Performance analysis and optimal parameters of a direct evaporative pad cooling system under the climate conditions of Morocco

Performance analysis and optimal parameters of a direct evaporative pad cooling system under the climate conditions of Morocco

Author’s Accepted Manuscript Performance analysis and optimal parameters of a direct evaporative pad cooling system under the climate conditions of Mo...

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Author’s Accepted Manuscript Performance analysis and optimal parameters of a direct evaporative pad cooling system under the climate conditions of Morocco Azzeddine Laknizi, Mustapha Mahdaoui, Abdelatif Ben Abdellah, Kamal Anoune, Mohamed Bakhouya, Hassan Ezbakhe www.elsevier.com/locate/csite

PII: DOI: Reference:

S2214-157X(18)30113-8 https://doi.org/10.1016/j.csite.2018.11.013 CSITE362

To appear in: Case Studies in Thermal Engineering Received date: 21 April 2018 Revised date: 3 October 2018 Accepted date: 19 November 2018 Cite this article as: Azzeddine Laknizi, Mustapha Mahdaoui, Abdelatif Ben Abdellah, Kamal Anoune, Mohamed Bakhouya and Hassan Ezbakhe, Performance analysis and optimal parameters of a direct evaporative pad cooling system under the climate conditions of Morocco, Case Studies in Thermal Engineering, https://doi.org/10.1016/j.csite.2018.11.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Performance analysis and optimal parameters of a direct evaporative pad cooling system under the climate conditions of Morocco

Azzeddine Laknizia,b, Mustapha Mahdaouic, Abdelatif Ben Abdellaha,b, Kamal Anounea,b, Mohamed Bakhouyab, Hassan Ezbakhed

aFaculty of science and technology of Tangier, Abdelmalek Essaâdi University – Tangier, Morocco bRenewable Energy and Advanced Materials Laboratory, International University of Rabat-Sale, Morocco cEquipe de Recherche en Transferts Thermiques & Énergétique - UAE/E14FST Département de Physique FST, Université Abdelmalek Essaâdi Tanger – Maroc dEnergy Laboratory, College of Science-Tetouan, Abdelmalek Essaâdi University-Morocco

Abstract In this study, a direct evaporative pad cooling system to be used for a poultry house is simulated under the climate conditions of six Moroccan cities. The objective is to optimize the parameters affecting the performance of the system for each city. The thermodynamic properties of humid air are extracted from the open source program CoolProp, and a computer program is used to perform a parametric study in order to find the impacts of the pad thickness and the frontal velocity on system’s performance. Simulations have been conducted on cellulosic pad cooling and obtained results show that the system has the potential to lower the outdoor temperature during the hottest period with a coefficient of performance higher than 80, while the average rate of water consumption is higher than 3.3 kilograms per hour.

Keywords: Direct evaporative pad cooling; cellulosic; simulated; coefficient of performance; Optimization.

Nomenclature Latin symbols Cap

Specific heat capacity, J/(kg K)

D

Pad thickness, mm

h

Enthalpy, kJ/kg

H

Height of the pad, mm

L

Width of the pad, mm

m

mass flow rate, kg/s

Ppad

Cooling capacity of the pad, W

Pfan

Power consumption of the fan, W

Ppump Power consumption of the pump, W ∆P

pressure drop, Pa

V

Velocity, m/s

Tin

inlet temperature, K

Tout

outlet temperature, K

Win

indoor temperature, kgw/kgda

Wout outdoor temperature, kgw/kgda Greek symbols η

Effectiveness,%

φ

Relative humidity, %

ρ

Density, kg/m3

Subscripts e

Evaporated

out

Outlet

int

inlet

iw

inlet wet bulb

1. Introduction Heat stress is considered as the most important problem in the poultry sector worldwide [1]. Hot and dry regions are the most exposed to heat stress, which contributes to high losses in terms of mortality and performance drop. In Morocco, losses for summer 2012 are estimated at 12% in terms of mortality and 25% in terms of performance drop (weight loss) [2]. In order to reduce these losses, it is imperative to adapt the temperature of the air inside poultry houses to adequate values through the use of an efficient energy system. Among the available cooling solutions, direct evaporative cooling is the widely used in poultry houses and greenhouses [3-8]. This widespread use of the direct evaporative cooling is due to its high efficiency and its eco-friendly aspect [9,10]. In addition to its application as an air conditioner, it has been used in many applications. In this context, it was used to improve the performance of the vapor compression air conditioner by coupling the pad cooling with the condenser, which will decreases the temperature of the condensing air [11-13]. Kabeel, Abdelgaied et al used the pad cooling to improve a solar thermal system desalination system based on membrane distillation [14]. Farmahini-Farahani et al. [15] conducted the exergy analysis of the direct and indirect evaporative cooling process based on experimental investigations. The study was done on different climates in order to choose the favored system. Kim et al. [16] are evaluated experimentally the energetic performances of an indirect and direct evaporative cooling assisted 100% outdoor air system installed in a building. An experimental study of the performances of a new direct evaporative cooling system by integrating a phase change material into a cylindrical tank was made by Panchabikesan et al. [17]. Direct evaporative cooling is an adiabatic cooling process in which enthalpy remains constant, the specific and relative humidity increase, and the dry bulb temperature decreases. During

this process, water is evaporated and the necessary heat of vaporization is extracted from the dry air which leads to air to be cold. Fig.1 shows a schematic of the thermal process on the psychrometric chart. State ‘1’ and ‘2’ represents the inlet-outlet conditions. Fig.2 shows a schematic of a direct evaporative cooling which consists of a pad made of cellulose paper (1), a fan (2), and a pump (3) for recirculation of the water from the reservoir (4) via PVC tube (5) to the sprinkler (6). To the best of authors knowledge, there are no known studies that deal with a direct evaporative cooling pad in Morocco. The previous studies are divided into three categories experimental, numerical and analytical investigations. Malli et al. [18]

conducted an

experimental investigation on two types of cellulosic pad cooling (5090 and 7090). They found that the pressure drop and the rate of the evaporated water increased with increasing the frontal velocity and the thickness of the pads. On the other side, the effectiveness and the variation of the humidity decreased by increasing the frontal velocity. Bishoyi et al. [19] conducted an experimental investigation on two types of evaporative pad cooling under the climate conditions of India and reported that evaporative pad cooling made of Honeycomb paper has an energy efficiency ratio and cooling capacity higher than pad cooling made of Aspen. Kovačević and Sourbron[9] developed a numerical model of evaporative pad cooling, they found that their model can predict the air outlet temperature, with a maximal error of 1.33 % compared with experimental data. Wang et al. [20] used the analytical expression of the effectiveness saturation coupled to fuzzy mathematical method to evaluate the suitability of evaporative pad cooling under the climate conditions of nine cities of China, they conclude that this system is very suitable in some cities and is more than suitable in the other cities. The main objective of this paper is to optimize the parameters that affect the performance of a direct evaporative cooling system. Extensive simulations have been performed during five successive months, from May to September, under the climate conditions of Morocco. The reminder of this paper is structured as follows. Section 2 presents the simulation of the direct evaporative pad cooling. In Section 3, the flowchart of the code program is presented. Obtained results are discussed and explained in Section 4. Conclusions and perspectives are given in Section 5. 2. Modeling of the direct evaporative pad cooling For the investigation, an ordinary fan-pad evaporative cooling is considered, which consists of a fan, a cellulosic pad paper, sprinkler, and water pump.

The considered system here consists of corrugated cellulose paper sheets (Model: CELdek®7090-15). Its height was 1000 mm and width of 600 mm and was taken from Munters Company[21] . The cooling capacity is calculated by the temperature difference at the inlet and the outlet: ( ̇ ̇ Where, ̇

(1) (2)

ρ

is the flow rate of air supply (kg/s),

the outdoor temperature,

)

is the specific heat capacity of air,

is

is the indoor temperature, ρ is the volume mass of air, V is the

velocity, L is the width of the pad cooling, and H is the height of the pad cooling. The mass of water evaporated is given by: ̇ Where,

and

̇

(

)

(3)

are the humidity ratio at the inlet and the outlet respectively.

The effectiveness saturation is given by the following equation: η Where

(4)

is the inlet dry bulb temperature,

is the outlet dry bulb temperature and

is

the inlet wet bulb temperature. The power consumption of the fan is calculated by the following equation: ̇ ρη Where ̇ the mass flow rate (kg/s), η

η

is the pressure drop in the circuit (Pa), η

(5) and

are the efficiency of the fan and motor respectively, and ρ is the volume mass of the air

(kg/m3). The motor and fan efficiencies are taken equal to 80%. The coefficient of performance which equals to the ratio of cooling effect to the electric power consumption is calculated by the following equation:

(6) The pump power consumption in this study is taken equal to 5% of the fan power consumption [22]. The effectiveness saturation and the pressure drop are obtained by curves performance presented by the manufacturer. These curves were digitized by using digitizer plot software[23] .According to the manufacturer, these two parameters are related to the velocity and the thickness of the pad as mentioned in table 1. 3. Program flowchart and operating conditions The program code was written in MATLAB, and for extracting the necessary thermodynamic properties of humid air, the CoolProp library [24] was linked to MATLAB. The input parameters for this program are the effectiveness saturation and the inlet conditions which are the weather conditions of the located city namely the outdoor dry temperature and the outdoor relative humidity. The effectiveness was calculated based on the pad thickness and the frontal air velocity. The outputs are the outlet temperature, the outlet relative humidity, the cooling capacity, the rate of the evaporated water, the electric power consumption of the fan and the COP. The program flowchart is shown in Fig.3. The climate in Morocco varies from city to another. Moroccan Agency for Energy Efficiency (AMEE) divides Morocco into six climatic zones [25]. Fig.4 shows the ambient temperature in each representative city of these six climatic zones. These weather data are generated by METEONORM software [26]. Furthermore, high temperatures that exceed 45 °C are recorded in several regions of Morocco during some periods of the year especially for the period of the wind of ‘chergui’ which is a hot and dry wind coming from the Sahara desert. It’s worth noting that the system operates in open loop without any recirculation. In fact, the moist is let out to the outdoor ambient, there is a separated inlet and outlet in the room. The recirculation of the air will cause a high relative humidity at the inlet, which consequently leads to a low performance of the pad. 4. Simulation results and discussion

The system performance was investigated during five successive months May, June, July, August, and September. The effect of pad thickness and the frontal velocity on the performance of the system is studied in each case of the aforementioned Moroccan cities. 4.1 Case of Marrakech Fig.3a shows the effect of the frontal velocity and the thickness of pad cooling on the cooling capacity. From this figure, the cooling capacity increases with increasing the thickness and frontal velocity. The air flow increases with increasing the frontal velocity which is proportional to the cooling capacity of the pad cooling Eq. (1) and by increasing the thickness of the pad cooling, the heat transfer area increases. Therefore, the increase of the cooling capacity is due to the increase in the air flow and the heat transfer area. The maximum value of the cooling capacity is obtained for a frontal velocity of 3 m/s and a thickness of the pad cooling of 300mm. Fig.3b shows the effect of the frontal velocity and the pad thickness on water consumption. As can be seen, the rate of water consumption increases with increasing the thickness and frontal velocity. The air flow increases with increasing the velocity and the mass transfer area increase with increasing the pad thickness which improves the rate of water evaporation. Therefore, the increase in the rate of water consumption is due to the increase of the air flow which is dry air and the mass transfer coefficient. The maximum value is obtained for a frontal velocity of 3 m/s and a pad thickness of 300mm. The minimum value is obtained for the frontal velocity of 1 m/s and a pad thickness of 100mm. This result confirms the experimental study of [19] concerning the effect of the frontal velocity and the thickness of the same type of pad cooling (CELdek 7090) on water consumption. Fig.3c shows the effect of the frontal velocity and the pad thickness at the rate of the operability of the pad cooling. From this figure, the rate of operability increases with increasing the thickness and by decreasing the frontal velocity. The rate of the operability depends on the temperature difference between the inlet and the outlet of the pad cooling which must be greater than 5°C. The increase in the thickness increases the heat transfer area and the decrease of the frontal velocity increase the residence time of the air in the cooling pad. Therefore, the increase in the rate of the operability is due to the increase of the heat transfer area and the decrease in the air flow rate.

Fig.3d shows the effect of the frontal velocity and the pad thickness on the coefficient of performance. As can be seen, the coefficient of performance decreases by increasing the thickness and the frontal velocity. By increasing the thickness and frontal velocity, the pressure losses and the air flow increases which lead to the electricity consumption of the fan to increase. The air flow and the pressure drops are proportional to the consumption of the fan Eq. (5). Therefore, the decrease of the COP is due to the increase in the electric consumption of the fan. The maximum value of the COP is obtained in the case of a frontal velocity of 1 m/s and a pad thickness of 100mm. An efficient pad cooling must have a high coefficient of performance, a lower water consumption rate, a high cooling capacity and a high rate of the operability. According to the results obtained in the city of Marrakech and for a passage of a pad cooling of 100mm to 150 mm, improvements by 12% in the cooling capacity and by 10% in the rate of operability are obtained while the water consumption increase by 12% and the COP decrease by 1.7%. Therefore, a cooling pad of 100mm with a frontal velocity of 1 m/s is recommended for the city of Marrakech. The same methodology is followed for other cities. The obtained results follow the same trend but with different values. 4.2 The results for the other cities Fig.5 shows the variation of the COP of the different size of the pad cooling during the considered period of May to September with different frontal velocities. As it can be seen, the maximum value of the COP is obtained for a frontal velocity of 1 m/s and a pad thickness of of 100 mm while the minimum is for a frontal velocity of 3 m/s and a thickness of 300 mm. Therefore, a pad cooling of 100mm which operates at frontal velocity of 1 m/s is recommended. The same figure shows the hourly variation of the inlet-outlet temperature of the recommended pad cooling. The outdoor temperature is lowered to an acceptable indoor temperature. The difference between the indoor and the outdoor temperature varied from a city to another this is due to the relative humidity which is a key parameter that influences and restricts the performance of the system.

The results of Agadir indicate that the COP range from 80 to14. For a pad cooling of 100mm, a high difference of temperature of 17.2 °C is obtained which lead to an outdoor temperature of 44°C to be lowered to 26.7°C. The rate of operability is 33.97%. In the city of Ifrane, the COP range from 165 to 28. The high difference of temperature obtained is 15°C which makes the indoor temperature equal to26°C instead of an outdoor temperature of 41°C. The rate of operability is 63%. In the city of Tangier, the COP varies between 90 and 15.The high difference of temperature is 9.5°C with an inlet temperature of 37°C and an outlet temperature of 27.5°C. The rate of the operability is 25.67% For Errachidia and Fez cities, the COP varies between 262 to 45 and 153 to 26 respectively. The highest difference of temperature is 17.79°C with an outdoor temperature of 42 and an indoor temperature of 24.2°C in Errachidia city and 16.17°C with an outdoor temperature of 45°C and an indoor temperature of 28.8 °C in Fez city. The rate of the operability is 96.18% and 54.47% for Errachidia and Fez cities respectively. Because of a high relative humidity occurs during this period in some cities which give a lowtemperature difference, therefore, the system will not operate. A minimum temperature difference of 5°C is adopted as a criterion for evaluating the operability of the system in this study. Fig.6 shows the variation of the rate of evaporated water of the recommended pad cooling during the considered period of May to September. As it can be seen, the maximum values occur in Errachidia and Marrakech cities which characterized by dry weather conditions. 18.4 kg/h is the maximum water consumption in Errachidia city while 17.8 kg/h is the maximum in Marrakech city. On average the rates of water consumption are 7.3 kg/h, 3.6 kg/h, 6.2 kg/h, 3.3 kg/h, 6.7 kg/h and 10.6 kg/h in Marrakech, Tangier, Fez, Agadir, Ifrane, Errachidia respectively. These results are in good accordance and confirm the performance of the system. Because water and humidity is the driving force of the direct evaporative systems in other words higher consumption of water means good cooling performance. Table 2 summarises the average water consumption, the average cooling capacity, the rate of operability based on the selected pad in each city.

The cooling capacity delivered by the system is between 2.3 kW and 7.4kW, which higher than the cooling capacity of mechanical compression air conditioner of 12000 Btu. The COP is higher than 80 which also higher than the COP of the mechanical compression air conditioner that not exceeds a value of 3. The rate of operability calculates the running time of the system during the considered period based on the minimum temperature of 5 °C. The lower obtained values occur in Agadir and Tangier which are two coastal cities where the humidity is higher. The higher value occurs in Errachidia which equals to 94.90% 5. Conclusions and perspectives In the present study, a physical model is introduced in order to study the performance of a direct evaporative cooling pad system under the climate conditions of six Moroccan cities. The effect of the frontal velocity and the thickness of pad cooling on the performance of this system are examined for each climatic zone. The results showed that the rate of water consumption and the cooling capacity increase with increasing the thickness and frontal velocity while the rate of operability increases with increasing the thickness and decreasing the frontal velocity. On the other hand, the coefficient of performance decreases with increasing the thickness and the frontal velocity. In fact, with a thickness of 100mm and a frontal velocity of 1 m/s, the simulation results show that the system’s performance differs from one city to another. For the COP, the maximum value is for the city of Errachidia followed by Marrakech, Tangier, Fez, and Ifrane with values greater than 80. On the other hand, the cooling power delivered by the system is greater than 2.3 kW with a maximum value of 7.4kW at Errachidia. In addition, the minimum evaporated water is about 3.3 kg/h is in Agadir and a maximum value of 10.6 kg/h is in Errachidia. From these results, it can be concluded that the system is suitable for the climatic conditions of Morocco. These results are also applicable to the residential sector. Our ongoing work focuses on the deployment and experimentation of the system in real testing cases and studies its performance in terms of energy consumption and comfort.

Acknowledgements The authors would like to express their appreciation to ‘‘IRESEN” for providing financial support to carry out this research

Conflict of Interest and Authorship Conformation: We have no conflict of interest to declare. All authors have participated in approval of the final version.

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Fig. 1: Schematic of (a) : the direct evaporative cooling process in the psychrometric chart, (b) an evaporative pad cooling

Start

Input Tin, ϕin, V,D

Manufacturer data sheet: L, H, η, P

CoolProp Software: Tiw=f(Tin, ϕin), hin(Tin,ϕin), Win=f(Tin,ϕin)

CoolProp Software: Tout=f(Tiw,Tin,η), hout=hin, Wout=f(Tout,hout), ϕout=f(Tout,hout)

Output Tout, ϕout, me, Ppad, Pfan, COP

End Fig.2: Flowchart of performance simulation of the evaporative pad cooling

Fig.3: Hourly dry bulb temperature variation in the six climatic zones of Morocco

Fig. 4: The effect of the frontal velocity and the thickness of pad cooling on: (a): the cooling capacity, (b): water consumption, (c): the rate of the operability and (d): the coefficient of performance

Fig. 5: COP and inlet-outlet temperatures in the Moroccan cities

Fig. 6: Water consumption in the Moroccan cities

Table 1: The effectiveness and the pressure drop for various pad thickness (Model: CELdek® 7090-15) Velocity (m/s)

D=100mm

D=150mm

D=200mm

D=300mm

82%

91%

94.80%

96.60%

0.5 1 1.5 2 2.5 3

5 Pa

5 Pa

5 Pa

6.4 Pa

80.40%

90.30%

94.20%

96.40%

9 Pa

13.3 Pa

17.8 Pa

25.7 Pa

78.60%

89.40%

93.70%

96.40%

20.4 Pa

29.8 Pa

40.6 Pa

59.8 Pa

76.40%

87.40%

92%

96.20%

35.5 Pa

54.8 Pa

73.3 Pa

106.5 Pa

74%

86.20%

91.70%

95.90%

56.9 Pa

83.3 Pa

116.5 Pa

165.6 Pa

71.20%

84%

90.40%

95.50%

81.5 Pa

124.7 Pa

166.9 Pa

200 Pa

Table 2: Performance indicators of the system City

me(kg/h)

P(kW)

COP

the rate of operability(ΔT> 5°C)

Marrakech

7.3

5.11

179

68.33%

Tangier

3.6

2.5

90

25.67%

Fez

6.2

4.3

153

54.47%

Agadir

3.3

2.3

80

33.97%

Ifrane

6.7

4.7

165

63%

Errachidia

10.6

7.4

262

96.18%