Difference of ventilaiton and infiltration losses between HB and steady-state

energyplus
energy

#1

Dear all, I am still comparing energy balance of HB with Steady-state calculation for an office space. I used ventilation losses=ventilation rate * air density* heat capacity of air * (wet bulb temperature - indoor air temperature), infiltration losses=infiltration rate * air density * heat capacity of air * (wet bulb temperature -indoor air temperature) for steady-state calculation. Here, heat capacity of air = 1000 J/kg.K, and air density changes with dry bulb temperature.

I made a figure for the hourly comparison between HB and Steady-state. In winter and transition seasons, ventilation and infiltration losses are almost the same. However, in summer, there are some times (during working time) that the ventilation and infiltration losses are postive, even outdoor air temperature is lower than inside air temperature.

Does anyone have an idea about why ventilation and infiltration losses are positive in these periods when outdoor temperature is lower than indoor air temperature?


#2

@Tiantian

It’s hard to say without more information, like the actual GH file, or what your weather file is.

That being said, in this case the increase in ventilation/infiltration energy might be due to the latent energy gains coming from the humid, summer air that you’re not accounting for in your calculation.

If you go to your EPOutput component, and toggle the Sensible Only option for the _loadType_ input, you can check if that matches your calculation better.

If your specified HVAC has no heat/mass recirculation, you may be able to account for this in your calculation with something like:
dehumidification_demand = ventilation_rate * air density * (indoor_hum - outdoor_hum) * latent heat of evaporation

S


#3

20181223-office-5-for validation-forum.gh (708.4 KB) 20181223-HB results-office-5-sensible-forum-Simply.xlsx (1.5 MB)

Dear Saeran,
Thanks a lot for your reply. I tried the ‘Sensible option’ for ‘loadType’. And the results get much similar. But there is still some moment that the ventilation losses in HB is positive.


I attached with the HB file and excel (simplified for size limit). I also tried to calculate dehumidification_demand with your formula, but its amount looks much higher than ventilation_losses (See excel)…Did I understand something wrong?


#4

@Tiantian

I figured out the problem by checking the Outdoor Air Inlet Node temperature (which you can get from the in.csv file Honeybee outputs). The positive ventilation/infiltration gains are occurring because you’re calculating the ventilation sensible load with wet bulb temperature. You can see here that the dry bulb is actually significantly hotter during the afternoon:

Swapping your wet bulb and dry bulb columns make the sensible portion of the ventilation energy match more closely:

I found that your infiltration loads are still slightly different, even after swapping the dry bulb for wet bulb. The problem seems to be that the infiltration air flow is not the same as what you are calculating in the excel sheet. Not sure why that is the case, but I took the infiltration mass air flow, from the in.csv file (after some modifications in the idf editor to get that output variable) and that seems to have fixed the problem:

So you will have to do some further debugging on why there is that discrepancy in the airflow rate for infiltration.

Finally, in terms of dehumidification demand, I actually meant absolute humidity, not relative humidity as you are calculating. So with units:

dehumidification_demand(W) = ventilation_rate(m3/s) * air_density(kg_dryair/m3_dryair) * delta_hum(kg_vapour/kg_dryair) * J/kg_vapour

I’m not sure if this’ll work, but it’s worth a shot.

S


#5

Dear Saeran,
Thanks a lot for you reply! and Merry Christmas!
For ventilaiton losses, I am confused about the wet-bulb and dry-bulb temperature. I found the following text on E+ website of https://bigladdersoftware.com/epx/docs/8-0/engineering-reference/page-031.html.


Does it mean that when I am calculating sensible loads, I should use dry-bulb temperature, and when latent loads are added, I should use wet-bulb temperature?

For infiltration losses, it is true that I used the average infiltration air flow, as I didn’t find the hourly infiltration air flow. I even cannot find IDF editor in my install file of E+… Can you show me briefly how you modify the IDF? If possible, can you also show me how to find the IDF editor?

For absolute humidity, is it possible also find it from the .csv file?
Thanks a lot!
Best


#6

@Tiantian ,

You are correct that the difference in outdoor dry bulb temperature and the indoor set point relates to the sensible load. Also the difference in outdoor wet bulb temperature and an indoor wet bulb humidity setpoint can realte to the latent load. However, the default for the ideal air system is to use a dew point (or humidity ratio) set point so, to calculate latent load, you should use the difference between the outdoor dew point and the indoor dew point set point (typically around d 12 C).

To get the hourly infiltration airflow, @saeranvasanthakumar is looking through the .rdd file that is output from the simulation and contains a list of all possible outputs from an E+ simulation that you can request (rdd stands for Result Data Dictionary"). There’s a Honeybee component under the 10 tab that parses this file for you and lets you search for keyords in it. You can then plug the relevant outputs related to infiltration into the simulationOutputs_ of the OpenStudio component and use the “Read Custom Result” component to read in the result values. You can request absolute humidity or dew point values from the simulation using this same method but as I said before, the indoor dew point set point should be constant and around 12 C.

@saeranvasanthakumar ,
I believe the reason why the infiltration air flow rate is not constant is that E+ tries to modulate the infiltration using the outdoor wind speed. I would have to check the E+ input output reference to be sure but I know that infiltration rates in the real world can vary greatly over time as the outdoor wind speed changes and the bouyancy effects increase with indoor/outdoor temperature difference.


#7

@chris

Thanks for jumping in, and connecting the missing pieces of the puzzle here. However, I’ve only ever done mass-energy balances by breaking up the sensible and latent components via dry bulb and the humidity ratio, and I don’t really understand how the dew point or wet bulb translates to latent loads as you are describing here.

Specifically, if you look at a psychrometric chart of an example scenario where we go from Tout (drybulb) = 30C W =0.013 to Tin (drybulb) = 25C, W=0.010, the enthapy of the difference in humidity ratio (independant of drybulb) is less then the enthalpy you would calculate by getting the difference of the dew point (QL1 and QL2 respectively).

Is there something I’m missing here?

@saeranvasanthakumar ,
I believe the reason why the infiltration air flow rate is not constant is that E+ tries to modulate the infiltration using the outdoor wind speed. I would have to check the E+ input output reference to be sure but I know that infiltration rates in the real world can vary greatly over time as the outdoor wind speed changes and the bouyancy effects increase with indoor/outdoor temperature difference.

Cool, indeed that would explain it.

S


#8

Dear Chris and Saeran,
I tried the way of Chris and found the Zone Infiltration Standard Density Volume Flow Rate (m3/s), and used to calculate infiltration losses. The infiltration losses of HB and our calculation are quite similar now:


I also found the Site Outdoor Air Humidity Ratio [kgWater/kgDryAir] and Zone Mean Air Humidity Ratio [kgWater/kgDryAir], and used them to calculate the dehumidification demand. Then compare the ventilation losses in HB with the sum of calculated ventilaiton-losses and dehumidification demand. But the difference is much bigger:

This is might be caused by the latent heat of evaporation, which I used 2264000 J/kg. I assume that the latent heat of evaporation should changed with pressure changes. Do you know how I can get the hourly value?

I used google drive to share the excel, as the limit of document on this website:
https://drive.google.com/file/d/1ccFTanId9RX4owd8Fm7ikr3gcxQZ9fWy/view?usp=sharing

Best
Tiantian


#9

@Tiantian

Sorry for the late response here. I took a look at your excel spreadsheet, and the load difference in your ventilation comparison is because you chose the wrong column in your excel spreadsheet: you’re comparing HB’s sensible load against your manually calculated total load.

image

Once you fix that, the steady state ventilation and HB simulation loads match reasonably well:

S


#10

Thanks a lot for your reply, Saeran. Never too late. I am sorry for the mistake. But I checked again, and found that there is always some times in summer when the difference between HB and my own calculation is quite different, for sensible ventilation, sensible infiltration and also for total ventilation with latent load. I compared their difference with %, i.e. (HB result-SS result)/ SS result *100.

I shared the excel with googledrive:

Do you have any idea for the big difference in summer time?

Best
Tiantian


#11

@Tiantian

I wouldn’t take the percentage change to judge the discrepancy, if you take the absolute difference between the two, you can see that there is small, but consistent offset for the loads at peak gains/loss periods. I.e for infiltration

It’s fairly subtle, so I suggest escalating this to the Unmet hours forum https://unmethours.com/ as I’m hitting the limits of knowledge of how EnergyPlus is modeling this to know what the difference could be.

One error I noticed is that the outdoor site dry bulb values you are getting from the EPW is not equivalent to the site outdoor air used by energyplus. If you look closely, you’ll see that the values in the epw are offset half an hour from your EP results. EP also will modify the outdoor air db based on its own factors (i.e zone height).

Plug a string with Output:Variable,*,Site Outdoor Air Drybulb Temperature,hourly; !- Zone Average [C] into your additionalStrings input, (as Chris describes above) to get the outdoor dry bulb that EP is using to calculate the temperature delta.

S


#12

@SaeranVasanthakumar,
Sorry for the very late response here. I wasn’t referring to the method by which you calculate latent load (you are right that in your description here). Rather, I was talking about when the ideal air system will remove moisture from the air vs when it will only remove sensible load. Because the system is trying to produce ideal air for cooling and this supply air is supposed to have a specific temperature and humidity setpoint, the latent load won’t show up unless the humidity ratio of the outdoor air is greater than that of the ideal supply air. The humidity setpoint of the ideal air system can be set based on the dewpoint of the supply air, its humidity ratio, its wet bulb temperature, or its relative humidity. But you wont get any latent load unless the humidity ratio of the outdoor air is above the location of the ideal supply air on the psychrometric chart.