#2038 CBECC-Com - Modeling Lab Systems and 100% OA systems

[Felicia Lee]

Version = 2016.2.1_868

Category = HVAC

I have some questions regarding lab system modeling in cbecc-com. (attach my model for your reference)

1. Exh. Control

Out of curiosity, I compare the energy consumption between variableflowvariable speed option VS variable flowCONSTANTspeed option.

         As shown below from the screenshot, my fan energy increased change from VFD to constant, which totally makes sense. However, my heating also increased dramatically, which I do not quite understand since both are variable flow. Could you explain a little bit further on this?

1. My system is variable speed exhaust fan, based on the simulation result, it shows that I could expect 40.25 heating savings, which I feel like is a little bit high, so I go into the model (idfs) to see where does that savings come from. What surprised me is that there are quite a bit discrepancy between baseline VS proposed on the zone mechanical ventilation rate as highlighted below.

However, in the model, I use standard 6ACH ventilation for the lab space in cbecc-com, could you help me look into it why there are discrepancy there?

        And the ventilation rate is not mataching 6ACH, take 1sr LER zone as an example. In cbecc-com, 6ach equals to 4021 CFM ===1.9 m3/s, however, the result shows ventilation rate with  5.12 m3/s for the proposed and 9.82m3/s for the baseline? The LER is a load driven space, is the airflow calculated based on the peak cooling load since it is a high internal load space.

1. The result table generated from E+ also indicate discrepancy between the proposed case VS baseline .

It surprised me that 1st conf space even has a different value 1.769ACH VS 1.677ACH.

[Nikhil Kapur]

DR : Re: 1)
This is a good observation that reveals a change I am recommending we make in the next release. I've tested the change, and am getting results more in-line with the expectation; i.e an increase in indoor fan energy, but relatively insignificant change in heating energy.

Re: 2) and 3)
The zone mechanical ventilation report is a tricky one to use to validate/compare ventilation rates, particularly when the loads of the zone and/or the system serving the zones is different (i.e. comparing baseline to proposed values).
Per the E+ documentation: (Fig 7)

The EnergyPlus summary report table is also not very useful for the comparison you are trying to make. Here is an UMLH topic on it:
https://unmethours.com/question/12171/how-are-outdoor-air-summary-report-numbers-getting-calculated/

For labs, the design ventilation rates between the baseline and proposed are intended to be the same, but exhaust fan controls may differ and the ventilation is modeled to track the exhaust.

On a separate topic: I noted that your fan efficiency appears to be specified to be quite high (see below). Have you confirmed this for your expected fan selec

[Nikhil Kapur]

12/8 User - Thanks so much for your response. This is really helpful. As a follow up question, based on my building size, my baseline model will be 100% OA system with minimum 6ACH ventilation according to T24.

How do we model a 100% OA system in cbecc-com? I tried to using cbecc-com to open the baseline model xml file, but could not find a clue how it achieve 100% OA system for the lab space

I am suspecting this might be the reason that there is a big discrepancy between my baseline and proposed ventilation rate for the load driven lab spaces. (fresh air is driven by load not ventilation)

Regarding the fan efficiency, I agree that 80% is a little unrealistic, I use that just as a placeholder to match the BHP data.

[Nikhil Kapur]

12/8 DR - To model a 100% OA system, the ventilation air flow rate specified at the Space should be equal to (or sum up to if there is more than one Space in a ThrmlZn) to the TrmlUnit primary air flow serving the ThrmlZn. In your model, you are using the 'AutoHardSize' rules to size TrmlUnit air flows, but have the the fan flow set to 180000 cfm.

To simulate a 100% OA system, you will need to set the fan air flow to equal the sum of the OA flows, or, go through and increase the ventilation air flow rates in the Spaces. For the latter, using your SpcFuncDefault is the fastest way to increase the ventilation air flows.In either case, the Supply Flow and Design OA Flow should be equal to each other (or at least w/in 1%, see below)

For the next release, I have been considering adding a way to more quickly default space ventilation/TrmlUnit air flows for 100% OA systems. My idea is to have a checkbox at the system that could be selected, and if selected, it would set the proposed TrmlUnit/System airflows equal to the zone ventilation air flow rates. I think this would be useful for setting up DOAS and lab systems more quickly in the CBECC interface. I can't guarantee it will be included in the next release, but I welcome any thoughts you might have on this capability.

[Nikhil Kapur]

Reopening as per user follow up questions:

User - That is an excellent idea adding a function to automatically size the system to 100% OA.
However, another issue I have with my proposed 100% OA system is

1. My proposed case real time outside air rate is modulating depends on the space load, for example, for the load driven lab spaces, the outside air rate fluctuate between 6 ach (ventilation ) to 10ach (peak cooling rate). How do we account for this scenario in cbecc-com? ( if we bump up the ventilation rate to 10 ach, could we choose to modulate ventilation rate based on load?)

2. My whole building including non-lab space (offices, commons) is using 100% OA system, which has the same issue with number 1, the non-lab spaces outside air rate will be modulating depends on the space load. What modeling method do you recommend accounting for this?

[Nikhil Kapur]

12/23 DR

1. Yes, if the peak air flow capacity of the system is greater than the minimum ventilation rate, the system should ramp up to meet the load as needed. Do you have a model where you don't think this is happening?
2. At first thought, I'd suggest a second system and isolation between the lab and non-lab spaces, but that isn't really relevant to modeling in CBECC.

You'd have to specify the design ventilation rate of the offices/core zones to be equal to the design flow of zone, and then modify the min flow of the terminal serving the zone to be equal to the min flow for the zone. Please send us your model.

[John Arent]

DR - regarding the reponse to the first user issue above, what is the recommended change?

"DR : Re: 1)
This is a good observation that reveals a change I am recommending we make in the next release. I've tested the change, and am getting results more in-line with the expectation; i.e an increase in indoor fan energy, but relatively insignificant change in heating energy"

IS there a rules change for the ACM?