I am running the prototype building energy models for medium size office buildings in energyplus. In the parametric study, I am testing the change in energy consumption with respect to an increase in cooling setpoint (72 to 77F) while keeping the heating setpoint constant 68F.
For an increase in cooling setpoint, I see the drop in cooling energy which is expected, however, most of the HVAC savings are coming from a drop in terminal electric heating energy. For example, for San Francisco, the increase in cooling setpoint from 72F to 77F is resulting in 68% drop in reheat coil energy. Am I missing something?
Normally your heating setpoint would be set a few degrees above the cooling set point. Therefore as your current off coil cooling temperature is higher then the heating setpoint it is preventing the heating units from operating.
Is your system assuming the increase in cooling setpoint will change your Air handler SAT output. Because you need less cold air your AHU will cool less, meaning your terminal units will need to heat less. Another thing to look for is to make sure that your heating setpoint didn't change. I think john is confused, cooling setpoints are supposed to be warmer than heating setpoints.
Thank you, John and Kelly.
John, in HVAC systems heating setpoint is lower than the cooling setpoint.
Kelly, that is a valid question. But no, I am not using SAT reset method. In both baseline and modified model simulations, the SAT is kept at 55F. Therefore, even if the return air is warmer with increased cooling setpoint, the central cooling coil is cooling the supply air to 55F in all cases. And yes, heating setpoint is kept the same at 68F in all simulations.
dehumidification wouldn't affect reheats if the incoming supply temp is always 55 F.
Perhaps by allowing the drift to go higher the room is allowing for more heat to accumulate, which could lower the amount of time that heating is needed because the drift is now covering an extra 5 degrees. So when the room has been allowed to get up to 77F you will have to wait for it to drift down to 68 F before the heating comes on. This could take longer, than if it the room was kept at 72 F. I'm not really sure how the program determines where your room is likely to sit on the spectrum of your setpoint deadband.
The medium office prototype uses Packaged VAV with electric reheat system.
San Francisco has mild climate. When you increase the cooling setpoint, the supply air flow rate will be reduced as well as the operation time of the system. Therefore, the cooling electricity consumption, electric reheat electricity consumption, fan electricity consumption should all be reduced.
The prototype uses autosize to determine the equipment capacity. Increasing the cooling setpoint will result in a smaller fan with smaller maximum air flow rate and electric power. The heating system uses the same fan. The air will be heated less by the fan, and it will require more natural gas to heat up the air. Therefore, the fan electricity consumption during winter time will be reduced, while the natural gas for heating will be increased.
Best Regards.
Thank all for your responses.
Fred thank you for the 'unmethours' suggestion. Lot of interesting discussions.
Yixing, I checked the sizing of VAV boxes before and after the setpoint change. You are right, the 'autosize' variable is reducing the design airflow, that drops the min 30% airflow when operating in reheat mode. Since the design airflow has dropped the min airflow dropped and that is leading to the drop in reheat energy.
After your suggestion, I changed the autosize variable and made all VAV box fixed size (according to the sizing done for 72F). This reduced the total energy savings from an increase in cooling setpoint from 72F to 77F. If my design airflow is fixed then the min airflow should also be constant and change in cooling setpoint now should not affect the reheat energy. However, after removing the autosize variable and fixing the VAV boxes I am still getting savings in reheat energy.
I have attached the fixed airflow VAV (after removing the autosize airflow variable from VAV box) file for your reference.
Baseline:FixedVAVAirflow_Setpoint72.idf
Modification: FixedVAVAirFlow_SizedFor72_Setpoint77.idf
e+ version: 8.6
Increase in cooling setpoint leading to drop in terminal heating energy. ... For example, for San Francisco, the increase in cooling setpoint from 72F to 77F is resulting in a 68% drop in reheat coil energy.
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