Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
EnergyPlus models the cycling of DX cooling coils through part load curves. A DX system has a startup loss and if the coil cycles, the startup losses for each on cycle are modeled through the PLF curve.
Coil:Cooling:DX:SingleSpeed,
WindACPLFFPLR, !- Part Load Fraction Correlation Curve Name
Curve:Quadratic,
WindACPLFFPLR, !- Name
0.85, !- Coefficient1 Constant
0.15, !- Coefficient2 x
0.0, !- Coefficient3 x**2
0.0, !- Minimum Value of x
1.0; !- Maximum Value of x
The coil will cycle based on the load to coil capacity ratio. For example, a 5000 W load with a 10000 W coil is cycle at part-load ratio (PLR) = 0.5. The PLF curves shows that the degradation = 0.85 + (0.5 * 0.15) = 0.925. The runtime fraction of this coil, the time is takes to startup and also meet the load, is RTF = PLR / PLF, or 0.54054. This means that the coil actually operates 54% of the time to meet a load equivalent to 50% of the capacity. As PLR decreases, there are slightly more losses and as PLR increases there are slightly less losses (no losses at PLR=1 where RTF=PLR=1). PLR is used to determine operational capacity while RTF is used to calculate the power consumed by the coil and fan. This model is roughly equivalent to operating the system based on a thermostat dead band.
If you really want to model Tstat deadband, use the EMS system to turn the system on and off at the appropriate zone temperatures. You can do this by simply setting the zone thermsotat temperature schedule values very high and low to turn the system off and on, respectively for cooling, or on and off, respectively for heating.
| | 2 | No.2 Revision |
EnergyPlus models the cycling of DX cooling coils through part load curves. A DX system has a startup loss and if the coil cycles, the startup losses for each on cycle are modeled through the PLF curve.
Coil:Cooling:DX:SingleSpeed,
WindACPLFFPLR, !- Part Load Fraction Correlation Curve Name
Curve:Quadratic,
WindACPLFFPLR, !- Name
0.85, !- Coefficient1 Constant
0.15, !- Coefficient2 x
0.0, !- Coefficient3 x**2
0.0, !- Minimum Value of x
1.0; !- Maximum Value of x
The coil will cycle based on the load to coil capacity ratio. For example, a 5000 W load with a 10000 W coil is will cycle at part-load ratio (PLR) = 0.5. The PLF curves shows that the degradation = 0.85 + (0.5 * 0.15) = 0.925. The runtime fraction of this coil, the time is takes to startup and also meet the load, is RTF = PLR / PLF, or 0.54054. This means that the coil actually operates 54% of the time to meet a load equivalent to 50% of the capacity. As PLR decreases, there are slightly more losses and as PLR increases there are slightly less losses (no losses at PLR=1 where RTF=PLR=1). PLR is used to determine operational capacity while RTF is used to calculate the power consumed by the coil and fan. This model is roughly equivalent to operating the system based on a thermostat dead band.
If you really want to model Tstat deadband, use the EMS system to turn the system on and off at the appropriate zone temperatures. You can do this by simply setting the zone thermsotat temperature schedule values very high and low to turn the system off and on, respectively for cooling, or on and off, respectively for heating.
