I am experimenting with using a setback scheme for supply air and chiller water temperatures, using Energy Plus's standard "RefBldgLargeOfficeNew2004_Chicago_SA_setback.idf" building model with weather file "USA_IL_Chicago-OHare.Intl.AP.725300_TMY3.epw". This is in Energy Plus 8.3.
The plan is that the supply air temperature setpoint will be regulated to match the warmest zone's requirements, rather than always set at 12.8 deg C. Then the chiller water temperature setpoint will be follow at 6.1 deg C below the supply air temperature setpoint, rather than being held at 6.7 deg C all the time.
The idea of this strategy is that the chiller water can rise to a higher temperature when there is low cooling load, increasing the efficiency of the chiller. In addition, if the zone air setpoints are raised, the setpoints for the supply air and chiller water will follow, and the chiller will reduce its work or even shutdown for a while. This would be useful for cutting electricity use and "coasting" in response to a short period of high electricity prices.
Concretely, I'm planning to implement this strategy in Energy Plus by replacing the SetpointManager:Scheduled controllers with"SetpointManager:WarmestTemperatureFlow for the VAV outlet nodes and SetpointManager:FollowSystemNodeTemperature for the chillers. The chillers' setpoint managers will be indexed to the temperature at the VAV_1 Supply Equipment Outlet Node, since there doesn't seem to be a way to index them to the VAV_* SAT setpoints themselves.
As a first stage, I have inserted the SetpointManager:FollowSystemNodeTemperature controllers in place of the SetpointManager:Scheduled controllers for the chillers. However, now when I run Energy Plus, I get warning messages like this:
** Warning ** CHILLER:ELECTRIC:REFORMULATEDEIR "COOLSYS1 CHILLER 1": The part-load ratio (0.000) is outside the range of part-load ratios (Y var) given in Electric Input to Cooling Output Ratio Function of Part-load Ratio bicubic curve = REFORMEIRCHILLER CARRIER 19XR 1259KW/6.26COP/VANES EIRFPLR
** ~~~ ** The range specified = 0.180 to 1.030. Environment=CHICAGO ANN CLG .4% CONDNS WB=>MDB HVAC Sizing Pass 1, at Simulation time=07/21 00:10 - 00:20
I'm not surprised that the part-load ratio would drop to 0.000 under this control strategy, but I am concerned that Energy Plus isn't recognizing this and just turning off the chillers. I think it is handling part-load ratios between 0 and 0.1 by cycling the chillers and between 0.1 and 0.2 by hot gas bypass (using the 0.2 point on the performance curve). In the default setup, I think it also manages to turn the chillers off on cool days without complaining. But for some reason, when the SetpointManager:FollowSystemNodeTemperature calls for no chilling, Energy Plus reports these errors, rather than recognizing this special case and shutting off.
Is there anything I can do to avoid this message? Or should I just ignore it? Or follow some different strategy entirely?
