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Abstract - An assisted evaporative cooling system is
proposed and analyzed to get higher cooling efficiency
in hot and arid environments such as Saudi Arabia. For
this purpose, a two-stage evaporative cooling
experimental setup consisting of a direct evaporative
cooling and indirect evaporative cooling was designed,
constructed, and tested.
The obtained data showed that, the hourly average
solar radiation flux incident outside and transmitted
into the greenhouse was 555.7 and 298.7 W/m2
,
respectively, consequently, the effective transmittance
of the covering material was on the average 53.75%.
The temperature and relative humidity of outdoor air,
respectively, were 37.3ºC and 21.8%, dry and wet-bulb
temperatures just leaving the cooling coil were 29.4ºC
and 18.8ºC, respectively, and dry-bulb temperatures of
air just leaving the direct evaporative cooling were
22.8ºC. The obtained results also revealed that, the
overall effectiveness of the combining cooling system
was more than 100%. Thus, this system
environmentally clean and energy efficient system,
which considered as an alternative to the mechanical,
vapour compression systems. It can also conclude that,
this combining cooling system can use in various
climatic conditions as an environmentally clean and
energy efficient system.
Introduction
The weather of Saudi Arabia has a long
summer season with high air temperature. Aircondition
system has become more popular and even
a necessity in life not only for human but also for
animal and plants to create comfortable environment,
and consumed a large amount of energy at the same
time (Qun Chen et al., 2010).
Evaporative cooling is one alternative to
mechanical vapor compression for air conditioning
applications. These systems usually require only a
quarter of the electric power that mechanical vapor
compression uses for air conditioning (Cerci, 2003).
Therefore, such systems will help to reduce
electricity requirements, and also contribute to
reducing greenhouse gas emissions.
Conventional evaporative cooling system can
decrease the process air temperature theoretically
approaching its wet bulb temperature, and has been
used as a low energy consuming device for various
cooling and air conditioning applications in
industrial, agricultural and residential sectors
(Costelloea, and Finn, 2003), (Maheshwari et al.,
2001) for providing low temperature medium fluid
(i.e. air, water, etc.).
Evaporative pad cooling is the most efficient
method for greenhouses cooling under arid
conditions (e.g., Saudi Arabia) (Al-Helal, 2001).
Controlling thermal environment in hot
weather areas is very challenging. Removing
excessive heat from the house requires using
evaporative cooling pads, misting or fogging systems
in conjunction with mechanical ventilation. In hot
and dry climates such as the case in Saudi Arabia,
evaporative cooling process is the most effective and
economical technique of air conditioning (Alodan
and Al-Faraj, 2005).
Still in order to improve the performance of
the evaporative cooling units, extensive research has
been conducted in analyzing the influences of such
factors as moist air velocity, temperature and
humidity (Muangnoi et al., 2008), water velocity and
temperature (Lemouari et al.2009 ), longitudinal heat
conduction (Hettiarachchi et al., 2007 ), heat and
mass exchanging materials properties (Zhao et al.,
2008 ), and geometries (Sureshkumar et al., 2008 )
on the efficiency of various traditional and novel
evaporative coolers, including plate/tube type indirect
evaporative cooler (Stoitchkov, and Dimitrov, 1998).
Several active or passive evaporative cooling systems
have been developed, including two-stage
indirect/direct evaporative cooling (Jain, 2007) (AlJuwayhel
et al., 2004). The general underlining
principle in all these is to maximize the refrigerating
effect in an evaporative cooling system.
Since, air temperature and humidity are the
two major parameters affecting thermal comfort
significantly, and only sensible load can be handled
by an evaporative cooling system, conventional
evaporative cooling system is suitable for dry and
temperate climate where the humidity is low
Two principle methods of evaporative cooling
are commonly used, the direct evaporative cooling
(DEC) and the indirect evaporative cooling (IEC).
DEC is the oldest, simplest, and the most widespread
form of evaporative air conditioning
Direct evaporative cooling system adds
moisture to the cool air, which also makes conditions
more uncomfortable for humans as (air) humidity
increases. On the other hand, an indirect evaporative
cooling system provides only sensible cooling to the
process air without any moisture addition. Therefore,
it is more attractive than direct evaporative system.
However, the cooling effectiveness is generally low,
around 40–60% (Maheshwari et al., 2001).
This is a major drawback of conventional
evaporative cooling. To overcome this drawback, the
enhancement of cooling effectiveness to provide
much lower outlet air temperature is an interesting
option for hot and humid climate conditions.
The underlying principle of DEC is the
conversion of sensible heat to latent heat. Through a
direct evaporative cooling system, hot outside air
passes a porous wetted medium. Heat is absorbed by
the water as it evaporates from the porous wetting
medium, so the air leaves the system at a lower
temperature. In fact, this is an adiabatic saturation
process in which dry bulb temperature of the air
reduces as its humidity increase (constant enthalpy).
Some of the sensible heat of the air is transferred to
the water and become latent heat by evaporating
some of the water. The latent heat follows the water
vapor and diffuses into the air. The minimum
temperature that can be obtained is the wet bulb
temperature of the entering air (Camargo et al.,
2005), (Heidarinejad, and Bozorgmehr, 2007–2008).
Direct evaporative cooling (DEC) involves no
change in the heat content of the air/water vapor
mixture. Rather, as water evaporates it takes away
heat from the air thus reducing its temperature. These
systems are based on the conversion of sensible heat
into latent heat of evaporated water, with the water
supplied mechanically. During the process, the total
heat (enthalpy) of the air remains the same. It is
known that one gram of water evaporated into 1 m3
of air space reduces its temperature by about 2.5 C.
The method of lowering the air temperature by
evaporation of water is thus the most effective way of
controlling the temperature and humidity inside a
greenhouse.
Indirect evaporative cooling (IEC) where air is
cooled via a heat exchanger – the temperature of air,
whose moisture content consequently remains
unchanged - can be classified in two types based on heat and mass transfer occurring in the heat
exchangers (Zhao et al., 2008).
In heating, ventilation, and air conditioning
(HVAC) systems, cooling coils unit (CCU) perform
an essential function by exchanging the cooling load
from the hot air to the chilled water loop by pushing
airflow through the coil. In addition, cooling coil unit
has utilized as pre-cooler systems to decrease
temperature of hot air. Totally, utilization of cooling
coils affects performance of HVAC systems
increasingly (Jin Guang-Yu et al., 2006 ;
Heidarinejad et al., 2010).
In this method, two streams of air are used;
namely alternative wet and dry passages, which are
separated from each other. Primary air is cooled in
dry passages which is separated from wet passages
where secondary air and water flow. Evaporation
occurs in wet passages and heat is removed from
primary air through impermeable separating wall and
evaporates water into the secondary air. Therefore,
heat and mass transfer due to evaporation in wet
passages and heat removal in dry passages are
simultaneous and practically inseparable. The air
leaving the dry side of the cooler has a lower wet
bulb temperature than the ambient. Consequently, it
would be advantageous to extract a fraction of this
cooled air and pass it through the wet side of the heat
exchanger instead of using ambient air.
Depending on the climatic conditions and the
application, combining indirect and direct
evaporative coolers might be appropriate to increase
the cooling capacity (Mazzei, and Palombo, 1999).
A two-stage air-conditioner combining
indirect and direct processes is gaining popularity in
places where the higher wet bulb temperatures (i.e.
higher ambient humidity) does not permit sufficiently
low indoor temperatures supplied from a simple
direct air-conditioner.
In this research work, the water in a storage
tank cooled by means of circulating the water
through the cooling pads throughout the nighttime.
During the next daylight, the cold water in the
storage tank used in the cooling coil unit as chilled
fluid to decrease temperature of outdoor air (precooling).
Then, the pre-cooled air with lower wetbulb
temperature passed through the direct
evaporative cooling system. The performance and
feasibility of such cooling system have analyzed in
this paper.
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