VRF heating issue: same heating power with different compressor speed at low heating load #10427
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Defect
Includes code to repair a defect in EnergyPlus
Issue overview
Cycling issue at low speed for heating mode
From 12:30 to 13:00, the compressors are running at the lowest speed in both cases, zone temperature are the same but both system have cycling ratio = 1. This shouldn't happen as they have different compressor curves corresponding to the lowest speed.
From 1/12 14:40-16:40, the the two files with different minimum speed both have cycling ratio = 1 and the same zone air temperature, under the same outdoor condition.
The defect files are attached here
simulation_with_different_speeed_update.zip
The problem is likely related to two issues in the cycling ratio calculation.
Similar to the cycling ratio problem in the cooling side. Whether the demand is less than minimum speed capacity is checked at two places. The first one will short-circuit and prevent the Te adjustment step (orange box in the following simplified flow diagram)
The following is the diagram in the documentation
When the demand is very low, the cycling ratio is calculated as follows. When Q_h_TU_PL <= CompEvaporatingPWRSpdMin, it will enter branch 1. When Q_h_TU_PL > CompEvaporatingPWRSpdMin, it will enter branch 2. C_cap_operation is generally close to 1. However, when Q_h_TU_PL is close to CompEvaporatingPWRSpdMin, depending on which branch it ended up in, the result could be very different. If we assume CompEvaporatingPWRSpdMin and CompEvaporatingCAPSpdMin have similar magnitude, and C_cap_operation is indeed close to one,
Defect files
US+SF+CZ4A+hp+crawlspace+IECC_2006_VRF_minSpd900.idf.txt
US+SF+CZ4A+hp+crawlspace+IECC_2006_VRF_minSpd1320.idf.txt
USA_NY_New.York-John.F.Kennedy.Intl.AP.744860_TMY3.epw.txt
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