Page 1

Desalination 220 (2008) 531–537

Presented at the conference on Desalination and the Environment. Sponsored by the European Desalination Society

and Center for Research and Technology Hellas (CERTH), Sani Resort, Halkidiki, Greece, April 22–25, 2007.

0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.desal.0000.00.000

Performance analysis of a new type desalination unit of heat

pump with humidification and dehumidification

Penghui Gao*, Lixi Zhang, Hefei Zhang

Institute of Air-Conditioning & Solar Energy, School of Dynamics and Energy,

Northwestern Polytechnical University, Xi’an, ShaanXi, 710072, China

Tel. +86-29-88474097; Fax +86-29-88495911; email: gaopenghui2004@126.com, gaopenghui2004@sina.com

Received 17 December 2006; accepted 3 January 2007

Abstract

Desalination with humidification and dehumidification process is deemed as an efficient and promising means

of utilizing the condenser and evaporator of heat pump to produce freshwater from seawater. This paper presents

a new type desalination unit driven by mechanical vapor compression pump which was designed and fabricated

by the Institute of Air-Conditioning & Solar Energy of the Northwestern Polytechnical University. The unit

utilized the heat from condenser and the cold from evaporator of heat pump adequately, and reclaimed most latent

heat. The air, firstly, was humidified in the humidifier with the alveolate structure, and then was cooled in the pre-

condenser and the evaporative condenser to produce freshwater. A mathematical model of the unit is presented, in

which the hydrokinetics method was used to study the flow and the heat and mass transfer inside the alveolate

humidifier. The effects of some of the operation such as flow rates, temperatures of cooling water and air, and

etc., were studied in detail. The comparison between the numerical and experimental results was accepted. The

desalination unit that is considered in the study produces freshwater 60kg/day with the less electric power that is

500W and is proven to be an efficient desalination device to obtain freshwater.

Keywords: Desalination; Humidification and dehumidification; Heat pump

1. Introduction

A freshwater shortage affects a large and

widespread number of residential communities

and industrial locations. Specially, in the place

where there are coastal areas and the quantity of

natural drinking water is limited, the poor capac-

ity desalters that apply robust technology and

demand relatively little attendance are popular

for those areas.

The desalination system that utilizes the “heat

pump” is already applied prevalently, which is a

concept of the energy utilization. Hawlader

analyzed the performance of a novel solar-assisted

*Corresponding author.

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P. Gao et al. / Desalination 220 (2008) 531–537

system by heat pump and obtained good water

production [1]. Slesarenko studied the using state

of heat pump in desalination plants [2]. This

paper presents a new desalination system of heat

pump which is coupled with application of solar

energy.

The use of heat pump could make the desalina-

tion unit structure compact and the solar energy

as renewable energy source make desalination

cost less and protect environment. By virtue of

using the heat pump and solar energy, the system

in this work can reduce operating costs and its

maintenance is simple and convenient. Moreover,

difference from the work reported in [3], the

process in this work is a closed-air cycle type

in which air is forced to pass through the solar

collector and the humidifier, then the humid air is

cooled in the pre-condenser and the evaporative

condenser. The hydrokinetics method is used to

study the flow and the heat and mass transfer

inside the alveolate humidifier.

2. Desalination system

The system is diagrammatically shown in

Fig. 1. It consisted of three parts. One part is solar

collector; the other is humidification–dehumidi-

fication portion including alveolate humidifier,

pre-condenser and the evaporative condenser;

and another part is heat pump component which

comprises compressor, air condenser, throttle and

the evaporator (i.e., evaporative condenser).

Fig. 1. Schematic sketch of desalination system. 1. solar air collector; 2. blower; 3. alveolate humidifier; 4. sprinkler;

5, 18. commutator; 6. cooling water inlet of pre-condenser; 7. cooling water outlet of pre-condenser; 8. outlet of seawater;

9. pre-condenser; 10. evaporative condenser; 11. drainer; 12, 13. freshwater box; 14. compressor; 15. throttle; 16. air

condenser; 17. aerofoil fan.

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P. Gao et al. / Desalination 220 (2008) 531–537

533

In the system, the air is heated through the solar

air collector, and then humidified in the alveolate

humidifier which is driven by the blower. Sub-

sequently the humid air is cooled when passing

through the pre-condenser and the evaporative

condenser, and the freshwater is obtained. The

low-temperature seawater first enters into the pre-

condenser to cool the humid air, then a portion

of the seawater is sprayed into the alveolate

humidifier to humidify air and the remains of

seawater is discharged. The air condenser gives

out quantity of heat to the cooled air. When at the

night or the condition of sunlight is not good,

this system can adjust the commutator to cease

the operation of solar air collector and make air

flow into the alveolate humidifier directly. The

direction of air flow is shown in Fig. 1.

The related parameters of the unit are as follow.

The compressor rated power is 450 W, COP of heat

pump is 3.0 and humidifier (mm) is 450 × 450 ×

300. The area of solar air collector is 2m × 1.6 m.

3. Mathematical model

The system is mainly composed of the alveo-

late humidifier, the condenser (pre-condenser and

evaporative condenser) and the solar air collector.

The mathematical model is established based on

the models of the components, and the mathemat-

ical model is given below.

3.1. Model of humidifier

The structure sketch of humidifier is shown

in Fig. 2. It consists of honeycomb paper which

is characterized by a large evaporation surface

per unit volume of packed material.

Assumptions to obtain the mathematical

model are

•Falling film is in laminar flow.

• No wavy occurs on the gas–liquid interface

in course of the gas–liquid flow. The heat and

mass transfer take place on the interface.

•Humid air is regarded as ideal gas.

3.1.1. Governing equations

The mathematical model of humidifier is

established by the hydrokinetics method, which

include mass conservation equation, momentum

conservation equation and energy conservation

equation. The equations are listed as follows (1),

(2) and (3):

(1)

Fig. 2. Sketch of the alveolate humidifier.

∂∂∂

∂

+

∂

+

∂

=

()()()

rrr

u

x

vw

zy

0

∂

∂

+∂

∂

+∂

∂

=

∂

∂

∂

∂

⎛

⎝⎜

⎞

⎠⎟

+

∂

∂

∂

∂

⎛

⎝⎜

⎞

⎠⎟+

∂

()()()

rrr

m

m

uu

x

uv

y

uw

z

∂

∂

x

r

u

x

y

u

y

∂ ∂

⎛

⎝⎜

⎞

⎠⎟−∂

∂

z

u

yx

m

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534

P. Gao et al. / Desalination 220 (2008) 531–537

(2)

Because this process meets the heat and mass

transfer of gas–liquid, the volume of fluid (VOF)

is a good approach to resolve the question. Hirt

and Nichols applied the VOF method to analyz-

ing the dynamics of free boundaries [4]. Using

the VOF method, the interface between the two

phases is tracked using a fractional volume

function F. The VOF function is averaged

over each computational cell: a value of unity

indicates the presence of one of the phases and

zero indicates its absence. The VOF function

satisfies

(4)

By the above CFD calculation, the distributing

of the flow field and temperature field inside the

alveolate humidifier was obtained. Based on the

above fields, the distributing of air humidity in

the alveolate humidifier was worked out by the

heat–mass transfer formulas (auxiliary equations)

which are shown later in the paper.

3.1.2. Auxiliary equations

One formula to predict the evaporation rate

at the water surface is recommended by Eames

et al. [5]

(5)

e1 is the Knudsen coefficient of evaporation. It can

be calculated by e1 = 2e/(2 − e). Here the coeffi-

cient of the evaporation can be calculated by

(6)

For flow in thin channels with disturbance poles

(typical velocity is about 1~4 m/s), Kettleborough

and Hsieh [6] suggested the coefficient of heat

transfer,

h = FVbW/m2 (7)

Here, F is 49 and b is 0.6 corresponding to a

13.4 mm spacing.

MacLaine-Cross and Banks [7] introduced

the concept of the wet bulb coefficient of heat

transfer to correct the heat transfer process of a

wet surface

h* = h(1 + el / Cpa) (8)

Where e is constant, given by

e = (wmax− wmin) / (Tmax− Tmin)(9)

Tmax and Tmin are respectively, the maximum and

minimum temperatures at the liquid–gas interface.

Here, Tmax is the higher one of the temperature

of gas flow and water flow. Tmin is the wet bulb

temperature of the process air. wmax and wmin are,

respectively, the maximum and minimum humid-

ity ratios corresponding to Tmax and Tmin.

∂

∂

+∂

∂

+∂

∂

=

∂

∂

∂

∂

⎛

⎝⎜

⎞

⎠⎟

+

∂

∂

∂

∂

⎛

⎝⎜

⎞

⎠⎟+

∂

()()()

rrr

m

m

vu

x

vv

y

vw

z

∂

∂

x

v

x

y

v

y

∂ ∂

⎛

⎝⎜

⎞

⎠⎟−∂

∂

−

z

v

z

p

y

g

mr

∂

∂

+∂

∂

+∂

∂

∂

∂

=

∂

∂

∂

∂

⎛

⎝⎜

⎞

⎠⎟

+

∂

∂

∂

∂

⎛

⎝⎜

⎞

⎠⎟+

∂

()()()

rrr

m

m

wu

x

wv

y

ww

zx

r

w

x

y

w

y

∂ ∂

⎛

⎝⎜

⎞

⎠⎟−∂

∂

z

w

zz

m

∂

∂

+∂

∂

+∂

∂

=

∂

∂

∂

∂

⎛

⎝⎜

⎞

⎠⎟

+

∂

∂

∂

∂

⎛

⎝⎜

⎞

()()()

rrr

uT

x

vT

y

wT

zx

k

c

T

x

y

k

c

T

y

p

p

⎠ ⎠⎟+

∂

∂

∂

∂

⎛

⎝⎜

⎞

⎠⎟+

z

k

c

T

z

S

p

h

( ) 3

∂

∂

+⋅ ∇ =

F

t

V

(

F

)0

WPP

M

RT

p

zsv

n

1

2

2

=()

⎛

⎝⎜

⎞

⎠⎟

−

e

1

e

p

r l

s

=

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

h*

RT

M

T

2

avg

1

2

n

2

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P. Gao et al. / Desalination 220 (2008) 531–537

535

3.2. Model of condenser

Freshwater is produced when the humid air

from the alveolate humidifier pass through the

condenser and is cooled under the dew point. The

condenser in the system is regarded as a parallel

heat exchanger. The thermal calculation equations

are listed hereinafter.

The effectiveness is expressed as

(10)

(11)

(12)

Where Cwb is the specific heat of the air, which

can be calculated by

Cwb= Cpa(1+el/Cpa) (13)

Where l is the latent heat, and e is calculated by

Eq. (9). The amount of heat transfer is

(14)

Keeping to the conversation of energy, the

outlet state of working liquid can be predicted

and the humidity of the outlet airflow can be

calculated as saturated air. Thus, the water pro-

ductivity is,

(15)

3.3. Solar air collector

A solar air collector was used to heat air in

the desalination system. The efficiency of the

component can be calculated using Eq. (16) [8].

(16)

Where Ti is the inlet air temperature of the solar

air collector, Ta is the ambient temperature, and

GT is the solar insolation density. The outlet air

temperature is calculated by the energy conser-

vation equation of the solar air collector.

4. Numerical analysis of the system and

discussion

Before analyzing, there is a point need to be

made clear that the solar insolation density used

is from the Institute of Renewable and Saving

Energy of Science & Technology of China Uni-

versity. The line of solar insolation density is

shown in Figs. 3, 4 and 5 by the dash-dot line.

Fig. 3 shows the effect of different mass flow

rate of air on the water production following the

change of the solar insolation density in the day-

time when the mass flow rate of cooling inlet

seawater is 300 kg/H and the cooling seawater

temperature is 18°C. Here, the time of desalination

operation is from 8 a.m. to 18 p.m.. From Fig. 3,

it can be seen that the freshwater production

increase with the solar insolation density strength-

ening and the freshwater production reduce with

eh

min

max

1 exp

−

NTU 1

1

=−+

⎡

⎣⎢

⎤

⎦⎥

⎧

⎨⎪

⎩ ⎪

⎫

⎬⎪

⎭ ⎪

⎛

⎝⎜

⎞

⎠⎟

+

(

(

)

)

(

(

? mc

?

?

?

mc

mc

mc) )

)

min

max

⎡

⎣⎢

⎤

⎦⎥

()()

???

mcmCm C

cmaxacinwb ps

min[1,]

=+ w

()()

???

mcmC m C

cmaxacin wbps

max[1,]

=+ w

Q mc

?

TT

c minh cout cin

()()

=−

e

mm

? w

wa cin cout

()

=−

w

hi

ia

T

0.7034.5625

=−

−

G

⎛

⎝⎜

⎞

⎠⎟

TT

Fig. 3. Effect of mass flow rate of air on freshwater

production.

Page 6

536

P. Gao et al. / Desalination 220 (2008) 531–537

the solar insolation density weakening. Simulta-

neity, the water production when the mass flow

rate of air is 200 kg/H is more than the water pro-

duction when the mass flow rate of air is 150 kg/H.

The maximal water production occurs from

13 p.m. to 14 p.m. when the mass flow rate of

air is 200 kg/H and its value is 4700 mL.

Fig. 4 presents the effect of different mass flow

rate of cooling inlet seawater on the water produc-

tion following the change of the solar insolation

density in the daytime when the cooling inlet

seawater temperature is 18°C and the mass flow

rate of air is 200 kg/H. From the diagram, the mass

flow rate of cooling inlet seawater has the clear

effect on the water output of the desalination

system. With the mass flow rate of cooling inlet

seawater increasing, the yield of freshwater

increases. The maximal water production appears

at the time from 13 p.m. to 14 p.m. when the

mass flow rate of air is 200 kg/H and the mass

flow rate of cooling seawater is 350 kg/H, and its

value is 5150 mL.

Fig. 5 shows the effect of different temperature

of cooling inlet seawater on the water production

following the change of the solar insolation

density in the daytime when the mass flow rate

of cooling seawater is 300 kg/H and the mass flow

rate of air is 200 kg/H. From Fig. 5, it is obtained

that the water production increases with the cool-

ing seawater temperature reducing. The maximal

value of water production is 4980 mL when the

mass flow rate of air is 200 kg/H and the tem-

perature of cooling inlet seawater is 13°C.

At night, the solar air collector is shut off and

the freshwater production mainly depends on

the “heat pump” system, and the yield ratio is

1300 mL/H steadily when the mass flow rate of

cooling seawater is 300 kg/H, the mass flow rate

of air is 200 kg/H and the cooling inlet seawater

temperature is 18°C. Obviously, adding water

yield of the daytime, the total water production

of the day is 60 kg at least.

5. Conclusion

A new desalination system of heat pump was

proposed and a mathematical model was pre-

sented. Simulation results were in agreement

with the experimental data. The results of this

study indicate that the mass flow rate of cooling

seawater and the cooling inlet seawater tempera-

ture have the apparent effect on the freshwater

yield of the desalination system. Approximately,

when the mass flow rate of cooling seawater is

larger and the cooling inlet seawater temperature

is lower, the water yield of system is more. It is

worthy of being attention that the matching the

Fig. 4. Effect of mass flow rate of cooling inlet seawater

on freshwater production.

Fig. 5. Effect of cooling inlet seawater temperature on

freshwater production.

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P. Gao et al. / Desalination 220 (2008) 531–537

537

mass flow rate of cooling seawater and the cool-

ing inlet seawater temperature for the system

desalination.

From the application standpoint, the system

produces freshwater 60 kg/day with the less elec-

tric power that is 500 W (compressor rated power)

and the desalination has the advantages that the

fee of maintenance is little and operating is easy.

If much freshwater is needed, the more power

system can be constructed to satisfy the require-

ment by the appropriate proportion. So, the system

is more fit for the people of the remote area to use.

Acknowledgment

The work that has been presented in this paper

was supported by the Chinese foundation com-

mittee of nature and science, under the projects,

No.50576078.

Symbol

u, v, w velocity vector of the X, Y, Z direc-

tions (m/s)

density (kg/m3)

viscidity dissipation item

temperature (K)

interface temperature (K)

average temperature (K)

saturated water vapor pressure at

interface (Pa)

water vapor pressure of air (Pa)

r

Sh

T

Tn

Tavg

Ps

Pv

Wz

evaporation rate of water at the interface

(kg/m2/s)

molecular weight of water

latent heat of water (kj/kg)

universal gas constant (8.314kj/kmol · K)

air specific heat at constant pressure

(kj/kg · K)

mass flow rate of air (kg/H)

water specific heat at constant pressure

(kj/kg · K)

mass flow rate of water (kg/H)

water productivity (kg/H)

M

l

R

Cpa

Cps

mw

References

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assisted heat pump desalination system, Desalina-

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V.V. Slesarenko, Heat pumps as a source of heat

energy for desalination of seawater, Desalination,

139 (2001) 405–410.

N.Kh. Nawayseh and M.M. Farid, Energy Con-

version Manage., 40 (1999) 1423.

C.W. Hirt and B.D. Nichols, Volume of fluid

(VOF) method for dynamics of free boundaries,

J. Comput. Phys., 39 (1981) 201–225.

I.W. Eames, N.J. Marr and H. Sabir, Int. J. Heat

Mass Transf., 40 (1997) 2963.

C.F. Kettleborough and C.S. Hsieh, J. Heat Transf.,

105 (1983) 366.

I.L. MacLaine-Cross and P.J. Banks, J. Heat Transf.,

103 (1981) 579.

C.L. Gupta and H.P. Garg, Performance studies of

solar air heaters, Solar Energy, 11 (1976).

[2]

[3]

[4]

[5]

[6]

[7]

[8]

? ma

? mc