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CBECC-Com: HVAC Question - Fan Energy & Fan Power #88

Open joesinger12 opened 4 years ago

joesinger12 commented 4 years ago

3/18/2020 CEC (From User): I had a couple more quick questions regarding results from CBECC-Com.

1.) I am surprised by some of the fan energy calculations that come from the software. Taking the included small office building as an example (020012-OffSml-CECStd.cibd19), it consumes 23 MWh of fan energy annually – it exceeds cooling/heating energy in most locations. I went and compared that against the results I’d get from one of the DOE reference buildings of the same type and I got closer to 5 MWh with those models. A quick review of the idfs and I can see there are definitely differences between them (CBECC-Com assumes a 965Pa pressure rise compared to 622.5 in DOE, CBECC-Com has higher flow rates and ventilation, etc.), but I haven’t yet found why the value is almost 5x as great. Do you have any insights into this? I’d like to understand why CBECC-Com calculates a higher fan energy and determine if that is deliberate and reflective of some real difference in building design for title 24 compliance. Attached is an .idf meant to represent 90.1-2016 construction from here: https://www.energycodes.gov/development/commercial/prototype_models

2.) To clarify from our previous discussion- I’m still a little confused about the fan power correction. This is because for each zone, the COP in energy plus is the same (taking that small office example again it’s 3.848) for every zone. Since those zones all have different cooling capacities, it means the fan/pump power being deducted is different in each zone in a way that produces exactly the same COP (e.g. I think in the core zone, ~440W of fan/pump energy are removed, in perim4, the fan energy is 313W to get that same 3.848). These are close to about half the fan energy for each zone, so maybe the fan efficiency is being used. Can you elaborate on that calculation? a. I notice the same thing happening for VRF systems – the COP in openstudio/energy plus is higher than entered in the interface. This is interesting to me, because my understanding of the VRF performance curves from FSEC was that they were curve fits of outdoor unit power, not including indoor unit fan power. So that indoor fan power may not need to be deducted. Am I misunderstanding this?

Reported by: joesinger12

Original Ticket: cbecc-com/tickets/3197

joesinger12 commented 4 years ago

Original comment by: joesinger12

joesinger12 commented 4 years ago

DR - 1) For this model, one reason why the fan power will be higher is due the HVAC availability schedule. In CBECC-Com, the ‘Office’ schedule is 122 hrs/wk (6am-midnight, 7 days/week), where as the 90.1 schedule is 65hrs/wk (6am-7pm, off on weekend). As already known, the CBECC schedules have the Office system running on Sundays, when it should be turned off. CEC has directed us to wait until 2.0 to fix this. Another reason is the 020012-OffSml-CECStd.cibd19 is in Sacramento, whereas the 90.1 example model provided is for San Diego. This will result in fan sizing differences.

2) Regarding the COP adjustment for (AHRI) fan power of SZAC systems in 020012-OffSml-CECStd.cibd19, the observation is correct. As defined in the NACM, the AHRI fan power is based on the normalized assumptions of 0.365 W/cfm and 400 cfm/ton, regardless of unit size. Therefore, the simulated COP for each unit, assuming the all have the same AHRI rated input efficiency, will end up with the same adjusted COP, regardless of unit size. Regarding VRF adjustment of AHRI rated EER/COP, the same AHRI fan heat assumptions are used for VRF as well. This was an outstanding question that was never resolved in the VRF development project. We have pointed out that it hasn’t been resolved on a few occasions (see VRF final report, excerpt below, attached email, and ticket 2972), and we’ve never gotten a response. Can we finally close the loop on this? I suspect we’d need to do some research into the rating method and industry outreach to confirm.

Original comment by: joesinger12

joesinger12 commented 4 years ago

Original comment by: joesinger12

joesinger12 commented 3 years ago

Original comment by: joesinger12

joesinger12 commented 3 years ago

Item 1) was addressed separately under ticket 3054 https://sourceforge.net/p/cbecc-com/tickets/3054/

Item 2) is still outstanding. Recent inquiry from PNNL suggested they had been looking at using 0.125 W/CFM for the indoor fan and 450 CFM/ton for cooling capacity.

Original comment by: joesinger12

joesinger12 commented 3 years ago

Original comment by: joesinger12

joesinger12 commented 3 years ago

Moving to Dev as this would be an ACM change and we could discuss this for inclusion in the next major release.

Original comment by: joesinger12

joesinger12 commented 1 year ago

Original comment by: joesinger12