Study of a solar powered solid adsorption–

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Renewable Energy 25 (2002) 417–430

www.elsevier.nl/locate/renene

Study of a solar powered solid adsorption–desiccant cooling system used for grain storage

Y.J. Dai *, R.Z. Wang, Y.X. Xu

Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai, 200030, People’s Republic of China

Received 14 August 2000; accepted 14 February 2001

Abstract

A hybrid solar cooling system, which combines the technologies of rotary desiccant dehu-midification and solid adsorption refrigeration, has been proposed for cooling grain. The keycomponents of the system are a rotary desiccant wheel and a solar adsorption collector. The

former is used for dehumidification and the later acts as both an adsorption unit and a solarcollector. The heating load from sunshine can thus be reduced to a greater extent since thesolar adsorption collector is placed on the roof of the grain depot. Compared with the solidadsorption refrigeration system alone, the new hybrid system performs better. Under typicalconditions, the coefficient of performance of the system is 0.4 and the outlet temperature is20°C. It is believed that the system can be used widely in the regions with abundant solarresources due to such advantages as environmental protection, energy saving and low operationcosts. Additionally, some parameters, for example, ambient conditions, the effectiveness of the heat exchanger and evaporative cooler, mass air-flow rate, etc., which affect system per-formance, are also analyzed. 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Adsorption refrigeration; Rotary desiccant dehumidification; Grain storage; Coefficient of per-

formance

1. Introduction

The control of insect and mould population growth is an essential element in grainstorage. The rate of population growth of both insects and moulds generally decreaseswith decreasing grain temperature and intergranular relative humidity (RH). The coo-

* Corresponding author. Tel.: 8621-62933250; fax: 8621-62933250.

E-mail address: [email protected] (Y.J. Dai).

0960-1481/02/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 0 - 1 4 8 1 ( 0 1 ) 0 0 0 7 6 - 3



418 Y.J. Dai et al. / Renewable Energy 25 (2002) 417 –430

Nomenclature

Ac Area of the solar collection surface (m2)

C p Specific heat of adsorbent (kJ/kg·K)

C pr Specific heat of refrigerant (kJ/kg·K)

C pm Specific heat of metal structure (kJ/kg·K)

COP Coef ficient of performance

F Factor of ef ficiency

H Enthalpy of moist air (kJ/kg)

I Density of solar radiation (W/m2)

k , n Constants in the D–A equation

m Mass of adsorbent (kg)

ma Mmass air-flow rate (kg/s)

mr Mass of refrigerant (kg)

mm Mass of metal structure (kg)

Pe Pressure of the evaporator (Pa)

Pc Pressure of the condenser (Pa)

qst Specific desorption heat (kJ/kg)

Qc Cooling capacity (kW)

Qre Regeneration heat of the rotary desiccant wheel (kW)

Qdes Desorption heat (kW)

RH Relative humidityS Effective irradiation density (w/m2)

T Temperature (K)

T a Ambient temperature (K)

T a1 Temperature at start of adsorption (K)

T a2 Temperature at end of adsorption (K)

T g1 Temperature at start of desorption (K)

T g2 Temperature at end of desorption (K)

T s Saturation temperature of the methanol (K)

ul Coef ficient of heat loss (w/m2·K)

x Adsorption rate (kg/kg) xmax Maximum adsorption rate (kg/kg)

Greek symbols

ev Effectiveness of evaporative cooler

h Effectiveness of heat exchanger

Subscripts

In Inlet conditionsOut Outlet conditions



419Y.J. Dai et al. / Renewable Energy 25 (2002) 417 –430

ling of stored grains suppresses the growth of insect pest populations, inhibits spoil-

age by fungi and helps to preserve grain quality. Now, grain chillers are essential

for use in Chinese grain depots. The most widely used method of cooling grain is

to blow the cold dry air produced by the grain chillers into the grain packs when

the temperature rises. This kind of device is characterized by a strong ability to cool,

and the temperature rapidly decreases, but has the drawbacks of high initial costs

and large electric power consumption, which leads to an increase in the cost of grain

storage. To realize the economic operation of the grain depot, the primary problem

is to reduce the cost of cooling grain. It is undoubtedly reasonable to cut the cost

by using solar energy for cold grain storage, since solar energy is the most abundant

and the most widely distributed energy resource in the world.

Research on solar powered solid adsorption refrigeration has been carried out

recently [1–5]. Critoph [6] evaluated some refrigeration–adsorption pairs for refriger-ation cycles. In view of practical applications, the solid refrigeration system using

activated carbon and methanol as a working pair has the advantages of low desorp-

tion temperature, operation reliability, few moving parts, operating duration, less

dependence on electric power, being environmental friendly, etc., and has the unique

potential of utilizing low grade heat resources such as solar energy and waste heat.

Shanghai Jiao Tong University (SJTU) [5] set up a solar ice-maker using methanol

and activated carbon based on a flat plate adsorption bed, which can produce 5–7

kg of ice with 20 MJ heat input per day per square meter to receive solar irradiation,

and has a coef ficient of performance (COP) up to 0.14. Of the techniques of solid

adsorption refrigeration, open cycle desiccant cooling is the maturest technique andhas been put into commercial application for rather a long time [7]. Ismail et al. [8]

introduced a solar desiccant bed grain cooling system in 1991. Li Chen and Thorpe

[9] successfully applied the technique of desiccant cooling into grain storage and

obtained a better result.

In this paper, a hybrid solar adsorption cooling system, which combines desiccant

cooling and closed cycle solid refrigeration, is presented for utilization in cold grain

storage. Performance of the system and the effects of some parameters are also

discussed. The development of adsorption refrigeration system suitable for depots is

of importance in reducing the cost of grain storage. On the one hand, the arrangement

of a solar collection adsorber is beneficial for reducing the heating load of the depots

because the solar collector is placed on the roof of the depot, on the other hand,

utilization of desiccant dehumidification to remove moisture in the depots lowers the

cooling load of grain chiller to a greater extent. Thus it is possible for this kind of

system to substitute a conventional grain refrigerator partially or totally. The novel

hybrid solar cooling system is expected to be applied in the regions with abundant

solar resources and is of great significance in decreasing the cost of grain storage

using renewable energy for China.

SJTU and the Nanjing Institute of Grain Science and Technology have carried out

research on hybrid solar cooling systems. The objective of this paper is to introducethe hybrid solar cooling system developed in SJTU, and to analyze system perform-

ance using a mathematical model.




2. System configuration and working principle

The novel hybrid solar cooling system is configured primarily by three subsystems,

namely: solid intermittent adsorption refrigeration; desiccant dehumidification; andcold storage, as shown in Fig. 1. Solid adsorption refrigeration includes a solar

adsorption bed, condenser, receiver and evaporator, etc.. Desiccant dehumidification

consists of a rotary desiccant wheel, a regenerative heater, and a thermal storage

heat exchanger. In cold storage, the grain itself is used as the cold storage material —a fan-coil unit is applied to supply cold air into the grain packs. Cooling waterflowing in the condenser comes from the water recycling in the evaporative cooler.

Here, activated carbon and methanol is selected as a working pair. The desorption

temperatures for both desiccant materials and activated carbon-methanol are all

within the range of 80–100°C.

The subsystem of solid adsorption, in which the adsorption bed is laid on the roof

of the grain depot, works intermittently. During the day the adsorption bed receives

solar irradiation and turns it into thermal energy, which leads to a rise in temperature.

The desorption process of methanol begins when the pressure of the bed is higher

than that of the condenser, which is determined by the temperature of cooling water.

The desorbed methanol turns into liquid state from vapor, and is collected in the

receiver. At night, natural convection and irradiation cool the adsorption bed and

causes the pressure to drop in the bed. The adsorbent begins to adsorb methanol

Fig. 1. Schematic of the hybrid solar cooling system.




again when the bed pressure is lower than that of the evaporator. A cooling effect

is thus produced when the refrigerant is evaporated within the evaporator. Parti-

cularly, the grain itself is the cold storage material, which stores the cold in the night

and becomes somewhat hot due to heat transfer from its surroundings in the day,this heat gained in the day is removed again at night by means of solid adsorption

refrigeration. Moisture produced by the grain is removed by desiccant dehumidifi-

cation, in which silica gel or a molecular sieve is often used as desiccant materials.

The regeneration process, which ensures that the subsystem works continually, is

driven by hot air heated by a solar heater or burning gas/coal. The air comes mainlyfrom the recycled air from the depot and its surroundings. The air is cooled in the

heat exchanger by other air, which is preliminarily cooled by an evaporative cooler.

There are two parts to the process: at night the air is induced into the evaporator

where the air takes the cold production away; during the day the air is sent to the

grain depot directly or passes through the evaporative cooler depending on the

humidity of air coming out of the desiccant wheel. The temperature of the regener-

ation air coming out of the rotary desiccant wheel is still somewhat high, and so

can assist in heating the adsorption bed with the help of auxiliary heat resources,

such as burning coal or oil. The rotary desiccant wheel, together with the evaporative

cooling, helps maintain the humidity of the grain depot at a constant level that is

suitable for grain storage. Fig. 2 shows the schematic diagrams of a rotary desicc-

ant wheel.

The unique features of the system are as follows:

Ability to deal with the problems of humidity in the grain depot due to the use

of desiccant dehumidification and evaporative cooling.

Realization of low operation cost

Overcoming the dif ficulties that solid adsorption refrigeration is inef ficient for

Fig. 2. Schematic diagram of the rotary desiccant wheel.




removing moisture from air and desiccant cooling cannot reduce the temperature

of air enough without changing the humidity.

3. Mathematical model

In order to predict the system performance, we established a mathematical modeldescribing this novel system based on earlier works. Here, a well-developed math-

ematical model reported in Refs. [10,11] was adopted to calculate the process of

rotary dehumidification. The concepts of evaporative cooler effectiveness and heat

exchanger effectiveness were used to describe the evaporative cooler and the heat

exchanger respectively, just as the method reported by Charoensupaya and Worek [12]. The authors would put emphasis on developing an appropriate model suitable

for the rapid evaluation of the solar adsorption collector. Particularly, the fundamen-

tals of solid adsorption refrigeration are illustrated in detail in Refs. [13,14]. Fig. 3

shows a basic cycle of adsorption refrigeration, which is classified into four sections,

namely: isosteric heating (ab); isobar heating (desorption, bc); isosteric cooling (cd);

and adsorption (da).

A one-dimensional model with respect to time is thus established to predict the

temperature variation of the solar adsorption collector. It is assumed that the adsorp-

tion equilibrium exists at any time since the cycling time period for solar adsorption

refrigeration is so long. Hence it is reasonable that the D–A equation is used todescribe the adsorption process. It is known from the fundamentals of solid adsorp-

tion refrigeration that the heating process of the refrigeration cycle can be divided

into isosteric heating and isobar heating.

The energy balance for the isosteric process is:

Fig. 3. Schematic of a basic cycle of adsorption refrigeration.




AcF [Sul(T T a)](mC p)e∂T

∂t (1)

Here F is ef ficiency factor of the solar adsorption bed, S is the effective solarirradiation, equal to incident radiation×optical ef ficiency. ul denotes the coef ficient

of the heat loss.

The energy balance for the isobaric process is:

AcF [Sul(T T a)]mqst

∂ x

∂t (mC p)e

∂T

∂t (2)

Here the first item on the right-hand side is adsorption heat, the second item is

sensible heat produced by temperature rising, x is the adsorption rate and is given

by the D–A equation:

x xmaxexpk T

T s1n (3)

where k and n are constants. For the desorption process, T s=T c. Rearranging Eq.

(2) gives:

AcF (SulT a) AcF ulT mqstkn

T c xmaxT

T c1n−1expk T

T c1n (4)

(mC p)e∂T

∂t

here

(mC p)emC p xmC prmmC pm

The method for obtaining the coef ficient of heat loss is similar to that of the flat

plate solar collector [15]. The thickness of the solar adsorption collector is 4.5 cm.

In order to enhance the process of heat transfer a fin type structure is employed with

a fin ef ficiency is 0.9. Under these conditions, using the method mentioned above,

one could obtain the temperature variations of the solar adsorption collector within

the daytime as shown in Fig. 4. The maximum temperature of the adsorption bed is

about 100°C. There is a turnover point corresponding to T g1, prior to this point and

an isosteric heating process exists, followed by an isobar heating process. We do

not want to consider the cooling process of the adsorption bed. For the adsorption

process, we use the concept of mean adsorption temperature, as used by Teng et

al. [14].

4. Performance analysis

The performance of the system was estimated using the above model. The con-

ditions are as follows: area of solar collection=20 m2; length of the rotary desiccant




Fig. 4. Variation in the adsorption bed temperature over 1 day.

wheel=20 cm; outer diameter=50 cm; and inner diameter=10 cm; ratio of the area

of air flowing to cross section area=0.85; and rotation speed of the wheel=5 rph.

Regular density silica gel was selected as the desiccant material and the heat

exchanger effectiveness and evaporative cooler effectiveness were set at 0.85 and

0.7, respectively. The cooling production is:

Qcma( H 1 H 2) (5)

where H 1 stands for the enthalpy of the inlet air and H 2 stands for the enthalpy of

outlet air. The COP is:

COPQc

Qre+Qdes

(6)

where Qc is the cooling production of the hybrid system, Qre is the regeneration heat

of the rotary desiccant wheel, and Qdes is the desorption heat for solid adsorption

refrigeration. The temperature of recycled air is 5°

C lower than that of the inlet state,the condensation temperature is taken as the ambient temperature, the evaporation

temperature is 8°C and the mean adsorption temperature is 40°C. Table 1 gives two

sets of results for typical cases.

It is clear that the system can process air at the temperature and humidity needed

Table 1

Performance of the system under two typical cases

T in (°C) RH in (%) ma (kg/h) T out (°C) RH out (%) Qc (kW) COP

35 40 650 16.0 60 6.58 0.42

25 70 650 13.0 61 5.99 0.34




for grain storage according to the criterion introduced in Ref. [8]. The COP of the

system reaches 0.42 and 0.34, respectively, for the two cases, which are both higher

than that of the solid adsorption refrigeration system alone.

5. Discussions on the effects of some parameters

It should be noted that the calculation conditions below are taken to be the same

as those mentioned earlier unless specifically stated.

5.1. Effect of ambient temperature and humidity

Fig. 5 shows the effects of the ambient temperature and humidity on the COP.

‘System’ in the figure represents the analysis results of system as a whole, and

‘adsorption’ means the results of adsorption refrigeration subsystem. Under the given

conditions, the COP of the solid adsorption refrigeration subsystem is weakly depen-

dent on the RH but increases with the ambient temperature. The ambient temperature

and humidity affect the system COP greatly —the higher the temperature and

humidity are — the higher the system COP will be. The reason for this lies in the

fact that the performance of the rotary desiccant wheel is intimately related to with

T in and RH in. The outlet temperature of the air varies significantly with changes in

ambient temperature. Fig. 6 shows that T out increases with the ambient temperature,

but increases only a little as RH increases for a fixed T in. Under the given conditions,the peak value of the system COP is about 0.34; about twice the minimum value,

0.155. This means that the system can perform better at higher temperatures. More-

over, it is found that the outlet temperature increases by no more than 2°C for four

cases in Fig. 4 when the RH changes from 20 to 80%. The outlet temperature is in

the range of 16–18°C, and 11–13°C for T in=25°C and 30°C, respectively.

Fig. 5. Impact of ambient temperature and humidity on COP.




Fig. 8. Impact of in on COP.

for decreasing the enthalpy of the air. Fig. 8 indicates that greater effectiveness of

the heat exchanger will improve the COP and cooling capacity. Variations in h

changes COP significantly. When h is 0.2, COP is about 0 and no cooling effect

is produced. The influence of the inlet temperature on the system performance is

small corresponding to a small h, but becomes strong with the increase of h.

5.3. Evaporation temperature

Fig. 9 shows the impact of the evaporation temperature (T e) on the COP; the COP

increases linearly with T e. When T e varies from 0 to 10°C, the COP increases by

Fig. 9. Impact of the evaporation temperature on COP.




Fig. 10. Impact of ma on COP.

0.05. Other research has also found that the higher the T e is, the better the system

performance will be [14]. In this system, variation in temperature changes the heat

rejected by condenser, the temperature of cooling water, and causes a change in

evaporative cooling, so it is thought that T e is still somewhat influential on desicc-

ant dehumidification.

5.4. Mass air- fl ow rate

Figs. 10 and 11 shows the effect of the mass air-flow rate on system performance.

The inlet RH for every considered case is always 70%. It is found that the greater

the mass air-flow rate is, the higher the system COP and the higher the outlet tem-

Fig. 11. Impact of ma on T out.




perature of the air will be. In the case of ma=1500 kg/h, the COP is about 0.53 and

the outlet temperature is 26.0°C for T in=35°C. The COP is 0.48 and T out=21°C for

T in=30°C. COP increases with an increase in air inlet temperature, but behaves

weakly under a small mass flow rate. A similar phenomenon could not be foundwith regard to the outlet temperature, which increases linearly as the mass air-flow

rate increases.

6. Conclusions

In this paper a hybrid solar powered solid adsorption–desiccant cooling system

with a 20 m2 solar collection area was investigated using numerical analysis. It wasindicated that this kind of hybrid solar cooling system using for grain storage is

acceptable from both technology and economic operation viewpoints. Numerical

simulation was conducted based on a mathematical model of solar adsorption

refrigeration, together with other well-developed models concerning the rotary

desiccant wheel, etc. The peak temperature value of the adsorption bed was about

100°C. The system removes the sensible heat and latent heat by means of solid

adsorption refrigeration and dessicant dehumidification, respectively. Under given

conditions, the COP of the system reaches 0.4, while the outlet temperature is

20°C. Hence any drawbacks of the poor ef ficiency of solar adsorption refrigeration

and higher outlet temperature for desiccant dehumidification will be overcome. It

was also found that the COP of the system increases with an increase in ambienttemperature and humidity, has a linear relationship with the effectiveness of evapor-

ative cooling, that is, the greater the effectiveness of the evaporative cooling, the

better the system performance will be. There also exists a parabolic relationshipbetween the COP and the effectiveness of heat exchanger; the COP increases with

the effectiveness of heat exchanger. Moreover, a higher mass air-flow rate is ben-

eficial for improving the COP, but is not helpful in decreasing the outlet temperature

of the air. The mass air-flow rate should be balanced between achieving a better

COP and in lowering the temperature of the cooling system.

Acknowledgements

This research is supported by the fund of the state Post Doctor Science and the

state Key Fundamental Research program under the contract No. G2000026309. Theauthors gratefully acknowledge the staff and students of SJTU who contributed to

this work.

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