Review of sustainable methods for atmospheric water harvesting

Abstract

The scope of this paper is to review different types of sustainable water harvesting methods from the atmospheric fogs and dew. In this paper, we report upon the water collection performance of various fog collectors around the world. We also review technical aspects of fog collector feasibility studies and the efficiency improvements. Modern fog harvesting innovations are often bioinspired technology. Fog harvesting technology is obviously limited by global fog occurrence. In contrast, dew water harvester is available everywhere but requires a cooled condensing surface. In this review, the dew water collection systems is divided into three categories: i) dew water harvesting using radiative cooling surface, ii) solar-regenerated desiccant system and iii) active condensation technology. The key target in all these approaches is the development of an atmospheric water collector that can produce water regardless of the humidity level, geographical location, low in cost and can be made using local materials.

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doi:10.1093/ijlct/ctz072 Advance Access publication 1
Review of sustainable methods for
atmospheric water harvesting
..............................................................................................................................................................
Hasila Jarimi1,2,*,RichardPowell
1and Saa Riat1
1Buildings, Energy and Environment Research Group, Department of Architecture and
Built Environment, Faculty of Engineering, The University of Nottingham, Nottingham
NG7 2RD, UK 2SolarEnergyResearchInstitute(SERI),UniversitiKebangsaanMalaysia,
43600 Bangi, Selangor, Malaysia
............................................................................................................................................
Abstract
The scope of this paper is to review dierent types of sustainable water harvesting methods from the
atmospheric fogs and dew. In this paper, we report upon the water collection performance of various
fog collectors around the world. We also review technical aspects of fog collector feasibility studies and
the eciency improvements. Modern fog harvesting innovations are oen bioinspired technology. Fog
harvesting technology is obviously limited by global fog occurrence. In contrast, dew water harvester is
available everywhere but requires a cooled condensing surface. In this review, the dew water collection
systems is divided into three categories: i) dew water harvesting using radiative cooling surface, ii) solar-
regenerated desiccant system and iii) active condensation technology. The key target in all these approaches
is the development of an atmospheric water collector that can produce water regardless of the humidity level,
geographical location, low in cost and can be made using local materials.
Keywords: atmospheric water harvesting; fog collector; biomimicry; innovative sustainable technology
∗Corresponding author:
hasila.jarimi@outlook.com
Received 29 June 2019; revised 30 October 2019; editorial decision 13 November 2019; accepted 13 November
2019
................................................................................................................................................................................
1. INTRODUCTION
Globally, the number of people lacking access to water is
2.1 billion, while 4.5 billion people have inadequate sanitation
and clean water source [1]. The latter, has led to risk of infected by
diseases, such as cholera and typhoid fever and other water-borne
illnesses. As a result, the world has witnessed 340 000 children
undervedieeachyearfromdiarrhealdiseasesalone[1]. Clearly,
water scarcity is an issue requiring urgent action. The situation is
exacerbated by climate change causing rainfall patterns to change
with some areas already experiencing prolonged droughts.
Worldwide,manymethodshavebeenusedtoharvestwater
such as through water desalination, ground water harvesting and
rain water collection and storage. Obviously, for these to work
liquid water must already be available, but when such supplies are
limited, harvesting atmospheric water becomes essential. There-
fore, not surprisingly, it is now receiving considerable attention
from researchers worldwide. This paper reviews this work, dis-
cussing the various water harvesting technologies and their per-
formance, both theoretical and experimental. Commercialized
atmospheric water harvesting technologies are also described.
We hope this review will help new workers wishing to enter
this important eld by providing introduction to state-of-the-art
technologies and inspire them to develop their own ideas for inno-
vative and sustainable atmospheric water harvesting technology.
We believe that general readers, with an interest in the welfare of
‘water poor’ people, will also nd this paper useful by showing
how emerging water harvesting systems can contribute to improve
living standards.
Figure 1 shows how atmospheric water harvesting technologies
may be classied. The rst category is harvesting water from fog,
i.e. a visible cloud water droplets or ice crystals that are suspended
in the air at or near the Earth’s surface [2]. It normally occurs due
to added moisture in the air or falling ambient air temperature.
Methods may be usefully divided into ‘traditional’ and ‘modern’.
The second collection category is the collection of water vapour.
Whilefogisvisibletoournakedeyes,watervapourisinvisibleand
is generated by the evaporation of liquid water or the sublimation
of ice. When water vapour condenses on a surface cooled temper-
ature below the dew point temperature of the atmospheric water
vapour, ‘dew water’ will be formed [3]. While fog water harvesting
system are more related to traditional concept using a mesh-
like structure, there are various technologies related to dew water
harvesting technique. The early studies involved passive systems
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H. Jarimi et al.
Figure 1. Categories of atmospheric water harvesting techniques.
using radiative condenser, but their low eciencies resulted in,
researchers introducing solar-regenerated desiccant methods to
enhance the moisture sorption and desorption, however, still has
notprovedonitsowntobesucient.Thus,researchindewwater
harvesting also covers integration with active cooling condenser
technology that covers the use of typical vapour compression
air conditioning system and most recently, thermoelectric cooler.
Due to the high in eciency of active cooling condenser systems,
at the end of this paper, readers will be presented with selected
commercially available technology on water harvesting technol-
ogy involving active cooling condensing system.
2. ATMOSPHERIC FOG HARVESTING
2.1. Fog collector inspired by traditional concept
Illustrated in Figure 2, the traditional fog collecting method is
very simple, comprising a mesh exposed to the atmosphere over
whichthefogisdrivenbythewind.Twopostsonguywiresare
used to support the mesh and cables to suspend the mesh. Water
droplets trapped by the mesh accumulateand drain under gravity
into the channels of the water collection system. Collectors can
be usefully classied as standard fog collectors (SFCs) and large
fog collectors (LFCs) [2]. SFCs are typically used in a small scale
exploratory studies to evaluate the amount of water that can be
collected for a specic condition. The collector has a typical size
of (1 ×1) m2surfacewithabaseof2mabovetheground[4].
LFCs, typically 12 m long and 6 m high has mesh covers the upper
4 m of the collector giving ∼48 m2of water collection area. They
are mainly used for actual harvesting installation. For maximum
eciency, fog collectors should be positioned perpendicularly to
the prevailing wind. Typically, LFSs produce 150 l to 750 l of water
adaydependingonthesite[5]. Reported in 2011, the cost for a
unit of 48 m2fog collectors is US$400 meanwhile, the 1 m2SFCs
cost from US$100 to US$200 to build depending on the country
and the materials [5].
Commercially available, Raschel-weave high-density polyethy-
lene mesh, commonly used for shading crops in hot climates, has
Figure 2. The basic concept of fog collector. Adapted with permission from [6]
Copyright (2013) American Chemical Society.
been a popular collector material, although other weaves such
as aluminet shade net have subsequently been investigated [7].
Illustrated in Figure 3, the standard Raschel mesh is black in
colour, with treated UV-resistance and has 35% shade coecient S
[8]. Shade coecient Sis the portion of the fog collector’s area that
is capable of capturing fog droplets and can be expressed using the
following equation [9]:
S=1−f,(1)
where fis dened as the ratio of mesh openings area to the total
screen area.
Along the longitudinal direction, the mesh lament is tied
up continuously, meanwhile transversely, we can see that the
laments are not continuous but knotted to the longitudinal one
[8]. A leading developer of fog harvesting technology based on
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Atmospheric water harvesting
Figure 3. An example of Raschel mesh used in a project by Fog Quest [10].
Raschel-weave shading mesh is the non-prot registered Cana-
dian charity, FogQuest, (www.foqguest.com), which ‘is dedicated
to planning and implementing water projects for rural communi-
ties in developing countries’. Their rst fog water harvesting expe-
rience dates from 1987. In addition to innovative fog collectors,
they have also included rainfall collectors to make optimum use
of natural atmospheric sources of water.
2.1.1. Selected projects from the past 30 years to current
Fogharvestingiscommoninaridandsemi-aridareasclosetothe
ocean where clouds are formed over the sea and pushed by the
prevailing winds towards the mainland. The clouds would be come
fog when they intercept with the surface of highlands near to the
sea. There are various fog collector installation, both for research
and real applications in dierent places such as Namib Desert,
Africa. The desert is well known for its potential in harvesting
water through fog collection. Mtuleni et al. [11] conducted an
interesting research to nd out the quality of the Namibian fog
water. Fourteen SFCs were studied at three Topnaar villages in
Namib Desert [11]. The highest water collection was 2.122 l/m2
at Klipneus village. In terms of the water quality, aer a non-
foggy period, the initial rinse of SFCs give turbid, brackish water
that contains 1630 mg NaCl [11]. The water was considered as
marginally t for human consumptions. Nevertheless, the subse-
quent water collected aer the initial rinse was found fairly cleaner
and has low salt content. In the Coquimbo region of Chile, in
1980s, a research project involving y 48 m2fog collectors was
conducted [12]. Forty-one new large fog collectors were installed
to provide fresh water supply for 100 families beneted, supported
initially by the foreign partners and then given over to the local
population in the 1990s [12]. However, due to the incompetency
of the local non-governmental organisation(NGO) in terms of
technical skills, the project was reported degraded. Large fog
collectors were also developed from 1995–99 utilized mainly for
reforestation and restoration of degraded coastal ecosystems near
the town of Mejia, Peru [13,14]. In Pachamama Grande, Ecuador
alargescaleprojectwasdevelopedsuchthat40LFCswerecon-
structed throughout 1995–97 with the collection eciencies are
Figure 4. Theexamplesofrobustmaterials.Le:isarobustmaterialwitha
stainless mesh, co-knitted with poly material. Right: a 3D net structure (1 cm
thickness) of poly material [2].
as high as 12 l per square metre per day [15]. Also in the 1990s, in
Oman, a major fog collector study was conducted. Daily average
collection rates were reported to be as high as 30 l/m2.However,
the large amount of water collected only happens during monsoon
season that occurs about only 2 months in a year. This was
considered as a huge limitation to the use of fog collectors in
that region [16]. The following Tab l e 1 listed more fog collection
projects carried out worldwide.
2.1.2. Fog collectors design
For LFCs, the prevailing wind imposes pressures on the mesh
which then imposes forces on the supporting structures and
nally weakening/break the foundation. Meanwhile, the mesh
and other components of LFCs can be damaged by UV radia-
tion and also other environmental factors. Lacking in rational or
engineered design process of LFCs being the main reason to the
collapse of LFCs under extreme weather. This apparently explains
the maintenance issue faced by the local people in managing fog
collectors [8]. In order to suit dierent environmental conditions
for examples for very windy sites, robust materials for the fog
collectors were made using stronger stainless steel mesh, co-
knitted with poly material. See Figure 4 [2].
VariouscollectordesignshavealsobeenresearchedbyLum-
merich and Tiedemann [22] in a eld study on the outskirts of
Lima (Peru) to address crucial aspects of economic competitive-
ness of fog water harvesting. Prior to the eld testing, ve small
scaleprototypeswithdierentshapesandmaterialsweretestedin
selecting the most eective fog collector structure. Following the
small scale testing, three dierent types of large scale fog collector
were investigated termed ‘Eiel’, ‘Harp; and ‘Diagonal Harp’. The
‘Eiel Collector’ is an example of a 3D collector that is used at
places with a rare condition with no unique wind direction asso-
ciated with the occurrence of fog. In their report, a three-winged
screencalledastropodwasintroducedasanimprovedmeansto
evaluate the amount of water yield by fog water harvesting. The
useofastropodallowedthemeasurementofthefavourablewind
direction and absolute amount of collected fog at the same time.
The fog collector designs and the description are summarized in
Table 2 .
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H. Jarimi et al.
Tab l e 1 . The selected fog collector projects worldwide.
State/country Project Year Size/design/cost Type of application/issues Water collection Ref
Falda Verde
Mountain, Peru
Locally designed fog
collector
1998–2001 1.5 m2
1mabovethesurface
- 100 l (day not stated) [17]
Alto Patache, Chile SFC fog collector 1987–2001 - Primarily for ecosystem and
climate research.
6lperday [17], [18]
Village of Tojquia in the
Wes t e r n Hi g h l and s
35 LFCs 2006–2012 - High wind speeds are an issue. Average of 6300 l of water per
day in 4–6 months in winter
dry season
[2]
Yemen (in the mountains
near Hajja,
inland from the Red Sea)
25 LFCs 2004 - Stopped aer a year due to
insucient monitoring at
community level and issues
related to occasional high
wind speeds.
4.5 l m−2per day over the
3-month dry winter period
[19]
North West of the island of
Tene r i fe
Four LFCs, in 2000s and four
more were added in 2011.
2000s and 2011 - The water is used for
domestic purposes in
the Forestry Commission
Oce, for irrigation for the
reforestation of endemic
laurisilva species and for
prevention of and ght
against forest res.
-[20]
Lima, Peru Project- 60 fog nets 2016 ‘Fog catchers’ nylon nets
designed. Cost: 500
USD per net
Supply free water to 250
households. The water is
not drinkable thus used to
sustain small scale farming,
wash clothes and to wash
households’ utensils for
poor families.
100 l of water per day, a saving of
almost 60% in water usage.
[21]
Tojquia, Guatemala FogQuest project-
35 large fog collectors
(LFCs)
Since 2006 to current
(2017)
40 square meters For community use Produces an average of about
200 l of water a day during the
winter dry season.
[10]
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Atmospheric water harvesting
Tab l e 2 . Selected fog collector designs [22].
The fog collector Size/design Type of application Advantage Maximum water collection
(litres/day)
Country
Eiel collector 4×8×0.3 m metal frame, two
separated layers of Raschel
50% net with 10 additional
stripes in between
Large scale experiment A 3D collector that is
advantageous for places
with no unique wind
direction associated with
the occurrence of fog.
2650 l per day during the
peak fog season.
Peru
Harp collector 2×4×0.3 m metal frame,
2256 m of 1.5 mm rubber
string vertically installed
Large scale experiment 200 l/day during peak season Peru
Diagonal Harp collector 2×4×0.3 m metal
frame, 1520 m of 1.5 mm
rubber string diagonally
installed
Large scale experiment 94.2 l/day during peak season Peru
Figure 5. The concept of the cloud harvester. The harvester is designed to catch
and condense fog into water droplets that in turn run down on a stainless steel
mesh into a gutter type extrusion leading to a water storage container [23].
Auniquedesignoffogcollectorcalledcloudharvesterhas
been designed by Choiniere-Shields [23], see Figure 5.Thecon-
cept of cloud harvester is based on a fog catcher that turn the
condense fog into water droplet. In comparison to the current
modelavailableonthemarket,theuniquepartofinthedesign
of cloud harvester is that it uses stainless steel mesh instead of
the polypropylene nets with an extra sheet under the net for
the water collection. The cloud harvester is expected to have a
better condensing eciency and much smaller than the similar
products that are currently on the market. The cloud harvester
hasapotentialwaterharvestingoutputof1loffreshwaterper
hourforeach10squarefeetofmesh[23].
Aiming to harvest water from the atmosphere to supply fresh
drinking water to the community in the developing world, a
unique wooden atmospheric water harvesting project called
Warka Water has been founded by Arturo Vittori [24]. The
project won the World Design Impact Prize 2015–16 at World
Design Capital(R) Taipei 2016 Gala [25]. Arturo and his team
have developed 12 dierent prototypes since 2012. Figure 6
shows an example of the prototype and its working principle.
Theteam’stargetistodevelopaprototypethatislightweight
(about 80 kg), easy and quick to build using local materials with-
out using scaolding and power tools. They intend to use bamboo
fortheframestructure,whilethewatercatchmentsystemwillbe
made from biodegradable mesh 100% recyclable materials. Fog
and dew, and also rainwater, will be collected when they strike the
meshandthentrickledownafunnelintoareservoiratthebase.
To prevent water evaporation, a fabric canopy will be used to cover
the lower section of the water collector. There is no indication
of the amount of water that can be produced by the prototype
since the project is still in the exploratory phase. However, the
aim of the project is to produce water from fog or highly humid
places between 50 to 100 l per day [26].
2.1.3. Fog collector eciency and feasibility studies
Afogwatercollectorwouldactasthebarriertothewind-driven
fog. However, a portion of the fog is unperturbed by the fog water
collector. Although there is a collision with the fog collector, it
cannot capture all the liquid water contained in the fog [9]. There
are losses due to:
(i) Fog passing around the fog water collector.
(ii) Fog passing through the openings of the mesh.
(iii) Droplets bouncing back into the airow.
For the fraction of the fog that is captured by the fog water
collector, we call this fraction as fog interception eciency [9].The
captured water droplet merged, move to the lower part of fog col-
lector, reached the water gutter and transported to the water tank.
However, at water gutter, there is a potential of re-entrainment or
watercanreturnbacktotheairoworsomewaterfromthemesh
slack, wrinkles and folds, may be entering the gutter and collected
at the water tank.
Thebasicscalculationforthefogwatercollectionhasbeen
discussed in Rivera [9]. To discuss the collector eciency, there
arefourimportantfactorsthatdeterminetheeciencyofthefog
collection and they are wind velocity, fog liquid water content,
droplet size distribution and mesh characteristics. The water col-
lector eciency ηcoll of a fog collector can be computed using the
following equation (2).
ηcoll =
˙
W"
coll
vo.LWC (2)
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H. Jarimi et al.
Figure 6. Water bamboo tower top: the working prototype and bottom: the concept [24].
Where ˙
W"
coll (kg/s
m2) is the water ow rate collected in the gutter
per unit screen area, vom
sis the unperturbed wind velocity
of the incoming fog/air ow and LWC kg
m3is the liquid water
content of the incoming fog/air ow.
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Atmospheric water harvesting
Additionally, Rivera [9] reported that we can also express the
collection eciency by considering the following conditions:
i. The aerodynamic collection eciency ηAC ,calculatedbased
on the amount of unperturbed fog droplets that would col-
lide with the fog’s mesh.
ii. The capture eciency ηcapt, to account for the fraction of
the aforementioned intercepted droplets that are actually
captured by the mesh wire.
iii. The draining eciency ηdr , to account for the fraction of the
water captured by the mesh that is collected by the gutter
sincesomeofthewatercanspillorre-entertheairow.
Therefore, the fog water collector can also be expressed using
the following equation [9]:
ηcoll=ηcaptηdrηAC (3)
Clearly,beforeinstallingafogcollector,itspracticalitymust
rst be assessed. A group of Iranian researchers [27]havedis-
cussed the feasibility of implementing fog collectors as a mean
to harvest water in their country. Their research has included
analysis on the data collected from 10 representative stations
located facing the Persian Gulf and Oman Sea. Among the impor-
tant parameters recorded were ‘hourly dry and wet temperature,
relative humidity, wind direction and velocity and the dew point
temperature’. The values were then used to calculate ‘the atmo-
sphere water vapour pressure, saturated vapour pressure and the
absolute humidity of the atmosphere’ and the feasibility of fog
harvesting system predicted by using the equation (4):
For RH ≥69%, WH3=(3×Mt×Uz×ηcoll ×3.6).
If RH <69%, →WH3=0(4)
Where RH is relative humidity measured by weather station,
WH isthepotentialwaterharvested(litrespersquaremetre
per day) and the subscript 3 represents for every 3 hours, an
inputvaluechosenbecausedataattherepresentativestationswas
recordered 3 hourly, and they assumed stable conditions were
achieved aer this period is achieved. U2is the wind velocity at
2 m height above the ground, Mtis the absolute humidity that
is dened as the humidity in grams per cubic meter of air in a
specic temperature (g/cm3).Thevaluesofwindspeedforeight
dierent wind directions were then investigated. Their analysis
haveshownpromisingresultsforwatercollectionatAbadanand
Chahabar station with the amount of potential collected water is
6.7 l/m2/day and 156.3 l/m2/day, respectively [27].
2.1.4. Studies on mesh topology
Toimprovefogcollectorperformance,understandingtheeects
of fog collector topology is a key as dened especially by the
mesh radius and mesh diameter. Collectors can be categorized
based on their bre radius Rand the half spacing of the bres
D[28], values that are important in the calculation of Stokes
coecient that is related to the collector eciency. Stokes number
typically determines the inertia of the moist air and its migration
across the streamline and thus indicates the eectiveness of the
fog collector design, thus a large Stokes number implies a higher
rate of water droplet collection [28]. However, this paper will
not further elaborate the equation used for the calculation of
Stoke coecient. Interested readers may refer to [29]forfur-
ther description. As previously discussed, Rivera [9]investigated
aerodynamic collection eciency (ACE). Rivera [9]considered
that two important characteristics of the mesh were the shade
coecient and the characteristics of the bres used to weave or
knit the mesh. He also discussed a simple superposition model in
analyzing the inuence of these parameters to Regalado and Ritter
[29] the ACE of the fog water collectors. Rivera [9]concludedthat
the ACE value can be increased by introducing concave shape to
thefogwatercollectorandimprovingtheaerodynamicsofthe
mesh bres. Regalado and Ritter [29] have performed a theoretical
analysis on wind catchers in the form of cylindrical structures
equipped with several screens of staggered laments to determine
their eciency. Like Rivera [9], these researchers also assessed the
aerodynamic eects of the water/fog impacting on the mesh.
2.1.5. Studies on surface wettability of a fog harvester
While most researchers focussing on the mesh topology, Park et
al. [6] have investigated the inuence of surface wettability char-
acteristics, length scale and weave density on the fog harvesting
capability of woven meshes. In their research, Park et al. [6]have
developed a model that combined the hydrodynamic and surface
wettability characteristics of a fog water collector in predicting
the overall fog collection eciency. From their modelling, later
validated against experimental results and depicted in Figure 7,
there are two limiting factors that will eect fog harvesting and
reducing the collection eciency; rst is the re-entrainment of
collected droplets into the prevalent wind, and second one is
the blockage of the mesh opening. However, they have con-
cluded appropriate tuning of the wetting characteristics of the
surfaces, reducing the radius of the wire and optimizing the wire
spacing will lead to more ecient fog collection. Additionally,
they have introduced family of coated meshes that have demon-
strated enhancement in the fog collecting eciency as high as
ve times of the conventional polyolen mesh. To coat the mesh,
quoted from the researchers’ paper [6], ‘a 1.7 wt.% 1H,1H,2H,2H-
heptadecauorodecyl polyhedral oligomeric silsesquioxane (uo-
rodecyl POSS) 98.3 wt.% poly(ethyl methacrylate) (PEMA, MW
= 515 kDa, Sigma Aldrich) solution in a volatile hydrochlorouo-
rocarbon solvent (Asahiklin AK-225, Asahi Glass Company) at a
concentration of 10 mg/m’ was used by the researchers. They rst
dipped the mesh in the solution for 5 minutes and then air dried
to evaporate the solution. To check the uniformity of the coating,
they have used scanning electron microscopic method and also
by contact angle measurements at several locations on the coated
surface. The aim of the coating is to decrease the contact-angle
hysteresis of the mesh wires that allows small droplets to easily
slide down into the collecting gutter when they were captured by
the mesh wires. Even in a mild fog with a droplet radius of 3 μm,
wind speed of 2 m/s and liquid water content of 0.1 g/m3,theuse
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H. Jarimi et al.
Figure 7. Factors aecting fog harvesting and reducing the collection eciency are (a) the re-entrainment of collected droplets in the wind and (b)blockageofthe
mesh. Adapted with permission from [6] Copyright (2013) American Chemical Society.
Figure 8. (a) The schematics of the experimental arrangement and (b) the photos of dierent materials used to the test surface wettability in fog harvesting with the
water droplets [30].
of optimal dip-coated mesh surface can collect ∼2lofwaterover
an area of 1 m2inaday[6].
Seo et al. [30] have investigated the eects of surface wettability
for both fog and dew harvesting. Their approach to fog harvesting
involves dierent test surfaces. A commercially available copper
was used in various wetting characteristics, see Figure 8b.The
wettability of surface is determined by the contact angle of the
liquid on the surface where the liquid-vapour meets the surface.
When a droplet is owing, the contact angle (Figure 9)canbe
classied as advancing or receding. The researchers showed that
the moisture harvesting performance was determined by the com-
bination of the moisture capture at the surface and the removal of
the captured water from the surface. In their study, they found
out that a large receding contact angle is a determining factor
in performance. Among all the surfaces tested, the oil-infused
surfaces with their large receding contact angle at a high super-
saturation condition exhibit the best fog harvesting performance.
Azad et al. [32] compared the fog collection performance
of three dierent categories of mesh sample for fog collection
performance:
Figure 9. Schematics represent advanced and receding angles from Weistron
[31].
i. Surfaces with ne microstructures and dierent coatings
can have markedly dierent wetting behaviours than
smooth surfaces. Therefore, in their research, they have
investigated smooth and microgrooved copper wire with a
diameter of 1.2 mm. They created the microgroove surface
using a sandpaper. Then, microgrooves were implemented
on the wire surface using Korn 80 sandpaper that contains
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Atmospheric water harvesting
Figure 10. (a)Coppercombsampleand(b) polyolen mesh (double layered)
scale bar 1 cm [32].
particles with the diameter of 190–265 μm. Illustrated in
Figure 10a,thecopperwires(10ofthem,withsmoothand
microgrooved structure) were soldered electrically on a
wire stick.
ii. Polyolen mesh samples that comes in three types,
hydrophilic mesh (attract water), superhydrophilic mesh
that was dip coated with an aqueous TiO2 solution and
dried at room temperature for 48 hours and ‘hydrophobic
mesh’ (repel water) that were prepared by dip coating the
polyolen mesh with a hydrophobizing agent and dried at
room temperature for 48 hours.
iii. Epoxy replication (replica) to replicate surface microstruc-
tures of Gunnera and Dendrocalamus under leaf surfaces
and a smooth glass (microscope slide). The glass replica
had a smooth surface, the Gunnera replica had a con-
vex shape microstructure and random channels with hairs
inside of the channel and the Dendrocalamus replica had
microgroove surface.
It was found that the amount of collected water by super-
hydrophilic mesh was ve times higher than the hydrophilic
polyolen mesh. Whereas water collection by hydrophobic mesh
was 2.5 times higher than the hydrophilic mesh. In the micro-
structured replica, water dripped 2–3 times higher than unstruc-
tured replica and smooth surface. In addition, the water was
collected more quickly for the micro-grooved copper wire than
smooth wires [32].
Rajaram et al. [33] studied ways to improve the capacity of fog
water collection by modifying the surface and geometrical shapes
of Raschel mesh structure as shown in Figure 11.Thesurface
modication includes coating the mesh using superhydrophobic
coating such as Teon, ZnO nanowires, NeverWet and hydrobead.
In general, when compared with the uncoated Raschel mesh,
the use of the coatings gives about 50% enhancement in the
collection eciency given by equation (3). Meanwhile, in terms
ofthemodicationtothegeometricalshapes,theyhaveincreased
the shade coecient of the Raschel mesh by developing a new
manufacturing method via a punching process. That has resulted
in reduction in the pore size and also the increase in the distance
between two inclined laments. The change in the geometrical
shape leads to another 50% of enhancement. In general, both
methods have collected water about two times that of a typical
Raschel mesh.
2.2. Biomimicry-inspired fog water harvesting
2.2.1. Animals and plants with special characteristics in harvesting
water from the ambient
In parts of the world, despite extreme water shortages result-
ing from the low annual rainfall, animals have evolved to sur-
vive in such conditions by acquiring special characteristics that
allow them to collect water from the fog or the atmosphere.
Namib desert beetles, such as Stenocara gracilipes (Figure 12),
for instance, survive by collecting water although the annual
rainfall is only 12 mm [34,35].The surface of the beetle’s back is
covered with a random array of smooth hydrophilic bumps and
microgrooves ∼0.5 mm in diameter and arranged at 0.5–1.5 mm
intervals. These bumps on the forewings are micro size (in micron
dimension) allowing water to condense and trickle directly to
their mouth. Both fog and dew water harvesting eciency are said
to increase with the combination of hydrophilic (water attracting)
and hydrophobic (water repelling) areas.
Other water harvesting animals are a lizard species known
as Moloch horridus [36](Figure 13).The lizard species is native
to hot and arid regions, which drinks water droplets collected
over its hydrophilic skin and that reach to its mouth by capillary
action. In contrast, a spider, Uloborus walckenaerius uses its web
(Figure 14) to collect water. A special structure formed a combi-
nation of its spindle-knot structure and the web joints. As seen
in Figure 15, the spindle knots have rough surface and the joints
have nanobrils that make it less rough. The transportation of the
waterdropletstowardstheroughspindle-knotsfromthejointsis
promotedbythedrivingforceresultingfromtheLaplacepressure
gradient and surface structural anisotropy [37].
Plants are also able to survive in arid climates by harvesting
water. An example is the endemic Namib desert grass called Stipa-
grostis sabulicola. The round shape of the plants’ stem are covered
with leaves whose surfaces are hydrophilic and have an irregular
construction. The water droplets travel from the leaves onto the
roots (Figure 16)viagroovesalongitscone-shapedstructure.A
combination of surface roughness, prickle hairs and wax prevent
the scattering of water droplets [39].
Many of the cactaceae (cactus) family living in hot and arid
regions also show great tolerance to water scarcity and capable
of water harvesting [40]. One species, Opuntia microdasys,from
the Chihuahuan Desert, has several characteristic with properties
thatprovideeectivefogcollection[41]. It has hair-like needles
(glochids) instead of spines on its large green leaves, thus reducing
exposure to sunlight, which limits the evaporation of water, thus
causing more storage of water. In this way, more water is stored
for longer survival [42].
The water collection mechanism of Lychnis sieboldii,aplant
speciesfromdrygrasslandinJapanhassurfacehairsthatshow
morphological changes when in contact with water, [43]. The
microbres in the hairs play a vital role in absorbing and releas-
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H. Jarimi et al.
Figure 11. Illustration and experimental results of mist ow (optical images) on two rectangular Rachel meshes with cylindrical bres (real images) conducted by
Rajarametal.[33].
Figure 12. Fog-baskingbehaviorofaNamibdesertbeetle.CourtesyofJames
Anderson/NSF/Creative Commons BY-NC-SA 2.0.
ing water by becoming cone-shaped when exposed to water but
changed to a perpendicularly twisted shape under dry conditions
as shown in Figure 17.
Asmalldesertmoss,Syntrichia caninervis from the Great Basin
in the western United States and the Gobi Desert in China, also
survives arid conditions by condensing water using its hairs.
The water condensation and the droplet formation are promoted
by the grooves and barbs on the hair surfaces. The condensed
water droplets will then travel from the tip to their base [44].
2.2.2. Biomimicry approach in atmospheric water harvesting
In recent decades, reports on bioinspired water harvesting have
emerged rapidly [45]. Inspired by the Namib beetles, Garrod et al.
[35] have investigated the inuence in the degree of hydrophilic-
ity/hydrophobicity of beetle backs in determining their over-
all micro-condensation eciency. In this research, the micro-
condensation eciency of fog water harvesting units has been
explored in terms of the chemical nature of the hydrophilic ‘pixels’
and their dimensions. Imitating the pattern on the back of the
beetle, they have applied plasma deposition method to make a
hydrophilic polymer array on a superhydrophobic background.
The performance of the surfaces as microcondensors were inves-
tigated by measuring the amount of water collected from a ne
mistin2hours.Thebumpyarraypatternsofthehydrophilic
and hydrophobic surfaces are concluded to be more ecient at
collecting suspended water droplets than a pure hydrophilic or
hydrophobic surface. The amount of water collected by surfaces
with bumpy array is more than 50% higher than the smooth
surfaces.
To imitate the hairs of the cactus and its surface, Cao et al. [46]
investigated a large-scale fog collector through integrating cactus
spine-like, hydrophobic, conical micro-tip arrays. The tip arrays
were arranged on a spherical hydrophobic cotton matrix, see
Figure 18a–d. For the fog collector, about 30–40 micro tips were
placed at each edge of the articial cactus at 4∼5mmdistance,see
Figure 18a and b. The experimental set up is shown in Figure 18d.
The distance between the fogging jet and the collector was set
at 3 cm. At fog velocity of 45∼50 cm/s, the biomimetic cactus-
inspired fog collector was reported to harvest ∼3mlofwaterin
10 minute. The results imply that at this wind speed, 100 cactus-
like fog collectors will be able to collect the water in 1.5 hours,
sucient drinking water for human survival. Clearly, a promising
device for collecting water in foggy regions.
More research on bio-inspired plants was conducted by Gürsoy
et al. [47] who replicated the surface of the Eremopyrum orien-
tale leaf, which displays an asymmetric-anisotropic directional
mist collection behavior underpinned by macroscale grooves,
microscale tilted cones (tilted in the direction of water ow)
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Atmospheric water harvesting
Figure 13. Molochhorriduslizardandthehydrophilicsurface[36].
Figure 14. Water droplets on spider web [38].
and nanoscale platelets to harvest water. The surface replication,
achieved using so lithography combined with either nanocoat-
ing deposition or functional nanoimprinting, was shown to be
highly-ecient for directional mist collection, compared to mist
water harvesting by at surfaces. In a dierent study, Gürsoy et al.
[48] have reported that non-woven and cotton brous materials
areshowntomimicthefogharvestingbehaviourofSalsola crassa
hairs, see Figure 19. In order to enhance the overall mist collection
eciency, they incorporated multiple length scale (hierarchical)
channel structures and tune the surface wettability by introducing
hydrophobic functionalization of the bres (in order to mimic
the leaf waxes of the plant Salsola crassa)usinginitiatedchemical
vapor deposition surface coatings or plasma-enhanced chemical
vapor deposition. The overall mist collection eciency can be
enhanced by over 300%.
An interesting fog water harvesting concept has been demon-
strated by Park et al. [49] on the design of the fog water harvesting
surface bioinspired by combining three dierent elements from
dierent species: Namib desert beetles, cacti and pitcher plants.
Inspired by the bumpy surface of Namib desert beetles, they have
performed modelling to optimize the radius of curvature and
cross-sectional shape of the water harvester surface to promote
condensation. Then, inspired by cactus spine, they integrated the
geometry with a widening slope in facilitating water droplet to
thecollectorinafasterratetoavoidadecreaseinthedroplet
size. Finally, they integrated the optimized bump radius and the
wide slope structures with a slippery nano-coated surface that is
inspiredbypitcherplants.Theroleoftheslipperysurfaceisto
promote coalescence droplet growth.
Shang et al. [50] mimic the special characteristics of the spider
web silk in order to harvest water. In their research, they have
developed a novel microuidic technology that can control the
size and spacing of the spindle knots in order to adjust the ow
rates. In this way, the size and spacing of the spindle knots can
be controlled and thus, the function of humidity-responsive water
capture can be obtained. As a result, some features are gained such
as thermally triggered water convergence, humidity-responsive
water capture that can be used for many applications.
3. DEW WATER HARVESTING
In fog water harvesting, the collection of water will occur when
the fog droplets impact and intercept with the collection surfaces.
However, the main limiting factor of harvesting water from the
fog droplets is the global fog occurrence that is highly dependent
on the geographical and metrological factors or conditions. Only
limited number of places experience environmental conditions
whereby the temperature of moist air could naturally drop below
its saturation temperature thus form fog. Not surprisingly there-
fore, on a global scale, fog is reported to be even less accessible
than seawater as an alternative source of freshwater [51]. Water
vapour is ubiquitous in the atmosphere, so, if condensed by cool-
ing, freshwater can be harvested at many locations. Nevertheless,
the condensation process is more thermodynamically compli-
cated than fog harvesting and as reported in Gido et al. [51], the
process involves a signicant release of heat.
Water droplets that are formed due to the condensation of
water vapour on a surface at temperature below its dew point
temperature are called dew water [3,52]. In this paper, dew water
harvesting processes are divided into three categories: i) passive
(radiative) cooling condenser, ii) solar-regenerated desiccant and
iii) water harvesting from air using active cooling condensation
technology. This review includes dew water collection under both
high and low humid air conditions.
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H. Jarimi et al.
Figure 15. Structure of the spindle-knot and joint [37].
Figure 16. Stipagrostis sabulicola in their natural habitat [39].
3.1. Water harvesting using radiative cooling
condenser (passive systems)
The principle of radiative cooling condenser is very simple.
Inspired from dew formation on plants in the morning, the
formation of dew is driven by radiation phenomena of the
surface of the materials. The formation of the dew is physical and
determined by the surface cooling without additional energy, and
the most important element being the power gradient between the
condenser outgoing radiative power and the sky radiative power
P[53] which is presented by the Stefan–Boltzmann law presented
in equation (5):
P=εσ(T)4.(5)
The radiative power per unit area P(W/m2) also depends on
the local surface temperature T(K). In equation (5), σis the
Stefan–Boltzmann constant (W/m2K4), and εis the emissivity of
the surface. Thus, to optimize the dew formation, as reported in
[52]citedin[3], one could:
(i) maximize the infrared wavelength emitting properties of the
surface to allow surface cooling at night;
(ii) increase the reectivity of the condensing surface to ensure
that the surface will not trap heat that will warm the con-
denser and resulting in evaporation during the day;
(iii) reduce the wind eect to the condenser by tilting the con-
denser surface;
(iv) increase the hydrophilic property of the surface, and this can
be achieved by applying hydrophilic coating to the surface
and lastly;
(v) reduce the heat inertia of the condensing surface to promote
change in temperature dierence and also as a means to
avoid heat transfer from the ground.
Studies on passive cooling system include investigation on
materials with low emissivity surfaces. Early study on the inu-
ence of condensing surface materials to the dew formation has
been investigated for Bahrain climatic condition [54]. Three mate-
rials: aluminium, glass and polyethylene foils were investigated as
the condensation surfaces. From their study, aluminium surfaces
were reported to have the highest amount of average dew collected
at 3 kg/m2per hour, followed by glass and polyethylene foils at
0.8 and 0.3 kg/m2per hour, respectively. Three dierent types
of condensing surface namely: i) galvanized iron (GI) sheet with
emissivity 0.23 and thickness 1.5 mm, ii) commercial aluminium
sheet with emissivity of 0.09 and thickness 1.5 mm and iii) PETB
lm(polyethylenemixedwith5%TiO2and2%BaSO4)UV
stabilized with emissivity 0.83 and thickness 0.3 mm have been
investigated, see Figure 20 [55]. The condensing surfaces were
tested as a radiative condenser at 1 m ×1 m in size installed at
the village of Kothara (23◦14 N, 68◦45 E, 21 m a.s.l.) that is a
part of the semi-arid coastal region of northwest India. The aim
of the project was to use the water harvesting system as a solution
to drinking water problem in that region that is well known with
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Atmospheric water harvesting
Figure 17. The morphology changes of hairs on the leave of Lychnis sieboldii [43].
Figure 18. The illustration of the (a) cactus-inspired device and (b) the water transpor tation pathway in the device. (c) The photographs of the cactus-inspired
continuous fog collector and (d) the photographs of collection process of the device [46].
Figure 19. ‘Fog collection mechanism of salsola crassa plant species and bioin-
spired brous water harvesting’ [48].
poor groundwater quality. From the daily data collected over
2-year period in 2004 and 2005, the quantity of water collected
on most (60%) nights varied more or less uniformly between
0.05 and 0.25 mm and there were two peaks. The peaks that
one of them centred over March–April (summer) and the other
over October (fall) shows water collection of 0.55 mm. From all
the three surfaces being tested, the highest collection was in the
PETB units (19.4 mm) followed by GI (15.6 mm) and aluminium
(9 mm).
Kothara village in the Kutch region now has India’s rst potable
large-scale water production plant designed to harvest atmo-
spheric moisture and process it into drinking water. The con-
densersweremadeofplanarpanelsusinghighemissivityplas-
tic lm insulated underneath that promotes cooling. In addi-
tion to dew water harvesting, the condenser are also capable
to collect rainwater. It was reported that the expected cost of
1lofbottledwateris0.5rupeewiththeexpectedyieldofl-
tered, treated potable water from the plant is 150 000 litres a
year [56].
Another important surface parameter that inuences the per-
formance of the passive system is the shape of the radiative con-
denser. As reported in Khalil et al. [3],amongtheearlyresearchers
who investigated various shapes of these passive condenser
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H. Jarimi et al.
Figure 20. Dierent types of condenser surfaces investigated by [55].
surfaces were Jacobs et al. [57] who investigated an inverted
pyramid shape. Investigated at the grassland of the Netherlands,
the authors concluded that their collector collected water 20%
more that the planar shape at angle 30◦.Researchers[58]have
performed a CFD simulation Computational Fluid Dynamics
using PHOENIXS to simulate the innovative designs proposed in
their study.
Reported in 2011, the world’s largest dew and rain water col-
lecting system was constructed in 2006 at Panandhro in the semi-
arid area of Kutch (NW India). Ridge-and-trough shape modules
have been chosen as the shape of the dew water collector [59]. The
performanceofthelargedewcondenserat850m
2net total surface
with 10 ridge-and-trough modules had a total output for 2007 of
6545 l, corresponding to 7.7 mm/day on average. The maximum
collection rate reported was 251.4 l/night (0.3 mm). In addition
to dew, the designed condenser could also collect rain (and, to a
lesser extent, fog).
In a passive system, natural convection between the condenser
surfaceandtheairowisnotfavouredsinceitwillreducethe
condensing eciency of the condenser system. Thus, a condenser
in a hollow form such as a funnel will reduce the free convection
along the surface since the heavier cold air will remain at the
bottom of the funnel due to gravity regardless of the wind direc-
tion [53]. The researchers have performed both simulation and
eld studies. From their simulations, cone angle ≈60◦give the
best condenser cooling eciency. Based on experimental work
and eld testing, a repetitive pattern of hollow shapes to pave
a planar or weakly curved roof surface, have been considered,
providing pleasing aesthetics and construction cost advantages.
The egg-box and origami types were specically investigated. The
prototypes were fabricated and installed at Les Grands Ateliers
(Villefontaine - France) during the ‘Chaleurs urbaines’ project
(ENSA de Grenoble - Métro).
3.2. Solar regenerated desiccant in water harvesting
(passive system)
Low yield is a key issue for the passive, radiative condenser system
because of its dependency on certain parameters, notably the sky
emissivity, the amount of water vapour in the air (relative humid-
ity),windspeedandtopographiccover[3]. Desiccant materials
such as silica gel, zeolites and CaCl2are hygroscopic and can
absorb moisture through adsorption and absorption process thus
increasing the amount of the dew water collected. As a result,
desiccant beds are now commonly being used in atmospheric
water harvesting applications. Figure 21 presents the generic pro-
cess of atmospheric water harvesting using desiccant. The process
may be explained as follows: the rst stage is water absorption
stage at night where the desiccant bed will absorb moisture from
humid air. The second stage is water desorption during the day by
heating the bed with solar radiation, which will regenerates the
desiccant by driving out water vapour. In the third and nal stage,
the evaporated water will then condensed into water droplets and
collectedinatank.
The advantages of a desiccant system over radiative condensers
include the hygroscopic capacity of the desiccant that enables
more ecient water collection, achieving low dew points without
the risk of freezing thus reducing operational cost [51]. Early
studies on solar regenerated systems involve desiccants such as
saw dust [61], silica gel [62] and recycled newspaper [63]. In
apatent,Ackerman[64] claimed a spiral water harvester con-
taining hydrophilic particles such as silica gel and tilted at an
angle that optimized water collection. To improve the atmo-
spheric water harvester performance, various collector designs
have been investigated by researchers and several are described
below.
3.2.1. Glass pyramid collector
Kabeel [65] described a glass pyramid collector (Figure 22)com-
prising: i) desiccant beds on shelves, ii) a slanting wall cover,
iii) a collection cone and iv) a condenser section mounted on
top of the pyramid, shading it from solar radiation. Sawdust and
cloth, saturated with CaCl2, were investigated as the desiccants.
Thecoversoverthebedsareopenovernightsothedesiccantcan
absorb water vapour from the air. During the day, the covers are
closed so the beds are heated by solar radiation driving o the
absorbed water, which condenses on the sides and especially at
the pyramid apex water, where it is collected by a central cone and
ows through a tube to an external container. The reported water
yield is 2.5 l/day/m3; the cloth bed showed better performance
than the sawdust bed system.
3.2.2. Corrugated surface
Based on the principle of desiccant moisture absorption at night
and simultaneous desorption (regeneration using solar energy)
and water vapour condensation during the day, Gad et al. [66]
introduced the use of an integrated desiccant/solar collector to
harvest water from humid air. In their study, a small air circulation
fan was used to force the ambient air to enter the glass-enclosed
solar collector during the evening (Figure 23). In the collector,
a thick layer of corrugated cloth was used as the desiccant bed.
The use of corrugated surface was meant to increase the heat and
mass transfer area during the absorption/desorption mechanism.
During the day, water vapour condensation will occur on the
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Atmospheric water harvesting
Figure 21. Wet desiccant technique for water production from atmospheric air [60].
Figure 22. (a) Photograph of the system used. (b) Pyramid with glass covers open at night (right) [65].
inner surface of the glass enclosing the solar collector. According
to the researchers, the solar driven system could produce 1.5 l of
fresh water per square meter per day.
3.2.3. Trapez oid al pr i sm
William et al. [67] designed a trapezoidal prism with CaCl2as the
desiccant (Figure 24) supported on sand and on dark cloth. For
the prism wall, transparent bre glass bolted to aluminium frames
was used while the top of the prism was an opaque material that
acted as a condenser and to facilitate collecting the condensate
water, the walls were slanting. The trapezoidal prism worked in
essentially the same way as the pyramidal system described above
in that moisture absorption occurred at night time and the solar
radiation driven desorption occurred during the day with the
evaporated water forming water droplets that collected in the
water tank. The system eciency was computed by considering
the total heat of evaporation to the total incident solar radiation
during the day time. The recorded daily total evaporated water for
cloth and sand bed achieved a maximum of 2.32 and 1.23 l per m2
at system eciency of 29.3% and 17.76%, respectively.
3.2.4. Solar glass desiccant box type system
In India, an atmospheric water harvesting system that named
‘solar glass desiccant box type system’ (SGDBS) with a capture area
of 0.36 m2was developed and investigated. The box was made of
a 3 mm single glaze glass; the desiccant bed was xed at 0.22 m at
inclination of 30◦. The desiccant bed was a composite material
using sawdust impregnated with CaCl2(Figure 25a,absorption
and Figure 25b, desorption). Three boxes were tested under the
Indian climatic conditions at NIT Kurukshetra, India [29◦58
(latitude) north and 76◦53(longitude) east] in October. The
researchers observed that the performance depend mainly on
the concentration of CaCl2, which generated 180 ml/kg/day at a
loading of 60% on the sawdust.
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H. Jarimi et al.
Figure 23. Schematic diagram of the experimental apparatus and the corrugated desiccant bed [66].
3.2.5. MOF porous metal-organic framework-801
Recently, the potential of harvesting water from humid air
as low as 20% have been investigated by researchers from
Berkeley and MIT [69]. Based on the same principal of intro-
ducing hygroscopic element to improve moisture uptake, the
researchers have developed an hygroscopic sheet using a kilo-
gram of dust-sized MOF porous metal-organic framework-801
[Zr6O4(OH)4(fumarate)6] crystals pressed into a thin sheet of
porous copper metal positioned between a solar absorber plate
(at the top) and a condenser plate (see Figure 26), both placed in
achamber[70].
ThedeviceisshowninFigure 26. At night aps are open, allow-
ing ambient air to enter the chamber. Water vapour diuses into
theporousMOFandisabsorbedonitsinternalsurfaceinclusters
of eight molecules, essentially tiny ‘cubic droplets’. In the morning,
with the chamber closed, natural sunlight (∼1kW/m
2) heats MOF
causing the water to desorb as vapour, which then condenses on
the bottom of the chamber [70] and the resulting liquid drains
to a collecting tank. Published results suggest that MOF-801 is
superior to other absorbents, being capable of generating 2.8 l of
water per kg and with the ability to operate a relative humidity
level as low as 20% [70].
3.3. Water harvesting from air using active cooling
condensation technology
The water harvesting systems described previously can be
described as ‘passive’, i.e. they are driven simply by solar heating
anddonotrequiretheinputofelectricorotherhigh-grade
power. In contrast, ‘active’ systems typically require electrically
powered compressors or vacuum pumps and the quantity of
water harvested in directly related to the input energy [3].
Activeharvestersrangeinscalefromthosesuitablefordomestic
drinking water (15–50 l per day) to industrial scale units for
irrigation (2000 l per day), outputs typically signicantly larger
thanpassivesystems.Thepowerconsumptionperkilogramme
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Atmospheric water harvesting
Figure 24. Schematic diagram of experimental test rig [67].
Figure 25. (a)and(b)thedesignoftheSGDBSand(c)theexperimentalsetup[68].
of water collected is a major concern for active systems and will
be aected by the ambient temperature, humidity and eciency
of ‘coolth’ recovery in the equipment. Leading active technologies
are described below.
3.3.1. Dehumidier using selective membrane
Water vapour is only a minor component of air in the atmosphere,
even at 30◦C/100% RH only 30.4 g is present, while at 10◦C/RH
100% the moisture content is 9.4 g/m3,sothemaximum
quantity of water that can be recovered by cooling between these
temperaturesis21g/m
3.However,thisrequirescooling1m
3
of air by 20 K that requires the removal of 24 kJ of heat plus
52.5 kJ of latent heat to condense the water. If the coolth of the
outgoing air aer condensation is not recovered, it represents a
signicant ineciency. To minimize the power requirement of the
dehumidication process, as shown in Figure 27,researchers[71,
72] have used water vapour selective membranes to separate the
water vapour component prior to cooling and condensation, thus
avoiding cooling the other atmospheric gases. The key element
of the system is the water-selective membrane that allows only
water vapour to pass through driven by a concentration gradient
imposed by the vacuum pump. The concept underlying the
membrane system is shown in Figure 28 in a dierent study by
Woods [73]. The researchers [74] found that with a 62 kW power
input, the harvester produce water at the rate of 9.19 m3/day,
a 50% better eciency than the equivalent system without
the membrane. In addition to improved energy eciency, the
selective membrane generated fresh water that cleaner than
water condensed directly out of the air. Other than selective
membranes, some researchers also use desiccants systems (liquid
and/or solids) to absorb the water vapour from an incoming air
stream. However, these methods require regeneration steps and
cyclic operation conditions reduce the rate of water production.
Furthermore, the use of spatially separated liquid desiccant dehu-
midication methods results in energy-intensive regeneration
and condensation processes [75].
Various selective membranes have been investigated. A
Singapore group investigated water vapour permeation through
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H. Jarimi et al.
Figure 26. The experimental setup [70].
Figure 27. The representation of the water vapour selective membrane in an
atmospheric water harvesting system [74].
membranes fabricated by impregnating poly(vinyl alcohol) (PVA)
with LiCl [76]. They concluded that higher LiCl contents and
lower temperature optimizes the water vapour permeance of the
membrane. With respect to humid condition, the tests showed
thatthemembranewassuitablefordehumidifyingairathigh
humidity conditions.
In a separate publication, the group compared two dierent
membranes, one containing LiCl and the other triethylene gly-
col (TEG) supported on PVA. The researchers concluded that
the water vapour permeability of the membranes increased with
increasing amounts of the hygroscopic component (LiCl or TEG),
because it lowered the diusion energy and thus the barrier to
permeation. The researchers further claimed that a membrane
withPVA/TEGishighlydurable,haslesscorrosiveproblemsand
more environmentally friendly in comparison to the membrane
with LiCl as the hygroscopic component [77].
Figure 28. Theconceptofwatervaporselectivemembrane[73].
3.3.2. Atmospheric water harvesting integrated with air
conditioning system and condensing coil
Active condensing systems, using the conventional reverse Rank-
ine cycle, operate in the same way as a dehumidier where passage
of moist air passed over a coil cooled by a refrigerant, causes
the water vapour to condense. The rate of the water production
depends mainly on the relative humidity and the air temperature.
Versions of the technology have been described in various aca-
demic papers and patents. For example, Lukitobudi [78]claimeda
mobile dehumidier unit that simultaneously produced drinking
water. Sawyer and Larson [79]whopresentedadisclosureunied
system that provides both air conditioning and atmospheric water
harvesting. Magrini et al. [80] have discussed in their paper the
advantage of water harvesting from the integration with an HVAC
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Atmospheric water harvesting
Figure 29. Ecolo Blue EB30 [82].
Figure 30. Atlanti s H2O Elite Atmospheric Water Generator [83].
system that also serves as the air conditioning system for a hotel
in a sub-tropical arid climate. Rather than having the condensate
water from an HVAC system wasted, the water is collected and
utilized. The researchers found that the integrated system water
produce ∼56% of the hotel water daily demand.
Another study into water harvesting from an air conditioning
system has been recently conducted by Dalai et al. [81] to maxi-
mize the amount of water vapour captured by a window air con-
ditioner, a process termed ‘atmospheric water vapour processing’
(AWVP).Thewaterwasclaimedtobesucientlygoodqualityfor
human consumption. With a power input of 160 watt and air ow
rate of 0.00623 m3/s,theamountofwatercollectedwasreported
to be as high as 1025 ml.
Ecolo Blue, a United States company, produces the EB30 com-
mercial unit based on dehumidier circuit to harvest atmospheric
water (Figure 29). To minimize contamination of the water by the
metals of the cooling coils, they are treated with a food grade
coating. The EB30 can generate up to 30 l of water from air over a
24 hour cycle with a unit cost of 1300 US dollars.
Another company, Atlantis Solar, oer the Atlantis H2O Elite
range of units providing atmospheric water harvesting from 100 l
up to 10 000 l per day (Figure 30)(Atlantis[83]).
3.3.3. Thermoelectric cooling in atmospheric water harvesting
The application of thermoelectric cooling (TEC) is being actively
investigated as an alternative approach to conventional Rank-
inecycleforwaterharvestingforexamplebyJoshietal.[84]
who constructed a prototype containing 10 Peltier components
(Figure 31).
To enhance the cooling performance, the researchers have
introduced an internal heat sink on the cold side to increase
the cooling rate and thus the condensation rate. Over a 10 hour
run, the TFWG with internal heat sinks showed 81% improve-
ment over in amount of water collected compared to the
TFWG without the heat sinks. Other parameters being inves-
tigated are electric current, air mass ow rate and air
humidity.
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H. Jarimi et al.
Figure 31. Le and middle: diagram of prototype and right: actual water harvester prototype [84].
Liu et al. [85] have investigated a portable water generator, with
two TECs. In their system, air is forced into the mixing chamber
and then humidied. The humidied air is then ow through
the TECs via the inlet air channel. At TECs, the temperature
of the inlet air was reduced by the cool surface of the TECs to
the dew point temperature and water condensation occurs. The
researchers investigated the relationships between inlet relative
humidity and air ow rates with the amount of the water generat-
ed/condensed. They concluded, not surprisingly, that the higher
theairrelativehumiditythehighertheamountofwatergenerated,
while increasing the air ow rate lowered the condensation rate,
possibly because the reduced contact time between the air ow
and the TEC degraded the heat transfer rate. Lui et al. [85]showed
that the maximum amount of generated water was ∼25.1 g/h with
0.216 m2of condensation surface and 58.2 W power input.
3.3.4. Innovative cooling condensation technology: concept and
prototype development
Exciting developments integrate cooling condensation technol-
ogy with wind energy source element. The water harvesting
billboard (2013) designed by University of Engineering and
Te ch n o l og y o f P er u ( Figure 32) contains ve generators that
extract moisture from air using an inverse osmosis ltration
system [86]. The water ows through the small ducts to a central
holding tank at the billboard’s base. Although the billboard
requires power supply, it could provide as much as 100 l of
drinking water per day.
EOLE WATER have introduced the WMS1000 wind turbine
(Figure 33) that harnesses wind energy to simultaneously drive
thecompressorofaRankinecycledehumidier-typesystemand
create an airow over the cold coil. With an electrical output of
30 kW, the WMS 1000 can produce up to 1000 l of drinking water
per day and requires no additional external electrical input [87].
Figure 32. Water harvesting billboard [86].
Over the past decade, Australia has suered severe droughts
causing considerable economic hardship to its famers. To alleviate
their plight, Edward Linacre has therefore invented the airdrop
water harvester [88]. Airdrop comprises a mast-like tube above
ground through which air is sucked and driven into an under-
ground metal coil by a wind-powered turbine. Since the earth
is at a lower temperature, it cools the air below its dew point
resulting in water vapour condensation. Liquid water collects in
a reservoir from where it is pumped to a network of irrigation
tubes to the plant roots, a very ecient method of distribution
since it minimizes water loss. The airdrop can harvest 11.5 ml of
water for every cubic meter of air in the driest deserts such as the
Negev in Israel, which typically has a relative humidity of 64%,
and can produce 1 l of water per day [88].Theairdropisalow-
tech solution that could be installed and maintained easily and it
is self-contained, using a combination of wind and solar power.
The turbine is generally wind powered, but when wind speeds are
lowitispoweredbysolarPVbueredbyabattery.
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Atmospheric water harvesting
Figure 33. The WMS1000 wind turbine from EOLE WATER [87].
4. DISCUSSIONS AND CONCLUSIONS
At least 2.7 billion people worldwide experience water scarcity,
a problem that is increasing and has the potential to cause con-
icts between countries as they compete for an increasingly short
resource. Clearly, this crisis needs tackling urgently and will be
compounded as climate change causes profound shis in rainfall
patterns. Although traditionally arid regions, such as the Middle
East will suer, developed countries are certainly not immune
as prolonged droughts in parts of Australia and California have
demonstrated. Not surprisingly therefore, harvesting atmospheric
water has received considerable attention from researchers world-
wide since starting with the traditional method of capturing water
from fog 50 years ago.
Thisreviewhasdescribedvarioustechnologiesinrapidly
developing eld we expect more to appear in the near future.
All have their merits and disadvantages with some being more
suited than others to specic situations. Fog harvesting systems
are simple, relying upon simple, relatively cheap materials that
may be obtained from indigenous natural resources. However,
fog only occurs in a limited number locations where rainfall is
low, so can only make a modest contribution to alleviating water
shortages.
Atmosphericwatervapourisaworld-wideresourceandis
available even in the driest climates. Passive harvesting devices
relyinguponradiativeheatloss,and,likefogcollectors,alsohave
advantage of being simple and not requiring an external power
source. The surface energies and topographies can be modied
to facilitate the collection of water and facilitating drainage. How-
ever, long term testing is required to check whether fouling, either
natural or man-made, might compromise performance over a
time scale of several years. Will regular cleaning be required? The
quantities of water that can be harvested by passive systems are
limited and are perhaps limited to providing drinking water to
small communities rather than large-scale applications such as
agricultural irrigation.
Desiccant-based water collection systems are more sophisti-
cated than radiation-based systems, but can collect more water for
a given size of unit. Although cheap absorbents can be fabricated
from sawdust and calcium chloride, recently developed modern
metal organic framework (MOF) materials are able to operate
with relative humidities as low as 20%, but will be more expen-
sive. The choice of absorbent will be determined by economics
versus technical eciency. The desiccant systems described in this
review rely upon thermal solar energy to drive the desorption
process, which is not a problem since most arid areas have plenti-
ful sunshine. Desiccant systems would benet from fans to drive
moist air over the beds on windless nights, which require solar
PV cells and batteries. All the systems reviewed rely upon aps to
openedandclosedmanually.Obviously,thisisnotaproblemfor
an experimental system, but for a production unit an automatic
vent opener typically used for greenhouses would allow water
harvesting with minimum of attention. Of course, it would need
to be installed to close the vent during the day and open at night,
the reverse of its normal operation.
‘Active’ water harvesting units that require the cooling of air
by the input of electric or mechanical energy are capable of
operating from scales of few litres to 1000s litres per day and can
be used for domestic water to agricultural irrigation. Whether
fossil fuel or nuclear, provide the power for condensation, it is
questionable whether this makes technical or economic sense
since such stations require large quantities of cooling water. If such
waterisavailablewhynotuseitdirectly.However,solarorwind
powerisreadilyavailableinanaridarea,usingitharvestwateris
potentially attractive. Furthermore, water can be readily stored; a
renewable energy installation might be scaled to supply both the
powerandthewaterforanaridlocality,withwaterharvesting
continuing when power demand was low. Water can also be used
for evaporative air conditioning systems so conceivably integrated
power and a/c systems might be designed. Maybe in arid climates,
we shall see the construction of fully self-contained dwellings that
do not rely upon any connections to public utilities? Of course,
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H. Jarimi et al.
there may more than one system installed, so that the house
derivesitspowerandwaterfromPVcells,whilethegardenis
watered by several ‘airdrop’ units scattered around the grounds.
For public buildings and facilities such as golf courses and where
adequate land is available, the EOLE WATER WMS1000 water
unit might be attractive because of its large scale.
Water harvesters based on the reverse Rankin cycle, operat-
ing on the same principle as present-day dehumidiers, require
a conventional refrigerant. Over the past 25 years, the major
refrigerants have been the Hydrouorocarbon (HFCs), but these
are now being phased-down and ultimately phased out because
of the high global warming potentials (GWP). The low GWP
replacements are the so-called ‘natural’ refrigerants, carbon diox-
ide, ammonia and hydrocarbons and the so-called ‘synthetic’
refrigerants the HFOs (hydrouoroolens), notably R1234yf and
R1234ze(E). Ammonia and hydrocarbons have well-known haz-
ards so increasing their applications in close proximity with the
public means they must be treated with caution. Carbon diox-
ide is non-ammable and has low toxicity, but of necessity has
to operate at high pressure supercritical conditions for part of
the cycle, which presents signicant thermodynamic eciency
problems. The two HFOs have low toxicity, are only marginally
ammable and can operate on a conventional reverse Rankine
cycle. However, they attract considerable opposition from cam-
paigning environmentalists who strongly advocate the ‘natural’
refrigerants, although, as presently sourced, these are just syn-
thetic as the HFOs being manufactured in large chemical plants.
AnyfutureworkonactivereverseRankinecycleharvestersshould
consider what refrigerants will be available in the future. The
‘airdrop’ system does not rely upon refrigerants or external power,
so is possible to develop a large-scale version? Maybe this is the
way forward? The TEC cooling systems also avoid the need to
choose a refrigerant, but are they as ecient and can they be
operated at large scales?
Several of the technologies we described above are essentially
laboratory studies; water harvesting technology is only now being
to be commercialized. If water is being collected for drinking
water then attention must be paid to potential contamination.
Fog nets, passive radiation and even desiccant collectors may be
fouled with algal and bacterial growth and bird droppings, so the
water obtained may need to be treated before being drunk. The
problem of legionnaire’s disease in a/c water tanks is well known.
Atmospheric pollution, such as soot particles, might also be a
hazard.Comparableproblemsmightoccurwithactivecollection
devices.
Dalai et al. [81] recognized the need to treat the water collecting
plates of their AWVP windowbox device with a coating that
prevented potential contamination of the water with metals to
ensure it was drinkable. This is an important point; chemical as
well as natural contaminants must be considered. Standard horti-
cultural Raschel fabric may contain additives, such as plasticisers
and UV stabilisers, that would contaminate collected fog water.
A food grade material might be specied, but would this survive
suciently long in the open air? In any case, natural contami-
nation accumulating during use might nullify the value of food
grade material. Fluorochemical coatings provide the highest water
repellency so they would seem to potentially useful for water
harvesting devices. However, it has been known for over 20 years
that they slowly release non-biodegradable peruoroalkylsulfonic
saltsthatcanaccumulateinthefatswithinorganisms.Theuse
of uorochemical coatings is therefore best avoided. For crop
irrigation,potablequalitywaterisnotrequiredsotheseproblems
are not issues, apart perhaps from the uorinated coatings.
Waterharvestingisatechnologywhosetimehascome.Clearly,
considerable challenges remain to optimize eciency and ensure
the delivery of water with a quality appropriate to its end use at
cost the customers can aord. These problems can be solved.
ACKNOWLEDGMENTS
This work was supported by Newton Fund Institutional Links
[grant number 261839879]
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