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Theoretical studies ofliquid desiccant columns

for hybrid air conditioner

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Introduction

Application of low humidityEssential forarchives, museums,etc.

Desirable for human comfort

Vapor compression systemDehumidification by vapor condensation

High COP but deep dehumidification notpossible

Desiccant cooling systemDehumidification by absorption of vapor

Deep dehumidification but low COP

Proposed hybrid systemDehumidification by both condensation and

absorption

High COP and more dehumidification 2

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Analysis of three circuits

a) Solution circuit

b) Vapor compression circuit

c) Air circuits Simulation of the system

Fabrication and testing of a 0.8 TR capacity LDVC hybrid

system

Objectives

3

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LITERATURE SURVEY

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Yadav, 1995Conducted experiments in conventional vapour compression

system and liquid desiccant cycle. It has an energy saving of 80% at

90% latent heat load.

Khalid et al., 1997Simulated an open cycle vapour absorption and liquid desiccantsystem using LiBr for the process of absorption and

dehumidification found suited for hot and humid climate

Kessling et al., 1998Liquid desiccant cooling systems enable efficient energy storage for

air dehumidification and air-cooling using low temperature heat.

Meunier, 1998Shows that solid sorption is very effective low grade cooling not

only for air conditioning but for deep-freezingalso.

Fanger, 1999 The perceived indoor air quality (IAQ) increases with a decrease inrelative humidity, as long as it Is kept between 30 to 70%

Techajunta et al.,

1999

Conducted experiments in a solar simulator on solid desiccant

regeneration and air dehumidification for air conditioning in a

tropical humid climate, and found that regeneration rate is strongly

dependent on insolation, and slightly affected by air flow rate.

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Armando et al., 2000 Used lithium bromide as the liquid desiccant, modeled an opensystem and observed the system to be more efficient in colder and

drier climates and that system efficiency can be improved by

employing indoor air re-circulation

Jain, 2001 The temperature required for regenerating the liquid desiccant islow; therefore solar energy can be utilized effectively.

Dieng and Wang2001

Conducted review on solar adsorption technologies and emphasizedthe possibility of using non-polluting materials and energy saving

(more than 50%) as the important characteristics .

Goktun and Deha Er,

2001

Conducted theoretical evaluation of the maximum overall

performance of a hybrid air conditioning system which consists of a

conventional vapour compression system (VCS) cascaded withsolar assisted vapour absorption system.

Ahmed, 2003Studied the desorption characteristics of liquid desiccant bed for

solar dehumidification for air conditioning systems. Air stream at a

low grade temperature is applied in the desorption process

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R2

Evaporator

Expn.

Valve

Condenser

R1

R3R4

Compressor

VAPOUR COMPRESSION SYSTEM

conditioned space ( Supply air )

A4A5A4

A1

Ar

A2

Pump2

D2

A6

Pump1

A3

Dehumidifier

D2

D3

Regenerator

D4

D1

WITH AIR CIRCUIT

AND DESICCANT LOOP

Block diagram of the proposed hybrid system

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dry bulb temperature

humidityratio

A2

A4

Ar

A1

A5

R3R2

Compressor

R1

R4

Evaporator

Expn.

Valve

Condenser

A1

A4

conditioned space

( Supply air )

A4A5A2

VAPOR COMPRESSION SYSTEM WITH AIR CIRCUIT

D1

D3Pump2

D2

A6

D2Pump1

D4

D4

A3

Regenerator

Dehumidifier

AND DESICCANT LOOP

A3

A6

h

Process diagram

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dZ

air out

maha+dha

Wa+dWat

a

+dta

mahaWata

solution in

air in solution out

Z

mshsxt

s

ms+dmhs+ dhsx+dxts+dts

9

Computational Model Counter flow

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Algorithm

1. Calculate the enthalpy of inlet air: ha= f(ta, Wa)

2. Calculate heat transfer coefficient:hc= f(v)

3. Estimate Lewis number: Le = (/D)2/3

4. Estimate mass transfer coefficient: hm= hc/CpLe

5. Estimate interface vapor pressure, humidity ratio and enthalpy:

pi= ps= f(xs, ts); Wi= 0.622 pi/(ptpi); hi= f(ts, Wi)

6. Estimate the moisture absorbed and total heat transfer:

dm= hmdA(Wi-Wa); dq = hcdA (hi-ha)/cpm

7. Estimate air properties at exit of the element:

Wa* = Wa+ dWa; ha* = ha+dha; ta* = f(ha* ,Wa* )

8. Estimate solution flow rate, concentration, enthalpy and temperature at exit of the

element:

ms* = ms+ dm = ms+dWa; xs* = 1-[((1-xs) ms-(madWa))/ms* ] ; hs* = hs+ (dq/ms* );

ta* = f(hs* ,xs* )10

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Parameter Range Mean value

Inlet air temperature,

taa1,oC

6-22 14

Inlet air specific

humidity, Waa1, g/kg6-10 8

Inlet air temperature,

tar1,o

C (reg)

30-50 45

Inlet air specific

humidity, War1, g/kg15-30 20

S/A flow ratio (%) 0.05 - 150 Varied

Table 1: Performance parameters Table 2: Fixed/operating parameters

11

Parameter value

Inlet solution concentration, xsa1,% 45

Inlet solution concentration, xsr1,% (reg) 30

Inlet solution temperature, tsa1,oC 20

Inlet solution temperature, tsr1,oC (reg) 20

Height of absorber and regenerator, cm 40

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Parameter Variation Influence Result

Air temperature increasesolution T

solution VP

dehumidification suppressed

solution concentration

Solution temperature

Air specific humidity increase air VP

dehumidification enhanced

solution concentration

Solution temperature

S/A ratio increase capacity

dehumidification enhanced

solution concentration

Absorber

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Parameter Variation Influence Result

Air temperature increasesolution T

solution VP

desorption increased

solution concentration

Solution temperature

Air specific humidity increase air VP

desorption suppressed

solution concentration

S/A ratio increase capacitydesorption enhanced

solution concentration

Regenerator

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COUNTER FLOW

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6 7 8 9 100.0

0.5

1.0

1.5

2.0

2.5

3.0

S/A=0.05% S/A=0.2% S/A=0.8% S/A=1.6% S/A=3.2% S/A=6.4%

Changeinsp.humidity,

Waa,g

/kg

Inlet air sp.humidity,Waa1

,g/kg

6 7 8 9 100

1

2

3

4

5

6

7

8 S/A=0.05% S/A=0.2% S/A=0.8% S/A=1.6% S/A=3.2% S/A=6.4%

Changeinairte

mperature,

taa,o

C

Inlet air sp.humidity,Waa1

,g/kg

4 6 8 10 12 14 16 18 20 22 24

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5 S/A=0.05%S/A=0.2%S/A=0.8%S/A=1.6%S/A=3.2%S/A=6.4%

Changeinsp.h

umidity,

Waa,g

/kg

Inlet air temperature,taa1

,oC

4 6 8 10 12 14 16 18 20 22 24

0

1

2

3

4

5

6

7

8

9

Changeinairtemperature,

taa,o

C

Inlet air temperature,taa1

,oC

S/A=0.05%S/A=0.2%S/A=0.8%S/A=1.6%S/A=3.2%S/A=6.4%

EFFECT OF INLET AIR SPECIFIC HUMIDITY (ABSORBER)

EFFECT OF INLET AIR TEMPERATURE (ABSORBER) 15

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6 7 8 9 10

15

20

25

30

35

40

45

S/A=0.05% S/A=0.2% S/A=0.8% S/A=1.6% S/A=3.2% S/A=6.4%Outletsolution

concentration,xsa2,%

Inlet air sp.humidity,Waa1

,g/kg

6 7 8 9 10

14

15

16

17

18

19

20

21

22 S/A=0.05% S/A=0.2% S/A=0.8% S/A=1.6% S/A=3.2% S/A=6.4%

Outletsolutiontemperature,tsa2,o

C

Inlet air sp.humidity,Waa1

,g/kg

4 6 8 10 12 14 16 18 20 22 245

10

15

20

25

30

35

40

45

S/A=0.05%S/A=0.2%S/A=0.8%S/A=1.6%S/A=3.2%S/A=6.4%

Outletsolution

concentration,xsa2,%

Inlet air temperature,taa1

,oC

4 6 8 10 12 14 16 18 20 22 248

10

12

14

16

18

20

22

24

S/A=0.05%S/A=0.2%S/A=0.8%S/A=1.6%S/A=3.2%S/A=6.4%Outletsolutiont

emperature,tsa2,o

C

Inlet air temperature,taa1

,oC

EFFECT OF INLET AIR SPECIFIC HUMIDITY (ABSORBER)

EFFECT OF INLET AIR TEMPERATURE (ABSORBER) 16

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14 16 18 20 22 24 26 28 30 32

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Changeinsp.humidity,War,

g/kg

Inlet air sp. humidity, War1

,g/kg

S/A=0.2%S/A=0.4%S/A=0.8%S/A=1.6%S/A=3.2%S/A=6.4%

14 16 18 20 22 24 26 28 30 320

2

4

6

8

10

12S/A=0.2%S/A=0.4%S/A=0.8%S/A=1.6%S/A=3.2%S/A=6.4%

Changeinairtemperature,tar,

oC

Inlet air sp. humidity, War1

,g/kg

34 36 38 40 42 44 46 48 50 520.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Changeinsp.h

umidity,War,

g/kg

Inlet air temperature, tar1

,oC

S/A=0.05%S/A=0.2%S/A=0.8%S/A=1.6%S/A=3.2%S/A=6.4%

34 36 38 40 42 44 46 48 50 52-1

0

1

2

3

4

5

6

7

8

9

10

11

12

13 S/A=0.05%S/A=0.2%S/A=0.8%S/A=1.6%S/A=3.2%S/A=6.4%

Changeinairtemperature,tar,

oC

Inlet air temperature, tar1

,oC

EFFECT OF INLET AIR SPECIFIC HUMIDITY (REGENERATOR)

EFFECT OF INLET AIR TEMPERATURE (REGENERATOR) 17

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14 16 18 20 22 24 26 28 30 3230

32

34

36

38

40

42

44

46

48

50

52

54 S/A=0.2%S/A=0.4%S/A=0.8%S/A=1.6%S/A=3.2%S/A=6.4%

Outletsolutionconcentration,

xsr2,%

Inlet air sp. humidity, War1

,g/kg

14 16 18 20 22 24 26 28 30 3231

32

33

34

35

36

37

38

39

40

41

42

43

44

45

S/A=0.2%S/A=0.4%S/A=0.8%S/A=1.6%S/A=3.2%S/A=6.4%

Outletsolutiontemperature,

tsr2,o

C

Inlet air sp. humidity, War1

,g/kg

34 36 38 40 42 44 46 48 50 5230

32

34

36

38

40

42

44

46

48

50

52

54 S/A=0.05%S/A=0.2%S/A=0.8%S/A=1.6%S/A=3.2%S/A=6.4%

Outletsolutionc

oncentration,

xsr2,%

Inlet air temperature, tar1

,oC

34 36 38 40 42 44 46 48 50 52

32

34

36

38

40

42

44

46

48

50

52

S/A=0.05%S/A=0.2%S/A=0.8%S/A=1.6%S/A=3.2%S/A=6.4%

Outletsolutiontemperature,

tsr2,o

C

Inlet air temperature, tar1

,oC

EFFECT OF INLET AIR SPECIFIC HUMIDITY (REGENERATOR)

EFFECT OF INLET AIR TEMPERATURE (REGENERATOR) 18

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ABSORBER

REGENERATOR

19

0 1 2 3 4 5 6 7

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Changeinsp.h

umidity,

W,g

/kg

S/A ratio,%

ts1

=12oC

ts1=18oCt

s1=24

oC

ts1

=30oC

ts1

=40oC

ts1

=50oC

0 1 2 3 4 5 6 7

0

1

2

3

4

5

Changeinsp.

humidity,Wr,

g/kg

S/A ratio,%

tsr1

=10oC

tsr1

=15oC

tsr1

=20oC

tsr1

=25oC

tsr1

=30oC

tsr1

=40oC

tsr1

=50oC

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COUPLED COLUMNS

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Block diagram of coupled columns

21

C i l d l C l d l

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Computational Model- Coupled columns

msahsaxstsa

msa+dmhsa+ dhsa

xs+dx

tsa+dtsa

dZ

marhar

Wartar

marhar+dhar

War+dWartar+dtar

Z

Regenerator

air in

Weak

solution out

msr+dmhsr+ dhsr

xw+dx

tsr+dtsr

msrhsrxwtsr

dZ

Strong

solution out

Weak

solution in

Strong

solution in

maahaa+dhaa

Waa+dWaataa+dtaa

maahaa

Waataa

Process

air in

Absorber Regenerator

Dehumdified

air out Humidified

air out

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Table 3. Operating parameters

Parameter Range Mean value

Process air temperature, taa1,oC 10-18 14

Process air specific humidity, Waa1, g/kg 6-10 8

Regeneration air temperature, tar1

, oC 35-50 45

Regenerator air specific humidity, War1

, g/kg 15-30 20

S/A flow ratio (%)

Absorber

Regenerator

0.3-1.6

0.3-1.6

0.8

0.8

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10 12 14 16 180.5

1.0

1.5

2.0

2.5

3.0

Decreaseofspecifichumidity

inabsorber,

Waa,g

/kg

Process air temperature , taa1

,oC

(S/A)r=0.4%

(S/A)r=0.6%

(S/A)r=0.8%

(S/A)r=1.0%

(S/A)a=0.8%

10 12 14 16 181

2

3

4

5

6

7 (S/A)a=0.8% (S/A)r=0.4% (S/A)

r=0.6%

(S/A)r=0.8%

(S/A)r=1.0%

Increaseofairtemperature

inabsorber,taa,o

C

Process air temperature, taa1

,oC

10 12 14 16 18

0.5

1.0

1.5

2.0

2.5(S/A)

a=0.8% (S/A)

r=0.4%

(S/A)r=0.6%

(S/A)r=0.8%

(S/A)r=1.0%

Increaseofspecifichumidity

inregenerator,War,g

/kg

Process air temperature, taa1

,oC

10 12 14 16 180

1

2

3

4

5

6

(S/A)a=0.8% (S/A)

r=0.4%

(S/A)r=0.6%

(S/A)r=0.8%

(S/A)r=1.0%

Decreaseofa

irtemperature

inregenerator,tar,

oC

Process air temperature, taa1

,oC

EFFECT OF PROCESS AIR TEMPERATURE (ABSORBER)

EFFECT OF PROCESS AIR TEMPERATURE (REGENERATOR)24

3 0 6

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6 7 8 9 100.0

0.5

1.0

1.5

2.0

2.5

3.0

Specific humidity of process air, Waa1

, g/kg

Decreaseo

fspecifichumidity

intheab

sorber,

Waa

g/kg

(S/A)a=0.2%

(S/A)a=0.4%

(S/A)a=0.8%

(S/A)a=1.6%

6 7 8 9 100

1

2

3

4

5

6

Increaseofairtemperature

intheabsorber,taa

oC

(S/A)a=0.2%

(S/A)a=0.4%

(S/A)a=0.8%

(S/A)a=1.6%

Specific humidity of process air, Waa1

, g/kg

6 7 8 9 100.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

(S/A)a=0.2%

(S/A)a=0.4%

(S/A)a=0.8%

(S/A)a=1.6%

Increaseofspecifichumidity

intheregen

erator,Waa

g/kg

Specific humidity of process air, Waa1

, g/kg

6 7 8 9 101

2

3

4

5

6

7

8

9

10

11

(S/A)a=0.2%

(S/A)a=0.4%

(S/A)a=0.8%

(S/A)a=1.6%

Decreaseofairtemperature

inther

egenerator,tar

oC

Specific humidity of process air, Waa1

, g/kg

EFFECT OF PROCESS AIR SPECIFIC HUMIDITY (ABSORBER)

EFFECT OF PROCESS AIR SPECIFIC HUMIDITY (REGENERATOR)25

3.0 6

(S/A) 0 2%

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14 16 18 20 22 24 26 28 30 320.0

0.5

1.0

1.5

2.0

2.5

(S/A)a=0.2%

(S/A)a=0.4%

(S/A)a=0.8%

(S/A)a=1.0%

(S/A)a=1.6%

Decreaseofspecifichumidity

intheabsorber,

Waa

g/kg

Specific humidity of regenerator air, War1

, g/kg

14 16 18 20 22 24 26 28 30 320

1

2

3

4

5

Increaseofairtemperature

in

theabsorber,taa

oC

(S/A)a=0.2%

(S/A)a=0.4%

(S/A)a=0.8%

(S/A)a=1.0%

(S/A)a=1.6%

Specific humidity of regenerator air, War1

, g/kg

14 16 18 20 22 24 26 28 30 32

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Increaseofspecifichumidity

intheregenerator,War

g/kg

Specific humidity of regenerator air, War1, g/kg

(S/A)a=0.2%

(S/A)a=0.4%

(S/A)a=0.8%

(S/A)a=1.0%

(S/A)a=1.6%

14 16 18 20 22 24 26 28 30 32

1

2

3

4

5

6

7

8

9

10

Specific humidity of regenerator air, W

ar1, g/kg

(S/A)a=0.2%

(S/A)a=0.4%

(S/A)a=0.8%

(S/A)a=1.0%

(S/A)a=1.6%

Decreaseof

temperature

intheregen

erator,taro

C

EFFECT OF REGENERATOR AIR SPECIFIC HUMIDITY (ABSORBER)

EFFECT OF REGENERATOR AIR SPECIFIC HUMIDITY (REGENERATOR)26

(S/A) =0 3%(S/A) 0 8% 6

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35 40 45 50

0.0

0.5

1.0

1.5

2.0

2.5

Decrea

seofspecifichumidity

inth

eabsorber,

Waa

g/kg

Temperature of regenerator air, tar1

oC

(S/A)r=0.3%

(S/A)r=0.4%

(S/A)r=0.6%

(S/A)r=0.8%

(S/A)a=0.8%

35 40 45 500

1

2

3

4

5

6(S/A)

a=0.8% (S/A)

r=0.3%

(S/A)r=0.4%

(S/A)r=0.6%

(S/A)r=0.8%

Increaseofairtemperature

inthe

absorber,taa

oC

Temperature of regenerator air, tar1

oC

35 40 45 50

0.0

0.5

1.0

1.5

(S/A)r=0.3%

(S/A)r=0.4%

(S/A)r=0.6%

(S/A)r=0.8%

(S/A)a=0.8%

Temperature of regenerator air, tar1

oC

Increaseof

secifichumidity

intheregen

erator,War

g/kg

35 40 45 50

0

1

2

3

4(S/A)

a=0.8% (S/A)r=0.3%(S/A)

r=0.4%

(S/A)r=0.6%

(S/A)r=0.8%

decreaseofairtemperature

intheregen

erator,tar

oC

Temperature of regenerator air, tar1

oC

EFFECT OF REGENERATOR AIR TEMPERATURE (ABSORBER)

EFFECT OF REGENERATOR AIR TEMPERATURE (REGENERATOR)27

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28

EFFECT OF PROCESS AIR TEMPERATURE ON SOLUTION CONCENTRATION

10 12 14 16 18

27

30

33

36

39

42

45

48

51

tar1=35o

C

tar1

=40oC

tar1

=45oC

tar1

=50oC

tar1

=35oC

tar1

=40oC

tar1

=45oC

Absorberexit

Solutionconcentration%

Process air temperature,taa1

oC

Regeneratorexit

(S/A)a

=(S/A)r

= 0.3tar1=50oC

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Conclusions

At high range of S/A ratio

Solution has to be necessarily pre-cooled.Cooling of air will only complement the dehumidification.

Change in humidity increases with increase in air specific

humidity, solution concentration and decrease in air

temperature at the inlet.

At low range of S/A ratio

Air has to be pre-cooled for the sustained

dehumidification of air.

Solution temperature has negligible influence ondehumidification.

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Desiccant to assist only in dehumidification

Entire cooling supported by compression system

Regeneration achieved by warm condenser air

Air properties decide the process whether it is absorption

or regeneration

Regeneration is possible at temperatures above 30oC

Thus the proposed hybrid system is feasible

30

Conclusions

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Thank You