JP Journal of Heat and Mass Transfer
Volume 4, Number 3, 2010, Pages 213-228.
This paper is available online at http://pphmj.com/journals/jphmt.htm
© 2010 Pushpa Publishing House
:phrases and Keywords solar energy, extraction, regeneration, air, absorption, liquid
desiccant, dew collection.
Received October 31, 2010
A TECHNICAL REVIEW ON THE EXTRACTION OF WATER
FROM ATMOSPHERIC AIR IN ARID ZONES
AHMED M. HAMED, A. E. KABEEL∗, E-SHAFEI B. ZEIDAN and AYMAN A. ALY
Department of Mechanical Engineering
Faculty of Engineering
Kingdom of Saudi Arabia
∗Mechanical Power Department
Faculty of Engineering
Fresh water supply is one of the most limiting conditions for the
populations of arid regions. The present paper covers the working
principles of systems and processes for extracting water from atmospheric
air. Moreover, a summary of the experimental and analytical studies
which investigate system performance has been made. Some new designs
that greatly expand the solar desiccant technique for absorption with
subsequent regeneration are also introduced. The research activities in this
sector are still increasing to solve the crucial points that make these
systems not yet ready to compete with other systems as water distillation.
AHMED M. HAMED et al.
Shortage of drinking water is chronic, sever, and widespread in the regions of
Northern Africa, Middle East, and Central and Southern Asia. The problem of
providing arid areas with fresh water can be solved by the following methods :
1. transportation of water from other locations;
2. desalination of saline water (ground and underground);
3. extraction of water from atmospheric air.
Transportation of water through these regions is usually very expensive, and
desalination depends on the presence of saline water resources, which are usually
rare in arid regions. Atmospheric air is a huge and renewable reservoir of water. This
endless source of water is available everywhere on the earth surface. The extraction
of water from atmospheric air has several advantages compared with the other
methods. Air as a source of water is renewable and clean and the amount of water in
atmospheric air is evaluated as 14000 km3, and the amount of fresh water in the earth
is only about 1200 km3 .
The extraction of water from atmospheric air can be accomplished by different
methods, the most common of these methods are cooling moist air to a temperature
lower than the air dew point, and absorbing water vapor from moist air using a solid
or a liquid desiccant, with subsequent recovery of the extracted water by heating the
desiccant and condensing the evaporated water.
In some regions of the world, dew water – if available – appears to be a simple
solution to complement sources of potable water. Dew water is indeed used by
plants and small animals where, in arid and semi-arid environment, it is significant to
sustain their activity.
Choice of methods is an engineering decision dependent on local climatic
conditions and economic factors such as capital, operating, and energy costs. On the
other hand, patented devices vary in scale and potable water output from small units
suitable for one person's daily needs to structures as large as multi-story office
buildings capable of supplying drinking water to an urban neighborhood. The
objective of the present work is to highlight the different technological processes
used for moisture or dew collection from the atmospheric air.
A TECHNICAL REVIEW ON THE EXTRACTION OF WATER … 215
2. Literature Review
2.1. The earth-collector
One of the first works dealing with water extraction from atmospheric air was
published in Russia . An apparatus consisting of a system of vertical and inclined
channels in the earth to collect water from atmospheric air by cooling moist air to a
temperature lower than its dew point has been proposed. The earth-water collector as
proposed by Kobayashi  is shown in Fig. 1. The earth essentially consists of three
layers. When the sun shines brightly, the surface of the earth becomes dry, creating a
dry layer. The depth of the dry layer varies according to the type of soil, amount of
rainfall, depth of the capillary layer, etc. Under the dry layer there lies a moist layer
that remains wet through capillary action as it is in contact with the underground
water. By capillary action, this water is sucked up to the surface of the earth through
tiny crevices in the soil. When the ground surface is heated by the sun, this water
dissipates in aqueous vapor. To collect this vapor in the form of droplets, a
quadrilateral frame with a glazing at a slope, called as earth-water collector, is used.
When the earth surface is heated by solar energy, the water vapor that evaporates
from the surface rises to the glazing by convection and it is condensed on the
underside of the glazing. The condensate flows along the glazing by gravity into
Figure 1. Principle of earth-water collector .
Different technological processes are proposed by numerous investigators to
extract water from the ambient air using solar energy as a power source. Flow
AHMED M. HAMED et al.
diagram of technological processes of separation of water from moist air using solar
energy which is presented in  is demonstrated in Fig. 2. In this diagram two main
methods are described by using solar energy. One of them is by cooling the
atmospheric air to a temperature lower than the dew point and the second by
sorption with subsequent regeneration. In the following subsections the operation of
such systems will be discussed.
Separation of water from
Air cooling to a temperature below
the dew point
Combined cycle (cooling and
Solar cooling system
Regeneration of sorbent
Water from air
Solar electric power plant
Figure 2. Flow diagram of technological process of separation of water from moist
air using solar energy .
2.2. The absorption-regeneration cycle
Description and analysis of the theoretical cycle for absorption of water vapour
from air with subsequent regeneration, by heating is presented in . A theoretical
limit for the maximum possible amount of water which can be collected from air
using the desiccant through the absorption regeneration cycle at certain operating
limits of ambient conditions, heat to be added to the desiccant during regeneration
and maximum available heating temperature could be evaluated through the analysis
of this cycle. The absorption regeneration cycle, which can be applied for the
production of water from atmospheric air, is shown in Fig. 3. The theoretical cycle is
plotted on the vapour pressure-concentration diagram for the operating absorbent
and consists of four thermal processes which are:
A TECHNICAL REVIEW ON THE EXTRACTION OF WATER … 217
1. Process 1-2: isothermal absorption of water vapour from air.
2. Process 2-3: constant concentration heating of the absorbent.
3. Process 3-4: constant pressure regeneration of absorbent.
4. Process 4-1: constant concentration cooling of absorbent.
This cycle can be applied in desiccant systems with different configurations and
different heat sources. As the purpose of this cycle is to produce water from air and
the input energy to the system is the heat added during the regeneration process, then
the efficiency of the cycle can be defined as the ratio of heat added to regenerated
vapour to the total heat added. Theoretical analysis showed that, strong and weak
solution concentration limits play a decisive role in the value of cycle efficiency.
However, a modified cycle is described and analyzed by Sultan . In this
modification, the practical considerations are taken into account.
Figure 3. Absorption-regeneration cycle .
2.3. Desiccant systems
Hall  proposed a system for the production of water from atmospheric air by
absorption using ethylene glycol as a liquid desiccant with subsequent recovery in a
solar still. The effects of temperature and humidity on the recovered water were
studied and the results presented in the form of a composition-psychometric chart,
but the paper does not provide any information about the mass of recovered water.
Sofrata  constructed a non-conventional system to collect water from air based on
an adsorption-desorption process using a solid desiccant. The paper also discussed
the feasibility of the application of air conditioning systems for collecting water from
moist air by cooling it to a temperature lower than the dew point. Alayli  used a
AHMED M. HAMED et al.
typical S-shaped composite material for absorption of moisture from atmospheric air
with subsequent regeneration using solar energy. Hamed  tested two methods to
extract water from atmospheric air using solar energy. The first method was based
on cooling moist air to a temperature lower than the air dew point using a solar
LiBr–H2O absorption cooling system. The second method was based on the
absorption of moisture from atmospheric air during the night using calcium chloride
solution as a liquid desiccant, with subsequent recovery of absorbed water during the
day. As a result of this study, the second method was recommended as a most
suitable application of solar energy for water recovery from air.
Abualhamayel and Gandhidasan  proposed a system for water recovery
from air. A schematic of this system is shown in Fig. 4. It consists of a flat,
blackened, tilted surface and is covered by a single glazing with an air gap of about
45 cm. The bottom of the unit is well insulated. At night, the strong absorbent flows
down as a thin film over the glass cover in contact with the ambient air. If the vapor
pressure of the strong desiccant is less than the vapor pressure of water in the
atmospheric air, mass transfer takes place from the atmosphere to the absorbent. Due
to absorption of moisture from the ambient air during the night, the absorbent
becomes diluted. The water-rich absorbent must be heated during the day to recover
the water from the weak absorbent. Therefore, during the day, the weak desiccant
flows down as a thin film over the absorber surface. The weak absorbent is heated
by solar energy, and the water that evaporates from the solution rises to the glass
cover by convection where it is condensed on the underside of the glass cover and
the absorbent leaving the unit becomes strong. The performance of the unit at night
depends on the potential for mass transfer, which is the difference in water vapor
pressure between the ambient air and desiccant.
Figure 4. Schematic of the unit proposed by .
A TECHNICAL REVIEW ON THE EXTRACTION OF WATER … 219
The performance of a desiccant/collector system with a thick corrugated layer of
blackened cloth to absorb water vapor at night from atmospheric air with subsequent
regeneration during the day, using solar energy, was reported by Gad et al. . Fig.
5 shows a schematic diagram of the experimental apparatus. It consists mainly of
three parts: a flat plate collector with a movable glass cover, a corrugated bed and an
air-cooled condenser consisting of two parallel flat plates. The inner surface of the
collector is a box of cross section m 0.3 m, 42.142.1
in height. Three square
openings of m, 0.20.2 × are distributed on one side of the box. In each opening, a
fan of 10 W power is supported. The box is insulated by a high density foam of 0.05
m thickness, covered with aluminum sheets which form, by riveting, the outside case
of the apparatus. A glass cover which has a square cross sectional area of 2
6 mm thickness is supported by a metallic frame to form the upper side of the
apparatus. The frame is hinged with the box from one side. To support the cloth
layers which comprise the bed, a metallic frame made of steel wires is used (Fig. 5).
The bed height is 0.2 m, the horizontal distance between each two successive steel
wires is 5 cm. The corrugation increases the absorption area to about 4.1 times the
area of the box. A steel frame with a tilt angle of 30° supports the apparatus. An air-
cooled surface condenser with two parallel flat plates and total surface area of 2
is connected to the solar collector from the back (north) side through a small steel
duct. The condenser is made of steel sheets of 0.5 mm thickness. A condensate
collection flask is located below the condenser. The condenser is connected to the
system to evaluate its effect on the system productivity and operation. Actual
recorded results show that the solar operated system can provide about 1.5 l of fresh
water per square meter per day.
The need for economical realization of solar-desiccant systems for water
production in arid areas is of great importance. Moreover, the inconvenience and
relatively high capital cost of the desiccant bed limits the utilization of such units in
large scale. In desert regions, mixing a sandy layer of the ground surface with
desiccant as a promising method to minimize the cost of the vapour absorption bed
was proposed . The sandy layer impregnated with desiccant is subjected to
ambient atmosphere to absorb water vapour in the night. During the sunshine period,
the layer is covered with a greenhouse where desiccant is regenerated and water
vapour is condensed on the transparent surface of the greenhouse or any other cold
surface. Prediction of the absorption cycle requires knowledge of the percentage
approach to saturation. In view of the design parameters of the absorption bed, the
AHMED M. HAMED et al.
desiccant to sand mass ratio is an important factor affecting the rate of absorption
and consequently the rate of water production. Extracting water from air by using
sandy bed solar collector system is explored by Kabeel . The system is studied
theoretically and experimentally to evaluate the performance of the sandy bed
impregnated with 30% concentration CaCl2 to produce water from moist air. It is
reported that the system can provide up to about 1.2 l fresh water per square meter of
glass cover per day.
Figure 5. Schematic diagram of the experimental solar-desiccant collector for water
recovery from air .
Figure 6. Desiccant pond for absorbing moisture from the air .
The application of solar concentration for frish water production from the
atmospheric air is reported in . The results obtained in the AQUASOLIS project
for assessing the use of solar trough concentration plants for applications other than
heating and cooling, in particular for the production of fresh water for human
A TECHNICAL REVIEW ON THE EXTRACTION OF WATER … 221
consumption and for agriculture for Mediterranean countries. Fig. 6 prsents an
apparatus for extracting moisture from the ambient air that includes a desiccant pond
for absorbing moisture from the air to produce a water rich desiccant . The
atmospheric vapour is absorbed in the absorber section and the weak desiccant is
circulated to the generator for heating and vapour condensation.
Figure 7. Water production from air using multi-shelves solar glass pyramid system
The capability of the glass pyramid shape with a multi-shelf solar system to
extract water from humid air is explored in . Two pyramids were used with
different types of beds on the shelves (Fig. 7). The beds are saturated with 30%
concentrated calcium chloride solution. The pyramid sides were opened at night to
allow the bed saturated with moist air and closed during the day to extract the
moisture from the bed by solar radiation. The bed in the first pyramid was made of
AHMED M. HAMED et al.
saw wood while it is made of only cloth in the second pyramid with the same
dimensions. The system was experimentally investigated at different climatic
conditions to study the effect of pyramid shape on the absorption and regeneration
processes. Preliminary results have shown that the cloths bed absorbs more solution
(9 kg) as compared to the saw wood bed (8 kg). Adopting this approach produces
.mday 1 2
Selective water sorbents developed at the Boreskov Institute of Catalysis
(Novosibirsk, Russia) for fresh water production from the atmosphere are reported
by Aristov et al.  The results of their lab-scale tests have demonstrated a
feasibility of the fresh water production with the output of 3-5 tones of water per 10
tones of the dry sorbent per day. Also, selective composite adsorbent for solar-driven
fresh water production from the atmospheric air is presented in . It is synthesized
by a patented ultra-large pore crystalline material MCM-41 as host matrices and
calcium chloride as a hygroscopic salt. Adsorption capacity of the new composites is
as high as 1.75 kg/kg dry adsorbent, which is higher than composites synthesized by
silica-gel and calcium chloride, and the adsorption rate of the new composites is also
found attractive. A solar-driven water production test unit using the new adsorbent is
also presented and tested. The experimental tests of this developed unit demonstrated
a feasibility of the fresh water production with the daily water productivity more
mkg solar collector area.
The production of water from air on a continuous, 24-hour basis using more
compact adsorption units by applying forced convection adsorption in packed porous
bed is proposed in . Figure 8 shows a typical plant layout arranged for this
method. The system operates in two modes; namely adsorption mode and desorption
mode. In the adsorption mode, ambient air is forced onto the sorbent bed where the
moisture is adsorbed and desiccant concentration decreases with time. At the top of
the sorbent bed, air is exhausted outside the system through the air exhaust valve,
which is opened during this mode of operation. At the end of this stage, inlet and
exhaust air valves are closed and the bed is isolated from the outside air. During the
desorption mode, sorbent bed is heated and desiccant is regenerated. Vapour
pressure on the desiccant air surface increases and as a result water vapour flows to
an air cooled condenser through the vapour valve. Simultaneous evaporation from
the bed and condensation on the condenser surface take place during desorption
mode. The condensate is collected through the condenser opening shown in figure.
A TECHNICAL REVIEW ON THE EXTRACTION OF WATER … 223
Regeneration ends when desiccant concentration in the bed reaches its initial value at
start of adsorption process. New cycle starts when bed temperature decreases to the
initial sorption temperature (ambient temperature).
Figure 8. Typical layout of the absorption/desorption system producing water from
2.4. Air cooling
A similar study was conducted analytically  for the climatic conditions of
UAE coastal regions, and it was reported that the quantity of fresh water obtained
depends on the properties of humid air, air velocity, cooling coil surface area, and
the heat exchange arrangement. It is to be noted that this system uses chlorinated
fluorocarbon compounds (CFCs) identified as contributors to depletion of the ozone
For typical hot humid weather (Jeddah, Saudi Arabia, 21o 23o N and 39o E),
Habeebullah reported that the daily variation of water yield showed to follow the
relative humidity pattern with minimum during midday hours. On the basis of actual
climatic data, the monthly estimated average water yield during August and
February were 509 and 401 ,mkg 2 respectively .
2.5. Dew collection
On clear nights, the moisture in the air begins to condense on any surface where
the temperature has fallen below the dew point due to radiation. Cloudiness, surface
temperature, air humidity, and wind speed influence the dew formation. This type of
AHMED M. HAMED et al.
water collection is possible whenever humid air and clear night time skies exist
simultaneously. This kind of dew formation can occur over large land areas in a
humid but clear environment as in coastal areas. Jacobs et al.  reported the
experimental results of a specially designed 1 m2 insulated planar dew collector, set
at a 30° angle from horizontal, covered with a thin (0.39 mm) polyethylene foil and
subsequently replaced with 4 mm polyvinyl chloride. A second dew collector, in the
shape of an inverted pyramid, was constructed to reduce the view angle to only the
nighttime sky (Fig. 9).
A simple surface energy-budget model and an aerodynamic model were used to
simulate the dew collected by both collectors. The planar collector collected about
90% of the dew at the grass cover while the pyramid collector collected about 1.20%
of the grass cover. The aerodynamic model was able to predict the amount of
collector data to within 50% for the planar collector and 60% for the inverted
pyramid collector. The pyramid collector design was able to collect about 20% more
dew than the inclined planar collector. They also reported that Both dew collectors
are efficient in collecting dew and the collected amounts are comparable with the
natural dew at a grass cover, despite the need for the dew drops to drain towards the
A project called Dew Equipment for Water (DEW) was initiated for a 15.1m2
roof in the island of Bisˇevo (Croatia), equipped with commercial plastic cover
selected for its superior dew collection properties (Fig. 10). Measurements of both
rain and dew water were performed over several years and data will be correlated
with meteorological data collected in situ. Preliminary measurements during the
period 21 April - 21 October 2005 showed that dew water contributed significantly,
26% of the total collected water .
To predict the performance of the dew collector, a steady state mathematical
model is developed by Gandhidasan and Abualhamayel . The dew collector
performance predicted with the model shows a good agreement with the
experimental findings. Experiments are conducted in Dhahran, Saudi Arabia, with
1 × dew collecting panel and about 0.22 2
mL of water is collected during a
single night of operation.
In the southwestern region of Kingdom of Saudi Arabia there is a potential to
provide an alternative source of freshwater. A fog collection project has been carried
out in Asir region of Saudi Arabia. Three Standard Fog Collectors (SFC) were
designed, manufactured and installed. Three different sites were chosen based on
A TECHNICAL REVIEW ON THE EXTRACTION OF WATER … 225
topography and altitude and data from April 2006 to April 2007 were obtained.
Measurements with the SFCs were made for regions with 2,260 to 3,200 m
elevation. The results indicated that at highest altitudes (at Alsooda), it is feasible to
obtain an average water production of 6.215 2
mL day over the studied period, and
in the lower altitudes, which are in Abha city, it is possible to collect more than 3.3
mL day .
Figure 9. The planar dew collector and the inverted pyramid collector. Both
collectors have condensing surfaces inclined at 30° .
AHMED M. HAMED et al.
Figure 10. The house in Salbunara bay, NW of Bisˇevo Island. The 17.1m2 roof
In the literature review of the extraction of water from air using air processors
, it is reported that each cubic meter of air throughout Earth’s 100-600 m thick
atmospheric boundary layer contains 4-25 g water vapour, potentially allowing water
supplies almost anywhere people inhabit. Absolute humidity (meteorological
normals) ranges from 4.0 g of water vapour per cubic metre of surface air in the
atmosphere (Las Vegas, Nevada, USA) to 21.2 3
mg (Djibouti, Republic of
Djibouti). An extensive research work is still needed to maximize the utilization of
this huge and endless source of water.
The technology of water extraction from atmospheric air is still at an early stage
compared with other systems such as water distillation. However, if the experience
of the studies carried out in desiccant cooling is applied in this area, improved and
more efficient units could be designed. Also, rapid development of appropriate and
reliable systems for water recovery from atmospheric air could be facilitated by
adapting financial investment and using friendly energy sources. Collecting dew is
still a viable option to get water from air, however, the application of dew collection
is restricted by the availability of dew.
A TECHNICAL REVIEW ON THE EXTRACTION OF WATER … 227
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