[BLDG-SIM] Boiler Performance Curves

[Michael Wilson]

I haven't used Doe 2.2, just Doe2.1e, but I suspect
the problems with the boiler are the same. There's a
bit of a glitch in the boiler model when it comes to
part load, as Doe2 considers the boiler capable of
cycling "OFF". When it does, no energy is consumed. Of
course in reality boilers aren't OFF unless somebody
shuts them down for summer. When they're not firing
they still have jacket and stack losses, which have to
be accounted for when the boiler fires again. But in
Doe2, a boiler with a higher part load ratio will
cycle "OFF" more often, reducing consumption. So a
boiler with a low part load ratio will be less
efficient than one with a high PLR, the opposite of
what it should be. I get around this by specifying
boilers with a minimum PLR of 0.001 and then
incorporating all the losses and the cycling action
into the boiler curve. 

Michael Wilson


--- Fred Porter <fporter at archenergy.com> wrote:

---------------------------------
Additionally:
First, in DOE2.x, if I'm not mistaken, the BOILER
"MIN-RATIO," or cyclingthreshold, holds the efficiency
(PLR modified EIR) constant below the specifiedPLR,
contrary to reality and intuition. I think this input
defaults to aPLR of 20%, just a bit above where boiler
efficiency starts to really godown the tubes. So this
needs to be dropped to a very low value(where
yourEIR-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 withthe 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 notshow great savings compared to a
33% low fire.

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

These require some modification and care in use for
purpose of developingDOE2 curves. Steady-state
efficiency may vary somewhat with load, producingthe
manufacturer's cited increase in efficiency at 50%
load. This cited efficiencyimprovment may also take
into account some assumed HW reset, much as ARINPLV
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
studyfor a new  elementary school, we are analyzing
fuel savings opportunitiesfor a  proposed boiler with
fully modulating burners as compared to a 
base-casesectional lo-hi-lo boiler.  So far, we have
found that, all  things beingequal, there are no
significant savings associated with such a  measure
(atleast 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 curveto
most base-case boiler calculations (perhaps
mistakenly?).   Does anyoneknow of the origins of this
curve or what it would have been based  upon? Most
manufacturers we have spoken with state that peak
boiler  efficiencyoccurs between 25% and 50% load. 
Using the default  curve, peak performanceoccurs at
full load and efficiency  decreases by nearly 10% at
half load.     Also, if this curve is in error, can
anyonesuggest  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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=====
Michael Wilson
455 Elphinstone Ave.
Gibsons, BC, V0N 1V1
604-886-9864 phone
604-676-2604 fax

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