study of a novel silica gel¨cwater adsorption chiller[1]. part i. design and perfor mance...

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Study of a novel silica gel–water adsorption chiller. Part I. Designand performance prediction

D.C. Wanga, Z.Z. Xiaa, J.Y. Wua,*, R.Z. Wanga,1, H. Zhaia, W.D. Doub

a Institute of Refrigeration and Cryogenics, Shanghai Jiaotong University, Shanghai 200030, China

b Jiangsu Shuangliang Air Conditioning Equipment Co. Ltd, Jiangsu 214444, China

Received 7 December 2004; received in revised form 28 February 2005; accepted 1 March 2005

Available online 23 May 2005

Abstract

A novel silica gel–water adsorption chiller is designed and its performance is predicted in this work. This adsorption chiller

includes three vacuum chambers: two adsorption/desorption (or evaporation/condensation) vacuum chambers and one heat pipe

working vacuum chamber as the evaporator. One adsorber, one condenser and one evaporator are housed in the same chamber

to constitute an adsorption/desorption unit. The evaporators of two adsorption/desorption units are combined together by a heat-

pipe heat exchanger to make continuous refrigerating capacity. In this chiller, a vacuum valve is installed between the two

adsorption/desorption vacuum chambers to increase its performance especially when the chiller is driven by a low temperature

heat source. The operating reliability of the chiller rises greatly because of using fewer valves. Furthermore, the performance of

the chiller is predicted. The simulated results show that the refrigerating capacity is more than 10 kW under a typical working

condition with hot water temperature of 85 8C, the cooling water temperature of 31 8C and the chilled water inlet temperature of

15 8C. The COP exceeds 0.5 even under a heat source temperature of 65 8C.

q 2005 Elsevier Ltd and IIR. All rights reserved.

Keywords: Design; Adsorption system; Water; Silica gel; Performance

Etude sur un nouveau refroidisseur a adsorption a gel de silice/eau.Partie I. Conception et prevision de la performance

Mots cle s : Conception ; Systeme a adsorption ; Eau ; Gel de silice ; Performance

1. Introduction

An adsorptioncooling systemis a noiseless, non-corrosive,

environmentally friendly and economical system. So many

researchers have made great efforts to study such a cooling

systemin order to commercialize it. Further, a silica gel–water

adsorption cooling system has more interests for it can be

driven by low-grade waste heat or solar energy. Much

interesting work has been done recently. A silica gel–water

adsorption chiller driven by the waste heat source has been

successfully commercialized in Japan, as reported by Saha [1].

Waste heat at the temperature of 50–90 8C abounds in

industry. It is seldom utilized, but usually discharged into

International Journal of Refrigeration 28 (2005) 1073–1083

www.elsevier.com/locate/ijrefrig

0140-7007/$35.00 q 2005 Elsevier Ltd and IIR. All rights reserved.

doi:10.1016/j.ijrefrig.2005.03.001

* Corresponding author. Tel.: C86 21 62933250; fax: C86 21

62933250.

E-mail address: [email protected] (J.Y. Wu).1 Is IIR-B2 vice president, and member of the Strategic Planning

Committee of IIR.

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environments at present. Compared with a single-stage

LiBr-water absorption chiller, a silica gel–water adsorptionchilled can effectively use such low waste heat without

corrosion or crystallization. Moreover, it needn’t consume

electric energy to drive the solution pump. Multi-bed, multi-

stage or dual-mode adsorption chillers were developed in

order to utilize low-grade waste heat. And many achieve-

ments have been obtained. Saha et al. [1] designed a three-

bed silica gel–water adsorption chiller and developed a

cycle simulation computer program to predict its perform-

ance. For a three-bed chiller, waste heat recovery efficiency

is about 35% higher than that of the two-bed system, and the

COP value is 0.38 with a driving source temperature at

80 8C and the coolant inlet and chilled water inlet

temperatures at 30 and 14 8C, respectively. Saha et al. [2]

also developed a three-stage adsorption chiller which

utilized about 50 8C waste heat as the driving source and

30 8C cooling water as the cooling source. Hamamoto et al.

[3] developed a two-stage adsorption refrigeration chiller

prototype driven by heat source temperature from 53 to

61 8C with the evaporator and condenser temperatures at 7

and 30 8C, respectively. In Ref. [4], a multi-bed regenerative

adsorption chiller design was proposed. The simulated

results showed that, using the same waste heat source, a

four-bed chiller generated 70% refrigerating capacity

improvement compared with a typical two-bed chiller, and

a six-bed chiller generated 40% refrigerating capacity

improvement compared with a four-bed chiller. Saha et al.[5] designed a dual-mode silica gel–water adsorption chiller

effectively, which could utilize low temperature solar or

waste heat sources with the temperature from 40 to 95 8C

and could offer two operation mode: the first mode with the

driving source temperature at 60–95 8C and the second

mode of an advanced three stage adsorption chiller with the

available driving source temperature at 40–60 8C. The

simulated results in Ref. [5] showed that the optimum COP

values were obtained at driving source temperatures from 50

to 55 8C for three-stage mode, and from 80 to 85 8C for

single-stage multi-bed mode. With the mass recovery cycle,

an adsorption chiller can be driven by the heat source of alower temperature. Under the same working condition, mass

recovery cycles have advantages to conventional single

stage cycle at low heat source temperature [6].

Heat and mass transfer is crucial to an adsorption chiller.

The performance of heat and mass transfer in adsorber must

be enhanced for the working pairs have a poor heat and mass

transfer coefficients. Vichan [7] added activated carbon to

the blue silica gel to improve the desorption rate and

regeneration temperature of the packed bed for a solar

powered adsorption cooling system. Eun et al. [8,9] used

composite blocks to enhance the heat and mass transfer and

led to enhancement of performances of the cooling systems.

Nomenclature

a constant in Van der Waals equation,

Pa m6 kgK2

A area, m2

b constant in Van der Waals equation, m

3

c specific heat, kJ kgK18CK1

COP coefficient of performance

KA performance coefficient of heat exchanger,

kW 8CK1

L latent heat of vaporization, kJ kgK1

M mass, kg

M e,0 mass of residual refrigerant in evaporator, kg

_m mass flow rate, kg sK1

n times of data acquisition in one cycle

P pressure, Pa

Q heat power, kW

R gas constant, J kgK1 K K1

T temperature, K v specific volume, m3 kgK1

x adsorption capacity, kg kgK1

D H isosteric heat of adsorption, kJ kgK1

t time, s

d, q, w flag functions

J mass, kg

Subscript

a adsorbent

ad adsorber

ads adsorptional aluminium

c condenser

chill chilled water

cool cooling water

cond condensing

cu cuprum

des desorption

e evaporator

evap evaporating

h heat, hot

in inlet

mr mass recovery

out outletref refrigeration

s silica gel, saturation

w water

wv water vapor

Superscript

i the order number of the data acquisition

interval

D.C. Wang et al. / International Journal of Refrigeration 28 (2005) 1073–10831074



Restuccia et al. [10] experimentally studied the properties of

the composite adsorbent–macroporous silica gel impreg-

nated with CaCl2.

Both the switching frequency and cycle time have great

influence on the performance of an adsorption chiller. Alam

et al. [11] used a novel simulation technique to study the

optimum switching frequency of an adsorption refrigerationsystem. The result showed that the optimum switching

frequency is much sensitive to the heat exchanger’s design

parameters. Chua et al. [12] developed a transient two-bed,

insulated adsorption chiller model to quantify the effects of

switching frequency and cycle time, they confirmed that the

resultant cyclic-steady-state condition is independent of the

initial refrigerant mass distribution in the beds and

evaporator.

The conventional or advanced chillers mentioned above

all have many vacuum valves. For example, a two-bed

conventional adsorption chiller [12,14] must have four or

five vacuum valves at least, and six vacuum valves arenecessary for a three-bed advanced adsorption chiller [1].

These vacuum valves have caused a low-reliability problem

in the commercialization. In this study, a novel silica gel–

water adsorption chiller is designed. This chiller is

combined by three vacuum chambers: two desorption/

adsorption working chambers and one heat pipe working

chamber. Such structure of the chiller decreases the number

of vacuum valves. There is only one vacuum valve installed

to execute mass recovery process between the two

desorption/adsorption working chambers. The operating

reliability of the chiller rises greatly. The main goal of this

work is to propose a novel silica gel–water adsorption

chiller and to testify its high effectiveness by means of a

simulation prediction.

2. Adsorption chiller design

2.1. System description

Fig. 1 shows the structure of the designed silica gel–

water adsorption chiller. This novel silica gel–water

adsorption chiller is composed of three working vacuumchambers including chambers A, B and C. Chambers A and

B are desorption/adsorption working chambers and each of

them comprises one condenser, one adsorber and one

evaporator (half water-evaporating surface of the chamber

C). So this chiller also seems to be combined by two single-

bed systems. Water is taken as the refrigerant. Practically,

the evaporator is one gravitation heat pipe. The heat media

of the heat pipe is methanol. Inside of the tubes in the cold-

end (upside) of the heat pipe are methanol condensing

surface. The water evaporation surface, outside of the tubes

in the upside, is divided into two parts by the clapboard in

the middle, which lie in chambers A and B, respectively.The hot-end (lower side) of the heat pipe is the methanol

evaporating side and is also the heat exchange side between

the chilled water and the methanol. In this paper, the upside

of the heat pipe is called water evaporator (WE), and the

lower side is named as methanol evaporator (ME). So the

whole evaporator seems to be combined by two WEs (WE 1

in chamber A and WE 2 in chamber B) and one ME. ME,

WE1 and WE2 will also be shown in Fig. 2.

In order to improve the performance and increase the

adaptability to a low temperature heat source, a vacuum

valve is installed between chamber A and chamber B to help

the chiller fulfill mass recovery process. The desorption/ad-

sorption process is completely controlled by the electric

valves in the heating/cooling water system. So such

Fig. 1. Schematic diagram of the designed silica gel–water adsorption chiller.

D.C. Wang et al. / International Journal of Refrigeration 28 (2005) 1073–1083 1075



structure of the system increases the reliability of the chiller

though the water evaporating and condensing area is

doubled, compared with the traditional two-bed system.

2.2. Components

The adsorber is one compact tube-fin heat exchanger.

The copper tubes and mass transfer channels, framed by

metal mesh and spring, are lined in arrays. The heat transfersurface is extended by aluminium fins. In the adsorbent side,

the heat transfer area of each adsorber is 3.3 m2 for the heat

transfer copper tubes and 60.6 m2 for the fins. The space

length between two fins is 2.5 mm as the same as that

recommended by Miltkau [13]. The mass transfer dimension

designed is shorter than 12 mm. And the maximum

dimension of heat transfer between the metal and silica

gel is 1.25 mm. In each adsorber, 50 kg microporous silica

gel with 0.50–1 mm in diameter is filled.

The condenser is a tube-shell heat exchanger, and the

shell is the same to that of the chamber A or B. Two

condensers are in series arrangement. The cooling water

flows into the second condenser from the first condenser,

and then into the adsorber. The condensing area is 2.8 m2 for

each condenser.

The evaporators are combined together by a heat-pipe

heat exchanger (HPHE), as shown in Fig. 2. The exterior

surface of the copper tubes in the WE is water-evaporatingsurface that is porous medium to enhance the evaporating

capability so that the volume of the evaporator can be

minished. And the internal surface of the copper tubes is the

methanol-condensing surface-plain pipe surface. The

methanol will evaporate on the exterior surface of the heat

exchange tubes in the ME of the HPHE and condense on the

internal surface of the tubes in one WE (for example, WE1).

This time WE1 is at work. Simultaneously, the WE2 collects

the condensate coming from the condenser through the

divider, so its temperature is higher than the temperatures of

the WE1 and the ME of the HPHE. As a result, the heat

exchange of the WE2 to the WE1 and the ME of the HPHEis isolated according to the working principle of a

gravitation heat pipe.

The structure of the heat pipe evaporator is more

complicated and more metal is consumed, compared with

the evaporator in the conventional system. However, the

water evaporators can automatically switch into evaporating

work, and at least two vacuum valves are decreased when

the heat pipe evaporator is adopted in the system.

Fig. 3 shows the cyclic process of the methanol inside the

heat pipe. The liquid methanol evaporates on the exterior

surface of the tubes in the ME, and the methanol vapor flows

into the tubes in the WE from one side. After being

Fig. 2. Schematic diagram of the heat-pipe combined evaporator.

Fig. 3. Schematic diagram of the loop heat pipe used in the

evaporator.




condensed, the condensate of the methanol then returns to

the ME from the other side through a U-shape tube. So this

cyclic process forms a loop. As a result, the evaporator is

also a loop heat pipe.

In the desorption/adsorption working chamber, the vapor

channel connecting the adsorber to the condenser and the

evaporator has a large through-flow sectional area. There-

fore, the mass transfer from the adsorber to condenser or to

the evaporator is enhanced and the potential refrigerating

capacity of the adsorbent rises.

This structure of the chiller causes one water evaporator

and one condenser idle anytime so as to decrease the

utilization ratio of the evaporators and the condensers, but

four vacuum valves for the switch between adsorption and

desorption process are spared. Thus, the reliability of the

system improves greatly.

3. Working principle

The cycle process of the designed silica gel–water

adsorption chiller is different from the conventional two-bed

single-stage adsorption chiller, especially in the desorption/

adsorption process. Thechiller is operatingin sixphases. As an

example, the chiller startups is from the desorption process for

the adsorber 1 and the adsorption process for the adsorber 2.

Then the whole cycle process can be described as:

3.1. Chamber A works in desorption process and chamber Bin adsorption process

In this phase, valves 2, 3, 6, 7 are opened, and other

valves are closed. Hot water flows through adsorber 1.

Adsorber 1 is heated to desorb and the vapor will condense

in WE 1 due to its lower temperature than the condenser 1.

The condensation process begins in the condenser 1 when

the pressure in chamber A is higher than the corresponding

saturation pressure of the condenser temperature. The

condensation process will only occur in the condenser 1

when the temperature of the condenser 1 is lower than that

of WE 1. During the desorption process, the condensate isdistributed on the evaporating surface of WE 1 by divider 1.

WE 1 acts as a collector. At the same time, adsorber 2 is

cooled to adsorb by the cooling water. Water in WE 2 is

vaporizing and the temperature of WE 2 decreases soon.

The methanol in chamber C starts to evaporate if the

temperature of WE 2 decreases lower than the correspond-

ing saturation temperature of the pressure in chamber C.

Then the chilled water will be cooled. In this phase, the

temperature of WE 1 is highest and that of WE 2 is lowest

among WE 1, WE 2 and ME. And the WE 1 and WE2 are

separated. So the heat is only transferred from ME to WE 2.

The time of this phase is heating/cooling time.

3.2. Mass recovery process from chamber A to chamber B

After the first phase, the vacuum valve for mass recovery

is opened. The vapor with a high pressure from adsorber 1

and WE 1 flows into chamber B with a high speed. Adsorber

1 continues to desorb and adsorber 2 continues to adsorb. In

this phase, the temperature of the WE 1 decreases and that of the WE 2 increases. The pressures in chambers A and B

trend to one value in the end of this phase. The time of this

phase is mass recovery time.

3.3. Heat recovery process from the adsorber 1 to the

adsorber 2

The vacuum valve for mass recovery is closed when the

pressures in chamber A and B nearly come to equilibrium.

Valve 11 is opened, then valves 2, 3, 6 are closed and valves

4, 9 are opened. The hot water flow runs through the bypass.

The cooling water flows into adsorber 1 and pushes theresident hot water in adsorber 1 into adsorber 2. Adsorber 1

is cooled by the cooling water and adsorber 2 is heated by

the resident hot water from adsorber 1. The heat is

recovered. This phase will finish when the resident hot

water from adsorber 1 is completely pushed into adsorber 2.

The time of this phase is heat recovery time, which is

determined by the flowrate of the cooling water.

3.4. Chamber B works in desorption process and chamber A

in adsorption process

Valve 8 is opened and valves 7, 9 are closed. Then valves

1, 5, 8 are opened and valve 11 is closed. Adsorber 1 is

cooled to adsorb and adsorber 2 is heated to desorb.

Condenser 2 and WE 1 start to work in condensation process

and evaporation process, respectively. This phase is similar

to the first phase.

3.5. Mass recovery process from chamber B to chamber A

This phase is similar to the second phase. The high-

pressure vapor in chamber B flows through the vacuum

valve into chamber A. Adsorber 1 adsorbs the vapor

desorbed from adsorber 2 and WE 2. The temperature of

WE 2 quickly decreases. Some refrigeration power is saved.The vacuum valve will be closed and this phase finishes

when the pressures in chamber A and B nearly come to

equilibrium.

3.6. Heat recovery process from adsorber 2 to adsorber 1

Valve 11 is opened and valves 1, 4 closed, then the hot

water flows through the bypass. Valve 2, 10 are opened and

valve 5 closed. The cooling water pushes the resident hot

water in adsorber 2 flowing into adsorber 1. As similar as the

third phase, the heat recovery process will finish when the

resident hot water from adsorber 2 is completely discharged




into adsorber 1. Then the chiller will operate in the first

phase again. The cycle process will go round and round

along phase 1/phase 2/phase 3/phase 4/phase 5/

phase 6/phase 1. The cycle time includes the heating/

cooling time, the mass recovery time, the heat recovery time

and the valves switching time. In this study, the valves

switching time is neglected, and 20 s is taken as the fixedheat recovery time.

The cycle process A/B/C/D/E/F/A in Fig. 4

is the ideal cycle process for this novel adsorption chiller.

The processes A–B and D–E are heat recovery processes.

The processes B/C and E/F represent that the chiller

works in the heating/desorption and cooling/adsorption

phases, respectively. The processes C/D and F/A are

mass recovery processes. Practically the heat exchange

capacity of all heat exchangers is finite, so the practical

cycle process for this novel adsorption chiller is along A/

B/n/C/q/D/E/m/F/p/A. The temperature

of the adsorber and the pressure in the adsorber areunceasingly changing in the whole cycle. Even so, the

work area of the chiller is still magnified largely compared

with the ideal basic cycle process B/C/E/F/B. The

desorption occurs in the whole process A/B/n/C/

q/D and the adsorption does in the whole process D/

E/m/F/p/A.

4. Mathematical model

A lumped parameter model is used in this study. All the

parameters used in the mathematical model are listed inTable 1. And the main assumptions are taken as follows:

(1) The temperature and the pressure are uniform through-

out the whole adsorber.

(2) The refrigerant is adsorbed uniformly in the adsorber

and is liquid in the adsorbent.

(3) The pressure difference between the adsorber and the

condenser or the evaporator is neglected.

(4) The heat conduction of the shell connecting the

adsorber to the condenser or the evaporator is neglected,

and the heat exchange between two WEs is absolutely

isolated.

(5) The system has no heat losses (or refrigerating output

loss) to the environment.

4.1. Adsorption equation

The adsorption equilibrium equation used in this model

is developed by Boelman [14]:

xZ0:346PsðT wÞ

PsðT sÞ

1 = 1:6

(1)

where Ps(T w) and Ps(T s) are respectively the corresponding

saturation vapor pressures of the refrigerant at temperaturesT w (water vapor) and T s (silica gel).

The saturation vapor pressure and temperature are

correlated as follows:

Ps Z0:0000888ðT K 273:15Þ3K 0:0013802ðT

K 273:15Þ2C 0:0857427ðT K 273:15Þ

C 0:4709375 (2)

4.2. Energy equations

Different from other two-bed system, the energy

equations for the condenser and the evaporator is more

complicated. The condensation process occurs firstly in the

evaporator during the desorption process because the

evaporator temperature is lower than the condenser

temperature and also the saturation temperature of the

refrigerant vapor. The condensation process will occur only

in the condenser after the evaporator temperature rises

higher than the saturation temperature.

(1) Energy balance for the adsorber/desorber

d

dtf½ M aðca C cp;w xÞ C ccu M tube;ad C cal M fin;adT ag

Z M aD H d x

dtC ð1 K dÞcwv M a

d xads

dtðT e K T aÞ

C _mwcp;wðT ad;in K T ad;outÞ (3)

T ad;out K T a

T ad;in K T aZ exp

KKAad

_mwcp;w

(4)

where

Fig. 4. P-T diagram for the ideal basic cycle, ideal and practical

cycles of this chiller.




dZ1; desorption process

0; adsorption process

((5)

(2) Energy balance for the condenser

ccu, M c,dT c

dt

Zd, K L , M a,d xdes

dt

C cwv, M a,d xdes

dt

,ðT c K T aÞC _mcool,cp:w,ðT cool;in K T cool;outÞ

ð6Þ

T cool;out K T c

T cool;in K T cZ exp

KKAc

_mcoolcp;w

(7)

(3) Energy balance for the evaporatorThe HPHE is taken as

one part, and the heat and mass transfer process of

inside is not taken into account.

ddt

½ðcp;w M e;w C ccu M eÞT eZ ð1 K dÞ

! K LM ad xads

dtC _mchillcp;wðT chill;in K T chill;outÞ

Cd qcp;wðT e K T cÞ M ad xdes

dtK ð1 K qÞ LM a

d xdes

dt

(8)

T chill;out K T e

T chill;in K T eZ exp

KKAe

_mchillcp;w

(9)

where

qZ1; T c%T e

0; T cOT e

((10)

4.3. Liquid refrigerant equilibrium in evaporator

d M e;w

dtZ M e;0 K M a

d x

dt(11)

4.4. Equilibrium equations in the mass recovery process

In the mass recovery process, the condensers will be in

idle, the evaporator in the desorbing chamber will begin

evaporating, and the evaporator in the adsorbing chamber

will condensate the water vapor acting as a condenser. The

desorption process of the desorber and the adsorption

process of the adsorber accelerate under the driving force of

the pressure difference between the desorbing chamber and

the adsorbing chamber. The different equations in this phase

from the desorption/adsorption process are listed in the

following.

Mass equilibrium:

K M ad xdes

dtC _me;evap Z _me;cond C M a

d xads

dtZ _mmr (12)

Energy equation in the evaporator

d

dt½ðcp;w M e;w C ccu M eÞT e

ZK L J C w _mchillcp;wðT chill;in K T chill;outÞ (13)

where

JZ

_me;evap; for desorbing chamber

_me;cond; for adsorbing chamber

((14)

Table 1

Physical property parameters used in the simulation

Symbol Value Unit

a 1714.2 Pa m6 kgK2

b 1.7!10K3 m3

cp,w 4.180 kJ kgK1 K K1

ca 0.924 kJ kgK1 K K1

cal 0.905 kJ kgK1 K K1

ccu 0.386 kJ kgK1 K K1

cwv 1.850 kJ kgK1 K K1

D H 2800 kJ kgK1

KAad (heating) 3570 W K K1

KAad (cooling) 3290 W K K1

KAc 6090 W K K1

KAe 3420 W K K1

L 2500 kJ kgK1

M a 50.0 kg

M tube,ad 21.8 kg

M fin,ad 10.9 kg

M c 15.2 kg M e 65.1 kg

R 461.5 J kgK1 K K1




and

wZ1; T e%T chill;in

0; T eOT chill;in

((15)

where _me;evap and _

me;cond are respectively the mass flow ratesof refrigerant evaporated in the evaporator of the desorbing

chamber and condensed in the evaporator of the adsorption

chamber.

The vapor is assumed as incompressible flow and the

flow resistance of the water vapor is neglected. The

pressures in the chambers can be calculated by the follows:

Pwv;des K Pwv;abs Zvwv _m2

mr

2 A2(16)

where A is the sectional area of the mass recovery channel.

The Van der Waals equation is introduced to calculate the

state parameters for water vapor:

Pwv Ca

v2

ðv K bÞZ RT wv (17)

4.5. Performance parameters equations

Refrigerating capacity:

Qref Z

Ð tcycle

0 cp;w _mchillðT chill;in K T chill;outÞdt

tcycle

(18)

Heating power:

Qh Z

Ð tcycle

0 cp;w _mhðT h;in K T h;outÞdt

tcycle

(19)

COPZQref

Qh

(20)

5. Results and discussions

5.1. Water temperature variations

Fig. 5 shows the temperature profiles of the hot, cooling

and chilled water. Phrases 1, 2 and 3 respectively represent

the heating/cooling process, the mass recovery process and

the heat recovery process. The hot water outlet temperature

approaches to the inlet temperature after the adsorber is

heated about 600 s. After this moment, the heat consumed

by the desorber will be very small. But the difference

between outlet and inlet temperature for the cooling water is

still 3 8C after the adsorber is cooled about 600 s. This

difference will reach 1.7 8C until the cooling time is

extending to 900 s. Heretofore, the adsorption ability of

the adsorber is still strong but the heating power becomes

very small. Therefore, the heating/cooling time of 900 s is

recommended for a typical working condition, especially

when a higher COP required. This result also can be found in

Fig. 6.From Fig. 5, an effective mass recover process can be

found. During the mass recovery process, the hot water

outlet temperature will decrease about 4 8C in 60 s for the

heat consumed by the desorber sharply increases, and the

cooling water outlet temperature increases about 1.5 8C for

large quantity of heat yields in the adsorber. So the mass

recovery has caused a quite forceful second-adsorption/de-

sorption process, which is slightly different from the results

of the previous work about the adsorption cooling system.

This process increases the cyclic adsorption capacity of the

adsorber/desorber. Thereby, the potential refrigerating

capacity of the chiller rises, though the chiller has no

refrigerating output in the mass recovery process.

5.2. Heating/cooling time

The refrigerating capacity and the COP variations with

the heating/cooling time are shown in Fig. 6. The COP

increases uniformly with extension of the heating/cooling

time under a driving heat source of 65 or 85 8C. This is

because a longer heating/cooling time causes much lower

Fig. 5. Temperature profiles of heat transfer fluids.

Fig. 6. Effect of the heating/cooling time on the refrigerating

capacity and the COP(180 s mass recovery time; cooling water inlet

temperature: 31 8C; chilled water inlet temperature: 15 8C).




consumption of driving heat, but the refrigerating capacity

decreases a little along with a longer heating/cooling time.

For a different heat source temperature at 65 or 85 8C, the

heating/cooling time has different effect on the refrigerating

capacity. The effect of the heating/cooling time is less

intense for a lower heat source temperature.

5.3. Mass recovery time

Figs. 7 and 8 show the effects of the mass recovery time

on the performance of the chiller. In the range of the mass

recovery time from 150 to 200 s, there is an optimal value

for the refrigerating capacity when the hot water tempera-

ture is 85 8C. But the optimal value of the refrigerating

capacity will be longer than 240 s if the hot water

temperature is 65 8C. This indicates a longer mass recovery

time is more important to improve the refrigerating capacity

of the chiller for a lower heat source temperature. The COP

gains its maximum value with the mass recovery time atabout 60 s for a hot water temperature at 65 or 85 8C. The

reason of the variation for the COP is because longer mass

recovery time increases the ratio of the time without

refrigerating output in one cycle besides the mass recovery

time increases the adsorption capacity of the adsorbent.

5.4. Operating temperatures

As a driving force, the hot water is crucial to the

performance of the chiller, especially to the refrigerating

capacity, as shown in Fig. 9. The refrigerating capacity

increases about 76% for a chilled water temperature at 158C

and 59% for a chilled water temperature at 20 8C when the

driving heat source is 85 8C, compared with a 65 8C heat

source. As for COP, the improvement is only 0.9–16% with

the hot water temperature variation from 65 to 85 8C,

because a higher hot water temperature causes a higher

heating power as well as a higher refrigerating capacity. For

this chiller, the optimal hot water temperature is in the range

between 80 and 85 8C for the COP.

Figs. 10 and 11 show the effects of the cooling water and

chilled water on the performance of the chiller. As similar as

the effect of the hot water temperature, the refrigerating

capacity is more sensitive to the cooling water temperature

or the chilled water inlet temperature than the COP. For atypical working condition with the chilled water inlet

temperature at 15 8C, the hot water temperature at 85 8C and

the cooling water inlet temperature at 31 8C, the refrigerat-

ing capacity is more than 10 kW. If the chilled water inlet

temperature rises to 20 8C, the refrigerating capacity reaches

12.6 kW. The corresponding SCPs under these two typical

working conditions are 104 and 126 W/kg, respectively.

5.5. Du hring diagram

Fig. 12 shows the Duhring diagram of the chiller. The

Duhring diagram for the adsorber of this chiller is muchdifferent from that of a conventional chiller with vacuum

valves for the adsorption/desorption switch [1]. In the

desorption process (phase 1/m/2 in Fig. 12) or the

adsorption process (phase 3/n/4), the pressure in

adsorption/desorption vacuum chambers are variational all

the time. So there is a peak (point m) for the desorption

process and a nadir (point n) for the adsorption process,

which are corresponding to an intense adsorption point and

an intense desorption point, respectively. The mass recovery

Fig. 7. Effect of the mass recovery time on the refrigerating capacity

(900 s heating/cooling time).

Fig. 8. Effect of the mass recovery time on the COP (900 s

heating/cooling time).

Fig. 9. Effect of the hot water temperature on the refrigerating

capacity and the COP (900 s heating/cooling time; 180 s mass

recovery time; cooling water inlet temperature: 31 8C).




process causes a less change of the pressure in the adsorber

than that in the desorber. This is because the pressure levelof the chiller is predominated by the temperature of the

evaporator during the mass recovery process rather than by

the condenser temperature in the desorption process.

6. Conclusions

The chiller designed in this work is a reliable and high-

efficient machine, though one evaporator and one condenser

are idle anytime. As the simulation results using one simple

model, the refrigerating capacity is more than 10 kW and the

COP exceeds 0.5 under the available working conditions.

The typical working conditions selected in the conclusions

are the cooling water inlet temperature of 31 8C and the

chilled water inlet temperature of 15 or 20 8C. Through the

analysis of the prediction results, some conclusions can be

drawn for this work as follows:

(1) The influence of the operating parameters on COP and

SCP of this novel system shows the similar trends

compared with the previous conventional system.

Differently from the previous works, for the novel

system, the hot water outlet temperature and the

adsorber temperature sharply change because of the

effective mass recovery process, and there are no

isobaric processes although the working process

because there no vacuum valves installed between the

adsorber and the condenser or the evaporator so that the

pressure of the system always changes.

(2) The refrigerating capacity is 6 kW under driving heat

source at 65 8C, and the improvement is 76% if the heat

source temperature rises to 85 8C. The COP exceeds 0.5

even under a 65 8C heat source. And the optimal hot

water temperature is in the range between 80 and 85 8C

for the COP.

(3) If the chilled water inlet temperature is 20 8C, a

12.6 kW refrigerating capacity and a 0.65 COP areobtained. This result is much valuable for utilization in

the field of no requirement for dehumidification.

(4) For a better refrigerating capacity as well as the COP,

900 s heating/cooling time and 180 s mass recovery

time are recommended.

(5) The mass recovery process has greater influence on the

performance with a lower heat source temperature, such

as 65 8C.

Acknowledgements

This work is supported by National Science Fund for

Distinguished Young Scholars of China under the contract

No. 50225621, the state Key Fundamental Research

Program under the contract No. G2000026309 and Shanghai

Shuguang Training Program for the Talents under the

contract No. 02SG11.

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Fig. 10. Effect of the cooling water temperature on the refrigerating


recovery time).

Fig. 11. Effect of the chilled water temperature on the refrigerating


recovery time).

Fig. 12. Duhring diagram of the novel chiller based on the simulated

results (900 s heating/cooling time; 180 s mass recovery time).




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study of a novel silica gel¨cwater adsorption chiller[1]. part i. design and perfor mance...

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