EnergyPlus assumes perfect control, i.e. set-points are met and followed instantaneously, subject only to capacity constraints. At the local loop level, a well-tuned PI (or PID) controller that controls equipment that responds quickly relative to the time-step will closely approximate this perfect control. So, in effect, EnergyPlus implements PI control for most HVAC control loops - exceptions include thermal storage tank control and zone temperature control. Most modern digital control systems in buildings are set up to implement PI control - how well these loops are tuned in practice is another matter.
Most of the pneumatic control systems found in older buildings implement proportional-only (P) control. The consequence of this is that the controlled variable is not controlled to a fixed set-point but varies across the 'proportional band', depending on the load. Consider a heating coil in an air handler that produces a 10 K temperature rise: if the supply air temperature set-point is 30 C and the proportional band is 5 K, the supply air temperature will be 30 C only if the load is zero, i.e. if the temperature of the air entering the coil is 30 C. If the entering air temperature is 15 C, the valve needs to open fully to meet the load. However, proportional-only control requires that the supply air temperature be (30 - 5) = 25 C in order for the valve to be commanded to be fully open. Thus, in this example, the supply air temperature varies from 25 C at fully load to 30 C at zero load. A bias can be introduced into the controller to shift this range, e.g. to 27.5 - 32.5 C or to 30 - 35 C, but the range is always determined by the proportional band. The proportional band can be reduced in the controller but the risk of hunting (oscillation) increases as the proportional band is increased. DOE-2 uses proportional-only control, reflecting industry practice when the program was first developed. (N.B. The proportional band should not be confused with the dead-band between heating and cooling.)
It MAY be possible to model proportional-only control in EnergyPlus using EMS but I haven't tried it. I'd be interested to hear from anyone who has tried it, or tries it in the future. A further complication is that there is often a non-linear relationship between the control signal and the equipment output. This can be reduced through good design, e.g. appropriate control valve selection for water coils, but this often not done in practice, making this an issue in modeling existing buildings.
Phil
On 5/15/2012 11:01 PM, ohchoen wrote:
Hi, I'd like to know that it is possible P or PI Control using EP.
is it possible??
-- Philip Haves Leader, Simulation Research Group Lawrence Berkeley National Laboratory One Cyclotron Road, MS 90R3111 Berkeley, CA 94720-8134 (510) 486 6512 (office) (510) 486 4089 (fax)
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