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Zone Ideal Loads mismatch

asked 2018-08-13 11:21:37 -0500

Tobias gravatar image

updated 2018-08-21 09:32:53 -0500

Hi guys

I can't get the following equation (for both heating and cooling) to match in my simulations (example file idf):

Zone Ideal Loads Supply Air Total Cooling Energy + Zone Ideal Loads Heat Recovery Total Cooling Energy

= Zone Ideal Loads Zone Total Cooling Energy+ Zone Ideal Loads Outdoor Air Total Cooling Energy

<=>

2312.1212797468634 + 0 = 8598.715438236986 + 77.11803171210939 [kWh/year]

EnergyPlus version 8.5

Source


----- Update ----

Have the same issue in version 8.9, also for both heating and cooling

idf


+++++ Info +++++

HVAC objects from expanded idf:

ZoneHVAC:IdealLoadsAirSystem,
   ZONENAME Ideal Loads Air System,            !- Name
   ,                                                     !- Availability Schedule Name
   ZONENAME Ideal Loads Supply Inlet, !- Zone Supply Air Node Name
   ,                                                !- Zone Exhaust Air Node Name
   40,                                   !- Maximum Heating Supply Air Temperature [C]
   13,                                   !- Minimum Cooling Supply Air Temperature [C]
   0.008,                !- Maximum Heating Supply Air Humidity Ratio [kg-H20/kg-air]
   0.0085,              !- Minimum Cooling Supply Air Humidity Ratio [kg-H20/kg-air]
   NoLimit,                          !- Heating Limit
   ,                                    !- Maximum Heating Air Flow Rate {m3/s}
   ,                                     !- Maximum Sensible Heating Capacity {m3/s}
   LimitFlowRate,               !- Cooling Limit
   autosize,                        !- Maximum Cooling Air Flow Rate {m3/s}
   ,                                     !- Maximum Total Cooling Capacity {m3/s}
   ,                                     !- Heating Availability Schedule Name
   ,                                     !- Cooling Availability Schedule Name
   None,                              !- Dehumidification Control Type
   0.7,                                !- Cooling Sensible Heat Ratio
   None,                            !- Humidification Control Type
   ZONENAMEOutdoorAirCntrl,       !- Design Specification Outdoor Air Object Name
   ZONENAME Ideal Loads Outdoor Air Inlet,  !- Outdoor Air Inlet Node Name
   None,                                     !- Demand Controlled Ventilation Type
   NoEconomizer,                        !- Outdoor Air Economizer Type
   None,                                     !- Heat Recovery Type
   0.7,                                         !- Sensible Heat Recovery Effectiveness
   0.65;                                       !- Latent Heat Recovery Effectiveness


ZoneHVAC:EquipmentConnections,
  ZONENAME,                                     !- Zone Name
  ZONENAME Equipment,                    !- Zone Conditioning Equipment List Name
  ZONENAME Ideal Loads Supply Inlet,   !- Zone Air Inlet Node or NodeList Name
  ,                          !- Zone Air Exhaust Node or NodeList Name
  ZONENAME Zone Air Node,                  !- Zone Air Node Name
  ZONENAME Return Outlet;                !- Zone Return Air Node Name

ZoneHVAC:EquipmentList,
  ZONENAME Equipment,                       !- Name
  ZoneHVAC:IdealLoadsAirSystem,        !- Zone Equipment Object Type
  ZONENAME Ideal Loads Air System,       !- Zone Equipment Name
  1,                                         !- Zone Equipment Cooling Sequence
  1;                                   !- Zone Equipment Heating or No-Load Sequence




ThermostatSetpoint:DualSetpoint,
  ZONENAME_HVAC Dual SP Control,     !- Name
  HeatStPoint_BP,                !- Heating Setpoint Temperature Schedule Name
  CoolingStPoint_BP;           !- Cooling Setpoint Temperature Schedule Name

ScheduleTypeLimits,
  HVACTemplate Any Number;                   !- Name

Schedule:Compact,
  HVACTemplate-Always 4,        !- Name
  HVACTemplate Any Number,   !- Schedule Type Limits Name
  Through: 12/31,                    !- Field 1
  For: AllDays,                          !- Field 2
  Until: 24:00,                          !- Field 3
  4;                                       !- Field 4

ZoneControl:Thermostat,
  ZONENAME Thermostat,             !- Name
  ZONENAME,                          !- Zone or ZoneList Name
  HVACTemplate-Always 4,            !- Control Type Schedule Name
  ThermostatSetpoint:DualSetpoint,      !- Control Object Type
  ZONENAME_HVAC Dual SP Control;       !- Control Name

Sizing:Zone,
  ZONENAME,       !- Zone or ZoneList Name
  SupplyAirTemperature,      !- Zone Cooling Design Supply Air Temperature Input Method
  13,                !- Zone Cooling Design Supply Air Temperature {C}
  ,               !- Zone Cooling Design Supply Air Temperature Difference {delta C}
  SupplyAirTemperature,    !- Zone Heating Design Supply Air Temperature Input Method
  40,             !- Zone Heating Design Supply Air Temperature {C}
  ,                !- Zone Heating Design Supply Air Temperature Difference {delta C}
  0.0085,      !- Zone Cooling Design Supply Air Humidity Ratio {kg-H20/kg-air}
  0.008,        !- Zone Heating Design Supply Air Humidity Ratio {kg-H2O/kg-air}
  ZONENAMEOutdoorAirCntrl,   !- Design Specification Outdoor Air Object Name
  ,            !- Zone Heating Sizing Factor
  ,            !- Zone Cooling ...
(more)
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Comments

To help others answer this question better, you should paste in the objects that appear in your expanded IDF (file ending in [name of your IDF].expidf). Since you are using the HVACTemplate:Zone:IdealLoadsAirSystem object, this will be expanded into one or multiple "standard" objects before running EnergyPlus.

Aaron Boranian gravatar imageAaron Boranian ( 2018-08-13 18:04:01 -0500 )edit

Also, be careful about using the correct documentation. Your model is for v8.5, but the documentation "Source" link you provide is for v8.7. Here is the same outputs section for v8.5.

Aaron Boranian gravatar imageAaron Boranian ( 2018-08-13 18:17:31 -0500 )edit

Thank you @Aaron Boranian ! I have updated the post with your suggestions

Tobias gravatar imageTobias ( 2018-08-14 01:41:18 -0500 )edit

2 Answers

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3

answered 2018-08-21 06:54:41 -0500

That relationship is only valid when the outdoor air is warmer and more humid than the zone air which only happens in the afternoons of Jul 14 and Aug 3 for this simulation with Copenhagen IWEC weather. The qualifier "When the economizer is not active . . . " should be expanded to "When the outdoor air and the supply air both require sensible and latent cooling these values are related by the following equation:" but that's rather confusing.

The missing link here is free cooling (sensible and/or latent) which is not currently reported as a separate value. The goal of the current outputs was to show what the real load components are

  • Zone Ideal Loads Zone Total Cooling Energy = the cooling delivered to the zone (both mechanical and free)
  • Zone Ideal Loads Outdoor Air Total Cooling Energy = the cooling load required to condition outdoor air (only accumulates when this is positive, free cooling is excluded here)
  • Zone Ideal Loads Supply Air Total Cooling Energy = supply air mechanical cooling required (includes the net outdoor air load)

The reporting currently separates out the Outdoor Air Sensible/Latent Cooling/Heating values independently, and then adds them together for the totals. So this balance can get even more confusing when the outdoor air is warmer and drier than the zone. If anyone is interested in the exact algorithm, see the code here.

Perhaps it would help to add new output variables for Zone Ideal Loads Outdoor Air Free Sensible Cooling, etc

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Comments

Thank you very much for the answer @MJWitte

I think it would be great to add the suggested output for completeness and transparency. To make sure that I have understand what you mean completely, the equation would then be:

Z.I.L. "Outdoor Air Free" T. C. E. +

Z.I.L. "Supply Air" T. C. E. +

Z.I.L. "Heat Recovery" T. C. E.

=

Z.I.L. "Zone" T. C. E. +

Z.I.L. "Outdoor Air" T. C. E.

(At all times)

I'm still a bit confused about the meaning of the output "Outdoor Air"

1) how is it different from "Supply Air" and why isn't it contained in "Zone".?

Tobias gravatar imageTobias ( 2018-08-22 06:03:52 -0500 )edit

Also diving more into the engineering reference a question comes to mind, with reference to:

Ideal Loads Air System - Engineering Reference

2) Is the exhaust air being mixed or staying separate from the supply air (for instance through the heat exchanger/recovery) - concerning pollutants such as CO2?

Tobias gravatar imageTobias ( 2018-08-22 06:21:16 -0500 )edit

0) No. The above equation will only be true "at all times" if you separate out sensible and latent. 1) If the system does not bring in any outdoor air, then the Zone values will equal the Supply Air values. If you add some fraction of outdoor air, and condition the zone to the same temperature and humidity, then the Zone values will not change, but the Supply Air (which is return air and outdoor air mixed) values will change. 2) The term "exhaust" can be confusing here. A Zone Exhaust Node is simply air leaving a zone. Whether is becomes return air or is exhausted outside depends on the system

MJWitte gravatar imageMJWitte ( 2018-08-23 14:23:19 -0500 )edit

Hello, I'm also having this issue and am still a little confused with the answer. With this equation from the reference: Zone Ideal Loads Supply Air Total Cooling Energy + Zone Ideal Loads Heat Recovery Total Cooling Energy = Zone Ideal Loads Zone Total Cooling Energy+ Zone Ideal Loads Outdoor Air Total Cooling Energy. For example an hour in January I will get the LS of the equation being larger than the RS and in that same hour the RS being larger than the LS. It says the total supply cooling is zero while the zone total is some number. In the cooling case, is that due to free cooling?

apapaky gravatar imageapapaky ( 2019-02-15 10:39:47 -0500 )edit

Most likely yes. Report the System Node temperatures and humidities to see what's happening to help understand.

MJWitte gravatar imageMJWitte ( 2019-03-05 15:53:36 -0500 )edit
1

answered 2018-10-01 08:25:58 -0500

lukanuts gravatar image

updated 2018-10-01 08:28:39 -0500

Hello I am experiencing a similar Issue in Eplus 8.9 with the total values of

Zone Ventilation Sensible/Latent HG/HLoss the sum of each component is not equal to the value calculated by eplus in Zone Total Ventilation HG/HL as per image attached. Has it some kind of concealed elements that is not clearly show up?

thanks!

image description

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Comments

Each load type (sensible, latent, and total) is binned separately into the gain and loss columns. If the air is cool and moist, for example, it will be sensible loss and a latent gain. The total for that time step could land in either the gain or loss column. I personally don't find the the total gain/loss columns to be very useful, others do.

MJWitte gravatar imageMJWitte ( 2018-10-04 15:02:32 -0500 )edit

Hi @MJWitte, for the is clear the presence of both loads, actually I am wondering why the total variables are not equal to the sum of each element, in this case latent+sensible gain/loss ? Should I have to doubt even the other totals?

lukanuts gravatar imagelukanuts ( 2018-10-05 02:40:14 -0500 )edit

Perhaps an example will help.

Hour 1 (outdoor air is cooler but more humid than indoor):
Sensible Loss = 10, Latent Gain = 2, Total Loss = 10-2 = 8, Total Gain = 0
Hour 2 (outdoor air is warmer and more humid than indoor): 
Sensible Gain = 5, Latent Gain = 4, Total Gain = 5+4=9, Total Loss = 0

Total for Hours 1&2:
Sensible Loss=10, Sensible Gain=5, Latent Loss=0, Latent Gain=6, 
Total Loss=8, Total Gain=9
MJWitte gravatar imageMJWitte ( 2018-10-11 15:21:36 -0500 )edit

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Asked: 2018-08-13 11:21:37 -0500

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Last updated: Oct 01 '18