Impact of Surface Area and Porosity on the Cooling Performance of

Evaporative Cooling Devices

by

Trang Luu

S.B., Massachusetts Institute of Technology (2018)

Submitted to the Department of Mechanical Engineering

in partial fulfillment of the requirements for the degree of

Master of Science in Mechanical Engineering

at the

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

September 2020

© Massachusetts Institute of Technology 2020. All rights reserved.

Author………………………………………………………………………………………………

Trang Luu

Department of Mechanical Engineering

August 31, 2020

Certified by…………………………………………………………………………………………

Daniel Frey

Professor of Mechanical Engineering

MIT D-Lab Faculty Research Director

Thesis Supervisor

Accepted by………...………………………………………………………………………………

Nicolas Hadjiconstantinou

Chairman, Department Committee on Graduate Theses

2

3

Impact of Surface Area and Porosity on the Cooling Performance of

Evaporative Cooling Devices

by

Trang Luu

Submitted to the Department of Mechanical Engineering

on August 31, 2020, in partial fulfillment of the requirements for the degree of

Master of Science in Mechanical Engineering

Abstract

Evaporative cooling devices are low-cost, low-energy solutions for post-harvest storage of fruits

and vegetables on farmlands. Surface area and porosity are two design parameters that affect the

cooling devices’ evaporation rate and cooling performance. Both design parameters lack prior

systematic testing that methodically varies levels of surface area and material porosity to

understand their effects on these devices’ cooling performance (e.g. maximum temperature drop,

duration of high internal relative humidity, cooling efficiency and total cooling). For fruits and

vegetables, storage environments with low temperature and high humidity are critical to reduce

deterioration. In this thesis, ridges were cut into the outer wall of pot-in-pot evaporative cooling

devices at four different interridge distances to vary total available surface area. Sawdust was

added to clay in different ratios to create devices with varying porosity. A new performance metric

of total cooling is also introduced to account for the maximum temperature drop and the total

duration of evaporative cooling. The surface area experiments reveal that adding corrugations on

the surface introduces competing effects between increased surface area for water evaporation and

decreased vapor concentration gradient inside of the corrugations’ troughs; consequently, among

the devices with corrugations, the amount of total surface area does not always correlate with

cooling performance. Between the devices with some surface corrugation and the device without

corrugation, the devices with corrugation do consistently achieve greater temperature drops.

However, the devices with corrugation are unable to maintain temperature drops and high levels

of internal relative humidity for as long as the device without corrugation. The porosity

experiments conclude that the greater the porosity in the device’s outer vessel, the greater the

maximum temperature drop. This is due to the reduced transport resistance during water and

moisture movement to the device’s surface. Higher percentages of porosity lead to faster

evaporation rates which deplete the amount of water inside the devices quicker and explain why

the temperature drops and internal relative humidity of the more porous devices do not last as long

as the temperature drops and internal relative humidity of the less porous devices. This thesis

investigates two design parameters of cooling devices and shows that increasing surface area and

porosity increases maximum temperature drops but decreases both the duration of temperature

drops and high internal relative humidity. Between the two design parameters, increasing porosity

is the more practical and less burdensome solution to improve the overall performance of

evaporative cooling devices for low-resource communities.

Thesis Supervisor: Daniel Frey

Title: Professor of Mechanical Engineering; MIT D-Lab Faculty Research Director

4

Acknowledgements

I wish to express my gratitude and appreciation to Professor Daniel Frey for his continuous support

of my study and research. I would also like to extend my deepest gratitude to Dr. Eric Verploegen

for his guidance, mentorship, and advice throughout the duration of my time at D-Lab. The

evaporative cooling devices used in these experiments were handmade by Jason Pastorello and the

work could not have been done without his pottery expertise. This thesis was conducted inside of

D-Lab, and I want to thank Jack Whipple for always extending a helping hand. The funding for

this work came from the generous support of the National Science Foundation Graduate Research

Fellowships Program.

I would like to thank my great friend, Krithika Swaminathan, who took the time to help me solve

hard problems and better articulate my research. I am grateful to Daniel Kreus for all the late night/

early morning help with setting up my experiments, editing my thesis and for his unwavering

support of my endeavors. I am thankful to Alejandro De La Parte Autrán for his support and for

his help building the initial momentum of this research project. I wish to extend my sincerest

thanks to Danyal Rehman for his insights into the data analysis. For answering all my MATLAB

questions, I want to thank my friends Pierre Walker and Chen Horng. For reading over my thesis

and helping me better express my ideas, I want to thank my friends Tyler Okamoto, Mark Chang,

and Carla Pinzón Gaytán.

Lastly, I would like to thank my parents, my sister, and my brother-in-law who have always been

my foundation in life.

5

Table of Contents

TABLE OF CONTENTS ............................................................................................................. 5

LIST OF FIGURES ...................................................................................................................... 6

LIST OF TABLE .......................................................................................................................... 7

INTRODUCTION......................................................................................................................... 8

EXPERIMENTAL DESIGN ..................................................................................................... 18

2.1. Instrumentation .............................................................................................................. 20 2.2. Experimental Metrics ..................................................................................................... 23 2.3. Experimental Methodology ........................................................................................... 25 2.4. Experiments ................................................................................................................... 27

RESULTS AND DISCUSSION ................................................................................................. 31

3.1. Surface Area Experiment ............................................................................................... 33 3.2. Effects of Porosity .......................................................................................................... 67 3.3. Porosity Mini-Experiments ............................................................................................ 95 3.4. Porosity versus Surface Area Design Parameters .......................................................... 98

CONCLUSION ......................................................................................................................... 101

SUPPLEMENTARY INFORMATION .................................................................................. 105

BIBLIOGRAPHY ..................................................................................................................... 110

6

List of Figures

Figure 1. Regions with Hot Desert Climates Based on Koppen-Geiger (1980-2016) -------------- 10

Figure 2. Population with Access to Electricity in 2016 ------------------------------------------------ 11

Figure 3. Pot-in-Pot Evaporative Cooler ------------------------------------------------------------------ 12

Figure 4. Sensor Placement Inside each Cooling Device ----------------------------------------------- 19

Figure 5. Ambient Sensors Placement -------------------------------------------------------------------- 20

Figure 6. Characterization of Surface Corrugations in Surface Area Experiment ------------------ 28

Figure 7. Evaporative Cooling Devices in Surface Area Experiments ------------------------------- 33

Figure 8. Wet-Bulb Temperature Comparison Between Surface Area Experiment 1 and 2 ------ 35

Figure 9. Internal Temperature Drop for Surface Area Experiment 1 -------------------------------- 37

Figure 10. Internal Temperature Drop for Surface Area Experiment 2 ------------------------------ 38

Figure 11. Mass Transfer Coefficient for Surface Area Experiment 1 ------------------------------- 44

Figure 12. Mass Transfer Coefficient for Surface Area Experiment 2 ------------------------------- 45

Figure 13. Evaporation Surface Temperature Drop for Surface Area Experiment 1 --------------- 48

Figure 14. Evaporation Surface Temperature Drop for Surface Area Experiment 2 --------------- 49

Figure 15. Cooling Efficiency for Surface Area Experiment 1 ---------------------------------------- 52

Figure 16. Cooling Efficiency for Surface Area Experiment 2 ---------------------------------------- 53

Figure 17. Total Cooling for Surface Area Experiment 1 ---------------------------------------------- 57

Figure 18. Total Cooling in Surface Area Experiment 2 ----------------------------------------------- 58

Figure 19. Relative Humidity in Surface Area Experiment 1 ------------------------------------------ 61

Figure 20. Relative Humidity in Surface Area Experiment 2 ------------------------------------------ 62

Figure 21. Evaporative Cooling Devices in the Porosity Experiments ------------------------------- 67

Figure 22. Wet-Bulb Temperature Comparison Between Porosity Experiment 1 and 2 ----------- 68

Figure 23. Internal Temperature Drop for Porosity Experiment 1 ------------------------------------ 72

Figure 24. Internal Temperature Drop for Porosity Experiment 2 ------------------------------------ 73

Figure 25. Evaporation Surface Temperature Drop for Porosity Experiment 1 --------------------- 77

Figure 26. Evaporation Surface Temperature Drop for Porosity Experiment 2 --------------------- 78

Figure 27. Mass Loss Rate and Mass Loss for Porosity Experiment 1 ------------------------------- 79

Figure 28. Mass Loss Rate and Mass Loss for Porosity Experiment 2 ------------------------------- 80

Figure 29. Cooling Efficiency for Porosity Experiment 1 ---------------------------------------------- 83

Figure 30. Cooling Efficiency for Porosity Experiment 2 ---------------------------------------------- 84

Figure 31. Total Cooling for Porosity Experiment 1 ---------------------------------------------------- 86

Figure 32. Total Cooling for Porosity Experiment 2 ---------------------------------------------------- 87

Figure 33. Relative Humidity in Porosity Experiment 1------------------------------------------------ 90

Figure 34. Relative Humidity in Porosity Experiment 2------------------------------------------------ 91

Figure 35. Moisture in Sand Gap for Surface Area Experiment 1 ---------------------------------- 105

Figure 36. Moisture in Sand Gap for Surface Area Experiment 2 ---------------------------------- 106

Figure 37. Mass Loss Rate and Mass Loss for Surface Area Experiment 1 ----------------------- 108

Figure 38. Mass Loss Rate and Mass Loss for Surface Area Experiment 2 ----------------------- 109

7

List of Table

Table 1. Internal Temperature Drop Cooling Performance for Surface Area Experiments ------- 39

Table 2. Evaporation Surface Temperature Drop Cooling Performance for Surface Area

Experiments --------------------------------------------------------------------------------------------- 50

Table 3. Cooling Efficiency Performance for Surface Area Experiments --------------------------- 54

Table 4. Total Cooling Performance for Surface Area Experiments --------------------------------- 59

Table 5. Relative Humidity of the Surface Area Experiments ----------------------------------------- 63

Table 6. Comprehensive Cooling Performance Metrics of Surface Area Experiments ------------ 66

Table 7. Internal Temperature Drop Cooling Performance for Porosity Experiments ------------- 74

Table 8. Evaporation Surface Temperature Drop Cooling Performance for Porosity Experiments

------------------------------------------------------------------------------------------------------------ 81

Table 9. Cooling Efficiency Performance for Porosity Experiments --------------------------------- 84

Table 10. Total Cooling Performance for Porosity Experiments -------------------------------------- 88

Table 11. Relative Humidity of the Porosity Experiments --------------------------------------------- 92

Table 12. Comprehensive Cooling Performance Metrics of Porosity Experiments ---------------- 94

Table 13. Devices in Mini-Experiment Comparing Porosity Due to Different Clay Material

Versus Porosity Created by Sawdust ---------------------------------------------------------------- 96

Table 14. Devices in Mini-Experiment Comparing Porosity Due to Different Clay Material ---- 97

Table 15. Comprehensive Cooling Performance Metrics of Surface Area And Porosity

Experiments ------------------------------------------------------------------------------------------- 100

8

Chapter 1

Introduction

Food loss and waste pose serious challenges for both developed and developing countries’

economic development and food security [1]. The Food and Agriculture Organization of the

United Nations (FAO) reported that 13.8% of all food globally is lost from post-harvest up to but

not including retail in 2019 [2]. At the same time, the number of chronically malnourished people

in the world has been rising since 2014 [3]. There are approximately 60 million more malnourished

people in 2019 than in 2014 globally—a total of nearly 690 million people or 8.9% of the world

population [3]. The COVID-19 pandemic may have added an additional 83 – 132 million people

to the total number of undernourished people in 2020 [3]. The underlying cause of the increased

undernourishment globally stems from deteriorating economic conditions that threaten food

affordability for low-income and vulnerable communities [3]. To increase affordability and

availability of nutritious foods, reducing harvest losses at the production level is an vital first step

[3]. Due to inadequate crop processing technology, lack of labor, and insufficient/unreliable cold

9

storage infrastructure throughout the supply chain, underdeveloped countries primarily suffer from

food loss and waste during the post-harvest storage and processing stage [1,3]. Post-harvest

storages, such as mechanical refrigerators and cold rooms, can reduce how quickly produce perish

but require expensive initial capital investment and reliable electricity to operate. Consequently,

they are inaccessible or unattractive to most farmers in underdeveloped communities [3,4].

Evaporative cooling devices are low-cost alternatives to mechanical refrigerators and cold rooms

that can increase the shelf life of fruits and vegetables by providing cooler and more humid storage

conditions. At high temperatures, the respiration rate, water loss, ethylene production, and

microbial development shorten the shelf life of the produce, and at low relative humidity, produce

tends to wilt, soften, and lose moisture content—all of which decrease the ability of farmers to sell

their produce [4,5]. Communities can use locally available material to manufacture and operate

these cooling devices without using electricity or requiring much training. These cooling devices

use the evaporation of water to create the cooling effect inside; thus, these devices work best in

dry and arid climates that are conducive to water evaporation [6]. Specifically, evaporative cooling

devices are well suited to benefit communities in the Near East and North Africa (NENA) 1 region

due to its desert climate (Figure 1) and limited access to electricity (Figure 2). In 2016, the Food

and Agriculture Organization (FAO) reported that the NENA region alone had 250 kg per capita

each year of food loss and waste; this constitutes up to 50% of all fruits and vegetables produced

in the NENA region each year [7,8]. Reducing the amount of food waste and loss will enable

farmers to have more food available for consumption and sale [9]. Furthermore, reducing food loss

1 Countries include: Algeria, Bahrain, Egypt, Iran (Islamic Republic of), Jordan, Kuwait, Lebanon, Libya,

Mauritania, Morocco, Oman, Qatar, Saudi-Arabia, Tunisia and the United Arab Emirates

10

and waste can decrease the price of food and reduce household expenditures, hence facilitating the

acquisition of other basic necessities related to health, education, and quality of life [3,9].

Figure 1. Regions with Hot Desert Climates based on Koppen-Geiger (1980-2016).

The red identifies the arid and dry climates around the world. These areas provide the

environmental conditions that maximize the cooling performance of evaporative cooling

devices. BWh indicates hot desert climates.2 The NENA region overlaps with the region

of hot desert climates near North Africa.

2 By Beck, H.E., Zimmermann, N. E., McVicar, T. R., Vergopolan, N., Berg, A., & Wood, E. F. -

"Present and future Köppen-Geiger climate classification maps at 1-km resolution". Nature Scientific

Data. DOI:10.1038/sdata.2018.214., CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=74673078

11

Figure 2. Population with Access to Electricity in 2016. The data represents percentage

of people who have electricity, both on-grid and off-grid, for their home.3 Part of the NENA

region overlaps with areas that largely do not have access to electricity.

Evaporative cooling devices come in two primary categories: large evaporative cooling chambers

and household coolers [10,11]. This thesis focuses on the latter because they cost less and require

less labor to construct for the users. The author anticipates that the findings on household coolers

can later be generalized for larger cooling chambers. For household coolers, the three most

common configurations are pot, cabinet, and curtain configurations [11–13]. The evaporative

cooling devices in the pot configuration are the most used. Coolers in the cabinet and curtain

configuration are adaptation of the common pot configuration design; the main difference among

the three lies in where water evaporates from. This thesis explores the cooling performance of

evaporative coolers in the pot configuration. Mohammed Bah Abba, an inventor, from Nigeria

created the most common cooler in the pot configuration, called the Pot-in-Pot Preservation

3 By World Bank – World Development Indicators. 2016. http://data.worldbank.org/data-catalog/world-

development-indicators

12

Cooling System (Zeer Pot) seen in Figure 3 [14]. Abba’s invention consists of an earthenware pot

placed inside a larger pot and has sand filling the gap between the two pots. Users place fruit and

vegetables in the inner pot and cover the whole configuration with a wetted cloth. Inside the inner

pot, evaporative cooling creates a cooled and humidified environment to extend the shelf life of

the stored fruits and vegetables.

Figure 3. Pot-in-Pot Evaporative Cooler. The Zeer Pot (pot configuration) consists of an

outer and inner clay vessel with wetted sand to fill the gap and a wet cloth to cover the top.

The porous vessels allow water to travel from the sand gap to the outer and inner surfaces

to evaporate. As the water evaporates from the lid and the surfaces, the internal temperature

decreases and the internal relative humidity increases.

Evaporative cooling performance is limited by geographical constraints because the cooling

fundamentally relies on water evaporation. Environment conditions such as relative humidity,

vapor pressure, air movement, and temperature affect rate of water evaporation and thereby the

cooling performance. Relative humidity is the percentage of water vapor in the air. It relates the

13

partial pressure of water vapor to the saturated water vapor pressure at the same temperature. For

water to evaporate, the vapor pressure at the liquid-air interface needs to be higher than the

surrounding air to create a concentration difference that drive the evaporation process [15]. As

water evaporates into vapor, the relative humidity immediately around the evaporative cooling

device gradually increases, reducing the driving force for further evaporation of water. With no

natural or forced air movement around the evaporative cooling devices to replace the saturated air

with drier air, the vapor pressure gradient would continue to decrease and the rate of evaporation

would continue to slow until it stops completely. There are two temperature measurements that

are relevant to evaporative cooling performance: dry-bulb temperature and wet-bulb temperature.

Dry-bulb temperature is the air temperature measured by ordinary thermometers. Most people

commonly refer to this as air temperature. Higher ambient air dry-bulb temperatures lead to more

sensible heat that water inside evaporative cooling devices can use to convert to latent heat for

evaporation [6,15]. Wet-bulb temperature measures air temperature at 100% relative humidity and

marks the lowest temperature obtainable via evaporative cooling. To measured it, drape a wet cloth

over a thermometer. The wet bulb temperature is lower than the dry bulb temperature, and the

difference between them represents the maximum decrease in temperature achievable using

evaporative cooling devices [4]. It is important to note that even at peak performance, evaporative

cooling devices cannot prevent rapid spoilage for commodities that will perish if not kept

consistently below 20 °C (such as medications, meat, and animal products) or products that need

low humidity to prevent mold growth (grains, coffee, and cereals) [10]. Evaporative cooling

devices best increase the shelf life and slow the physiological deterioration of fruits and vegetables

by providing cooler and more humid storage conditions to reduce wilting, water loss, and over-

ripening [4].

14

Water evaporation from porous materials such as clay pots occurs in two stages [16–18]. In the

first stage, water evaporates from the wetted surface directly into the atmosphere, and in the second

stage, when there is disruption in the capillary water flow to the surface, moisture evaporates

within the porous medium and then vapor diffuses to the surface [16–18]. The main two factors

that influence the two stages of evaporation are respectively, the ambient conditions that affect

water vapor exchange with air at the surface and the topology of pore spaces inside the porous clay

pot [16,17]. For evaporative cooling devices, surface area and porosity are two design parameters

that influence the devices’ rate of evaporation and can be optimized to improve cooling

performance. Increasing the amount of surface area for water evaporation will increase the rate of

evaporation [6,13,19]. Scaling the overall evaporative cooling device up in size will provide more

surface area over which evaporation can occur, which in theory should provide more cooling, but

in practice, increasing the overall size of the cooling device also increases the internal volume the

cooling device needs to cool. Thus, to study the effects of how just the surface area influence

cooling performance, the internal volume of the cooling device must remain constant. Gustafsson

et al. conducted a study on the effects of hanging an evaporative cooling device to allow for

evaporation from the bottom of the pot, thereby exploring the effects of increasing the surface area

over which evaporation can occur without increasing the internal volume [20]. Gustafsson et al.

found that the hanging pot reached a 64% temperature decrease relative to the its wet bulb

temperature, 9% greater than the device placed on the ground and attributed the result to extra

exposure to airflow from hanging the pot [20]. The result of the previously mentioned study shows

that increasing surface area for evaporation lead to better cooling performance, but the design is

difficult to implement practically on farmlands. Gustafsson et al. incorporated forced convection

with wind velocity of 3-3.5 m/s into the experiment [20]. The introduction of forced convection

15

requires electricity that not all farmers can access. Furthermore, the weight of evaporative coolers

makes them impractical to hang off the ground. Another approach to studying how increasing the

surface area without increasing the internal volume affect the cooling performance involves

constructing evaporative cooling devices with ridges on the outer vessel. The added surface

corrugations increase the surface area of the outer vessel without significantly changing the overall

volume of the device. This thesis will compare the cooling performance of cooling devices with

different total amount of available surface area for evaporation.

It is common knowledge that the outer vessels of evaporative cooling devices should have enough

porosity to allow water to pass through to the surface from the sand gap to evaporate. To date, no

literature is available detailing the effect of varying the porosity level of the inner and outer

material of evaporative cooling devices on the cooling performance of the devices. There have

been studies on the effects of varying levels of porosity on water evaporation from porous samples.

Unno et al. compared the evaporation behavior of a single water droplet on top of porous and non-

porous bodies of epoxy resin and found that evaporation rate increased by 1.2 times when the

droplet was on the surface of a porous body [21]. Aboufoul et al. found that pore space topology

influence water evaporation from porous asphalt [16]. The experiments showed that relatively

large pores weaken capillary forces and shortened the stage-1 evaporation, but increased

continuous air void connection and evaporation rates during stage-2 evaporation [16]. The

experiments conducted by Aboufoul et al. is more similar to the experiments in this thesis than the

experiments by Unno et al. but still differ in that the evaporation was only allowed out the top

surface [16]. Regarding evaporative cooling devices, there have been studies using different

materials for the inner vessel of the cooling device such as clay (porous) versus plastic and metal

(non-porous). Salaudeen et al. compared the clay pot-in-pot configuration with a tin pot-in-pot

16

configuration in Nigeria and discovered that the tin pot-in-pot did achieve between 2 °C – 3.5 °C

greater temperature drop and 10% - 11% higher relative humidity than the clay pot-in-pot

throughout the nine days of experiment [22]. The tin-in-pot showed a greater temperature drops

during the cooler morning and night, than during the hotter afternoon because as Salaudeen et al.

suggests, tin serves as a good conductor for heat transfer from the environment. Nevertheless,

Salaudeen et al. ruled out using tin as a practical material due to tin’s chemical reaction with the

stored fruits which increased the ripening process [22]. Verploegen et al. conducted a similar

experiment that compared the performance of a clay pot in clay pot cooler with a plastic pot in

clay pot cooler and a metal pot in clay pot cooler [23]. Once again, the metal container in the clay

pot achieved the greatest average temperature decrease at 2.2 °C and 57% cooling efficiency

followed by the plastic container in clay pot at 1.8 °C with 47% cooling efficiency and then the

clay pot-in-pot at 1.7 °C with a 43% cooling efficiency. Verploegen et al. noted that because the

experiments with plastic and metal inner pots did have greater temperature drop and because they

are easier to clean in case of fungal growth than porous material, they make viable alternatives to

using clay internal pot [23]. The two previously mentioned studies explored the effects of using

inner vessels made of material with different conductivity and indirectly investigated the effects

of porosity at the extreme end. These studies differ from the experiments in this thesis in that they

did not vary the porosity of the outer vessels. In the work presented here, the porosity of the clay

body was varied by mixing sawdust into the clay to alter its porosity after firing. With

consideration to practicality, using sawdust to increase the porosity level in the evaporative cooling

device does not require additional electricity usage or cost. Sawdust can be locally sourced in most

places for little or no additional cost to clay pot makers and farmers.

17

To measure the cooling in the surface area and porosity experiments, this thesis uses common

metrics of performance which include temperature drop from ambient conditions, relative

humidity inside the device, and cooling efficiency. In addition to the common metrics of

performance, the author has developed a new metric to find the total amount of cooling by

computing the total area in between the device’s internal temperature curve and the ambient

temperature curve. This performance metric allows readers to compare cooling devices that have

large temperature drops at the beginning of the experiment but quickly dry out to cooling devices

that maintain a steady level of cooling for a longer time period. The thesis also includes practical

time metrics such as the time taken for the internal temperature drop to return to zero and the

internal relative humidity to measure below 80%. The time metrics are analyzed in relationship to

the amount of water added and the relative humidity threshold of 80% was chosen to better

compare the devices’ performance. All the performance metrics are evaluated in a series of four

experiments that systematically vary surface area and porosity. The author hypothesizes that as

surface area and porosity increase, the maximum temperature drops of the evaporation cooling

devices also increase. This work will enable practical improvement to the design, functionality,

and evaluation of evaporation cooling devices in rural, arid environments for low-resource

communities.

18

Chapter 2

Experimental Design

In order to systematically evaluate the effects of surface area and porosity on the cooling

performance of household evaporative cooling devices, 3 temperature and relative humidity (joint)

sensors, 3 moisture sensors, and 1 load cell are used per device. An additional 4 sensors are placed

in the ambient environment around the experiments. Water bottles are used as thermal loads inside

the cooling devices. Figure 4 provides the location of the sensors on each cooling device and

Figure 5 shows the location of the ambient sensors and load cells. For the surface area

experiments, there are 4 cooling devices to represent the levels of variation, and in the porosity

experiment, there are 3. Each level of variation is evaluated using common measuring points such

as temperature and relative humidity difference from ambient and cooling efficiency. In addition

to these common metrics, this thesis also includes time metrics: when the temperature differences

19

return to zero, when relative humidity inside the device measures below 80%, and a newly

developed method to quantity the total cooling the device has over the course of the experiment.

Figure 4. Sensor Placement Inside Each Cooling Device. The white squares represent

temperature and humidity (joint) sensors. They are placed inside the middle of the cooling

device, inside the lid, and strapped on the outside in the middle of the cooling device. The

triangles represent moisture sensors that are placed in the sand of the cooling device, in the

lid, and strapped on the outside in the middle of the cooling device. Water bottles surround

the temperature sensor inside the middle of the pot. The entire cooler sits on top of a load

cell.

20

Figure 5. Ambient Sensors Placement. Four ambient sensors measure the ambient

environment temperature and humidity around the evaporative cooling devices.

2.1. Instrumentation

Two microcontrollers control the sensors measurements: the microcontroller onboard SenSen

sensor data collection units (http://www.sensen.co/) and the ATmega2560 on board the Arduino

Mega 2560. Arduino and Processing software read and record the measured data. Data from the

SenSen sensor data is collected every 300 seconds and data from the Arduino sensors is collected

every 52-53 seconds. The sensors on the SenSen data collection unit are used to supplement the

sensors measuring ambient temperature and relative humidity controlled by the Arduino Mega.

The SenSen data collection unit is a standalone data collection box. It has 2 temperature and

relative humidity sensors and 2 moisture sensors.

21

Temperature and Humidity Sensors

DHT22 sensors from Adafruit measure the temperature inside the middle of the evaporative

cooling device, inside the lid of the device, and outside the device (Figure 5). Four sensors

measure the ambient conditions of the experiments. The sensors are rated to measure

temperature from -40 to 80 °C with ± 0.5 °C accuracy, and measure relative humidity from 0-

99.9% with ± 2-5% accuracy.

One BME280 sensor is used as an ambient sensor on the Sensen data collection unit. It

measures temperature with ±1.0 °C accuracy and humidity with ±3% accuracy.

Before the start of every experiment, all 22 temperature and humidity sensors are bundled

together and placed inside a closed box for calibration purposes. The calibration period for

each experiment lasts between 14 – 21 hours. During that time frame, all the temperature and

relative humidity sensors should have the same measurements. As part of the calibration

process, a calibration shift factor is calculated for each of the 22 temperature sensors. Each

experiment yields a separate calibration shift factor which is then applied to the respective

experiment. The calibration shift factors range from -0.32 to 0.62 °C for temperature and from

-2.75% to 3.91% for relative humidity. The shift factor values are close to the error range of

the sensors.

Relative humidity values that read 99.9% do not have shift factors applied to them to avoid

situations where the data reads constant humidity at 79.3% for example. The relative humidity

reading realistically will only stay a constant value at saturation. Relative humidity values

greater than 99.9% after the calibration shift factors are applied are set to 99.9% because

relative humidity is a ratio and anything above 100% is not physically feasible.

22

Moisture sensor

SparkFun Soil Moisture Sensors are used to measure the moisture level in the sand, in the top,

and outside each device. The sensors output an integer between 0 (dry) and 880 (wet) and their

magnitudes indicate the level of wetness. The moisture sensors are not calibrated and are used

to determine wetness and dryness. In this thesis, their units are assigned as Moisture Units.

Load cell

The load cell is from JiuWu (Model number: YZC-1B) and has a maximum capacity at 50kg.

It has a combined error of ± 0.015kg. The combined error specifies the hysteresis of the load

cell. The temperature effect on sensitivity is 8 x 10-4 kg/°C and its creep is 0.015 kg/ 30

minutes. The experiment is conducted indoors and the temperature changes through the course

of the experiments is less than 10 °C so the error due to temperature changes is minimal.

However, the creep tendency of the load cell does add errors to the mass measurements because

of how long the experiments lasted. The first surface area experiment lasts 306 hours. The first

porosity experiment lasts 331 hours. The second surface area and porosity experiment last 613

hours. The creep tendency introduces error into the load cell data that all of the devices

experience but the maximum and minimum required offsets to reset the load cells back to zero

at the start of each experiment is one to two orders of magnitude lower than the amount of

water added. Additionally, no conclusion is drawn from load cell measurements near the end

of the experiments. The load cells are tested for accuracy and precision prior to their usage.

To conduct the accuracy test, known weights (5 lbs., 10 lbs., 25 lbs.) are used to verify the load

cell reading of individual weights and combinations of weights. Verifying the accuracy of the

combinations of weights is important because the weights could not be exactly the labeled

weight. The precision test involves incrementally adding and removing water balloons of

23

known weights. Before starting each experiment in the study, each of the six load cells were

tared to read 0 kg.

2.2. Experimental Metrics

The metrics for quantifying cooling performance include:

Relative humidity [%]

Relative humidity is the percentage of water vapor in the air. It relates the partial pressure

of water vapor to the saturated water vapor pressure at the same temperature.

Wet-bulb temperature difference from ambient temperature [ºC]

Wet-bulb temperature measures air temperature at 100% relative humidity and marks the

lowest temperature obtainable via evaporative cooling. To calculate the wet bulb

temperature, the following formula was used [24]:

𝑇𝑤𝑒𝑡−𝑏𝑢𝑙𝑏 = 𝑇𝑑𝑟𝑦−𝑏𝑢𝑙𝑏 ∗ 𝑎𝑟𝑐𝑡𝑎𝑛 [0.151977 ∗ (𝑅𝐻 + 8.313659)12]

+ 𝑎𝑟𝑐𝑡𝑎𝑛(𝑇𝑑𝑟𝑦−𝑏𝑢𝑙𝑏 + 𝑅𝐻) – 𝑎𝑟𝑐𝑡𝑎𝑛(𝑅𝐻 – 1.676331)

+ 0.00391838 ∗ (𝑅𝐻)32 ∗ 𝑎𝑟𝑐𝑡𝑎𝑛(0.023101 ∗ 𝑅𝐻) – 4.686035

[Eq. 1]

where,

𝑇𝑤𝑒𝑡−𝑏𝑢𝑙𝑏 is the wet-bulb temperature in [ºC]

𝑇𝑑𝑟𝑦−𝑏𝑢𝑙𝑏 is the air temperature given by a thermometer not exposed to direct

sunlight, measured in [ºC].

24

𝑅𝐻 is the relative humidity of the air around the pot, measured in [%].

Cooling Efficiency [%]

Cooling efficiency is the percentage of maximum temperature drop achieved by the device.

A hundred percent indicates that the temperature inside the evaporative cooling device has

dropped to the wet-bulb temperature. It is calculated using the following equation: [25].

𝜀 = 𝑇𝑎,𝑑𝑏−𝑇𝑝,𝑑𝑏

𝑇𝑎,𝑑𝑏−𝑇𝑎,𝑤𝑏 𝑥 100 [Eq. 2]

where,

𝜀 is the cooling efficiency, measured in [%]

𝑇𝑎,𝑑𝑏 is the ambient air dry-bulb temperature, measured in [℃]

𝑇𝑝,𝑑𝑏 is the internal pot air dry-bulb temperature, measured in [℃]

𝑇𝑎,𝑤𝑏 is the ambient air wet-bulb temperature, measured in [℃]

Total Cooling [°C x Hour].

The total cooling metric allows for better comparison between cooling devices that have

large temperature drops at the beginning of the experiment but quickly dry out versus

cooling devices that maintain a steady level of cooling for a longer time period. The total

cooling is calculated by computing the total area in between the device’s temperature curve

and the ambient temperature curve. Its units are [°C x Hour].

Time metrics [Hour]

o The time it takes to achieve the maximum temperature drop.

o The time it takes for the internal relative humidity to drop below 80%.

25

2.3. Experimental Methodology

Before the start of every experiment, all 22 of the temperature and humidity sensors are gathered

into an enclosed container for calibration purposes. The sand used in the previous experiments are

mixed among 3 large buckets and left overnight to dry. Jute sacks are used as the lids of the

evaporative cooling devices because of how widely available they are in Sub-Saharan and East

Africa. The jute sacks require overnight soaking to fully absorb water.

All moisture sensors are tested to ensure that none has deteriorated, and all the load cells are tared

to 0 before each experiment. Before loading the sand into each evaporative device, samples are

taken from each of the 3 sand buckets to ensure that the sand density from each bucket is within

0.05 grams/cm3 of each other. Sand from each bucket is used to fill each cooling device. This mixes

sand even more to further ensure the same sand consistency across all devices. The inner vessel

sits concentrically inside the outer vessel with its opening plane at maximum 1 cm below the

opening plane of the outer vessel. Sand fills the gap between the inner and outer vessel.

Next, the jute sacks that were left to soak overnight are taken out of water and hanged to eliminate

excess water. The jute sack should not be dripping with water when placed on top of the

evaporative cooling device. While the jute sack hangs, the next step is to add 8 oz. water bottles

inside the inner vessels to serve as thermal mass. Each device is packed with as many water bottles

as possible without crossing above the inner vessel’s opening plane. Adding the water bottles

before placing the sensors allow the water bottles to act as a point of reference for sensor

placement. In the first experiment of the surface area and porosity experiments, the temperature

sensor inside the middle of the inner vessel of each device is freely placed above and below one

layer of water bottles. In the second surface area and porosity experiments, the same temperature

26

sensor in the middle of the inner vessel is tied to a water bottle to reduce temperature differences

due to sensor placement. The temperature sensor on the outer vessel of each device is strapped in

place with rubber bands, and the sensor to measure the lid (the jute sack) is set aside because the

lid has not been added yet. One moisture sensor is placed in the sand of the evaporative devices.

Another moisture sensor is wrapped in paper towel and strapped to the outside of each device’s

outer vessel. The moisture sensor measuring the lid is also set aside to be added later. Lastly, the

four ambient sensors are placed in the same location for all of the experiments (Figure 5).

After all of the sensors are placed, each damp jute sack is measured and the weight of the heaviest

jute sack is noted. Water is then added to each of the other jute sacks so that all the jute sacks have

the same amount of water. The re-wetted damp jute sack is not yet placed on top of the evaporative

cooling device until water has been added to device’s sand gap. Because cooling begins the

moment water evaporates from the device, adding water to the sand gap and jute sack is always

the last steps. Each device in the experiments has the same sand to water ratio to account for the

differences among all the devices’ volumes. The variations in volumes are due to the nature of

handmade clay pots. Because the volume of each device is not exactly the same, the absolute

amount of water added varies among the devices. After water is added to a device’s sand gap, its

jute sack is added. To cover the top of the devices, each jute sack is folded in such a way that

ensured complete coverage of each outer vessel’s opening. The sensors measuring the temperature

and relative humidity and moisture of the lids are now added. The sensor for temperature and

relative humidity is placed in one of the jute sack folds and the moisture sensor is clipped in the

middle of the jute sack.

The experiments start after all the cooling devices have been watered and all the jute sacks are

placed on top. No changes are made to the set up for the duration of the experiment. The

27

experiments end when the moisture sensor in the sand returns to zero. The second experiment in

both the porosity and surface area experiments ran longer to further observe the devices’ cooling

performance.

2.4. Experiments

Surface Area Experiment

The evaporative cooling devices used in the experiments exploring the effects of surface area

have corrugations on their outer vessel. The corrugations are created with ridges. Figure 6

shows the characterization of the corrugations. The surface area experiments compare four

devices with different amount of surface area available for water evaporation on their outer

vessels. Adding the ridges increases the surface area over which water can evaporate, and the

smaller the interridge distance, the more surface area is added to the outer vessel. The author

hypothesizes that adding more surface area over which water can evaporate will increase the

rate of evaporation and thereby the amount of cooling.

28

Figure 6. Characterization of Surface Corrugations in Surface Area Experiment. The

ridges on the outer vessels add more surface area to the devices without increasing the

volume. The devices are labeled based on the ratio between the devices’ surface area with

corrugation and their surface area without corrugation to account for the differences in

volume among the devices.

29

The surface area experiment is done twice to check for repeatability. Between the first and

second surface area experiment, the devices are shifted to different positions to ensure that the

cooling performance is not a result of the device’s location. The first experiment ran for 306

hours and the second experiment ran for 613 hours.

Porosity Experiment

The evaporative cooling devices in the experiments investigating the effects of porosity are all

made from terracotta clay. To achieve the incremental percentage of porosity among the outer

vessels, sawdust is added in different clay to sand ratio. Adding sawdust to wet clay changes

the final outer vessel’s porosity because during firing stages, the added sawdust burns out,

leaving cavities. The more sawdust mixed into the wet clay, the more cavities the final vessels

have and the higher the porosity. The sawdust added went through a 60-mesh sieve. The

percentages of porosity tested are 3.14% (no saw dust added), 7.20% (40 clay: 1 sawdust), and

11.22% (20 clay: 1 sawdust). To calculate the percentage of porosity, the following steps are

taken:

1. Weigh the dry samples of each pot

2. Put the samples in water and boil them for about 15 minutes

3. Take the sample out of water individually and shake off any excess water. The sample

is ready to be weighed when there is no water dripping from it. Do not use a cloth to

dry the excess water because the cloth may soak up water from inside the sample.

4. Weigh the wet sample and dry the scale between each sample.

5. Porosity is calculated by

30

𝑃𝑜𝑟𝑜𝑠𝑖𝑡𝑦 = 𝑊𝑒𝑡 𝑊𝑒𝑖𝑔ℎ𝑡−𝐷𝑟𝑦 𝑊𝑒𝑖𝑔ℎ𝑡

𝐷𝑟𝑦 𝑊𝑒𝑖𝑔ℎ𝑡 𝑥 100 [Eq. 3]

The effects of using terracotta with different percentages of porosity are explored in two

different experiments. In the first experiment, all the internal storage vessels are made up of

the 3.14% porosity terracotta clay, while only the outer vessels vary in porosity. In second

experiment, the porosity percentages of the inner and outer vessels are the same. The first

experiment ran for 331 hours and the second experiment ran for 613 hours. The author

hypothesizes that adding more porosity will increase the rate of evaporation and the maximum

temperature drop.

31

Chapter 3

Results and Discussion

Two main competing processes affect the cooling performance of evaporative cooling devices: the

heat transfer out of the device due to water evaporation and the heat transfer into the device via

convection and radiation from ambient environment. In evaporation, mass transfer and heat

transfer are tied together. The direction of the heat and mass transfer depends on the partial vapor

pressure and the temperature difference between the surface of the devices and the ambient air

[26]. The evaporation flux from the surface of the devices depends on the capillary flow of water

through the device to the surface of the clay walls and on the water vapor diffusion across the

boundary layer formed at the surface [27–30]. As the water in the devices depletes, the network of

liquid filled pores in the clay walls becomes disconnected and the evaporation flux becomes

dependent on the rate of vapor diffusion from within the sand gap and the clay wall to the outer

surface [16–18].

32

For all the devices, four distinct zones are observed: Zone 1—Cooling Transient, Zone 2—Cooling

Steady State, Zone 3—Drying Transient, Zone 4—Drying Steady State. Each evaporative cooling

device enters and exits each zone at a different time. For clarity, only the general divide of the

zones is displayed on the figures. The divide is determined by observing the changes in the internal

temperature profiles. It is important to note that because all the devices were conducted at room

temperature, the temperature drop from ambient is not as large as it would have been in hotter

environment.

In Zone 1, the cooling effect due to evaporation is in a transient state. The water added in the sand

gap is still diffusing into the sand and through the clay wall of the device via capillary action [27–

29]. Water can evaporate from three different surfaces in each device: the internal storage vessel’s

surface, the outer vessel’s surface, and the lid’s surface. In Zone 2, the majority of the clay wall

has been wetted and the device reaches its maximum cooling performance. The cooling effect

remains steady but not constant because of the daily temperature fluctuation in the test

environment. In Zone 3, the device enters another transient state. Here, the evaporative cooling

effect decreases as there is less water to evaporate. As the network of liquid filled pores inside the

device becomes disconnected, the capillary actions that supplied the outer vessel’s surface with

water decrease [31]. This causes the internal temperature drop from ambient and the internal

relative humidity to also begin decreasing. In Zone 4, the device is another dynamic steady state.

The internal temperature and the relative humidity have not fully equilibrated with the ambient

conditions. As the outer vessel’s surface dries out, the vapor concentration gradient between the

surface and the environment continues to decrease, and a vapor pressure gradient develops inside

the cooling devices’ sand gap and clay wall [31]. The latter gradient causes evaporation to occur

inside the sand gap and clay wall and drives diffusion towards the outer surface [31]. The cooling

33

effect in Zone 4 is smaller and slower because the evaporation rate is lower. As more time passes,

the internal temperature and relative humidity eventually do return to ambient conditions, signaling

the end of Zone 4.

3.1. Surface Area Experiment

The evaporative cooling devices used in the two Surface Area experiments are shown in Figure

7. Within each experiment, the same sand to water ratio is the same. The differences in the devices’

dimensions from being individually handmade account for the differences in total absolute amount

of water added to each device. The devices are labeled based on the ratio between the device’s

total surface area with added corrugation and the theoretical total surface area of the same device

without corrugation.

Figure 7. Evaporative Cooling Devices in Surface Area Experiments. The corrugated

surface on the evaporative cooling devices increases the surface area over which water can

evaporate without increasing the volume internally. Each device is labeled based on the

ratio between the device’s surface area with corrugation and the device’s surface area

without corrugation. The color of the devices matches the color on the plots.

34

Wet-bulb Temperature

Figure 8 compares the wet-bulb temperature drop from ambient of the two surface area

experiments. The two experiments are conducted in the exact same indoor space, yet their wet-

bulb temperature profiles differ. As a result, the cooling performance of individual devices cannot

be compared across experiments. However, general trends, similarities, and differences that repeat,

despite the wet-bulb temperature difference, provide insight into the cooling device’s performance.

For the first experiment, the general four zones are: 0 – 15 hours, 15 – 90 hours, 90 – 230 hours,

and 230 hours onward. For the second experiment, the general four zones are 0 – 60 hours, 60 –

230 hours, 230 – 310 hours, and 310 hours onward. The general four zones approximate each

device’s four zones. The lengths of the zones differ between the first and second surface area

experiment because of ambient conditions. The oscillations seen in the data are caused by daily

temperature fluctuation in the testing environment. The missing data between 50-61 hours in the

first experiment corresponds to a temporary halt of the data recording software due to an automated

computer restart.

35

Figure 8. Wet-Bulb Temperature Comparison Between Surface Area Experiment 1

and 2. The wet-bulb temperature is the lowest temperature achievable through evaporative

cooling. The difference in wet-bulb temperature between the first and second experiment

is large enough to affect the cooling performance of the devices. This difference prevents

the devices from being compared individually across experiments, but the general trends,

repeated similarities and differences are noted and analyzed.

36

Internal Temperature

Figure 9 and Figure 10 show the internal temperature drop for the two surface area experiments.

Table 1 summarizes key performance metrics of the devices in the two experiments. In Zone 1—

Cooling Transient, the added water wets the entirety of each cooling device as it disperses through

the sand and clay walls. While some water settles down at the bottom of the cooling device, a large

majority of the water binds to the sand, keeping the sand gap and adjacent clay surfaces wet.

Moisture data of the sand gap is provided in the Supplementary Information S.1. The temperature

drops of all the devices in Zone 1 are similar to each other because all the devices’ evaporation

rate is limited by the same ambient conditions that determine how quickly the saturated air near

the surface of the devices can be replaced with drier air [18,28]. The effect of surface area on the

cooling performance is not seen until Zone 2 when the majority of the outer surfaces have been

wetted and the cooling effect is at its maximum. The temperature drop profiles of the devices begin

deviating from each other in Zone 2 because the added corrugations change the dynamics of vapor

transport on the surface, which then affects water evaporation rate. All the cooling devices with

added surface corrugation have greater internal temperature drops than the device without

corrugation. The corrugation adds more surface area that allows for more water evaporation to

occur. The net mass losses and mass loss rates of both surface area experiments reflect that the

water evaporation rate is higher for the devices with surface corrugation. This information is

included in the Supplementary Information S.2.

Among the devices with some surface corrugation, the addition of more corrugation does not

always result in a greater temperature drop. In the first surface area experiment, the 2.59x Device

has the greatest temperature drop, followed by the 1.52x Device instead of the expected 1.77x

Device. In the second experiment, the 1.77x Device has the greatest temperature drop followed by

37

the 2.59x Device and the 1.52x Device. The inconsistency in performance between the two

experiments likely results from the competing effects introduced by the added surface corrugation.

Figure 9. Internal Temperature Drop for Surface Area Experiment 1. The lines

indicating the four zones approximate when each device enters and exits each zone. In

Zone 2, the devices with the added corrugation have greater temperature drops than the

device without. Among just the devices with corrugated surface, the temperature drop does

not always increase with more corrugation. This suggests that using corrugation to increase

surface area introduces other competing factors that affect the cooling performance.

38

Figure 10. Internal Temperature Drop for Surface Area Experiment 2. The

temperature drop of the devices with corrugated surface is greater than the temperature

drop of the device without corrugation. This is also observed in the first experiment. The

device that has the greatest temperature drop differs between the two experiments. This

further suggests that corrugated surfaces increase surface area but also introduce other

competing effects that influence cooling performance.

39

Table 1. Internal Temperature Drop Cooling Performance for Surface Area

Experiments. The maximum temperature drops and the time they occur is listed for all

devices in both experiments. The device with the maximum temperature drop is

highlighted. Temperature drop difference of greater than 0.5 ℃ are outside the sensor error

and are more significant. The total amount of water added and the time when the

temperature drops first return to zero are also included. The temperature drop of the 1x

Device in Experiment 2 does not return to zero within the experiment timespan. The time

it takes the devices in the second experiment to reach their max temperature drop is almost

2-3 times longer than in the first experiment because of differences in ambient conditions.

Experiment

Number Devices

Amount of

Water

Added

[kg]

Max

Internal

Temperature

Drop

[°𝐂]

Time Max

Internal

Temperature

Drop

[Hour]

Time

Temperature

Drop

Returns to

Zero

[Hour]

1 1x Device 2.84 5.72 14 218

1 1.52x Device 3.43 6.56 63 216

1 1.77x Device 3.03 5.35 62 123

1 2.59x Device 3.13 7.24 62 213

2 1x Device 3.06 4.28 41 Never

2 1.52x Device 3.32 5.81 135 360

2 1.77x Device 3.06 5.42 207 360

2 2.59x Device 3.13 5.68 207 360

Gao et al. and Haghighi et al. both conducted experiments on the effects of evaporation from wavy

surfaces [28,32]. Gao et al. studied the effects of evaporation from a bed of sand with a single

“wave” in natural convection [28]. The experiments conducted by Gao et al. found that a steeper

“wave” caused lower vapor concentration gradient at the surface and led to lower evaporation rates

[28]. Haghighi et al. conducted experiments on evaporation from wavy sand surfaces in turbulent

airflow and also found that evaporation flux is suppressed in the trough of the “wave” and that the

evaporation rates depend on the ratio of height to distance between each “wave” due to the

competing effects between increased evaporating surfaces and reduced vapor concentration

gradient [32]. There are differences between the experiments on the wavy surfaces and the surface

area experiments conducted in this thesis that were taken into consideration before extrapolating

40

the studies’ findings. Together, the differences prevent a direct extrapolation for some of the

studies’ findings, but the insight that wavy geometry reduces vapor concentration gradient in the

troughs of the corrugation can explain the competing effects seen in the surface area experiment.

The differences include the water diffusion direction, the shape and scale of the corrugation, the

evaporation surface material, the presence of heat transfer due to radiation, and the presence of

controlled airflow. In the surface area experiments of this thesis, the capillary gradient required for

water diffusion to the outer evaporation surface is smaller because water does not have to act

against gravity to reach the evaporation surfaces. The effect of gravity is mostly seen when the

device is running low on water because there is more vertical distance between the water and the

evaporation surface [28,32]. The extrapolation of the findings from the study is restricted to when

the cooling rate is constant near the beginning. The corrugations on the devices in this thesis are

not sinusoidal; they are better approximated as square waves. Nevertheless, the wavy surface still

serves as a good approximation of evaporation behavior over a wavy surface and as a reference

for the corrugations. Although the scale of the corrugations in the studies is larger, the ratio of the

wave’s height to width in Gao et al. experiments and ratios of corrugated surface area to

corresponding flat-surface area in the Haghighi et al. experiments are respectively comparable to

the corrugation in this thesis. In the surface area experiments, water evaporates from the surface

of clay pots which is more rigid and less permeable than sand. This means that in the surface area

experiments of this thesis, water has more resistance to get to the surface, but once water is on the

surface, how the corrugation affects water evaporation is comparable to the effects seen in the

studies by Gao et al. and Haghighi et al. The presence of radiation increases the temperature of the

device which increases evaporation rate but does not completely negate the effect of the

corrugation on the vapor concentration gradient. The presence of controlled airflow in the Gao et

41

al. and Haghighi et al. experiments simplified the dynamics because air always flowed in one

direction. This does not completely describe the devices in the surface area experiments of this

thesis because the devices are exposed to air flow from random directions. Haghighi et al. did

include data on airflow that are perpendicular and parallel to the corrugation that better aid in

understanding the airflow dynamics of the devices [32].

To further understand the competing effects due to corrugation, Figure 11 and Figure 12 present

the mass transfer coefficient of the devices in the surface area experiments. The mass transfer

coefficient quantifies how much water vapor has diffused from near the device’s surface to the

environment, normalized by the area and concentration gradient. The higher the mass transfer

coefficient, the more unsaturated air is replenished near the surface of the device to allow for more

evaporation to occur. It is calculated as follows [18,26,31]:

ℎ𝑚 =𝐸𝑒𝑣𝑎𝑝

𝑀𝑤𝑅

(𝑃𝑠𝑎𝑡,𝑠𝑢𝑟𝑓𝑎𝑐𝑒

𝑇𝑠𝑢𝑟𝑓𝑎𝑐𝑒𝑅𝐻𝑠𝑢𝑟𝑓𝑎𝑐𝑒−

𝑃𝑠𝑎𝑡,𝑎𝑚𝑏𝑇𝑎𝑚𝑏

𝑅𝐻𝑎𝑚𝑏) [Eq. 4]

where,

ℎ𝑚 is the mass transfer coefficient, measured in [𝑚

𝑠]

𝐸𝑒𝑣𝑎𝑝 is the evaporation flux, measured in [𝑘𝑔

𝑚2𝑠]

𝑀𝑤 is the molar mass of water, 0.018 [𝑘𝑔

𝑚𝑜𝑙]

𝑃𝑠𝑎𝑡,𝑠𝑢𝑟𝑓𝑎𝑐𝑒 is the saturated vapor pressure of air at the surface, measured in [Pa]

𝑇𝑠𝑢𝑟𝑓𝑎𝑐𝑒 is the temperature of air at the surface, measured in [K]

𝑅𝐻𝑠𝑢𝑟𝑓𝑎𝑐𝑒 is the relative humidity of air at the surface, measured in [%]

𝑃𝑠𝑎𝑡,𝑎𝑚𝑏 is the saturated vapor pressure of ambient air, measured in [Pa]

𝑇𝑎𝑚𝑏 is the temperature of ambient air, measured in [K]

42

𝑅𝐻𝑎𝑚𝑏 is the relative humidity of ambient air, measured in [%]

𝑅 is the ideal gas constant, 8.314 [𝐽

𝑚𝑜𝑙 𝐾]

Buck’s equation is used to find the saturated vapor pressure at the surface and in the environment

[33]:

𝑃𝑠𝑎𝑡(𝑇) = (100)6.1121𝑒((18.678− 𝑇

234.5)(

𝑇

257.14+𝑇))

for liquid water T > 0 ºC [Eq. 5]

where,

𝑃𝑠𝑎𝑡(𝑇) is the saturated vapor pressure, measured in [Pa]

𝑇 is the temperature at the specific location, measured in [ºC]

The mass transfer coefficient of both experiments is steadiest in Zone 2 because that is when the

evaporation flux is steadiest. The negative and increased values of the mass transfer coefficient in

Zone 3 and 4 are mathematical results that occur when the concentration of water vapor in the

ambient air is greater than or close to the concentration in the air near the surface. This mostly

occurs when the devices have started to deplete their water sources.

Adding more surface area over which water can evaporate, in isolation, will increase the

evaporation rate, but in practice, the geometry of the corrugation causes a competing effect that

reduces vapor concentration gradient inside the trough of the corrugation; this creates a bottleneck

in the water evaporation processes [34,35].

In the first experiment, the mass transfer coefficient of the devices varies inversely with the

increased surface area. In the second experiment, the same pattern is observed but to a lesser

degree. This inconsistency in performance of the surface corrugation devices is likely due to the

43

patterns of airflow around the devices given the openness of the experiment’s location. Haghighi

et al. observed that the airflow dynamics in parallel to the troughs resemble that of over a flat

surface [32]. Because the devices in the surface area experiments of this thesis are exposed to

varied airflow with no set direction, the changes in airflow, combined with the varying amount of

irregularities in the added corrugation that come from the fact that the devices are handmade,

explain the inconsistent results across the two experiments. The surface area experiments show

that adding some corrugation to flat surfaces consistently resulted in greater temperature drops,

but reducing the interridge distance to add more corrugation also reduced the vapor concentration

gradient from the surface to the ambient environment which slows evaporation. This suggests that

the parameters of the corrugations could be optimized to maximize cooling performance.

44

Figure 11. Mass Transfer Coefficient for Surface Area Experiment 1. The mass

transfer coefficient is how fast saturated air is replenished with drier air near the surface of

the device. A lower mass transfer coefficient reflects a low evaporation flux because the

concentration of lingering saturated air at the device’s surface decreases water evaporation.

Adding corrugation to the devices causes the mass transfer coefficient to decrease because

the geometry reduces the vapor concentration at the surface [32]. Zone 2 has the steadiest

mass transfer coefficient because the evaporative cooling rates are steadiest there. Near the

end of the experiments, the large increases and negative values of the mass transfer

coefficient come mathematically from the decrease in concentration gradient between the

ambient air and the air near the surface as the devices have less water.

45

Figure 12. Mass Transfer Coefficient for Surface Area Experiment 2. The mass

transfer coefficient profiles in experiment 2 have the same inverse relationship with

increased surface corrugation but not as distinctly due to differences in airflow ambient

conditions. The negative values and large increased in value near the end are caused as the

depleting water source decreases the concentration gradient. The increased in values near

the beginning results from ambient conditions.

In Zone 3—Drying Transient, the temperature drops of all the devices with surface corrugations

begin to decrease as they have less remaining water to evaporate; consequently, their surfaces

begin to dry. Their rates of evaporation are limited by the reduced partial vapor pressure gradient

inside the troughs of the corrugation. In Zone 3, the effect of having a greater absolute amount of

water added has the most influence on the device’s cooling performance. Each device has a

different amount of water added but maintains the same sand to water ratio to account for the

differences in dimensions of each device. The more water a given device has, the longer it is

46

expected to maintain evaporative cooling. Nonetheless, the devices with the added corrugated

surfaces have a higher absolute amount of water added than the device without corrugation, yet

their temperature drops start to decrease sooner than the device with no added corrugated surface.

This occurs because devices with corrugated surfaces have faster rates of evaporation, which leads

to a faster depletion of total water reserve. In the first experiment, the temperature drop profile of

the 1.77x Device deviates from the other devices’ profiles, measuring temperatures greater than

ambient temperature for an initial time period before returning to similar temperature drop values

as the other devices. When the temperature drop profile of the 1.77x Device measures above zero,

it still captures the same undulation as the other devices; however, the data is offset, indicating

sensor hardware error. Table 1 summaries key performance metrics.

Entering Zone 4—Drying Steady State, all the devices continue to approach a temperature drop of

zero as the remaining water is still evaporating. The thermal mass of the devices also helps

maintain the coolness. The cooling effect in Zone 4 is not as large as in the other zones, but it lasts

longer because the evaporation rate is low. The evaporation rate is now limited by how quickly

water vapor inside the sand gap and clay wall can diffuse to the surface [18,28]. In Zone 2, the

evaporation rate was limited by how quickly water vapor can diffuse away from the outer surface

through air, which is faster than the water vapor transport dynamics through the device. In

experiment 2, the runtime is twice as long, and near the end of Zone 4, the temperature drops of

all the devices approach zero.

47

Temperature on Evaporation Surface

Figure 13 and Figure 14 display the temperature on the evaporation surface of each device as a

function of time. The temperature drop profiles on the evaporation surfaces best reflect the

temperature changes due to water evaporation. The greater the temperature drop, the more

evaporative cooling that occurred. In both experiments, the temperature drops of the devices with

surface corrugation are greater than the temperature drop of the 1x Device. This further supports

that adding surface corrugations leads to more evaporation and evaporative cooling. Table 2

displays key performance metrics of the temperature drop on the evaporation surface. Among the

devices with added corrugation, the temperature drop performances of the individual devices are

not consistent between experiment 1 and experiment 2. The inconsistency is attributed to the

competing factors that arise from increasing available surface area. Adding corrugations will

increase the total surface area for more water evaporation but will decrease the vapor pressure

gradient inside the troughs.

48

Figure 13. Evaporation Surface Temperature Drop for Surface Area Experiment 1.

The temperature on the evaporation surface reflects the amount of heat transfer due to

evaporation. The devices with the surface corrugation all have greater temperature drop

than the device without. This signifies there is more evaporative cooling that occurs on the

surface of the devices with the added corrugation. Among the devices with added

corrugation, the devices with more surface area added record greater temperature drops.

49

Figure 14. Evaporation Surface Temperature Drop for Surface Area Experiment 2.

The temperature drop profiles on the evaporation surface in experiment 2 have less

differences among them than in experiment 1 because of the ambient conditions. In Zone

2, the 1x Device has a smaller temperature drop than the devices with added surface

corrugation. This is also observed in experiment 1. Among the devices with added

corrugation, the 1.52x Device that had the lowest temperature drop in experiment 1 now

has the greatest temperature drop. The inconsistency in performance of the devices with

surface corrugation is because of the competing effect between increasing surface area for

evaporation and decreasing the mass transfer coefficient.

50

Table 2. Evaporation Surface Temperature Drop Cooling Performance for Surface

Area Experiments. The maximum temperature drop on the surface and the time it occurs

are presented. The greatest temperature drop is highlighted. The time the devices in the

second experiment take to reach the maximum temperature drop is almost 2-3 times as

long as in the first experiments due to ambient conditions.

Experiment

Number Devices

Amount of

Water

Added

[kg]

Max Surface

Temperature

Drop

[°𝐂]

Time Max

Surface

Temperature

Drop

[Hour]

1 1x Device 2.84 4.24 12

1 1.52x Device 3.43 4.91 62

1 1.77x Device 3.03 5.39 62

1 2.59x Device 3.13 5.49 62

2 1x Device 3.06 3.00 41

2 1.52x Device 3.32 4.27 135

2 1.77x Device 3.06 4.07 207

2 2.59x Device 3.13 4.28 207

51

Cooling Efficiency

Figure 15 and Figure 16 illustrate the cooling efficiency profiles of the different devices. For both

experiments, the greatest efficiency occurs during Zone 2—Cooling Steady State—and aligns with

when the cooling devices have their lowest temperature drop. In Zone 3, the cooling efficiency of

all the devices except for the 1x Device’s drops as the internal temperature drop decreases. In

Zones 3 and 4, the 1x Device records the greatest cooling efficiency because it has the greatest

temperature drop. The 1x Device still has water for evaporative cooling due to its slower rate of

evaporation. The cooling efficiency profiles of the first and second experiments share similar

trends and observation as show in Table 3. The maximum cooling efficiency of the devices occur

before their maximum internal temperature drops in experiment 1 but after their maximum internal

temperature drops in experiment 2. The difference in performance is due to the differences in

ambient conditions. The cooling efficiency captures what percentage of maximum temperature

drop (wet-bulb temperature drop from ambient) the devices achieved so it is not unexpected that

the time the maximum cooling efficiency occurs does not align with when the maximum internal

temperature drop occurs.

52

Figure 15. Cooling Efficiency for Surface Area Experiment 1. The maximum cooling

efficiency occurs in Zone 2 and before the maximum internal temperature drop. The 1x

Device has the lowest cooling efficiency of all the devices in Zone 2 but near the end of

Zone 3, it has a highest cooling efficiency. The 1x Device’s slower evaporation rate enables

it to still have water to evaporate. The 1.77x Device has a negative cooling efficiency

because it recorded internal temperature above ambient temperature due to sensor hardware

error as previously discussed.

53

Figure 16. Cooling Efficiency for Surface Area Experiment 2. The maximum cooling

efficiency in experiment 2 occurs in Zone 2. The devices’ maximum cooling efficiency

occurs after their maximum internal temperature drops. The devices with the surface

corrugation have the maximum cooling efficiency overall. In Zone 3 and 4, the 1x Device

has a greater cooling efficiency because it still has water to evaporate due to its slower

evaporation rate.

54

Table 3. Cooling Efficiency Performance for Surface Area Experiments. The max

cooling efficiency and the time the efficiency occurs is presented with the maximum

efficiency highlighted.

Experiment

Number Devices

Amount of

Water

Added

[kg]

Max

Efficiency

[%]

Time Max

Efficiency

[Hour]

1 1x Device 2.84 52.39 13

1 1.52x Device 3.43 58.50 40

1 1.77x Device 3.03 45.59 39

1 2.59x Device 3.13 65.28 40

2 1x Device 3.06 40.95 184

2 1.52x Device 3.32 69.98 182

2 1.77x Device 3.06 61.22 182

2 2.59x Device 3.13 66.10 182

55

Total Cooling

The total cooling performance metric accounts for the duration of the evaporative cooling to better

compare the devices with slower but longer rates of evaporation, i.e. the 1x Device to the other

devices. The 1x Device has a smaller temperature drop overall but maintains evaporative cooling

for a longer time period than the others. Figure 17 and Figure 18 show the total cooling as a

function of time for the two experiments. In both experiments, the total cooling profiles of the

devices with surface corrugations have two distinct slopes determined by what factors limit their

evaporative cooling. In Zone 2 of both experiments, the entirety of the devices’ surface becomes

wet and the evaporation rate is limited by how quickly vapor saturated air can be replaced with

drier air; temperature, relative humidity, and air flow controls this stage [18,28]. As the devices

lose more water, and the surfaces of the devices begin to dry out, the evaporation rate is now

limited how quickly water vapor inside the sand gap and clay wall can diffuse to the surface; the

porosity, permeability, and tortuosity of the sand and clay wall dictate the evaporation rate in this

stage [18,28]. The slope at the beginning of the experiment is steeper than later in the experiment

because water vapor on the surface can diffuse into the ambient air quicker than it can diffuse

through the sand gap and clay wall. How quickly each device with corrugated surface reaches the

transition point where the slope changes depends on when the liquid filled cavities near the outer

surface become disconnected because capillary forces that brought water to the outer surface to

evaporate stop [31]. The 1x Device reaches its transition point much later than the devices with

corrugated surfaces even though it has the least amount of absolute water added. The 1x Device

maintains its cooling effect for longer than the others because its rate of evaporation was smaller

in the previous zones; it still has more water left to evaporate and sustain capillary forces to draw

water to its outer surface.

56

The total cooling of the devices in the first experiment reveals that the 1.52x Device has a lower

total cooling amount until Zone 3 when it surpassed the 2.59x Device. The crossover occurs

because the 2.59x Device receives 0.26 kg less total amount of water than the 1.52x Device and

has a higher rate of evaporation at the beginning of the experiment. The 1x Device receive 0.59 kg

less than the 1.52x Device but at the end of Zone 4, its total cooling is comparable to 2.59x and

1.52x Device. Table 4 presents the total cooling achieved by the end of the experiments. The 1x

Device total cooling performance presents an interesting practical opportunity for users in water

scarce, hot, and arid environment. The device does not provide as great of a temperature drop but

maintains it cooling effects for longer with less water added.

The 1.77x Device exhibits unexpected result that is also reflected in Figure 9, the internal

temperature drop figure. The performance of the 1.77x Device is attributed to sensor error so it is

not included in the analysis. The second experiment lasts twice as long as the first experiment to

further observe the devices’ cooling performance. In the second experiment, the total cooling of

1x Device surpasses the total cooling of the 1.77x Device near the end of the experiment.

57

Figure 17. Total Cooling for Surface Area Experiment 1. The total cooling performance

metrics accounts for how long evaporative cooling lasts. The 1x Device has a smaller

temperature drop than the other devices but by the end of the experiment, its total cooling

is comparable to the others. The 1.77x Device total cooling profile reflects the sensor

hardware error.

58

Figure 18. Total Cooling in Surface Area Experiment 2. The total cooling of the devices

with the surface corrugation share similar profiles. The point where the profile changes

slope at the end of Zone 2 aligns with when the devices run low on water. The 1x Device

has the least amount of water added but is able to maintain cooling effect and achieve a

comparable total cooling amount at the end of the second experiment. Its low evaporation

rate allows it to still have water to evaporate when the other devices have already

evaporated the majority of their water supply. The 1x Device surpasses the total cooling of

the 1.77x Device near the end of the experiment.

59

Table 4. Total Cooling Performance for Surface Area Experiments. The total cooling

at the end of the experiments is provided. The 2.59x Device in experiment 1 has the greatest

total cooling until near the middle of Zone 3 when it is surpassed by the 1.52x Device. The

total cooling amount of the 1.77x Device in the first experiment is omitted because of the

sensor hardware error. In experiment 2, the 1x Device surpasses the 1.77x Device near the

end of the experiment.

Experiment

Number Devices

Amount of

Water

Added

[kg]

Total

Cooling

[°𝐂 × 𝐇𝐨𝐮𝐫]

Time Total

Cooling

[Hour]

1 1x Device 2.84 811 306

1 1.52x Device 3.43 923 306

1 1.77x Device 3.03 - 306

1 2.59x Device 3.13 890 306

2 1x Device 3.06 1097 613

2 1.52x Device 3.32 1205 613

2 1.77x Device 3.06 1059 613

2 2.59x Device 3.13 1152 613

60

Relative Humidity

At the start of the experiment, the added water begins evaporating into the internal storage vessel

of the device and out into the environment. Figure 19 and Figure 20 display the relative humidity

inside the internal storage chamber, on the surface, and in the environment around the device. The

relative humidity inside the device increases faster than the relative humidity on the surface

because the inside is covered by a lid. When air in the inner vessels reaches saturation, more water

begins to evaporate from the outer surface because the saturated air inside the internal storage

vessel has less capacity to hold more water vapor. The relative humidity and the partial pressure

of water vapor increases around the vicinity of the device as more water evaporates from the

outside surface. The difference between the partial pressure of water vapor at the surface and in

ambient environment creates a concentration gradient that moves the water vapor away from the

device to allow for more evaporation.

In experiment 1, the relative humidity measurements of the 1.52x Device and the 2.59x Device

suggest that the measurement sensors have gotten wet inside the device. It is unrealistic for any

devices to measure 99.9% relative humidity in Zone 4 as they all have porous lids that cannot

contain water vapor for over 230 hours. The 99.9% relative humidity measurements inside of the

1.77x Device is a result of the sensor hardware error. It is also unrealistic for the relative humidity

inside the 2.59x Device to increase in Zone 4. The internal relative humidity data for the devices

with surface corrugation is thus omitted from analysis.

In experiment 2, the devices with corrugation all drop below 80% relative humidity before the 1x

Device. This is attributed to the differences in surface area. The devices with surface corrugation

evaporate water faster than the 1x Device so that by Zone 4, these devices run low on water even

though they all have greater absolute amounts of water added. Among the devices with surface

61

corrugation, adding more surface area increases how long the internal relative humidity lasted.

This is because added corrugation can decrease the concentration gradient at the surface which can

slow evaporation. Table 5 summaries the key relative humidity metrics inside and on the

evaporative cooling surface.

Figure 19. Relative Humidity in Surface Area Experiment 1. The relative humidity

levels inside and on the evaporative surface of the device are greater than the ambient

environment relative humidity because of water evaporation. The internal relative humidity

is greater than on the surface because of the wetted lid. The relative humidity inside and

on the surface decreases over time because the devices run out of water to evaporate. The

2.59x Device dips below 80% relative humidity first because its higher levels of

evaporation means that it has less water to evaporate. The saturated relative humidity of

the 1.52x and 1.77x Device suggests that the measurement sensors may have encounter

water inside the device that wetted the measurement sensor because it is unlikely for any

cooling device with a porous top to maintained saturated levels of relative humidity until

Zone 4.

62

Figure 20. Relative Humidity in Surface Area Experiment 2. The relative humidity

levels inside and on the surface of the device is higher than the ambient relative humidity.

This is also seen in Experiment 1. The difference between the relative humidity on the

surface and in the ambient is smaller in experiment 2 than in experiment 1. The effect of

this difference is reflected in the other performance metrics as well. The internal relative

humidity of the 1x Device remain above 80% for longer than all devices with surface

corrugation. This is due to how the 1x Device has a smaller evaporation rate than the others

so it is able to maintain evaporation until the end of Zone 4.

63

Table 5. Relative Humidity of the Surface Area Experiments. The relative humidity

(RH) at the end of the experiment inside the device is provided along with the time when

the relative humidity first measured below 80%. No data is presented from the devices with

corrugations in the first experiment on the time the relative humidity measures below 80%

and on the final measurement of the internal relative humidity measurement at the end of

the experiment because the data does not realistic represent the relative humidity inside

those devices.

Experiment

Number Devices

Amount

of Water

Added

[kg]

Time <80%

Internal RH

[Hour]

Internal RH

at the End

[%]

1 1x Device 2.84 222 63.27

1 1.52x Device 3.43 - -

1 1.77x Device 3.03 - -

1 2.59x Device 3.13 - -

2 1x Device 3.06 485 46.98

2 1.52x Device 3.32 303 28.07

2 1.77x Device 3.06 311 32.73

2 2.59x Device 3.13 335 31.41

The devices in the surface area experiments have shown that adding corrugations results in greater

rates of evaporation, higher max cooling efficiencies, and greater temperature drops internally

compared to a device that does not have corrugation. However, adding more corrugation does not

consistently lead to better cooling performance. The corrugation geometry introduces competing

effects between the increased available surface area for water evaporation and lowered

concentration gradient for mass transport of saturated air inside the troughs of the corrugation

[28,32]. When the concentration gradient is reduced the diffusion in the boundary layer becomes

the bottle neck in the evaporation process.

The inconsistency in cooling performance between experiments among the devices with

corrugations is due to differences in temperature, relative humidity, and airflow in the ambient

environments. For the devices in the surface area experiments, airflow particularly has a large

64

influence because the direction of the airflow over the corrugations impacts the concentration

gradient in the troughs [32]. When the air flows perpendicular to the corrugation, it induces an

inverse relationship between the amount of added surface area and the mass transfer coefficient;

when the air flows parallel to trough, it has the same airflow patterns as flowing over a flat-surface

[32]. Since the devices in the surface area experiments are exposed air flow with no set direction,

the cooling performance is varied depending on ambient conditions. The corrugations on the

devices itself have irregularities in their dimensions that also contribute to the inconsistent cooling

performance.

Between the two experiments, the devices with some corrugation consistently have a greater

temperature drop internally than the device without corrugation; however, more corrugation does

not always lead to better cooling performance. This suggests that the parameters of the corrugation

can be optimized for better cooling performance. The length, width, orientation and entrance angle

of the corrugation can impact the cooling performance. Table 6 shows a comprehensive table of

all the performance metrics.

The relative humidity inside the device without corrugation lasts the longest with the least absolute

amount of water added. Its evaporation rate is slower than the devices with corrugation so it is able

sustain its water supply until near the end of each experiments. The device without corrugation has

the smallest temperature drop but overall, its total cooling performance is comparable to the other

devices. For users in water scarce area, the device without corrugation may be more appropriate.

For users with more access to water, adding more water at the end of Zone 2 will enable the devices

to sustain its maximum temperature drop and high internal relative humidity for longer.

In general, adding corrugations to the outer vessels increases maximum temperature drop, but there

are practical tradeoffs for users to consider. To create uniform surface corrugations on clay pots,

65

additional equipment such as corrugation cutting tools and pottery wheels are required. The users

will need access to more skilled potter to create the corrugations as well. Adding corrugations will

increase the total weight and the total amount of time required to make a single pot. The

corrugations are more susceptible to being chipped so they necessitate additional care in handling

and transportation. The added weight and additional required care further burdens clay pot

vendors. A survey of clay pot and container vendors in Burkina Faso found that only a third of

vendors would deliver clay pots to their customer because of how heavy and fragile clay pots are

[23]. Depending on the final geometry of the added corrugation and the availability of skill labor,

evaporative cooling devices with added corrugations may not be practical or accessible.

66

Tab

le

6.

Co

mp

reh

ensi

ve

Co

oli

ng

Per

form

an

ce

Met

rics

of

Su

rface

A

rea

Exp

erim

ents

. T

he

max

imum

v

alu

e o

f ea

ch

per

form

ance

met

ric

is h

ighli

gh

ted. F

or

the

tem

per

ature

met

rics

, any v

alue

that

is

wit

hin

the

sen

sor

erro

r o

f ±

0.5

°C

of

the

max

imum

val

ue

is a

lso

hig

hli

gh

ted

. T

he

"nev

er"

term

is

appli

cable

only

wit

hin

the

tim

espan

of

the

resp

ecti

ve

exp

erim

ent.

Th

e co

oli

ng

per

form

ance

met

rics

th

at a

re m

ost

pra

ctic

al f

or

use

rs a

re t

he

inte

rnal

tem

per

ature

dro

p,

the

tim

e b

efo

re t

he

inte

rnal

tem

per

ature

dro

p r

etu

rns

to z

ero

and

ho

w l

ong h

igh l

evel

s of

inte

rnal

rel

ativ

e hum

idit

y l

ast.

In b

oth

exp

erim

ents

, th

e dev

ices

wit

h s

urf

ace

corr

ug

atio

n h

ave

gre

ater

inte

rnal

tem

per

ature

dro

ps

bu

t did

not

sust

ain t

hei

r over

all

evap

ora

tive

cooli

ng

fo

r as

lo

ng

as

the

dev

ice

wit

ho

ut

surf

ace

corr

ug

atio

n.

The

dev

ice

wit

hout

corr

ugat

ion a

lso m

ainta

ins

rela

tive

hum

idit

y a

bo

ve

80%

fo

r lo

ng

er t

han

the

dev

ices

wit

h c

orr

ug

atio

n.

Th

ere

is a

tra

de-o

ff b

etw

een h

avin

g g

reat

er i

nte

rnal

tem

per

atu

re d

rop

an

d l

on

ger

per

iod

s of

inte

rnal

cooli

ng

an

d h

igh

lev

els

of

inte

rnal

rel

ativ

e hum

idit

y.

For

use

rs i

n w

ater

-sca

rce

area

, th

e dev

ice

wit

ho

ut

corr

ug

atio

n m

ay b

e m

ore

appro

pri

ate.

Inte

rna

l

RH

at

the

En

d

[%]

63

.27

- - -

46

.98

28

.07

32

.73

31

.41

Tim

e

<8

0%

Inte

rna

l

RH

[Ho

ur]

22

2

- - -

48

5

30

3

31

1

33

5

Tim

e

To

tal

Co

oli

ng

[Ho

ur]

30

6

30

6

30

6

30

6

61

3

61

3

61

3

61

3

To

tal

Co

oli

ng

[°C

× H

ou

r]

81

1

92

3

-

89

0

10

97

12

05

10

59

11

52

Tim

e M

ax

Eff

icie

ncy

[Hou

r]

13

40

39

40

18

4

18

2

18

2

18

2

Max

Eff

icie

ncy

[%]

52.3

9

58.5

45.5

9

65.2

8

40.9

5

69.9

8

61.2

2

66.1

0

Tim

e

Max

Su

rface

Tem

p

Dro

p

[Hou

r]

12

62

62

62

41

135

207

207

Max

Su

rface

Tem

p

Dro

p

[°C

]

4.2

4

4.9

1

5.3

9

5.4

9

3

4.2

7

4.0

7

4.2

8

Tim

e

Tem

p

Dro

p

Ret

urn

s

to Z

ero

[Hou

r]

218

216

123

213

Nev

er

360

360

360

Tim

e

Max

Inte

rnal

Tem

p

Dro

p

[Hou

r]

14

63

62

62

41

135

207

207

Ma

x

Inte

rnal

Tem

p

Dro

p

[°C

]

5.7

2

6.5

6

5.3

5

7.2

4

4.2

8

5.8

1

5.4

2

5.6

8

Am

ou

nt

of

Wa

ter

Ad

ded

[kg

]

2.8

4

3.4

3

3.0

3

3.1

3

3.0

6

3.3

2

3.0

6

3.1

3

Dev

ices

1x

Dev

ice

1.5

2x

Dev

ice

1.7

7x

Dev

ice

2.5

9x

Dev

ice

1x

Dev

ice

1.5

2x

Dev

ice

1.7

7x

Dev

ice

2.5

9x

Dev

ice

Exp

Nu

mb

er

1

1

1

1

2

2

2

2

67

3.2. Effects of Porosity

Figure 21 summarizes the devices used in the two porosity experiments. In the first experiment,

all the inner storage vessels are the same material. In the second experiment, the inner and outer

vessels of each device share the same level of porosity. Only the 7.20% porosity and the 11.22%

porosity devices use sawdust to form their porosity. All three of the devices share the same

terracotta base clay.

Figure 21. Evaporative Cooling Devices in the Porosity Experiments. The increased

levels of porosity allow for higher levels of water permeability and greater rates of

evaporation. The devices are labeled based on their porosity level and are colored to match

the plots.

68

Wet-bulb Temperature

Figure 22 compares the wet-bulb temperature of the two porosity experiments. The wet-bulb

temperature profile of the second porosity experiment is the same as the temperature profile of the

second surface area experiment because those experiments were conducted in parallel. Similarly,

to the surface area experiments, the differences between the temperature profiles prevent direct

comparison of the cooling devices across experiments. The devices have a greater cooling potential

in the first experiment due to a lower wet-bulb temperature profile.

Figure 22. Wet-bulb Temperature Comparison Between Porosity Experiment 1 and

2. The wet-bulb temperature represents the greatest cooling achievable through evaporative

cooling. The difference in wet-bulb temperature between the first and second experiment

is large enough to prevent a direct comparison across experiments, but the general trends,

repeated similarities and differences are noted and analyzed.

69

Internal Temperature

The internal temperature drop increases as the porosity in the clay wall increases as shown Figure

23 and Figure 24. The four zones depicted on the figures are also an approximation determined

by observation of changes in the internal temperature drop profile. Each device enters and exits

each zone at a separate time based on its internal properties.

In the previous section on surface area experiments, all devices have the same composition; as a

result, all devices share similar temperature drops at the beginning of the experiments. Their

evaporation rates are limited by how quickly the saturated air near the surface of the devices can

be replaced with drier air [18,28]. However, in the porosity experiments, the devices with more

porosity in their clay wall have greater vapor concentration because their clay walls have more

cavities to hold water. A greater vapor concentration at the surface leads to a greater concentration

gradient, which in turn drives the evaporation rate faster. In Zone 1 of the first porosity experiment,

the two devices with greater porosity have greater temperature drops than the device with lower

porosity due to the differences in vapor concentration at the surface of each device.

The devices in the first experiments all have internal vessels at 3.14% porosity. In the second

experiment, the porosity levels of the internal vessels match the porosity levels of the outer vessel.

For the 7.20% porosity and 11.22% porosity devices, the increase in porosity levels of the internal

vessels decrease the concentration gradient in these devices’ sand gap between experiment 1 and

2. The decreased concentration gradient inside the sand gap, in combination with the lowered

cooling potential as evident by the wet-bulb temperature profiles in Figure 22, explain why near

the beginning of Zone 1 of the second porosity experiment, the temperature drops of the three

devices are similar to each other. The temperature drop profiles of the three devices separate as the

added water wets more of the outer surfaces. The temperature drop profile of the 11.22% porosity

70

device deviates from the other profiles first because its higher percentage of porosity in its outer

vessel allows for greater capillary flow to wet its outer vessel’s surface quicker. The temperature

drop profile of the 7.20% porosity device separates from the 3.14% porosity later in the middle of

Zone 2.

In Zone 2, the temperature drop profiles reach a steady cooling rate. The oscillation occurs because

of the daily fluctuation of temperature in the indoor testing space. The difference in temperature

drop in Zone 2 across devices is due to the influence of porosity on water transport resistance. As

more water evaporates from the surface, the devices with more porosity are able to rewet their

surface quicker because there is more voids near the surface to hold water [16]. As expected, the

device with the highest porosity (11.22% porosity device) achieves the greatest temperature drop.

It was able to achieve the greatest temperature drop even with 0.45 kg less water than the 7.20%

porosity device. The device with the lowest porosity (the 3.14% porosity device) has the lowest

temperature drop but its cooling effect lasts the longest in both experiments. In experiment 1, Zone

2 of the 3.14% porosity device lasts until the end of the experiment, and in experiment 2, Zone 2

of the 3.14% porosity device lasts until around the 300-hour mark. It sustains evaporative cooling

for longer than the other devices because its lower porosity slows its evaporation rate. As a result

of the slower rates, the 3.14% porosity device has more water left by the end of the experiment

even though it has the least absolute amount of water added.

The maximum temperature drops in the second experiment of the 7.20% porosity and the 11.22%

porosity devices are not as large as their drops in experiment 1 due to lower cooling potential and

differences in their internal vessels’ material porosity between experiment 1 and 2. In experiment

2, the internal vessels of the 7.20% porosity and the 11.22% porosity have greater percentages of

porosity than in experiment 1. The greater percentage of porosity allows for water to evaporate

71

into the inner storage vessel more easily and reduce the concentration gradient inside of the sand

gap of the devices and the overall evaporation rate that influences the internal temperature drops.

The temperature drops in the devices with higher levels of porosity begin decreasing due to

depleted levels of water in Zone 3. The water inside the devices with more porosity experience

less transport resistances across the clay wall. As the devices dry, the evaporation rates slow due

to reduced water availability near the evaporation surface and disruption to the continuous network

of liquid-filled pores inside the clay walls [17]. The moisture content in the device is greatest near

the bottom because of gravity and close to the internal vessel because the water near the surface

evaporates first. The evaporation rate of the 11.22% porosity device decreases first because the

11.22% porosity device has the least remaining water due to its greater rate of evaporation in the

previous zones and because it has a lower absolute amount of water added than the 7.20% porosity

device. Table 7 summarizes the cooling performance of the devices in both experiments.

In Zone 4, the temperature drops of the two devices with more added sawdust gradually approach

zero. They have not reached zero yet because there is still moisture inside the device that is

evaporating and the device’s thermal mass provides cooling inertia [31]. Although the devices

have not completely dried out, their evaporation rates are lower than in the previous zones.

72

Figure 23. Internal Temperature Drop for Porosity Experiment 1. The temperature

drop increases as the porosity of the device’s outer clay wall increases. The effect of the

added porosity is seen immediately in Zone 1 when the temperature drops of all the devices

deviate from each other. The greatest temperature drop occurs in Zone 2 when the

evaporative cooling is the steadiest. The device with the lowest porosity maintains its

evaporative cooling for the longest while the two devices with sawdust begin drying out in

Zone 3 as evident by the temperature drop decrease. Zone 2 of 3.14% porosity device lasts

until around the 300-hour mark. Near the end of the experiment, the temperature drop of

the devices with sawdust begin decreasing to zero while the 3.14% porosity device without

sawdust continues its constant evaporative cooling rate. It still has water to evaporate due

to its lower evaporative rate throughout the entire experiment.

73

Figure 24. Internal Temperature Drop for Porosity Experiment 2. The internal

temperature drops of the devices with higher porosity are greater than the temperature

drops of the devices with lower porosity throughout of Zone 1 and Zone 2. In Zone 3, the

temperature drops of the higher porosity devices decrease as water depletes until Zone 4

where their evaporative cooling is still present but at a lower rate. The second experiment

lasts for twice as long as the first experiment to observe the dynamics of the devices. At

the end of the second experiment, all devices share a similar temperature drop and just

started to reach zero.

74

Table 7. Internal Temperature Drop Cooling Performance for Porosity Experiments.

The maximum temperature drop and the time it occurred is listed for each device in both

experiments. The total amount of water added along with the time when the temperature

drop first returns to zero is also included. The maximum temperature drop is highlighted.

The devices with the lowest amount of porosity never had their temperature drop return to

zero. In both experiments, the devices with the highest level of porosity achieved the

greatest temperature drop.

Experiment

Number Devices

Amount of

Water

Added

[kg]

Max

Temperature

Drop

[℃]

Time Max

Temperature

Drop

[Hour]

Time

Temperature

Drop

Returns to

Zero

[Hour]

1 3.14% Porosity 2.91 4.41 280 Never

1 7.20% Porosity 3.85 8.43 89 218

1 11.22% Porosity 3.39 9.45 62 219

2 3.14% Porosity 3.06 4.28 41 Never

2 7.20% Porosity 3.73 5.60 135 360

2 11.22% Porosity 3.61 7.67 109 360

75

Temperature on Evaporation Surface

The temperature drop measured on the evaporation surface is a direct indication of evaporative

cooling. A greater temperature drop indicates a greater evaporation rate. Figure 25 and Figure 26

display the temperature drop profile on the evaporation surface for the devices in each experiments.

The devices that have higher levels of porosity achieve greater temperature drops than the device

with lower porosity, as expected, in both experiments. In the first experiment, between just the

devices with sawdust, it is unexpected that the 7.20% porosity device has the same temperature

drop on the surface as the 11.22% porosity device in Zones 1 and 2 when their internal temperature

drops do not share the same overlap. The discrepancy does not occur in experiment 2 even though

there is an overlap between the 3.14% porosity and the 7.20% porosity devices near the beginning.

The overlap in experiment 2 is reflected in the internal temperature drop measurements. A possible

explanation for the discrepancy seen in experiment 1 could be the presence of a temperature

gradient inside the devices that then affects temperature sensor readings depending on their

placement. The sensors were generally placed in the same area with respect to the water bottles

added inside as thermal mass, but variation could have occurred. To cross-validate the cooling

performance of the devices in experiment 1, Figure 27 displays the mass loss and the evaporation

rate. The evaporation rates of the 7.20% porosity and the 11.22% porosity devices are remarkably

similar, which indicates that their internal temperature drop should have recorded similar values.

Figure 28 shows the mass loss and evaporation rate of the second porosity experiment. The

11.22% porosity device has a greater evaporation rate than the 7.20% porosity device. The 3.14%

porosity and the 7.20% porosity devices share an overlap in evaporation rate that is reflected in

both the temperature drop profile on the surface and inside the device. In both experiments, the

76

evaporation rates of the two sawdust devices are higher than the evaporation rate of the device

without sawdust.

Zones 3 and 4 of both experiments’ evaporation surface temperature drop profiles share similar

trends with the internal temperature drop profiles. The devices with sawdust have the biggest

temperature drop decrease in Zone 3 because they had the greatest evaporation rates in Zone 2.

The temperature drop of the devices with sawdust experience a more gradual decrease for the

duration of Zone 4. The temperature drop of the 3.14% porosity device did not return to a

temperature drop of zero in the first experiment. In the second experiment that ran for 613 hours,

the temperature drop on the surface of the 3.14% porosity device does approach zero.

The inconsistency in when the devices reach their greatest temperature drop between the two

experiments is attributed to ambient conditions and the differences in internal vessel’s porosity.

The 3.14% porosity device is the same in both experiments, but the 7.20% porosity and the 11.22%

porosity devices have more porous internal vessels in the second experiment. The two devices with

sawdust (7.20% porosity and 11.22% porosity devices) took more time before reaching their

maximum temperature drop in the second experiment because the devices in the second

experiment have less transport resistance in the inner vessels’ material. In the first porosity

experiment, the inner vessels of the 7.20% porosity and the 11.22% porosity devices have higher

transport resistance that led to the development of a larger gradient on the surface of each device’s

inner vessel. The concentration gradient in the sand gap drives more water to the outer surface,

which has higher porosity and lower transport resistance. Table 8 shows the temperature drop on

the evaporation surface for both experiments.

77

Figure 25. Evaporation Surface Temperature Drop for Porosity Experiment 1. The

temperature on the evaporation surface reflects the amount of heat transfer due to

evaporation. The devices with higher levels of porosity both record greater temperature

drop than the device with lower porosity which reflects the influence of porosity on

evaporation. Between just the devices with sawdust added, it is unexpected to see that the

7.20% porosity device has the same temperature drop as the 11.22% porosity device when

its internal temperature drop is lower. This is attributed to a sensor placement difference.

78

Figure 26. Evaporation Surface Temperature Drop for Porosity Experiment 2. The

temperature drop profiles on the surface of the devices in the second porosity experiment

reflect the same trends as the internal temperature drop profiles. The temperature drop

increases when porosity increases. The overlap of the 3.14% porosity and the 7.20% at the

beginning is also reflected in the internal temperature drop. This overlap is caused by a

combination of internal structure and ambient conditions that resulted in the two devices

sharing similar cooling performance.

79

Figure 27. Mass Loss Rate and Mass Loss for Porosity Experiment 1. A curve (dotted

line) was fitted to the mass loss of each cooling device. In the legend, the normalized root

mean square error is given for each curve fit. The derivative of the curve fit is the

evaporation rate because water evaporation is the only mass transfer that occurs. The

evaporation rate of the 7.20% porosity and the 11.22% porosity device are very similar and

further confirms that their internal temperature difference is a function of sensor placement.

Both the devices with higher porosity clay walls records greater rates of evaporation than

the 3.14% porosity device.

80

Figure 28. Mass Loss Rate and Mass Loss for Porosity Experiment 2. A curve was

fitted to the mass loss of each cooling device. In the legend, the normalized root mean

square error is given for each curve fit. The derivative of the curve fit is the mass loss rate

and synonymous with the evaporation rate because water evaporation is the only mass

transfer that occurs. The 3.14% porosity and the 7.20% porosity share an overlap of

evaporation rate at the start of Zone 2 that is reflected in the internal temperature drop and

the evaporative surface temperature drop.

81

Table 8. Evaporation Surface Temperature Drop Cooling Performance for Porosity

Experiments. The maximum temperature drops on the surface and the time they occur are

presented. The greatest temperature drop is highlighted. The devices with the greatest

porosity have the greatest temperature drop. It takes the devices with sawdust longer to

reach their maximum temperature drop in the second porosity experiment than in the first

because of differences in ambient conditions and internal structures that slow the rates of

evaporation.

Experiment

Number Devices

Amount of

Water

Added

[kg]

Max

Surface

Temperature

Drop

[℃]

Time Max

Surface

Temperature

Drop

[Hour]

1 3.14% Porosity 2.91 3.46 280

1 7.20% Porosity 3.85 7.30 39

1 11.22% Porosity 3.39 7.47 62

2 3.14% Porosity 3.06 3.00 41

2 7.20% Porosity 3.73 4.74 135

2 11.22% Porosity 3.61 6.42 108

82

Cooling Efficiency

The cooling efficiency of the devices in both experiments increases with higher levels of porosity.

For the devices with sawdust, their greatest cooling efficiencies occur in Zone 2, after which their

efficiency begins decreasing in Zone 3 as their surfaces dry. Although the cooling efficiency of

the 3.14% porosity device is low, it is the most stable and lasts the longest due to its slower

evaporation rate. In the second experiment, the longer experimental run time shows how the

cooling efficiencies of the devices gradually decrease to zero as less water is present for

evaporative cooling to occur. The negative cooling efficiencies at the end of the experiment occur

when the temperature inside the devices measure hotter than the ambient temperature.

Table 9 provides the maximum cooling efficiency values and the time they occur. The cooling

efficiency of the devices in the second experiment is generally lower than in the first experiment

because of the differences in the porosity of the internal storage vessel between the two

experiments and ambient conditions. The time that the maximum cooling efficiency and the

maximum internal temperature drop occur do not always align because the cooling efficiency is

what percentage of the maximum total cooling (wet-bulb temperature) the device captured. In the

first experiment, the maximum cooling efficiency occurs at the same time as the greatest maximum

temperature drop for the 3.14% porosity and the 7.20% porosity devices. The maximum cooling

efficiency for the 11.22% porosity device occurs sooner than its greatest temperature drop by 24

hours. In the second experiment, the 11.22% porosity device is the only device where the

maximum cooling efficiency occurs at the same time as its maximum temperature drop. The 3.14%

porosity device’s maximum cooling efficiency occurs 143 hours after its maximum temperature

drop and the 7.20% porosity device’s maximum cooling efficiency occurs 47 hours after its

maximum temperature drop.

83

Figure 29. Cooling Efficiency for Porosity Experiment 1. The cooling efficiency of the

devices show that the devices with more porosity have greater cooling efficiency until

around Zone 3 when water in the devices with higher porosity runs low. The cooling

efficiency of the 11.22% porosity device decreases fastest because it has the fastest

evaporation rate. It also has less total amount of water added than the 7.20% porosity

device. The 3.14% porosity device has the most consistent cooling efficiency that lasted

until the end of the experiment due to its slower evaporation rate.

84

Figure 30. Cooling Efficiency for Porosity Experiment 2. The cooling efficiency of the

devices in the second porosity experiment shows that the devices with more porosity have

greater cooling efficiency. The second experiment ran for almost twice as long as the first

and reveals how long the cooling effect in Zone 4 lasts as compared to Zone 1-3.

Table 9. Cooling Efficiency Performance for Porosity Experiments. The max cooling

efficiency and the time the efficiency occurred is presented with the maximum efficiency

highlighted.

Experiment

Number Devices

Amount of

Water

Added

[kg]

Max

Efficiency

[%]

Time Max

Efficiency

[Hour]

1 3.14% Porosity 2.91 37.24 280

1 7.20% Porosity 3.85 59.36 89

1 11.22% Porosity 3.39 66.61 38

2 3.14% Porosity 3.06 40.95 184

2 7.20% Porosity 3.73 54.63 182

2 11.22% Porosity 3.61 69.07 109

85

Total Cooling

The total cooling metric takes into account the maximum temperature drop as well as the length

of evaporative cooling process. It is expected that the devices with a higher absolute amount of

water added will have greater total cooling but that is not always the case. Figure 31 and Figure

32 show the total cooling in the first and second experiment.

In the first experiment, the total cooling of the 7.20% porosity device and the 11.22% porosity

device should be more similar, and their differences may be a result of sensor placement as

previously discussed. Nonetheless, 11.22% porosity device has a lower total amount of water

added and still achieves total cooling comparable to the 7.20% porosity device. Both the devices

with greater percentages of porosity achieve greater total cooling than the device with lower

porosity percentage. In second experiment, the 11.22% porosity device achieves the greatest total

cooling even though it has a lower absolute amount of water added than the 7.20% porosity device.

This is an important finding for users in water scarce area.

Each total cooling profiles has two distinctive slopes. The slope changes based on what factors

limit the devices’ evaporation rates. In fact, the evaporation rate figures (Figure 27 and Figure

28) have similar profiles to the total cooling plots. In Zone 2, the added water wets the entirety of

the devices’ surface, and the devices’ evaporation rates are limited by the water vapor diffusion

away from the surface. As the surface dries and the network of liquid filled pores is disrupted, the

evaporation rate is limited by how quickly water vapor or moisture can diffuse through the device

to the evaporation surface. The slope at the beginning is steeper than the slope at the end because

water vapor diffuses through air faster than it does through solids. Near the end of the second

experiment, the gap in the total cooling profiles between the 7.20% porosity and the 11.22%

porosity increases because water vapor can diffuse through more porous materials with less

86

transport resistance. In experiment 1, the total cooling of the 3.14% porosity device is a linear line

because its lower evaporation rate sustains constant cooling until the end of the experiment. The

second experiment allows more time for all devices to dry, and near the end of Zone 4, the total

cooling profile slope of the 3.14% porosity device decreases. Table 10 provides the total cooling

amount at the end of the experiment.

Figure 31. Total Cooling for Porosity Experiment 1. The total cooling performance

metrics accounts for how long evaporative cooling lasts in addition to maximum

temperature drop for each device. The two devices with the highest porosity records higher

total cooling than the 3.14% porosity. Between just the sawdust devices, the higher total

cooling amount of the 11.22% porosity device could be a factor of sensor placement as

previous discussed. The total cooling profile of the two devices with sawdust have two

clear slopes that the total cooling profile of 3.14% porosity device lacks. The 3.14%

porosity device is still evaporating water as fast as the air around its surface allows. With

more time, the 3.14% porosity device will also exhibit the same plateauing behavior as

seen in the second experiment as it runs out of water.

87

Figure 32. Total Cooling for Porosity Experiment 2. The 11.22% porosity device clearly

achieves higher total cooling than the other lower porosity devices. It also received a lower

absolute amount of water added than the 7.20% porosity device. The longer experimental

run time of the second experiment shows how the 3.14% porosity device just began

plateauing near the end. The point where the slope changes for the devices coincides with

when the evaporative cooling is no longer limited by the ambient conditions but by the

internal structures of each device. The more porous the device is, the easier it is for water

vapor to diffuse through the device and the greater the evaporative cooling effects. That

explains why the 11.22% porosity device reach its critical point before both of the other

porosity device but is still able to maintain a greater total cooling amount.

88

Table 10. Total Cooling Performance for Porosity Experiments. The total cooling at

the end of the experiments is provided. The devices with the greater amount of porosity

achieve the greater total cooling even if their internal temperature drop near the end of the

experiment is lower than the internal temperature drop of the 3.14% porosity device.

Experiment

Number Devices

Amount of

Water

Added

[kg]

Total

Cooling

[°𝑪 × 𝑯𝒐𝒖𝒓]

Time Total

Cooling

[Hour]

1 3.14% Porosity 2.91 776.50 331

1 7.20% Porosity 3.85 1018.71 331

1 11.22% Porosity 3.39 1065.61 331

2 3.14% Porosity 3.06 1096.73 613

2 7.20% Porosity 3.73 1160.62 613

2 11.22% Porosity 3.61 1411.73 613

89

Relative Humidity

Figure 33 and Figure 34 show the relative humidity inside and on the evaporation surface of each

device during the two experiments. A higher relative humidity on the surface signifies greater

evaporation activity. A higher relative humidity inside the device is desirable for practical purposes

of storing fruits and vegetables. In both experiments, the relative humidity on the surface of the

devices with added sawdust is higher than the relative humidity on the surface of 3.14% porosity

device. The devices with more porosity allow water to flow to the evaporation surface with less

transport resistance which result in a higher evaporation rate and higher relative humidity

measurements on the surface. As more water evaporates from the devices with higher levels of

porosity, near the end of the experiment, there is less water available to evaporate into the internal

storage vessels to maintain high levels of relative humidity. This explains why the internal relative

humidity of the 3.14% porosity device lasts the longest.

Between the first and second porosity experiment, the difference in porosity of the internal storage

vessels changes the internal relative humidity profiles. In experiment 1, the 7.20% porosity and

the 11.22% porosity device show similar patterns and values in their internal relative humidity

profiles, particularly as the humidity drops below 80%. In the second experiment, there is less

overlap because the higher levels of porosity in the internal storage vessel of the 11.22% device

causes its relative humidity to return to ambient faster than the 7.20% porosity device. Table 11

summarizes the relative humidity inside and on the surface of the devices.

90

Figure 33. Relative Humidity in Porosity Experiment 1. Added water in the device can

evaporate into the internal vessel or out into the environment. The relative humidity inside

the internal vessel is higher than on the relative humidity on the surface because there is a

dampened top that covers the internal vessel. There is also water that evaporates into the

internal vessel from the dampened top. The relative humidity inside the 3.14% porosity

device never dips below 80% even though it has the least amount of added water. Its lower

porosity levels allow it to sustain high levels of relative humidity for longer. The 3.14%

device also has a lower steady evaporation rate as evident by its relative humidity

measurements on its surface. The devices with higher porosity have higher levels of

relative humidity on the surface. These devices do not sustain high levels of relative

humidity for as long as the lower porosity device.

91

Figure 34. Relative Humidity in Porosity Experiment 2. The relative humidity of the

second experiment shows different trends than in the first experiment. The difference in

porosity of the internal storage vessels causes the internal relative humidity of 11.22%

porosity device to return to ambient quicker than the relative humidity of the 3.14% and

the 7.20% porosity device. At the beginning of the experiment, the difference between the

relative humidity on the surface and in the ambient environment is smaller than in

experiment 1. This difference is due to different ambient conditions. Both experiments

show that the relative humidity on the surface is greater for the device with more porosity

and that the relative humidity inside the 3.14% device lasts the longest.

92

Table 11. Relative Humidity of the Porosity Experiments. The relative humidity (RH)

at the end of the experiment inside and on the surface is provided along with the time when

the relative humidity first measured below 80%. The device that has the longest time above

80% is highlighted. The 3.14% porosity device never recorded internal relative humidity

levels under 80%.

Experiment

Number Devices

Amount of

Water

Added

[kg]

Time <80%

Internal RH

[Hour]

Internal

RH at the

End

[%]

1 3.14% Porosity 2.91 Never 89.40

1 7.20% Porosity 3.85 243 50.21

1 11.22% Porosity 3.39 241 40.79

2 3.14% Porosity 3.06 485 46.98

2 7.20% Porosity 3.73 364 31.47

2 11.22% Porosity 3.61 364 25.96

The porosity experiments show that the evaporative cooling devices with higher levels of porosity

in their outer vessels achieve greater maximum temperature drop, higher cooling efficiency, and

larger overall total cooling. As the porosity increases, the evaporation rate also increases because

the water moves more readily through the clay outer wall. As the devices dry, having a larger

porosity is still advantageous because the water vapor can diffuse through the clay wall more

easily. The trade-off of having more porosity includes the internal relative humidity not lasting as

long because there is less water left in the devices to evaporate into the internal chamber. For users

with more access to water, adding water near the end of Zone 2 will allow the devices to sustain

their maximum temperature drops and high levels of internal relative humidity for longer.

The evaporative cooling devices in the porosity experiments mix sawdust in clay to create

additional pores, but any organic material that can burn away when the clay pots undergo firing

will serve the purpose. A 60-mesh sieve was used on the sawdust before mixing it into clay. Adding

any organic matter to the clay also has the benefit of producing uniformly burnt pots and reducing

93

fuel and power consumption because the additives double as fuel during firing [36]. Adding

sawdust also reduces the total weight of the devices [36].

Besides the organic pore-formers, no other material is needed to increase porosity in clay pots.

There is a factor of time required for experimenting on what amount of sawdust can be added to

what amount of the specific local clay but no other higher skill level is required. Overall, increasing

cooling performance by increasing porosity with sawdust presents a practical opportunity to

improve the cooling devices. A comprehensive table of all the performance metrics is included in

Table 12.

94

Tab

le 1

2. C

om

pre

hen

siv

e C

ooli

ng P

erfo

rman

ce M

etri

cs o

f P

oro

sity

Exp

erim

ents

. T

he

max

imum

val

ue

of

each

per

form

ance

met

ric

is h

igh

lig

hte

d.

Fo

r th

e te

mper

ature

met

rics

, an

y v

alue

that

is

wit

hin

the

senso

r er

ror

of

±0

.5 °

C o

f th

e m

axim

um

val

ue

is

also

hig

hli

gh

ted

. T

he

"nev

er"

term

is

appli

cable

only

wit

hin

the

tim

espan

of

the

resp

ecti

ve

exper

imen

t. T

he

coo

lin

g p

erfo

rman

ce

met

rics

th

at a

re m

ost

pra

ctic

al f

or

use

rs a

re t

he

inte

rnal

tem

per

ature

dro

p,

the

tim

e bef

ore

the

inte

rnal

tem

per

atu

re d

rop

ret

urn

s to

zero

and

ho

w lo

ng

hig

h lev

els

of

inte

rnal

rel

ativ

e hum

idit

y las

t. I

n b

oth

exper

imen

ts, t

he

dev

ices

wit

h h

igh

er p

erce

nta

ge

of

poro

sity

hav

e gre

ater

in

tern

al t

emp

erat

ure

dro

ps

but

do n

ot

sust

ain t

hei

r over

all

evap

ora

tive

cooli

ng

for

as l

on

g a

s th

e d

evic

e w

ith

lo

wer

per

cen

tag

e of

poro

sity

. T

he

dev

ice

low

er p

oro

sity

per

centa

ge

also

mai

nta

ins

rela

tive

hum

idit

y a

bo

ve

80%

for

lon

ger

. A

s w

ith

th

e

dev

ices

in

the

surf

ace

area

ex

per

imen

ts,

ther

e is

a t

rade-o

ff b

etw

een h

avin

g g

reat

er i

nte

rnal

tem

per

atu

re d

rop

an

d l

ong

er p

erio

ds

of

inte

rnal

coo

lin

g a

nd

hig

h l

evel

s of

rela

tive

hum

idit

y. F

or

use

rs i

n w

ater

-sca

rce

area

, th

e d

evic

e w

ith l

ess

po

rosi

ty m

ay

be

mo

re

appro

pri

ate.

In

tern

al

RH

at

the

En

d

[%]

89

.4

50

.21

40

.79

46

.98

31

.47

25

.96

Tim

e

<8

0%

Inte

rna

l

RH

[Ho

ur]

Nev

er

24

3

24

1

48

5

36

4

36

4

Tim

e

To

tal

Co

oli

ng

[Ho

ur]

33

1

33

1

33

1

61

3

61

3

61

3

To

tal

Co

oli

ng

[°C

× H

ou

r]

77

6.5

10

18

.71

10

65

.61

10

96

.73

116

0.6

2

14

11

.73

Tim

e M

ax

Eff

icie

ncy

[Hou

r]

28

0

89

38

18

4

18

2

10

9

Max

Eff

icie

ncy

[%]

37.2

4

59.3

6

66.6

1

40.9

5

54.6

3

69.0

7

Tim

e

Max

Su

rface

Tem

p

Dro

p

[Hou

r]

280

39

62

41

135

108

Max

Su

rface

Tem

p

Dro

p

[°C

]

3.4

6

7.3

0

7.4

7

3

4.7

4

6.4

2

Tim

e

Tem

p

Dro

p

Ret

urn

s

to Z

ero

[Hou

r]

Nev

er

218

219

Nev

er

360

360

Tim

e

Max

Inte

rnal

Tem

p

Dro

p

[Hou

r]

280

89

62

41

135

109

Ma

x

Inte

rnal

Tem

p

Dro

p

[°C

]

4.4

1

8.4

3

9.4

5

4.2

8

5.6

0

7.6

7

Am

ou

nt

of

Wa

ter

Ad

ded

[kg

]

2.9

1

3.8

5

3.3

9

3.0

6

3.7

3

3.6

1

Dev

ices

3.1

4%

Po

rosi

ty

7.2

0%

Po

rosi

ty

11

.22

%

Po

rosi

ty

3.1

4%

Po

rosi

ty

7.2

0%

Po

rosi

ty

11

.22

%

Po

rosi

ty

Exp

Nu

mb

er

1

1

1

2

2

2

95

3.3. Porosity Mini-Experiments

Two mini-experiments are conducted to add more context to how porosity affects cooling

performance. These two experiments are not conducted in a systematic framework as they only

represent 2 levels of comparison instead of the 3 or 4 used in the main experiments; nonetheless,

they still provide points of references. All the devices were conducted in parallel and share the

same sand to water ratio. The first mini-experiment compares the cooling effect among devices

with different levels of porosity created by sawdust and a device that has a different percentage of

porosity because it is made from different clay material. The second mini-experiment explores

how porosity from different clay materials affect cooling performance.

Porosity due to Different Clay Material versus Porosity due to Sawdust

The devices’ key metrics and properties are in Table 13. Temperature on the surface is the metric

used to compare the devices because it is the more representative of the cooling performance. The

devices in porosity experiment 1 are compared with a device that has inner and outer vessels made

from a different clay material, EM 210; the clay porosity is 6.60%. The surface temperature and

internal relative humidity trends in this mini experiment are the same as in the two main porosity

experiments. The maximum surface temperature drop varies with porosity, and the time the

internal relative humidity drops below 80% inversely varies with porosity.

96

Table 13. Devices in Mini-Experiment Comparing Porosity due to Different Clay

Material versus Porosity Created by Sawdust. The devices with more porosity record

greater temperature drop on the surface and shorter time span of high internal relative

humidity. The added 6.60% porosity device fits into the trend found in the first porosity

experiment.

Porosity

Formation

Devices

(Outer

Porosity)

Inner

Porosity

[%]

Amount of

Water

Added

[kg]

Max

Surface

Temperature

Drop

[℃]

Time Max

Surface

Temperature

Drop

[Hour]

Time

<80%

Internal

RH

[Hour]

Sawdust 3.14% Porosity 3.14% 2.91 3.46 280 Never

EM 210 6.60% Porosity 6.60% 3.40 6.63 35 249

Sawdust 7.20% Porosity 3.14% 3.85 7.30 39 243

Sawdust 11.22% Porosity 3.14% 3.39 7.47 62 241

97

Porosity due to Different Clay Material

Two clay bodies of different outer material porosity, T 417 and EM 106 are compared as shown

in Table 14. Their porosity levels are 3.50% and 4.08% respectively without adding any sawdust.

The 4.08% porosity device’s cooling performance supports the findings that increasing porosity in

clay material will lead to greater temperature drop. Its internal relative humidity did not stay above

80% relative humidity for as long as the internal relative humidity inside the 3.50% porosity

device; however, that is expected based on the trends seen in the porosity experiments with sawdust

pots. This suggests that porosity due to using different clay materials or due to adding sawdust will

reflect the same trends.

Table 14. Devices in Mini-Experiment Comparing Porosity due to Different Clay

Material. Between the two devices, the 4.08% porosity device (EM 106) records greater

temperature drop on its evaporative cooling surface. The 3.50% porosity device (T 417)

dips below 80% relative humidity before the 4.08% but it hovers around that point until

the end of the experiment while the 4.08% humidity continue to decrease.

Porosity

Formation

Devices

(Outer

Porosity)

Inner

Porosity

[%]

Amount

of Water

Added

[kg]

Max

Surface

Temperature

Drop

[℃]

Time Max

Surface

Temperature

Drop

[Hour]

Time

<80%

Internal

RH

[Hour]

T 417 3.50%

Porosity 3.50% 3.14 3.08 254 55

EM 106 4.08%

Porosity 4.08% 2.49 6.01 14 189

98

3.4. Porosity versus Surface Area Design Parameters

The second experiments of the surface area and porosity experiments are conducted at the same

time, so the individual devices with added surface corrugation and sawdust can be directly

compared. Table 15 details the cooling performance of all the devices in the second experiment

of the surface area and porosity experiments. The 1x Device and the 3.14% porosity device are the

same device. The 7.20% porosity and the 11.22% porosity devices have an average of 0.5 kg of

water more than the average amount of water added to the devices with surface corrugation. The

additional amount of water added explains why the devices with added sawdust have internal

relative humidity measurements above 80% for an average of 47 hours longer than the devices

with added corrugation.

The 11.22% porosity device achieves internal temperature drop approximately 2°C greater than

the average temperature drops of all the devices with surface corrugations and reaches its

maximum temperature drop faster due to its higher levels of porosity. As a tradeoff for its fast

evaporation rate, the temperature drop of the 11.22% porosity device returns to zero within the

same 360-hour mark as all the devices with the surface corrugation even though it has more water.

The 7.20% porosity device has similar temperature drops within the sensor error range as all the

devices with surface corrugation and its internal temperature drop also returns to zero within the

same 360-hour mark even though it has the most amount of water added at 3.73 kg. The 7.20%

porosity device also has the lowest maximum efficiency among all the devices.

The performance metrics of the devices suggest that a high enough porosity percentage is

necessary to achieve greater evaporative cooling than only adding surface corrugation. The devices

with surface corrugation also sustain evaporative cooling for as long as devices with sawdust with

99

less water, making them a more attractive option for users in water scarce areas. For users not

constrained by availability of water, watering the devices on a cycle will further sustain the

maximum temperature drop and high internal relative humidity of the devices, making the 11.22%

porosity device an even more attractive option because it has the greatest maximum temperature

drop.

Another factor for users to consider when choosing between the two design parameters is that

adding surface corrugations require more skill labor and equipment than adding sawdust to clay;

therefore, while the devices with corrugation can sustain evaporative cooling for as long as the

devices with sawdust with a lower amount of water added, constructing corrugation may not be

accessible and too burdensome for some users.

100

Tab

le 1

5. C

om

pre

hen

siv

e C

oo

lin

g P

erfo

rman

ce M

etri

cs o

f S

urfa

ce A

rea a

nd

Poro

sity

Ex

per

imen

ts. T

he

max

imu

m v

alue

of

each

per

form

ance

met

ric

is h

ighli

ghte

d.

For

the

tem

per

ature

met

rics

, an

y v

alue

that

is

wit

hin

th

e se

nso

r er

ror

of

±0

.5 °

C o

f th

e

max

imu

m v

alu

e is

als

o h

igh

lighte

d.

The

"nev

er"

term

is

appli

cable

only

wit

hin

the

tim

esp

an o

f th

e re

spec

tiv

e ex

per

imen

t. T

he

cooli

ng

per

form

ance

of

all

the

dev

ices

indic

ates

that

a h

igh e

nough p

erce

nta

ge

of

poro

sity

is

nec

essa

ry t

o a

chie

ve

inte

rnal

tem

per

ature

gre

ater

th

an t

he

tem

per

ature

dro

ps

of

the

dev

ices

wit

h s

urf

ace

corr

ugat

ion.

The

per

form

ance

met

rics

als

o s

ho

w t

hat

the

dev

ices

wit

h s

urf

ace

corr

ug

atio

n c

an s

ust

ain e

vap

ora

tive

cooli

ng f

or

as long a

s th

e dev

ices

wit

h s

awd

ust

wit

h les

s w

ater

add

ed.

The

dev

ices

wit

h s

awd

ust

hav

e g

reat

er tota

l co

oli

ng a

nd a

re a

ble

to s

ust

ain inte

rnal

rel

ativ

e h

um

idit

y a

bo

ve

80%

fo

r lo

ng

er b

ecau

se

they

hav

e m

ore

add

ed w

ater

.

Inte

rna

l

RH

at

the

En

d

[%]

46

.98

28

.07

32

.73

31

.41

46

.98

31

.47

25

.96

Tim

e

<8

0%

Inte

rna

l

RH

[Ho

ur]

48

5

30

3

31

1

33

5

48

5

36

4

36

4

Tim

e

To

tal

Co

oli

ng

[Ho

ur]

61

3

61

3

61

3

61

3

61

3

61

3

61

3

To

tal

Co

oli

ng

[°C

× H

ou

r]

10

97

12

05

10

59

11

52

10

96

.73

11

60

.62

14

11

.73

Tim

e M

ax

Eff

icie

ncy

[Hou

r]

184

182

182

182

184

182

109

Max

Eff

icie

ncy

[%]

40.9

5

69.9

8

61.2

2

66.1

40.9

5

54.6

3

69.0

7

Tim

e

Max

Su

rface

Tem

p

Dro

p

[Hou

r]

41

135

207

20

7

41

135

108

Max

Su

rface

Tem

p

Dro

p

[°C

]

3

4.2

7

4.0

7

4.2

8

3

4.7

4

6.4

2

Tim

e

Tem

p

Dro

p

Ret

urn

s

to Z

ero

[Hou

r]

Nev

er

360

360

360

Nev

er

360

360

Tim

e

Max

Inte

rnal

Tem

p

Dro

p

[Hou

r]

41

135

207

207

41

135

109

Ma

x

Inte

rnal

Tem

p

Dro

p

[°C

]

4.2

8

5.8

1

5.4

2

5.6

8

4.2

8

5.6

7.6

7

Am

ou

nt

of

Wa

ter

Ad

ded

[kg

]

3.0

6

3.3

2

3.0

6

3.1

3

3.0

6

3.7

3

3.6

1

Dev

ices

1x

Dev

ice

1.5

2x

Dev

ice

1.7

7x

Dev

ice

2.5

9x

Dev

ice

3.1

4%

Po

rosi

ty

7.2

0%

Po

rosi

ty

11

.22

%

Po

rosi

ty

Exp

Nu

mb

er

2

2

2

2

2

2

2

101

Chapter 4

Conclusion

This thesis investigated the effects of surface area and porosity on the cooling performance of

household evaporative cooling devices. In the surface area experiments, corrugation was cut into

the walls of cooling devices to create more surface area over which water can evaporate without

altering the internal volume. To systematically vary the additional surface area, the distance

between the ridges was increased by 0.25 cm starting from 0.25 cm and up to 0.75 cm, inclusive.

With less distance between ridges, more surface area was added to the device. For the porosity

experiments, sawdust was mixed into wet clay at different clay to sawdust ratios to create

incremental percentages of porosity at 3.14% (no sawdust added), 7.20% (40 clay: 1 sawdust), and

11.22% (20 clay: 1 sawdust). Adding sawdust to wet clay created cavities and increased porosity

because the sawdust burned away during the firing stages of making the clay pots.

102

The work conducted in the surface area experiment showed that adding surface fluctuation

increases the rate of evaporation and maximum temperature drops when compared to devices

without surface fluctuation. However, adding more corrugation did not consistently lead to greater

temperature drops. The geometry of corrugation introduced competing effects between increased

available surface area for water evaporation and decreased vapor pressure concentration inside the

troughs. The device without corrugation had the smallest temperature drop but the steadiest and

longest-lasting cooling effects. Although the device without corrugation had less total water added

at the beginning of the experiment, it was able to maintain its internal relative humidity for longer

than the devices with corrugation. For users in water-scarce areas, the device without corrugation

could be a better choice given the constraints.

The work done on the porosity experiment concluded that increasing porosity in the outer vessel

leads to greater temperature drop and higher cooling efficiency—even with a lower amount of

water added. The increased level of porosity in the device’s clay wall decreased the internal

transport resistance for water and moisture to transfer to the evaporation surface, which then led

to increased vapor pressure concentration on the surface to facilitate evaporation. The tradeoffs

with having higher porosity in the outer vessels include shorter periods of evaporative cooling and

shorter periods of high levels of relative humidity inside the devices.

To further the investigation on the cooling effect of surface area and porosity the author

recommends conducting additional tests in a closed chamber where temperature, relative humidity,

and airflow could be controlled. The devices’ dimensions should be more uniform so that the same

amount of sand and water can be added to each device. The temperature and relative humidity

sensors should have greater accuracy and the load cell should have lower creep error percentage.

More temperature sensors should be added inside the cooling chamber and the placement of the

103

sensors should be uniform across all devices. To further investigate surface area effects,

experiments that focus on the optimization of the corrugation’s length, width, height, and angle

will allow for greater cooling performance and enable the creation of a model that could predicts

response given different parameter values. For porosity, the experiments in this thesis have shown

the advantages of adding porosity but the practicality of implementing the study findings remain

unknown. Experiments that focus on investigating the maximum amount of sawdust that can be

added without compromising the integrity of the structure or the usability of the pot, i.e. the pot

fragility, and identifying what other locally available organic material can be used as pore-formers

will further the practical implementation of the findings in this thesis.

Both increasing surface area and porosity can improve cooling performance of evaporative cooling

devices. When deciding between which design parameter to implement, the important factors to

consider include: the cooling performance achievable, the water use efficacy, and the practicability

of design parameter implementation. The device with the highest percentage of porosity (11.22%

porosity device) achieved a temperature drop of ~2°C greater than the average temperature drops

of all the devices with added corrugations. The 7.20% porosity device achieve similar temperature

drop within the sensor error range of the devices with surface corrugations. The difference in

cooling performance of the two porosity devices in comparison to the devices with corrugations

suggests that a minimum porosity percentage is required for cooling devices to achieve greater

cooling performance than the devices with surface corrugation. The devices with corrugations had

less water added but were still able to sustain their total evaporative cooling for as long as the

devices with added sawdust. This is an important finding for users in water scarce areas. For users

with less water constraints, continuously adding water near the end of Zone 2 will allow the devices

to sustain their maximum temperature drops and high levels of internal relative humidity for

104

longer, making the devices that have greater maximum internal temperature drop more attractive.

Of the three factors to consider, the practicability of implementation carries the most weight

because it determines accessibility. Increasing porosity presents a more practical and less

burdensome solution to increasing cooling performance as compared to adding surface

corrugation. Adding corrugation to the outer clay pots requires additional equipment and skill

levels while adding sawdust to create more porosity only require a single time investment to

pinpoint ideal sawdust to clay ratio. Devices with sawdust are lighter, require less fuel to burn and

burn more uniformly [36]. Sawdust can be locally sourced in most places and any fine organic

material that can burn out during firing can also be used in place of sawdust. The cooling

performance and practicality of devices with sawdust present a viable opportunity to improve

household evaporative cooling devices. For future researches, the author hypothesizes that

increasing both surface area and porosity in individual evaporative cooling devices can lead to

greater cooling performance.

In this thesis, the effects of surface area and porosity on cooling performance were systematically

investigated and evaluated using multiple metrics. A new metric measuring total cooling was

developed to account for the length of the evaporative cooling as well as the maximum temperature

drop. The work done in this thesis furthered the understanding of how the design parameters of

surface area and porosity impacted cooling performance. The study’s finding can serve as the

foundation for future experimentation, optimization, and design of those design parameters to

improve performance of evaporative cooling devices.

105

Supplementary Information

Supplementary Information S.1. Moisture Sensor Data in the Sand Gap.

Figure 35. Moisture in Sand Gap for Surface Area Experiment 1. The moisture sensor

in the sand gap is located near the top. The sand gap of all devices stays wet until near the

end of Zone 2. The device without added surface area maintains its moisture for the longest

time because of its lower evaporation rate. The devices with more surface corrugation dry

faster due to their higher rates of evaporation. The absolute amount of water added to the

sand gap also influence how long moisture lasts.

106

Figure 36. Moisture in Sand Gap for Surface Area Experiment 2. The moisture sensor

data in experiment 2 display hardware sensor malfunction. There is a lot more noise in the

data starting in Zone 2 and continuing to Zone 3 for all the sensors. Most notably, the sand

gap moisture sensor for the 1.52x Device records high level of moisture in Zone 3 that is

not realistic.

107

Supplementary Information S.2. Mass Loss and Mass Loss Rate for Surface Area

Experiments

Figure 37 and Figure 38 display the mass loss and the mass loss rate of the first and second surface

area experiment. The mass loss data for each device is fitted with a polynomial curve of the 5th

order, and the normalized root mean square error is provided in the legend. The derivative of the

mass loss data curve fit is the mass loss rate. In the evaporative cooling devices, the only mass

transfer that occurs is water evaporation, so the mass loss rate is the water evaporation rate. This

temperature drop in Zone 4 was maintained for the longest time over the course of the experiment.

108

Figure 37. Mass Loss Rate and Mass Loss for Surface Area Experiment 1. A curve

was fitted to the mass loss of each cooling device. In the legend, the normalized root mean

square error is given for each curve fit. The derivative of the curve fit is the mass loss rate.

Since water evaporation is the only mass transfer that occurs, the mass loss rate is the rate

of water evaporation. In Zone 2, where the maximum temperature drop happened, the

devices with added surface area all have greater rates of water evaporation as compared to

the device without corrugation.

109

Figure 38. Mass Loss Rate and Mass Loss for Surface Area Experiment 2. The same

curve fitting technique from experiment 1 is used to experiment 2. The rate of water

evaporation of the devices with added surface area also are greater than those of the device

without added surface area. A difference between experiment 1 and experiment 2 is how

the 1.52x Device records a greater rate of evaporation than the 2.59x Device. The greater

rate of evaporation also corresponds with the 1.52x Device’s greater temperature drop. This

difference in performance is due to the competing effects introduced by the corrugation.

110

Bibliography

[1] Ishangulyyev, R., Kim, S., and Lee, S. H., 2019, “Understanding Food Loss and Waste—Why

Are We Losing and Wasting Food?,” Foods, 8(8).

[2] FAO, 2019, The State of Food and Agriculture 2019. Moving Forward on Food Loss and

Waste Reduction, Rome.

[3] FAO, IFAD, UNICEF, WFP, and WHO, 2020, The State of Food Security and Nutrition in

the World 2020. Transforming Food Systems for Affordable Healthy Diets, FAO, Rome.

[4] Basediya, A. lal, Samuel, D. V. K., and Beera, V., 2013, “Evaporative Cooling System for

Storage of Fruits and Vegetables - a Review,” J Food Sci Technol, 50(3), pp. 429–442.

[5] Gross, K. C., Wang, C. Y., and Saltveit, M., 2016, The Commercial Storage of Fruits,

Vegetables, and Florist and Nursery Stocks, United States Department of Agriculture.

[6] Odesola, I. F., and Onwuka, O., 2009, “A Review of Porous Evaporative Cooling for the

Preservation of Fruits and Vegetables.,” The Pacific Journal of Science and Technology, 10(2).

[7] FAO, 2017, Near East and North Africa Regional Overview of Food Insecurity 2016, Cairo.

[8] Smolak, J., 2020, “Tackling Food Loss and Waste in the Near East and North Africa” [Online].

Available: http://www.fao.org/neareast/perspectives/food-waste/en/. [Accessed: 02-Jul-

2020].

[9] Lipinski, B., Hanson, C., Lomax, J., Kitinoja, L., Waite, R., and Searchinger, T., 2013,

Reducing Food Loss and Waste, World Resources Institute.

[10] Verploegen, E., Sanogo, O., and Chagomoka, T., 2018, “Evaporative Cooling

Technologies for Improved Vegetable Storage in Mali,” MIT D-Lab.

[11] Ndukwu, M. C., and Manuwa, S. I., 2014, “Review of Research and Application of

Evaporative Cooling in Preservation of Fresh Agricultural Produce,” Int J Agri & Biol Eng,

7(5), pp. 85–102.

[12] Noble, N., 2003, “Practical Action Technology Challenging Poverty: Evaporative

Cooling.”

[13] Liberty, J. T., Okonkwo, W. I., and Echiegu, E. A., 2013, “Evaporative Cooling: A

Postharvest Technology for Fruits and Vegetables Preservation,” IJSER, 4(8), pp. 2257–2266.

[14] Oluwasola, O., 2010, “Pot-in-Pot Enterprise: Fridge for the Poor.”

[15] Lienhard, IV, J. H., and Lienhard, V, J. H., 2020, A Heat Transfer Textbook, Phlogiston

Press, Cambridge, MA.

[16] Aboufoul, M., Shokri, N., Saleh, E., Tuck, C., and Garcia, A., 2019, “Dynamics of Water

Evaporation from Porous Asphalt,” Construction and Building Materials, 202, pp. 406–414.

[17] Lehmann, P., Assouline, S., and Or, D., 2008, “Characteristic Lengths Affecting

Evaporative Drying of Porous Media,” Phys. Rev. E, 77(5), p. 056309.

[18] Prime, N., Housni, Z., Fraikin, L., Léonard, A., Charlier, R., and Levasseur, S., 2015, “On

Water Transfer and Hydraulic Connection Layer During the Convective Drying of Rigid

Porous Material,” Transp Porous Med, 106(1), pp. 47–72.

[19] Kale, A., and Sundaram, K., 2014, “Experimental Evaporative Coolers for Vegetable

Preservation,” Jawaharlal Nehru University, New Delhi, p. 6.

[20] Gustafsson, K., and Simson, H., 2016, “An Experimental Study on an Evaporative Cooler

for Hot Rural Areas,” KTH School of Industrial Engineering and Management.

111

[21] Unno, N., Yuki, K., Inoue, R., Kogo, Y., Taniguchi, J., and Satake, S., 2020, “Enhanced

Evaporation of Porous Materials with Micropores and High Porosity,” JTST, 15(1), pp.

JTST0007–JTST0007.

[22] Salaudeen, K., Anugwom, U., Olayemi, F. F., and Fidelis, A., 2013, “Effect of NSPRI Tin-

in-Pot Compared with Pot-in-Pot Evaporative.Pdf,” International Journal of Engineering and

Technology, 2(1), pp. 63–69.

[23] Verploegen, E., Ekka, R., and Gill, G., 2019, “Evaporative Cooling for Improved Fruit &

Vegetable Storage in Rwanda & Burkina Faso.”

[24] Stull, R., 2011, “Wet-Bulb Temperature from Relative Humidity and Air Temperature,”

Journal of Applied Meteorology and Climatology, 50(11), pp. 2267–2269.

[25] Watt, J., 1986, EVAPORATIVE AIR CONDITIONING HANDBOOK, Chapman and Hall,

New York and London.

[26] Poós, T., and Varju, E., 2020, “Mass Transfer Coefficient for Water Evaporation by

Theoretical and Empirical Correlations,” International Journal of Heat and Mass Transfer, 153.

[27] Shokri, N., Lehmann, P., and Or, D., 2010, “Liquid-Phase Continuity and Solute

Concentration Dynamics during Evaporation from Porous Media: Pore-Scale Processes near

Vaporization Surface,” Phys. Rev. E, 81(4), pp. 046308-1-046308–7.

[28] Gao, B., Davarzani, H., Helmig, R., and Smits, K. M., 2018, “Experimental and Numerical

Study of Evaporation From Wavy Surfaces by Coupling Free Flow and Porous Media Flow,”

Water Resources Research, 54(11), pp. 9096–9117.

[29] Whitaker, S., 1977, “Simultaneous Heat, Mass, and Momentum Transfer in Porous Media:

A Theory of Drying,” Advances in Heat Transfer, J.P. Hartnett, and T.F. Irvine, eds., Elsevier,

pp. 119–203.

[30] Comings, E. W., and Sherwood, T. K., 1934, “The Drying of Solids VII, Moisture

Movement by Capillarity in Drying Granular Materials,” Industrial & Engineering Chemistry,

26(10), pp. 1096–1098.

[31] Nasrallah, S. B., and Perré, P., 1988, “Detailed Study of a Model of Heat and Mass Transfer

during Convective Drying of Porous Media,” International Journal of Heat and Mass Transfer,

31(5), pp. 957–967.

[32] Haghighi, E., and Or, D., 2015, “Evaporation from Wavy Porous Surfaces into Turbulent

Airflows,” Transp Porous Med, 110(2), pp. 225–250.

[33] Buck, A. L., 1981, “New Equations for Computing Vapor Pressure and Enhancement

Factor.,” Journal of Applied Meteorology, 20, pp. 1527–1532.

[34] Zilker, D. P., and Hanratty, T. J., 1979, “Influence of the Amplitude of a Solid Wavy Wall

on a Turbulent Flow. Part 2. Separated Flows,” Journal of Fluid Mechanics, 90(2), pp. 257–

271.

[35] Zilker, D. P., Cook, G. W., and Hanratty, T. J., 1977, “Influence of the Amplitude of a

Solid Wavy Wall on a Turbulent Flow. Part 1. Non-Separated Flows,” Journal of Fluid

Mechanics, 82(1), pp. 29–51.

[36] Okunade, E. A., 2008, “The Effect of Wood Ash and Sawdust Admixtures on the

Engineering Properties of a Burnt Laterite-Clay Brick,” Journal of Applied Sciences, 8, pp.

1042–1048.