National Diploma in Mechanical Engineering
P2 Report- Radiative Sky Cooler
By
Ashton Lee Seloka
Student Number: 215207025
Co-Ordinator
Mr. Pitso Tebele
Mentor
Mr. Eugene Coetze
I swear that this is the original work of the author(s). All information obtained directly or
indirectly from other sources has been fully acknowledged.
Signature:…AL SELOKA
Date: 26 November 2020
TITLE PAGE
STUDENT DETAILS
Name(s):
Ashton Lee
Surname:
Seloka
Student No:
215207025
Email Address:
ashtonleeseloka@gmail.com
Cell:
0670816111
TECHNICAL PROJECT DETAILS
Project Title:
Radiative sky cooler
Field:
HVAC
Duration:
COORDINATOR DETAILS
Name:
Pitso Tebele
Email:
tebelep@cput.ac.za
Discipline:
Experiential Learning Co-ordinator
Department:
Mechanical
Faculty:
Engineering
Institution:
Cape Peninsula of Technology
ii
COMPANY DETAILS
Company Name:
AERSA air conditioning
Address:
2 square street Stikland industrial
Cape Town
7530
Landline:
087 630 1518
MENTOR DETAILS
Name:
Eugene Coetze
Email Address:
Eugene@aersa.co.za
Position:
Maintenance Engineer
Qualification:
Btech Mechanical engineering
Experience:
8 years
iii
DECLARATION
Name and Surname:
Ashton Lee Seloka
Student Number:
215207025
I Ashton lee Seloka, Student number: 215207025 registered for the ND mechanical
engineering qualification at the Cape Peninsula University of Technology declare that all
work presented in this research report unless otherwise referenced or stated is my own.
I have to the best of my ability tried to acknowledge the original authors by providing
references throughout the entirety of this document indicating ownership of their original
information.
Signature: AL Seloka
Copyright© Cape Peninsula University of Technology
iv
Acknowledgment
Firstly, I would like to provide my greatest gratitude to the Lord Almighty who throughout all
the trials I have faced during my journey from student to qualified technician has provided me
with the strength and courage needed to face the challenges and set backs presented to me on this
journey. I thank you for your faithfulness and dedication.
To my family, I thank you for making every sacrifice conceivable to allow me to peruse my
dreams, sometimes at the sacrifice of your own. Without your love and support this journey
would not be possible. Thank you for your unwavering support on my journey from a brighteyed dreamer to a soon to be qualified mechanical engineering technician. It is my hope that I
may provide you the same support you have always afforded me.
To the Cape peninsula university of technology, A big thank you to all lecturers and staff I have
encountered on my journey thank you for instilling your passion for engineering into every
student who walks through your classrooms. A special thank you to Ms. Lynn and Mr. Paul
Senda for always fighting for a better learning environment for all students, your impact is
lasting.
To Aersa air conditioning, I am grateful that you have allowed me into your amazing
organization. Thank you for providing me with an opportunity to better the lives of myself and
those around me while allowing me to grow at my own pace.
To my Mentors and fellow staff members, I thank you all for welcoming me into your team and
more importantly fostering my love for engineering. I thank you all individually for your
patience and willingness to help me no matter how small my problems may seem. Your input
and knowledge are invaluable to me and I stand in awe of your capabilities and professionalism.
Lastly to Kelly, thank you for always being my number 1 cheerleader and supporter as well as
sacrificing alongside me to accomplish my dreams. Your love and support have motivated me
more than you could ever know.
v
Abstract.
Globally the demand for air-conditioning has grown due to the increasing global average
temperatures and the development of third world countries creating greater standards of living.
Airconditioning and HVAC systems come at great cost to the environment requiring high energy
inputs and the use of hydrofluorocarbons to produce cooling. The use of conventional
mechanical air-conditioning systems produces both direct (Release of greenhouse gases into the
atmosphere) and Indirect (burning of fossil fuels) pollution to the environment. The release of
greenhouse gases into the atmosphere is responsible for increasing global temperatures which in
turn results in a greater demand for air-conditioning and comfort cooling, the result is a solution
which causes and expands the problem.
Due to the problems caused by conventional mechanical cooling systems, Innovative solutions
are required to firstly provide cooling and secondly reduce the damage done to the atmosphere
through conventional methods. A few emerging technologies are shining a light on the possible
future of air-conditioning through using passive methods requiring no input to produce cooling.
Radiative sky cooling is a technology which provides free sub ambient cooling through
exploiting the atmospheric window in which the atmosphere is transparent between the mid
infrared range of 8-13 micrometers Resulting in the rejection of waste heat via infrared to a
infinitely big cold heat sink namely space without increasing the local ambient temperature.
Radiative sky cooling technology relies on near white and black body infrared imitators to
produce its passive cooling process. Many designs using highly emissive films and paint are
currently in development.
The student was tasked in assessing the feasibility of radiative sky cooling technology and to
provide designs capable of cooling either a gas or fluid. This report will focus on the design of a
passive radiative sky cooler capable of cooling a fluid or gas to sub ambient temperatures for use
in air-conditioning application. Methods of radiative cooling are explored throughout this text
providing the reader with an introduction to radiative sky cooling technologies.
The free nature of radiative sky cooling has generated a great deal of interest in the technology.
The technology is still to be perfected and like any new emerging technology still has its
obstacles namely low power density, therefore large surface areas are required to provide
sufficient cooling to commercial and residential spaces.
This report aims to shine a light and spark interest in the possibility of a greener solution to an
ever-increasing problem faced not only by south Africans but globally.
vi
Contents
.................................................................................................................................................................. 1
TITLE PAGE .................................................................................................................................................... ii
DECLARATION .............................................................................................................................................. iv
1)
Introduction .......................................................................................................................................... 1
1.1)
Global cooling demand ................................................................................................................. 1
1.2)
Global electrical consumption ...................................................................................................... 1
1.3)
Environmental effect of convention air conditioning ................................................................... 2
1.4)
Problem statement ....................................................................................................................... 2
1.5)
Constraints .................................................................................................................................... 2
1.6)
Alternatives ................................................................................................................................... 2
1.7)
Literature survey ........................................................................................................................... 3
1.7.1)
Aili et al-A kW-scale, 24-hour continuously operational, radiative sky cooling system. .......... 3
1.7.2)
Bhatia et al- Passive directional sub-ambient daytime radiative cooling ................................. 3
1.7.3)
Eicker and Dalibard - Photovoltaic–thermal collectors for night radiative cooling of building
3
1.7.4)
Hu et al - Field investigation of a hybrid photovoltaic-photothermic-radiative cooling system
4
1.7.5)
Jarimi, Powell and Riffat - Review of sustainable methods for atmospheric water harvesting4
1.7.6)
Kou et al- Daytime Radiative Cooling Using Near-Black Infrared Emitters ............................... 4
1.7.7)
Kwan et al - Enhanced cooling by applying the radiative sky cooler to both ends of the
thermoelectric cooler ............................................................................................................................... 4
1.7.8)
Raman et al - Passive radiative cooling below ambient air temperature under direct sunlight
5
1.7.9)
Zhang et al - Energy saving and economic analysis of a new hybrid radiative cooling system
for single-family houses in the USA .......................................................................................................... 5
1.7.10) Zhao, Hu, Ao and Pei- Performance evaluation of daytime radiative cooling under different
clear sky conditions ................................................................................................................................... 5
1.7.11) Zhao et al- Development of a single-phase thermosiphon for cold collection and storage of
radiative cooling........................................................................................................................................ 5
1.8)
Setting of objectives...................................................................................................................... 6
1.8.1) Concept formation .......................................................................................................................... 6
1.8.2) Final concept ................................................................................................................................... 7
vii
1.8.3) Research ......................................................................................................................................... 7
1.8.4)
Calculations ............................................................................................................................... 7
1.8.5) Material and component selection ................................................................................................ 8
1.9)
Constraints .................................................................................................................................... 8
1.10)
2.)
Planning of project .................................................................................................................... 9
Investigation.................................................................................................................................... 10
2.1)
2.1.1)
Theoretical considerations.......................................................................................................... 10
Concept formation .................................................................................................................. 10
2.1.1.2) Concept 1: Gas radiative sky cooler ........................................................................................... 10
2.1.1.3) Concept 2: Fluid radiative sky cooler (Final concept) ................................................................ 10
2.2) Research into fluid Radiative sky coolers ........................................................................................ 11
2.2.1) What is radiative sky cooling ........................................................................................................ 11
2.2.2) Where is Radiative sky cooling utilized......................................................................................... 12
2.2.3) Radiative sky cooler Benefits ........................................................................................................ 12
2.3) Working Principle............................................................................................................................. 13
2.4) Atmospheric window ....................................................................................................................... 15
2.5) Emissivity ......................................................................................................................................... 15
3 Design of a kW scale radiative sky cooler ................................................................................................ 15
3.1)
Problem statement ..................................................................................................................... 16
3.2)
Radiative reflectors. .................................................................................................................... 16
3.2)
Material consideration................................................................................................................ 17
3.2.1) Corrosion ...................................................................................................................................... 17
3.2.1) Common forms of corrosion prevention ...................................................................................... 18
3.3) Climate consideration and effect .................................................................................................. 18
3.4)
3.4.1)
3.5)
Design methodology and goals ................................................................................................... 20
Goal setting ............................................................................................................................. 20
Mathematical interpretation of goals......................................................................................... 21
3.5.2) Radiative sky cooler calculations .................................................................................................. 22
3.6) Design .............................................................................................................................................. 25
4)
Recommendations .............................................................................................................................. 28
5)
Conclusion ........................................................................................................................................... 29
APPENDIX .................................................................................................................................................... 30
Bibliography ................................................................................................................................................ 31
viii
ix
1) Introduction
With the demand for air conditioning predicted to rise to accommodate the everincreasing climates caused by global warming. Air conditioning forms the basis of human
comfort for both industry and in the home and greatly effects productivity.
Globally air conditioning accounts for a large percentage of power consumed in the
developing and developed world. With finite resources alternatives need to be considered
to continue to provide the required level of air-conditioning to both plants and the home
1.1)
Global cooling demand
With the global population estimated to reach 8.5 Billion souls by the year 2030 (UN
News Centre,2015) and the global demand for cooling in developing countries on the rise
due to modernization (Krull,2016). HVAC and air-conditioning applications will begin to
find itself more commonplace in homes and commercial properties to ensure comfortable
living / working Environments.
Global warming or the steady rise in average global temperature due to the buildup of
pollutants in the atmosphere will be a major contributing factor in the rise of global
cooling demand. Global average temperatures since the preindustrial era of the 19th
century have already risen by 2’C,
1.2)
Global electrical consumption
Air conditioning and comfort cooling is responsible for up to 40 % of energy
consumption used in modern buildings. In 2007 a study conducted by EC,s joint research
center concluded that 11% or 313TWh of the European unions power consumption was
dedicated to HVAC (Knight,2012).
Power consumption in the European union is planned to be reduced by approximately
20% in the year 2020 (Knight,2012).With the standards of living increasing globally,
greater reliance placed on HVAC systems y developing countries such as South Africa
are also plagued by high electrical consumption due to HVAC systems. (Eskom,2015)
states that HVAC systems are incorrectly utilized to a great extent through firstly lack of
knowledge secondly overuse thirdly poor maintenance and lastly due to ageing
equipment. (Eskom,2015) has determined that HVAC is responsible for 26 % of
electrical consumption in the commercial sector and 20 % in the Industrial sector.
With the ever increasing demand for HVAC systems in both Industry and the home more
than ever energy efficient solutions are required to firstly lesson the load on the limited
existing infrastructure and lastly to reduce the dependency om dwindling fossil fuels
while promoting greener solutions and innovations.
1
1.3)
Environmental effect of convention air conditioning
Conventional refrigeration/air-conditioning operates on the Rankine cycle which utilizes
a compressor to transfer heat energy from one point while removing it from another. This
process is not possible without a medium of heat transfer namely refrigerants.
Refrigerants range from the natural occurring variety ammonia, CO2, and water to
manmade synthetic refrigerants namely hydrofluorocarbons or commonly termed HFC’s.
HFC’s are man made greenhouse gases which are used from solvents to aerosols The
most common use for HFC’s is in the HVAC industry as a refrigerant, due to their
nontoxic nature and high performance in refrigeration systems providing a high kw/kg
ratio.
Although HFC’s are widely considered the go to refrigerant in the HVAC industry for the
past 26 years (EPA, n.d.) they are responsible for increasing global warming
1.4) Problem statement
The main function of this project is to create a passive cooling solution for industrial and
air conditioning applications to be placed on the roof top of buildings which Is capable of
lowering the temperature of a fluid/gas below the ambient dry bulb temperature. The unit
will be designed to operate during daytime hours by radiating energy into space through
the atmospheric window under direct sunlight. The KW/m^2 and feasibility of the cooler
are to be determined.
1.5)
•
•
•
•
•
•
Constraints
The radiative sky cooler must produce 1 kW of cooling.
A minimum of 5’C reduction in inlet temperature must be achieved at outlet.
A flow rate capable of running a 1kW fan coil unit.
A single unit may not exceed 2m/2m
The Body is to be made of a weather resistant material
Produce cooling in direct sunlight.
1.6) Alternatives
Currently passive cooling is produced by either dry coolers or cooling towers. Dry
coolers utilize fluid/gas heat exchangers which exchange heat energy by passing air over
coils, the airflow is provided by fans. Dry coolers are limited by the ambient air
temperature and cannot provide cooling below the ambient air temperature.
Cooling towers utilize evaporative cooling by passing a high temperature fluid over a
high surface area cooling medium causing evaporative cooling to take place at the cost of
high-water consumption. Cooling towers provide cooling at near wet bulb temperatures.
2
1.7)
Literature survey
A literature survey was conducted to assess the feasibility and limitations on current
radiative sky cooler technology while considering the implication and effects radiative
sky cooler technologies present. This section provides a summary of the collected
research information while providing the key aspects of each source.
1.7.1) Aili et al-A kW-scale, 24-hour continuously operational, radiative sky cooling
system.
A 24 hour continuously operating radiative sky cooler at the kW scale was Investigated
through both modelling and experimental study. The Impact on flow rate on both fluid
temperature drops, and cooling power is investigated.
The developed prototype could produce 80 w/m^2 of cooling during daylight hours and
120 w/m^2 during nighttime operation.
It was observed that a tradeoff between net cooling power and temperature drop was
present and determined by flow rate.
The cooling generated was comparable to the monthly air-conditioning electricity use per
household
1.7.2) Bhatia et al- Passive directional sub-ambient daytime radiative cooling
A novel method for passive radiative sky cooling is presented by exploiting the
confinement of solar irradiation from the sky to produce cooling below ambient
temperature. This novel approach demonstrates that radiative sky cooling is possible with
methods other than complex and expensive nanophotonic structures.
The study concluded that the producing of a low-cost radiative sky cooler is possible and
the overall design may remain simple while still providing sub ambient cooling. The
authors state that one of the great constraints encountered in the development of their
passive directional sub ambient radiative cooler was the need for solar tracking.
1.7.3) Eicker and Dalibard - Photovoltaic–thermal collectors for night radiative cooling
of building
The Development of a new PVT system capable of producing both cooling as well as
power generation was investigated and implemented in a Zero energy building. The
radiative sky cooler was tested under the climatic conditions of Madrid, Spain and was
showed to produce 40-45 W/m^2. The device was proposed to be utilized by providing
additional heat rejection of a compression chiller unit and low electrical energy
production.
3
1.7.4) Hu et al - Field investigation of a hybrid photovoltaic-photothermic-radiative
cooling system
Exploiting the natural heat sink provided by space, A practical hybrid photovoltaicphotothermic-radiative cooling device was developed capable of producing heating and
electricity during nighttime operation as well as providing cooling and electrical
generation during daytime operations. It was determined that on average the efficiency of
electrical generation was approximately10.3%. The effects of both clear and overcast sky
conditions were considered.
The device is poised as a passive green solution to providing both cooling and heating at
their respective periods, as well as providing free electrical energy generation for both
building and agricultural fields providing a trifunctional collector.
1.7.5) Jarimi, Powell and Riffat - Review of sustainable methods for atmospheric water
harvesting
An emphasis is placed on water collection in the natural world and considers bioinspired
technologies capable of replicating naturally occurring phenomenon. The Study focuses
on the collection of dew and fog from the atmosphere, Radiative sky cooling is
introduced through the Namibian desert beetle which utilizes radiative cooling to allow
the formation of dew on its back.
The mechanisms of how the Namibian desert beetle produces radiative cooling is
expanded upon.
1.7.6) Kou et al- Daytime Radiative Cooling Using Near-Black Infrared Emitters
An investigation on the use of thin multilayer films which exploit the atmospheric
window to allow radiative cooling under direct sunlight conditions providing sub ambient
cooling temperature.
It is demonstrated that a polymer coated fused silica mirror acts as an approximate black
body in the mid-infrared region thus providing average cooling below ambient of
approximately 8.3’C under direct sunlight conditions and during nighttime operation of
the device.
1.7.7) Kwan et al - Enhanced cooling by applying the radiative sky cooler to both ends
of the thermoelectric cooler
Peltier modules are electrical devices utilizing 2 dissimilar metals which when a current
pass through produces cooling and heating on thee apposing faces. A design utilizing two
4
Peltier modules was conceived which can generate electricity and rejecting waste heat
through radiative sky cooling.
Through experimentation it was shown that sub ambient cooling using the dual Peltier
radiative cooling system provided up to 10k cooling below ambient air temperatures
while providing 40 W/m^2.
1.7.8) Raman et al - Passive radiative cooling below ambient air temperature under
direct sunlight
In the united states of America air condition applications account for approximately 15%
of electrical energy consumed in buildings. Focus on development of radiative sky
cooling systems could result in great energy savings due to the passive nature of the
technology. By radiating heat through the atmospheric window 8-13 micrometers under
direct sunlight the cooler was able to produce cooling of 4.9’C under ambient conditions
providing 40.1 watt per square meter.
1.7.9) Zhang et al - Energy saving and economic analysis of a new hybrid radiative
cooling system for single-family houses in the USA
Radiative sky cooling technology is receiving a great deal of development and attention
due to its free nature, providing radiative sky cooling in place of mechanical air
conditioning systems in buildings incurs great energy savings. The authors acknowledged
that the implementation of radiative sky cooling is slowed due to the high initial capital
cost and the ability to only provide nocturnal cooling. It is estimated that the hybrid
radiative cooled cold storage system is capable of saving between 26%-46% on annual
air-conditioning cost.
1.7.10) Zhao, Hu, Ao and Pei- Performance evaluation of daytime radiative cooling under
different clear sky conditions
Radiative sky cooling is highly affected by the sky conditions. The focus of this study is
to determine exactly what impact different atmospheric conditions have on daytime
radiative sky cooling. A general roadmap for the regional and seasonal effects of
radiative sky cooling is produced through this study.
1.7.11) Zhao et al- Development of a single-phase thermosiphon for cold collection and
storage of radiative cooling
The author contemplates the design of a single phase thermosyphon for cold collection
and storage. Unlike conventional nocturnal radiative cooling systems which are driven by
pumps a novel design which puts to work the natural buoyancy force to facilitate heat
transfer thus negating the need for electrical input. The device was shown to develop 105
5
watt per meter square while cooling water 10.3 ‘C below ambient. The Device was
shown to have an efficiency of approximately 96.8%.
1.8)
Setting of objectives
With the ever-increasing demand on air-conditioning systems and electrical consumption
new technologies and innovative approaches are required to ensure the sustainable use of
finite resources and fossil fuels. Due to the use of HFC’s and the high rate of pollution
being released into the atmosphere global considerations must be made to move to
greener solutions to meeting air-conditioning demands.
The student was tasked to undertake a study into a device which in future could provide
benefits to firstly the company, secondly the environment and lastly to reducing the
already burdened South African national grid.
Radiative sky cooling is a relatively new technology capable of meeting all requirements
tasked to the student, The technology allows for the free production of cooling by
utilizing the natural transparency of the atmosphere in the range of 8-13 micrometers (
atmospheric window) where heat energy can be radiated to space which is used as an
infinitely large cold heatsink without affecting the ambient temperature.
The main objective of this project will be to develop a device capable of providing
passive air-conditioning for small to medium offices and residential spaces.
The following are a list of objectives to be met by this project.
1.8.1) Concept formation
Radiative sky cooling technology has many different variants from the thermoelectric
assisted cooler to the use of materials to radiate waste energy into the heat sink of space.
After considering the main purpose of this project the student has decided to narrow their
focus on a device capable of producing passive radiative cooling.
The device will be placed on rooftops and used to cool single rooms in residential and
office spaces by providing passive air-conditioning. The student has decided to design
two concepts namely a device capable of cooling fluid and storing for when required and
lastly a device capable of cooling gases in an enclosed space
6
The student has considered two concepts namely.
• A device capable of producing passive cooling of water.
• A device capable of producing passive cooling of gas in an enclosed space.
Although the basis of the two concepts are similar their executions are unique.
1.8.2) Final concept
The final concept selected by the student was a result of both the ease of design,
performance of the radiative sky cooler as well as the economic considerations of
manufacturing the device. The final device was capable of meeting most of the design
criteria.
1.8.3) Research
Research was conducted by the student to determine if radiative sky cooling technology
is applicable to solve the problems faced by developing countries such as south Africa.
The problems encountered in developing countries are the increased demand for airconditioning in both residential and commercial spaces as well as the demand placed on
the national grid by air-conditioning.
Due to the free nature of Radiative sky cooling presented in the research, radiative sky
cooling is an innovative solution to the global demand for comfort cooling in ever
increasing climates.
In commercial and industrial spaces radiative sky coolers can provide immense saving in
electrical cost by lower total consumption and peak demand required by these spaces.
Radiative sky cooling is picking up interest at an ever-increasing rate due to the
possibility it provides for a greener global solution to global warming by reducing the
need for HFC’s and their damage to the ozone layer.
1.8.4) Calculations
To create an accurate mathematical model of the operation of a radiative sky cooler,
A few highly important parameters will need to be calculated namely.
•
The temperature drops below ambient.
7
•
•
•
•
•
•
•
•
•
•
Solar irradiance.
Heat rejection through radiation.
Flow rate required for maximum cooling capacity.
The pressure drop across the system.
Reynolds number
Pipe losses, minor and major losses including component and fitting losses.
The pump selection
The dimensions of a single unit
Cooling power of an array of radiative sky coolers
Electrical savings
1.8.5) Material and component selection
Materials selected will play an important role in the effectiveness of the device.
To operate effectively approximate white body and black body infrared emitters will be
required. The outer casing of each device must be weather resistant as the device will be
exposed to the elements outdoors.
Insulation will be a crucial component to avoid heat gain from the environment while
blocking convection and conduction. The conduit for transfer of the fluids will be of a
highly conductive material to allow maximum heat transfer to the device.
Selection of films or highly reflective paints are required to exploit the atmospheric
window and allow radiative heat transfer to take place between the working fluid and the
cold of space.
An itemized list of required material is as follows.
•
•
•
•
•
Aluminum sheet metal
Copper piping
High emissivity film.
Insulation
Fittings and joints.
1.9)
Constraints
• The radiative sky cooler must produce 1 kW of cooling.
• A minimum of 5’C reduction in inlet temperature must be achieved at outlet.
• A flow rate capable of supplying a fan coil unit.
• A single unit may not exceed 2m/2m
• The Body is to be made of a weather resistant material
• Produce cooling in direct sunlight of approximately 5’C
8
1.10) Planning of project
The project commenced on the 13 of July 2020 and 3 months where allocated to allow the
student research and design a prototype radiative sky cooler capable of providing sub ambient
cooling to either a fluid or gas under direct sunlight conditions.
Below is the planning schedule presented by the student summarizing each phase of
development for the project over the allocated 3-month period from 13th of July 2020 until the 1st
of October 2020.
Week 1 (13-19 July) Planning
The discussion of the project had taken place on the first allocated week between the student and
mentor. The student was tasked with planning and breaking down the project to mee the required
deadline. Calendar dates where provided by the student stating the start and ending of each phase
of the project.
Week 2 (20-26 July) Concept formation
The second week of the project was dedicated to the formation of concepts by the student. The
student had to formulate two concepts namely a radiative sky cooler for the cooling of gas and a
device capable of cooling a fluid below ambient air temperature.
Week 3-5 (27 July – 16 August) Research
Over 3 weeks research was compiled by the student to provide a solid foundational
understanding to the operation and design of a radiative sky cooler. The student was challenged
to extend their grasp of radiative heat transfer as well as ask questions to their mentor when
explanation was required.
Week 6-8 (17 August – 6 September) Material study
As radiative sky cooling technologies require material capable of high emissivity and reflection
of radiation between the mid infrared range of 8-13 micrometers, the student was tasked with
determining low cost and readily available materials capable of producing sub ambient cooling
under direct sunlight.
Week 9-10 (7 September -20 September) Design
The designs where developed over a 2-week period by the student utilizing solid works to
generate the technical drawings and designs of the units. Calculations where completed by the
student to determine the feasibility of each design.
One week was dedicated to the design of a radiative sky cooler designed to cooling a fluid below
ambient air temperature.
One week was dedicated to the design of a radiative sky cooler capable of cooling a gas below
ambient air temperature.
Week 11-12 (21 September – 1 October) Report
9
The report was compiled by the student over a two-week period presenting the students research
designs and findings. The report attempts to document the formation of a viable radiative sky
cooler while introducing the reader to the concept of radiative sky cooling as well as the
applications of such a device for the cooling passive cooling of fluids or gases. The student
attempted to provide concise and accurate information.
Task
Planning
Concept formation
Research
Material study
Design
Report writing
Start Date
13/07/2020
20/07/2020
27/07/20
17/08/2020
07/09/2020
21/09/2020
End Date
19/07/20
26/07/2020
16/08/2020
06/09/2020
20/09/2020
01/10/2020
Figure 1: Gant chart
2.)
Investigation
2.1) Theoretical considerations
This section places a probe in the theoretical aspects and process of radiative sky cooling. Due to
the complex nature of Radiative sky cooling many considerations are required to fully design and
simulate the capabilities of such a system. Research into the process of radiative sky cooling was
conducted and calculations where completed to determine possible cooling yields furthermore
the benefits and short comings of radiative sky cooling are explored to aid in the formation and
design.
2.1.1) Concept formation
Two concepts will be briefly discussed and investigated due to each concept focusing on the sub
ambient cooling of gas or fluid both concepts will be assessed. The concepts formulated are as
follows.
2.1.1.2) Concept 1: Gas radiative sky cooler
Concept 1 focused on the cooling of an enclosure of gas by exploiting the atmospheric window
through a device capable of passively producing radiative sub ambient cooling.
2.1.1.3) Concept 2: Fluid radiative sky cooler (Final concept)
10
Concept 2 focused on a device capable of cooling a fluid medium to sub ambient levels by
exploiting the atmospheric window and allowing waste heat to be radiated into space without the
aid of electrical inputs.
The remainder of the report will be based on concept two, a device capable of passively cooling
a fluid to sub ambient temperatures under direct sunlight by exploiting the atmospheric window.
2.2) Research into fluid Radiative sky coolers
The following information is based on the research completed regarding the passive nature and
limitations of radiative sky coolers with a focus on the cooling of fluid below ambient
temperatures.
2.2.1) What is radiative sky cooling
According to (Kou et al., 2017) The process of radiative cooling is a natural phenomenon in
which one body of high temperature dissipates heat across a distance to a second body of lower
temperature via thermal radiation, The coldest known body known to man is the universe with an
approximate temperature of only 3 degrees kelvin.
Due to the cold nature of the universe ingenious methods have been devised to exchange waste
heat energy between objects on earth and the universe. (Bhatia et al., 2018) explains that passive
radiative day time cooling under direct sunlight takes advantage of the atmospheric transparent
wavelength allowing radiative access to the cold upper atmosphere through the atmospheric
window.
Figure 2: atmospheric window (Bhatia et al., 2018)
11
Radiative sky cooling is a phenomenon which naturally occurs in which a 2 bodies exchanges
heat across vast distances through radiation. Since ancient times nighttime radiative cooling has
been exploited to achieve cooling below ambient for example the formation of ice (Bhatia et al.,
2018).
2.2.2) Where is Radiative sky cooling utilized.
Radiative sky cooling is a technology with many application from the preservation of food and
medication, but due to large surface area required the focus of radiative sky cooling applications
has been in the building industry to provide air-conditioning due to the large available space on
roofs as well as to provide energy savings to the building, According to (Zhang et al., 2018)
approximately 45-68% of air-conditioning energy consumption can be saved in medium sized
office buildings over conventional mechanical cooling by utilizing radiative sky coolers pictured
below.
Figure 3: Radiative sky cooler plant integrated into building (Zhang et al., 2018)
Radiative sky cooling devices and technologies are gaining traction due to their free nature and
as the push for greener building and practices are becoming more common place greater research
still will be placed into the implementation of radiative sky cooling in commercial and industrial
spaces.
2.2.3) Radiative sky cooler Benefits
Every emerging technology has to be assessed to determine if the benefits outweigh the obstacles
such a technology may present. South Africa is a country plagued by shortage of electricity and
this has a great impact on the economy as a whole and industry.
12
Radiative sky cooling has the potential of offering the following benefits.
1) Reduced energy consumption
Eskom estimates that annually approximately 26% of the electricity supplied to the
commercial sector is consumed through HVAC. The power supplied to the industrial
sectors of south Africa paint a similar picture with approximately 20% being consumed
by HVAC (Eskom).
Proper implementation of radiative sky cooling can greatly lesson the load on energy
consumption in south Africa allowing greater economic opportunity and lowering
electrical costs across the board. The use of radiative sky cooling can also aid in the
lowering of peak demand and therefore the result of load shedding taking place.
2) Reduced Maintenance.
Due to the passive nature of radiative sky cooling devices no moving parts are required
therefore radiative sky cooling systems require virtually no maintenance due to their
simple construction.
Compared to mechanical cooling systems which utilize compressors and are required to
be serviced annually to allow longevity of the unit, Radiative sky coolers provide a plug
and play system capable of operating without the need for maintenance
The simple design results in low cost installations where only a few pipes are required to
be connected compared to the commissioning required for chilled water systems
3) Environmentally friendly
Unlike conventional chilled water systems radiative sky coolers do not require the use of
refrigerants rather a clever combination of different materials is utilized to produce
cooling through the natural exploitation of the atmospheric window.
Radiative sky coolers reduce both the direct and indirect pollution caused by
conventional mechanical cooling systems and therefore contribute to global warming in a
far reduced portion compared to mechanical cooling devices.
2.3) Working Principle
Radiative sky coolers are devices which obey the three laws of thermodynamics but only one of
the laws will be required to be elaborated on namely the conservation of energy
13
That energy exiting and entering a system must be of equal magnitude and therefore create an
energy balance. The construction of radiative sky cooler allows for only radiative heat transfer
between the universe and the device by blocking both convection and conduction from affecting
the fluid as well as reflecting solar irradiance. Figure 4 illustrates the working principle of
radiative sky cooling technology.
Figure 4: Operating principle of radiative sky cooler (Zhang et al., 2018)
Radiative sky coolers allow a working fluid to be cooled by releasing the heat energy present in
the fluid only by radiation thanks to the surrounding insulation blocking convection and
conduction from taking place. Solar irradiance is reflected by a high emissivity film/material
capable of allowing radiation energy to pass through the atmospheric window cooling the fluid.
The energy balance is as follows.
𝐸𝑛𝑒𝑟𝑔𝑦 𝑖𝑛𝑡𝑜 𝑡ℎ𝑒 𝑠𝑦𝑠𝑡𝑒𝑚 = 𝐸𝑛𝑒𝑟𝑔𝑦 𝑜𝑢𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑦𝑠𝑡𝑒𝑚
𝑄𝑤𝑎𝑡𝑒𝑟(𝑖𝑛) + 𝑄𝑠𝑜𝑙𝑎𝑟 𝑖𝑟𝑟𝑎𝑑𝑖𝑎𝑛𝑐𝑒 = 𝑄𝑅𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒 𝑐𝑜𝑜𝑙𝑖𝑛𝑔 + 𝑄𝑤𝑎𝑡𝑒𝑟(𝑜𝑢𝑡)
𝑄𝑅𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒 𝑐𝑜𝑜𝑙𝑖𝑛𝑔 = 𝑄𝑠𝑜𝑙𝑎𝑟 𝑖𝑟𝑟𝑎𝑑𝑖𝑎𝑛𝑐𝑒 + ((𝑄𝑤𝑎𝑡𝑒𝑟(𝑖𝑛) − 𝑄𝑤𝑎𝑡𝑒𝑟(𝑜𝑢𝑡))
Since the solar irradiance is almost completely reflected the cooling capacity generated by the
system is equivalent to.
𝑄𝑅𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒 𝑐𝑜𝑜𝑙𝑖𝑛𝑔 = 𝑄𝑤𝑎𝑡𝑒𝑟 (𝑖𝑛 − 𝑜𝑢𝑡)
𝑄𝑅𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒 𝑐𝑜𝑜𝑙𝑖𝑛𝑔 = 𝑀𝐶𝑝∆(𝑇𝑖𝑛 − 𝑇𝑜𝑢𝑡)
Where
M = mass
Cp = coefficient of heat transfer
Tin = water inlet temperature
Tout = water outlet temperature
Radiative sky coolers operate most effectively during night as there is little to no solar irradiance
required to be reflected. Because nighttime temperatures are far lower than daytime
14
temperatures, nocturnal radiative sky cooling produces lower temperatures than its daytime
counterpart resulting in increased cooling capacity.
2.4) Atmospheric window
The atmospheric window represents a bandwidth of the wavelengths in which atmospheric gases
absorb little to no energy from radiation passed through the atmosphere.
Three major regions of wavelength comprise of the Visible window (0.3 -0.9 μm) which contains
all visible light and colours, the second major wavelength is the infrared window (8-13μm) in
which the atmosphere is demanded transparent allowing radiation energy to pass through freely
without increasing the local ambient temperature of the gas particles encountered. The Third
major window is the microwave windows at wavelengths greater than 1mm. (American
Meteorological society, 2012)
High humidity can cause the atmosphere to become opaque in the infrared region as water
molecules continuously absorb energy (American Meteorological society, 2012)
2.5) Emissivity
Emissivity is a material or finish property which shows the ability of a body to emit radiation
energy between two bodies in the hopes of neutralizing or averaging both temperatures out while
obeying the phenomenon of heat energy moving from a high heat energy sink to a low heat
energy sink.
Emissivity can be seen as the comparison between a perfect emitter or blackbody versus the
radiation emitted from a materials surface, emissivity or the capability to emit radiation is
presented on a dimensionless scale from 0 – 1 (NPL, 2020).
Emissivity is affected by not the type of material but also the surface finish of a material, Shinier
or polished metals will have a low emissivity and will approximately mimic a white body
radiative emitter while oxidizes metals having a dull or mat appearance will have a high
emissivity thus approaching that of a black body radiative emitter (NPL, 2020)..
3 Design of a kW scale radiative sky cooler
The following body of work represents the design process undertaken to meet the requirements
for a radiative sky cooler capable of passively cooling a fluid below ambient air condition.
The device will require no electrical power input and will rely on exploiting the atmospheric
window to transfer heat energy through radiation to the universe which acts as an infinitely large
cold heat sink
15
3.1)
Problem statement
The first part of providing a solution to a problem is to fully understand the problem. The
problem statement provides a clear and concise statement describing the correct operation and
performance criteria required of the devised solution.
The problem statement is as follows.
The main function of this project is to create a passive cooling solution for industrial and air
conditioning applications to be placed on the roof top of buildings which Is capable of lowering
the temperature of a fluid below ambient temperature. The unit will be designed to operate
during daytime hours by radiating energy into space through the atmospheric window under
direct sunlight. The KW/m^2 and feasibility of the cooler are to be determined.
The device or array of devices must be capable of generating 1 to 3 kw of cooling at sub ambient
temperatures. The Device will utilize cold storage tanks to store cooling energy as cooling will
not be required throughout the entirety of the day.
3.2)
Radiative reflectors.
A crucial component of a radiative sky cooling devices are radiative reflectors. Radiative
reflectors are films, paints, polished metals capable of reflecting solar irradiance and resulting in
negligible heat gain to the system.
Commonly reflective films are utilized to cover the sun facing windows of building and provide
a natural cooling effect to the interior spaces. According to (conveniencegroup, n.d) the use of
window cooling films can reduce electrical cooling cost by up to 40%, With this substantial
saving the power of radiative reflectors are displayed to full effect.
Research into the application of silicone aerogels have yielded many possible uses for the
substance ranging from thermal insulation and fire protection device, According to (Bheekhun,
Abu Talib and Hassan, 2013) Silicone aerogels are capable of providing no reflective loss when
light enters the aero gel, therefore acting as a near perfect radiative reflector. The draw back of
aerogel based radiative reflectors is the high material cost due to the complexities associated with
production of aerogels therefore silicone aerogels are still manufactured in small quantities for
specialist or novel applications.
A simple and cheap solution to radiative reflectors is the use of low emissivity paints capable of
reflecting high amounts of solar radiation energy. Low emissivity paints can be directly applied
to the walls to building and more commonly the roof of buildings. Apply high emissivity paints
16
to the roof of buildings results in a passive cooling solution while not consuming valuable realestate on rooftops and requires no bulky equipment. According to (Fantucci and Serra, 2017), the
use of low emissivity paints can result in a summer electrical cooling cost saving of anywhere
between 25%-70%. Low emissivity paints provide a cheap solution to radiative sky cooling but
require re application yearly as the paint begins to deteriorate due to the exposure of harsh UV
rays and rain.
3.2) Material consideration
As radiative sky coolers are required to be exposed to outdoor conditions to operate correctly,
Consideration is required to the selection of materials capable of withstanding the harsh climatic
conditions and fluctuations the device will experience over its lifetime.
3.2.1) Corrosion
corrosion is the formation of compounds on the surface of metals when exposed to oxygen, water
or an electrolyte. Materials are rarely present in their pure states and are often found as ores
during the mining process. Corrosion is the process in which refined material undergoes a natural
formation of more stable forms such as oxides. Corrosion can be seen as the release of energy
used in the refining process which in turn returns the substance to its natural state
Reactivity of metal Reactivity is the measure of how easily a relatively pure substance forms
compounds ie How noble the metal is. When in the presence of oxygen / electrolytes, the
reactivity of the metal will determine the rate at which the new compound (oxide/rust) forms on
the surface (BBC-Bitesize, n.d.).
To ensure a long operating life span consideration towards the effects of the different types of
corrosion are as follows.
Uniform corrosion- Occurs when metal is exposed and comes into contact with oxygen/ water.
Uniform corrosion takes place at the same rate across entire surface of a metal object. If left
untreated uniform corrosion will inevitably result in the loss of strength in the part and reduction
of weight. An example of uniform corrosion is surface rust (corrosionclinic, n.d.).
Pitting corrosion- Pitting corrosion takes place when alloys are exposed to chemicals in the
environment such as chlorides which strips the naturally produced protective oxide layer .Pitting
corrosion is considered the most common form of corrosion found on aluminum, magnesium
alloys. Pitting corrosion results in localized corrosion confined to multiple small areas across the
face of the surface resembling holes. (corrosionclinic, n.d.).
Galvanic corrosion- When two dissimilar metals are in contact whilst in the presence of an
electrolyte example sea water. The potential difference between the two dissimilar metals results
in the formation of a galvanic circuit. Current flows to the more noble metal while corroding the
less noble metal (corrosionclinic, n.d.). Galvanic corrosion forms the basis for sacrificial anodes
to be discussed next.
17
3.2.1) Common forms of corrosion prevention
• Galvanizing- Is the process of providing a protective outer coating to steel and iron
components and structures by providing a sacrificial zinc layer. According to (Britannica,
1998) galvanizing can result in protection of corrosion for up to 30 years and can be
applied either through hot dipping or electrolytic application.
•
Sacrificial magnesium anode- According to (Pathak, Mendon, Blanton and Rawlins,
2012) Electrochemically it is seen that magnesium is one of the most reactive metals and
is commonly employed to provide cathodic protection to crafts submerged in seawater
such as ships, submarines as well as structures such as bridges and decks. Magnesium is
provided as a coating as a sacrificial anode.
•
Protective coating- paint powder coating according to (Bhadu, Guin, Singh and
Choudhary, 2013) Is the process in which a layer of plastic polymer is bonded to the
exterior surface of steel components and is capable of providing a robust and easily
applied solution for the protection of iron and steel structures providing a thick and
uniform coating
3.3) Climate consideration and effect
Because passive radiative sky cooling device allow for sub ambient cooling the maximum
cooling capacity attainable is highly dependent on the regional climatic conditions. Therefore, it
is important to have accurate weather data to correctly model the capabilities of a radiative sky
cooler.
The weather data collected was for the cape town region and is presented in the below table.
The table represents design temperatures for each month of the year.
18
Figure 5: Cape town weather data (ASHRAE)
Interpreting the above data, it is clear that the hottest month of the year is in February with a
design temperature of 33’C. Due to this high temperature even with a high sub ambient cooling
being achieved the design outlet temperature would be roughly between 4-8’C lower than the
ambient. An alternative solution would be to utilize nocturnal radiative cooling with
insulated/double walled storage tanks capable of storing the energy for daytime use.
An important factor to consider is the effect relative humidity has on the so-called clarity of the
atmospheric window, as stated by (American Meteorological society, 2012) High relative
humidity or cloudy sky can result in the infrared wavelength becoming opaque. With this
consideration in mind radiative sky cooler technology will operate most efficiently providing the
greatest amount of cooling in regions where low relative humidity is present. With the relative
humidity of Cape town averaging at 76% (Weather & Climate, n.d.), a better candidate for
radiative sky cooling would be the Northern cape where the relative humidity is maintained at
approximately 49%. (Weather & Climate, n.d.)
The Annual relative humidity profile for both Cape town and the northern cape are presented
below.
Figure 6: Cape Town Vs Northern Cape relative humidity (Weather & Climate, n.d.)
19
Although climate conditions have a limiting effect on the application of radiative sky cooling.
Radiative sky cooling still provides a promising technology capable of having a great energy
saving potential globally.
3.4) Design methodology and goals
The first step in providing a solution to a problem is to fully define and simplify what one is
asked, this step is handled through the problem statement which clearly emphasizes the key
points highlighted by the challenge. For ease of reading please find the Problem statement
repeated below.
The main function of this project is to create a passive cooling solution for industrial and air
conditioning applications to be placed on the roof top of buildings which Is capable of lowering
the temperature of a fluid/gas below the ambient dry bulb temperature. The unit will be designed
to operate during daytime hours by radiating energy into space through the atmospheric window
under direct sunlight. The KW/m^2 and feasibility of the cooler are to be determined.
With a Cleary defined problem the second step would be to acquire information on the topic to
develop an In depth knowledge of both the terminology and operating principles used to
formulate a solution to the challenge highlighted in the problem statement. Research provides the
current state of a technology highlighting the limitations and capabilities while as well as the
development of and progression of the concepts and methods of implementation.
Thirdly the individual aspects affecting the technology are to be collected and assessed to ensure
proper implementation of the concept taking into account how each factor affects overall
performance.
Fourthly goals are to be set to pre-define required performance figures and provide a targeted
approach to sizing and providing the correct capacities.
Goals are to be interpreted in mathematical language to firstly test the validity of the goal and
secondly improve upon expected results.
Lastly a design is to be developed to provide a solution to the problem stated in the problem
statement.
3.4.1) Goal setting
The following is a brief list of goals to be accomplished by the radiative sky cooling system.
1.
2.
3.
4.
The device must have the capabilities of cooling a fluid to sub ambient temperatures.
The only electrical allowable electrical input shall be to a pump.
The device must be capable of passively cooling flowing water.
The system must be nocturnally charged and be capable of storing sufficient cooling
capacity in insulated tanks for daytime operation.
5. The system must be capable of providing 1 kw’s of cooling capacity for approximately 1
hours a day.
20
6. A 1 kw Fan coil unit will be used to provide the required cooling.
3.5) Mathematical interpretation of goals
The following section provides the process and calculations performed to reach required goals
listed above.
3.5.1) Sizing of storage tanks.
A 1 kW AERMEC FCZ100U Fan coil unit was selected.
Figure 7: Aermec FCZ100U fan coil unit
The selected fan coil unit was capable of providing 1 kw of cooling at a flow rate of 0.0478 l/s
with water in/out temperatures of 12’C/7’C.
To provide 1 kw of cooling over a 1-hour period the total storage capacity was determined
𝐿
× 𝑇𝑖𝑚𝑒
𝑠
𝐿
𝑇𝑜𝑡𝑎𝑙 𝑠𝑡𝑜𝑟𝑎𝑔𝑒 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 = 0.0478 × 3600 𝑠𝑒𝑐𝑜𝑛𝑑𝑠
𝑠
𝑇𝑜𝑡𝑎𝑙 𝑠𝑡𝑜𝑟𝑎𝑔𝑒 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 =
𝑇𝑜𝑡𝑎𝑙 𝑆𝑡𝑜𝑟𝑎𝑔𝑒 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 = 172 𝑙𝑖𝑡𝑟𝑒𝑠
With the total storage capacity being determined, A 200 L Fiorini VKGE-HC cold storage tank
was selected. The tanks are constructed out of rolled carbon steel with weather resistant
aluminum outer coating and High density rigid polyurethane foam to ensure that the tank will be
capable of retaining the available cooling energy.
21
Figure 8: Fiorini VKGE-HC tanks (fiorini,2020)
3.5.2) Radiative sky cooler calculations
Stefan Boltzmann’s Law is a mathematical representation of the amount of radiation energy per
unit area of a black body is capable of exchanging with its environment at absolute temperature
to the fourth power. Stefan Boltzmann’s law (Byjus, n.d.).
A blackbody is capable of absorbing all energy radiated onto it. Perfect black bodies do not
occur in nature and therefore physical bodies are described as grey due to the fact that not all the
energy received is absorbed. The factor by which a body variates from a perfect black body is
called the emissivity.
Stefan Boltzmann’s Law is presented as follows
𝑃 = 𝐴𝜀𝜎𝑇 4
Where:
P (watt)= the radiated energy per square meter of a body at temperature Kelvin
𝜺 (Emissivity) = a dimensionless number from 0-1 that presents the efficiency of a body to
radiate and absorb energy.
𝝈 (Stefan Boltzmann’s constant) = 5.67𝑥10−8 𝑊𝑚−2 𝑇 −4
22
The Stefan Boltzmann equation represents outgoing radiative energy only. To determine the
radiative energy supplied by the atmosphere the modified swine bank model for downward
thermal radiation is utilized. (Goforth et al,2002).
The modified Swine bank model is as follows
𝑃𝑡ℎ𝑒𝑟𝑚𝑎𝑙 = (1 + 𝐾𝐶2)8.78 × 10^(−13)𝑥(𝑇)^5.852𝑋(𝑅𝐻)^0.07195
Where
P(thermal) = Downward radiation in Watt per square meter.
K= 0.34 @ Cloud height of <2 km
K= 0.18 @ Cloud height of 2m<X>5m
K=0.06 @ Cloud height of >5 m
C= cloud cover factor (0 clear sky & 1 for overcast)
T= ambient temperature in degrees kelvin
RH = Percentage of relative humidity.
The design conditions have been selected as follows.
•
•
•
•
•
•
•
•
Operation will be performed nocturnally
The surface area of the emitter will be 0.7 meters square.
The emitter material will be anodized aluminum.
The emissivity of the emitter will be 0.77
The location cape town
RH = 76%
Design month February
Average nighttime temp = 17.9 ‘C
Maximum radiation released by the emitter
𝑃 = 𝐴𝜀𝜎𝑇 4
𝑃 = 0.7 ∗ 2 ∗ 𝑥0.77𝑥5.67𝑥10−8 (17. 9 + 273)4
P=218.85 watt for 2x units
Downward radiation clear nights sky
𝑃𝑡ℎ𝑒𝑟𝑚𝑎𝑙 =225.3 watt
23
Net cooling capacity = Radiated – received energy
Net cooling capacity = 212 watt
Flowrate through unit
𝑄 = 𝑀𝑐𝑝∆𝑇
Where
Q=cooling capacity
M = mass flow rate
CP= Coefficient of heat transfer (4.2 for water)
∆𝑇= temperature difference.
Required condition
Inlet water temp = 17.9’C
Outlet water temp=10’C
𝑄 = 𝑀𝑐𝑝∆𝑇
0.212 = 𝑀𝑥4.2𝑥(10 − 17.9)
𝑀 = 0.0064 𝑘𝑔/𝑠
Time to Cool total tank volume
𝑇𝑖𝑚𝑒 =
𝑇𝑎𝑛𝑘 𝑀𝑎𝑠𝑠
𝑀𝑎𝑠𝑠 𝑓𝑙𝑜𝑤
𝑇𝑖𝑚𝑒 =
200
0.0064
𝑇𝑖𝑚𝑒 = 31250 𝑠𝑒𝑐𝑜𝑛𝑑𝑠
𝑇𝑖𝑚𝑒 = 8.6 𝐻𝑜𝑢𝑟𝑠
Therefore, the radiative sky cooler is capable of cooling a 200-liter cold storage tank in
approximately 8.6 hours. This stored energy is capable of providing approximately an hour of
cooling at 1 kW.
24
3.6) Design
To provide the required cooling 2 x radiative sky cooler will be required. The following section
displays the design of the unit.
Emitter
The radiative sky coolers will each have an emitter of 0.7 square meters constructed of anodized
aluminum with emissivity of 0.77 each.
Figure 9: Anodized aluminum emitter
Frame
Galvanized steel structure utilizing square tubing of 25x25 mm. The frame is angled at 30
degrees to reduce direct sunlight.
25
Figure 10: Galvanized square tubing frame
Reflector
Polished aluminum low emissivity reflector to focus radiation in the direction of the emitter.
The pipework carrying the fluid to be cooled runs across reflector.
Figure 11: reflector and pipework
Galvanized sheet steel container
26
The container supporting the emitter and housing the reflector comprises of galvanized sheet
steel of 5 mm thick.
Figure 12: Container
Assembly
Figure 13: Assembled view
The beauty of he radiative sky cooler is the modular design, capable of producing large cooling networks when used
in array the possibilities of providing large scale cooling are endless.
27
Simple System schematic
Figure 14: Simple system diagram
4)
Recommendations
Although the student has tried to compile this report in the most logical and comprehensive
fashion possible, improvements are always possible. Improvements are the driving force of
innovation and provides the reader a solid foundation for continued work. The following is a list
of improvement the overall project.
•
•
•
Introduction to topics and writing style.
o Due to the complex topics explained in this report, it is the student’s responsibility
to produce information which is both simple and precise in nature.
o The writing style and tone greatly effect the manner in which a body of work is
perceived.
Planning.
o Accurate time management greatly affects both the quality of work as well as the
depth in which a project can be explored. Planning between team members allows
for easy communication and guidelines for progression between all parties.
Teamwork.
28
o Efficient teamwork creates an environment in which each member involved in the
project takes responsibility for his or her input to the project. Teamwork affects
moral and can greatly improve the speed and overall quality of the work
produced. Teamwork results in shorter turn around times for projects as well as
provides confidence amongst each of the team members.
•
•
•
Drafting and Design
o Efficient design results in a product or device capable of meeting all requirements
as well as providing low cost of maintenance manufacturing.
o Accurate drawing and BoQ result in easy ordering of materials and greater
manufacturing precession resulting in less waisted material.
o Cad drafting and designs must be drawn in a manner in which multiple designers
may collaborate.
Experience and skill development.
o Active skill development result in knowledgeable solutions being provided as
well as technical confidence in problem solving.
Study on energy saving
o A study provided on energy savings could result in improved reception of a
product or device. Such a study could stand as economic justification to
implement a solution.
5) Conclusion
In this report the student was tasked over a three-month period to evaluate and design a
device capable of providing energy savings.
The student was tasked to undertake a study into a device which in future could provide
benefits to firstly the company, secondly the environment and lastly to reducing the
already burdened South African national grid.
The impact of conventional mechanical refrigerant devices on the environment was
investigated by the student and it was determined that a huge percentage of electrical
consumption was generated for the use in HVAC systems.
Refrigerants and their implications where briefly touched on in an effort to understand
the role the emission of greenhouse gases played in global warming. It was determined
that the release of HFC gases into the atmosphere has a detrimental impact on the
environment and was directly responsible for global warming.
A study into the different factors affecting Radiative sky coolers where explored and
elaborated by the student for the purpose of the reader.
In conclusion the designed device proved effective in generating and storing cooling
capacity in insulated tanks. It was shown that 200 liters of fluid was required to provide 1
hour of cooling at 1 kw. The device was capable of charging the cold storage tanks in
approximately 8.6. Although improvement can still be made the device was deemed to be
s success.
29
Radiative sky cooling provides a green solution to the ever-growing thirst for comfort
cooling in both the industrial, commercial and residential sector while reducing both
indirect and direct pollution created by conventional mechanical cooling devices.
APPENDIX
30
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