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Pros and Cons of operating a DOAS EW continuously

Stanley A. Mumma, Ph.D., P.E. Prof. Emeritus, Architectural Engineering

Penn State University, Univ. Park, PAsam11@psu.edu

ASHRAE June meeting: Montreal, 2011

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.

Learning Objectives for this Session

•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.

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Observation:some:

are not properly controllingenthalpy wheels (EW)

• energy modelers,

• energy analysis software,

• manufacturers, and

• owners

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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 IllustrationConsider 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.

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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 increases by 45,755 TH EW off. 55 hrs.

If on, cooling use increases 115

TH.

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Conclusion: operating the EW in KC all the time for a 10,000 scfm OA system equipped with a 70% effective () 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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PostludeTwo 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

TH.

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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 penalty, and is not best.

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Exploring measurement errorin 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 choice!

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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

0

50

100

150

200

250

300

V0 - BaselineEnergy Budget

V1 - LightingLoad

Reduction to0.75 W/ft2

V2 - ElectricLoad

Reduction to1.0 W/f2

V3 - PassiveHaus Insulation

Package;Insulation,

Windows andAir Tightness,Reduced OA

V4 - EfficientVAV; Increase

Chiller andBoiler

Efficiencies,Var SpeedHigh-Eff

Pumps/Fans

V5 - EnergyRecovery

(ERV) with V4

V6 - IndirectEvap (IDEC)Precooling

with V4

V7 - DedicatedOutside Air

System(DOAS)

V8 - DOASwith ERV

V9 - DOASwith IDEC

V10 - DOAS,ERV, andRadiant

V11 - DOAS,ERV, and FreeCooling Chiller

V12 - DOAS,ERV, and

GSHP

V13 - GSHP,ERV, and VAV

So

urc

e E

UI

(kB

tu/f

t2)

1A_Miami_FL

5A_Chicago_IL

7A_Duluth_MN

Office Only

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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 ComparisonMiami, FL

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

0 1 2 3 4 5 6 7 8 9 10 11 12 13EEM step

kBtu

/ft2

Modeling errors

DOAS corrections

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Pro of EW always “on”

• 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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