Large discrepancy between modelled HVAC and district heating/cooling

asked 2016-03-01 08:28:40 -0500

Sanya
21 ●1

I am trying to optimise exterior shading design for an office building. To make a head start with building and climate, I have chosen large office building from the ASHRAE's 90.1-2013 Commercial Building Prototype Models, to be simulated with Chicago climate. Further, in order to speed up simulations as I'd like to employ genetic algorithm in optimisation, I have decided to delete all HVAC related details from the model and replace them with IdealLoadsAirSystem for each zone, such as:

ZoneHVAC:IdealLoadsAirSystem,
Basement Purchased Air, !- Name
, !- Availability Schedule Name
Basement Inlet Nodes, !- Zone Supply Air Node Name
, !- Zone Exhaust Air Node Name
50, !- Maximum Heating Supply Air Temperature {C}
13, !- Minimum Cooling Supply Air Temperature {C}
0.015, !- Maximum Heating Supply Air Humidity Ratio {kgWater/kgDryAir}
0.01, !- Minimum Cooling Supply Air Humidity Ratio {kgWater/kgDryAir}
NoLimit, !- Heating Limit
, !- Maximum Heating Air Flow Rate {m3/s}
, !- Maximum Sensible Heating Capacity {W}
NoLimit, !- Cooling Limit
, !- Maximum Cooling Air Flow Rate {m3/s}
, !- Maximum Total Cooling Capacity {W}
, !- Heating Availability Schedule Name
, !- Cooling Availability Schedule Name
ConstantSupplyHumidityRatio, !- Dehumidification Control Type
, !- Cooling Sensible Heat Ratio {dimensionless}
ConstantSupplyHumidityRatio, !- Humidification Control Type
, !- Design Specification Outdoor Air Object Name
, !- Outdoor Air Inlet Node Name
, !- Demand Controlled Ventilation Type
, !- Outdoor Air Economizer Type
, !- Heat Recovery Type
, !- Sensible Heat Recovery Effectiveness {dimensionless}
; !- Latent Heat Recovery Effectiveness {dimensionless}

The problem now is that there is a large discrepancy between results obtained for heating and cooling loads. The initial ASHRAE model requires 10.8 GJ of electricity and 5325.81 GJ of natural gas for heating, and 2748.36 GJ of electricity for cooling. However, the IdealLoadsAirSystem version reports only 550.65 GJ for district heating and 21417.95 GJ for district cooling. Hence almost 10 times less energy for heating and almost 8 times more energy for cooling?! What am I doing wrong here? Convincing demystifications are welcome! What has to be done to keep IdealLoads (and shorter simulation times), yet to obtain more realistic ratio of heating/cooling loads?

If you're interested, you can see the modified IdealLoads version of the model at https://www.dropbox.com/s/3xcsr4ighrg...

Thanks in advance, Sanya Stevanovic

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answered 2016-03-01 09:48:39 -0500

Annie Marston
4297 ●4 ●31 http://www.resustain.com

I am not sure how you calculated your first energy results. If you were using the model that was provided and didn't change anything and looked at the energy usages then these aren't the loads, these are end use energy with all the inefficiencies included. The ideal heating loads object does not include inefficiencies, it assumes that everything is working perfectly, no heating and cooling at the same time, boilers and chillers have a COP of 1 and so on.

The other thing you might want to check on is that the amount of outdoor air your model is bringing in is the same in both cases. This could also be causing some discrepancy.

These are my first two thoughts. I hope they help a little bit.

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My main concern is that EnergyPlus reports district cooling end use to be approximately 40 times larger than that of district heating, when IdealLoads is used instead of actual HVAC (which more expectedly reports heating end use to be approximately twice as much as cooling end use). The outdoor air is the same as in the original ASHRAE model. I do not see enough COPs around that would be able to explain this.

Result table for modified model, if needed, is here: https://www.dropbox.com/s/6q23ibfck4a...