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Simulation of an adsorption solar cooling system
Energy 36 (2011) 530e537
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Simulation of an adsorption solar cooling system H.Z. Hassan a, A.A. Mohamad a, *, R. Bennacer b a b
Mechanical and Manufacturing Engineering, Schulich School of Engineering, University of Calgary, 2500 univ. Drive, NW, Calgary, Alberta, Canada T2N 1N4 University de Cergy-Pontoise, L2MGC F-95000 Cergy-Pontoise, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 February 2010 Received in revised form 3 August 2010 Accepted 6 October 2010 Available online 3 November 2010
A more realistic theoretical simulation model for a tubular solar adsorption refrigerating system using activated carbonemethanol (AC/M) pair has been introduced. The mathematical model represents the heat and mass transfer inside the adsorption bed, the condenser, and the evaporator. The simulation technique takes into account the variations of ambient temperature and solar radiation along the day. Furthermore, the local pressure, and local thermal conductivity variations in space and time inside the tubular reactor are investigated as well. A Cþþ computer program is written to solve the proposed numerical model using the finite difference method. The developed program covers the operations of all the system components along the cycle time. The performance of the tubular reactor, the condenser, and the evaporator has been discussed. Time allocation chart and switching operations for the solar refrigeration system processes are illustrated as well. The case studied has a 1 m2 surface area solar flat plate collector integrated with a 20 stainless steel tubes containing the AC/M pair and each tube has a 5 cm outer diameter. In addition, the condenser pressure is set to 54.2 kpa. It has been found that, the solar coefficient of performance and the specific cooling power of the system are 0.211 and 2.326 respectively. In addition, the pressure distribution inside the adsorption bed has been found nearly uniform and varying only with time. Furthermore, the AC/M thermal conductivity is shown to be constant in both space and time. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Solar energy Adsorption Refrigeration Modelling Simulation Activated carbonemethanol
1. Introduction Solar radiation is by far the largest and the most world’s abundant, clean and permanent energy source. The amount of solar radiation intercepted by the Earth’s surface is much higher than the annual global energy use. The energy available from the sun ð821015 WÞ is greater than about 5200 times the global world’s need in 2006. In recent years, many promising technologies have been developed to harness the Sun’s energy. These technologies help in environmental protection, economizing energy, and sustainable developments which are the major issues of the world in the 21st century. One of these important technologies is the solar cooling system that makes use of either absorption or adsorption technologies. Due to the environmental problems of chlorofluorocarbons (CFCs) emissions, adsorption cooling technologies are more attractive field of research and development than the conventional vapour compression refrigeration systems. The international and local policies nowadays are directed towards replacing the traditional
* Corresponding author. Tel.: þ1 403 220 2781; fax: þ1 403 282 8406. E-mail address: [email protected] (A.A. Mohamad). 0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.10.011
refrigeration systems with environmentally friendly ones that can be operated by new and renewable energy sources [1]. The solar adsorption cooling systems are good alternative since they operate with environmentally benign refrigerants that are natural, free from CFCs and therefore they have a zero ozone depleting potential (ODP). Furthermore, these refrigerants satisfy the Kyoto protocol on global warming, the Vienna Convention for Protection of the Ozone Layer (1985), and Montreal Protocol on Substances Depleting the Ozone Layer (1987). Adsorption cooling systems are characterized by their simple control and the absence of vibration and corrosion problems. The wide range of heat source temperatures that can be used 50 C 600 C, and the low operation and maintenances costs make these systems more attractive [2]. Although the adsorption chiller systems have these advantages, their drawbacks are the intermittent operation, the requirements of special designs to maintain high vacuum, the large volume and weight relative to traditional refrigeration systems, the low specific cooling power (SCP) and the low coefficient of performance (COP) [3]. Besides the poor heat and mass transfer within the adsorbent, the adsorption deterioration of the adsorbent is also vital to the development and applications of the adsorption refrigeration technology [4]. However, enhancement of heat and mass transfer properties in the adsorbent bed,
H.Z. Hassan et al. / Energy 36 (2011) 530e537
Nomenclature A C hceamb Imax L Wo x Cp Cv D hgoeamb k Lv M m mcl n P Qe R T t tsr tss U Ugoepo
Upoeamb
Area [m2] Specific heat [J kg1 K1] Convection heat transfer coefficient between the condenser and the ambient [Wm2 K1] The maximum solar radiation intensity [W m2] Length [m] Constant in the DubinineAstakhov (DeA) equation (Eq. (6)) [m3 kg1] Adsorbate concentration ratio [kg/kg] Specific heat at constant pressure [J kg1 K1] Specific heat at constant volume [J kg1 K1] Constant in the DubinineAstakhov (DeA) equation (Eq. (6)), or diameter [m]. Convection heat transfer coefficient between the outer glass cover and the ambient [W m2 K1] Coefficient of thermal conductivity [W m1 K1] Latent heat of vaporization [J kg1] Total mass [kg] Mass [kg] Mass of liquid methanol that leaves the condenser towards the evaporator [kg] Constant in the DubinineAstakhov (DeA) equation (Eq. (6)) Pressure [pa] Refrigeration effect [J] Radius [m] Temperature [K] Time [sec] The time of the day when sun rises [sec] The time of the day when sun sets [sec] Overall heat transfer coefficient [W m2 K1] Overall heat transfer coefficient between the outer surface of the glass cover and the outer pipe surface based on the glass cover surface area [W m2 K1] Overall heat transfer coefficient between the outer surface of the pipe and the ambient based on the glass cover surface area [W m2 K1]
increasing the adsorption properties of the working pairs and a better heat management during the adsorption cycle lead to a more efficient system [2]. The performance of the adsorption cooling system depends mainly on the working pairs used. A good designed system should have the characteristics of large adsorption capacity, large change of adsorption capacity with temperature variation, and a more flat desorption isotherm. Moreover, the refrigerant should have a large latent heat per unit volume, no toxicity, no flammability, no corruption, and good chemical and thermal stability [4]. The most widely used working pairs are activated carbonemethanol, activated carbon fiberemethanol, activated carboneammonia, zeoliteewater, silica gelewater, calcium chlorideeammonia and composite adsorbenteammonia pairs. Anyanwu [3] presented a review of the practically realized solid adsorption solar refrigeration cycles. He classified the cycles according to the adsorbate utilized as: cycles with water as refrigerant, cycles using fluorocarbon as refrigerant, cycles using ammonia as refrigerant and cycles with alcohols as refrigerant. Activated-carbon is the most widely used adsorbent reported in literature due to its extremely high surface area and micro pore volume. Moreover, Critoph [5] and Critoph and Vogel [6] found charcoal a preferable and
Upoepi
V
531
Overall heat transfer coefficient between the outer and inner surfaces of the pipe based on the pipe inner surface area [W m2 K1] Volume [m3]
Greek letters d Thickness [m] DH Isosteric heat of adsorption [Jkg1]. 3 Porosity of the medium [-] r Density, kgm3 s Stefan Boltzmann constant [W m2 K4] s Transmissivity [-] 3 Emissivity [-] a Absorptivity q Volume fraction of the adsorbed phase [-] Subscripts a Adsorbed phase. amb Ambient cond Condensation d Desorbed eff Effective evap Evaporation or evaporator g Glass cover or the gas phase l Liquid o Out p Pipe pi Inner surface of the pipe s Solid phase or saturation po Outer surface of the pipe Abbreviations AC/M Activated carbonemethanol CFCs Chlorofluorocarbons COP Coefficient of performance ODP Ozone depleting potential SCOP Solar coefficient of performance SCP Specific cooling power [Wkg1]
recommended adsorbent for solar cooling. Activated carbonemethanol pair has been studied by many researchers [7e9]. This pair is characterized by high latent heat of vaporization of methanol and low desorption temperature. However, the operating temperature is one of the limitation that constrain the operation of the activated carbonemethanol pair cooling systems. At temperatures more than 150 C, the methanol decomposes into dimethyl. However, a very low decomposition reaction rate is found to happen at temperatures near 120 C, Yuan [10]. The simulation done by Eric [11] showed that the thermal decomposition of methanol did occur in the normal solar powered adsorption refrigeration system. The rate of the decomposition depends on the presence of various alloys such as aluminium and copper alloys. Moreover, the decomposition products may be one of the main reasons for the diminishing performance of the solar ice maker. The solar adsorption refrigeration system usually operates with the basic simple cycle. The basic cycle is a single-bed, intermittent, and has a low efficiency and low cooling effect. Many attempts have been reported in the literature to develop and design a semicontinuous operation and to increase the performance and efficiency of the cycle. These developments include heat recovery or the regeneration cycle [12e15], mass recovery cycle [16e18], heat and
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mass recovery cycle [13,19], thermal wave cycle [20], forced convective thermal wave cycle [21e23], cascade cycle [24e29], multi-bed cycle [30e32], multi-stage cycle [33e37], dual-mode cycles [38]. An explained review for these cycles is presented in [3,39]. 2. Physical model The adsorption cooling system likes the basic vapour compression refrigeration machine except that the power compressor is replaced with a thermal compressor or a reactor. This reactor is composed of a type of porous medium that has the ability to adsorb the refrigerant. The working principle of the basic reactor cycle is represented in the Clapeyron diagram, Fig. 1. The adsorbenteadsorbate cycle consists mainly of four phases; pressurization process at a constant volume (isosteric heating phase), desorption at constant pressure (isobaric heating phase), depressurization at constant volume (isosteric cooling phase), and adsorption at constant pressure (isobaric cooling phase). At the beginning of the day, state A, the reactor is isolated from both the condenser and the evaporator by valves c and e and is completely charged with the refrigerant. The pressure inside the reactor initially equals the evaporator pressure Pevap. and its temperature is uniform and equals the ambient temperature Tamb. When the reactor starts to heat up by the incident solar radiation, both pressure and temperature inside the adsorbent are elevated. This constant concentration heating phase continues till point B where the pressure reaches a value that equals the condenser pressure Pcond. This period is equivalent to the compression in the classical vapour compression refrigeration cycle. At state B, valve c is opened and the adsorbate starts to desorb and flow towards the condenser. During this isobaric heating phase, the temperature continues to increase, and the adsorbate concentration continues to decrease as more adsorbate is being freed from the reactor. When the adsorbate temperature reaches the maximum value Tmax at state C, valve c is closed and the reactor starts the third phase. When the solar flux decreases, the reactor is cooled down at constant volume and constant isoster till the pressure inside the reactor decreases to the evaporator pressure Pevap. , point D. The last phase of the reactor cycle starts at the night, point D, when valve e is opened and the refrigerant flows towards the reactor. The adsorption process continues while the reactor is cooled at the constant evaporator pressure till the higher cycle isoster at point A.
Fig. 2. Configuration diagram of the solar adsorption cooling reactor.
The solar refrigeration system presented in this study has three main components: the solar collector/reactor, Fig. 2, the condenser, and the evaporator. The solar collector/reactor is constituted of a clear plane glass sheet cover, and the activated carbonemethanol pair which is contained in the annular space between a two coaxial collector pipes. The inner pipe is perforated in order to ease the flow of methanol into and out from the activated carbon granules and to avoid pressure drops and temperature differences along the collector tubes as well. These pipes are integrated in the plane solar collector and are coated with a selective painting to allow a good absorption of solar radiation and low emission. The lateral and rear sides of the collector are insulated. The inner pipes are connected to a common methanol inlet and outlet headers. The adsorbent bed is cooled during the adsorption process when the collector glass cover plate is opened by natural convection and by radiation from the collector plate and tubes. The condenser pipe is steel and is cooled by natural convection and by radiation as well. 3. Mathematical model The presented mathematical model simulates the real working of the solar adsorption cooling system taking in account the variation of solar radiation and ambient temperature along the day. Furthermore, the energy and mass balances for every element of the system are taken into account as well. 3.1. Solar radiation and ambient temperature modelling Both desorption and isosteric heating phases depend strongly on the quantity of incident solar radiation. In addition, the adsorption
Fig. 1. Clapeyron diagram for a conventional adsorption cycle.
H.Z. Hassan et al. / Energy 36 (2011) 530e537
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and the isosteric cooling phases besides the condenser operation are dependent on the ambient temperature. Therefore, in order to simulate the system in a more realistic manner, the solar radiation and ambient temperature are modelled in this study as variant along the day. These climatic conditions are taken for Calgary town, Canada (latitude ¼ 51.12 and longitude ¼ 114.01) where a summer day in July 30, 2009 is chosen. The apparent sun rise time, apparent sun set time, and the solar noon during this day are at 5:59:00, 21:25:00, and 13:42:30 respectively [43]. The intensity of solar radiation as a function of the time is given by:
lower temperature layers. This mass transfer results in the heat transfer process. That is because the gas that will be adsorbed on cold layers is hot. Therefore, the adsorption and desorption mass transfer through the porous medium is accompanied with a heat transfer and vice versa. The conservation of mass equation and the first law of thermodynamics for an open system are applied to a layer of radial coordinates r and thickness dr in the adsorbent bed. Both the mass and heat balance equations are merged together to give the following combined heat and mass transfer equation within the adsorbent bed:
pðt tsr Þ Is ðtÞ ¼ Imax sin tss tsr
P vT ð1 3Þrs Cs þ qra Ca ð3 qÞ rg Cvg þ T vt # " 2 v T 1 vT vP P vra vq 3 q q D þ ð ¼ Keff þ Þ þ H þ ra DH þ ra vt vt vt vr 2 r vr
(1)
where, tsr < t< tss Imax is taken to be equal 900 W/m2. Furthermore, the real hourly temperature variation on the same day is taken from [44]. The temperature data is fitted in a form of a polynomial from the 8th degree and is inserted to the simulation program. 3.2. Energy balance for the solar collector The solar radiation is absorbed at the external surface of every tube. This heat flux is transmitted by conduction, through the pipe wall in the inward radial direction to the internal walleadsorbent interface, then towards the adsorbent-refrigerant reactive medium. A one-dimensional transient model for heat balance is suggested for the glass cover and the pipe. The heat balance for the glass cover is given by the following equation:
rg Cg dg Ag
vTgo ¼ ag Ag Is ðtÞ þ Ugopo Ag Tpo Tgo vt 4 4 s Tpo Tgo pRpi Lp hgoamb Ag Tgo Tamb þ 1 1 þ 1
4 T4 s Tgo amb
1
3g
þ
1
3amb
3p
3g
In this equation, the adsorbate density is modelled as varying with temperature. Moreover, the pressure is taken as variant throughout the adsorbent bed layers. It is assumed that the gaseous phase behaves as an ideal gas. The effective thermal conductivity of the adsorbent bed is also taken as variant and is evaluated by the following formula:
Keff ¼ Ks ð1 3Þ þ Ka q þ Kg ð3 qÞ
(5)
where ks,ka, and kg are the thermal conductivities of the solid, adsorbate, and gas phases respectively.
3.4. Adsorption equilibrium model The adsorption equilibrium equation of the activated carbon/ methanol as an adsorbent/adsorbate pair presented in this study, is described by the DubinineAstakhov (DeA) equation of state. The adsorbed mass of methanol per unit mass of activated carbon is given by:
n Ps ðTÞ X ¼ ra ðTÞWo Exp D Tln P
1 (2)
Neglecting the heat conduction between the adsorbent bed and the condenser and evaporator, the energy balance for the steel pipe is given by:
rp Cp Vg
(4)
vTpo ¼ sg ap Is ðtÞLp Dpo Ugopo Ag Tpo Tgo vt 4 4 s Tpo Tgo pRpi Lp Upopi pDpi Lp Tpo Tpi 1 1 þ 1 Ag
3p
4 Tpo
s Upoamb Ag Tpo Tamb
1
3g
3g
þ
Tg4
1
3p
pRpi Lp
1 Ag
(6)
The values of the constants Wo, D, and n are given in Table 1. The thermodynamic properties of methanol are taken from Methanex Corporation [40]. In order to complete the mathematical formulation of the model, the methanol properties are fitted as polynomials and merged with the mathematical model.
3.5. Condenser, evaporator and system performance The mass of methanol gas desorbed from the adsorption bed goes to the condenser where it is cooled and condenses to liquid. The condenser is cooled by the convection and radiation to the ambient air. The combined mass and energy conservation equation of the condenser is given by:
(3) Table 1 Constants of the DubinineAstakhov (DeA) equation, Jing and Exell[7].
3.3. Combined heat and mass transfer equation in the adsorbent bed It is clear that, higher temperature layers in the adsorbent bed desorb the methanol gas. This desorbed gas is adsorbed on the
Charcoal
Wo[Lit/kg]
D 105
n
207E4 Chinese LSZ30 LH Thai MD6070
0.36546 0.405 0.860 0.988
14.96202 31.972 25.74 88.98
1.34 1.26 1.321 1.12
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H.Z. Hassan et al. / Energy 36 (2011) 530e537
½mc Cc þ Md ðtÞCd
vTc ¼ md ðtÞ Lv ðPc Þ þ Cpg Tg Tcond vt Ac hcamb ðTcond Tamb Þ 4 s Tc4 Tamb 1 1 þ 1 Ag
3c
(7)
3amb
The cooling effect is calculated by the following equation:
Qe ¼ mcl Lv Tevap hl Tc;o þ hl Tevap
(8)
The parameters used in this present work to assess the adsorption refrigeration system performance are the solar coefficient of performance SCOP and the specific cooling power SCP. These parameters are defined by the following equations:
SCOP ¼ Z
Qe
(9)
Is ðtÞAg dt
The SCP is the ratio between the cooling effect and the cycle time per unit mass of adsorbent:
SCP ¼
Qe tcycle ms
simulate the behaviour of the solar adsorption cooling system. For each time step, the program updates the ambient temperature and the solar insolation based on the suggested climatic scheme. Based on the new conditions every time step, the temperature, pressure, concentration ratio, volume fraction, desorbed and adsorbed mass inside the reactor, and the condenser temperature are updated. At the end of the cycle, which is adjusted to be within 24 h period, the overall system performance parameters are determined. These parameters include the solar coefficient of performance, the specific cooling power, and the evaporation effect. The main data and design parameters input to the developed Cþþ code and used in this simulation are depicted in Table 2. 5. Results and discussion Based on the proposed mathematical and numerical models and the input data presented in Table 2, the output results for only one cycle of operation are presented in this section. The obtained results and the overall system performance are estimated in this study by considering a number of 20 pipes backed with the activated carbonemethanol pair and placed in the tubular reactor. Each tube is 1 m in length and the surface area of the solar flat plate
(10)
where tcycle is the complete cycle time.
Table 2 The main parameters used in the case study. Symbol Parameter
3.6. Initial and boundary conditions The operation of the solar cooling system is strongly dependant on the variation of the climatic conditions. This means that, the performance of this system changes day by day. Moreover, the initial conditions of the system for a given cycle are defined by the final conditions for the previous one. In this work we deal with the first operating cycle in which the system is considered in thermal equilibrium with the ambient. Therefore, the initial pressure and temperature are given by:
Pðt ¼ 0Þ ¼ Pevap
¼ Psat Tevap
Tðt ¼ 0Þ ¼ Tgo ðt ¼ 0Þ ¼ Tpo ðt ¼ 0Þ ¼ Tpi ðt ¼ 0Þ ¼ Tc ðt ¼ 0Þ ¼ Tamb The boundary conditions used to solve equation (4) are as follows :
vTpi ¼ Upopi Tpo Tpi vr
1: At r ¼ Rpi ;
Keff
2: AT r ¼ Ri ;
vT ¼ 0:0 vr
The second boundary condition assumes an adiabatic condition at the inner tubular reactor surfaceegas interface [41,42]. This can be explained by the poor thermal conductivity of methanol gas which is about 0.016 W/m K. 4. Numerical method of solution In the present work, the proposed mathematical model is discretized using finite difference approximation method. The resulting system of equations is solved explicitly to obtain the numerical solution for the variables at every time step. A computer code based on this numerical scheme is written in Cþþ in order to
Glass cover Cg Specific heat of the glass cover rg Density of the glass cover Glass cover thickness tg Coefficient of thermal conductivity kg sg Transmissivity ag Absorptivity 3g Emissivity Air gap tgab Air gap thickness Air thermal conductivity kair Back insulation Insulation thickness tins Insulating material thermal conductivity kair Stainless steel pipe Specific heat of stainless steel Cp rp Density of stainless steel Coefficient of thermal conductivity kp ap Absorptivity of the pipe coating material 3p Emissivity of the pipe coating material Outer diameter of the pipe Dpo Inner diameter of the pipe Dpi Pipe length Lp Number of pipes np Adsorption bed 3 Porosity of the AC Specific heat of carbon Cs Thermal conductivity of carbon ks rs Density of carbon Inner diameter of the inner coaxial pipe Dip Methanol properties Specific heat of liquid methanol Ca Cpgas Specific heat of methanol vapour atp ¼ cons: Specific heat of methanol vapour at v ¼ cons: Cvgas Thermal conductivity of liquid methanol ka Thermal conductivity of methanol vapour kgas Condenser Condenser Pressure Pcond Condenser tube length Lc Roct Condenser tube outer radius Condenser tube inner radius Rict Specific heat of the condenser metal Ccm rcm Density of the condenser metal Evaporator Evaporator Pressure Pevap
Value
Unit
750 2500 0.003 1.4 0.95 0.05 0.9
[J kg1 K1] [kg m3] [m] [W m1 K1] [] [] []
0.1 [m] 0.0263 [W m1 K1] 0.1 0.038
[m] [W m1 K1]
480 8055 15.1 0.98 0.1 0.05 0.048 1.0 20
[J kg1 K1] [k gm3] [W m1 K1] [] [] [m] [m] [m] []
0.7 711 1.6 2000 0.01
[] [J kg1 K1] [W m1 K1] [k gm3] [m]
2534 1820 1560 0.2022 0.0163
[J kg1 K1] [J kg1 K1] [J kg1 K1] [W m1 K1] [W m1 K1]
54,209 20 0.0025 0.002 480 8055
[Pa] [m] [m] [m] [J kg1 K1] [k gm3]
3967
[Pa]
H.Z. Hassan et al. / Energy 36 (2011) 530e537
Fig. 3. Variation of climatic data with the day time.
collector glass cover is 1 m2. Moreover, the condenser pressure is set to a value of 54.21 kpa which corresponds to a saturation temperature of 50 C. Generally, it is found that the solar coefficient of performance of this adsorption cooling system attains a value of 0.211. Furthermore, the specific cooling power and the evaporator cooling effect values are found to be 2.326, and 4175 kJ respectively. Fig. 3 shows the variation of the solar incident radiation and the ambient atmospheric temperature with the day hours. These data simulate a summer day in July 30, 2009, for Calgary, Canada as discussed in Section 3.1. The time allocation chart for the solar refrigeration system is illustrated in Fig. 4. This chart indicates the rank of each process, its period of operation, and its start and end time. The timing schedule is affected strongly by the climatic conditions, cycle initial conditions, and the system design parameters. Moreover, the behaviour of any phase of the reactor affects the operation of the following other phases and therefore the condenser, evaporator, and the overall cycle of the system. Therefore, the matching among the system elements is an important issue that insures a proper operation of the cooling system and satisfies a daily complete working cycle. Fig. 5 represents the evolution of the volume averaged temperature of the adsorbent bed along the cycle period. The period of the cycle is 24 h starting at 6:00 in the morning which is the apparent sun rise time. It is noticed that, during the isosteric heating phase, the reactor continues to absorb the continuously increasing solar radiation. This results in a rapid increase in the adsorption bed temperature from about 10.8 C to 60 C in a time
Fig. 4. The time allocation of the solar cooling system.
535
Fig. 5. Variation of average adsorption bed temperature with time.
period of about 3.55 h. The pressure inside the activated carbonemethanol pair approaches the condenser pressure of 54.209 kpa at the temperature of 60 C. At this moment, methanol starts to desorb from the adsorbent and flows towards the condenser. During this process, heat energy is extracted from the adsorption bed due to the endothermic character of the desorption process. Therefore, the rate of heating and the associated temperature increase rate during this isobaric desorption phase is lower than the first phase as seen from Fig. 5. The desorption process continues for a time period of 5.4 h and stops when methanol reaches 120 C, the temperature at which it starts to decompose. At this moment, the solar flat plate collector glass cover is opened and the reactor is kept away from the sun rays to allow beginning of the cooling phase of the adsorption bed. Heat transfers from the reactor by means of both natural convection and by radiation with a high driving potential from the high reactor temperature to the low temperature of the surroundings. This results in the sudden dramatic decreasing rate in the reactor temperature, as noticed in Fig. 5, from 120 C to 61 C in a short time period of 0.74 h. When the temperature reaches 61 C, the corresponding reactor pressure at this low isosteric process will be the same as the evaporator pressure and adsorption process has to begin. Since the adsorption process is exothermic and the driving potential for heat transfer is lower than the previous phase, a lower decreasing rate for the reactor temperature is remarked in Fig. 5. The cooling of the adsorption bed continues till it reaches a temperature of 16.3 C at the end of the cycle. It is clear that, this temperature is not the same as the corresponding value at the beginning of the cycle. This is because the reactor has not reached a complete thermal equilibrium with the ambient. Therefore, this will affect the next operating cycle of the system which has an initial temperature of 16.3 C. The variation of both the average pressure and the adsorbed mass inside the activated carbonemethanol pair with time is illustrated in Fig. 6. The average pressure increases from the initial evaporator pressure, 3.967 kpa, to the condenser pressure, 54.209 kpa during the first phase. It reaches a plateau while the desorption process takes place then sharply drops again to the evaporator pressure at the beginning of the adsorption process. Although the pressure computation in this study is modelled as variant with the radial direction inside the tubular reactor, its observed gradients are found to be very small and therefore can be neglected. Consequently, it is a good assumption to consider a uniform pressure distribution inside the adsorption bed. The amount of adsorbed methanol at the beginning of the operating cycle is 5.1 kg and it reaches an amount of 1:13 kg at the end of desorption process. That is, the total effective mass of methanol freed from the tubular reactor and circulates in the condenser and
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H.Z. Hassan et al. / Energy 36 (2011) 530e537
Fig. 6. Variation of the average pressure and the adsorbed mass inside the adsorption bed.
the evaporator is about 3.97 kg. Consequently, the effective methanol mass ratio, which is the fraction of the adsorbate total initial mass that circulates in the cycle, is about 0.778. During the adsorption process, the methanol mass inside the reactor continues to increase till it reaches a value of 4.5 kg at the end of this cycle and beginning of the next cycle, Fig. 6. The local values of the thermal conductivity inside the adsorbent bed layers are determined each time step. It is found that, the local thermal conductivity does not vary so much from a layer to a layer and therefore it can be considered uniform in the adsorption bed. The change of the adsorbent bed volume averaged effective thermal conductivity along the cycle time is seen in Fig. 7. Generally, the average conduction coefficient attains a low value that ranges from a maximum of about 0.528 W/mK to a minimum of about 0.5 W/mK. Since the range of variation is very small, it can be verified that the adsorbent bed thermal conductivity is both uniform and does not vary with time. Fig. 8 illustrates the condenser temperature changes during the day. It is clear that, before the adsorption process, the condenser temperature change follows that of the corresponding ambient temperature. As the vapour of methanol starts to flow towards the condenser, a remarkable sudden temperature increasing rate is seen, Fig. 8. This increase is due to the increasing desorbed methanol vapour mass that has a high temperature. The condenser reaches a maximum temperature of about 40 C after which its temperature begins to go down. The condenser temperature decreases in a rapid rate during the third phase. That is because the flow of methanol vapour is stopped from one side and the ambient temperature starts
Fig. 8. Condenser temperature variation with time.
to decrease providing a higher driving potential for the condenser cooling process from the other side. This cooling process continues by means of both free convection and radiation until the liquid methanol reaches about 33 C at the beginning of the adsorption process. The liquid methanol exits from the condenser to enter the evaporator after it is throttled in the expansion valve to the evaporator pressure. As the condenser begins to discharge its content of methanol, its temperature starts to follow the ambient temperature changes along the remaining period of the day. 6. Conclusion In this study we have introduced a more actual theoretical model to simulate the adsorption cooling system powered by the solar energy. The system uses the activated carbonemethanol as the working pair. The mathematical and numerical models for the adsorbent bed, condenser, and evaporator has been programmed be the Cþþ language. The real ambient temperature and solar radiation variations are taken into account. The variation of the adsorbent bed average temperature, pressure, adsorbed mass, and its thermal conductivity with time are discussed. Moreover, the condenser temperature variation, the time allocation and switching operations and the overall cooling system performance parameters are investigated as well. For the case studied, it is found that, the solar coefficient of performance reaches a value of 0.211. Moreover, the specific cooling power is estimated to about 2.326. It can also be seen from this work that, the pressure is nearly uniform inside the adsorption bed and the activated carbonemethanol thermal conductivity is found to be constant in space and time. The authors would like to acknowledge the support of Solar Buildings Research Network and NSERC. Acknowledgement The authors would like to acknowledge the support of Solar Buildings Research Network and NSERC. References
Fig. 7. Change of the effective thermal conductivity of the adsorbent bed along the cycle.
[1] Anyanwu EE. Environmental pollution: restructuring the refrigeration industry as a way out. Environment Protection Engineering 2000;26 (4):17e28. [2] Wang RZ, Oliveira RG. Adsorption refrigeration e an efficient way to make good use of waste heat and solar energy. Progress in Energy and Combustion Science 2006;32(4):424e58. [3] Anyanwu EE. Review of solid adsorption solar refrigerator I: an overview of the refrigeration cycle. Energy Conversion and Management 2003;44 (2):301e12.
H.Z. Hassan et al. / Energy 36 (2011) 530e537 [4] Wang DC, Li YH, Li D, Xi YZ, Zhang JP. A review on adsorption refrigeration technology and adsorption deterioration in physical adsorption systems. Renewable and Sustainable Energy Reviews 2010;14(1):344e53. [5] Critoph RE. Performance limitations of adsorption cycles for solar cooling. Solar Energy 1988;41(1):21e31. [6] Critoph RE, Vogel R. Possible adsorption pairs for use in solar cooling. International Journal of Ambient Energy 1986;7(4):183e90. [7] Jing H, Exell RHB. Simulation and sensitivity analysis of an intermittent solarpowered charcoal/methanol refrigerator. Renewable Energy 1994;4(1):133e49. [8] Ogueke NV, Anyanwu EE. Design improvements for a collector/generator/ adsorber of a solid adsorption solar refrigerator. Renewable Energy 2008;33:2428e40. [9] Leite APF, Daguenet MD. Performance of a new solid adsorption ice maker with solar energy regeneration. Energy Conversion and Management 2000;41 (15):1625e47. [10] Yuan ZH. Charcoal methanol adsorption refrigeration. Bangkok, Asian Institute of Technology 1988, M. Eng. Thesis ET-88e10. [11] Hu EJ. A study of thermal decomposition of methanol in solar powered adsorption refrigeration machines. Solar Energy 1998;62(5):325e9. [12] Vasiliev LL, Mishkinis DA, Antukh AA, Vasiliev Jr LL. Solar-gas solid sorption heat pump. Applied Thermal Engineering 2001;21(5):573e83. [13] Wang RZ. Performance improvement of adsorption cooling by heat and mass recovery operation. International Journal of Refrigeration 2001;24(7):602e11. [14] Critoph RE. Simulation of a continuous multiple-bed regenerative adsorption cycle. International Journal of Refrigeration 2001;24(5):428e37. [15] Chahbani MH, Labidi J, Paris J. Modeling of adsorption heat pumps with heat regeneration. Applied Thermal Engineering 2004;24(2e3):431e47. [16] Alam KCA, Akahira A, Hamamoto Y, Akisawa A, Kashiwagi T. A four-bed mass recovery adsorption refrigeration cycle driven by low temperature waste/ renewable heat source. Renewable Energy 2004;29(9):1461e75. [17] Akahira A, Alam KCA, Hamamoto Y, Akisawa A, Kashiwagi T. Mass recovery adsorption refrigeration cycle-improving cooling capacity. International Journal of Refrigeration 2004;27:225e34. [18] Luo HL, Dai YJ, Wang RZ, Wu JY, Xu YX, Shen JM. Experimental investigation of a solar adsorption chiller used for grain depot cooling. Applied Thermal Engineering 2006;26(11e12):1218e25. [19] Wang W, Qu TF, Wang RZ. Influence of degree of mass recovery and heat regeneration on adsorption refrigeration cycles. Energy Conversion and Management 2002;43(5):733e41. [20] Sward BK, LeVan MD, Francis M. Adsorption heat pump modeling: the thermal wave process with local equilibrium. Applied Thermal Engineering 2000;20(8):759e80. [21] Critoph RE. Forced convection enhancement of adsorption cycles. Heat Recovery Systems & CHP 1994;14(4):343e50. [22] Critoph RE. Forced convection adsorption cycle with packed bed heat regeneration. International Journal of Refrigeration 1999;22:38e46. [23] Hai-Ming Lai. An enhanced adsorption cycle operated by periodic reversal forced convection. Applied Thermal Engineering 2000;20:595e617. [24] Douss N, Meuiner FE, Sun LM. Predictive model and experimental results for a two absorber solid adsorption heat pump. Industrial & Engineering Chemistry Research 1988;27(2):310e6. [25] Douss N, Meunier F. Experimental study of cascading adsorption cycles. Chemical Engineering Science 1989;44(2):225e35.
537
[26] Douss N. Experimental study of adsorption heat pump cycles. International Chemical Engineering 1993;33(2):207e14. [27] Chua HT, Ng KC, Malek A, Kashiwagi T, Akisawa A, Saha BB. Modeling the performance of two-bed, sillica gel-water adsorption chillers. International Journal of Refrigeration 1999;22(3):194e204. [28] Khan MZI, Alam KCA, Saha BB, Hamamoto Y, Akisawa A, Kashiwagi T. Parametric study of a two-stage adsorption chiller using re-heat-The effect of overall thermal conductance and adsorbent mass on system performance. International Journal of Thermal Sciences 2006;45(5):511e9. [29] Liu Y, Leong KC. Numerical study of a novel cascading adsorption cycle. International Journal of Refrigeration 2006;29(2):250e9. [30] Chua HT, Ng KC, Malek A, Kashiwagi T, Akisawa A, Saha BB. Multi-bed regenerative adsorption chiller-improving the utilization of waste heat and reducing the chilled water outlet temperature fluctuation. International Journal of Refrigeration 2001;24(2):124e36. [31] Saha BB, Koyama S, Lee JB, Kumahara K, Alamc KCA, Hamamoto Y, et al. Performance evaluation of a low-temperature waste heat driven multi-bed adsorption chiller. International Journal of Multiphase Flow 2003;29 (8):1249e63. [32] Qenawy AM, Mohamad AA. Simulation of double-bed cooling and heating hybrid solar adsorption refrigeration cycle. In: 2nd Canadian solar buildings conference, Calgary; 2007. [33] Bidyut BS, Elisa CB, Takao K. Computational analysis of an advanced adsorption-refrigeration cycle. Energy 1995;20(10):983e94. [34] Saha BB, Boelman EC, Kashiwagi T. Computational analysis of an advanced adsorption refrigeration cycle. Energy 1995;20:983e94. [35] Saha BB, Kashiwagi T. Experimental investigation of an advanced adsorption refrigeration cycle. ASHRAE Transactions 1997;103(2):50e8. [36] Saha BB, Akisawa A, Kashiwagi T. Solar/waste heat driven two-stage adsorption chiller: the prototype. Renewable Energy 2001;23(1):93e101. [37] Kashiwagi T, Saha BB. Experimental investigation of an advanced adsorption refrigeration cycle. ASHRAE Transaction Research; 1997:51e8. [38] Saha BB, Koyama S, Kashiwagi T, Akisawa A, Ng KC, Chua HT. Waste heat driven dual-mode, multi-stage, multi-bed regenerative adsorption system. International Journal of Refrigeration 2003;26:749e57. [39] Qenawy AM, Mohamad AA. Current technologies and future perspectives in solar powered adsorption systems. In: Canadian solar buildings conference, Montreal; 2004. [40] Corporation Methanex. Technical information & safe handling guide for methanol. Version 3.0. See also:, http://www.methanex.com/environment/ technical.html; September 2006. [41] Wu WD, Zhang H, Sun DW. Mathematical simulation and experimental study of a modified zeolite 13X-water adsorption refrigeration module. Applied Thermal Engineering 2009;29:645e51. [42] Al-Mers A, Mimet A. Numerical study of heat and mass transfer in adsorption porous medium heated by solar energy: Boubnov-Galerkin method. Heat Mass Transfer 2005;41:717e23. [43] U.S. Department of Commerce. National Oceanic & Atmospheric Administration. NOAA Research. Earth System Research Laboratory. Global Monitoring Division. NOAA Solar Calculator.http://www.esrl.noaa.gov/gmd/grad/solcalc/ [44] Environment Canada. National Climate Data and Information Archive. www. climate. Hourly Data Report for July 30, 2009. www.climate.weatheroffice. gc.ca
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Simulation of an adsorption solar cooling system H.Z. Hassan a, A.A. Mohamad a, *, R. Bennacer b a b
Mechanical and Manufacturing Engineering, Schulich School of Engineering, University of Calgary, 2500 univ. Drive, NW, Calgary, Alberta, Canada T2N 1N4 University de Cergy-Pontoise, L2MGC F-95000 Cergy-Pontoise, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 February 2010 Received in revised form 3 August 2010 Accepted 6 October 2010 Available online 3 November 2010
A more realistic theoretical simulation model for a tubular solar adsorption refrigerating system using activated carbonemethanol (AC/M) pair has been introduced. The mathematical model represents the heat and mass transfer inside the adsorption bed, the condenser, and the evaporator. The simulation technique takes into account the variations of ambient temperature and solar radiation along the day. Furthermore, the local pressure, and local thermal conductivity variations in space and time inside the tubular reactor are investigated as well. A Cþþ computer program is written to solve the proposed numerical model using the finite difference method. The developed program covers the operations of all the system components along the cycle time. The performance of the tubular reactor, the condenser, and the evaporator has been discussed. Time allocation chart and switching operations for the solar refrigeration system processes are illustrated as well. The case studied has a 1 m2 surface area solar flat plate collector integrated with a 20 stainless steel tubes containing the AC/M pair and each tube has a 5 cm outer diameter. In addition, the condenser pressure is set to 54.2 kpa. It has been found that, the solar coefficient of performance and the specific cooling power of the system are 0.211 and 2.326 respectively. In addition, the pressure distribution inside the adsorption bed has been found nearly uniform and varying only with time. Furthermore, the AC/M thermal conductivity is shown to be constant in both space and time. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Solar energy Adsorption Refrigeration Modelling Simulation Activated carbonemethanol
1. Introduction Solar radiation is by far the largest and the most world’s abundant, clean and permanent energy source. The amount of solar radiation intercepted by the Earth’s surface is much higher than the annual global energy use. The energy available from the sun ð821015 WÞ is greater than about 5200 times the global world’s need in 2006. In recent years, many promising technologies have been developed to harness the Sun’s energy. These technologies help in environmental protection, economizing energy, and sustainable developments which are the major issues of the world in the 21st century. One of these important technologies is the solar cooling system that makes use of either absorption or adsorption technologies. Due to the environmental problems of chlorofluorocarbons (CFCs) emissions, adsorption cooling technologies are more attractive field of research and development than the conventional vapour compression refrigeration systems. The international and local policies nowadays are directed towards replacing the traditional
* Corresponding author. Tel.: þ1 403 220 2781; fax: þ1 403 282 8406. E-mail address: [email protected] (A.A. Mohamad). 0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.10.011
refrigeration systems with environmentally friendly ones that can be operated by new and renewable energy sources [1]. The solar adsorption cooling systems are good alternative since they operate with environmentally benign refrigerants that are natural, free from CFCs and therefore they have a zero ozone depleting potential (ODP). Furthermore, these refrigerants satisfy the Kyoto protocol on global warming, the Vienna Convention for Protection of the Ozone Layer (1985), and Montreal Protocol on Substances Depleting the Ozone Layer (1987). Adsorption cooling systems are characterized by their simple control and the absence of vibration and corrosion problems. The wide range of heat source temperatures that can be used 50 C 600 C, and the low operation and maintenances costs make these systems more attractive [2]. Although the adsorption chiller systems have these advantages, their drawbacks are the intermittent operation, the requirements of special designs to maintain high vacuum, the large volume and weight relative to traditional refrigeration systems, the low specific cooling power (SCP) and the low coefficient of performance (COP) [3]. Besides the poor heat and mass transfer within the adsorbent, the adsorption deterioration of the adsorbent is also vital to the development and applications of the adsorption refrigeration technology [4]. However, enhancement of heat and mass transfer properties in the adsorbent bed,
H.Z. Hassan et al. / Energy 36 (2011) 530e537
Nomenclature A C hceamb Imax L Wo x Cp Cv D hgoeamb k Lv M m mcl n P Qe R T t tsr tss U Ugoepo
Upoeamb
Area [m2] Specific heat [J kg1 K1] Convection heat transfer coefficient between the condenser and the ambient [Wm2 K1] The maximum solar radiation intensity [W m2] Length [m] Constant in the DubinineAstakhov (DeA) equation (Eq. (6)) [m3 kg1] Adsorbate concentration ratio [kg/kg] Specific heat at constant pressure [J kg1 K1] Specific heat at constant volume [J kg1 K1] Constant in the DubinineAstakhov (DeA) equation (Eq. (6)), or diameter [m]. Convection heat transfer coefficient between the outer glass cover and the ambient [W m2 K1] Coefficient of thermal conductivity [W m1 K1] Latent heat of vaporization [J kg1] Total mass [kg] Mass [kg] Mass of liquid methanol that leaves the condenser towards the evaporator [kg] Constant in the DubinineAstakhov (DeA) equation (Eq. (6)) Pressure [pa] Refrigeration effect [J] Radius [m] Temperature [K] Time [sec] The time of the day when sun rises [sec] The time of the day when sun sets [sec] Overall heat transfer coefficient [W m2 K1] Overall heat transfer coefficient between the outer surface of the glass cover and the outer pipe surface based on the glass cover surface area [W m2 K1] Overall heat transfer coefficient between the outer surface of the pipe and the ambient based on the glass cover surface area [W m2 K1]
increasing the adsorption properties of the working pairs and a better heat management during the adsorption cycle lead to a more efficient system [2]. The performance of the adsorption cooling system depends mainly on the working pairs used. A good designed system should have the characteristics of large adsorption capacity, large change of adsorption capacity with temperature variation, and a more flat desorption isotherm. Moreover, the refrigerant should have a large latent heat per unit volume, no toxicity, no flammability, no corruption, and good chemical and thermal stability [4]. The most widely used working pairs are activated carbonemethanol, activated carbon fiberemethanol, activated carboneammonia, zeoliteewater, silica gelewater, calcium chlorideeammonia and composite adsorbenteammonia pairs. Anyanwu [3] presented a review of the practically realized solid adsorption solar refrigeration cycles. He classified the cycles according to the adsorbate utilized as: cycles with water as refrigerant, cycles using fluorocarbon as refrigerant, cycles using ammonia as refrigerant and cycles with alcohols as refrigerant. Activated-carbon is the most widely used adsorbent reported in literature due to its extremely high surface area and micro pore volume. Moreover, Critoph [5] and Critoph and Vogel [6] found charcoal a preferable and
Upoepi
V
531
Overall heat transfer coefficient between the outer and inner surfaces of the pipe based on the pipe inner surface area [W m2 K1] Volume [m3]
Greek letters d Thickness [m] DH Isosteric heat of adsorption [Jkg1]. 3 Porosity of the medium [-] r Density, kgm3 s Stefan Boltzmann constant [W m2 K4] s Transmissivity [-] 3 Emissivity [-] a Absorptivity q Volume fraction of the adsorbed phase [-] Subscripts a Adsorbed phase. amb Ambient cond Condensation d Desorbed eff Effective evap Evaporation or evaporator g Glass cover or the gas phase l Liquid o Out p Pipe pi Inner surface of the pipe s Solid phase or saturation po Outer surface of the pipe Abbreviations AC/M Activated carbonemethanol CFCs Chlorofluorocarbons COP Coefficient of performance ODP Ozone depleting potential SCOP Solar coefficient of performance SCP Specific cooling power [Wkg1]
recommended adsorbent for solar cooling. Activated carbonemethanol pair has been studied by many researchers [7e9]. This pair is characterized by high latent heat of vaporization of methanol and low desorption temperature. However, the operating temperature is one of the limitation that constrain the operation of the activated carbonemethanol pair cooling systems. At temperatures more than 150 C, the methanol decomposes into dimethyl. However, a very low decomposition reaction rate is found to happen at temperatures near 120 C, Yuan [10]. The simulation done by Eric [11] showed that the thermal decomposition of methanol did occur in the normal solar powered adsorption refrigeration system. The rate of the decomposition depends on the presence of various alloys such as aluminium and copper alloys. Moreover, the decomposition products may be one of the main reasons for the diminishing performance of the solar ice maker. The solar adsorption refrigeration system usually operates with the basic simple cycle. The basic cycle is a single-bed, intermittent, and has a low efficiency and low cooling effect. Many attempts have been reported in the literature to develop and design a semicontinuous operation and to increase the performance and efficiency of the cycle. These developments include heat recovery or the regeneration cycle [12e15], mass recovery cycle [16e18], heat and
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H.Z. Hassan et al. / Energy 36 (2011) 530e537
mass recovery cycle [13,19], thermal wave cycle [20], forced convective thermal wave cycle [21e23], cascade cycle [24e29], multi-bed cycle [30e32], multi-stage cycle [33e37], dual-mode cycles [38]. An explained review for these cycles is presented in [3,39]. 2. Physical model The adsorption cooling system likes the basic vapour compression refrigeration machine except that the power compressor is replaced with a thermal compressor or a reactor. This reactor is composed of a type of porous medium that has the ability to adsorb the refrigerant. The working principle of the basic reactor cycle is represented in the Clapeyron diagram, Fig. 1. The adsorbenteadsorbate cycle consists mainly of four phases; pressurization process at a constant volume (isosteric heating phase), desorption at constant pressure (isobaric heating phase), depressurization at constant volume (isosteric cooling phase), and adsorption at constant pressure (isobaric cooling phase). At the beginning of the day, state A, the reactor is isolated from both the condenser and the evaporator by valves c and e and is completely charged with the refrigerant. The pressure inside the reactor initially equals the evaporator pressure Pevap. and its temperature is uniform and equals the ambient temperature Tamb. When the reactor starts to heat up by the incident solar radiation, both pressure and temperature inside the adsorbent are elevated. This constant concentration heating phase continues till point B where the pressure reaches a value that equals the condenser pressure Pcond. This period is equivalent to the compression in the classical vapour compression refrigeration cycle. At state B, valve c is opened and the adsorbate starts to desorb and flow towards the condenser. During this isobaric heating phase, the temperature continues to increase, and the adsorbate concentration continues to decrease as more adsorbate is being freed from the reactor. When the adsorbate temperature reaches the maximum value Tmax at state C, valve c is closed and the reactor starts the third phase. When the solar flux decreases, the reactor is cooled down at constant volume and constant isoster till the pressure inside the reactor decreases to the evaporator pressure Pevap. , point D. The last phase of the reactor cycle starts at the night, point D, when valve e is opened and the refrigerant flows towards the reactor. The adsorption process continues while the reactor is cooled at the constant evaporator pressure till the higher cycle isoster at point A.
Fig. 2. Configuration diagram of the solar adsorption cooling reactor.
The solar refrigeration system presented in this study has three main components: the solar collector/reactor, Fig. 2, the condenser, and the evaporator. The solar collector/reactor is constituted of a clear plane glass sheet cover, and the activated carbonemethanol pair which is contained in the annular space between a two coaxial collector pipes. The inner pipe is perforated in order to ease the flow of methanol into and out from the activated carbon granules and to avoid pressure drops and temperature differences along the collector tubes as well. These pipes are integrated in the plane solar collector and are coated with a selective painting to allow a good absorption of solar radiation and low emission. The lateral and rear sides of the collector are insulated. The inner pipes are connected to a common methanol inlet and outlet headers. The adsorbent bed is cooled during the adsorption process when the collector glass cover plate is opened by natural convection and by radiation from the collector plate and tubes. The condenser pipe is steel and is cooled by natural convection and by radiation as well. 3. Mathematical model The presented mathematical model simulates the real working of the solar adsorption cooling system taking in account the variation of solar radiation and ambient temperature along the day. Furthermore, the energy and mass balances for every element of the system are taken into account as well. 3.1. Solar radiation and ambient temperature modelling Both desorption and isosteric heating phases depend strongly on the quantity of incident solar radiation. In addition, the adsorption
Fig. 1. Clapeyron diagram for a conventional adsorption cycle.
H.Z. Hassan et al. / Energy 36 (2011) 530e537
533
and the isosteric cooling phases besides the condenser operation are dependent on the ambient temperature. Therefore, in order to simulate the system in a more realistic manner, the solar radiation and ambient temperature are modelled in this study as variant along the day. These climatic conditions are taken for Calgary town, Canada (latitude ¼ 51.12 and longitude ¼ 114.01) where a summer day in July 30, 2009 is chosen. The apparent sun rise time, apparent sun set time, and the solar noon during this day are at 5:59:00, 21:25:00, and 13:42:30 respectively [43]. The intensity of solar radiation as a function of the time is given by:
lower temperature layers. This mass transfer results in the heat transfer process. That is because the gas that will be adsorbed on cold layers is hot. Therefore, the adsorption and desorption mass transfer through the porous medium is accompanied with a heat transfer and vice versa. The conservation of mass equation and the first law of thermodynamics for an open system are applied to a layer of radial coordinates r and thickness dr in the adsorbent bed. Both the mass and heat balance equations are merged together to give the following combined heat and mass transfer equation within the adsorbent bed:
pðt tsr Þ Is ðtÞ ¼ Imax sin tss tsr
P vT ð1 3Þrs Cs þ qra Ca ð3 qÞ rg Cvg þ T vt # " 2 v T 1 vT vP P vra vq 3 q q D þ ð ¼ Keff þ Þ þ H þ ra DH þ ra vt vt vt vr 2 r vr
(1)
where, tsr < t< tss Imax is taken to be equal 900 W/m2. Furthermore, the real hourly temperature variation on the same day is taken from [44]. The temperature data is fitted in a form of a polynomial from the 8th degree and is inserted to the simulation program. 3.2. Energy balance for the solar collector The solar radiation is absorbed at the external surface of every tube. This heat flux is transmitted by conduction, through the pipe wall in the inward radial direction to the internal walleadsorbent interface, then towards the adsorbent-refrigerant reactive medium. A one-dimensional transient model for heat balance is suggested for the glass cover and the pipe. The heat balance for the glass cover is given by the following equation:
rg Cg dg Ag
vTgo ¼ ag Ag Is ðtÞ þ Ugopo Ag Tpo Tgo vt 4 4 s Tpo Tgo pRpi Lp hgoamb Ag Tgo Tamb þ 1 1 þ 1
4 T4 s Tgo amb
1
3g
þ
1
3amb
3p
3g
In this equation, the adsorbate density is modelled as varying with temperature. Moreover, the pressure is taken as variant throughout the adsorbent bed layers. It is assumed that the gaseous phase behaves as an ideal gas. The effective thermal conductivity of the adsorbent bed is also taken as variant and is evaluated by the following formula:
Keff ¼ Ks ð1 3Þ þ Ka q þ Kg ð3 qÞ
(5)
where ks,ka, and kg are the thermal conductivities of the solid, adsorbate, and gas phases respectively.
3.4. Adsorption equilibrium model The adsorption equilibrium equation of the activated carbon/ methanol as an adsorbent/adsorbate pair presented in this study, is described by the DubinineAstakhov (DeA) equation of state. The adsorbed mass of methanol per unit mass of activated carbon is given by:
n Ps ðTÞ X ¼ ra ðTÞWo Exp D Tln P
1 (2)
Neglecting the heat conduction between the adsorbent bed and the condenser and evaporator, the energy balance for the steel pipe is given by:
rp Cp Vg
(4)
vTpo ¼ sg ap Is ðtÞLp Dpo Ugopo Ag Tpo Tgo vt 4 4 s Tpo Tgo pRpi Lp Upopi pDpi Lp Tpo Tpi 1 1 þ 1 Ag
3p
4 Tpo
s Upoamb Ag Tpo Tamb
1
3g
3g
þ
Tg4
1
3p
pRpi Lp
1 Ag
(6)
The values of the constants Wo, D, and n are given in Table 1. The thermodynamic properties of methanol are taken from Methanex Corporation [40]. In order to complete the mathematical formulation of the model, the methanol properties are fitted as polynomials and merged with the mathematical model.
3.5. Condenser, evaporator and system performance The mass of methanol gas desorbed from the adsorption bed goes to the condenser where it is cooled and condenses to liquid. The condenser is cooled by the convection and radiation to the ambient air. The combined mass and energy conservation equation of the condenser is given by:
(3) Table 1 Constants of the DubinineAstakhov (DeA) equation, Jing and Exell[7].
3.3. Combined heat and mass transfer equation in the adsorbent bed It is clear that, higher temperature layers in the adsorbent bed desorb the methanol gas. This desorbed gas is adsorbed on the
Charcoal
Wo[Lit/kg]
D 105
n
207E4 Chinese LSZ30 LH Thai MD6070
0.36546 0.405 0.860 0.988
14.96202 31.972 25.74 88.98
1.34 1.26 1.321 1.12
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H.Z. Hassan et al. / Energy 36 (2011) 530e537
½mc Cc þ Md ðtÞCd
vTc ¼ md ðtÞ Lv ðPc Þ þ Cpg Tg Tcond vt Ac hcamb ðTcond Tamb Þ 4 s Tc4 Tamb 1 1 þ 1 Ag
3c
(7)
3amb
The cooling effect is calculated by the following equation:
Qe ¼ mcl Lv Tevap hl Tc;o þ hl Tevap
(8)
The parameters used in this present work to assess the adsorption refrigeration system performance are the solar coefficient of performance SCOP and the specific cooling power SCP. These parameters are defined by the following equations:
SCOP ¼ Z
Qe
(9)
Is ðtÞAg dt
The SCP is the ratio between the cooling effect and the cycle time per unit mass of adsorbent:
SCP ¼
Qe tcycle ms
simulate the behaviour of the solar adsorption cooling system. For each time step, the program updates the ambient temperature and the solar insolation based on the suggested climatic scheme. Based on the new conditions every time step, the temperature, pressure, concentration ratio, volume fraction, desorbed and adsorbed mass inside the reactor, and the condenser temperature are updated. At the end of the cycle, which is adjusted to be within 24 h period, the overall system performance parameters are determined. These parameters include the solar coefficient of performance, the specific cooling power, and the evaporation effect. The main data and design parameters input to the developed Cþþ code and used in this simulation are depicted in Table 2. 5. Results and discussion Based on the proposed mathematical and numerical models and the input data presented in Table 2, the output results for only one cycle of operation are presented in this section. The obtained results and the overall system performance are estimated in this study by considering a number of 20 pipes backed with the activated carbonemethanol pair and placed in the tubular reactor. Each tube is 1 m in length and the surface area of the solar flat plate
(10)
where tcycle is the complete cycle time.
Table 2 The main parameters used in the case study. Symbol Parameter
3.6. Initial and boundary conditions The operation of the solar cooling system is strongly dependant on the variation of the climatic conditions. This means that, the performance of this system changes day by day. Moreover, the initial conditions of the system for a given cycle are defined by the final conditions for the previous one. In this work we deal with the first operating cycle in which the system is considered in thermal equilibrium with the ambient. Therefore, the initial pressure and temperature are given by:
Pðt ¼ 0Þ ¼ Pevap
¼ Psat Tevap
Tðt ¼ 0Þ ¼ Tgo ðt ¼ 0Þ ¼ Tpo ðt ¼ 0Þ ¼ Tpi ðt ¼ 0Þ ¼ Tc ðt ¼ 0Þ ¼ Tamb The boundary conditions used to solve equation (4) are as follows :
vTpi ¼ Upopi Tpo Tpi vr
1: At r ¼ Rpi ;
Keff
2: AT r ¼ Ri ;
vT ¼ 0:0 vr
The second boundary condition assumes an adiabatic condition at the inner tubular reactor surfaceegas interface [41,42]. This can be explained by the poor thermal conductivity of methanol gas which is about 0.016 W/m K. 4. Numerical method of solution In the present work, the proposed mathematical model is discretized using finite difference approximation method. The resulting system of equations is solved explicitly to obtain the numerical solution for the variables at every time step. A computer code based on this numerical scheme is written in Cþþ in order to
Glass cover Cg Specific heat of the glass cover rg Density of the glass cover Glass cover thickness tg Coefficient of thermal conductivity kg sg Transmissivity ag Absorptivity 3g Emissivity Air gap tgab Air gap thickness Air thermal conductivity kair Back insulation Insulation thickness tins Insulating material thermal conductivity kair Stainless steel pipe Specific heat of stainless steel Cp rp Density of stainless steel Coefficient of thermal conductivity kp ap Absorptivity of the pipe coating material 3p Emissivity of the pipe coating material Outer diameter of the pipe Dpo Inner diameter of the pipe Dpi Pipe length Lp Number of pipes np Adsorption bed 3 Porosity of the AC Specific heat of carbon Cs Thermal conductivity of carbon ks rs Density of carbon Inner diameter of the inner coaxial pipe Dip Methanol properties Specific heat of liquid methanol Ca Cpgas Specific heat of methanol vapour atp ¼ cons: Specific heat of methanol vapour at v ¼ cons: Cvgas Thermal conductivity of liquid methanol ka Thermal conductivity of methanol vapour kgas Condenser Condenser Pressure Pcond Condenser tube length Lc Roct Condenser tube outer radius Condenser tube inner radius Rict Specific heat of the condenser metal Ccm rcm Density of the condenser metal Evaporator Evaporator Pressure Pevap
Value
Unit
750 2500 0.003 1.4 0.95 0.05 0.9
[J kg1 K1] [kg m3] [m] [W m1 K1] [] [] []
0.1 [m] 0.0263 [W m1 K1] 0.1 0.038
[m] [W m1 K1]
480 8055 15.1 0.98 0.1 0.05 0.048 1.0 20
[J kg1 K1] [k gm3] [W m1 K1] [] [] [m] [m] [m] []
0.7 711 1.6 2000 0.01
[] [J kg1 K1] [W m1 K1] [k gm3] [m]
2534 1820 1560 0.2022 0.0163
[J kg1 K1] [J kg1 K1] [J kg1 K1] [W m1 K1] [W m1 K1]
54,209 20 0.0025 0.002 480 8055
[Pa] [m] [m] [m] [J kg1 K1] [k gm3]
3967
[Pa]
H.Z. Hassan et al. / Energy 36 (2011) 530e537
Fig. 3. Variation of climatic data with the day time.
collector glass cover is 1 m2. Moreover, the condenser pressure is set to a value of 54.21 kpa which corresponds to a saturation temperature of 50 C. Generally, it is found that the solar coefficient of performance of this adsorption cooling system attains a value of 0.211. Furthermore, the specific cooling power and the evaporator cooling effect values are found to be 2.326, and 4175 kJ respectively. Fig. 3 shows the variation of the solar incident radiation and the ambient atmospheric temperature with the day hours. These data simulate a summer day in July 30, 2009, for Calgary, Canada as discussed in Section 3.1. The time allocation chart for the solar refrigeration system is illustrated in Fig. 4. This chart indicates the rank of each process, its period of operation, and its start and end time. The timing schedule is affected strongly by the climatic conditions, cycle initial conditions, and the system design parameters. Moreover, the behaviour of any phase of the reactor affects the operation of the following other phases and therefore the condenser, evaporator, and the overall cycle of the system. Therefore, the matching among the system elements is an important issue that insures a proper operation of the cooling system and satisfies a daily complete working cycle. Fig. 5 represents the evolution of the volume averaged temperature of the adsorbent bed along the cycle period. The period of the cycle is 24 h starting at 6:00 in the morning which is the apparent sun rise time. It is noticed that, during the isosteric heating phase, the reactor continues to absorb the continuously increasing solar radiation. This results in a rapid increase in the adsorption bed temperature from about 10.8 C to 60 C in a time
Fig. 4. The time allocation of the solar cooling system.
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Fig. 5. Variation of average adsorption bed temperature with time.
period of about 3.55 h. The pressure inside the activated carbonemethanol pair approaches the condenser pressure of 54.209 kpa at the temperature of 60 C. At this moment, methanol starts to desorb from the adsorbent and flows towards the condenser. During this process, heat energy is extracted from the adsorption bed due to the endothermic character of the desorption process. Therefore, the rate of heating and the associated temperature increase rate during this isobaric desorption phase is lower than the first phase as seen from Fig. 5. The desorption process continues for a time period of 5.4 h and stops when methanol reaches 120 C, the temperature at which it starts to decompose. At this moment, the solar flat plate collector glass cover is opened and the reactor is kept away from the sun rays to allow beginning of the cooling phase of the adsorption bed. Heat transfers from the reactor by means of both natural convection and by radiation with a high driving potential from the high reactor temperature to the low temperature of the surroundings. This results in the sudden dramatic decreasing rate in the reactor temperature, as noticed in Fig. 5, from 120 C to 61 C in a short time period of 0.74 h. When the temperature reaches 61 C, the corresponding reactor pressure at this low isosteric process will be the same as the evaporator pressure and adsorption process has to begin. Since the adsorption process is exothermic and the driving potential for heat transfer is lower than the previous phase, a lower decreasing rate for the reactor temperature is remarked in Fig. 5. The cooling of the adsorption bed continues till it reaches a temperature of 16.3 C at the end of the cycle. It is clear that, this temperature is not the same as the corresponding value at the beginning of the cycle. This is because the reactor has not reached a complete thermal equilibrium with the ambient. Therefore, this will affect the next operating cycle of the system which has an initial temperature of 16.3 C. The variation of both the average pressure and the adsorbed mass inside the activated carbonemethanol pair with time is illustrated in Fig. 6. The average pressure increases from the initial evaporator pressure, 3.967 kpa, to the condenser pressure, 54.209 kpa during the first phase. It reaches a plateau while the desorption process takes place then sharply drops again to the evaporator pressure at the beginning of the adsorption process. Although the pressure computation in this study is modelled as variant with the radial direction inside the tubular reactor, its observed gradients are found to be very small and therefore can be neglected. Consequently, it is a good assumption to consider a uniform pressure distribution inside the adsorption bed. The amount of adsorbed methanol at the beginning of the operating cycle is 5.1 kg and it reaches an amount of 1:13 kg at the end of desorption process. That is, the total effective mass of methanol freed from the tubular reactor and circulates in the condenser and
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H.Z. Hassan et al. / Energy 36 (2011) 530e537
Fig. 6. Variation of the average pressure and the adsorbed mass inside the adsorption bed.
the evaporator is about 3.97 kg. Consequently, the effective methanol mass ratio, which is the fraction of the adsorbate total initial mass that circulates in the cycle, is about 0.778. During the adsorption process, the methanol mass inside the reactor continues to increase till it reaches a value of 4.5 kg at the end of this cycle and beginning of the next cycle, Fig. 6. The local values of the thermal conductivity inside the adsorbent bed layers are determined each time step. It is found that, the local thermal conductivity does not vary so much from a layer to a layer and therefore it can be considered uniform in the adsorption bed. The change of the adsorbent bed volume averaged effective thermal conductivity along the cycle time is seen in Fig. 7. Generally, the average conduction coefficient attains a low value that ranges from a maximum of about 0.528 W/mK to a minimum of about 0.5 W/mK. Since the range of variation is very small, it can be verified that the adsorbent bed thermal conductivity is both uniform and does not vary with time. Fig. 8 illustrates the condenser temperature changes during the day. It is clear that, before the adsorption process, the condenser temperature change follows that of the corresponding ambient temperature. As the vapour of methanol starts to flow towards the condenser, a remarkable sudden temperature increasing rate is seen, Fig. 8. This increase is due to the increasing desorbed methanol vapour mass that has a high temperature. The condenser reaches a maximum temperature of about 40 C after which its temperature begins to go down. The condenser temperature decreases in a rapid rate during the third phase. That is because the flow of methanol vapour is stopped from one side and the ambient temperature starts
Fig. 8. Condenser temperature variation with time.
to decrease providing a higher driving potential for the condenser cooling process from the other side. This cooling process continues by means of both free convection and radiation until the liquid methanol reaches about 33 C at the beginning of the adsorption process. The liquid methanol exits from the condenser to enter the evaporator after it is throttled in the expansion valve to the evaporator pressure. As the condenser begins to discharge its content of methanol, its temperature starts to follow the ambient temperature changes along the remaining period of the day. 6. Conclusion In this study we have introduced a more actual theoretical model to simulate the adsorption cooling system powered by the solar energy. The system uses the activated carbonemethanol as the working pair. The mathematical and numerical models for the adsorbent bed, condenser, and evaporator has been programmed be the Cþþ language. The real ambient temperature and solar radiation variations are taken into account. The variation of the adsorbent bed average temperature, pressure, adsorbed mass, and its thermal conductivity with time are discussed. Moreover, the condenser temperature variation, the time allocation and switching operations and the overall cooling system performance parameters are investigated as well. For the case studied, it is found that, the solar coefficient of performance reaches a value of 0.211. Moreover, the specific cooling power is estimated to about 2.326. It can also be seen from this work that, the pressure is nearly uniform inside the adsorption bed and the activated carbonemethanol thermal conductivity is found to be constant in space and time. The authors would like to acknowledge the support of Solar Buildings Research Network and NSERC. Acknowledgement The authors would like to acknowledge the support of Solar Buildings Research Network and NSERC. References
Fig. 7. Change of the effective thermal conductivity of the adsorbent bed along the cycle.
[1] Anyanwu EE. Environmental pollution: restructuring the refrigeration industry as a way out. Environment Protection Engineering 2000;26 (4):17e28. [2] Wang RZ, Oliveira RG. Adsorption refrigeration e an efficient way to make good use of waste heat and solar energy. Progress in Energy and Combustion Science 2006;32(4):424e58. [3] Anyanwu EE. Review of solid adsorption solar refrigerator I: an overview of the refrigeration cycle. Energy Conversion and Management 2003;44 (2):301e12.
H.Z. Hassan et al. / Energy 36 (2011) 530e537 [4] Wang DC, Li YH, Li D, Xi YZ, Zhang JP. A review on adsorption refrigeration technology and adsorption deterioration in physical adsorption systems. Renewable and Sustainable Energy Reviews 2010;14(1):344e53. [5] Critoph RE. Performance limitations of adsorption cycles for solar cooling. Solar Energy 1988;41(1):21e31. [6] Critoph RE, Vogel R. Possible adsorption pairs for use in solar cooling. International Journal of Ambient Energy 1986;7(4):183e90. [7] Jing H, Exell RHB. Simulation and sensitivity analysis of an intermittent solarpowered charcoal/methanol refrigerator. Renewable Energy 1994;4(1):133e49. [8] Ogueke NV, Anyanwu EE. Design improvements for a collector/generator/ adsorber of a solid adsorption solar refrigerator. Renewable Energy 2008;33:2428e40. [9] Leite APF, Daguenet MD. Performance of a new solid adsorption ice maker with solar energy regeneration. Energy Conversion and Management 2000;41 (15):1625e47. [10] Yuan ZH. Charcoal methanol adsorption refrigeration. Bangkok, Asian Institute of Technology 1988, M. Eng. Thesis ET-88e10. [11] Hu EJ. A study of thermal decomposition of methanol in solar powered adsorption refrigeration machines. Solar Energy 1998;62(5):325e9. [12] Vasiliev LL, Mishkinis DA, Antukh AA, Vasiliev Jr LL. Solar-gas solid sorption heat pump. Applied Thermal Engineering 2001;21(5):573e83. [13] Wang RZ. Performance improvement of adsorption cooling by heat and mass recovery operation. International Journal of Refrigeration 2001;24(7):602e11. [14] Critoph RE. Simulation of a continuous multiple-bed regenerative adsorption cycle. International Journal of Refrigeration 2001;24(5):428e37. [15] Chahbani MH, Labidi J, Paris J. Modeling of adsorption heat pumps with heat regeneration. Applied Thermal Engineering 2004;24(2e3):431e47. [16] Alam KCA, Akahira A, Hamamoto Y, Akisawa A, Kashiwagi T. A four-bed mass recovery adsorption refrigeration cycle driven by low temperature waste/ renewable heat source. Renewable Energy 2004;29(9):1461e75. [17] Akahira A, Alam KCA, Hamamoto Y, Akisawa A, Kashiwagi T. Mass recovery adsorption refrigeration cycle-improving cooling capacity. International Journal of Refrigeration 2004;27:225e34. [18] Luo HL, Dai YJ, Wang RZ, Wu JY, Xu YX, Shen JM. Experimental investigation of a solar adsorption chiller used for grain depot cooling. Applied Thermal Engineering 2006;26(11e12):1218e25. [19] Wang W, Qu TF, Wang RZ. Influence of degree of mass recovery and heat regeneration on adsorption refrigeration cycles. Energy Conversion and Management 2002;43(5):733e41. [20] Sward BK, LeVan MD, Francis M. Adsorption heat pump modeling: the thermal wave process with local equilibrium. Applied Thermal Engineering 2000;20(8):759e80. [21] Critoph RE. Forced convection enhancement of adsorption cycles. Heat Recovery Systems & CHP 1994;14(4):343e50. [22] Critoph RE. Forced convection adsorption cycle with packed bed heat regeneration. International Journal of Refrigeration 1999;22:38e46. [23] Hai-Ming Lai. An enhanced adsorption cycle operated by periodic reversal forced convection. Applied Thermal Engineering 2000;20:595e617. [24] Douss N, Meuiner FE, Sun LM. Predictive model and experimental results for a two absorber solid adsorption heat pump. Industrial & Engineering Chemistry Research 1988;27(2):310e6. [25] Douss N, Meunier F. Experimental study of cascading adsorption cycles. Chemical Engineering Science 1989;44(2):225e35.
537
[26] Douss N. Experimental study of adsorption heat pump cycles. International Chemical Engineering 1993;33(2):207e14. [27] Chua HT, Ng KC, Malek A, Kashiwagi T, Akisawa A, Saha BB. Modeling the performance of two-bed, sillica gel-water adsorption chillers. International Journal of Refrigeration 1999;22(3):194e204. [28] Khan MZI, Alam KCA, Saha BB, Hamamoto Y, Akisawa A, Kashiwagi T. Parametric study of a two-stage adsorption chiller using re-heat-The effect of overall thermal conductance and adsorbent mass on system performance. International Journal of Thermal Sciences 2006;45(5):511e9. [29] Liu Y, Leong KC. Numerical study of a novel cascading adsorption cycle. International Journal of Refrigeration 2006;29(2):250e9. [30] Chua HT, Ng KC, Malek A, Kashiwagi T, Akisawa A, Saha BB. Multi-bed regenerative adsorption chiller-improving the utilization of waste heat and reducing the chilled water outlet temperature fluctuation. International Journal of Refrigeration 2001;24(2):124e36. [31] Saha BB, Koyama S, Lee JB, Kumahara K, Alamc KCA, Hamamoto Y, et al. Performance evaluation of a low-temperature waste heat driven multi-bed adsorption chiller. International Journal of Multiphase Flow 2003;29 (8):1249e63. [32] Qenawy AM, Mohamad AA. Simulation of double-bed cooling and heating hybrid solar adsorption refrigeration cycle. In: 2nd Canadian solar buildings conference, Calgary; 2007. [33] Bidyut BS, Elisa CB, Takao K. Computational analysis of an advanced adsorption-refrigeration cycle. Energy 1995;20(10):983e94. [34] Saha BB, Boelman EC, Kashiwagi T. Computational analysis of an advanced adsorption refrigeration cycle. Energy 1995;20:983e94. [35] Saha BB, Kashiwagi T. Experimental investigation of an advanced adsorption refrigeration cycle. ASHRAE Transactions 1997;103(2):50e8. [36] Saha BB, Akisawa A, Kashiwagi T. Solar/waste heat driven two-stage adsorption chiller: the prototype. Renewable Energy 2001;23(1):93e101. [37] Kashiwagi T, Saha BB. Experimental investigation of an advanced adsorption refrigeration cycle. ASHRAE Transaction Research; 1997:51e8. [38] Saha BB, Koyama S, Kashiwagi T, Akisawa A, Ng KC, Chua HT. Waste heat driven dual-mode, multi-stage, multi-bed regenerative adsorption system. International Journal of Refrigeration 2003;26:749e57. [39] Qenawy AM, Mohamad AA. Current technologies and future perspectives in solar powered adsorption systems. In: Canadian solar buildings conference, Montreal; 2004. [40] Corporation Methanex. Technical information & safe handling guide for methanol. Version 3.0. See also:, http://www.methanex.com/environment/ technical.html; September 2006. [41] Wu WD, Zhang H, Sun DW. Mathematical simulation and experimental study of a modified zeolite 13X-water adsorption refrigeration module. Applied Thermal Engineering 2009;29:645e51. [42] Al-Mers A, Mimet A. Numerical study of heat and mass transfer in adsorption porous medium heated by solar energy: Boubnov-Galerkin method. Heat Mass Transfer 2005;41:717e23. [43] U.S. Department of Commerce. National Oceanic & Atmospheric Administration. NOAA Research. Earth System Research Laboratory. Global Monitoring Division. NOAA Solar Calculator.http://www.esrl.noaa.gov/gmd/grad/solcalc/ [44] Environment Canada. National Climate Data and Information Archive. www. climate. Hourly Data Report for July 30, 2009. www.climate.weatheroffice. gc.ca