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
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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
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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.
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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.
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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.
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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.
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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
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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
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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.
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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.
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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.
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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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