Optimum start/ stop
To save energy air conditioning systems should start at the latest possible time and should stop as soon as possible.
The optimum start/ stop function makes use of the EOV statement to provide optimum start/ stop functions. The comfort zone is defined by the setting OPT-05 and shifts with the calculated room temperature setpoint "CalcRoomTmpSp".
Time schedule preparation
When you configure the time schedule, set the switching points for system start/ stop to the latest possible time because the strategy automatically advances these times, if necessary.
CARE: You must set the optimization flag of the occupied period "Occ_Period" to "yes" in the time schedule.
WARNING: Be sure, that the optimizing flag of the datapoint "Occ_Period" is enabled within the controller time schedule, otherwise the optimum start/ stop and the automatic pre-start function of the night purge routine will not be activated.
AX/ NX: the optimization is continuously ON. To switch off the optimization, set the input ""OptimEnable" to 0.
Automatic changeover from heating to cooling season
Within this Plant Strategy macro there is a long-period filter time averaging, defined by parameter OPT-07, which is used to define whether the optimizing-statement EOV should calculate optimum start/ stop for heating or for cooling. When the average outside air temperature, e.g. over 3 days, equals a value higher than the value, defined by parameter OPT-08, then optimum start/ stop will act for cooling. When the value of the averaged outside air temperature equals OPT-08 minus a fixed differential of 0.1K, then optimum start/ stop will act for heating.
An example of the smooth filtered outside air temperature is shown in the next figure.
Description |
Default value |
OPT-07: long period filter time htg-/ clg-mode |
72 h |
OPT-08: long period filter switch-point htg-/ clg-mode |
15 °C |
Enable Optimization
The optimum start/ stop for heating and cooling will be enabled based on the input "OptimEnable" and outdoor temperature
The "OptimEnable" signal is available in the control strategy macro with comfort zone and transmits the following values:
For other strategies enter the value value 999 at this input.
The optimization for heating will be enabled if:
The optimization for cooling will be enabled if:
Enable optimum start heating
Optimum start for heating will be enabled if all of the following conditions are true::
Optimum start heating will end:
When the room temperature enters the comfort zone, after optimum start has been executed, the optimum start function will be terminated after the value of the setting OPT-06 has been passed.
During the optimum start condition, the "PlantMode" will show a value of optimum start ("1"). By adjusting the parameter, it is possible to alter the comfort zone, as shown in the next figure.
Note:
If no boost termination is required, the value of the setting OPT-05 should be set at least to a value which equals the value of 2* OPT-06.
Enable optimum stop heating
Optimum stop for heating will be enabled if all of the following conditions are true::
Enable optimum start cooling
Optimum start for cooling will be enabled if all of the following conditions are true::
Optimum start cooling will end:
Enable optimum stop cooling
Optimum stop for cooling will be enabled if all of the following conditions are true::
Calculated room temperature setpoint
The Plant Strategy macro uses of the internal datapoint "CalcRoomTmpSp", which is provided by the Control Strategy-macro. During unoccupied periods (PlantMode = 0, 1, 2 or 3), it is possible that an additional signal will be received via the signal "OptimEnable".
Note: When you use another control strategy without comfort zone, enter a the value 999 at the input "OptimEnable" to enable the optimization based on heating-/ cooling-modes which depend on the long term filtered outside air temperature as described in the chapter - "Automatic changeover from heating to cooling season".
Optimized start-up in heating mode
When the system is in heating mode, optimum start displaces the switch-on time, to guarantee that the room temperature setpoint is reached by start-up. The displacement of the switch-on point depends on the difference between the room temperature setpoint and actual room temperature. The optimum start assumes a linear room model.
Setting EOV-01 sets the lowest pre-heat time before start-up. The maximum value of EOV-01 is limited to 1080 min (18 hours). The minimum limitation of early switch-on time is 0 minutes. When the room temperature setpoint corresponds exactly to the actual room temperature, the system start-up matches the switching point in the time schedule. Between these two limits, advancement of system start-up is calculated with the following formula:
Early start-up = (CalcRoomTmpSp - RoomRetAirTemp - 1/2 comfort zone) * EOV-02 (preheat factor)
The preheat factor indicates how many minutes the system requires to compensate for a room temperature deviation of 1 degree. Defined by the setting EOV-02, but adaptation can independently correct it. See chapter "Adaptation of factors".
Optimized stop in heating mode
If the time schedule contains a switching point that shuts down the air-handling unit while it is in heating operation, optimum stop calculates an early switch-off time to save heating energy.
Optimum stop calculates the early shutdown time using a linear characteristic curve. The early switch-off time takes its maximum value, if the outdoor temperature is equal to the room temperature setpoint. In this case, the building losses are zero and the room temperature will stay equal to the setpoint value even if the air-handling unit is shut down. There is no optimum stop if the outdoor temperature is less than or equal to the minimum outdoor temperature (parameter EOV-03). Between these limits, optimum stop calculates the early switch-off time as follows:
Stop time = (EOV-01 + tcorr)* (outdoor temp. - EOV-03)/(CalcRoomTmpSp - EOV-03)
Where tcorr = (1/2 comfort zone + RoomRetAirTemp - CalcRoomTmpSp) * EOV-04 (optimum stop factor)
The correction factor equals room temperature minus setpoint multiplied by the optimum stop factor. The correction factor changes the slope of the previously defined characteristic curve as a function of the control difference: If the room temperature equals the room temperature setpoint, the characteristic curve remains unchanged. If the room temperature is greater than the room temperature setpoint, the characteristic curve is steeper and the system shuts down earlier. If room temperature is less than the room temperature setpoint, the characteristic curve is less steep and the system shuts down later. The following figure illustrates this relationship.
The setting EOV-04 (optimum stop factor) weights the influence of the control difference on the characteristic curve. If the setting EOV-04 is zero, optimum stop calculates early shutdown based on outdoor temperature only.
Optimized start-up in cooling mode
When the system is in cooling mode, optimum start displaces switch-on point to guarantee that the room temperature setpoint is reached at start-up. The displacement of the switch-on point depends on the difference between the room temperature setpoint and actual room temperature. Optimum start assumes a linear room model.
The minimum pre-cool time is set by EOV-05 (lowest cooling time for opt. cooling). EOV-05 has a range of 0 to 1080 minutes. When the room temperature setpoint corresponds exactly to the actual room temperature, and EOV-06 is set to zero, the system start-up matches the switching point in the time schedule. Between these two limits, optimum start calculates the pre-cool time with the following formula:
Pre-cool time = (1/2 comfort zone + RoomRetAirTemp - CalcRoomTmpSp) * EOV-06 (pre-cool factor)
The pre- cooling factor indicates how many minutes the system requires compensating for a deviation of 1 K. The pre-cool factor is defined by the setting EOV-06, but optimum start can independently correct it. See Adaptation in this section.
Optimized stop in cooling mode
If the time schedule contains a switching point that shuts down the air-handling unit while it is in cooling operation, optimum stop calculates an early switch-off time to save cooling energy.
Optimum stop calculates the early shutdown time using a linear characteristic curve. The early switch-off time takes its maximum value, if the outdoor temperature is equal to the room temperature setpoint. In this case, the building losses are zero and the room temperature will stay equal to the setpoint value even if the air-handling unit is shut down. There is no optimum stop if the outdoor temperature is greater than or equal to the maximum outdoor temperature (parameter EOV-07). Between these limits, optimum stop calculates the advance time as follows
Stop time = (EOV-05+ tcorr)* (EOV-07- outdoor temp.)/(S.3.18 - CalcRoomTmpSp)
Where tcorr = (CalcRoomTmpSp - RoomRetAirTemp - 1/2 comfort zone) * EOV-08 (optimum stop factor)
The correction factor equals setpoint minus room temperature multiplied by the optimum stop factor. The correction factor changes the slope of the previously defined characteristic curve as a function of the control difference: If the room temperature equals the room temperature setpoint, the characteristic curve remains unchanged. If the room temperature is less than the room temperature setpoint, the characteristic curve is steeper and the system shuts down earlier. If the room temperature is greater than the room temperature setpoint, the characteristic curve is less steep and the system shuts down later. The following figure illustrates this relationship.
The setting EOV-08 (optimum stop factor) weights the influence of the control difference on the characteristic curve. If the setting EOV-08 is zero, optimum stop calculates early shutdown based on outdoor temperature only.
Adaptation of factors
For the early switch-on and switch-off time calculation of the switch-on point in heating/ cooling operation, optimum start/ stop uses a linear model of controlled operation. In the heating mode, the setting EOV-02 (pre-heat factor) defines this model. The factor indicates how much time is required to overcome a control difference of 1K. In the cooling mode, the setting EOV-06 (pre-cool factor) defines this model. The two modes require different curves because cool-down and heat-up occur at different rates. For example, in a factory, the factor for rapid cool-down is always larger than the factor for rapid heat-up because cooling down by 1K takes longer than heating up by the same amount. This effect is because the heat from machines supports heating up, while it works against cooling down. The strategy can adapt both factors automatically to the actual circumstances. This adaptation occurs when you select "New start" or enable for the factor adaptation with the setting EOV-09. The strategy adapts the factors with decreasing weighting, that is, the new factors calculated after the first optimization may change EOV-02/ EOV-06 significantly, the factors calculated after the second optimization somewhat less, and so on. After the first successful optimization, the strategy sets EOV-09 to enable (0). You can prevent or interrupt adaptation by setting EOV-09 to disable (1).
Description |
Default value |
OPT-01: optimum start heating; 1 = enable |
1 |
OPT-02: optimum stop heating; 1 = enable |
0 |
OPT-03: optimum start cooling; 1 = enable |
0 |
OPT-04: optimum stop cooling; 1 = enable |
0 |
OPT-05: comfort zone around on calculated room setpoint |
4 K |
OPT-06: comfort zone diff. for boost termination |
1 K |
OPT-07: long period filter time htg-/ clg-mode |
72 h |
OPT-08: long period filter switch-point htg-/ clg-mode |
15 °C |
EOV-01: minimum preheat time for heating |
0 min |
EOV-02: preheat factor |
10 min/K |
EOV-03: minimum OAT for optimum stop (htg mode) |
10 °C |
EOV-04: optimum stop factor for heating |
10 min/K |
EOV-05: minimum pre-cool time for cooling |
0 min |
EOV-06: pre-cool factor |
10 min/K |
EOV-07: maximum OAT for optimum stop (clg mode) |
25 °C |
EOV-08: optimum stop factor for cooling |
10 min/K |
EOV-09: adaptation 0 = enable |
2 |
Display parameters
The calculated start and end time of optimum start, as well as the last optimum start date are displayed by parameters DISP-01, DISP-02 and DISP-03 respectively.
Description |
Default value |
DISP-01: start time optimum start |
- |
DISP-02: end time optimum start |
- |
DISP-03: date last optimum start |
- |