Design and performance simulation of a new.pdf

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Renewable Energy 27 (2002) 401415www.elsevier.com/locate/renene

Design and performance simulation of a newsolar continuous solid adsorption refrigeration

and heating hybrid system

X.J. Zhang, R.Z. Wang *

Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200030, China

Received 24 April 2001; accepted 21 June 2001

Abstract

In this paper, the design of a new continuous solid adsorption refrigeration and heating

hybrid system driven by solar energy was proposed, and its performance simulation and analy-sis were made under the normal working conditions. Some performance parameters of thesystem were obtained, and the effects of water mass in water tank on the systems COPcooling,COPheatingetc. were discussed. The simulation indicated: the system could refrigerate continu-ously with such a design, and at the conditions of that the daily sun-radiation is 21.6 MJ, themean ambient temperature is 29.9C, the evaporating temperature is 5C, the heat-collectingcoefficient of upper bed h is 60%, and the heat-transfer coefficient between lower bed andambient a is 2 W/m2 K, by day a hybrid system of single combined bed could furnish 30 kghot water of 47.8C, and had a mean COPcoolingof 0.18, a mean COPheatingof 0.34, a continuousmean SCPa of 17.6 W/kg, a continuous mean SCPc of 87.8 W/m

2, and a continuous meanSHPc of 165.9 W/m

2; and at night it had a cooling capacity of 0.26 MJ/kg of adsorbent, and

a cooling capacity of 1.3 MJ/m2 of heat-collecting area. 2002 Elsevier Science Ltd. Allrights reserved.

Keywords:Solar energy; Continuous type; Adsorption refrigeration; Hybrid system

* Corresponding author. Tel./fax: +86-21-62933250.

E-mail address: [email protected] (R.Z. Wang).

0960-1481/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 0 - 1 4 8 1 ( 0 1 ) 0 0 1 4 8 - 3


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402 X.J. Zhang, R.Z. Wang / Renewable Energy 27 (2002) 401415

Nomenclature

I(t) solar radiant intensity (W/m2)

A heat-collecting area of the upper bed (m2)

h heat-collecting coefficient of the upper bedMa mass of adsorbent (kg)

Mb mass of adsorber material (kg)

Mwt water mass in water tank (kg)

cpa isobaric specific heat of adsorbent (J/kg K)cprl isobaric specific heat of adsorbate liquid (J/kg K)cpb isobaric specific heat of adsorber material (J/kg K)cprg

isobaric specific heat of adsorbate vapor (J/kg K)cpw isobaric specific heat of cooling water (J/kg K)

x0 maximum of adsorption capacity (kg/kg)

x1 adsorption capacity of adsorbent in upper bed (kg/kg)

x2 adsorption capacity of adsorbent in lower bed (kg/kg)

k1, n1 adsorption parameters related with desorbing of the working pair

k2, n2 adsorption parameters related with adsorbing of the working pair

ha adsorption heat (J/kg)

hd desorption heat (J/kg)

Ta1 initial temperature of adsorption (K)

Ta2 final temperature of adsorption (K)Tg1 initial temperature of desorption (K)Tg2 final temperature of desorption (K)Twt mean water temperature in water tank (K)

pco condensing pressure (pa)

pev evaporating pressure (pa)

Tco condensing temperature (K)

Twin inlet temperature of cooling water (K)

Twout outlet temperature of cooling water (K)

Tam ambient temperature (K)

Tev evaporating temperature (K)a heat-transfer coefficient between lower bed and ambient (W/m2 K)

L latent heat of vaporization of adsorbate (J/kg)

Gw mass flow rate of cooling water (kg/s)Qco heat loss of adsorbate in condenser (W)

Qcooling cooling capacity of adsorbate in evaporator (W)COPcooling solar cooling coefficient of performance of the hybrid systemCOPheating solar heating coefficient of performance of the hybrid systemSCPa continuous mean specific cooling power of adsorbent in the daytime (W/kg)SCPc continuous mean specific cooling power of heat-collecting area in

the daytime (W/m2)SHPc continuous mean specific heating power of heat-collecting area in

the daytime (W/m2)


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403X.J. Zhang, R.Z. Wang / Renewable Energy 27 (2002) 401415

1. Introduction

The coefficient of performance of solid adsorption refrigeration system has been

improved greatly due to a great number of experimental and theoretical researchessince the recent years. But the commercialization of solid adsorption refrigeration

system is retarded by low thermal conductivity of adsorbent and low specific coolingpower of the system [1].

By application of water, methanol, and ammonia, the solar adsorption refrigeration

system employs the intermittence of solar energy and adsorption refrigeration art-fully, and many prototypes and products have been reported [25]. But such coolingtype only refrigerates during the night, and in the summer day when larger cooling

capacity is needed, the method of cold storage must be adopted which will lead the

larger volume and higher cost of solar adsorption refrigeration system.

To improve the COP and cooling capacity of solid adsorption refrigeration system

effectively, the more rational adsorption cycle and structure of adsorber should be

built. In this paper, based on lots of experimental and theoretical researches of solid

adsorption refrigeration system, the design and performance analysis of a new con-

tinuous solid adsorption refrigeration and heating hybrid system driven by solar

energy was proposed. By comparison with the intermittent solar solid adsorption

refrigeration system, the new hybrid system has many advantages such as refrigerat-

ing continuously even in the daytime, cooling as well as heating, and higher

energy efficiency.

2. Design and principle of the hybrid system

2.1. Design of the hybrid bed

Solar solid adsorption bed is always designed according to the fabrication of plate-type heat collector. Such design has two disadvantages as following: (1) The encap-

sulation thickness of adsorbent is so large that the temperature difference of adsorbent

between surface layer and bottom in the bed is too big for adsorbent to adsorb or

desorb evenly; (2) Only be used in intermittent cycle with desorbing in the daytimeand adsorbing at night.

To overcome the two disadvantages of traditional adsorption bed, the hybrid

adsorption bed is combined by two adsorption beds which are covered by glass layer

similar with solar collector shown in Fig. 1. To avoid the mutual thermal effect, anadiabatic layer is placed between the two beds. Such hybrid bed is devised like a

slat and is made of alloy, which will make for keeping vacuity and increasing heat-

transfer areas. Black lacquer is coated on the surface of the bed. A U-type flume isdevised as the bed wall through which the cold water flows to cool adsorbent thatis adsorbing. When the two beds are in a state of saturated adsorption/desorption

respectively, the two beds exchange their positions through a rotation of 180 by anaxis, still with the upper bed desorbing and the lower bed adsorbing for refrigerat-

ing continuously.


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Fig. 1. The design of hybrid adsorption bed.

2.2. Working principle of the hybrid system

The schematic design of solar continuous solid adsorption refrigeration and heating

hybrid system is shown in Fig. 2. The system consists of a hybrid adsorption bed

array, water tank, condenser, evaporator, liquid receiver, throttle valve, valves and

so on.

In an ideal process, the working principle is shown in Fig. 3. In the morning, the

upper bed absorbs solar energy as a heat collector. As time going, the temperature

of adsorbent in upper bed rises. When the temperature of adsorbent rises up to a

temperature (Ta2Tg1) which causes the vapor pressure of the desorbed refrigerant

up to the condensing pressure (pev

pco), desorption at constant pressure is initiated,the desorbed vapor is condensed in the condenser and collected in the receiver. The

liquid flows into the evaporator via a throttle valve. The temperature of adsorbentin upper bed continues rising due to solar heating. After 2 h, when the temperatureof adsorbent reaches the expected desorption temperature 8090C (Tg1Tg2), thefirst heating of upper bed ends, and the hybrid bed is rotated by 180. The upperbed is substituted for the lower bed, which is heated by solar energy now, and the

Fig. 2. Continuous solid adsorption refrigeration and heating hybrid system driven by solar energy.


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Fig. 3. The Clapeyron diagram of ideal adsorption cycle.

upper bed is cooled by cold water in water tank which will circulates from tank tobed by its natural convection.

Conveniently, we still call the up bed upper bed, and the low bed lower bed.

The temperature of adsorbent in lower bed is reduced rapidly (Tg2Ta1), and the

vapor pressure of refrigerant drops too (pcopev). Evaporation, i.e. refrigeration could

happen if the connecting valve is open. The cooling by cold water and air naturalconvection causes the temperature of adsorbent in lower bed to drop (Ta1Ta2). In

the evening, by several cycles the water will reaches high temperature, and can be

used by the family.

The thermodynamic cycle for adsorption refrigeration can be demonstrated in a

pTx diagram shown as Fig. 3. But in such hybrid cycle, because the temperatureof cooling water in water tank is rising after every cycle, Ta2, Tg1, Tg2, and Ta1 will

have increasing values. So the cycle will not be repeated exactly due to the dynamic

change of the several parameters.

In general, when the upper bed desorbs, the lower bed adsorbs, and the hybridsystem refrigerates and heats simultaneously and continuously. At night, because the

ambient temperature is low, all the two beds will adsorbe refrigerants and have

refrigeration effects.

3. Theoretical analysis of the hybrid system

Based on the same design parameters of the two beds and the homogeneous tem-

perature of adsorbent in the adsorbing/desorbing process, this paper will make a

theoretical analysis on the system of a single hybrid adsorption bed. There are fivesubsystems in the hybrid system: (1) the upper bed (signed by bed 1), (2) the lower

bed (signed by bed 2), (3) condenser, (4) evaporator, (5) and water tank.


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3.1. Unsteady energy equations of subsystems

For the upper bed, the unsteady energy equation is

I(t)Ah (Macpa x1Macprl Mbcpb)dT1

dthdMa

dx1

dt, (1)

where I(t) is the solar radiant intensity (W/m2), A is the heat-collecting area of the

upper bed (m2), h is the heat-collecting coefficient of the upper bed, Ma is the massof adsorbent (kg), cpa is the isobaric specific heat of adsorbent (J/kg K), x1 is theadsorption capacity of adsorbent in upper bed (kg/kg), cprl is the isobaric specificheat of adsorbate liquid (J/kg K), Mb is the mass of adsorber material (kg), cpb is

the specific heat of adsorber material (J/kg K), and hd is the desorption heat (J/kg).x1, hd can be calculated by the two following equations [6]:

x1 x0expk1T1Tco1n1, (2)

which is called D-A equation, where k1,n1are the adsorption parameters related with

desorbing of the working pair, x0is the maximum of adsorption capacity (kg/kg), and

Tco is the condensing temperature equal to the ambient temperature (K); and

hd LT1

Tco, (3)

where L is latent heat of vaporization of adsorbate (J/kg).For the lower bed, the unsteady energy equation is

Gwcpw(TwinTwout) aA(TamT2) (Macpa x2Macprl Mbcb)dT2

dt(4)

haMadx2

dt,

where Gw is the mass flow rate of cooling water (kg/s), cpw is the isobaric specificheat of cooling water (J/kg K), Twin is the inlet temperature of cooling water (K),

Twout is the outlet temperature of cooling water (K), Tam is the ambient temperature

(K), a is the heat-transfer coefficient between lower bed and ambient (W/m2 K), x2is the adsorption capacity of adsorbent in lower bed (kg/kg), and hais the adsorption

heat (J/kg).

x2, ha can be calculated by the two following equations:

x2 x0expk2T2Tev1n2, (5)

which is called D-A equation, where k2, n2 are the adsorption parameters related

with adsorbing of the working pair, x0 is also the maximum of adsorption capacity

(kg/kg), and Tev is the evaporating temperature (K); and

ha LT2

Tev. (6)


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For the water tank, the unsteady energy equation is

Gwcpw(TwoutTwin) MwtcpwdTwt

dt. (7)

where Mwt is the water mass in water tank (kg), Twt is the mean water temperature

in water tank (K).

For the condenser, the unsteady energy equation is

Qco Ma[cprg(TcoT1)L]dx1

dt, (8)

where Qcois the heat loss of adsorbate in condenser (W), cprgis the isobaric specificheat of adsorbate vapor (J/kg K).

For the evaporator, the unsteady energy equation is

Qcooling Ma[Lcprl(T2Tev)]dx2

dt(9)

where Qcooling is the cooling capacity of adsorbate in evaporator (W).

3.2. Coefficient of performances of the hybrid system

The solar cooling coefficient of performance of the hybrid system is expressed as:

COPcooling

Qcoolingdt

I(t)Adt. (10)

The solar heating coefficient of performance of the hybrid system is expressed as:

COPhealing

MwtcpwdTwtdt

dt

I(t)Adt. (11)

4. Performance simulation of the hybrid system

4.1. Results of simulation

This paper makes a performance simulation and analysis on the system of a single

hybrid adsorption bed on one day in the summer in Shanghai when the solar radiant


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Fig. 4. Solar radiant intensity I variable with time in the daytime.

intensity I and the ambient temperature Tamare shown in Figs. 4 and 5, respectively.

The activated carbonmethanol working pair is adopted in the hybrid system inwhich the hybrid adsorption beds are overturned after every two hours during the

day. The parameters for calculation and design are shown in Table 1, and the results

of simulation are shown in Figs. 614.

Fig. 5. Ambient temperature Tam variable with time.


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Fig. 7. COPheating variable with water mass in water tank in the daytime.

Fig. 8. Final temperature of upper bed Tg2 variable with water mass in water tank in the daytime.


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Fig. 9. Final mean water temperature in water tank Twt variable with water mass in the daytime.

Fig. 10. Specific cooling power of adsorbent SCPavariable with water mass in water tank in the daytime.


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Fig. 11. Specific cooling power of heat-collecting area SCPc variable with water mass in water tank in

the daytime.

temperature for the family use at night, with rising of water temperature in the water

tank,the final temperature of adsorbent in lower bed Ta2 is equal to the mean watertemperature in water tank Twt after heat exchanging between the cooling water and

the lower bed. With the mean water temperature in water tankTwtrising,Ta2is raised

after every cycle. Fortunately in the next process of solar heating the final tempera-ture of adsorbent in upper bed Tg2is raised too, which will offset the drop of COPcoo-

ling

due to the rising offinal temperature of adsorbent in lower bed Ta2

. As are shown

in Figs. 6, 7, 1012, as the water mass in water tank rises, the COPcooling, COPheating,SCPa, SCPc and SHPc also rise during the day. As are shown in Figs. 8, 9, 13 and

14, as the water mass in water tank rises, the temperature Tg2andTwtin the daytime,

the cooling capacity of per kg adsorbent at night, and the cooling capacity of per

m2 heat-collecting area at night will reduce. Therefore an optimal value of water

mass in water tank exists, which will make the COPcooling, COPheating, SCPa, SCPcand SHPc in the daytime, the water temperature in water tank after 1 day and the

cooling capacity at night be appropriate values for an optimal design. Considering

the actual solar application, the optimal value of water mass in water tank of the

hybrid system in this paper is 30 kg from the simulation, and other performanceparameters are shown in Table 2.


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Fig. 12. Specific heating power of heat-collecting area SHPc variable with water mass in water tank inthe daytime.

Fig. 13. Cooling capacity of per kg adsorbent variable with water mass in water tank at night.


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Fig. 14. Cooling capacity of per m2 heat-collecting area variable with water mass in water tank at night.

Table 2

Simulation results when the water mass in water tank is 30 kg

Names of performance parameter Value

Final water temperature increment in water tank in the daytime (C) 22.8

Mean cooling COPcooling in the daytime 0.18

Mean heating COPheating in the daytime 0.34

Continuous mean specific cooling power of adsorbent in the daytime SCPa (W/kg) 17.6

Continuous mean specific cooling power of heat-collecting area in the daytime SCPc 87.8

(W/m2)

Continuous mean specific heating power of heat-collecting area in the daytime SHPc 165.9

(W/m2)

Cooling capacity of per kg adsorbent at night (MJ) 0.26

Cooling capacity of per m2 heat-collecting area at night (MJ) 1.3

5. Conclusion

This paper has proposed a design scheme of new solar continuous solid adsorption

refrigeration and heating hybrid system and has made a performance simulation and

analysis on the system. The relations between cooling/heating coefficient and solarradiation time and the optimal value of water mass in water tank of the hybrid system

are found. This paper shows that such hybrid system has a great-applied value andthe theoretical work on the system in this paper will make for the optimization of

performance parameters of the next experimental prototype.


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Acknowledgements

This work is supported by the state Key Fundamental Research Program under

the contract No. G2000026309 P.R.C., the Teaching and Research Award Programfor Outstanding Young Teacher in High Education Institutions of MOE P.R.C., the

Research Fund for Doctoral Program of Higher Education under the contract No.

2000024808 P.R.C., and the Subsidy Plan to Core Teachers in High Education under

the contract No. GGJT02003 P.R.C..

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methanol pairs. Acta Energiae Solaris Sinica 1995;16(2):15561.

[3] Critoph RE. Gasfired air conditioning using a carbonammonia convective thermal wave cycle. In:

Proceedings of International Ab-Sorption Conference, Montreal, 1996:353360.

[4] Meunier F. Adsorption heat pump technology: possibilities and limits. In: Proceedings of the Int.

Sorption Heat Pump Conference, Munich, 1999:2535.

[5] Wang RZ, Li M et al. An energy efficient hybrid system of solar powered water heater and adsorption

ice maker. Solar Energy 2000;68(2):18995.

[6] Critoph RE. Performance limitations of adsorption cycles for solar cooling. Solar Energy

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Design and performance simulation of a new.pdf