
> What is a realistic pull down rate? 10 min? 1hr? As stated before, ASHRAE requires less than 300 hrs per year, for hours SP not met. I could run a parametric simulation testing different averaging intervals to get best results and set the Averaging Window this way. But no one else is doing this, so how do they size?
In my home, I am lucky to get 1F / hr, but my AC is undersized. I think
2 or 3 F per hour is realistic.
Is it 300 hrs occupied or any time? If you start the AC early enough,
the SP is met before occupants arrive. If it's anytime, then pull down
is an issue.
> Also, I would have to decide on a fixed tolerance for the SP not met reporting, before the parametric study. James uses 0.5ºC from what I remember. The default is 0.2ºC (0.5ºF from DOE). Allowed by ASHRAE 55 is 1.1ºC (2ºF) peak to peak variation over a 0.25hr period. Minimum throttling range for low-temperature radiant systems is 0.5ºC.
I'm not sure I understand "peak to peak variation over 0.25 hrs up to
1.1C". What's wrong with using Std 55 max tol for SP not met?
>
> ASHRAE 90.1 2010 defines
> temperature control throttling range:
> as the number of degrees that the room temperature must change to go from no-heating to full heating or from no-cooling to full cooling.
> unmet load hour:
> an hour in which one or more zones is outside of the thermostat setpoint plus or minus one half of the temperature control throttling range. Any hour with one or more zones with an unmet cooling load or unmet heating load is defined as an unmet load hour.
Now here they talk about any "hour". Since E+ has sub-hourly time steps,
and counts during the fractional hours, I guess it's fair to include
sub-hourly "not met" hours in the total. If hrs are counted when
unoccupied, then the pull down is important. For hourly simulations
(DOE-2), maybe that first hour's "avg" zone temp gets closer to the SP
and hrs not met are smaller? Should the "hrs not met" be increased for
sub-hourly simulations for this very reason (not likely to happen)?
What would happen if the AC is scheduled on all year (wkend and wkday)?
With a constant set point (no SB)? Both the baseline and proposed would
see the same schedules, and then energy savings could still be compared
(energy magnitude would increase, % savings would decrease).
I don't do 90.1 simulations, so I am sure someone will tell me if I am
way off the mark with those last comments.
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