Mass recovery four-bed adsorption refrigeration cycle.pdf

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Mass recovery four-bed adsorption refrigeration cycle

with energy cascading

Akira Akahira *, K.C. Amanul Alam, Yoshinori Hamamoto,Atsushi Akisawa, Takao Kashiwagi

Department of Mechanical Systems Engineering, Tokyo University of A&T, 2-24-16 Naka-cho, Koganei-shi,

Tokyo 184-8588, Japan

Received 10 March 2004; accepted 12 October 2004

Available online 16 December 2004

Abstract

The study investigates the performance of a four-bed, silica gel–water mass recovery adsorption refrig-

eration cycle with energy cascading. In an adsorption refrigeration cycle, the pressures in adsorber and

desorber are different. The mass recovery cycle utilizes the pressure difference to enhance the refrigerant

mass circulation. Proposed cycle has three main characteristics; (1) The cycle consists of two single-stage

cycles. (2) Hot and cooling water is used in each single-stage cycle with cascading. (3) Adsorber/desorber

heat exchanger of one cycle is connected with another adsorber/desorber heat exchanger of other cycle.

Specific cooling power (SCP) and coefficient of performance (COP) were calculated by cycle simulation

computer program to analyze the influences of operating conditions. The proposed cycle was compared

with the single-stage cycle in terms of SCP and COP. The results show that SCP and COP of proposed cycle

with cascading chilled water are superior to that of conventional, single-stage cycle and the proposed cycle

has high advantage at low heat source temperature.

Ó 2004 Elsevier Ltd. All rights reserved.

Keywords: Adsorption system; Silica gel; Mass recovery; Cascading; Performance; Calculation

1359-4311/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.applthermaleng.2004.10.006

* Corresponding author. Tel./fax: +81 042 388 7282.

E-mail address: [email protected] (A. Akahira).

www.elsevier.com/locate/apthermeng

Applied Thermal Engineering 25 (2005) 1764–1778

mailto:[email protected]

mailto:[email protected]



Nomenclature

A area (m2

)C specific heat (kJ kgÀ1 KÀ1)Dso surface specific heat (m2 sÀ1)E a activation energy (J kgÀ1)F mass flow (kg sÀ1)K heat transfer coefficient (W mÀ2 KÀ1)L latent heat of vaporization (J kgÀ1)P s saturated vapor pressure (Pa)q fraction of refrigerant which can be adsorbed by the adsorbent (kg kgÀ1)q* fraction of refrigerant which can be adsorbed by the adsorbent under saturation

condition (kg kgÀ1)QIN driving heat (kJ)Qchill cooling output (kJ)Qst isosteric heat of adsorption (J kgÀ1)Rgas gas constant (J kgÀ1 KÀ1)Rp average radius of a particle (m)t time (s)tcycle cycle time (s)W weight (kg)

Subscripts

ads adsorptional aluminumc condenser

chill chilled watercu copper

cw cooling waterdes desorption

e evaporatorfHex fin (aluminum)hex adsorber/desorber heat exchanger

hw hot waterin inletkHex heat transfer tube (cupper)

out outlets silica gelw water

wv vapor

A. Akahira et al. / Applied Thermal Engineering 25 (2005) 1764–1778 1765



1. Introduction

Adsorption chiller has lower environmental impact and large energy saving potential, because

the system does not use the fossil fuel and electricity as driving source. Especially, the cycle withsilica gel–water pair utilizes the heat at temperatures below 85 °C as driving source. Waste heat of

near environmental temperature is used to drive the adsorption refrigeration cycle.In the adsorption chiller, it is important to have a good adsorbent–adsorbate pair. Water,

methanol or ammonia has been used as adsorbate, and zeolite or activated carbon has been

combined with the adsorbate as adsorbent. For example, the zeolite–water pair was studied byRothmeyer et al. [1] and Douss and Meunier [2] for heating. The zeolite–ammonia pair was inves-tigated by Critoph and Turner [3]. The activated carbon–methanol pair was studied by Pons and

Guilleminot [4], and the activated carbon–ammonia pair was investigated by Jones and Christo-philos [5]. The systems were either gas-fired or solar-heat-driven. The silica gel–water pair, how-

ever, can utilize the heat sources with temperatures of less than 100°

C [6,7]. It is advantage foradsorption chiller to be operated with low heat source temperature. To use lower temperatureheat source, Saha et al. [8,9] proposed three-stage and two-stage adsorption cycles and investi-

gated the performance of the cycles. Results show that two and three-stage adsorption chillerscould be operated by low temperature heat source between 40 and 60 °C, but the COPs are obsti-

nately low.Pons and Poyelle [10] studied the influence of mass recovery processes in conventional two

beds adsorption cycle to improve the cooling power. In the study, zeolite–water pair was

used, the heat source temperature was 240 °C; their heat source temperature is higher thanthe temperature of low level waste heat and the adsorbent–adsorbate pair is not suitable forusing low temperature waste heat. Recently, Wang [11] investigated the performances of vapor

recovery cycle with activated carbon–methanol as adsorbent–adsorbate pair and demonstratedthat the mass recovery cycle is effective for the low regenerating temperature. The cycle with acti-vated carbon–methanol can be operated with low heat source temperature and the cycle has the

advantage for freezing lower than 0 °C, for example, ice maker. In the present analysis, it isassumed to use the waste heat of temperature below 100 °C for heat source to provide effectivecooling. Therefore, silica gel–water pair is selected due of its ability to be operated with lower

temperatures.In our previous study [12], the performance of two-bed mass recovery cycle with silica gel–water

pair was investigated and improvement of performance was confirmed. Mass recovery processutilizes the pressure difference between adsorber and desorber. Therefore, the bigger the differ-

ence of pressure, the more the refrigerant that could be move from desorber to adsorber. Inthe present study, four-bed mass recovery cycle is proposed to increase the pressure difference.In such a cycle, hot and cooling water is used with cascading flow from one desorber or adsorber

to other desorber or adsorber, where the movement of refrigerant from desorber to adsorber isaccelerated.

The present treatment is concerned with silica gel–water adsorption chiller using pressuredifference of the adsorber/desorber heat exchangers to accelerate adsorption/desorption process.

In the study, the performances of the novel system are compared with those of the conventionalsingle-stage cycle. The effects of operating temperature and cascading chilled water on the perfor-

mance are also presented.

1766 A. Akahira et al. / Applied Thermal Engineering 25 (2005) 1764–1778



2. Working principle

2.1. Four-bed mass recovery cycle without heating and cooling

The schematic of proposed cycle is shown in Fig. 1. The cycle consists of two single-

stage adsorption cycles, that is, four pairs of heat exchangers, namely an evaporator (EVA1)-adsorber (HEX1) and a condenser (COND1)-desorber (HEX2), EVA2-HEX3, COND2-HEX4.HEX2 connects HEX3 through the valve V9 and HEX1 connects HEX4 through valve V10.

Upper part of mode A on Fig. 1 (HEX1, HEX2, COND1 and EVA1) is denoted as upper cycleas well as lower part (HEX3, HEX4, COND2 and EVA2) is denoted as lower cycle. Hot waterused in upper cycle flows into lower cycle, and cooling water used in lower cycle flows into upper

cycle.Upper part of four-bed mass recovery cycle (HEX1, HEX2, COND1 and EVA1) is similar to

single-stage cycle. The cycle added the valve to connect HEX1 and HEX2 is two-bed mass recov-ery cycle. These mass recovery cycles have six operational modes; Fig. 1 shows half cycle of four-bed mass recovery cycle that has only three modes. The second and fourth modes are mass

recovery process. In this process on two-bed mass recovery cycle, HEX1 is connected withHEX2. If HEX1 is adsorber and HEX2 is desorber in mode A, the pressures of adsorber/desorber

heat exchangers at the beginning of mode B are equal to those in mode A. For example, adsorberconnects with evaporator. Therefore, the pressure of adsorber is equal to that of evaporator.Should the evaporator vapor temperature be 10 °C, the pressure of evaporator is about

1.2 kPa. The pressure of desorber, which is equal to the condenser pressure, is about 4.1 kPa,when the condenser vapor temperature is 30 °C. Because of the pressure difference of the beds,adsorption/desorption process will occur automatically without any heating and cooling applica-

tion. Thus, the desorbed refrigerant from desorber will move to adsorber; the high refrigerantmass circulation provides better performances.

Four-bed mass recovery cycle utilizes same principle. Moreover, the refrigerant mass circula-

tion will be higher than the conventional two-bed mass recovery cycle due to the higher pressuredifference in the present cycle with cooling water cascading on condenser. Fig. 1(a) presents themode A of four-bed mass recovery cycle; in mode A, valves V1, V4, V5, V8, V9 and V10 are

closed in such a manner that EVA1-HEX1 and EVA2-HEX3 are in adsorption process andCOND1-HEX2 and COND2-HEX4 are in desorption process. Refrigerant (water) in evaporator

is evaporated at the temperature (T eva) and thus removing Qeva from the chilled water. The evap-orated vapor is adsorbed by adsorbent (silica gel bed), and cooling water circulated to the beds

removes the adsorption heat, Qads. The desorption–condensaion process takes place at pressure(P cond). The desorbers (HEX2 and HEX4) are heated up to the temperature (T des) by Qdes, pro-vided by the driving heat source. The resulting refrigerant vapor is cooled down to temperature(T cond) in the condenser by the cooling water, which removes the heat, Qcond. When the refrigerant

concentrations in the adsorber as well as in the desorber are at near their equilibrium levels, thecycle is continued by changing into mode B.

In mode B, adsorber (HEX1) and desorber (HEX2) of upper cycle are connected with desorber

(HEX3) and adsorber (HEX4) through the valves V9 and V10, respectively. In this mode, no bedinteracts with the evaporator or condenser. The pressures of adsorber and desorber at the begin-

ning of mode B are equal to those in mode A. Each bed in mode A operates at different pressure




levels (Fig. 2). The process is same as mass recovery process of conventional two-bed mass

recovery cycle.

V3V1

V2V4

V6

V7

V8

V10

V9

V5

HEX1

HEX2

HEX3HEX4

COND1

COND2

EVA1

EVA2

Refrigerantvapor

Cooling

waterIn/Out

Coolingwater

In/Out

HotwaterIn/Out

HotwaterIn/Out

Chilledwater In Chilledwater Out

Coolin Water

HotwaterIn/Out

HotwaterIn/Out

V3V1

V2V4

V6

V7

V8

V10

V9

V5

HEX1HEX2

HEX3HEX4

COND1

COND2

EVA1

EVA2

Coolingwater

In/Out

Coolingwater

In/Out


Coolin Water

(a) ModeA

(b) ModeB

(c) ModeC

Refrigerantvapo

V3 V1

V2V4

V6

V7

V8

V10

V9

V5

HEX1HEX2

HEX3HEX4

COND1

COND2

EVA1

EVA2


Coolin Water

Fig. 1. Schematic of four-bed adsorption refrigeration cycle with mass recovery process.




Mode C is a warm-up process. In this mode, all valves are closed. HEX1 and HEX3 are

heated up by hot water, and HEX2 and HEX4 are cooled by cooling water. When the pres-sures of desorber and adsorber are nearly equal to the pressures of condenser and evapora-tor respectively, then valves between adsorbers and evaporators as well as valves between

desorbers and condensers are opened to flow the refrigerant. The latter mode is denoted asmode D.

It is noted that the right side of the system is in desorption process and left side is in adsorptionprocess. In next mode, the mode E is similar as mode B. Mode F is warm up process as mode C.

The mode is the last process and after the mode, it returns to mode A.

2.2. Mass recovery cycle with heating and cooling

The cycle aims at providing better cooling capacity by applying hot and cooling water in massrecovery process. Therefore, mode A, C, D, and F are same with the cycle without heating and

cooling. In mode B and E, all valves except V9 and V10 are closed. For example, HEX1 andHEX3 are adsorber and HEX2 and HEX4 are desorber, therefore, HEX1 and HEX3 are cooleddown by cooling water and HEX2 and HEX4 are heated by hot water. The input of hot and cool-

ing water increases a quantity of adsorbate moving from desorber to adsorber, which will lead the

system to provide better cooling capacity.

3. Mathematical modeling

The working principle of conventional chiller is available in Ref. [13,14]. In the present study, it

is assumed that the temperature and pressure are uniform throughout the whole adsorber, and thesystem has no heat losses to the environment, i.e. well insulated. According to these assumptions,the dynamic behavior of heat and mass transfer inside the different components of the adsorption

chiller can be written as:

Upper Cycle

Bottom Cycle

Bed Temperature

W a t e r V a p o r P r e s s u r e

Concentration100%

Fig. 2. Conceptual Duhring diagram for four-bed mass recovery cycle.




3.1. Energy balance in adsorber/desorber

The energy balance in adsorber can be written as

d

dt ½fW s Á ðC s þ C wv Á qÞ þ ðC cu Á W kHex þ C al Á W fHexÞg Á T ads

¼ Qst Á W s Ádq

dt þ F water Á C water Á ðT water in À T water outÞ ð1Þ

The outlet temperature of cooling water can be expressed as

T water out ¼ T ads þ ðT water in À T adsÞ Á expÀ K ads Á AHex

F water Á C water

ð2Þ

The energy balance in desorber can be described by identical equations, where the term for heat

transfer fluid (water) temperature (T water) and the subscript ads, respectively, denote the coolingwater and adsorber upon adsorption, and the hot water and desorber upon desorption.

3.2. Energy balance in condenser

The condenser energy balance equation can be written as

C cu Á W c ÁdT c

dt ¼ À L Á W s Á

dqdes

dt þ C w Á W s Á

dqdes

dt Á T c À C wv Á W s Á

dqdes

dt Á T c þ F c Á C w

Á ðT cool in À T cool outÞ ð3Þ

The outlet temperature of cooling water is given as

T cool out ¼ T c þ ðT cool in À T cÞ Á expÀ K c Á Ac

F c Á C c

ð4Þ

3.3. Energy balance in evaporator

The energy balance in evaporator is expressed as

d

dt fT e Á ðC w Á W ew þ C cu Á W eÞg ¼ À L Á W s Á

dqads

dt À C w Á W s Á

dqdes

dt Á T c À C wv Á W s Á

dqads

dt Á T e

þ F e Á C w Á ðT chill in À T chill outÞ ð5Þ

The outlet temperature of chilled water can be written as

T chilled out ¼ T e þ ðT chilled in À T eÞ Á expÀ K e Á Ae

F e Á C e

ð6Þ

3.4. Adsorption equilibrium

The adsorption equilibrium for silica gel type A/water vapor can be expressed by the followingcorrelation:




qÃ ¼ AðT sÞ ÂPsðTwÞ

PsðTsÞ

BðT sÞ

ð7Þ

where AðT sÞ ¼ A0 þ A1 Â T s þ A2 Â T 2s þ A3 Â T 3s ð8Þ

BðT sÞ ¼ B0 þ B1 Â T s þ B2 Â T 2s þ B3 Â T 3s ð9Þ

The numerical values of A0–A3 and B0–B3 are determined by the least square method appliedto experimental data; the numerical values have been discussed by Saha et al. [13]. The typical

values of physical property parameters used in the calculations are shown in Table 1.

3.5. System performance equations

The cooling capacity is expressed byCooling capacity ¼

Qchill

t cycle

ð10Þ

where

Qchill ¼

Z t 0

C w Á F e Á ðT chill in À T chill outÞdt ð11Þ

Table 1

Physical property values

Symbol Value Unit

Ac 3.73 m2

Ae 1.91 m2

AHex 2.46 m2

W c 27.28 kg

W e 12.45 kg

W kHex 64.04 kg

W fHex 51.20 kg

W sHex 47.00 kg

K c 4115.23 W m2 KÀ1

K e 2557.54 W m2 KÀ1

K ads 1602.56 W m2 KÀ1

K des 1724.14 W m2 KÀ1

Rgas 4.62 · 102

J kgÀ1

KÀ1

E a 2.33 · 106 J kgÀ1

Dso 2.54 · 10À4 m2 sÀ1

Rp 0.3 · 10À3 m

Qst 2.80 · 106 J kgÀ1

C s 924 J kgÀ1 KÀ1

C w 4180 J kgÀ1 KÀ1

C wv 4190 J kgÀ1 KÀ1

C cu 386 J kgÀ1 KÀ1

C al 905 J kgÀ1 KÀ1

L 2.50 · 106 J kgÀ1




The COP value is defined by the following equation:

COP ¼Qchill

QIN

ð12Þ

where

QIN ¼

Z t 0

C w Á F hw Á ðT hot in À T hot outÞdt ð13Þ

It is well known that cooling capacity increases as silica gel weight increases if other conditionremains same. If the performance of four-bed cycle is compared with one of two-bed cycle, it isnecessary to calculate cooling capacity per silica gel weight; which is defined as SCP. SCP can

be expressed by the following equation:

SCP ¼Cooling capacity

W sHex

ð14Þ

4. Results and discussion

In the preceding sector, a novel strategy in mass recovery process is proposed and cycle simu-lation runs are performed to evaluate the performance of the proposed mass recovery cycle. Silica

gel–water adsorption refrigeration cycle is designed for utilizing low grade waste heat. Therefore,it is important to utilize the low temperature waste heat as driving source. From this point of view,investigation was conducted for hot water of temperature between 50 and 80 °C. Standard oper-

ating conditions are cited in Table 2 [15]. In all figures, legend ÔwithoutÕ and ÔwithÕ are used whereÔwithout

Õmeans the cycle in which hot and cooling water are not supplied during mass recovery

process while ÔwithÕ means hot and cooling water are supplied during mass recovery process.

4.1. Performance comparison

Fig. 3 shows the effect of heat source temperature on COP and cooling capacity of the proposedsystem. In the calculation, mass flow rate of chilled water is constant at 0.7 kg sÀ1. From the

Table 2

Standard operating conditions

Parameter Value Unit

Hot water in Temperature 70 °C

Flow 1.7 kg sÀ1

Cooling water in Temperature 30 °C

Flow (Ads + Cond) 3.0 (1.7 + 1.3) kg sÀ1

Chilled water in Temperature 14 °C

Chilled water out Temperature 7 °C

Cycle time 1200 s

Adsorption/desorption 490 s

Mass recovery 80 s

Pre-heating/pre-cooling 30 s




figure, it can be seen that the COP values of the cycle without heating/cooling have the best value

and the values are always higher than those of the cycle with heating/cooling. Cooling capacity of each cycle increases as the heat source temperature increases.

Fig. 4 shows the predicted and experimental specific cooling power (SCP) of proposed mass

recovery. The input data of temperature is same with that of standard operating condition.Adsorption/desorption process time is 420 s, mass recovery process time is 60 s and pre-heat-

ing/pre-cooling process time is 30 s. As the prototype experimental machine has the maximumflow rate capacity, therefore, for the figure we considered mass flow rate of hot water is 1.5 kg s À1,

one of cooling water is 2.3 kg sÀ1 and one of chilled water is 0.6 kg sÀ1. It is seen that SCP is

superimposed with the measured SCP at any heat source temperature and it could be claimedthat our calculation agrees well with experimental data. Some values of experimental resultsare higher than those values of simulation results. The chiller operates by continuously switching

0

0.2

0.4

0.6

0.8

1

40 50 60 70 80 90

Heatsource temperature (°C)

C O P [ - ]

0

5

10

15

20

25

30COP (without)

COP (with)

Cooling capacity (without)

Cooling capacity (with)

chilled waterinlet temp.=14°C

cooling water inlet temp.=30°C

C o o l i n g c a p a c i t y ( k W

)

Fig. 3. Effect of heat source temperature.

0

20

40

60

80

100

120

140

160

180

50 60 70 80

Heat source temperature (°C)

S C P ( W

k g - 1 )

Simulation (masswithout)

Simulation (masswith)

Simulaiton (single-stage)

Experiment (mass without)

Experiment (mass with)

Experiment (single-stage)

chilled water inlet temp.=14°Ccooling water inlet temp.=30°C

Fig. 4. Comparison with experimental result on SCP.




its adsorber/desorber heat exchangers between the adsorption and desorption mode. Therefore,

there are some differences between experimental condition and standard condition. That is whysome experimental results are higher than those values of simulation results.

If the performance of one cycle is compared with that of other cycle, it is important to considerthe outlet temperature of chilled water because performances is strongly dependent on the evap-

orating temperature. Saha et al. [8] showed that the more the mass flow rate of chilled water, thehigher the COP value and cooling capacity. Nonetheless, if the cycle is operated with high flowrate of chilled water for getting high performance, it causes to increase the outlet temperature

of chilled water. Therefore, it is impractical to utilize high flow rates of chilled water; the outlettemperature of chilled water is important and should be used same value for comparison amongthe cycles. Therefore, cyclic average chilled water outlet temperature is fixed at 7 °C for the

following results.Fig. 5 shows the improvement ratio of COPs and cooling capacity on the cycle with heating/

cooling to the cycle without heating/cooling. From the figure, it is seen that improvement ratioof cooling capacity is 23.5% if heat source temperature is 60 °C. The lower the heat source tem-perature, the higher the effect of mass recovery process with hot and cooling water. Nevertheless,

the COP value decreases as the heat source temperature decreases. Therefore, if there is enoughinput heat, that is, waste heat, the mass recovery cycle with heating and cooling has advantage

to conventional mass recovery cycle.

4.2. Effect of cascading chilled water

Mass recovery process utilizes the pressure difference between the adsorber and desorber. Thepressure of adsorber depends on the pressure of evaporator and chilled water inlet temperature

decides the pressure of evaporator. If chilled water flows to evaporator from upper cycle to lowercycle, inlet temperature of lower cycle will be lower than that of upper cycle. Therefore, the pres-sure difference between desorber on upper cycle and adsorber on lower cycle will be higher than

that with chilled water parallel flow; it seems that the amount of refrigerant that moves from de-sorber to adsorber increases. Fig. 6 shows the results of the COP value and cooling capacity withparallel or cascading flow of chilled water. In the calculation, mass flow rate of chilled water is

-30

-20

-10

0

10

20

30

50 60 70 80 90


I m p r o v e m e n t r a t i o ( % )

COP

Cooling capacity

chilled water inlet temp.=14°C

cooling water inlet temp.=30 °C

Fig. 5. Effect of supplying hot and cooling water (average chilled water outlet temperature is 7 °C).




controlled to obtain average outlet temperature of chilled water at 7 °C. In the cascading cycle,chilled water is cooled twice; outlet temperature of chilled water with cascading flow is lower than

that with parallel flow. Mass flow rate of chilled water with cascading flow is more than that withparallel flow. Cooling output increases as mass flow rate increases. Therefore, the cycle with cas-cade flow has the advantage over the cycle with parallel flow. Especially, chilled water outlet tem-perature increases as heat source temperature decreases; cascading chilled water is effective for

operating the chiller at low heat source temperature.

4.3. Comparison with conventional, single-stage cycle

Fig. 7 shows the effect of heat source temperature on COP and SCP of mass recovery cycle with

cascading or parallel flow on chilled water and single-stage cycle. From the figure, COP values of

(b) Cooling capacity

0

100

200

300

50 60 70 80 90


Parallel (without)

Parallel (with)Cascading (without)

Cascading (with)

(a) COP

0

0.1

0.2

0.3

0.4

0.5

0.6

50 60 70 80 90

Heatsource temperature (°C)

C O P [ - ]

Parallel (without)

Parallel (with)

Cascading (without)

Cascading (with)





S C P ( W k g

- 1 )

Fig. 6. Effect of cascading chilled water (average chilled water outlet temperature is 7 °C).




the mass recovery cycle without heating/cooling and with cascading chilled water are alwayshigher than those of single-stage cycle. For example, if heat source temperature is 70 °C, improve-ment ratio of COP value on mass recovery cycle without heating/cooling and with cascadingchilled water is 30.3%. In the cycle with heating/cooling and with cascading chilled water, the val-

ues are higher than those of single-stage cycle at the heat source temperature lower than 70 °C.SCP values of two mass recovery cycles in which chilled water is used with cascading flow arealways higher than those of single-stage cycle at any heat source temperature. If heat source tem-perature is 70 °C, the improvement ratio of the cycle with heating/cooling and cascading chilled

water is 15.8%. Moreover, the lower the heat source temperature, the higher the improvementratio. Therefore, the cycle with cascading chilled water has the advantage over single-stage cycle

(a) COP

0

0.1

0.2

0.3

0.4

0.5

0.6

50 60 70 80 90


C O P [ - ]

single-stage

mass parallel (without)

mass parallel (with)

mass cascade (without)

mass cascade(with)

(b) SCP

0

20

40

60

80

100

120

50 60 70 80 90


S C P ( W k g - 1 )

single-stage

mass parallel (without)mass parallel (with)

mass cascade (without)mass cascade (with)





Fig. 7. Comparison with single-stage cycle (average chilled water outlet temperature is 7 °C).




at low heat source temperature and it is found that the mass recovery process has the effect on the

performance at low heat source temperature. For the mass recovery cycle with heating/cooling,the improvement ratio of SCP with chilled water parallel flow is 1.8% at heat source temperature

70 °C; the value for the cycle without heating/cooling is À5.9%. In the proposed mass recoverycycle, hot and cooling water flows to adsorber/desorber heat exchanger with cascading: hot water

flows to lower cycle from upper cycle and cooling water flows to upper cycle from lower cycle.Therefore, hot water inlet temperature on lower cycle is lower than that of upper cycle; coolingwater inlet temperature on upper cycle is higher than that of lower cycle because of adsorption

heat on lower cycle. The performance of upper cycle is lower than that of the single-stage cyclewith cooling water inlet temperature 30 °C and the performance of lower cycle is lower than thatof single-stage with hot water inlet temperature 70 °C. Therefore, it is natural the performance of

proposed cycle is lower than single-stage cycle if chilled water is used with parallel flow. The pro-posed cycle has the advantage for conventional single-stage cycle with chilled water cascading and

considering the fixed average chilled water outlet temperature.

5. Conclusion

The mass recovery cycles with silica gel–water pair were presented and the effects of operatingconditions were investigated. For the same operating conditions, mass recovery cycles haveadvantage to conventional single-stage cycle at low heat source temperature with considering fixed

average chilled water outlet temperature. The performance of the cycle with cascaded chilledwater flow is better than that of the cycle with parallel chilled water as the chilled water is coldtwice and outlet temperature is lower than that of the cycle with chilled water parallel flow. That

is why the proposed mass recovery cycle is superior than the conventional single-stage cycle withconsidering the fixed average chilled water outlet temperature and cascading chilled water. Cool-ing capacity of mass recovery cycle with heating and cooling is higher than that of conventional

single-stage cycle. This advantage is higher at low heat source temperature. COP values of pro-posed cycle are also higher than those of single-stage cycle if heat source temperature is lower than70 °C. Therefore, the mass recovery cycle with silica gel–water pair will be effective for using low

temperature waste heat in the future.

Acknowledgement

The authors wish to acknowledge the financial support provided by the New Energy and Indus-trial Technology Development Organization (NEDO), Japan, to conduct the present research.

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Mass recovery four-bed adsorption refrigeration cycle.pdf