[BLDG-SIM] Boiler Performance Curves

[Steve Gates]

The default boiler part-load performance curve is based on the ICES reports (Integrated Community Energy Systems) prepared by Oak Ridge National Laboratory in the 1970's.  This single curve is almost certainly not representative of all boilers.  In fact I would expect that the part-load performance of atmospheric boilers might be significantly different than from forced-draft boilers, where the percentage of excess air at part-load firing can be better controlled. 

The default MIN-RATIO is 0.25 for all fuel-fired boilers.  For versions 2.1e and earlier, Fred is correct in stating that the program holds the efficiency constant at part-load ratios below this value.  Version 2.2/eQUEST/PowerDOE address this issue by applying part of the START-UP-TIME to the portion of the hour the boiler is cycling.

If you have part-load data that includes the effect of cycling, then the best approach is to create a curve including the cycling portion of the range (use the CURVE-FIT command to generate the coefficients), and set the MIN-RATIO to zero or a very small fraction.  

Using a curve-generating routine other than the 2.2's CURVE-FIT algorithm may not give the correct results; DOE-2 expects that the part-load curve to give a value of exactly 1.0 at a part-load ratio of 1.0, and the CURVE-FIT algorithm ensures this (see the documentation for more information).  If you generate the curve using a different program, such as Excel, the best fit often does not yield the value of 1.0 at PLR = 1.0  There is a version of the program in development that can use any best-fit curve; this release will be available later this year.

The fuel-fired boilers do not have default curves correcting performance for supply and ambient ambient temperature (CAPACITY-FT, HIR-FT),  so the AQUASTAT-T setpoint should have no effect unless you also define the curves; these hooks were added in case someone had data and wanted to model this effect.

DOE-2.2 allows you to create libraries of boilers, chillers, VAV systems, etc.; similar to the material, construction, and window libraries of 2.1e (store the components in USRLIB.DAT).  If you have performance data for components that you would like to share, why not post them to this site so that others can include them in their libraries?
  ----- Original Message ----- 
  From: Fred Porter 
  To: BLDG-SIM at gard.com 
  Sent: Wednesday, February 12, 2003 4:58 PM
  Subject: [BLDG-SIM] Boiler Performance Curves


  Additionally:
  First, in DOE2.x, if I'm not mistaken, the BOILER "MIN-RATIO," or cycling threshold, holds the efficiency (PLR modified EIR) constant below the specified PLR, contrary to reality and intuition. I think this input defaults to a PLR of 20%, just a bit above where boiler efficiency starts to really go down the tubes. So this needs to be dropped to a very low value(where your EIR-fPLR curve goes wacko) if you roll your own curves.

  Aquastat temp (under BOILER), and HW reset scheds(HW-LOOP) can also have  major effects. Actually you need to create another custom curve(HIR-FT), and reduce the aquastat minimum, if you want the HW temp to interact with the boiler, which it most certainly does. See the DOE2.2 BOILER docs.

  Also, if you set up your model with optimistic assumptions (most defaults), you may miss many hours at very low (<5%) heating loads.

  In any case, most modulating boilers go down to 20 or 25%, and so may not show great savings compared to a 33% low fire.

  A method of calculating part load efficiency for boilers is outlined in the spreadsheets that can be downloaded from:
  http://www.keh-ee.com/cycling.htm

  These require some modification and care in use for purpose of developing DOE2 curves. Steady-state efficiency may vary somewhat with load, producing the manufacturer's cited increase in efficiency at 50% load. This cited efficiency improvment may also take into account some assumed HW reset, much as ARI NPLV chiller points include condenser "relief," at the lower loads.

  GOOD LUCK! In consideration of the other thread: Even with 3D IFC/DXF interfaces, architects (are any still reading) should not attempt this.

  Fred

  Wade McLaughlin wrote:

    Hello All,

    As part of a comprehensive energy study for a new elementary school, we are analyzing fuel savings opportunities for a proposed boiler with fully modulating burners as compared to a base-case sectional lo-hi-lo boiler.  So far, we have found that, all things being equal, there are no significant savings associated with such a measure (at least not for the boiler size and models we've been considering).

    We use PowerDOE for most of our energy simulations.  In the course of this study, we have come to question the default boiler part-load heat-input-ratio curve ( HW-Blr-HIR-fPLR) found in Bdllib.dat.  Until now, we have applied this curve to most base-case boiler calculations (perhaps mistakenly?).  Does anyone know of the origins of this curve or what it would have been based upon?  Most manufacturers we have spoken with state that peak boiler efficiency occurs between 25% and 50% load.  Using the default curve, peak performance occurs at full load and efficiency decreases by nearly 10% at half load.

    Also, if this curve is in error, can anyone suggest a method for developing a more reasonable curve estimate?

    Thanks,
    Wade

    Wade McLaughlin
    DMI
    450 Lexington St.
    Newton, MA  02466
    p 617-527-1525 x103
    f  617-527-6606
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