Hi everyone,
I am trying to implement an economizer based on OA enthalpy on a baseline model that has an economizer set on OA temperature and have issue because I have an unexpected increased in gas usage on the proposed case.
The system I am working on consists of a VAV box containing a heating and cooling coil respectively attached to a Hot water loop and a chilled water loop (operating from March to October). It has terminal reheat coils at the zone level.
-The baseline economizer system is set on Outdoor air temperature high limit of 50F because of the low discharge air temperature of 52F.
-The proposed case economizer is set on an enthalpy high limit of 23BTU (and an OA temperature high limit of arbitrarily 100F because we want the economizer to be only limited by the enthalpy). The discharge air temperature was also reset because the system will be economizing with air temperature higher than the initial discharge temperature of 52F. It was reset from 52F to 63F as the Outdoor temperature varies from 52 to 73F.
Results:
-1st run: When running the discharge air setpoint reset based on OAT alone, we obtain substantial electrical and gas usage gain.
-2nd run: When running the discharge air setpoint reset based on OAT + the economizer based on OA enthalpy, we obtain even higher gain in electrical consumption but a lower gas usage gain.
Results numbers:
Baseline:
Elec: 14,582,599 kwh
Gas: 646,752 therms
1st run:
Elec: 14,439,619 kwh
Gas:587,782 therms
2nd run:
Elec: 14,319,200kwh
Gas: 607,876 therms
I do not understand why there is a gas usage increase on the 2nd run in comparison to the 1st run.
Has anyone been through the same issue? It would be very helpful to share any kind of info.
Thank you for your time,
Regis Bruant
Regis,
I have a theory why your gas consumption goes up when you incorporate the discharge air temperature reset, but I don't know how to get around it. Perhaps someone more familiar with the DOE-2.2 code and algorithms can chime in. I believe it has to do with the difference between discharge air temperature and mixed air temperature.
Based on the economizer examples in the DOE-2.2 documentation "Volume 3: Topics" (p. 121-124) the economizer is controlled to a mixed air temperature setpoint. (TAPPXX in the FORTRAN code below.)
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I suspect that the economizer control is not changed by resetting the discharge air temperature setpoint. Perhaps the mixed air temperature setpoint is back-calculated from the discharge air temperature setpoint by subtracting the fan heat. I don't know for sure. Raising the discharge air setpoint might be causing the heating coil to come on and warm the air from the mixed air temperature to the higher discharge air temperature. That's my theory.
I have not found a way to access the mixed air temperature setpoint through eQUEST. Do any of you people more familiar with DOE-2 know of any BDL lines that could be added to the .inp file to adjust the mixed air temperature setpoint so that the economizer will not overcool?
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Keith Swartz, P.E., LEED AP