Towards Prediction of Artificial Monolayer Performance for Water

Conservation

Pam Pittaway & Nigel Hancock

National Centre for Engineering in Agriculture

University of Southern Queensland, Toowoomba.

ARTIFICIAL MONOLAYER TECHNOLOGY:

• Potential for cost-effective water saving; BUT

• Averaged daily data indicates highly variable performance.

THIS PRESENTATION:

• Understand the cause of highly variable performance to predict optimal conditions for cost-effective monolayer application.

ARTIFICIAL MONOLAYER TECHNOLOGY IN PRACTICE

VARIATION IN MONOLAYER FIELD TRIAL PERFORMANCE (Craig et al 2005)

Location Storage size(km2)

Trial & monitoring period

(days)

Evaporation Reduction (%)

University of Southern Queensland, Toowoomba, Qld

0.0001 1- 6 days2- 8 days3- 6 days4- 7 days5- 7 days

38%17%10%38%40%

Capella Qld 0.042 1- 9 days2- 8 days3- 7 days4- 8 days

0%0%0%0%

Cubby Station, Dirranbandi, Qld

1.2 1- 5 days2- 10 days3- 8 days

31%27%0%

ARTIFICIAL MONOLAYERS FOR EVAPORATION REDUCTION

• Monomolecular fatty alcohol films compressing at water surface to retard evaporative loss

• Long-chain, saturated fatty alcohols form continuous condensed film

• Condensed film retards molecular transfer across liquid thermal and gaseous boundary layers

• Wind speeds >6 m sec-1 disrupt films

1. METEOROLOGICAL DRIVERS OF EVAP LOSS AT MACRO-SCALE (Hancock et al. 2011)

2. PLUS DRIVERS AT MONO-MOLECULAR SCALE (Hancock et al. 2011)

• Damping capillary waves reduces wind shear RG reduced and eddies (Rayleigh-Benard convection) RL reduced.

3. IMPACT OF ARTIFICIAL MONOLAYER AT MONO-MOLECULAR SCALE

Liquid thermal boundary layer(LTBL)

(Figure 7.1 Davies and Rideal 1963)GAS PHASE

A condensed monolayer increases RG , RI & RL

• Cold surface film –thermally unstable, strong eddies reduce RL.

• Warm surface film –thermally stable, no eddies increase RL.

4. IMPACT OF MICROMETEOROLOGY ON RESISTANCE TO EVAPORATIVE LOSS

Liquid thermal boundary layer(LTBL)

(Figure 7.1 Davies and Rideal 1963)GAS PHASE

5. METEOROLOGICAL DRIVES AT THE MACRO SCALE

Q* radiation flux

QE turbulent

latent heat flux

QH sensible

heat flux

(Δ heat storage)

(Δ water current heat transfer)(Fig 3.14 Oke 1987)

6. METEOROLOGICAL DRIVERS AT THE MICRO SCALE

If θa – θw> 0 induces a cold surface film (θ0–θw<0),small RL

induces evap loss.

Increasing wind speed to 1.5 m sec-1

increases the cold surface film, reducing RG & RL, increasing evap loss.

1= reservoir2 = 0 ms-1 wind3 = 0.5 ms-1 wind4 = 1.5 ms-1 wind

surface – subsurface C

Air – subsurface C

Fig 2.5, Gladyshev (2002)

6. METEOROLOGICAL DRIVERS AT THE MICRO SCALE concluded:

• Air–subsurface water (θa – θw) is a surrogate of QH

• Surface–subsurface water (θ0 – θw) is a surrogate of Liquid Thermal Boundary Layer resistance

• (θ0 – θw) <0 = cold surface film (thin LTBL, < RL)

• (θ0 – θw) >0 = warm surface film (thick LTBL, > RL)

TRIALS: IMPACT OF PHYSICAL COVERS ON MICROMETEOROLOGY & RESISTANCE

TO EVAPORATIVE LOSS

Trial 1 black Atarsan cover on x2 tanks, monolayer on x1 tank

Trial 2 white shade cloth cover on x2 tanks, monolayer on x1 tank

1: Pyranometer; 2: Air temperature and relative humidity probe; 3: Net radiation sensor; 4: Infrared temperature sensor; 5: Floating thermocouple; 6-9: Fixed thermocouples.

10m

0.1m

0.3m

0.5m

0.6m

0.7m

1.2

m

0.8

m

1 2 3

4

5

7

6

8

9

INSTRUMENTATION ABOVE AND UNDER PHYSICAL COVERS

NOT TO SCALE

Atarsun cover

18/1/10 25/1/10 1/2/10 8/2/10 15/2/10 22/2/10 1/3/10

Ra

infa

ll (m

m)

0

20

40

60

80

100W

ate

r d

ep

th (

mm

)

400

500

600

700

800

900

1000

White Cover

22/2/10 1/3/10 8/3/10 15/3/10 22/3/10 29/3/10

Wa

ter

de

pth

(m

m)

400

500

600

700

800

900

1000

Ra

infa

ll (m

m)

0

20

40

60

80

100

Covered tank depth Uncovered tank depth rainfall

EFFECT OF PHYSICAL COVERS ON EVAPORATIVE LOSS:

Black cover >> effective in reducing evap loss but …….

ADDING C18OH MONOLAYER NO IMPACT

Black

DIURNAL ENERGY BALANCE FOR SHALLOW WATER (Fig 3.15 Oke 1987)

Shallow water Japan

(QG is soil heat flux)

Atarsun Cover

14/2/2010 12 15/2/2010 12 16/2/2010 12 17/2/2010 12

Tem

pe

ratu

re (0C

)

0

20

40

60

80

Re

lativ

e H

um

idity

(%

)

0

20

40

60

80

100

White Cover

7/3/2010 12 8/3/2010 12 9/3/2010 12 10/3/2010 12

Tem

pe

ratu

re (0C

)

10

20

30

40

50

60

70

80

Re

lativ

e H

um

idity

(%

)

40

60

80

100

Temperature Relative Humidity

IMPACT OF COVERS ON QH UNDER LOW WIND (<6m sec-1)

Black cover absorbs & re-radiates heat (>> QH?)

White cover reflects heat (< < QH?)

Black

IM PACT OF MONOLAYER ON QE\ &/OR QH

(hourly data, for 3 days wind <6 m s-1)

NO EVIDENCE OF

ADDITIONAL EVAPORATION

REDUCTION.

-10 0 10 20 30 40

Tw

f,c - T

w,0

.5,c (

C)

-4

-2

0

2

4

6

8Clean Tank 1 C18OH Tank 2

Ta,c - Tw,0.5,c (C)

-6 -4 -2 0 2 4 6 8 10 12

Tw

f,c -

Tw

,0.5

,c (

C)

-1.5

-1.0

-0.5

0.0

0.5

1.0

Black cover

White cover

(θa – θw)

(θ0 –

θw)

(θ0 –

θw)

IMPACT OF COVERS ON LIQUID THERMAL BOUNDARY LAYER AT THE MACRO SCALE

Atarsun cover

Wa

ter

tem

pe

ratu

re (0 C

)18

20

22

24

26

28

Wa

ter

tem

pe

ratu

re (0 C

)

18

20

22

24

26

28

Uncovered

18/1/10 25/1/10 1/2/10 8/2/10 15/2/10 22/2/10 1/3/10

Wa

ter

tem

pe

ratu

re (0 C

)

18

20

22

24

26

28

22/02/10 1/03/10 8/03/10 15/03/10 22/03/10 29/03/10

Wa

ter

tem

pe

ratu

re (0 C

)

18

20

22

24

26

28

0.1 m 0.3 m 0.5 m surface

Black cover water thermal gradient

White cover water iso-thermal

IMPACT OF COVERS ON LIQUID THERMAL BOUNDARY LAYER concluded

Date

22/3/10 29/3/10 5/4/10 12/4/10 19/4/10 26/4/10

Tw

f,c

- Tw

, 0.5

(0 C

)

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Atarsun cover White shade cloth no cover

Warm surface film (thick LTBL)

Cold surface film (thin LTBL)

black

CONCLUSIONS

1. Monomolecular films increase R in liquid thermal (RL) & gaseous (RG) boundary layers

2. Calm conditions with thermally stable LTBL (warm surface film),

RL > Rmonolayer (no effect)

3. Light wind, thermally unstable LTBL,

RL < Rmonolayer (water savings)

1. Hourly analysis is ESSENTIAL to interpret R and drivers of evaporation (QH, QE, Q*)

THANK YOU