Comparative experimental investigation of novel organic materials for direct evaporative cooling applications in hot-dry climate

Journal Pre-proof Comparative experimental investigation of novel organic materials for direct evaporative cooling applications in hot-dry climate Pervin Abohorlu Doğramacı, Devrim Aydın PII:

S2352-7102(19)32833-5

DOI:

https://doi.org/10.1016/j.jobe.2020.101240

Reference:

JOBE 101240

To appear in:

Journal of Building Engineering

Received Date: 13 December 2019 Revised Date:

1 February 2020

Accepted Date: 1 February 2020

Please cite this article as: P.A. Doğramacı, D. Aydın, Comparative experimental investigation of novel organic materials for direct evaporative cooling applications in hot-dry climate, Journal of Building Engineering (2020), doi: https://doi.org/10.1016/j.jobe.2020.101240. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.

CRediT authorship contribution statement

Pervin Abohorlu Doğramacı: Data curation, Formal analysis, Investigation, Methodology, Software, Validation, Visualization, Writing – original draft. Devrim Aydın: Conceptualization; Formal analysis, Investigation, Methodology, Project administration, Resources, Supervision, Visualization, Writing – review & editing.

Comparative experimental investigation of novel organic materials for direct evaporative cooling applications in hot-dry climate Pervin Abohorlu Doğramacı1* and Devrim Aydın2 1

Cyprus International University, Faculty of Fine Arts, Design and Architecture, Department of Interior Architecture, Haspolat-Lefkosa, Mersin 10, Turkey 2

Eastern Mediterranean University, Faculty of Engineering, Department of Mechanical Engineering, Gazi Magusa, Mersin 10, Turkey

⁎ Corresponding author: E-mail address: [email protected] (P. Abohorlu Doğramacı)

Abstract In the last decade, energy consumption for air conditioning applications has been dramatically rising as a result of the growing global population and increasing comfort demand. Consequently, direct evaporative cooling (EC) technology is emerging as an alternative to vapour compression air conditioners due to its lower environmental impacts, less energy consumption and lower operating costs. EC pad is one of the most important components of such systems and in present day corrugated cellulose paper pads, produced through industrial processing, are mostly used. Despite such materials are efficient, production of them requires energy and advanced machinery resulting in associated costs. Therefore, new organic EC materials, which are locally available, abundant and efficient are needed for making EC applications economically and environmentally more viable. Accordingly, in this study five new natural porous materials including eucalyptus fibres (EF), ceramic pipes (CP), yellow stone (YS), dry bulrush basket (DBB) and Cyprus marble (CM) were experimentally investigated for EC applications in hot-dry climate (RH<20%). Within the study, materials were tested in a wind tunnel at different air velocities varying in the range of 0.1-1.2 m/s. According to the study results EF and CP were found as the most promising candidates with effectiveness values varying in the range of 72-33% and 68-26% for the air velocities between 0.1→1.2 m/s. In contrast to the cooling effectiveness, cooling capacities showed an increasing trend with the rise of air velocity and they were found in the range of 0.13→0.71 kW and 0.12→0.55 kW for EF and CP respectively. Among the other investigated materials, YS was found competitive with these materials with effectiveness and cooling capacity in the range of 1

46%→22% and 0.08→0.48 kW. On the other hand, DBB and CM showed relatively poor performances where effectiveness and cooling capacity obtained with them were below 35% and 0.45 kW respectively. Study results also demonstrated an inversely proportional correlation between cooling capacity and effectiveness. According to this correlation, optimal mass flow rate for system operation was found as 0.063 kg/s, which was valid for all tested materials and which is only the function of inlet air temperature and relative humidity. Such correlation will be useful for performance optimization in EC applications. Keywords: direct evaporative cooling material, wind tunnel, porous ceramic, direct evaporative cooling, organic material Contents Nomenclature ......................................................................................................................................... 3 1. Introduction ....................................................................................................................................... 4 2. Materials and Methods ..................................................................................................................... 9 2.1 Instrumentations and Experimental Materials ............................................................................... 9 2.2 Experimental Procedures ............................................................................................................. 13 2.3 Thermodynamic analysis of evaporative cooling process ........................................................... 15 3. Results and Discussion .................................................................................................................... 17 3.1 Temperature variations ................................................................................................................ 17 3.2 Relative humidity variations........................................................................................................ 18 3.3 Water evaporation rate variations................................................................................................ 20 3.4 Cooling effectiveness variations.................................................................................................. 22 3.5 Average performance parameters ................................................................................................ 23 4. Conclusion ........................................................................................................................................ 26 References ............................................................................................................................................ 27

2

Nomenclature m2 kJ/kg°C

Cp

= cross section area = specific heat at constant pressure

Cpda

= specific heat of dry air

kJ/kg°C

Cpv

= specific heat of water vapour

kJ/kg°C

COPs

= system coefficient of performance

---

h

= enthalpy

(kJ/kg)

ℎ

= specific enthalpy of humid air

(kJ/kg)

h1

= specific enthalpy of inlet air

(kJ/kg)

h2

= specific enthalpy of outlet air

(kJ/kg)

lho

= heat of evaporation of water

kJ/kg

= mass flow rate of air

kg/s

= humidity ratio

kgwv/s

= sensible cooling capacity

kW

= temperature

°C

T

w

= inlet temperature

°C

= outlet temperature

°C

= inlet wet bulb temperature

°C

= velocity of air

m/s

= electric input power

kW

= absolute humidity of air

kgwv/kgda

= outlet absolute humidity

kgwv/kgda

= inlet absolute humidity

kgwv/kgda

Abbreviations ASHRAE

= American Society of Heating, Refrigerating and Air-Conditioning Engineers = cooling capacity

kW

CM

= Cyprus marble

COP

= coefficient of performance

CP

= ceramic pipes

DBT

= dry bulb temperature

DBB

= dry bulrush basket

°C 3

EF

= eucalyptus fibres

ER

= evaporation rate

EC

= evaporative cooling

RH

= relative humidity

OOC

= optimal operating conditions

YS

= yellow stone

WBT

= wet bulb temperature

gr/s

%

°C

Greek symbols ∆

= temperature difference

°C

= cooling effectiveness (Eff)

%

= density of air

kg m−3

1. Introduction Among the global total, buildings in European Union (EU) are responsible for 40% of the CO2 emissions and energy consumption. By 2020, the greenhouse gas emissions are aimed to be reduced by 20%. Therefore, fossil fuel sourced energy consumption should be reduced and renewable

energy

usage

should

be

enhanced

in

order

to

supply

safe

energy, technological growth and to provide employment opportunity [1-2]. In the last decade, with the increasing population and comfort demand, air conditioning became one of the major energy consuming applications. Vapour compression air conditioners dominate the cooling sector due to their high efficiency, practicality and technological maturity [3]. On the other hand, market for evaporative coolers is expected to show a sharp growth to £20 billion in next five years, whilst it was £5.5 billion in 2013 [4]. This is mainly due to the reason that the EC uses natural fluids; water and air, therefore has lower environmental impacts when compared to vapour compression systems operating with synthetic refrigerants. Besides the higher performance especially in dry climates, low initial and operating costs and simple configuration are other promising aspects of EC technology.

4

Considering that the buildings are responsible for nearly half of the global energy consumption, it is vital to enhance the use of renewable sourced heating, cooling and electric generation systems in building sector for achieving sustainable development. Besides being a practical cooling method, EC is also an energy-efficient and low-cost cooling technique, therefore have been utilized since the ancient centuries. In EC operation, air becomes cooler when it contacts and evaporates the water, thereby creates cooler buildings and urban spaces. In this context, several passive and active EC systems have been recently investigated for different applications. Oropeza-Perez [5] simulated passive EC performance under Mexico City climate conditions. It was found that indoor temperature drop of 8 °C could be achieved by implementing this cooling strategy. In another study, Mahmood et al. [6] demonstrated that EC could be a low cost opportunity for short term storage of agricultural products. Oliveira et al. [7] suggested that combining EC with finned tube heat exchanger could enhance the heat transfer performance in fluid cooling applications. Luo et al. [8] proposed indirect-direct evaporative cooling and ultrasonic atomizing humidification units for environmental control of historical sites. Rio et al. [9] investigated passive EC method integrated with louver to improve the outdoor microclimate. Shao et al. [10] experimentally studied the integrated EC with loop thermosyphon. Esparza L. et al. [11] tested the potential of wet fabric device as an evaporative cooling roof pond. It was concluded that proposed wet fabric device could be used under different climate conditions to reduce the building cooling load. Kiyaninia et al. [12] investigated solar photovoltaic-based direct evaporative air-cooling system. Alharbi et al. [13] tested the novel hollow porous ceramic cuboids-heat pipes for EC applications. Results showed that the wet bulb effectiveness and the dew point effectiveness were 1.05 and 0.73, respectively. Sellami et al. [14] developed a new mathematical model for analysing the performance of a porous evaporative cooler. It was found that increasing the thickness and porosity of ceramic layer enhances the cooling performance. The EC pad material plays a vital role for increasing the cooling efficiency of the EC system. The desired evaporation material should have high water holding capacity, high porosity, large surface area, low weight, high thermal conductivity, also should be low cost, noncorrosive and easily accessible. In the last decade research is ongoing to develop new pad materials for improving efficiency and cooling potential of EC systems. Metal, wood, plastic, ceramics, CELdek pads, cellulosic

5

pad, aspen pad and rigid medium pad are mostly investigated evaporation materials for EC systems [15-16]. Nowadays, manufacturers are using pads made from cellulose based paper or plastic fibres in order to obtain more efficient EC system for extreme climatic conditions and for different applications. Despite these materials are efficient, they are complicated therefore difficult to manufacture also expensive due to the fabrication and processing costs. Therefore, locally available materials can be promising alternatives for EC system. Since local materials are easy to find and abundant in most locations; they could be a low cost opportunity to be utilized in less developed hot countries. Environmentally friendly porous organic materials such as different natural stones and porous agricultural materials with good water holding capacity and long life span could be a new direction of research on EC technology towards achieving sustainable cooling in buildings. In Figure 1, various previously investigated EC materials are categorized.

Polymer hollow fibre [23]

PVC [24] Corrugated plastic sheet [25] Aspen pad [17] Wood chips [18] Rice husk [19] Vermiculite [20] CELdek pad [21] Kraft paper [22] NSSC paper [22]

Organic-based materials

Plastic-based synthetic materials

Porous metal plate[26] Metal mesh [26] Sintered metal [26] Metallic Foams [26] Wicked metal [26] Metal wool [27] Metallic-based materials

Coconut coir [28] Eucalyptus fibres [29] Palash fibres [30] Jute [31] Wood fibres [32] Cotton fibres [33] Luffa [34] Gunny sack [35] Natural fibre-based materials

EC Materials

Figure 1. Classification of EC materials [17-39]

6

Bulk Charcoal [17] Pumice stone [36] Siliceous shale [37] Volcanic ash [37] Porous ceramic [38] Roof Brick [39]

Stone-based materials

Organic materials are mainly obtained from natural resources therefore, they are considered as environmentally friendly and sustainable matrices to be used in EC applications. The suitable organic EC materials including aspen, rice husk, vermiculite…etc. can hold large quantities of water for evaporation. Nada et al. [40] conducted experiments to evaluate the performance of a new EC pad made of cellulose papers in bee-hive structure resulting 84% saturation efficiency. Dhamneya et al. [41] conducted an experimental investigation to examine the performance of aspen fibers as cooling media. It was concluded that the highest saturation efficiency was 97% for triangular cooling pad configuration. Rong et al. [42] used cellulose pad to investigate the performance of evaporative cooling in the wind tunnel. As a result, the variation of temperature drop was obtained when the air velocity was the lowest speed of 0.25 m/s. The main disadvantages of such materials are the low thermal conductivity and low heat capacity. As an alternative material, plastic-based synthetic materials could be used for EC as they are light and cheap compared to commercial materials. Regarding the hollow fibre as a commercial material; although it is not easy to manufacture, it has large surface area which is essential for heat and mass transfer [23-25]. Chen et al. [23] concluded that the wet bulb effectiveness of the EC system with hollow fibre bundles was between 0.3 and 0.45. Martinez et al. [43] carried out an experimental investigation on EC performance of plastic mesh. It was concluded that the maximum cooling efficiency was 80.5% with 250-mm thick pad. The metal based evaporation materials are mostly made from aluminium, copper and steel. They can be easily shaped as plate however, keep less water due to the less capillary force. In order to increase capillary force of the material, the porous structure is created (i.e. through iodization process) instead of flat surface [26, 44]. Khond et al. [45] investigated four different cooling materials including stainless steel wire mesh pad. The results showed that the lowest cooling efficiency was 50% for stainless steel wire mesh compared to the highest wood wool (87.5%). Kovacevic and Sourbron [46] studied the numerical model of heat and mass transfer for metallic-compact direct evaporative cooler. The study showed that higher cooling efficiency is obtained when the EC pad thickness is >90 mm. Natural fibre-based materials could be used as potential EC matrices since they have high porosity, penetrability and water holding capacity. Although they are cheap and light weight, the growth of bacteria could be occurred due to the longer contact time of natural fibre materials with water. In order to prevent the bacterial growth, the material should be cleaned and sterilised [26]. Also, by modification of the pore size of fibre, the bacteria and fungi 7

penetration could be prevented [23]. Abohorlu Doğramacı et al. [29] studied the performance of EF as a new EC material. Results showed that; at low air velocity, cooling efficiency was 71% with 11.3°C air temperature difference (ΔT). Akintunji et al. [47] experimentally investigated the coconut coir fiber as cooling media, resulting with higher cooling efficiency (64.7%) by reducing the air mass flow rate. The stone-based EC materials are durable against atmospheric conditions and can resist to chemical degradation particularly when they exposed to outdoor environmental conditions. Due to the porous structure of some organic stones such as charcoal, siliceous shale, porous ceramic, volcanic ash, porous clay and pumice, they can hold large quantities of water. Charcoal is a carbonaceous material, which includes 85 to 98% of carbon and which has water absorption potential [17]. Due to the higher capillarity of volcanic ash, it can absorb more water [37]. Khater and Ezzat studied the water absorption capacity of engineered stones based geopolymer composites. Percentage water absorption capacities of the investigated materials were found between 6.29-6.43% [48]. Nayak et al. [49] investigated the impact of water spray over asphalt based roof surfaces on the building thermal load. It was found that spraying 5 kg of water at a uniform rate for 40 minutes can reduce the cooling load of a building by 10%. Previous studies investigating the experimental performance of different evaporation materials are summarized in Table 1.

Table 1. Previous studies on EC materials

Researcher

Material

∆T (°C)

Cooling capacity (kW)

Laknizi (2019) [50]

Cellulosic pad

Wijaksana et al. (2018) [51]

Gunny sack

Sudprasert and Sankaewthon, (2018) [52]

Porous material with Rice husk

1.0-2.7

-

Warke and Deshmukh (2017) [53]

Cellulose pad (150mm)

1.0-2.7

1.6

Abohorlu Doğramacı et al. (2019) [29]

Eucalyptus fibres

6.6-11.3

0.1-0.6

8

4.6

2.3-7.4 4.6

Chen et al., (2018) [23]

Polymer hollow fibre

3.6-5.2

0.1-5

Harby and Al-Amri (2019) [54]

Corrugated papers

-

8.3

Clay plates covered by jute fiber film

14.6

-

Abdullah et al. (2019) [55]

In the literature, different studies were conducted to examine the performance of several EC pads as presented above. In these studies, mostly plastic and metallic based materials, produced through industrial processing, have been investigated. In order to make future EC systems truly renewable and sustainable, new materials which are low-cost, efficient, environmentally-friendly and easy to find are required. In this context, authors have performed preliminary investigations on EF previously [29]. Based on the obtained promising results with that material, new natural materials including yellow stone (YS), Cyprus marble (CM), ceramic pipes (CP) and dry bulrush basket (DBB) were identified and a comparative investigation between the selected candidates was proposed within this study. These local materials have been experimentally investigated to compare their performances (for direct EC applications in hot-dry climate) in terms of air temperature difference, relative humidity, absolute humidity ratio, cooling capacity, water evaporation rate, cooling efficiency and COP. It is worth mentioning that aforementioned organic EC materials have not been previously investigated in the literature for EC applications. In addition, presented study demonstrates new important correlations for performance optimization of EC systems, which is also missing in the literature. These correlations could be useful for determining optimal mass flow rates in EC applications.

2. Materials and Methods 2.1 Instrumentations and Experimental Materials This study used cross section of 292 mm width and 292 mm height open wind tunnel (HM170) measurements under controlled environmental conditions. The schematic diagram of the wind tunnel is presented in Fig. 2. Firstly, a heater with the capacity of 2 kW was located in the air blower section. The air is heated to 35 °C, in order to mimic the climate. Subsequently, uniform airflow was provided by the honeycomb structure which is located at

9

the entrance of the duct. The testing materials were placed in the measuring section. The water supply method was developed to continuously provide water on testing materials.

1- Inlet contour 2- Flow straightener 3- Nozzle 4- Measuring section 5- Testing material 6- Water tank 7- Display and control unit 8- Diffuser 9- Switch cabinet 10- Inclined tube manometer 11- Axial fan

Figure 2. Schematic diagram of the open wind tunnel and the specimen

During the testing of the materials, dry and wet bulb temperatures, air velocity and relative humidity were measured by using sensors which monitor the environmental condition of the testing area. Table 2 lists the types of sensors used in the experimentation with their accuracies. The sensors have been calibrated before the testing was started. Table 2. Characteristics of experimental sensors Sensor type

Sensor model

Sensor accuracy

Specification range

Temperature

Type-K Thermocouple

± 1.5 °C

-50 ~ + 250 °C

Relative humidity

Kestrel 4000 weather meter

±3%

5 ~ 95 %

WBT

Kestrel 4000 weather meter

± 2 °C

-29 ~ 70 °C

Air flow

Testo 405 V1 anemometer

± 5 % ± 0.1 m/s

0 ~ 10 m/s

Relative humidity

Metrel 6201 multinorm

±2%

0 ~ 60 %

Temperature

Metrel 6201 multinorm

± 0.2 °C

20 ~ 60 °C

Water temperature

Multi-thermometer

± 1 °C

-50 ~ + 300 °C

10

The errors and uncertainties of the experimental results were calculated based on the propagation of error of formula as explained by Moffat (1988) and Taylor (1997) [56, 57]. For determining the uncertainties, the source of errors (temperature, relative humidity…etc.) from the experimental measurement were determined. The general formulation for analysing the uncertainty is presented in Eq. (1):

[1]

Where

represents the overall uncertainty of the calculated parameter (i.e. cooling

efficiency) and

are the set of parameters which are directly measured (i.e.

temperature, relative humidity) during the experiments. Here,

are describing

the errors in the measurement of such parameters. In Eq. 1, each measured variable affecting the final calculated parameter is subject to uncertainty. For calculating the uncertainty of each variable, the partial derivative of

with respect to

is multiplied by the uncertainty value

for that variable. Finally, by summing up the square of the uncertainty of each variable and getting the square root of the total gives the total uncertainty of the calculated parameter. According to the performed uncertainty analysis, overall uncertainties of the temperature and relative humidity measurements also the uncertainty in the calculation of cooling efficiency is presented in Table 3. Table 3. Errors for uncertainty analysis Variable

Error

Temperature

Cooling

Relative

difference (°C)

efficiency (%)

humidity (%)

1.3%

7.1%

8.3%

In the performed study, EF, CP, YS, DBB and CM were used as EC materials (Figure 3). Prior to the testing, all materials were prepared to make their geometry and dimensions suitable to be placed inside the wind tunnel. Since they are different types of materials, different preparation process has been applied for each material. Firstly, CM was cut into 11

shapes for fitting in the tunnel. Following that, CP were shaped and dried. They were put into a kiln in order to be hardened. When they were taken from the kiln, they soaked into water until fizzing stop. The wooden structure was then formed to hold them up during the test. Afterwards, the porous YS was cut to fit in the tunnel and small holes were then drilled. The grooves were also created on the surface of the stone, in order to increase its surface area, thereby to enhance its water absorption capacity for EC process. Subsequently, EF were collected from the barks of the eucalyptus tree. The fibres were then separated and embedded into a mesh wire frame for testing. The last material, DBB is a natural fibre collected from the farms, river or dams. It is traditionally used to carry and store the food. In the manufacturing process, the bulrushes are firstly dried out and they are then woven by hands to make baskets. Therefore, they were used as basket in the experimentation. During the experimentation, the testing materials should be kept wet by adding water. With this purpose, a water pipe was placed on top of the materials (Fig. 4), which was connected to a 20 W water pump inside the water tank. When water from the pipe was dripped onto the material, the excess water was collected inside the water tank and recirculated back.

(a) Cyprus marble

(b) Ceramic pipes

(d) Yellow stone

(c) Dry-bulrush basket

(e) Eucalptus fibres

Figure 3. The local testing materials

12

2.2 Experimental Procedures In the experiments, meteorological parameters were adapted as an average summer day conditions. Therefore, the performances of the materials were studied by setting the inlet air temperature to around 35-36°C and the relative humidity to around 20%. The moistened cotton was wrapped around the thermocouples to record the wet bulb temperature (WBT). RH was also measured by a microclimate probe. Air velocity was measured by anemometer which was placed at the back side of the testing material. A 2 kW heater was positioned in front of the blower in order to increase the inlet air temperature to the desired level. Figure 4 and 5 show the schematic diagram and view of the experimental testing rig.

Cool air T2

2

T2 < T1 w1 < w2 RH1 < RH2

w2 RH2 h2

Hot dry air T1

1

w1 RH1 h1

a) Water tank

T1

T2

T4

T3

b) Figure 4. a) Concept of EC with porous materials and b) measuring points of sensors 13

Figure 5. General view of the experimental testing rig

The tests were started when the testing materials were fully saturated with water. In addition to this, saturation of material should also be in steady conditions in order to ensure that it is kept wet during the test. Therefore, the test measurements were taken into consideration after 1 hour of initial start to ensure that system is operating steadily. In each experiment, measurements were recorded in every 5 minutes and totally 19 readings were recorded by all sensors for 90 minutes of testing duration. Air velocity was set at 0.1, 0.3, 0.6, 0.9 and 1.2 m/s to compare the performance of materials in terms of temperature difference, relative humidity, cooling efficiency, cooling capacity, absolute humidity, evaporation rate and COP at different air velocities. In order to achieve the same experimental conditions for results comparison with different EC pad configurations, measured outlet air velocities were considered in the calculation of air mass flow rate in each testing. Thereby in the thermodynamic performance analyses of different pads, only the rate of air flow passing across the EC pad was taken into account. As an example, for analysing the different EC pad performances at 0.1 m/s wind velocity, fan speed was adjusted at such rate that at the exit of the wind tunnel, measured velocity was reaching to 0.1 m/s during the testing of all EC materials. The same procedure was applied in all experiments with different air velocities for each material. This method allows making the analyses more accurate by consideration of the pressure drop across the system, which occurs due to the porosity of EC pads and their resistance to air flow. Such method of using exit 14

velocity as the flow velocity was also utilized in previous studies [58]. Furthermore, during the experiments, exit velocity was recorded continuously and it showed a minimal fluctuation, differing at a range of ±%5 than the set value. Therefore, in the analyses the exit velocity was considered as a constant value at the set condition. Repeatability and reliability are also crucial for the experimental studies. Therefore, each experiment was run three times in order to ensure near steady state conditions and to obtain correct readings. All tests were conducted in almost at the same environmental conditions.

2.3 Thermodynamic analysis of evaporative cooling process The EC performances of the tested materials were evaluated based to the equations given below: The temperature drop of air across the EC pad is obtained with Equation 2; ∆ =

−

[2]

The cooling efficiency (saturation effectiveness) is calculated via Equation 3 as described by ASHRAE (2001) [59]:

(

= (

) " ) × –

100

[3]

Where; T₁ and T₂ are dry bulb temperatures of inlet and outlet air, while T΄₁ represents the wet bulb temperature of inlet air. In the performed study T1 and RH1 were set to 35 °C and 20% corresponding to T΄₁=19.1 °C and T₂ variations were measured during the experiments to calculate the effectiveness for each material. In order to determine the cooling capacity and COP variations during the EC process, mass flow rate of air across the EC pad should be determined. It was calculated by multiplying the measured air velocity at the outlet of the wind tunnel, V, with the average density of air, , and cross section area,

, of material sample as shown in Equation 4. In the experiments, air

density was varied between 1.14-1.20 kg/m3. Accordingly, average air densities were calculated based on the measured temperatures in each experiment. Obtained average densities were than used for determining the air mass flow rate. =

[4]

15

Sensible cooling capacity is obtained based the multiplication of air mass flow rate, specific heat capacity and obtained temperature drop across the EC pad as given in Equation 5: =



(

Where,

(

−

)

is the mass flow rate and

[5] (

is the specific heat capacity of air. During the

experiments air specific heat capacity varied between 1.004-1.006 kJ/(kg.K) due to the temperature variations of air in the range of 20-35 ˚C. Accordingly, average air specific heat capacity was determined and used in the analyses based on the calculated average air temperature in each experiment. The enthalpy of humid air is calculated via Equation 6: ha = 1.006.T+ w.(2501+1.85.T)

[6]

Where; T and w are temperature (°C) and absolute humidity (kgH2O/kgda) of dry air. For the temperature range of (-10)–(40) °C, average specific heat of dry air (Cpda) and water vapor (Cpv) are 1.005 kJ/kg °C and 1.85 kJ/kg °C respectively. Water latent heat of evaporation (Lho) is 2501 kJ/kg. Absolute humidity w, is the amount of water vapor per unit kg of dry air and could be determined with Equation 7 [60] as a function of T and RH of air: 17.62 - ./ - 6.112 - 1-2 3243.12 + 7 100% = 216.7 - [7] 273.15℃ + Coefficient of performance (COP) of the EC process is described as the ratio of obtained cooling capacity,

, to the electric power input,

, used during experiments. The total

power input is obtained by calculating the sum of the power consumption of fan and water pump used in the experimental system. >

<= = @?

[8]

A

Water evaporation rate in EC system is determined by multiplying the mass flow rate and absolute humidity difference of air (∆w) as illustrated in Equation 9; =

(



−

)

[9]

16

3. Results and Discussion 3.1 Temperature variations In Figures 6(a-e), outlet air temperature variations for the testing of different EC materials are presented. Peak value of y-axis (35 °C) represents the inlet temperature of the air. As seen from the figures, in all experiments, achieved ∆T considerably decreased with the increase in air velocity. On the other hand, among all tested materials, EF showed superior performance where outlet temperature varied between 23.6 – 29.7 °C for the air velocity range of 0.1→1.2 m/s. For the same velocity range, outlet air temperature between 24.2 →30.9 °C was obtained with CP. (a)

(b)

(c)

(d)

(e)

17

Figure 6. Outlet air temperature variations for the testing of different EC materials Testing results showed that; EC performance of EF and CP are close to each other and both materials are capable of providing ∆T>10 °C at low air velocities (vair = 0.1 m/s). YS, which is a natural porous rock, provided competitive performance with EF and CP at high air velocities (> 0.9 m/s). However, at low air velocities, it is observed that EC capacity of YS is less promising compared to EF and CP. For the increasing order of air velocity, outlet temperature varied between 27.6 →31.4 °C with the use of YS as EC material. Other two candidate materials, DBB and CM showed poor EC performances, due to their limited porosity and evaporation capacity. Furthermore, impact of air velocity on temperature drop is found very low with the use of these materials. For DBB and CM, outlet temperature varied between 29.5 °C→31.6 °C and 29.9 °C →32.7 °C. The drop of the ∆T, with the velocity increase between 0.1→1.2 m/s, was also found 6.2 °C, 6.6 °C, 3.8 °C, 2.1 °C and 2.3 °C for EF, CP, YS, DBB and CM respectively. Results showed that the most important characteristics expected from the EC materials are; (i) high surface area for fast water evaporation, (ii) spongy nature / high porosity for high water absorption capacity and (iii) good thermal conductivity for effective heat absorption from the air. Among the investigated materials, EF, despite having low conductivity, it is fibrous therefore has high surface area also it has large water absorption capacity resulting in good EC performance. CP has good thermal conductivity, however less evaporation surface area and less water absorption capacity when compared to EF. On the other hand, YS has a porous structure but its surface area and water holding capacity are lower. That material was used in the block form by drilling holes for air passage in this study. In future studies, it could be crushed into small pieces to enhance the surface area thereby increase the water evaporation rate. DBB and CM have high surface area, however, due to their limited porosities; these materials showed very limited water absorption capability therefore achieved temperature drops were low when compared to EF, CP and YS. 3.2 Relative humidity variations In EC applications, achieved temperature drop (See: Figure 6) is related with the humidity increase of air across the EC pad. Obtained outlet air RH variations are confirming this (See: Figure 7 (a-e)). In all experiments, inlet air RH was set to 20% and outlet air RHs were measured to evaluate the RH variations across the EC pad. With the use of EF and CP, outlet RH exceeds 60% at air velocity of 0.1 m/s, whereas for YS, it fluctuated between 45-50%. 18

For the DBB and CM, outlet RH was below 40%, explaining the poor performance of these materials. (a)

(b)

(c)

(d)

(e)

Figure 7. Obtained outlet air RH variations

It is evident from the results that EF has the highest humidification performance among all tested materials at low air velocities. However, at high air velocities, obtained outlet humidity was nearly 30% and in close approximation for all materials. Similar characteristics were also obtained for temperature variations as previously presented in Figure 1. Therefore, it could be

19

concluded that the results are confirming the linear psychometric correlation between the relative humidity and temperature changes during the EC process.

3.3 Water evaporation rate variations Calculated water evaporation rates at different air velocities are presented in Figures 8 (a-e). As seen from the figures, evaporation rate showed an increasing trend with the increase in air velocity for all materials. For EC and CP, it was nearly steady around 0.05 g/s at air velocity of 0.1 m/s. In contrast, at highest velocity of 1.2 m/s, large fluctuations of evaporation rate were observed between 0.22-0.35 g/s and 0.20-0.27 g/s for EF and CP respectively. (a)

(b)

(c)

(d)

(e)

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Figure 8. Calculated water evaporation rates at different air velocities

Results showed that; water evaporation capacities of EF and CP are same at low air velocities but at high air velocities EF provides higher evaporation rate when compared to CP. The YS demonstrated very close evaporation rates (0.15-0.25 g/s) for the air velocity range of 0.6-1.2 m/s. At lower air velocities a gradual drop of evaporation rate is observed for YS. Other tested materials, DBB and CM, provided low evaporation rates at all velocities in comparison with EC, CP and YS. For the air velocity of 0.1 m/s, evaporation rate was in the range of 0.02-0.03 g/s for both DBB and CM. At highest air velocity (1.2 m/s), it varied between 0.16-0.22 g/s and 0.14-0.17 g/s for DBB and CM respectively. In the experiments, the rate of water dispatch was kept same for all materials to ensure that all tests were performed under similar operating conditions. Despite the rate of water supply was kept equal for all materials, the water consumption (evaporation) rate was found different for each material. Furthermore, an increasing trend of water evaporation rate was observed with the increasing air velocity for all tested materials. For EC, CP, YS, DBB and CM, average hourly water consumption rates were between 0.196-1.075 kg/h, 0.184-0.840 kg/h, 0.1270.728 kg/h, 0.094-0.686 kg/h and 0.087-0.561 kg/h for the air velocity variation between 0.1 1.2 m/s. Experimental results showed that EF provides the highest water consumption rate among all tested materials. However, it was also found that, this material provides the highest cooling capacity in the range of 0.10 – 0.71 kW. This outcome demonstrates the direct relation between the water consumption (evaporation) rate and cooling capacity in EC applications. In EC process, achieving high cooling capacity with minimum water consumption is a desired condition. In this regard, the ratio of cooling capacity to water consumption rate was also analysed for the tested materials. It was found that, cooling capacity to water consumption rate ratio shows a decreasing trend with the increasing air velocity (0.1→1.2 m/s). However, the range of variation was found similar (2.3 → 2.4 kJ/g) for all tested materials. This outcome demonstrates that, increase of the air velocity and water supply rate enhance the cooling capacity at a certain level for all investigated materials. However, increasing these parameters also enhances the rate of water consumption per unit kW of cooling achieved. Therefore, air velocity and water supply rate should be optimized in any particular EC design for reducing the amount of water consumption while achieving high cooling capacity at the same time. 21

3.4 Cooling effectiveness variations In EC applications one of the most important performance measures is the cooling effectiveness. It is the ratio of the obtained temperature drop to the theoretical maximum achievable temperature drop across the evaporation pad. During the experiments, highest effectiveness values were obtained in the range of 70-80% with the use of EF at air velocity of 0.1 m/s. As expected, with the increase of air velocity, effectiveness gradually dropped for all tested materials. For EF, CP, YS, DBB and CM highest and lowest effectiveness values were obtained as 76→23%, 71→22%, 49→18%, 35→19% and 34→15% (Figure 9). (a)

(b)

(c)

(d)

(e)

Figure 9. Effectiveness variations at different air velocities 22

It was obtained that; EC and CP provide high effectiveness values at low air velocities. However, it is also clear that there is still room for improvement for achieving effectiveness values close to 100% with the use of these materials. In this context, beside the material performance, the design of EC system, water supply rate, density, porosity, configuration and dimensions of evaporation pad also the air velocity have vital importance. As the scope of this research was to compare the performance of new evaporation materials, performance optimization of the EC system is beyond the scope of the study and it could be investigated as a further work. 3.5 Average performance parameters Based on the experimental results presented in Figures 6-9, average values of the performance parameters at different air mass flow rates were calculated for the tested materials and presented in Figures 10-12. Here it should be noted that the illustrated mass flow rate variations in the range of 0.0113 →0.1357 kg/s corresponds to the velocity variations between 0.1→1.2 m/s. Figure 10 shows the obtained average ∆T over 90 min testing duration for different materials at different flow rates. As clearly seen, for all materials, highest temperature drop of air is obtained at mass flow rate of 0.0113 kg/s. With the increasing mass flow rate, average ∆T showed a decreasing trend where for EF, CP, YS, DBB and CM it varied between 11.3→5.21 ˚C, 10.7→4 ˚C, 7.3→3.5 ˚C, 5.4→3.3 ˚C and 5.0→2.72 ˚C respectively. Among all tested materials, largest average ∆T variation is observed with EF and CP, where YS, DBB and CM were found comparatively steadier. EF and YS provided ∆T>10˚C at lowest mass flow rate, however at highest mass flow rate, ∆T was lower than 6 ˚C for both materials. The reason of that is; with the increasing air mass flow rate, the evaporation surface area becomes insufficient to evaporate the necessary amount of water. Thereby temperature drop of air across the EC pad remains low. In the experiments, the volume of the materials was nearly 0.8 × 10-3 m3. While this volume was found sufficient to operate with mass flow rate up to 0.02 kg/s, for mass flow rates >0.1 kg/s, volume of EC pad should be >2 ×10-3 m3 to achieve ∆T>10 ˚C during the EC process.

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Figure 10. Average ∆T variation at different mass flow rate

Despite achieved ∆T is important in EC applications, obtained cooling capacity and cooling effectiveness are also expected to be high. However, obtained results showed that cooling capacity and cooling effectiveness are indirectly proportional to each other and in order to obtain optimal operating conditions (OOC), mass flow rate should be optimized. Figure 11 shows an important correlation between cooling capacity and cooling effectiveness. As seen, with the increasing mass flow rate of air, cooling capacities show an increasing trend whilst cooling effectiveness values gradually decrease for all tested materials. The highest average cooling capacity (0.71 kW) and effectiveness (0.72) were obtained with EF at mass flow rates of 0.1357 kg/s and 0.0113 kg/s respectively. Both cooling capacity and effectiveness varies between 0→1 and the mass flow rate, where the value for both parameters intersecting is only the function of inlet air conditions (T, RH). This value is also independent than the EC design, configuration or the utilized EC material. As seen from the Figure 11, for the inlet conditions of air (35˚C, 20%) considered in this study, intersection point of cooling capacity and effectiveness for all materials is 0.063 kg/s (mass flow rate). Such correlation could be useful for determining the optimum mass flow rate, cooling capacity and effectiveness value in any particular EC application. In addition, the correlation between these three parameters could be useful for developing operational control methods and algorithms for EC systems. For instance, an automatic fan speed controller could be integrated to EC system, that could autoadjust the air mass flow to an optimum rate by sensing the instant temperature of the conditioned environment via temperature sensors. Thus, both reduction in energy usage and 24

ease of operation could be achieved. In addition, produced cooling output and effectiveness of the system could be available to the end user via a display screen. This could increase the awareness of the end users on their energy consumption patterns thereby could enable increasing energy efficiency in residential and commercial applications.

Figure 11. Obtained cooling effectiveness and cooling capacity trends

Besides the correlation between cooling capacity and effectiveness, another relation is also obtained between ∆w and COP variations with the changing mass flow rate. As depicted in Figure 12, with the increase in mass flow rate, ∆w shows a decreasing trend while cooling COP gradually increases for all materials. Both values intersecting at mass flow rate of 0.04 kg/s. At that flow rate, COP (-) and ∆w (g/kg) were highest for EF and CP at the value of 3.8. At higher flow rates, COP becomes steadier while ∆w linearly drops for all tested materials. The highest COP during the experiments was obtained with EF (COP= 5.56) at air mass flow rate of 0.1357 kg/s. On the other hand, COP with the use of CP reached to a peak of 4.84 at mass flow rate of 0.068 kg/s. Later on it showed a drop and stabilized at COP ~ 4.3. Similarly, COP with YS had a peak value of 4.48 at air mass flow rate of 0.1 kg/s and it dropped to 3.76 with the increase in mass flow rate to 0.1357 kg/s. DBB and CM showed lower COP trends where highest COP achieved with these materials were 3.66 and 3.30 at mass flow rate of 0.1 kg/s respectively. With the increase in mass flow rate, absolute humidity difference of air (∆w) across the EC pad dropped for all materials. The ∆w varied between 4.79→2.20 g/kg, 4.52→1.72 g/kg 25

3.11→1.49 g/kg for EF, CP and YS respectively. For DBB and CM, the variation of ∆w was less considerable where it dropped in the range of 2.30→1.40 and 2.12→1.14 respectively (Please see: Figure 12).

Figure 12. Absolute humidity difference and COP trends at different mass flow rates

4. Conclusion In present study, performances of EF, CP, YS, DBB and CM were investigated for direct EC applications in hot-dry climate. Several performance parameters (temperature difference, relative humidity, water evaporation rate, cooling effectiveness and COP) at different air velocities were analysed for comparing material performances. According to the study results, average ∆T showed a decreasing trend with the increase in air velocity (or mass flow rate), where for EF, CP, YS, DBB and CM it varied between 11.3→5.21 ˚C, 10.7→4 ˚C, 7.3→3.5 ˚C, 5.4→3.3 ˚C and 5.0→2.72 ˚C respectively. Contrarily, evaporation rate increased with the increase in air velocity for all materials. Based on the corresponding results, amount of water consumption was found as the highest for EF, which varied between 0.196→1.075 kg/h, for the air velocity range of 0.1-1.2 m/s. Furthermore, highest average cooling capacity (0.71 kW) and effectiveness (0.72) were obtained with EF at air velocities of 1.2 m/s and 0.1 m/s respectively. The peak COP value was also determined as 5.56 for the same material. On the other hand, amount of water consumption per unit cooling output was found nearly 0.42 g/kJ and it was in close approximation for all tested materials.

26

Results also demonstrated an inversely proportional correlation between cooling capacity and effectiveness. Both parameters are important in EC operation and they are desired to be high. Therefore, optimum operating condition is considered as the value of mass flow rate where cooling capacity and effectiveness curves intersect. This mass flow rate was determined as 0.063 kg/s, which was valid for all tested materials and which was only the function of inlet air temperature and relative humidity. Another inversely proportional correlation was obtained between COP and ∆w. It was observed that both parameters vary between 0→5 and ∆w (g/kg) shows a decreasing trend while cooling COP gradually increases with the increase in mass flow rate. Results showed that values for both parameters become equal at mass flow rate of 0.041 kg/s. Obtained correlations in this study demonstrate that; in any particular application, depending on the input air conditions and desired output parameters, EC system operation could be optimized by adjusting the mass flow rate. Consequently, besides the investigations on novel EC materials, further studies on development of advanced models for EC optimization could also be a direction for future research.

Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declaration of conflicting interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Highlights -

New organic materials were experimentally investigated for EC applications.

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A correlation between cooling capacity and effectiveness has been found.

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Highest cooling capacity was obtained as 0.71 kW with EF at 1.2 m/s air velocity.

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At lowest air velocity of 0.1 m/s, effectiveness of EF was the highest at 0.72.

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Water consumption rate varied between 0.09-1.08 kg/h with increasing air velocity.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Author Names: Pervin Abohorlu Doğramacı Devrim Aydin