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COP for ASHRAE 90.1-2016 Appendix G Baseline

asked 2021-12-27 23:36:05 -0500

mattkoch's avatar

updated 2022-02-21 20:45:19 -0500

It is my understanding that a sizing run for the baseline building must be done. The COP (for OS:Coil:Cooling:DX:SingleSpeed) is to be determined via COP = 7.84E-8 × EER × Q + 0.338 × EER, where EER is from an appropriate table in Appendix G, based on Q, and Q is (presumably) the result of the sizing run. Both EER and Q are to be "net" (includes fan power), while COP is "gross" (excludes fan power).

Said Q, obtained from the sizing run, might be the OS:Sizing:System Cooling Design Capacity (I think this might be net, let's call it Qn). Alternatively, it might be the OS:Coil:Cooling:DX:SingleSpeed Rated Total Cooling Capacity (though I think this might be gross, let's call it Qg).

Either way, prior to the sizing run, the COP for OS:Coil:Cooling:DX:SingleSpeed is just a best guess. It is then to be adjusted via the above equation, using a suitable Q, which may be Qn or Qg. If one were to do another sizing run with this updated COP, the new Qn or Qg would be different from the original Qn or Qg, giving a different COP, and so forth. This would have to be repeated a few times to "converge", I suppose, if it even did.

I believe even for the great and almighty Appendix G, such iteration would go too far. Instead, is it correct to just use a ballpark Q after the first (and only) sizing run to obtain and update the COP? if so, what is to be used - Qn or Qg?

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answered 2022-09-08 13:24:30 -0500

COP will not be used until AFTER the sizing run is complete. The "sizing run" is ideal in that only a zone supply air temperature is required to complete sizing. No COP is needed in this step. The program uses a heat balance to determine a load using occupants, lights, equipment, infiltration, etc. to determine the zone load. Then the program uses the user entered supply air temperature to calculate a zone air mass flow rate as Mdot,zone = Qzone / (Cp,supply air * (Tsupply air - Tzone)).

At the end of the sizing run the zone load (Q) and air mass flow rate (Mdot) are known quantities. Qload is net.

From this point forward the components can be autosized and will use the zone air mass flow rate for sizing the fan and coils. The zone load, Q, from the sizing run is not used to size the components. It's more of a check to compare with other metrics (e.g., if there is no outdoor air mixer blending air at the coil inlet then the coil capacity should roughly size equal to the zone load, cooling capacity will include fan heat, heating capacity will not include fan heat. The fan will simply size to meet the maximum zone cooling and heating air mass flow rate. The coils will be sized using the zone air mass flow rate and the coil entering/mixed and exiting air temperatures and humidity ratios that are used to calculate coil entering/exiting enthalpy (e.g., Qcoil = Mdot,zone * (Hcoil,in - Hcoil,out)). This calculation will provide the coil capacity. Qcoil is gross capacity. Now that a coil capacity is known the power of the system can be determined using EER/COP.

COP is not required for a sizing run. COP is used to set the properties (power) of the HVAC system once the coil capacity is known, which happens after a sizing run.

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Well, that is quite enlightening! So, basically, use a sizing run with ideal air loads turned on to get zone mass flow rates, then hard-size these mass flow rates and let everything else be auto-sized with ideal air loads turned off and real systems in place? This still leaves me unsure about which Q to use for COP = 7.84E-8 × EER × Q + 0.338 × EER? LEED does not care about Q, only about power, doesn't it?

mattkoch's avatar mattkoch  ( 2022-09-08 20:01:23 -0500 )edit

There are no systems used during zone sizing. What you see in the docs that say "ideal system" does not mean the "ZoneHVAC:IdealLoadsAirSystem", it means the program will meet the zone load exactly given a supply air temp and heat balanced zone load. If you want a COP that applies to a coil (HVAC system) then use Qcoil (capacity) to calucalate LEED COP.

rraustad's avatar rraustad  ( 2022-09-08 20:20:47 -0500 )edit

@rraustad, can you clarify this equation:

Qcoil = Mdot,zone * (Hcoil,in - Hcoil,out)

Is H enthalpy (in J)? If so, how does the product of the mass flow rate and enthalpy equal cooling capacity? Shouldn't Mdot,zone be instead:

Qcoil = Mdot,zone * (cp * (Tcoil,in - Tcoil,out) + lv * (Wcoil,in - Wcoil,out))

[Edit: fix quote formatting].

saeranv's avatar saeranv  ( 2022-09-09 03:01:41 -0500 )edit

I am confused now. But perhaps there is a shortcut. Does the following call to an openstudio-standards function accomplish the right thing?

standard = Standard.build("90.1-2016")
standard.model_run_sizing_run(model,Dir.pwd.to_s)

And if so, and these are only zone sizes, how would I combine them into system sizes, other than assuming they are coincident and adding them all up? I know that COP only comes in at the back end, but a COP I must have for/from ASHRAE 90.1 Appendix G.

mattkoch's avatar mattkoch  ( 2022-09-09 09:15:46 -0500 )edit

With Q in [W], mdot in [kga/s], h in [J/kga], p in [Pa], T in [K] and w in [kgw/kga], the following is accurate:

Q = mdot * (hout - hin)
hout = h(pDA,TDA,wDA)
hin = h(pMA,TMA,wMA)

The following with cp in [J/kgaK] and dhv in [J/kgw] is just an approximation, as cp itself depends on wDA and wMA:

hout-hin = cp*(TDA-TMA) + (wDA-wMA)*dhv

You can see that by multiplying and subtracting the following:

hout = (cpa + wDA*cpw)*TDA + wDA*dhv
hin = (cpa + wMA*cpw)*TMA + wMA*dhv

Unless your cp accounts for the individual cpa in [J/kgaK] and cpw in [J/kgwK], it's complicated

mattkoch's avatar mattkoch  ( 2022-09-09 09:31:10 -0500 )edit
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answered 2022-09-08 12:44:24 -0500

saeranv's avatar

I'm not familiar with what Appendix G requires for sizing runs, but I've always seen the ideal loads air system used for them, which conceptually makes a lot more sense than computing the COP recursively from some assumed COP.

With the ideal loads system, the sizing run Q is simply the peak thermal energy demand from the system's zones, and issues like the fan energy or COP simply don't exist. Is there a reason why using the ideal loads air system Q wouldn't work here?

Note: I realize this is an old question, but I think resolving this would still be helpful.

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Thank you for looking into such an old question! Very interesting and appreciated. As indicated above, I am still puzzled as to which Q to use to look up EER and then convert both to COP. I don't think the ideal air zone loads can simply be added up, because they are not coincident?

mattkoch's avatar mattkoch  ( 2022-09-08 20:07:11 -0500 )edit

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Asked: 2021-12-27 23:36:05 -0500

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Last updated: Sep 08