Smart Grid and Renewable Energy, 2012, 3, 67-71
http://dx.doi.org/10.4236/sgre.2012.31010 Published Online February 2012 (http://www.SciRP.org/journal/sgre)
67
Modeling and Simulation of Evaporative Cooling System i n
Controlled Environment Greenhouse
Faten Hosney Fahmy, Hanaa Mohamed Farghally, Ninet Mohamed Ahmed, A. A. Nafeh
Electronics Research Institute, Giza, Egypt.
Received August 2nd, 2011; revised September 6th, 2011; accepted September 13th, 2011
ABSTRACT
Greenhouses are used for the main purpose of improving the environmental conditions in which plants are grown. There
are many parameters can affect the growing of plants inside greenhouse, such as air temperature and relative humidity.
The adjustment of these parameters is achieved by selecting appropriate control actions. This work proposes a control-
ling technique for greenhouse indoor temperature and relative humidity. The proposed greenhouse cooling system tem-
perature controller is designed to adjust the air volume flow rate in pad-fan cooling system to fix the greenhouse indoor
temperature at 20˚C and 70% relative humidity. The designed control technique is realized to ensure the required and
continuous operation of the greenhouse. Moreover, this work present, a complete mathematical modeling and simula-
tion of cooling system is introduced. In addition, a computer model based on MATLAB SIMULINK software has been
used to predict the temperature and relative humidity profiles inside the greenhouse. The results are realized the re-
quirements of the greenhouse cooling system environment.
Keywords: Pad-Fan; Evaporative Cooling System; PI Controller and Greenhouse
1. Introduction
One of the benefits of cultivating plants in a greenhouse
is the ability to control all aspects of the growing envi-
ronment. Two of the major factors influencing plant
growth are the temperature and relative humidity [1].
Different plant species have different optimum growing
temperatures and humidity. Medicinal herbs (e.g. Marjo-
ram) often need a temperature of 20˚C and about 70%
relative humidity to grow [2]. Low or high temperatures
may cause herb stress, inhibit growth, or promote the
dropping of herb leaves. Also, when the humidity is low,
this will impede the growth of the herb by stopping
photosynthesis and will eventually cause the herb to wilt.
On the other hand, high humidity will increase the poten-
tial for spreading diseases [2]. Therefore, a temperature
and humidity controllers should be used for avoiding the
previously harmful effects. The emphasis of this paper is
concerned with the control of the temperature and relative
humidity of a proposed greenhouse. Also, a complete
mathematical modeling and MATLAB SIMULINK mo-
del for the different components of the evaporative cool-
ing system in the proposed greenhouse is presented in
this paper.
2. The Proposed Greenhouse
The proposed greenhouse in this work consists mainly of
eight components as shown in Figure 1. These compo-
nents are polyethylene white coating, water impermeable
plastic material cover, woven water-porous shade curtain
material, aluminum pad, cool air fan, sumps, pump and
soil. In this work, the proposed cooling system consists
1. polyethylene white coating reflects 50% of radiation; 2. water im-
permeable plastic material cover; 3. greenhouse shading (woven wa-
ter-porous shade curtain material); 4. aluminum pad; 5. cool air fan; 6.
sumps; 7. pump; 8 soil.
Figure 1. The proposed greenhouse.

Modeling and Simulation of Evaporative Cooling System in Controlled Environment Greenhouse
68
mainly, of four components. These components are alu-
minum pad, cool air fan, pump and sump. This cooling
system can maintain a greenhouse interior temperature
and relative humidity to about 20˚C and 70% respect-
tively.
3. Cooling System Mathematical Modeling
3.1. Greenhouse Energy Balance
In order to study the variables, which determine the
greenhouse air temperature, a simplified version of the
greenhouse energy balance is formulated.
Usually, the greenhouse microclimate is represented
by the climate in the middle of the enclosure. The energy
balance in the middle area of the greenhouse can be
written as following [2-4]:
  
,1c nii opi o
gg
AQ
RKTTC
AA

 
 
 
 
 

TT (1)
where Rn,i is the incoming net radiation (in W·m–2),

is the ratio of latent heat flux to net radiation. Conse-
quently, the term “

,1 ni
R


” is the part of the incom-
ing net radiation which is transformed to sensible heat
contributing to the greenhouse air temperature increase,
To and Ti are the outside and inside greenhouse air tem-
peratures respectively (in ˚C), K (in W·m–2·˚C–1) is the
heat exchange coefficient of the greenhouse cover (by
convection and conduction), which depends on the cover
type and the wind speed, Ac is the greenhouse cover sur-
face area (in m2), Ag is the greenhouse ground surface
area (in m2), Q is the ventilation air flow rate (in m3·s–1),

is the air density (in kg·m–3), and Cp is the specific
heat of air at constant pressure (J·kg–1·˚C–1). Replacing
the term Q/Ag by Va, which represents the greenhouse
ventilation rate in (m3·s–1·m–2) of floor area, assuming
that Rn,i is very close to the incoming solar radiation Rs,i,
and that K is a function of the wind speed (K = K1 + K2u).
Equation (1) becomes:
   
,12
1c
niio paio
g
A
RK KuTTCVTT
A


 





(2)
where K1 and K2 are constants and u is the outside air
speed in m·s–1. Taking into account that Rs,i = τRs,o,
where Rs,o is the outside solar radiation (in W·m–2) and τ
is the greenhouse transmissivity to solar radiation. Thus,
using Equation (2), the greenhouse air temperature Ti can
be calculated by the following relation:



,
12
1
so
io
cg pa
R
TT
A
AKKu CV



  (3)
Dividing both numerator and denominator of second
part of Equation (3) by the term “

1cg
A
AK” we take:

 


,1
21 1
1
1
so c g
io
p
ac
RAAK
TT
g
K
KuCV KAA





 (4)
If we replace the term
 
1
1cg
K
AA  
 by 1

,
the term


21
K
K by 2

and the term



 1pcg
CKAA 



 by 3

, then we obtain:
1,
23
1
so
io
a
R
TT uV



 (5)
Equation (5) is a simplified version of the greenhouse
energy balance must be applied for the determination of
the unknown parameters 1

, 2

and 3

. Accordingly,
measurements of Ti, To, Rs,o,

, u and Va are needed, in
order to calibrate Equation (5) and statistically determine
the values of 1

, 2

and 3

.
3.2. Pad-Fan Subsystem Mathematical Modeling
3.2.1. Fan Mathematical Modeli ng
Pad-fan systems are commonly used for cooling the en-
vironment inside greenhouses to be suitable for growing
plants (e.g., nurseries, residential and commercial vege-
tables or flower production, etc). Fans push outside air
toward a wet pad, bringing cooled and humidified air
into the greenhouse. Typically, the wet pads and fans are
located on opposite walls so that the evaporative cooled
air is pulled from one end of the structure to the other.
A linear model of a simple fan consists of a mechanic-
cal equation and electrical equation as determined in the
following [5,6].
d
d
a
aaaa
I
VEIRL
t
  (6)
d
d
m
eLmm
TTBJt

   (7)
22
fan
PNDv (8)
where N is fan speed, D is fan diameter, v is specific
weight of air (11.82 N/m2), Ra is armature resistance (Ω),
La is armature inductance (H), Va is terminal voltage (V),
J is moment of inertia (kg·m2), B is damping factor of
mechanical system (N·m·s). Ia is armature current (A),
TL is load torque (N·m), Te is developed torque, and ω
is speed of rotation (rpm).
3.2.2. Pad Mathematical Modeli ng
The cooling efficiency ηc of the evaporative pad cooling
system is defined by [7,8]:
,
,,
db o
c
db owb o
TT
TT


 (9)

Modeling and Simulation of Evaporative Cooling System in Controlled Environment Greenhouse
69
where Tdb,o and Twb,o are the dry and wet bulb tempera-
tures of the air outside the greenhouse in ˚C, and T is the
dry bulb temperature of the cooled air passing over the
wet pad in ˚C. Equation (9) works well for evaporative
pad cooling systems because the cooling process (an
adiabatic process) occurs nearly at a constant wet bulb
temperature of the outside air. Equation (9) can be rear-
ranged as [7]:
control signal of the greenhouse fan and pad system vc.
The greenhouse inside temperature and humidity (Tin &
Hin) are the feedback signals to PI the controller.



,,,,
1
dboc dbowboc dbocwbo
TTT TTT   ,  (10)
The actual greenhouse inside temperature (Tin) is com-
pared with the reference temperature value (Tref = 20˚C)
through a comparator to give an error signal, which is
introduced to the system controller. In this case, the con-
troller uses the input error signal e to improve the system
response by producing the suitable control signal of the
greenhouse fan and pad system vc. Also, second com-
parator is used to compare the actual relative humidity to
the reference relative humidity (RH = 0.7) to obtain the
error signal for the PI controller. The humidity controller
operates between dehumidify and humidify modes for
removing unwanted atmospheric moisture accumulating
within the greenhouse or to add the needed moisture to
the air by means of humidification. In the dehumidifying
mode, high humidity conditions will activate moisture
fan until level drops approximately to 70% in relative
humidity. In the humidifying mode this control operates
pad-fan humidifying system by activating switches and
motors until the relative humidity increased to 70%.


,,db ocdbowb o
TT TTT   ,
,

(11)
where ∆T is the temperature drop of the air through
evaporative pad cooling systems in ˚C. Equations (10)
and (11) indicate that both T and ∆T depend only on the
dry and wet bulb temperatures of the outside air at a con-
stant cooling efficiency. By assuming a value of 80% for
the efficiency of evaporative pad cooling systems Equa-
tions (10) and (11) can be written as:
,
0.2 0.8
db owb o
TT T (12)

,,
0.8 db owb o TTT (13)
During this operation of the greenhouse cooling sys-
tem, different situations may appear.
4. Control Strategy and Simulation of the
Greenhouse 4.1. First Situation: Humidity (RH < 0.7)
The evaporative pad-fan cooling system must have ade-
quate controller for the operator to be able to adjust the
greenhouse environment to provide the best growing
conditions for the selected herb and a comfortable envi-
ronment for worker. Two conventional controllers (PI
controllers) [9-11] are employed to maintain, optimal
temperature and relative humidity (Tref = 20˚C & Href =
0.7) inside the greenhouse at any time and to overcome
the load effect of the outdoor undesirable climatic condi-
tions. The block diagram that describes the control strat-
egy is illustrated in Figure 2. Also, the MATLAB SI-
MULINK block diagram of the greenhouse cooling sys-
tem is shown in Figure 3.
The pad-fan system switches on to increase the required
relative humidity.
4.2. Second Situation: Dehumidify (RH > 0.7)
The pad-fan system switches off while roof and side
greenhouse vents are opened for ventilation. In addition,
a roof fan is used to replace the air inside the greenhouse
(loaded with humidity) with the dry air outside the green-
house.
5. Results and Discussions
The cooling system has two controllers which control the
greenhouse inside temperature and relative humidity. PI
The input signal to the suggested controllers is the
system error e, while, the output action is the required
Figure 2. Block diagram of the greenhouse controlled system using PI controller.

Modeling and Simulation of Evaporative Cooling System in Controlled Environment Greenhouse
70
Tin
NPfan volume flow rate
pad syst em
In1 Out1
Tref 2
1000
Tref 1
20
To Work space2
t
To
Relay
Rela t ive Hum i di t y
In1 Out1
RHref
0.70
RH cont rol l er
PID
Product
Mositure Fan
Tl
Va
W
Ia
Logical
Operator
NOT
Gain 2
-K-
Gain 1
-K-
Fcn5
f(u )
Fcn3
f(u )
Fcn2
f(u)
Fcn1
f(u)
.05*u^ 2
Fan Dc Motor
Tl
Va
W
Ia
0
10
0
Clock2
Clock1
Clock
power cur ve1
power curve
PID
Figure 3. MATLAB SIMULINK bloc k diagram of the greenhouse cooling system.
control technique is proposed to fix the greenhouse in-
door temperature and relative humidity at the optimal
value for growing the marjoram herb. The proposed con-
troller is fine tuned and used to achieve a good regulatory
response. The fine-tuned parameters of the temperature
and relative humidity controllers are Kp = 10, KI = 0.0001,
and Kp = 0.1, KI = 0.2 respectively.
The greenhouse outside air temperature during 24
hours is shown in Figure 4. While, Figure 5 indicates
the response of greenhouse inside temperature. It is no-
Figure 4. Ambient temperature.
Figure 5. Greenhouse indoor temperature with control tech-
nique.
ticed that, the greenhouse inside temperature tracks the
reference temperature very well. On the other hand, air
flow rate of the fan is indicated in Figure 6. It also ob-
served from this figure that the maximum air flow rate of
the fan occurs around noon at the corresponding maxi-
mum ambient temperature, while the minimum flow rate
occurs at the mid night at the corresponding minimum
ambient temperature. Also, the controlled relative hu-
midity is shown in Figure 7, which indicates a very good
performance, since there is a small overshoot, fast

Modeling and Simulation of Evaporative Cooling System in Controlled Environment Greenhouse 71
Figure 6. Air flow rate of the fan.
Figure 7. Greenhouse relative humidity with PI controller.
settling time and good tracking performance.
6. Conclusions
An evaporative cooling system is presented, in this work,
to reduce the air temperature inside the greenhouse that
affects the greenhouse environment and consequently the
growing of cultivated plants. A control technique (PI
controller) is proposed to fix the greenhouse inside tem-
perature and relative humidity at the optimal values (i.e. ,
20˚C and 70% respectively) that are suitable for growing
of marjoram herb. The fine-tuned parameters of PI con-
troller are, Kp =10, KI = 0.0001, and Kp = 0.1, KI = 0.2
respectively.
The proposed cooling system temperature controller is
designed to adjust the air volume flow rate of the fan by
adjusting the speed of the fan motor in pad-fan system; to
fix the greenhouse inside temperature at 20˚C. On the
other hand, the humidity controller operates between
dehumidify and humidify modes for removing unwanted
atmospheric moisture accumulating within the green-
house or to add the needed moisture to the air by means
of humidification, to fix the greenhouse inside relative
humidity at 70%. Also, a mathematical modeling and
MATLAB SIMULINK model for the different compo-
nents of the evaporative cooling system is presented in
this paper.
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