Abstract—Air-conditioning plays an essential role in ensuring
occupants thermal comfort. However, building’s electricity bills
have become unaffordable. Yet the commercially dominant
cooling systems are intensively power-consuming ones, i.e. vapor
compression systems. This paper aims to review the recent
developments concerning evaporative cooling technologies that
could potentially provide sufficient cooling comfort, reduce
environmental impact and lower energy consumption in
buildings. An extensive literature review has been conducted
and mapped out the state-of-the-art evaporative cooling systems.
The review covers direct evaporative cooling, indirect
evaporative cooling and combined direct-indirect cooling
systems. The indirect evaporative coolers include both wet-bulb
temperature evaporative coolers and dew point evaporative
coolers have been of particular interest because of high thermal
performance. The dew point evaporative coolers have shown
great potential of development and research opportunity for
their improved efficiency and low energy use.
Index Terms—Evaporative cooling, Effectiveness, Dew point,
Dry bulb temperature.
Energy demand worldwide for buildings cooling has
increased sharply in the last few decades, which has raised
concerns over depletion of energy resources and contributing
to global warming. Current energy demand estimates stands at
between 40 and 50% of total primary power consumption. In
hot climate countries, the highest share of building energy use
is mainly due to space air conditioning using traditional
HVAC systems. For example, in the Middle East, it accounts
for 70% of building energy consumption and approximately
30% of total consumption. Nowadays, buildings air
conditioning has become a necessity for people life and plays
a vital role in ensuring indoor comfort levels. Hence,
improving the efficiency of cooling technologies are essential,
particularly ones that have the potential, i.e. high performance,
low power consumption .
Currently, mechanical vapor compression coolers (MVC)
are commercially dominant despite their intensive energy use
and low performance in hot climate. In contrast, evaporative
cooling systems are more environmentally friendly as they
consume less energy and their performance improves as air
temperature increases and humidity decreases. Table I shows
a comparison of coefficient of performance (COP) values of
Manuscript received May 4, 2014; revised June 10, 2014. This
publication was made possible by NPRP grant No. 4-407-2-153 from the
Qatar National Research Fund (a member of Qatar Foundation). The
statements made herein are solely the responsibility of the authors.
O. Amer and R. Boukhanouf are with The University of Nottingham,
Department of Built Environment, Nottingham, UK (e-mail:
H. Ibrahim is with Qatar University, Department of Architecture and
Urban Planning, Doha, Qatar (e-mail: email@example.com).
several cooling cycles. However, the main drawback of the
evaporative cooling is their high dependency on the ambient
air conditions. Since the temperature difference between the
dry- and wet-bulb temperatures of the ambient air is the
driving force of evaporative cooling. For mild and/or humid
climate this difference is small, therefore, leads to limited
cooling capacity .
II. EVAPORATIVE COOLING TECHNOLOGY
Evaporative cooling is a heat and mass transfer process that
uses water evaporation for air cooling, in which large amount
of heat is transferred from air to water, and consequently the
air temperature decreases. Evaporative coolers could be
classified into: (1) Direct evaporative coolers, in which the
working fluids (water and air) are in direct contact; (2)
Indirect evaporative coolers, where a surface/plate separates
between the working fluids; (3) Combined system of direct
and indirect evaporative coolers and/or with other cooling
cycles . Fig. 1 illustrates a general classification of main
types of evaporative cooling systems for building cooling.
TABLE I: COP VALUES OF SOME OF AIR-CONDITIONING SYSTEMS 
Fig. 1. A classification of evaporative cooling systems in building cooling
A Review of Evaporative Cooling Technologies
O. Amer, R. Boukhanouf, and H. Ibrahim
2014 APCBEES Nottingham Conferences Proceeding
III. DIRECT EVAPORATIVE COOLING (DEC)
This system is the oldest and the simplest type of
evaporative cooling in which the outdoor air is brought into
direct contact with water, i.e. cooling the air by converting
sensible heat to latent heat. Ingenious techniques were used
thousands of years ago by ancient civilizations in variety of
configurations, some of it by using earthenware jar water
contained, wetted pads/canvas located in the passages of the
Direct evaporative coolers in buildings vary in terms of
operational power consumption from zero power to high
power consumption systems. DEC systems could be divided
into: Active DECs which are electrically powered to operate
and Passive DECs that are naturally operated systems with
zero power consumption. DEC is only suitable for dry and hot
climates. In moist conditions, the relative humidity can reach
as high as 80%, such a high humidity is not suitable for direct
supply into buildings, because it may cause warping, rusting,
and mildew of susceptible materials .
A. Active DEC Systems
The active direct evaporative coolers are electricity-driven
systems, however, it use a fraction of power for air and water
circulation. So, it is considered much less energy intensive
than other traditional cooling technologies, with energy
saving up to 90% . A typical direct evaporative cooler
comprises of evaporative media (wettable and porous Pads),
fan blows air through the wetted medium, water tank,
recirculation pump and water distribution system, as
illustrated schematically in Fig. 2-a. The direct evaporative
cooling is an adiabatic cooling process, i.e. the total enthalpy
of the air is constant throughout the process, as shown in Fig.
2-b. The water absorbs the sensible heat from the supply air
and evaporates causing the air temperature decreases and its
humidity to increase. .
Theoretically, the supply air could be cooled to 100%
effectiveness, but in such process a wet-bulb effectiveness of
70%-80% only is achievable because of short contact time
between the two fluids, insufficient wettability of the pads and
due to the fact that the circulated water and the supply air will
reach an equilibrium point that is equal to the wet-bulb
temperature of the supply air. Eventually the system would
not be able to cool down the incoming air lower than its
wet-bulb temperature. The wet-bulb effectiveness could reach
range between 70-95% in most current commercial DEC
coolers and mainly as a function of the type and thickness of
evaporative media, working climate, and supply air flow-rate
According to ASHRAE Handbook-HVAC Systems and
Equipment (2008) active DEC could be divided according to
types of wet media into: Random media DEC, Rigid media
DEC and Remote media DEC, as shown in Fig. 3.
However, active DEC coolers can be classified in terms of
water distribution system type: spray (also called air washer),
slinger (a rotating wheel), and drip (Misting) system .
Table II show the main types of active DEC systems:
Fig. 2. Structure, working principle and psychometric chart of a direct
evaporative cooler 
TABLE II: MAIN TYPES OF ACTIVE DEC SYSTEMS 
Excelsior or plastic
fiber/foam supported by
Hard to clean.
Blocks of corrugated
High initial cost.
Random or rigid Pads
mounted on wall or roof
Fig. 3. Types of DEC system Pads 
B. Passive DEC Systems
Passive cooling techniques use natural phenomena,
energies, and heat sinks for cooling buildings without the use
of mechanical apparatus consume electrical energy. However,
small fans and pumps could be required. Passive DEC is
relied on the climate which means the techniques applied for
hot and humid regions are different from those for hot and
arid areas. This technology is able to reduce indoor air
temperature by about 9 ℃ . The main types of passive
direct evaporative cooling building integrated systems are:
1. The Mashrabiya
The mashrabiya is a traditional Islamic architecture
element used for natural ventilation and cooling of buildings
without requiring any energy. It is wooden screens/windows
provides shad, protection from the sun and allows breezes to
flow through into the building for cooling purpose. Fig. 4-a
shows a mashrabiya system coupled with porous water-jugs to
provide evaporative cooling effect for a dwelling and cooling
water inside jugs for water drinking. .
2. Wind Towers
The wind tower, also called wind catcher is a traditional
passive cooling technique of buildings, existed hundreds of
2014 APCBEES Nottingham Conferences Proceeding
e. the air tower. Basic structure of a wind tower is
illustrated Fig. 4-b. A capped tower with one face opening or
multi-face openings at the top of tower, the tower is placed on
the roof of a dwelling. Wind towers/catchers could be divided
according airflow patterns inside the tower into: downward
airflow towers and upward airflow towers.
Downward airflow wind tower
between windward side and leeward sides of the tower. The
tower catches the ambient air which enters at top of the tower
and flow through it to the building providing fresh air. Water
could be introduced into the tower geometry by several means
i.e. water pool at bottom of tower, porous jars filled with
water located in the tower airstream or wetted pads hanged at
the top of the tower (Fig. 4-b).
Upward airflow wind tower is driven by temperature
difference between building interior and the outside
environment. In this system, the air is drawn upwards via wind
tower. Because of positive pressure on one sides of the
building the hot air could be drown down via underground
channels or water fountains before entering to the building as
cooled air, while the hot interior air rises upward via the
openings of the wind tower , .
Roof pond is a building-integrated evaporative cooling
technique. It can contribute highly to mitigate heat by cooling
the roof passively; therefore, the indoor air is cooled without
increasing its moisture and reducing the energy consumption
and heat gain during daytime. A typical roof pound consists of
water pool in plastic or fiber-glass container stored on top of
the roof of the building. The pond could be covered by a
removable cover, a fixed cover or a fixed floating installation.
A basic configuration of a shaded roof-pond system is shown
in Fig. 4-c. During summer, the ambient air flow over the
pond causes the water to evaporate, thus, cools the pond and
the roof structure which act as a heat sink of the building
interior. During winter, the pond is emptied and the shaded
openings are closed. Roof-pond cooling systems may
incorporate water spraying system to enhance evaporative
IV. INDIRECT EVAPORATIVE COOLING (IEC)
The primary idea of the indirect evaporative coolers is
cooling by decreasing air sensible heat without changing its
humidity, which is a distinctive advantage over DEC systems.
A common IEC unit comprises of: a heat exchanger (HX),
small fan, pump, water tank, and water distribution lines, as
illustrated in Fig. 5-a. Indirect evaporative coolers are
classified into: Wet-bulb temperature IEC systems and Sub
wet-bulb temperature ICE systems. .
A. Wet-bulb Temperature IEC System
Wet-bulb temperature IEC system are packaged unit of
flat-plate-stack, cross-flow heat exchanger, the most common
configuration and flow pattern, which can lower air
temperature close to, but not below, the wet-bulb temperature
of the inlet air. Fig. 5-b shows a schematic drawing of the
working principles of a typical HX configuration of a
wet-bulb temperature IEC system which comprises of several
Fig. 4. Types of passive cooling systems 
Fig. 5. IEC structure,working principle and its psychometric chart , 
pairs of adjacent channels: wet passages of the working
(secondary) air and dry passages of the supply (primary) air.
Heat transfer occurs between the two working fluids through a
heat conductive plate, therefore, the supply air is cooled
sensibly with no additional moisture introduced into the
cooled supply air stream. While, heat transfer mechanism
between the working air and water in wet channels is by latent
heat of water vaporization. The wet-bulb effectiveness of this
system is in the range of 4080%, which is lower than that of
the DEC systems . Different types of IEC systems are
existed which can be classified, according to the type of heat
exchanger (HX), into: plate-type IEC, tubular-type IEC and
heat pipe IEC as summarized below:
Plate-type HX based IEC: This type of heat exchanger is
the most commonly used configuration, that is,
flat-plate-stack HX with cross- or counter-flow arrangement
of the primary and secondary airstreams. Fig. 6-a illustrates
schematically a basic plate-type IEC system. Several
researches conducted evaluation of energy saving , ,
mathematically modeling of the heat transfer process and
performance evaluation -, studying the effects of
channels dimensions, humidity ratio, primary and working air
velocities, and plate wettability percentage on the efficiency
of the system , . However, the cooling effectiveness
of plate type IEC system s is only in the range of 5080%.
Tubular-type HX based IEC: This configuration is usually
built of circular tubes, as shown in Fig. 6-b. However, other
tubular shapes have been used such as elliptical and
rectangular tubes . A common configuration consists of a
bundle of round tubes mounted in a cylindrical or rectangular
shall, where the primary air flows inside the tubes and the
secondary air flow across and/or along the tubes in the normal
direction to the primary air, while the water is sprayed over
the external surface of the tubes. So that it could offer more
uniform water film over the tubes and less pressure losses
2014 APCBEES Nottingham Conferences Proceeding
comparing with plate-type IEC. Usually, the tubes are made
of either polymer, metal, porous ceramic, PVC, or aluminum
. Another common configuration is a tube-fin HX based
IEC in which round or rectangular tubes are fitted with
outside fins by soldering, brazing, or welding. For example,
Velasco Gomez et al.  introduced tube-fin HX based
indirect evaporative cooler, that is, a bundle of porous
ceramic tubes fitted with flat metallic fins. The results showed
air dry-bulb temperature reduction of 9-14 ℃was achieved.
The system can be used for heat recovery in air conditioning
Heat pipe HX based IEC: Heat pipe is a light, simple and
thermally conductive device available in shapes and sizes, can
be applied to transport heat from the primary to secondary air
passages for cooling applications. The configuration of heat
pipe can be any type from thermo-syphon, cryogenic, rotating
and revolving, flat plate and capillary pumped loop heat pipe
. In this structure, the heat pipe based IEC, the condenser
section of heat pipe is used in the secondary air (wet) channel,
and the evaporator section is used in primary air (dry) channel,
as shown in Fig. 7.
Limited studies ,  carried out evaluating the
performance of the heat pipe based IEC systems for building
cooling. A finned heat pipe was used to increase convective
heat transfer between the primary air and the heat pipe, with
different methods of heat eliminations from the condenser
sections such as water sprayer on condensation section
surface, the outdoor air is precooled by air washer before
passed through the condensation section, the use of porous
ceramic water container fitted around condensation section to
assure even distribution of water. Also, In the literature many
research studies conducted on heat pipes applications in
building cooling includes HVAC systems , , , and
, and heat recovery systems , , , and .
Fig. 6. Schematics of plate-type and Tubular-type based IEC 
Fig. 7. Schematic of heat pipe based IEC 
B. Sub Wet-bulb temperature IEC Systems
To overcome some of the disadvantages of DEC systems
and to enhance the effectiveness of wet-bulb temperature IEC,
introduced a new design of the heat exchanger of IEC system
. The Maisotsenko-cycle (M-cycle) based IEC system is a
combination of a cross-flow, multi-perforated flat-plate HX
and evaporative cooling, in which, the secondary air is
precooled in the dry channel before it is diverted to pass
through the wet channel to achieve further heat transfer with
the dry channel. Thus, the primary air temperature is lower
than wet-bulb temperature and approach dew-point
temperature of the incoming air. So, it is called Dew point
IEC. The wet-bulb effectiveness is in the range of 110%
-122% and a dew-point effectiveness of 55%-85%. Although
the M-cycle heat exchanger has 1030% higher effectiveness
than that of the conventional heat exchangers, its operation is
still facing some limitations; the secondary air is not fully
cooled as high proportion of it is gradually diverted early into
the wet channels, and cross-flow is unfavorable pattern for
heat exchangers. An experimental tests of the M-cycle based
IEC system showed that its dew-point/wet-bulb effectiveness
was only around 5060% and 8090% respectively .
Several research studies conducted to develop and modify
the thermal process of the M-cycle IEC to overcome the
above mentioned drawbacks and to enhance the efficiency
and increase the thermal performance. Zhao et al. 
introduced a new counter-flow heat and mass exchanger
based on M-cycle of a dew-point evaporative cooling system.
In this structure, unlike the cross-flow Maisotsenko-cycle heat
exchanger, holes are located at end of flow channels as
presented in Fig. 8. The product air flows through and along
the dry channels losing sensible heat to wet channels and at
the end of dry channels part of cooled product air is delivered
to the conditioned space and the remaining air is diverted to
the adjacent wet channels as cold working air transferring heat
latently with the water and sensibly with the product air in the
dry channel. It was found that the wet bulb effectiveness
achieve up to 130% and dew-point effectiveness of up to 90%.
Furthermore, a comparative study between cross-flow and
counter-flow M-cycle base IEC system showed that the
counter-flow arrangement achieved around 20% higher
cooling capacity and 1523% higher dew-point and wet-bulb
effectiveness respectively under the same geometrical sizes
and operational conditions. Contradictory, the cross-flow
exchanger has 10% higher performance which is due to an
increase in power consumption of counter-flow heat
Additionally, Zhao et al.  and  conducted a
feasibility study in China and the UK respectively, using the
proposed dew-point IEC system in .
Fig. 8. Working principle and psychometric chart of a sub wet-bulb IEC 
2014 APCBEES Nottingham Conferences Proceeding
It was found that the dew-point IEC system is applicable for
most of the UK and china regions, particularly where the
climate is dry. Tap water is suitable as feed water with an
adequate temperature for cooling and its consumption rate
ranged from 2.1 to 3 l/kWh cooling output. The system
cooling output is in the range of 3.1-4.3 W/m3/h air flow rate.
Also, Rogdakis et al.  theoretically and experimentally
evaluated the performance of an M-cycle based IEC system at
Greek climate condition. It was found that the Maisotsenko
cycle can be applied for most Greek cities without intensive
consuming of electricity and water, the effectiveness ranged
between 97% and 115%, while water consumption was in the
range of 2.5 3.0 l kW/h.
It is noteworthy that research done on the dew-point IEC
systems is still at its early stage. To date, most theoretical and
experimental works are based on the principles of the M-cycle.
Most highly ranked papers and innovative research work of
dew-point evaporative cooling systems are summarized in the
following Table III.
V. INDIRECT-DIRECT EVAPORATIVE COOLING (IDEC)
Since DEC have higher effectiveness but humidity
increases indoors while IEC have lower effectiveness and the
humidity is constant, a combination of both systems or in
conjunction with other cooling technologies can be a potential
and achieve the best characteristics of both systems, such as
cooler supply air at a lower relative humidity, higher
efficiency and controlled humidity. The main components of
IDEC system are heat exchanger of IEC unit, evaporative pad
of DEC unit, water recirculation system, water reservoir, and
blowers, as shown in Fig. 9.
TABLE III: LIST OF RECENT CONTRIBUTION AND DEVELOPMENTS OF DEW-POINT IEC SYSTEMS
Cross-flow M-cycle based IEC. A
multi-perforated flat-plate-stacked heat
and mass exchanger made of Aluminum.
Polyethylene as water-proof material.
cellulose-blended fiber as wettable material
Assessment of the performance of a
Coolerado Cooler in terms of air flow,
cooling effectiveness and power
Zhao et al.
Counter-flow M-cycle based IEC. A
polygonal plate-fin HX made from flat
sheets and corrugated triangular
cross-section fins as flow guides.
Introduce of a novel counter-flow HX of
M-cycle based IEC. And optimization
of the geometrical sizes and operating
conditions of the cooler
Zhan et al.
Cross-flow M-cycle based IEC.
Comparative study between cross- and
counter-flow IEC to identify the in
performance, effectiveness and cooling
capacity of both systems under the same
Counter-flow M-cycle based IEC.
et al. , 
Counter-flow M-cycle based IEC. HX
consists of flat-sheets-stacked structure.
Polyethylene as water-proof material, and
thin-film cotton sheet as wettable material
Evaluation of the performance at
different inlet air conditions of various
climate conditions. Developed a
numerical model to simulate the heat
and mass transfer processes and
optimize system parameters
Counter-flow plate-type exchanger based
on M-cycle indirect evaporative cooler
Experimental test of the thermal
performance of a module product IEC
for residential and commercial cooling
applications for a wide range of
Cui et al. 
Counter-flow flat-plate-stacked HX of IEC.
The HX comprises of a dry channel and
two adjacent wet channels with a
Investigation of the thermal
performance under varying inlet
conditions. Also, studied the effect of
the channels surface roughness and
effect of using the return air as working
air on the overall performance
Plate-type indirect evaporative cooler
Study of heat and mass transfer process
using a numerical analysis for four
flow-configuration of HX of IEC
system to evaluate the performances
et al. 
The exchanger of the IEC is a counter-flow,
plate-type HX consists of two adjacent
channels with hollow, rectangular porous
ceramic panels as heat transfer wall (wet
medium) filled with water
Evaluation of the performance, cooling
capacities and other characteristics of
the cooling system
Lee et al. 
Counter-flow M-cycle based IEC. The HX
is a plate-fin type HX, comprises of
multiple pairs of Aluminum finned
channels brazed with a thin flat plate
Evaluation of the performance of the
system at various operation conditions.
2014 APCBEES Nottingham Conferences Proceeding
Fig. 9. Two-stage IDEC system 
The effectiveness ranges from 90% to 115%. However, the
high initial cost and system complexity are the obvious
drawbacks . The common types of the IDEC systems are:
Two-stage IDEC: in this configuration, the IDEC
comprises of IEC stage followed by a DEC stage. The first
indirect stage (state 1) cools the outdoor air, which is then
flows through a direct stage (state 2) for further cooling to
below its wet-bulb temperature, but with additional moisture
added (state 3), as illustrated on the psychometric chart Fig.
9-b. The effectiveness is in the range of 90120%, but water
consumption increases by 55% , . Other two-stage
IEC-DEC configurations reported by ,  achieved
effectiveness of 109%-116%.
Three-stage IDEC: this system consists of two-stage IDEC
system in conjunction with a cooling cycle. For example, a
solid desiccant dehumidification with an IEC and/or DEC unit
, ,  gives COP of around 20. Several
configurations have been reported: An IEC, cooling coil and
DEC stage , . An IEC and a DEC system to provide
sensible and adiabatic cooling coupled with a desiccant
system for dehumidification (Fig. 10) can offer energy saving
of 54%-82% over the conventional cooling systems .
Multi-stage IDEC: a hybrid system of two-stage IDEC
coupled with more than one cooling cycle. For instance,
combined system of two-stage DEC-IEC coupled nocturnal
radiative cooling and cooling coil (Fig. 11) has higher
effectiveness than two-stage evaporative cooling system, with
energy saving is between 75-79% compared to MVC systems
Fig. 10. Schematics of solid desiccant and evaporative cooling systems 
Fig. 11. Hybrid system of radiative cooling, cooling coil, and two-stage
IEC-DEC system 
Using water for evaporation as a mean of decreasing air
temperature is considerably the most environmentally
friendly and effective cooling system. In this paper a review
of evaporative cooling technology that could be efficiently
applicable in building air-conditioning was carried out.
Indirect evaporative coolers showed higher values of
effectiveness and are more economical in terms of energy
consumption saving, particularly the breakthrough brought
about by the M-cycle based dew-point IEC system. However,
combined IDEC systems have similar performance or even
higher but their system complexity and high initial cost are
the major limitation. Recent works concerning indirect
evaporative cooling based on Maisotsenko-cycle have shown
considerable potential towards enhancing the performance
and cooling capacity of IEC system for building cooling.
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Mr O. Amer is a PhD degree candidate in the
department of the Built Environment, University of
Nottingham. His main research topic is sustainable
cooling technologies. Mr. Amer has an MSc in New
and Renewable Energies, University of Durham, UK.
2011. BEng degree in mechanical Engineering,
Misurata University, Libya, 2005.
He also held a position of a lecturer in Mechanical
engineering department, Misurata University, Libya between 2011-2012.
Dr R Boukhanouf is a lecturer in sustainable energy
technologies at the Department of Built Environment,
University of Nottingham. His experience in research
and teaching in the area of energy efficient and low
carbon technologies extends for over 15 years. He
obtained his PhD in 1996 from the University of
Dr. Boukhanouf worked on numerous research
projects funded by industry and government agencies
in the area of small scale combined heat and power, active and passive
heating and cooling systems for buildings, and advanced heat transfer
enabling devices. He published a number of journal and conference papers
and is named as the inventor in six international patents.
Dr. H. Ibrahim is associate Professor at Qatar
University. Dr. Ibrahim has a long and established
research experience including managing green
construction, carbon abatement in construction
industry using knowledge based programming, and
preservation of traditional architectural and urban
heritage of Qatar. The latter being particular an ass-on
advantage for reconciling the integration of new low
carbon technologies with the traditional architectural
2014 APCBEES Nottingham Conferences Proceeding