Packaged Rooftop with air economizer and gas heating (question about heating consumption)

answered 2019-01-25 09:04:44 -0500

updated 2019-01-25 09:06:01 -0500

Economizer controls should be optimized to provide maximum free cooling while minimizing the need for heating. Changing the economizer control type will essentially change the way the economizer operates and therefore provide different amounts of outdoor air during the simulation. To see how economizer controls affect heating energy use add report variables and plot them after a simulation. For this test, plot OA volume (or mass) versus heating coil heating energy and compare across the different economizer controls. Replace the asterisk in the reports below with the node name of the OA node in the OA mixer and the name of the heating coil to get just the data needed. See the rdd file to be sure of the actual reports available for each simulation. After running the simulation the rdd file will contain all available report variables for a specific simulation.

Output:Variable,*,System Node Standard Density Volume Flow Rate,hourly; !-HVAC Avg [m3/s]
Output:Variable,*,System Node Mass Flow Rate,hourly; !- HVAC Average [kg/s]

Output:Variable,*,Heating Coil Heating Energy,hourly; !- HVAC Sum [J]

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answered 2019-01-30 17:23:22 -0500

dahdo1m
31 ●2

updated 2019-01-31 11:55:30 -0500

I have created the suggested plot. It shows the volumetric air flow on the x-axis and the heating coil energy consumption on the y-axis. The plot is clearly different for each economizer control type (especially dry bulb vs enthalpy types).

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My question is, how do I know if my results are acceptable knowing that we were expecting the heating energy consumption to be independent of the control type? Is it unrealistic to expect the heating to be independent of the economizer control type especially that the heating setpoint is the same for all economizer types?

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This plot appears to show that FE and DE use greater heating energy than FDB or DDB. Between 0.35 m3.s OA during economizer operation and 1 m3/s there is essentially no additional heating energy which means you are getting the free cooling most of the time.

This plot also shows that when economizer is not active (at 0.22 m3/s OA flow) that there is a difference in heating energy. I would ask why. I would guess that the supply air flow rate is different between these simulations. You might also plot heating coil energy vs supply flow rate to see if something reveals itself.

rraustad ( 2019-01-31 07:40:30 -0500 )edit

I checked the supply air flow rate, it was exactly 0.982 m3/s for all economizer settings. I'll add the new plot to the previous answer.

dahdo1m ( 2019-01-31 11:54:56 -0500 )edit

The SA plot shows that there is no difference in flow rate between simulations (which means any difference in heating energy is not due to different supply air flow rates). So the differences in heating energy is due solely to economizer operation. Focusing for a minute on heating differences with no economizer operation (Figure 1, OA flow = 0.22), if SA and OA flow rates are the same then why is there a difference in heating energy? Maybe you should add a table in your question that shows what the magnitude of the differences are so a reviewer has more information on your results.

rraustad ( 2019-01-31 12:04:09 -0500 )edit

Based on my understanding, heating consumption is going to differ from one timestep to the next based on the dynamic change in inputs and the need for heating at certain times. Would you like me to insert a table showing SA and OA in addition to heating energy? I can also strip Fig. 1 to show the no economizer case alone.

dahdo1m ( 2019-01-31 13:20:09 -0500 )edit

I think all information is useful until the differences are identified. A table showing differences in each simulation would help.

rraustad ( 2019-01-31 13:55:15 -0500 )edit

I'm running the same file 5 times using (eppy), inputs other than economizer type are fixed. High economizer limits are specified as T_OA=23.889C and h_OA=65128J/kg for the file. I created the file using Open Studio's reference buildings measure. I don't believe there is any differences in simulations other than economizer type since I'm using the same file.

dahdo1m ( 2019-01-31 14:03:25 -0500 )edit

Can you compare 2 of the files to make sure that's the only difference? I use WinDiff to compare 2 text files.

rraustad ( 2019-02-01 12:01:31 -0500 )edit

I compared both files, the economizer type was the only difference between the two.

Like I said before, I'm using only one file to be simulated, the only thing that is changed for every run is the economizer type.

dahdo1m ( 2019-02-05 15:18:11 -0500 )edit

So now you know that the economizer controls are causing these differences. Compared to a simulation with no economizer, any change in economizer controls should result in less cooling energy with minimal change in heating energy. Remember that the Controller:OutdoorAir input for Economizer Minimum Limit Dry-Bulb Temperature can/should be used for any economizer to limit the change in heating energy.

rraustad ( 2019-02-06 07:25:12 -0500 )edit

I set the minimum limit to 55 F (12.778 C), heating consumptions retrieved from the summary tables are as follows (GJ): NoEconomizer: 0.36 FixedDryBulb: 0.27 DifferentialDryBulb: 0.25 FixedEnthalpy: 0.27 DifferentialEnthalpy: 0.26

Heating Coil consumption retrieved from the variables spreadsheet (Sum of hourly values) are as follows (GJ): NoEconomizer: 0.2856 FixedDryBulb: 0.2162 DifferentialDryBulb: 0.2036 FixedEnthalpy: 0.2162 DifferentialEnthalpy: 0.2106

dahdo1m ( 2019-02-06 13:09:52 -0500 )edit

You need to compare both heating and cooling energy to see how the total energy consumption changes with economizer control. You can play with the setting to see which works best.

rraustad ( 2019-02-07 07:36:04 -0500 )edit

Big Picture: in most cases, the cooling energy does drop while using economizers, which is the expected outcome from using economizers. That's what I'm basing my work on.

I just want to make sure that I'm doing my simulation right, and seeing that heating consumption changed depending on economizer type makes me skeptic about my cooling results.

dahdo1m ( 2019-02-07 12:29:31 -0500 )edit

Sounds like your doing it right, just helping find answers.

For outdoor air systems there is a temperature set point manager at the OA mixer mixed air node controlling the OA damper (damper modulates to meet this temperature set point). Look at this variable to see how it is controlling the OA damper. There might be interaction when using economizer on how the OA damper is controlled. Also wonder if you could set economizer minimum temp to 99 to replicate the no economizer case to see if those 2 results matched.

Output:Variable,"mixed air node name",System Node Temperature Setpoint,timestep

rraustad ( 2019-02-07 12:39:33 -0500 )edit

They matched after setting the minimum limit to 99

dahdo1m ( 2019-02-11 18:10:15 -0500 )edit

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