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Chiller blocked at rated capacity

asked 2018-01-10 19:28:13 -0500

updated 2018-01-16 17:15:09 -0500

I really need some help here about the Chiller:EIR component in EnergyPlus. I've tried to go read the engineering references but I still don't get it.

I'm using an electric air-cooled chiller to model an air to water heat pump in cooling mode.

My problem is that the part load ratio won't ever reach 1. Even if my PLR max is 1, the chiller capacity is blocked at rated capacity.

I've tried to change the maximum PLR, then the evaporator cooling rate will be blocked at maxPLR*rated capacity but it feels wrong to put a max PLR higher than 1, that's not what supposed to happen.

A little screenshot to help you understand my problem :

image description

In this exemple the rated capacity is 9050W, when the cycling ratio is 1 the chiller should be working at full capacity (~10500 as it's my capacity multiplier*rated capacity) but it won't. I've tried to completely suppress any cycling by puting the min PLR and unloading ratios at 0, but it won't work either.

EDIT : And there is a screenshot of the operation with PLRmax = 1.2

image description

Now the cooling capacity is blocked at 1.2 *rated capacity = 1.2 * 9050= 10860 W

In this case the chiller can reach a PLR of 1, but also can go higher and that's the problem. It then provides more cooling than the available capacity.

My question is:

  • To reflect the real operation (What's really going on), what should I do?
  • Am I not looking at the right outputs?

To me, it looks like the max PLR should actually be 1, should I adjust the maximum PLR in the object so it's actually the case or is it wrong? Is there an other way?

Chiller object:

image description

And this is the active operation scheme:

image description

Also I'm not sure if that would make a difference, but there is a Fluid to Fluid heat exchanger object between the radiant floor and the chiller, so there are 2 different loops with different flows. (Chiller > heat exchanger & heat exchanger > radiant floor). I've checked the demand request for both these loops and it's higher than 9kW.

I hope you'll be able to give me some insight, I really need some help to get that so all my results are'nt false. :(

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Please post the text for the Chiller:Electric:EIR object.

Molly Curtz's avatar Molly Curtz  ( 2018-01-11 16:10:17 -0500 )edit

Yes sorry ! It's added :)

Maryl J. Courtine's avatar Maryl J. Courtine  ( 2018-01-11 16:50:54 -0500 )edit

Is the load request to the chiller greater than 9050W? What is the active PlantEquipmentOperationScheme?

Molly Curtz's avatar Molly Curtz  ( 2018-01-11 21:17:13 -0500 )edit

Yes, the load request is way higher, like ~20kW for this exemple. For the operation scheme, I've edited the original post :)

Maryl J. Courtine's avatar Maryl J. Courtine  ( 2018-01-11 21:52:38 -0500 )edit

What is the actual assigned load on the chiller? Can you include that variable as well?

Anna Osborne Brannon's avatar Anna Osborne Brannon  ( 2018-01-11 23:19:53 -0500 )edit

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answered 2018-01-17 11:05:30 -0500

You have stumbled across an issue with plant equipment where the equipment passes information back to the plant so that the plant knows the available capacity of the equipment. For example, the EIR chiller passes the rated information to the plant and the plant then uses that information to choose the equipment load to be met. Here is the offending code:

MinCap = ElectricEIRChiller( EIRChillNum ).RefCap * ElectricEIRChiller( EIRChillNum ).MinPartLoadRat;
MaxCap = ElectricEIRChiller( EIRChillNum ).RefCap * ElectricEIRChiller( EIRChillNum ).MaxPartLoadRat;
OptCap = ElectricEIRChiller( EIRChillNum ).RefCap * ElectricEIRChiller( EIRChillNum ).OptPartLoadRat;

Notice that this information does not include the CapFT information, just the rated (or reference) capacity multiplied by the min/max/opt PLR. The plant then says, "hey, this equipment can only give me MaxCap, so I will request MaxCap from this equipment". To speed things up, this information is only passed to the plant once during the simulation, hence, there was no need to evaluate the CapFT curve. What you have proven is that this logic is incorrect at conditions where CapFT > 1, but at least you know why now.

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Thanks for the insight! Will you file a bug report? Or, should we? Please advise how to do that if we should.

Molly Curtz's avatar Molly Curtz  ( 2018-01-17 11:15:45 -0500 )edit

Issue #6436. This is a long overdue issue.

rraustad's avatar rraustad  ( 2018-01-17 12:04:25 -0500 )edit

answered 2018-01-12 03:02:45 -0500

updated 2018-01-12 03:04:45 -0500

I think you actually want to put a max PLR greater than 1. Perhaps 1.2 or so.

If you read the Engineering Reference for Electric Chiller Model Based on Condenser Entering Temperature, you'll see that it is actively being limited to the max PLR you define:

The part-load ratio is then calculated as the ratio of the evaporator heat transfer rate to the available chiller capacity. The part-load ratio is not allowed to be greater than the maximum part-load ratio specified by the user or less than zero as follows:

$$PLR = \max{(0.0, \min (\frac{\dot{Q}_{evap}}{\dot{Q}_{avail}},PLR_{max}))}$$

Interestingly, looking at the IDF example files briefly, I couldn't find a chiller with a PLR greater than 1, though 1.1 or 1.2 is often used for Boiler:HotWater.

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Thank you a lot for the answer !

I increased the PLR so the new maximum would be higher and that works. The only problem I can see with that solution is that sometimes the PLR will actually be higher than 1 and the chiller will provide more cooling that it should be able to, no?

I looked at the example files, and there actually are a lot of chillers (air-cooled) with 1.15 as the maximum PLR, so that comforted me a little.

Maryl J. Courtine's avatar Maryl J. Courtine  ( 2018-01-12 07:07:34 -0500 )edit

It will provide more cooling that it would be able at rated conditions. It's not at rated conditions, so that's not violating physics :)

Julien Marrec's avatar Julien Marrec  ( 2018-01-12 11:21:06 -0500 )edit

Out of curiosity @Mary J. Courtine, can you point me to a specific IDF example file that has a chiller with a max PLR over 1?

Julien Marrec's avatar Julien Marrec  ( 2018-01-13 05:45:20 -0500 )edit

@Julien Marrec, @Maryl J. Courtine, it doesn't seem to me that this answer quite explains what is happening. "PLRmax" is user defined as one (1). But we see the reported PLR variable from the simulation is always less than one in the posted table, implying that the term Qdot,evap/Qdot,avail (which is less than one) is limiting the allowable PLR (per the equation posted in this answer). Isn't Qdot,avail equal to the rated capacity times the CAPf(T) curve output? If that is correct than why shouldn't the evaporator power increase up to the temp mod. available ouput, resulting in a PLR of one?

Molly Curtz's avatar Molly Curtz  ( 2018-01-14 00:24:54 -0500 )edit

That's why I was asking about the required load and plant operation scheme - I was wondering if the load assigned to the chiller evaporator was being limited by some other means. Setting a max PLR of one doesn't seem like it logically should limit the chiller output to the rated capacity value. I would expect it to limit output to whatever the current timestep, temp adjusted, available capacity value is.

Molly Curtz's avatar Molly Curtz  ( 2018-01-14 00:31:22 -0500 )edit

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Asked: 2018-01-10 19:28:13 -0500

Seen: 721 times

Last updated: Jan 17 '18