Re: Bls: Bls: Bls: [EnergyPlus_Support] Fan Electric Energy Doesn't Change After Increasing the Leaving Chilled Water Temperature Reference

["Jean Marais jeannieboef@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx>]

I see this as follows:

If you raise the supply air temperature of the system providing the cooling air, you will need more air to extract the same amount of heat so the fan energy should increase.

By lowering the cooling coil water supply temperature, it will not automatically reduce the supply air temperature because you are now blowing more air through a coil that is sized differently.

You may want to look at the
@ air mass flow * Cp * (Supply air temp - extract air temp)
and the
@ water mass flow * Cp * (water in temp - water out temp) - energylostinCondensationWater

On 23 Jun 2016 10:26 am, "Annisa Nurul Hidayah hidayah.annisanurul@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

 

Hi Ivan, 

Thank you for your detailed explanation 

It really help me to understand the problem

Regarding your first question, it is my mistake to give unclear information. What I mean in the title of this post is "total" fan electric energy. I have several zones in this model and the total fan electric energy for all zones is the same for both models. But, if I pay more attention in the separate zone fan electric energy, some zones have higher fan electric energy and some zones have lower fan electric energy. But, the "total" is almost the same. And as I wrote in my previous email, I thought the fan electric energy for all zones should be higher after increasing the the leaving chilled water temperature setpoint from 7 C to 10 C. And later, I found that the fan electric energy related to the supply air temperature. That's why I mentioned about supply air temperature and cooling coil temperature on my previous email.  

Yes, your assumption is right, I used fan coil as the HVAC system plus DOAS for the fresh air. And you are also right that I only controlled the temperature, not the humidity. As the result, the humidity ratio in the zone reach 99%. As you mentioned above, it caused the supply air temperature to be below the cooling coil outlet temperature. But, What if the real system only has thermostat? and doesn't have anything to control the humidity? Do you think the simulation result already represent the real system condition?

And as I mentioned earlier that the sizing supply air temperature related to fan electric power, means that it will also related to fan electric energy. Do you think I should change the sizing supply air temperature when I increase the leaving chilled water temperature?

Thanks again

    

Pada Senin, 20 Juni 2016 20:42, "Ivan Korolija ivankorolija@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> menulis:

 

Hi Annisa,

I went back to your initial question and I need further clarifications. You mentioned that the fan electric energy doesn't change (that is the title of your post). However, further in the mail you said that different zones have different fan consumption as a function of chilled water temperature:

"think it is weird because the higher leaving chilled water temperature (10 C) model can produce lower supply air inlet temperature. And of course, It will impact the fan electric energy (in this case, I use the fan:onoff for the supply fan). But, it doesn't happen to all zones. Some zones in 10 C leaving chilled water temperature have higher supply inlet temperature compare to the 7 C model. So, in those zones, the fan electric energy are higher as expected."

Can you clarify this because it is in collision with previous statement: "But, the problem is in fan electric energy. Both models have the same fan electric energy"

In addition, it would be useful to know which HVAC system is modeled. I assume it is fan coil but I am not sure. Is there any fresh air or not? 

The main reason you got confused, in my opinion, is that you are not considering enthalpy of the air, both inlet and outlet. While the zone air temperature is controlled, the humidity is not. This affects the coil performance since the cooling coil can be completely dry, completely wet or partially wet.

Let's make a very simplified example. If we assume that both the chilled water flow rate and air flow rate are the same in both cases (chilled water temperatures 7/12C and 10/15C), which means that the amount of heat taken from the air is the same (assume 10 kJ/kg), and the supply air dry-bulb temperature is 24C, we can identify three coil's behaviors. 

1) When the supply air dew point temperature is lower than the coil surface temperature:

In this case only the sensible heat transfer occurs. The leaving air state will be the same in both case (see attached chart).

2) There is a condensation on the cooling coil, which means there is a latent heat removal. We can approximate that the change of the air stream temperature follows the straight line connecting the air inlet state and the average chilled water temperature. If the same amount of heat is removed (10 kJ/kg) with both coil's temperature regimes than the leaving air dry bulb temperature will be higher in case of lower chilled water temperature (2out' > 2out'').

3) This can be reversed if inlet air is very humid (third case).

It is important to notice that, despite the total heat removal is the same, the ratio between sensible and latent heat varies as a function of chilled water temperature in both cases 2 and 3.

However, this simplified example assumes that the conditions of the air are driven by the cooling coil. In EnergyPlus it is vice versa. The coil performance (flow rates, temperatures) are driven by zone loads at each time step (unless overridden by control or fixed system parameters). For each time step the sensible and latent cooling loads are calculated and passed to the HVAC system (coils, chillers, etc.). The HVAC system manager (if I am correct) calculates conditions of the supply air by taking into consideration the physics of HVAC equipment. Having in mind that the zone dry bulb temperature is controlled (unless you control humidity too), the humidity of the zone air can vary which I would expect to happen in your model. For each time step you'll probably have different zone air humidity depending on the chilled water temperature regime. That is why you cannot conduct the analysis just by looking into dry bulb temperatures.

Cheers,

Ivan

On Monday, June 20, 2016 6:03 AM, "Annisa Nurul Hidayah hidayah.annisanurul@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

 

Hi Jean and Ivan

After checking the idf, I found that both models used cross flow heat exchanger type.  

Pada Senin, 20 Juni 2016 11:54, "Jean Marais jeannieboef@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> menulis:

 

Thanks Ivan. I missed that. A good control would be to monitor the temperature of the medium going in. Checking it even further is possible if you have some other variables to calculate the enthalpy change of waterside and air side.

On 19 Jun 2016 11:42 am, "Ivan Korolija ivankorolija@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

 

The model 2 outputs are possible in counter flow heart exchangers. 

Sent from Yahoo Mail on Android

 

Hi Jean, 

Yes, you are right, both models' coils are being autosized. I already checked all inputs in coil:cooling:water, like design water/air flow rate, design inlet/outlet water and air temperature and design inlet/outlet air humidity ratio. All those inputs are being autosized.  

I would like to check the NTU-effectiveness or bypass factor of the model, but I couldn't find any input or output showing that object. Can you please give me a clue, where do I can find the detail of that object?

I also already tried to set the design water inlet temperature in coil:cooling:water object from autosize to be the same as the chilled water leaving temperature setpoint, in this case 10 C. But, the result is just the same as before.  

 

Another thing, I already show the temperature and mass flow rate output of the model. I take one zone as an example and the results for one time-step are as follows.

  

a. model 1 - with leaving chilled water temperature 7 C

    cooling coil inlet temperature = 7 C

    cooling coil outlet temperature = 12.01 C

    chilled water mass flow rate in coil = 0.104 kg/s

    cooling supply air temperature = 13.762 C

    supply air mass flow rate = 0.131 kg/s

b. model 2 - with leaving chilled water temperature 10 C

    cooling coil inlet temperature = 10 C

    cooling coil outlet temperature = 15.05 C

    chilled water mass flow rate in coil = 0.0999 kg/s

    cooling supply air temperature = 13.423 C

    supply air mass flow rate = 0.127 kg/s

I think there is something strange in model 2 output, is it possible to get supply air temperature (13.4 C) which is lower than the cooling coil outlet temperature (15.05 C)? Or do you think there is something wrong with the model or my understanding? 

Thank you so much for your help 

Pada Kamis, 16 Juni 2016 12:00, "Jean Marais jeannieboef@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> menulis:

 

This is the juicy stuff of simulation world. See if you can check the supply air coil effectiveness (NTU method) and read up on bypass factor (not that it applies directly to your inputs). If both models' coils are being autosized, it most certainly will give different behaviour.

On 14 Jun 2016 11:42 am, "Julien Marrec julien.marrec@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

 

I misunderstood your initial email, I apologize for that. I thought you were getting lower than 10C in the 10C case, an indication that your SPM would have been wrong.

The supply air temperature (or SAT) will be depending on both the chilled water temperature, water flow rate, and air flow rate (this one is determined by the demand of each zone). I would output at the Detailed timestep the node temperature and mass flow rates for both the chilled water loop and air loop to understand what's going on in detail.

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

LinkedIn (en) : www.linkedin.com/in/julienmarrec
LinkedIn (fr) : www.linkedin.com/in/julienmarrec/fr

2016-06-14 9:38 GMT+02:00 Annisa Nurul Hidayah hidayah.annisanurul@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx>:

 

Hi Julien, 

Thanks for the reply and explanation

Yes, I want the chiller to supply 10 C of water and I already add SetpointManager:Scheduled with a schedule of 10 C. 

And I also checked the output for chilled water loop supply outlet and it is already 10 C.

I am just a bit confused with the zone supply inlet temperature for the 10 C chilled water temperature. 

Because, with the same chilled water flow rate for both models (10 C and 7 C) and almost the same dT (delta temperature/ temperature difference), the supply inlet temperature for the 10 C is lower than the 7 C.

For example, the 7 C chilled water temperature with "X" flow rate can produce 13.1 C supply air temperature (the return chilled water temperature around 13.2 C), but the 10 C chilled water temperature with the same flow rate can produce lower temperature, let say 12.9 C (with return chilled water temperature around 16.4 C). 

NB: Both models are calculated with the same initial condition.  

Pada Selasa, 14 Juni 2016 13:45, "Julien Marrec julien.marrec@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> menulis:

 

Hi,

The laws of physics are pretty formal about it, energy and mass must be conserved. As Lavoisier said, nothing is created, nothing is lost, everything is transformed.

If you're only using a chilled water coil in an air loop, you cannot produce lower supply air temperature than the inlet chilled water temperature (provided that the entering air temperature at the coil is not even colder...)

I think your slight confusion comes from the term "Reference leaving chilled water temperature" (see here). Reference conditions are different than operating conditions.

Basically you specify the chiller characteristics at a reference condition, and then you use a set of curves - that all should have a value of 1.0 at said reference condition - to calculate the characteristics of the chiller at a different operating point, operating point that will be calculated at each timestep given the control you have specified.

In your case, I'd specifically check the setpoint manager you assigned to the chiller plant loop. If you actually want the chiller to supply 10C water, then you should add a SPM:Scheduled with a schedule of 10C on the supply outlet node of the plant loop.

As far as how E+ calculates things, you can always check the Engineering Reference Guide or the source code if you happen to not find what you're looking for (it usually is a last resort situation). Sizing:XXX objects do one thing: they size things. It happens during the sizing runs, at which point these values (airflow and water flow rates for example) are "hardcoded" and then the annual simulation begins.

Hope this helps,

Best,

Julien

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

LinkedIn (en) : www.linkedin.com/in/julienmarrec
LinkedIn (fr) : www.linkedin.com/in/julienmarrec/fr

2016-06-14 8:22 GMT+02:00 hidayah.annisanurul@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx>:

 

Hi Groups, 

I just simulated 2 models in energyplus. 

The first model is aircooled electric chiller with reference leaving chilled water temperature 7 C. 

And the other model is the same as the first model, but with reference leaving chilled water temperature 10 C. 

I already updated both models with new EIRFPLR based on their specifications.

 

Based on simulation result, cooling/chiller electric energy for the 10 C temperature is lower than the 7 C temperature, which I think it makes sense. 

But, the problem is in fan electric energy. Both models have the same fan electric energy. I think, based on the theory, The 10 C temperature should have higher fan electric energy than the 7 C temperature. 

And I already checked the supply inlet temperature from the output:variable, 

for some zones, the supply inlet temperature in 10 C model is lower than the 7C model. (For example, at the same time and date, in 7 C model, the supply inlet temperature is 13 C, meanwhile, in 10 C model, the supply inlet temperature is only 12 C). I think it is weird because the higher leaving chilled water temperature (10 C) model can produce lower supply air inlet temperature. And of course, It will impact the fan electric energy (in this case, I use the fan:onoff for the supply fan). 

But, it doesn't happen to all zones. Some zones in 10 C leaving chilled water temperature have higher supply inlet temperature compare to the 7 C model. So, in those zones, the fan electric energy are higher as expected.  

Does anyone knows how energyplus calculate the supply inlet temperature for each timestep? And Does the cooling supply air temperature setpoint in sizing:zone also impact the supply inlet temperature for the next timestep? 

And does anyone have the same experience? or any idea? I really need any kind of help

Thanks

  

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Posted by: Jean Marais <jeannieboef@xxxxxxxxx>

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