Transcript Slide 1

Pros and Cons of
operating a DOAS
EW continuously
ASHRAE June meeting:
Montreal, 2011
Stanley A. Mumma, Ph.D., P.E.
Prof. Emeritus, Architectural Engineering
Penn State University, Univ. Park, PA
[email protected]
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Learning Objectives for this Session
•
•
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Recognize enthalpy wheel control errors.
Learn the correct enthalpy wheel control action for all OA conditions.
Learn the extent of energy penalty when improperly controlled.
Learn the importance of maximizing air side free cooling.
ASHRAE is a Registered Provider with The American Institute of Architects
Continuing Education Systems. Credit earned on completion of this program
will be reported to ASHRAE Records for AIA members. Certificates of
Completion for non-AIA members are available on request.
This program is registered with the AIA/ASHRAE for continuing professional
education. As such, it does not include content that may be deemed or
construed to be an approval or endorsement by the AIA of any material of
construction or any method or manner of handling, using, distributing, or
dealing in any material or product. Questions related to specific materials,
methods, and services will be addressed at the conclusion of this presentation.
Observation:
some:
• energy modelers,
• energy analysis software,
• manufacturers, and
• owners
are not properly controlling
enthalpy wheels (EW)
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An EW that is “on” all the time in;
• software, or
• equipment,
leads to serious errors in
• assessing, or
• realizing,
efficient energy performance.
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An Illustration
Consider a 10,000 scfm OA system operating 24/7,
365 days/year in Kansas City, MO.
A balanced flow EW is employed with an
effectiveness of 0.7 (note: effectiveness is defined
as the ratio of the actual heat transfer to the heat
transfer with an infinite area heat exchanger).
The RA to the EW is assumed to be fixed at 75F DBT
and 50% relative humidity.
The constant required SA DBT and WBT are 48F,
i.e. 50 grains/lbmDA to meet the space latent load.
The energy performance of a properly controlled EW
is compared with one incorrectly “on”, or operating,
all the time!
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In this psychrometric hot and
humid OA region, the EW
should run full speed.
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In this psychrometric humid OA
region between the RA enthalpy
and the required SA humidity
ratio, the EW should be off.
Operating the EW in this region
elevates the enthalpy of the OA
entering the CC and hence the
energy use.
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In this psychrometric warm (hotter
than the space) but dry OA region
(mechanical dehumidification is not
necessary to meet the space latent
load using this air directly), the EW
should be off. Operating the EW in
this region elevates the humidity ratio
of the OA entering the CC requiring
latent cooling in addition to sensible
cooling, increasing the energy use
above that needed to sensibly cool
the air when the EW is off.
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In this psychrometric cool
(cooler than the space)
but dry OA region
(mechanical
dehumidification is not
necessary to meet the
space latent load using
this air directly), the EW
should be off. Operating
the EW in this region
elevates the humidity ratio
of the OA entering the CC
requiring latent cooling in
addition to sensible
cooling, increasing the
energy use above that
needed to sensibly cool
the air when the EW is off.
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In this psychrometric cold (colder
than the required SA DPT) and dry
OA region (mechanical
dehumidification is not necessary to
meet the space latent load using
this air directly), the EW should
modulate as necessary to avoid
overcooling or meet the design SA
DBT (48F in this illustration).
Modulating the EW in this region to
just avoid overcooling, enables the
system to do most, if not all, of the
cooling as an economizer allowing
the cooling plant to be off.
Mechanical cooling is required when
the EW is operating full speed. 10
Hot humid OA, 2,666 hrs. EW
on either method, no difference
EW should be off! 1,255 hrs. If
EW on, cooling use increases by
10,500 Ton Hrs (TH).
EW should be off! 1,261
hrs. If EW on, cooling use
increases 18,690 TH
EW speed to modulate to
hold 48F SAT. 3,523 hrs. If
EW full on, cooling use
off. TH
55 hrs.
increases byEW
45,755
If on, cooling use
increases 115
TH. 11
Conclusion: operating the EW in
KC all the time for a 10,000 scfm OA
system equipped with a 70%
effective (e) EW will consume 75,060
extra TH of cooling per year. At 1
kW/ton and $0.15/kWh—this
represents $11,260 of waste, and
takes us far from NZE buildings.
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Postlude
Two items of EW control that were not
discussed include:
– Wheel cleaning when EW off. One way of
accomplishing cleaning is to energize the wheel
once an hour for 1 min, resulting in about 20 air
flow reversals for wheel cleaning.
– Frost prevention. There are many ways to do
this. One is discussed in the ASHRAE paper at
this link:
http://doas-radiant.psu.edu/4428.pdf
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Maximizing free cooling w/ DOAS
• In the illustration presented above, it was assumed
that the SA DBT could not drop below 48F, however
it is not uncommon with just ventilation air (i.e.
DOAS) that 48F is not cold enough to meet all the
space sensible loads.
• However allowing the SA temperature to drop below
48F at the diffusers can be problematic. The OA
can be tempered with the sensible cooling
equipment without loosing free cooling. Such an
example is presented at this link:
http://doas.psu.edu/IAQ_summer_05.pdf.
Radiant panels are discussed in this ASHRAE
article, but any hydronic terminal unit can do the
same be it fan coil, chilled beam, fan powered box,
etc.
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One might ask: Why not base EW
control on DBT rather than enthalpy?
• To illustrate, consider one common DBT
control that uses 75F DBT as the on/off
switch point when it is hot (i.e. hotter than the
required SA DPT, 48F in this example). That
is to say, when the OA DBT is >75F, the EW
operates full speed, and when 48F < OA
DBT < 75F the EW is off.
• When the OA DBT drops below 48F, the EW
speed is modulated to hold a 48F DBT SAT.
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EW should be on! 1,048 hrs. If
EW off, cooling use increases by
9,540 Ton Hrs (TH).
EW should be off! 72 hrs.
If EW on, cooling use
increases 1 TH
EW should be off.
55 hrs. If EW on,
cooling use
increases 115
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TH.
Discussion of the 3 red regions where
the EW is operating incorrectly
• From an energy use perspective, this control is an
improvement over operating the EW all the time.
• However energy recovery is lost for 1,048 of the
2,666 hours when the OA is hot and humid (i.e. OA
enthalpy > RA enthalpy).
• It seems inconceivable to render the EW useless
for 40% of the hours in that hot and humid region.
• The penalty for operating the EW in the triangle
with this control is not wise, but has minimal energy
penalty.
• Finally, operating the EW any time it is dry, as
shown in red, imposes an unnecessary energy
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penalty, and is not best.
Exploring measurement error
in the control of EWs.
• If EW control during the hot humid times is
enthalpy based, a +5% RH error
(absolute) in measurements was explored.
• If EW control during the hot humid times is
DBT based, a 1F error in measurement
was explored.
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+5% error in RH reading. Causes EW
to be off when it should be on. 206
hours, 270 extra TH of cooling needed,
costing $40.45 when cooling uses 1
kW/ton and energy costs $0.15/kWh
-5% error in RH reading. Causes EW to
be on when it should be off. 34 hours, 25
extra TH of cooling needed, costing $3.80
when cooling uses 1 kW/ton and energy
costs 0.15/kWh
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If a DBT error of 1F caused the EW to
operate above 76F rather than 75F, that
1F band contains 153 hours of data. It
would increase the cooling load by 2,255
TH and increase the operating cost by
$338 assuming 1 kW/ton cooling
performance and $0.15/kWh utility cost.
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Sensor selection conclusion.
• Error in the RH readings cause error in the
enthalpy computation, but holds them near
the space enthalpy. Consequently, the
impact on energy use is small.
• Error in the DBT readings occur in a narrow
vertical band. However that band contains
OA points with exceedingly high enthalpy.
Consequently the impact on energy use is
substantial by comparison.
• Considering both error, and energy savings,
it is clear that enthalpy control is the proper
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choice!
Example of the modeling error
• The following slide was prepared by a
modeling group to illustrate energy saving
potential of various energy efficiency
methods (EEM). The erroneous part is
encircled.
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Brigade (Office) EEM Comparison
300
Office Only
250
Source EUI (kBtu/ft2)
200
150
100
50
1A_Miami_FL
5A_Chicago_IL
7A_Duluth_MN
0
V 0 - B aseline
Energy B udget
V 1 - Light ing
Load
Reduct ion t o
0.75 W/ f t 2
V 2 - Elect ric
Load
Reduct ion t o
1.0 W/ f 2
V 3 - Passive
Haus Insulat ion
Package;
Insulat ion,
Windows and
A ir Tight ness,
Reduced OA
V 4 - Ef f icient
V A V ; Increase
Chiller and
B oiler
Ef f iciencies,
V ar Speed
High-Ef f
Pumps/ Fans
V 5 - Energy
Recovery
(ERV ) wit h V 4
V 6 - Indirect
Evap (IDEC)
Precooling
wit h V 4
V 7 - Dedicat ed
Out side A ir
Syst em
(DOA S)
V 8 - DOA S
wit h ERV
V 9 - DOA S
wit h IDEC
V 10 - DOA S,
ERV , and
Radiant
V 11 - DOA S,
ERV , and Free
Cooling Chiller
V 12 - DOA S,
ERV , and
GSHP
V 13 - GSHP,
ERV , and V A V
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Modeling error example cont’d
• The next slide for Miami, FL was reproduced.
The black line is the “as modeled” results
with the EW (ERV) control errors.
• The blue line is the result of correcting the
EW control errors.
• Significantly, erroneous conclusions result
from sloppy modeling—to the detriment of
our energy, IEQ, and economic wellbeing.
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Office EEM Comparison
Miami, FL
210
200
190
180
170
160
kBtu/ft2
150
140
130
120
110
100
90
Modeling errors
80
DOAS corrections
70
60
0
1
2
3
4
5
6
7
EEM step
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9
10
11
12
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Pro of EW always “on”
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•
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Easy to model
Modeling inputs easy
Lower first cost
No thought required
That’s the way we have always done it
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Con of EW always “on”
• Huge waste of energy, both summer and
winter.
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Conclusion
EWs must be controlled,
based on measured OA and RA enthalpy,
when the OA humidity ratio exceeds the
design SA humidity ratio,
for efficient operation.
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