Hello, I am trying to simulate a simple box zone with all adiabatic surfaces aside from the roof to estimate changes in its properties on internal space energy usage. The part of the exercise I struggle with is getting a steady/ constant internal temperature (20oC) across the tested period. Temperature setpoints, program, schedules, and geometry look correct. Yet it seems like the system does not recognize cooling fully. Can you please point me towards the solution? Thanks in advance!
I should probably add an error that gets thrown when you set the cooling and heating setpoint to the same value. The cooling setpoint really needs to be higher than the heating setpoint. Even if it’s just half a degree, difference this should be fine to get energy plus to simulate correctly.
Thank you for your reply. I tried to set up the cooling set point 1 degree higher. The default roof material seems to be working fine. But when I place custom material in, it seems like the system loses the ability to autosize to the assumed set points. Would you have any suggestions where the error is?
The EnergyPlus autosize calculation is still working but the value that it’s coming up with is not good for meeting the setpoint at all hours given that just a little more solar load than the summer design day shining on such a poorly uninsulated roof is enough to push the autosized system past its limits. Putting aside the fact that any building energy code wouldn’t let you do this level of insulation for any permanent structure and assuming that this model is for something more experimental, I would suggest hard-sizing the system if you are trying to get it to work with a space like this, which is prone to big fluctuations in loads.
you assumed correctly, that’s for an experiment of “how energy is needed would change if…”. From what you are saying, I understand that there is no way to autosize such an uninsulated scenario. Do you mind elaborating more on what you mean by hard-sizing the system or pointing me towards possible threads on the forum that were touching on that?
You would just need to use a DDY with design days that are like the week of the annual energy simulation that you are running. The DDY files that ship with the EPW are just for the 99.6% worst temperature condition and they are not necessarily going to have the worst solar condition falling on a poorly insulated roof… You can build your own design days and DDY file with the Ladybug components and apply it to the energy simulation with the HB Sizing Parameter component.
Alternatively, you can hard size the ideal air system by specifying a number for the heating and cooling capacities of the ideal air system using the HB Ideal Air component.
My issue was solved by changing the components input from differentialdrybulb to noeconomizer and autosize to nolimit. The previous error came out due to my misunderstanding that “autosize” will size the system load to maintain internal conditions from setpoints, that’s what “NoLimit” achieved. When referring to description in the component it says “autosized” value will be calculated during EnergyPlus sizing. I wonder how HB comes up with it? That would be about it. Thank you for your help on that!
Glad that you figured out that you can remove all limits of the ideal air size as long as you remove the air side economizer. To explain what this means:
The default autosizing behavior is performed by EnergyPlus running a short simulation using only the conditions in the .ddy file (next to the .epw) before any annual simulation begins. This short simulation always happens before the annual one in order to determine the size of the HVAC equipment and it mimics how a mechanical engineer would go about sizing the heating and cooling equipment to meet the thermostat setpoint for almost all of the time. These sizes can have notable impacts on energy use so it is important to estimate them accurately. I say “almost all of the time” because it’s often not practical or cost effective to size a HVAC system for the worst possible hour because that hour does not last long (eg. the thermal lag of the building will help you out) and because occupants having to deal with temperatures a degree higher or lower than the thermostat for a few hours usually isn’t worth the extra thousands of dollars it might cost to beef up the HVAC further.
Granted, when you use an ideal air system with “No Limit”, there’s no need for this autosizing calculation. But, by default, the ideal air system is autosized because it includes an air side economizer. Economizers will ramp up the outdoor ventilation air for cases where the room needs cooling and the outdoor air is cooler than the room air. This oftentimes saves a lot on annual cooling energy and this is why economizers are pretty standard requirements in contemporary energy codes (and why it is the default for the ideal air component). However, if the amount of outdoor air that you could mix in through the economizer had “No Limit,” then it’s unclear how much outdoor air can be used to cool down the room to cool it down. So this is why we need to autosize the default ideal air system using the criteria in the .ddy file.
