residential water heater test procedures: why you should care · 2016. 9. 20. · recovery...

Residential Water Heater Test Procedures: Why You Should Care

Jay BurchNational Renewable Energy Laboratory

Residential Energy Efficiency Meeting 2010

Presentation Overview

I. BackgroundII. Current Test: Simulated useIII. Using test results in modelsIV. A few pitfallsV. Proposed new test: Input-outputVI. Appendix: Model calibration


I. Background

Building America Program:

• Domestic water heating:– 13% of residential source energy– Percentage of load increases with envelope/HVAC efficiency– Options: storage tank: gas/elec; tankless, condensing, heat pumps, solar

• Water Heater (WH) options analysis– Teams compare and recommend products– Currently, test results apply only to the test use volume/patterns– Realistic/BA draws very different from the test – Need to simulate under BA conditions

• Problem: deriving simulation inputs from WH test data

Presenter

Presentation Notes

Good economics result from good performance, low cost, and long life. Can we extend this success with some degradation of performance from operating at higher temperatures?


II. Current Water Heater Test

Doe Standard Test conditions:

• One day duration• 64.3 gal draw (> average?)• 6 equal draws 1 hr apart• Same draw for all sizes• Tset = 135 oF• Tin = 58 oF• Tenv = 67.5 oF

• Correction made for stored energy change over cycle: δEstore = Cstore δTstore,avg

Presenter

Presentation Notes


Standard vs. Realistic Draws


Draws: Volume vs. Time

0

5

10

15

20

48 60 72 84 96

Time [hr]

Dra

w V

olum

e [G

al]

Doe Standard Test Realistic Draws

Test results

• Energy Factor: EFstd-test = Eout,day/Eaux,day (E = energy)• Eout,day = Mdrawcp∆Tout-in = 41,092 Btu/day• Eaux,day = auxiliary energy used over the day

• Recovery Efficiency: REgas = Eout,dr/Eaux,dr• Eout,draw = energy withdrawn in one10.6 gal draw• Eaux,draw = energy input recovering from that draw• Lower than conversion efficiency because of tank losses• REelec ≡.98 for all tanks (by fiat)

• Input power: Paux = measured auxiliary input

• Measured Vst, Ust not reported: change?


Energy Factor Uncertainty

• Energy Factor: EFstd-test = ± ~.01-.02 (Lutz ~1999)

• Dominant error: Tstore,avg as avg. of six point sensors– Used in stored energy correction: δEstore = Cstore δTstore,avg

– Bottom sensor near or in thermocline at bottom element in electric tanks

• Compare ± ~.02 to min-max range ~.89 - ~.96 for electric WH

Residential Energy Efficiency Meeting 2010Plot from M. Thomas

ASHRAE2010

Sensor avg.


III. Tank Models

• Algebraic Models– Time-integrated energy balance:

– Eaux= draws + losses = Mcp∆Tset-in+UA ∆Tset-env ∆tperiod

– E.g.: WATSMPL

• 1 Dimensional finite difference• Account for stratification, draw and heat source dynamics, • Still simple: appropriate for annual simulations• E.g.: TRNSYS

• 2-3 Dimensional finite element (CFD)• T,v fields; hardware design tools• Slow: inappropriate for annual simulations

Presenter

Presentation Notes



Electric TankCold inHot out

Thermal shorts

node 1

node 2

node N-1

Electric elements

Skin insulation

TTenvenv

Uskin node N

Volume

ηconv ≡ 1

Height of element

UAnode,therm-shorts

Presenter

Presentation Notes



Gas Tank

Cold inHot out

Gas Burner/pilot

Convection loop

Additional inputs

UAflue Central flue

ηconv

Presenter

Presentation Notes


Key parameter: Conversion Efficiency

• Conversion efficiency: ηconv = Pto-water/Paux-in (P=power)• Pto-water = Ctank(dTtank,avg/dt) only when Ttank,avg = Tenv

– Tank losses are NOT to be included

• RE includes tank losses during the recovery period

• For ELECTRIC: ηconv ≡ 1 • For GAS: ηconv must be calculated from RE, UA



Inputs from Test Data

Electric tank:UAt,elec = Qout,day(1/EF–1)/[(∆Tt-env∆tday)(fabove – fbelow ∆Tin-env /∆Tset-env )]

ηconv ≡1

Gas tank:UAt,gas = (RE/EF–1)/[∆Tset-env(∆tday/Qout,day–1/(PauxEF))]ηconv = RE + UAtank,gas(∆Tset-env)/Paux

ηconv = ~ RE + .01-.02

Presenter

Presentation Notes


Spreadsheet Tool


Implements input formulae derivation, with paper on methodftp://ftp.nrel.gov/pub/solar_waterheat-out/wh_model

IV. Pitfalls Using Current Test Data

Common Blunders:• RE is not equal to ηconv

• ηconv is the model input for efficiency, not RE

• EF is not ηconv • Eauxiliary = Ethermal/ηconv is correct• Eauxiliary = Ethermal/EF is wrong



Error in U value vs. EF

• δU/U = EF*δ(EF)/(1 – EF)

0

0.1

0.2

0.3

0.4

0.4 0.5 0.6 0.7 0.8 0.9 1

Frac

tion

Erro

r in

U d

U/U

Energy Factor

Error in U value vs. EF

δ(EF)=.02

Bad news for electric

Sensitivity to Element Location


2

2.5

3

3.5

0 2 4 6 8 10

Infe

rred

U v

alue

(Btu

/hr-

ft2-

F)

Unheated Volume (gallons)

Sensitivity to unheated volume(in electric tanks only)

Element at 10 inch

V. New Test Method: Input/Output


Output (Btu/hr)

Input (Btu/hr) Steady state

Cyclic, low load point“Standby” Or

“Idle” Loss

High use/steady state

No use/draw = idle

Low use rate/cyclic

From T. Butcher ASHRAE2010

Model Inputs from I/O


Output (Btu/hr)

Input (Btu/hr) Steady state

Cyclic, low load point“Standby” Or

“Idle” Loss

High use/steady state

No use/draw = idle

Low use rate

Y-intercept:UA=Pinput-nodraw/∆Ttank-env

Slope of line:ηconv = 1/slope

19

Input-Output Validation

draw 6

draws 1-5

0

5000

10000

15000

20000

25000

30000

0 5000 10000 15000 20000 25000

output (BTU/hr)

inpu

t (BT

U/hr

)

Low and High Use Pointscalculatedcalculations

y = 1.2857x + 750.14

efficiency ( total output / total input ) = 58.1%

High Rate of Use

Low Rate of Use From J. Lutz ASHRAE2010

I/O Method Validation


y = 1.0735x + 211.95R2 = 0.999

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

0 10000 20000 30000 40000 50000 60000 70000 80000 90000

output (Btu/hr)

inpu

t (Bt

u/hr

)

Tankless water heater

From T. Butcher ASHRAE2010

I/O Method Advantages

• Allows any use profile• Get Pout, Pin = m*Pout + b

• UA, ηconv error should decrease• Applies to tank water heaters and tankless

(some issues)

• But: Heat pump water heaters? Condensing?


Conclusions

• Currrent test method: simulated use test• Methods for input parameters laid out• Uncertainty in U value large for higher EFs

• Future test method: Input-Output test• Under development: 1-2 years from now?• Major changes for heat pumps?• Reduction in parameter uncertainty


Thank you for listening.Questions?

Time Allowing:Model calibration tests and tankless


Model-based Test-Rate


Generic Model

Real Water Heater

Test Protocol

Test Data

Calibrated Model

Any rating conditions

Calibrated Model

Rating

Model-based:Test ⇒ Calibrate ⇒ Rate

Tankless Water Heaters


Losses between draws demand modeling of internal mass and losses to ambient

Tankless Thermal Model

Measured value

Tenv

mcp (Tout –Tin) .

CTWH

UA

ηconv

.Qgas,in

...N hx nodesCold in Hot out

Model Parameter

This model has three parameters

Presenter

Presentation Notes

UA is total, UA/node is UA/N. Similarly, C_TWH is the total capacitance, split equally between nodes, Cnode = C_twh/N

Example Test Protocol


Temperatures

Flow rates

Temperature Decay with heat off: capacitance signal

Hot in, low flow ⇒∆Thx: UA signal

Steady state burn: efficiency signal

Inlet water

Oulet water

Inlet water

Nat. Gas

Tankless: Sensitivity to Draw Profile

Draw net efficiency is sensitive to:• Delay between draws• Length of draw

Presenter

Presentation Notes

This shows efficiency is highly dependent on the length of the draw and length of time since the last draw.

A Tale of Two Draw Patterns

Building America 2 Bdrm benchmark64 gal/day, 30 draws

Net Efficiency: 77%

Building America 2 Bdrm low use22 gal/day, 12 draws

Net Efficiency: 65%

Tankless efficiency from DOE test is 0.80

Validation Draw Profiles

Residential Energy Efficiency Meeting 2010From P. Glanville,

ASHRAE2010

I/O validation: tankless

Residential Energy Efficiency Meeting 2010From P. Glanville,

ASHRAE2010

Tankless Conclusions

• Currrent test method:• Badly overestimates performance (6 big draws)

• Proposed I/O method:• ~ 3% underprediction with draws having long delays• Can eliminate error• Appears acceptably-accurate

• Potential model-based method:• Accommodates any draw patterns/delays• Simplest possible test, but complex analysis• Demands that generic model exists


residential water heater test procedures: why you should care · 2016. 9. 20. · recovery...

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