[BLDG-SIM] Boiler Performance Curves

[Fred Porter <fporter@xxxxxxxxxxxxxx>]

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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