Citation: Soussi, M.; Chaibi, M.T.;
Buchholz, M.; Saghrouni, Z.
Comprehensive Review on Climate
Control and Cooling Systems in
Greenhouses under Hot and Arid
Conditions. Agronomy 2022,12, 626.
https://doi.org/10.3390/agronomy
12030626
Academic Editor: Carmine Guarino
Received: 8 January 2022
Accepted: 24 February 2022
Published: 3 March 2022
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agronomy
Review
Comprehensive Review on Climate Control and Cooling
Systems in Greenhouses under Hot and Arid Conditions
Meriem Soussi 1,* , Mohamed Thameur Chaibi 1, Martin Buchholz 2and Zahia Saghrouni 1
1National Research Institute for Rural Engineering, Water and Forestry (INRGREF), Agronomic Sciences and
Techniques Laboratory (LR16 INRAT 05), University of Carthage, Ariana 2049, Tunisia;
2Department of Architecture, Faculty of Planning Building Environment, Technical University of Berlin,
*Correspondence: [email protected]; Tel.: +216-20-242-677
Abstract:
This work is motivated by the difficulty of cultivating crops in horticulture greenhouses
under hot and arid climate conditions. The main challenge is to provide a suitable greenhouse indoor
environment, with sufficiently low costs and low environmental impacts. The climate control inside
the greenhouse constitutes an efficient methodology for maintaining a satisfactory environment that
fulfills the requirements of high-yield crops and reduced energy and water resource consumption.
In hot climates, the cooling systems, which are assisted by an effective control technique, constitute
a suitable path for maintaining an appropriate climate inside the greenhouse, where the required
temperature and humidity distribution is maintained. Nevertheless, most of the commonly used
systems are either highly energy or water consuming. Hence, the main objective of this work is to
provide a detailed review of the research studies that have been carried out during the last few years,
with a specific focus on the technologies that allow for the enhancement of the system effectiveness
under hot and arid conditions, and that decrease the energy and water consumption. Climate control
processes in the greenhouse by means of manual and smart control systems are investigated first.
Subsequently, the different cooling technologies that provide the required ranges of temperature and
humidity inside the greenhouse are detailed, namely, the systems using heat exchangers, ventilation,
evaporation, and desiccants. Finally, the recommended energy-efficient approaches of the desiccant
dehumidification systems for greenhouse farming are pointed out, and the future trends in cooling
systems, which include water recovery using the method of combined evaporation–condensation, as
well as the opportunities for further research and development, are identified as a contribution to
future research work.
Keywords:
greenhouse; control methods; cooling; dehumidification; energy consumption; water
recovery; arid areas
1. Introduction
Protected agriculture is a growing activity that is spreading throughout all the world
continents, and that covers an area that was estimated to be 5.2 million ha in 2014 [
1
].
Greenhouses that are covered either by plastic films in mild climates, or by glass or rigid
plastic in temperate and cold climates, extend over an area that averages 4.7 million ha
in the temperate regions of Europe, Asia, and America, and over an area that averages
364,000 ha in the Mediterranean, as well as over an area that averages 156,000 ha in the
tropical and subtropical regions [1].
Hot and dry areas are characterized by hot summers, with high solar radiation and air
temperatures. The protected agriculture in these areas has important economic value. The
greenhouse sector is also considered to be the highest fossil energy consumer among the
agricultural activities, as well as the highest consumer of water, especially if it uses evapora-
tive cooling. Therefore, the increasing limits on the supply of conventional energy sources,
Agronomy 2022,12, 626. https://doi.org/10.3390/agronomy12030626 https://www.mdpi.com/journal/agronomy
Agronomy 2022,12, 626 2 of 31
the fading groundwater stocks, and the groundwater salinization, show the vulnerability
of this economic sector.
In hot climate regions, such as in the Middle East and North African (MENA) re-
gions, and the Gulf Cooperation Countries (GCC), where the protected crop areas reached
13,000 ha and 71,297 ha, respectively, in 2014 [
1
], efficient greenhouse cooling is required
during the hot seasons to bring the internal temperature to levels that are suitable for crop
growth. Hence, traditional cooling systems in greenhouse agriculture have been adopted
in regions of harsh and variable weather conditions, and they have improved the related
climatic greenhouse parameters. Nonetheless, the traditional cooling processes, which
involve natural ventilation and passive cooling techniques, were difficult to control, and no
ideal climate conditions were guaranteed for the crops, which prohibited the cultivation of
many crops, especially those that were not resistant to the high levels of temperature [
2
,
3
].
Similarly, at high levels of relative humidity, which are commonly occurring inside green-
houses in hot countries, especially during the night period, the risk for condensation on
the leaves is high, and, therefore, the risk of botrytis and fungal and bacterial diseases
is increased.
The transpiration rate of plants is affected by the amount of moisture in the air, which
inhibits plant health, growth, and development. A relative humidity ranging between
80 and 90% during the day, and between 65 and 75% during the night, is generally rec-
ommended, but is difficult to reach technically [
4
]. The climate control of greenhouses
refers to the regulation of the main environmental parameters inside the greenhouse in
all growing climates in order to meet the crop’s growth requirements, which provides
growers with the chance to enhance the crop quality and to increase the yield. In the
last few years, numerous research works have been focused on the climate control inside
greenhouses, which is assisted by computing technology via control hardware, such as
sensors, controllers, and actuators, which allow for the monitoring of the temperature, the
relative humidity, the solar radiation, and the carbon dioxide (CO
2
) concentration [
5
], as
well as for the coordination of the equipment operation [5–7].
The cooling and heating processes, which aim to maintain the greenhouse environment
under optimal conditions, and which are suitable for crop growth, have the highest share
of energy consumption, at about 65 to 85% [
8
]. The cooling energy consumption in the
Mediterranean region is about 100,000 kWh/ha per year [
1
], while no numerical estimate
for the hot regions is available in the existing literature; however, the amount of energy
consumed definitely reaches much higher values in the hot countries. For instance, the
cooling energy consumption for protected crops in the United Arab Emirates ranged from
10.43 to 14.67 kWh/m
2
during the production cycle, while the water consumption for the
evaporative cooling ranged from 2.6 to 3.5 times the amount of water that is required for
irrigation [9,10].
This energy consumption is mainly based on electric power, which is essentially pro-
duced from fossil energy, which is increasingly being depleted, which threatens the energy
supply. Consequently, selecting and developing appropriate climate control techniques and
more efficient cooling systems that aim to decrease the high amounts of energy and water
consumption is considered to be the major challenge in the upcoming years, particularly in
hot and arid countries.
As the existing literature on protected cropping fields lacks sufficient focus on green-
house climate control and the investigation of cooling systems in hot and arid regions, the
present work attempts to address this gap by providing a thorough review of the climate
control methods, as well as the cooling techniques, on the basis of the humidification
and dehumidification of greenhouses under these environmental conditions. A detailed
review of the research studies carried out during the last few years is provided, with a
specific focus on the emerging technologies that have been proposed and the experiments
in horticulture under hot and arid conditions. The main scope of this review is to support
decisions according to the use of humidification/dehumidification and climate control
practices, and to ensure that the technologies advance in the right direction by promoting
Agronomy 2022,12, 626 3 of 31
reliable and relevant solutions for the needs of the greenhouse farming communities in
hot and arid countries. Hence, this paper discusses the several cooling processes that
allow for taking advantage of the great potential of renewable energies, which are mostly
unexploited in arid areas. Finally, the future trends in cooling systems are presented, and
the opportunities for further research and development are identified as a contribution to
future research.
2. Control of Greenhouse Operation
The climate management of the greenhouse environment depends on many parame-
ters, such as the solar radiation, the air temperature (T), the relative humidity (RH), and
the carbon dioxide (CO
2
) concentration. By controlling and regulating these parameters,
the growing conditions for the crop, as well as valuable energy savings and water use
regulation, could be achieved. However, the interaction of the parameters that affect each
other, and their dependance on the changing ambient climate, makes the monitoring and
the climate control complex. For instance, the relative humidity is dependent on the amount
of moisture that is continuously being released by the plants, on the soil evaporation, as
well as on the temperature, which is, in turn, reliant on the solar radiation and the meteoro-
logical conditions. Hence, monitoring such parameters is a real challenge in greenhouse
climate control.
2.1. Control Parameters
2.1.1. Temperature
The temperature and relative humidity are interdependent parameters that should be
controlled simultaneously. These two parameters strongly affect the growth of crops. The
main role of the temperature is to ensure the expansion of the leaves of the crops at a young
age [
11
]. Young crops are more affected by heat, as the leaf area and the root development
are insufficient for high evaporation as a reaction to heat stress. With a continuous growth
system, and by using the permanent presence of mature and younger crop stands, this
problem might be resolved. Increased humidity in closed greenhouses can relieve the
problem of evaporation for the insufficient roots of young crops.
According to Nelson [
12
], the healthy growth of most plants is observed for tempera-
tures ranging between 10 and 24
◦
C, and the ideal growth for greenhouse plants is observed
for temperatures varying between 15 and 30
◦
C [
13
]. Nelson [
12
] found that, under sunny
conditions, the difference in the temperature that is needed for crop growth between the
day and night times should be around 8 to 10
◦
C. The optimum temperature growth levels
vary according to the plant species. Table 1summarizes the temperature requirements for
selected greenhouse crops in hot and arid climates.
Heat stress is always related to the overall climate conditions, as well as to the water
availability and to the means of the CO
2
and nutrient concentrations in the air and water.
By this standard, the terminus of an optimum growth temperature is a fluctuating term.
Cooling is the most expensive and most energy-consuming intervention, and it is ahead
of all the other solutions. Accordingly, the provision of a level for good growth at high
temperatures must be considered in all stages of growth, before considering the technical
measures of space cooling.
2.1.2. Humidity
The relative humidity control inside the greenhouse is vital to the maintenance of
an optimal climate for crop growth, especially in closed greenhouses, where the water
vapor has to be withdrawn by condensation or absorption processes in order to maintain
healthy conditions. This is considered to be the most complicated climatic parameter
to be controlled. The relative humidity also triggers the air indoor temperature, as the
self-cooling evaporation of the vegetation is dependent on the vapor pressure deficit in the
air. Amani et al. [
5
] found that the optimal relative humidity inside the greenhouse should
range between 60 and 80%.
Agronomy 2022,12, 626 4 of 31
Table 1. Climate requirements for selected greenhouse crops in hot and arid regions.
Optimal T (◦C) Optimal RH (%) Optimal DLI
Reference
Day Night (mol m−2d−1)
Tomato 23–27 13–16 50–80 15–53 [14–17]
Pepper 22–30 14–16 50–70 20–30 [18–21]
Cucumber 25–30 16–18 70–90 20–53 [17,19,21]
Lettuce 24–28 13–16 60–80 16–40 [17,21,22]
Aubergine 25–28 14–16 50–60 40–55 [17–19,21,23]
Cabbage 15–16 2 70–80 42 [21,24]
Courgettes 20–22 17–18 65–80 NA [19]
Beans 22–26 16–18 70–80 19–24 [19]
Peas 25–30 16–18 70–80 42 [21]
Strawberry 20–26 13–16 50–65 17–20 [25–27]
Melon 32 14–20 65–75 58–64 [17,21,28,29]
Vadiee and Martin [
30
] analyzed the influence of the humidity rate on the crop growth.
A lower relative humidity rate inhibits the crop growth, induces water stress in the crops,
and reduces the stem length and leaf sizes. In climates with high moisture levels, the vapor
condensation on the greenhouse cover increases and condensate decreases cause a decrease
in the transmittance of the normal incident radiation of the covering materials, which
reaches 23% [
31
]. Moreover, the excess of the moisture in the greenhouse is considered to
be one of the main factors that causes the fungi’s appearance, as well as other serious crop
diseases [
4
,
32
]. On the other hand, the problem of the condensation droplets from the roof
might be solved by the provision of a modified greenhouse shape, with an increased slope
for droplet removal, especially if higher amounts of humidity can be tolerated in order
to reduce the complexity of the control. The use of a covering material with a high drip
resistance can also provide an effective solution to condensation dripping, as it contains
special additives that prevent the formation of small water droplets, and it makes the condensate
stream down in a continuous thin layer onto the sides of the greenhouse [
4
,
33
,
34
]. During the
night, the roof temperature is lower than the leaf temperature, and condensation mainly
appears on the roof. The removal of condensate can be considered an easy method of air
dehumidification.
It is important to stipulate that the ideal humidity levels for a plant are highly depen-
dent on the water stresses, extreme weather conditions, the danger of fungus/pest/insect
attack, the maturity stage, and the plant growth stage [
5
,
13
]. Hence, the ideal growth of
a plant requires ideal humidity levels, which are reported in terms of the vapor pressure
deficit (VPD), which is defined as the difference between the water vapor pressure at
saturation (P
sat
), and the actual water vapor pressure at the temperature of the greenhouse
(P) [
5
]. The relationship between the temperature and humidity always has to be reflected.
For example, a 20
◦
/80% relative humidity means that another 3 g of water can be absorbed
by one m
3
of air, while, at 30
◦
/80%, it is 5.5 g, and, at 40
◦
, it is already 11.2 g, and at this
increased level, it is much more apart from the saturation.
A high vapor pressure deficit increases for low humidity and high temperature values,
which, in turn, increase the plant transpiration and vice versa. As is shown in Figure 1,
the VPD that is required for greenhouse plant growth ranges from 0.45 to 1.25 kPa, and
it ideally varies from 0.8 to 0.9 kPa [
13
]. Therefore, it is extremely important to use the
appropriate technology to control the relative humidity, the temperature, and the resulting
VPD, especially inside a closed greenhouse, in order to avoid crop damage and to guarantee
their proliferation.
Agronomy 2022,12, 626 5 of 31
Agronomy 2022, 12, x FOR PEER REVIEW 5 of 32
Figure 1. Normal and ideal greenhouse growth zones according to vapor pressure deficit levels [13].
2.1.3. Carbon Dioxide Concentration
The CO2 enrichment in the greenhouse is a crucial parameter since it has a positive
effect on the crop growth, provided that other growth factors, such as the water supply,
the nutrient supply, and especially the sunlight exposure, are sufficiently satisfied. Better
control of the required CO2 concentration is beneficial to increasing crop growth rates and
to enhancing the production quality [35,36]. The CO2 amounts should be supplied to the
greenhouse crops to compensate for the strong reduction in the CO2 by photosynthesis,
especially when good ventilation measures are lacking.
The benefit of the CO2 enrichment is reflected mainly in the higher crop production
through photosynthesis enhancement. Kirschbaum [35] states that, under elevated CO2
concentrations, the photosynthetic rate increase can exceed 50%, when compared to plants
grown under normal CO2 concentrations. Drake et al. [37] proved, through 60 experiments
that were performed under a CO2 enrichment of around 700 µmol mol−1, that the increase
in the photosynthesis of plants reaches 58%, compared to plants under ambient condi-
tions.
Several previous studies have quantified the CO2 amount to supply in greenhouses,
and they have proven that the economic-based optimal CO2 concentration for healthy crop
growth ranges between 700 and 900 µmol mol−1, depending on the plant species
[34,35,38,39]. At CO2 concentrations higher than 1000 µmol mol−1, growth reduction and
leaf injuries can occur for some species [35]. Under increased CO2 concentrations, multiple
positive effects on plants have been reported in the literature, such as increased numbers
of leaves, lateral branching, and plant heights, and advanced flowering dates and high
fruit yields of good quality [35,40–42]. Even crops with partially closed stomata can up-
take sufficient amounts of CO2, and can thereby provide considerable growth rates, even
under increased temperatures or reduced water availability.
Sanchez-Guerrero et al. [43] studied the effect of CO2 enrichment on cucumber cul-
ture in a greenhouse during the autumn/winter of the Mediterranean climate. A CO2 en-
richment was applied to the greenhouse to maintain a CO2 concentration ranging between
600 and 700 µmol mol−1 when the vents were closed, and of 350 µmol mol−1 when the vents
were open. The mean daytime values averaged between 400 and 500 µmol mol−1, while,
Figure 1.
Normal and ideal greenhouse growth zones according to vapor pressure deficit levels.
Reprinted with permission from Ref. [13]. Copyright 2016 Elsevier.
2.1.3. Carbon Dioxide Concentration
The CO
2
enrichment in the greenhouse is a crucial parameter since it has a positive
effect on the crop growth, provided that other growth factors, such as the water supply,
the nutrient supply, and especially the sunlight exposure, are sufficiently satisfied. Better
control of the required CO
2
concentration is beneficial to increasing crop growth rates and
to enhancing the production quality [
35
,
36
]. The CO
2
amounts should be supplied to the
greenhouse crops to compensate for the strong reduction in the CO
2
by photosynthesis,
especially when good ventilation measures are lacking.
The benefit of the CO
2
enrichment is reflected mainly in the higher crop production
through photosynthesis enhancement. Kirschbaum [
35
] states that, under elevated CO
2
concentrations, the photosynthetic rate increase can exceed 50%, when compared to plants
grown under normal CO
2
concentrations. Drake et al. [
37
] proved, through 60 experiments
that were performed under a CO
2
enrichment of around 700
µ
mol mol
−1
, that the increase
in the photosynthesis of plants reaches 58%, compared to plants under ambient conditions.
Several previous studies have quantified the CO
2
amount to supply in greenhouses,
and they have proven that the economic-based optimal CO
2
concentration for healthy crop
growth ranges between 700 and 900
µ
mol mol
−1
, depending on the plant species [
34
,
35
,
38
,
39
].
At CO
2
concentrations higher than 1000
µ
mol mol
−1
, growth reduction and leaf injuries
can occur for some species [
35
]. Under increased CO
2
concentrations, multiple positive
effects on plants have been reported in the literature, such as increased numbers of leaves,
lateral branching, and plant heights, and advanced flowering dates and high fruit yields
of good quality [
35
,
40
–
42
]. Even crops with partially closed stomata can uptake sufficient
amounts of CO
2
, and can thereby provide considerable growth rates, even under increased
temperatures or reduced water availability.
Sanchez-Guerrero et al. [
43
] studied the effect of CO
2
enrichment on cucumber culture
in a greenhouse during the autumn/winter of the Mediterranean climate. A CO
2
enrich-
ment was applied to the greenhouse to maintain a CO
2
concentration ranging between
600 and 700
µ
mol mol
−1
when the vents were closed, and of 350
µ
mol mol
−1
when the
vents were open. The mean daytime values averaged between 400 and 500
µ
mol mol
−1
,
while, in the nonenriched greenhouse, they ranged between 285 and 300
µ
mol mol
−1
[
43
,
44
].
Sanchez-Guerrero et al. found that the dynamic control strategy of the CO
2
enrichment
Agronomy 2022,12, 626 6 of 31
of the greenhouse culture permits an increase in the fresh yield of up 19%, compared to
the nonenriched one. Ohyama et al. [
45
] mention that, in European countries, the CO
2
concentrations are controlled in order to maintain the equilibrium between the CO
2
content
inside the greenhouse and the atmosphere. However, the control of the CO
2
concentration
is not usually set in hot climates, where systems with high ventilation rates are employed.
Very high levels of CO
2
(>1000 ppm) are only feasible in closed greenhouses. Even in
semiclosed greenhouses, the ventilation losses are high, and the supply of CO
2
at this level
is uneconomical. Furthermore, given the close relationship between the ventilation rate
and the CO
2
concentration inside the greenhouse, it is strongly recommended that these
two parameters should be controlled simultaneously [30].
2.1.4. Solar Radiation
Solar radiation is the first and main climate parameter for evaluating the suitability
of a region for protected cropping [
4
]. Direct solar radiation that is intercepted in the
greenhouse is the first source of heat gain, and it makes the highest contribution to the
increase in the daytime temperature of the protected cropping environment. Moreover,
important amounts of solar radiation are intercepted and stored in the soil that are to
be released during the night [
9
]. Inside greenhouses in dry climates, the solar radiation
passing beside the leaf area is directly transferred into sensible heat, and it heats up the
air surrounding the soil. The solar radiation that is intercepted is also the driving force
behind the photosynthesis process, and it harnesses the solar radiation energy and turns it
into the chemical energy that is necessary for plant metabolism, growth, and development.
The part of electromagnetic radiation that can be used as the source of energy for photo-
synthesis by green plants is the photosynthetically active radiation (PAR). It is expressed
either in terms of the photosynthetic photon flux density (PPFD) (mmol photons,
m−2s−1
),
or in photosynthetically active radiation (PAR) (mmol m
−2
s
−1
or W/m
2
) [
46
]. PAR ir-
radiance depends upon several factors, such as the meteorological seasonal conditions,
the geographical conditions, and the conceptual greenhouse characteristics. The cumula-
tive daily PAR radiation incident of the plants is known as the daily light integral (DLI)
(mol m
−2
d
−1
). The typical indoor greenhouse DLI values that are required for different
crops in hot and arid zones under periods of long daylight (16 h) are presented in Table 1.
2.2. Greenhouse Climate Control Methods
The climate control in the greenhouse is mainly used to guarantee the optimal envi-
ronmental conditions for crop growth, and to minimize the water and energy consumption.
There are different types of control systems (manual, automatic, or intelligent control),
where the greenhouse is equipped with Internet data monitoring [
47
,
48
]. The hardware of
the control system involves sensors, controllers, and actuators. The controlled parameters,
namely, the T
◦
, RH, CO
2
concentration, and the air flowrate, are measured by multiple
sensors that are placed inside and outside the greenhouse at different positions, including
at the planting level, as well as at the inlets and the outlets of the control components. The
control components are mainly the cooling, heating, and ventilation systems, as well as the
shading and fogging systems.
Detailed studies have been carried out to investigate the coupling characteristics
between the environmental parameters, and they show that, in order to enhance the
accuracy of the control strategy for low-energy greenhouses, it is necessary to study the
multiparameter coupling [
47
,
49
]. Several control algorithms and numerical models have
been performed to simulate the complexity of the control of the greenhouse environment.
The accuracy of these models ensures the control management and minimizes the energy
consumption whilst providing a pleasant environment for the plants. Beveren et al. [
50
]
found that by using the optimal control method, the cooling energy consumption and the
CO2injection could be reduced by up 15% and 10%, respectively.
Hence, effective control methods, particularly smart greenhouse monitoring, have
become a new trend for ensuring clean crop production. With these advanced control
Agronomy 2022,12, 626 7 of 31
methods, which are integrated into the greenhouse and combined with renewable resources,
the energy consumption in the greenhouse will be negligible.
3. Cooling Systems in Greenhouses
The temperature and relative humidity control inside the greenhouse are operated
by different cooling technologies, such as ventilation, external cooling systems using heat
exchangers, and evaporative and desiccant systems. The cooling processes can be classi-
fied into two main categories: “passive” and “active” systems. “Passive” cooling in the
greenhouse mainly refers to the design approach (shape, covering materials, openings,
passive night cooling of the soil) to reducing the temperature inside the greenhouse without
the consumption of additional water or power. “Active” cooling refers to all the cooling
systems that use electrical equipment, such as pumps, fans, and heat pumps. The integra-
tion of passive cooling techniques followed by active cooling could simultaneously ensure
adequate conditions for crop growth and a decline in the energy consumption.
3.1. Passive Cooling Systems
Several design features strongly contribute to the decrease in the cooling requirements,
namely, the shape and the geometry of the greenhouse, its location and orientation, the
covering materiel, and the pattern of the openings. According to Choab et al. [
51
] and
Sethi [52], independently of the considered location, the Quonset form corresponds to the
minimal values of the temperature and solar collection, as opposed to the uneven-span
shape, which allows for maximum solar capture, as well as high heat records. As for the
orientation, it is concluded in many comparative studies that the E-W orientation is more
convenient at all latitudes, except near the equator, where summer is weakly insolated
compared to winter. Stanciu et al. [
53
] studied the effect of the greenhouse orientation,
with respect to the E-W axis, on its required heating and cooling loads for an even-span
greenhouse of 180 m
2
at 44.25
◦
N latitude. They found that, for the E-W orientation,
the cooling and heating requirements were reduced by 125 kWh/day in June, and by
87 kWh/day in January, respectively. These energy savings for the E-W orientation, with
respect to the N-S axis, in both the summer and winter periods, has been confirmed by many
previous studies [
52
,
54
–
56
]. Other directions have been proven to be more appropriate
when the wind direction to avoid storm losses is considered. For instance, in southern
Algeria, greenhouses are S–N oriented because of the direction of stormy winds.
In hot and arid regions, greenhouses have several cladding material types, such
as glass, fiberglass, polyethylene, and polycarbonate, with various optical and thermal
performances. Shading and reflection are the basic concepts in the reduction and avoidance
of intense solar radiation, and they therefore reduce the cooling requirements. A reduction
in the greenhouse air temperature of 10% is guaranteed by a 50% shaded roof [
57
]. Several
shading methods have been used and investigated, such as roof shading, external shading,
and different material shading nets. It was proven that external shading is more effective
in radiation control than under-roof and side-wall placements [
58
]. For hot climates, white
painted roofs that are alternatively combined with external black shading nets, or internal
aluminized screens, can guarantee the necessary shading [
59
]. Painting or spraying the
greenhouse with white paint, which is known as “whitewashing”, is also a common
shading practice for regulating the greenhouse climate in hot and arid regions, and even
in cold regions with periods of intense heat [
59
–
61
]. At the start of the hot season, lime or
white painting cover is sprayed onto the external surface of the glass or plastic-covered
greenhouse, which allows for a general level of shading. Whitewashing is, therefore, a
passive and low-cost shading technique that reduces the indoor temperature, the VPD, and
the light supply [
62
]. The resulting minimized photosynthesis is accepted, as overheating
is the more stressful problem during heat waves.
Specific covering materials can also provide the appropriate shading/reflection effect,
namely, colored plastic films [
62
,
63
], and near infrared radiation (NIR)-filtering plastic films.
The selection of the appropriate covering material is realized on the basis of the amount
Agronomy 2022,12, 626 8 of 31
of intercepted radiation, as well as the type of crops. The minimum transmissivity of the
covering material is about 0.7 [
64
]. Al-Amri [
65
] demonstrated that the thickness of the
covering materials ranges between 0.1 and 0.2 mm for ultraviolet-stabilized polyethylene,
between 6 and 10 mm for polycarbonate sheets, and 4 mm for glass. Several covering
materials are also under experimentation in order to prove their optical performances.
NIR-filtering films are recommended for harsh climates since they reflect the near infrared
radiation without affecting the crop photosynthesis or the plant growth. As a result, the
air temperature inside the greenhouse is lowered by 5
◦
C, and the energy requirements
for cooling are reduced by 8% [
66
,
67
]. However, the use of movable NIR reflectors during
wintertime is recommended to prevent increases in the heating requirements [66,67].
Applying passive cooling strategies at the greenhouse design step helps the growers
to reduce the cooling requirements and provides more suitable operating conditions for
active cooling systems.
3.2. Ventilation Systems
Ventilation systems are commonly used to maintain a suitable environment inside
the greenhouse, especially for air dehumidification and for decreasing the temperature.
Two types of ventilation systems are used in greenhouses: natural ventilation and forced
ventilation. Natural ventilation is carried out mainly via the roof or the side-wall openings,
without any external input. Hence, it is the simplest and most cost-effective ventilation
technique for controlling the humidity and temperature. However, in greenhouse environ-
ments with high humidity levels, in certain conditions, forced ventilation is required and
can be carried out using air fans.
3.2.1. Natural Ventilation
Natural ventilation inside the greenhouse is driven by the wind and the internal
buoyancy that is created by the air density gradient. The air density gradient is created by
increasing the temperature and moisture values. Natural ventilation can be considered as a
passive cooling system since it is based on a greenhouse design that does not resort to the use
of equipment. Many works have been carried out on natural greenhouse ventilation systems,
which are controlled by the window openings at the side-wall and roof levels [
68
–
72
]. The
researchers found that the climate of greenhouses is highly impacted by the rate of the air
exchange through natural convection, which mainly depends on the area of the openings.
Therefore, the total area of the vents should be 15 to 30% of the ground area [
72
], with
simultaneously different types of openings (side, ridge, roof), rather than one unique type
of vent (Figure 2).
Agronomy 2022, 12, x FOR PEER REVIEW 8 of 32
VPD, and the light supply [62]. The resulting minimized photosynthesis is accepted, as
overheating is the more stressful problem during heat waves.
Specific covering materials can also provide the appropriate shading/reflection effect,
namely, colored plastic films [62,63], and near infrared radiation (NIR)-filtering plastic
films. The selection of the appropriate covering material is realized on the basis of the
amount of intercepted radiation, as well as the type of crops. The minimum transmissivity
of the covering material is about 0.7 [64]. Al-Amri [65] demonstrated that the thickness of
the covering materials ranges between 0.1 and 0.2 mm for ultraviolet-stabilized polyeth-
ylene, between 6 and 10 mm for polycarbonate sheets, and 4 mm for glass. Several cover-
ing materials are also under experimentation in order to prove their optical performances.
NIR-filtering films are recommended for harsh climates since they reflect the near infrared
radiation without affecting the crop photosynthesis or the plant growth. As a result, the
air temperature inside the greenhouse is lowered by 5 °C, and the energy requirements
for cooling are reduced by 8% [66,67]. However, the use of movable NIR reflectors during
wintertime is recommended to prevent increases in the heating requirements [66,67].
Applying passive cooling strategies at the greenhouse design step helps the growers
to reduce the cooling requirements and provides more suitable operating conditions for
active cooling systems.
3.2. Ventilation Systems
Ventilation systems are commonly used to maintain a suitable environment inside
the greenhouse, especially for air dehumidification and for decreasing the temperature.
Two types of ventilation systems are used in greenhouses: natural ventilation and forced
ventilation. Natural ventilation is carried out mainly via the roof or the side-wall open-
ings, without any external input. Hence, it is the simplest and most cost-effective ventila-
tion technique for controlling the humidity and temperature. However, in greenhouse en-
vironments with high humidity levels, in certain conditions, forced ventilation is required
and can be carried out using air fans.
3.2.1. Natural Ventilation
Natural ventilation inside the greenhouse is driven by the wind and the internal
buoyancy that is created by the air density gradient. The air density gradient is created by
increasing the temperature and moisture values. Natural ventilation can be considered as
a passive cooling system since it is based on a greenhouse design that does not resort to
the use of equipment. Many works have been carried out on natural greenhouse ventila-
tion systems, which are controlled by the window openings at the side-wall and roof lev-
els [68–72]. The researchers found that the climate of greenhouses is highly impacted by
the rate of the air exchange through natural convection, which mainly depends on the area
of the openings. Therefore, the total area of the vents should be 15 to 30% of the ground
area [72], with simultaneously different types of openings (side, ridge, roof), rather than
one unique type of vent (Figure 2).
Figure 2. Different types of greenhouse openings: (a) roof vents; (b) ridge vents; (c) side vents.
The indoor climate conditions also depend on many factors, such as the greenhouse
design and direction, the wind direction and speed, the solar radiation, the temperature
Figure 2. Different types of greenhouse openings: (a) roof vents; (b) ridge vents; (c) side vents.
The indoor climate conditions also depend on many factors, such as the greenhouse
design and direction, the wind direction and speed, the solar radiation, the temperature
gradient between the inside and outside air, and the plant evapotranspiration. Accordingly,
natural ventilation is not sufficient to maintain the desired climate and it becomes more
energy intensive, especially in cold periods [
5
]. For instance, in Sweden and Spain, the
energy consumption designed for greenhouse dehumidification via natural ventilation is
estimated to be between 20 and 30% of the total energy consumption [5,71,73].
Agronomy 2022,12, 626 9 of 31
3.2.2. Forced Ventilation
Forced ventilation is ensured by fans or ventilators that are used for heat removal and
for the relative humidity control (Figure 3). According to A. Santosh et al. [
74
], maintaining
a suitable rate of ventilation is also essential to avoiding the inequitable CO
2
distribution,
and to equalizing the inside greenhouse temperature to the outside temperature. The
forced ventilation can ensure a homogeneous air distribution inside the greenhouse better
than natural ventilation can [
75
]. It is commonly used on summer days in hot areas to
dehumidify and cool the greenhouse. Forced ventilation ensures the control of the indoor
environment to prevent overheating in greenhouse growing environments, and it can either
replace other common cooling systems, for instance, fans and pad systems, or contribute
to lowering their energy consumption [
76
,
77
]. According to Flores-Velazquez et al. [
78
],
maintaining a comfortable climate inside the greenhouse is induced by combining the roof
opening and the ventilators, which is better than using forced ventilators only.
Agronomy 2022, 12, x FOR PEER REVIEW 9 of 32
gradient between the inside and outside air, and the plant evapotranspiration. Accord-
ingly, natural ventilation is not sufficient to maintain the desired climate and it becomes
more energy intensive, especially in cold periods [5]. For instance, in Sweden and Spain,
the energy consumption designed for greenhouse dehumidification via natural ventila-
tion is estimated to be between 20 and 30% of the total energy consumption [5,71,73].
3.2.2. Forced Ventilation
Forced ventilation is ensured by fans or ventilators that are used for heat removal
and for the relative humidity control (Figure 3). According to A. Santosh et al. [74], main-
taining a suitable rate of ventilation is also essential to avoiding the inequitable CO2 dis-
tribution, and to equalizing the inside greenhouse temperature to the outside tempera-
ture. The forced ventilation can ensure a homogeneous air distribution inside the green-
house better than natural ventilation can [75]. It is commonly used on summer days in hot
areas to dehumidify and cool the greenhouse. Forced ventilation ensures the control of
the indoor environment to prevent overheating in greenhouse growing environments,
and it can either replace other common cooling systems, for instance, fans and pad sys-
tems, or contribute to lowering their energy consumption [76,77]. According to Flores-
Velazquez et al. [78], maintaining a comfortable climate inside the greenhouse is induced
by combining the roof opening and the ventilators, which is better than using forced ven-
tilators only.
Figure 3. A greenhouse equipped with forced ventilation devices at the Institute of Arid Regions
(Kebili, Tunisia).
3.3. Heat Exchangers
The heat transfer is one of the most influential driving processes of the cooling sys-
tems in greenhouses, especially in hot climates, where the heat flow received by the green-
house is highly prominent. In cooling systems, the heat transfer occurs between the green-
house inner air and a second fluid at a lower temperature, which is the cold source of the
exchange. This can be not only ambient air, but also cooling water, e.g., from deep ocean
water, or water that is cooled within an external cooling tower.
3.3.1. Air-to-Air Heat Exchangers
An air-to-air heat exchanger is recognized as a heat recovery ventilator (HRV), and it
constitutes an alternative technique to the condensation mechanism, and it is preferably
used in cold or mild climates. Many works have considered that HRVs are an efficient
technique for the dehumidification of the greenhouse, particularly during cold periods.
Moreover, the air-to-to-air heat exchange process is more valuable in closed greenhouses,
where the energy inside the greenhouse is extracted and replaced by the incoming cool
air. Air-to-air heat exchangers are usually made up of plastic or metallic plates, and they
Figure 3.
A greenhouse equipped with forced ventilation devices at the Institute of Arid Regions
(Kebili, Tunisia).
3.3. Heat Exchangers
The heat transfer is one of the most influential driving processes of the cooling systems
in greenhouses, especially in hot climates, where the heat flow received by the greenhouse
is highly prominent. In cooling systems, the heat transfer occurs between the greenhouse
inner air and a second fluid at a lower temperature, which is the cold source of the exchange.
This can be not only ambient air, but also cooling water, e.g., from deep ocean water, or
water that is cooled within an external cooling tower.
3.3.1. Air-to-Air Heat Exchangers
An air-to-air heat exchanger is recognized as a heat recovery ventilator (HRV), and it
constitutes an alternative technique to the condensation mechanism, and it is preferably
used in cold or mild climates. Many works have considered that HRVs are an efficient
technique for the dehumidification of the greenhouse, particularly during cold periods.
Moreover, the air-to-to-air heat exchange process is more valuable in closed greenhouses,
where the energy inside the greenhouse is extracted and replaced by the incoming cool air.
Air-to-air heat exchangers are usually made up of plastic or metallic plates, and they operate
either in the crossflow or the counterflow mode. Furthermore, the air stream circulation and
the turbulence require additional ventilation, specifically in no-wind conditions. Hence,
this cooling process entails heat exchanger costs, as well as ventilation equipment and the
related energy costs [79].
Numerous studies have dealt with the effectiveness of heat exchangers. For instance,
Campen et al. [
80
] investigated the effect of using the exchanger in the greenhouse on
economic incomes. According to Amani et al. [
5
], operating the air-to-air exchanger during
the early morning and in the summertime becomes more economically relevant, as it
Agronomy 2022,12, 626 10 of 31
requires more ventilation input to increase the dehumidification capacity. A rotary air-to-
air exchanger, which was modeled by Maslak and Nimmermark [
71
] and was intended
for greenhouse dehumidification, exhibits around a 70% effectiveness. They found that,
compared to natural ventilation, this process leads to a reduction of up to 17% of the overall
energy consumption. Thus, the air-to-air heat exchanger is an efficient way to maintain a
comfortable climate in greenhouses in cold and hot climates. This exchanger could also be
coupled with another cooling process to cover the cost and to achieve valuable results.
3.3.2. Air-to-Liquid Heat Exchangers
Cooling systems that involve air-to-liquid heat exchangers generally use cold water as
the cooling medium, such as seawater, or cooled water that is circulated from a compression
or an absorption chiller, or day-to-night cold storage.
Several theoretical and experimental studies have been carried out to investigate the
various cooling mediums and their operational limits. For instance, in India, Sethi and
Sharma [
62
,
81
] performed an experiment on a cooling and heating system using the aquifer
water at 24
◦
C as the heat transfer fluid in the air-to-liquid heat exchanger to ensure the
greenhouse heating on winter nights and cooling during summer days. The aquifer coupled
with the cavity flow heat exchanger system successfully maintained the greenhouse air
temperature at between 7 and 9
◦
C above the outside air temperature during winter nights,
and between 6 and 7
◦
C below the outside air temperature during extreme summer days.
The relative humidity was also decreased by 10–12% in the winter, and it averaged 60%
during extreme summer days, while it was about 25% on the outside.
Albright and Behler [
82
] modeled and performed an experiment on two air-to-liquid
heat exchangers that were joined as an air liquid-to-air heat exchanger in a 240 m
2
green-
house in the United States. The experimented liquid was a mixture of 50% water and a com-
mon automotive antifreeze, with a specific heat of 3140 J/kg K. The achieved heat exchanger
effectiveness values ranged between 0.4 and 0.5. According to Buchholz [
79
], the green-
houses cooling systems that use air-to-cold-water heat exchangers require
50 L h−1m−2
at
a temperature difference of 20
◦
C. This is traduced by 0.5 m
3
day
−1
m
−2
of cold water for
10 h of the cooling operation. Hence, the energy required for pumping this amount of cold
water is huge, particularly in the case of noncoastal regions. An important fact is also to
be taken into consideration in hot and arid conditions, which is, namely, the heat loss in
the piping that supplies the heat exchanger by the liquid cooling medium, especially in
extremely hot temperatures. Consequently, emphasis should be placed on the nature and
the cost of the piping material and insulation.
3.4. Heat Pump Cooling Systems
Heat pumps, or mechanical refrigeration systems, are electrically driven units that
operate according to the vapor compression process for either heating or cooling pur-
poses. Thus, controlled condensation on a cold surface that causes air dehumidification
is guaranteed by mechanical cooling units. The main disadvantage of these electrically
driven units is the significant amount of energy that they consume. In fact, condensation
is initiated when the warm indoor air is cooled down to its dew point. Consequently, the
amount of energy consumption consists of the energy needed for cooling the air, plus the
energy needed for the phase change from vapor to liquid water, which corresponds to
680 kWh/m
3
[
79
]. The majority of the research agrees that, although mechanical cooling is
efficient at controlling the temperature, humidity, and CO
2
levels, particularly in closed
greenhouses, the energy consumption remains intensive and also uneconomical, except
if the heat pumps are operated in the heating mode too, or if the heat released during
the cooling mode is also used, e.g., for hot water production for domestic or industrial
use [73,83–86].
Absorption heat pumps, which are commonly known as “absorption chillers”, are also
used for cooling, either in buildings or in greenhouses. Absorption chillers are thermally
driven units compared to electric heat pumps. Depending on the energy source, several
Agronomy 2022,12, 626 11 of 31
types of absorption chillers are distinguished, such as: direct-fired, hot water, steam, and
exhaust gas chillers. Absorption chillers have expansive integration in the industrial and
buildings fields, but few research cases have been performed on the integration of this
process in greenhouse cooling. For instance, Campiotti et al. [
87
] studied an absorption
plant that consisted of a single-effect water/lithium bromide absorption chiller, which was
coupled to evacuated tube solar collectors that were used for cooling a 300 m
2
greenhouse
in Italy. Campiotti et al. [
87
] found that this system provided satisfactory results when
considering the achieved energy savings. It is important to note that the absorption system
was performed to cover solely the cooling of the air volume surrounding the crop, and not
the entire greenhouse.
According to several experimental and numerical studies, the main advantage of
absorption chillers is their capability to be coupled with solar thermal collectors (Figure 4).
However, the adoption of solar collectors to generate high temperatures and to operate the
system efficiently, particularly for agriculture use, is often constrained by economic and
technical barriers [79,88,89].
Agronomy 2022, 12, x FOR PEER REVIEW 11 of 32
kWh/m3 [79]. The majority of the research agrees that, although mechanical cooling is ef-
ficient at controlling the temperature, humidity, and CO2 levels, particularly in closed
greenhouses, the energy consumption remains intensive and also uneconomical, except if
the heat pumps are operated in the heating mode too, or if the heat released during the
cooling mode is also used, e.g., for hot water production for domestic or industrial use
[73,83–86].
Absorption heat pumps, which are commonly known as “absorption chillers”, are
also used for cooling, either in buildings or in greenhouses. Absorption chillers are ther-
mally driven units compared to electric heat pumps. Depending on the energy source,
several types of absorption chillers are distinguished, such as: direct-fired, hot water,
steam, and exhaust gas chillers. Absorption chillers have expansive integration in the in-
dustrial and buildings fields, but few research cases have been performed on the integra-
tion of this process in greenhouse cooling. For instance, Campiotti et al. [87] studied an
absorption plant that consisted of a single-effect water/lithium bromide absorption chiller,
which was coupled to evacuated tube solar collectors that were used for cooling a 300 m2
greenhouse in Italy. Campiotti et al. [87] found that this system provided satisfactory re-
sults when considering the achieved energy savings. It is important to note that the ab-
sorption system was performed to cover solely the cooling of the air volume surrounding
the crop, and not the entire greenhouse.
According to several experimental and numerical studies, the main advantage of ab-
sorption chillers is their capability to be coupled with solar thermal collectors (Figure 4).
However, the adoption of solar collectors to generate high temperatures and to operate
the system efficiently, particularly for agriculture use, is often constrained by economic
and technical barriers [79,88,89].
Figure 4. An absorption cooling system coupled with parabolic trough solar collectors [88].
3.5. Evaporative Cooling
Evaporative cooling systems, which are based on the conversion of sensible heat into
latent heat by means of water evaporation, are commonly used to maintain a comfortable
climate for crop growth in hot and dry areas. There are many types of evaporative cooling
systems that are used to cool greenhouses, such as fan and pad systems, fogging systems,
and roof evaporation cooling systems. According to Ganguly et al. [90], the integration of
an evaporative cooling system with natural and forced ventilations, shading, and dehu-
midification, provided a favorable climate for better crop yields. Ghani et al. [64] confirm
that the evaporative cooling coupled with ventilation devices (fans, roof openings) is more
Figure 4.
An absorption cooling system coupled with parabolic trough solar collectors. Reprinted
with permission from Ref. [88]. Copyright 2013 Elsevier.
3.5. Evaporative Cooling
Evaporative cooling systems, which are based on the conversion of sensible heat into
latent heat by means of water evaporation, are commonly used to maintain a comfortable
climate for crop growth in hot and dry areas. There are many types of evaporative cooling
systems that are used to cool greenhouses, such as fan and pad systems, fogging systems,
and roof evaporation cooling systems. According to Ganguly et al. [
90
], the integration of
an evaporative cooling system with natural and forced ventilations, shading, and dehu-
midification, provided a favorable climate for better crop yields. Ghani et al. [
64
] confirm
that the evaporative cooling coupled with ventilation devices (fans, roof openings) is more
efficient for climate control in greenhouse in hot climates [
91
]. Table 2summarizes the
performances of the different types of evaporative cooling systems that are used in green-
houses under hot and arid climate conditions. Different systems permit a decrease in the
temperature and contribute to the energy efficiency.
Agronomy 2022,12, 626 12 of 31
Table 2. Evaporative cooling systems used in greenhouses in arid and hot regions.
Types Locations Advantages Limitation Performance
Indirect–direct
evaporative cooling
(IDEC) unit using
groundwater
Baghdad, Iraq [60] Water saving
Dependent on the
groundwater
availability
Higher cooling efficiency
as compared to the
direct evaporative cooler
Fan and pad
evaporative
cooling system
Khartoum, Sudan [92]Low energy
consumption
Short life cycle
of the pads
Cooling efficiency
up to 90%
Fan and pad systems Shanghai [93]Energy saving and
easily adjustable
Not sufficient in
humid climates,
and it is necessary
to add shading
Decrease in
temperature
to 27–29 ◦C
Fan and pad systems Oman [94]Energy saving and
fresh water production
Decrease in water
temperature of 3 ◦C,
and increase in the
RH to 100%
3.5.1. Fan and Pad Systems
The fan and pad evaporative cooling system is considered to be an efficient control
and cooling method for greenhouse climates where the temperature exceeds 40
◦
C. Its
principle is to place a wet pad and fans in opposite positions in the greenhouse (Figure 5).
The water evaporation through the wet pad material leads to a decrease in the temperature,
and to the humidification of the air inside the greenhouse [
64
]. There are two types of
evaporative cooling systems: direct and indirect. Indeed, a direct evaporative cooling effect
is obtained through a crossflow water-to-air heat exchanger. The air that is ventilated by
the fans circulates through a porous material, and the surface is humidified by a water
drip that is pumped vertically via a hydraulic pump. However, the indirect evaporative
cooler is a heat exchanger that decreases the temperature while maintaining a constant
humidity level.
Agronomy 2022, 12, x FOR PEER REVIEW 13 of 32
Figure 5. Fan and pad cooling systems for greenhouses at the Institute of Arid Regions (Kebili, Tu-
nisia).
Rafique et al. [95] report that the use of only an indirect evaporative cooler in hot and
humid climates is not sufficient, and that the use of only a direct evaporative cooler is not
economical. For this reason, they propose a combination of the two configurations. Hui
and Cheung [96] found that this combination led to the attainment of cool air better than
using one type of evaporative cooler only. The cooling system parameters, such as the pad
area, the fan power, the flowrates, the distribution system, and the pump capacity, should
be rigorously calculated and selected in order to guarantee the sufficient humidification
of the pad, and to avoid pad clogging. For instance, according to Kittas et al. [34], it is
recommended that the pad area correspond to 1 m2 per 20–30 m2 of the greenhouse area,
and that the air flowrate range from 120 to 150 m3 per m2 of the greenhouse area per hour.
3.5.2. Roof Evaporative Systems
Roof evaporative cooling is performed by circulating a thin film of water throughout
the greenhouse roof surface (Figure 6). The solar thermal energy, which is absorbed
through the external surface, is therefore reduced, and the roof, and the surrounding air
under the roof, are cooled. As a result, the air temperatures are lowered, and the humidity
increases inside the greenhouse. Hence, this system operates with high effectiveness un-
der hot and dry conditions [34].
Figure 5.
Fan and pad cooling systems for greenhouses at the Institute of Arid Regions (Kebili, Tunisia).
Agronomy 2022,12, 626 13 of 31
Rafique et al. [
95
] report that the use of only an indirect evaporative cooler in hot and
humid climates is not sufficient, and that the use of only a direct evaporative cooler is not
economical. For this reason, they propose a combination of the two configurations. Hui
and Cheung [
96
] found that this combination led to the attainment of cool air better than
using one type of evaporative cooler only. The cooling system parameters, such as the pad
area, the fan power, the flowrates, the distribution system, and the pump capacity, should
be rigorously calculated and selected in order to guarantee the sufficient humidification
of the pad, and to avoid pad clogging. For instance, according to Kittas et al. [
34
], it is
recommended that the pad area correspond to 1 m
2
per 20–30 m
2
of the greenhouse area,
and that the air flowrate range from 120 to 150 m
3
per m
2
of the greenhouse area per hour.
3.5.2. Roof Evaporative Systems
Roof evaporative cooling is performed by circulating a thin film of water throughout
the greenhouse roof surface (Figure 6). The solar thermal energy, which is absorbed through
the external surface, is therefore reduced, and the roof, and the surrounding air under the
roof, are cooled. As a result, the air temperatures are lowered, and the humidity increases
inside the greenhouse. Hence, this system operates with high effectiveness under hot and
dry conditions [34].
Agronomy 2022, 12, x FOR PEER REVIEW 14 of 32
Figure 6. Schematic of cooling by roof evaporative system [97].
All of the numerical and experimental studies that have been performed on roof
evaporative systems assert that using this system efficiently could be a cost-effective so-
lution to improving climate conditions for greenhouse cooling under hot and arid climate
conditions [98–100]. Limited studies have been conducted to investigate the performance
of roof evaporative cooling for greenhouses in hot and dry regions. The reported results
show that the addition of the roof water flow over the greenhouse roof can lower the
greenhouse inner temperature by up to 6 °C [98,101]. Willits and Peet [102] found that the
application of a film of water under an external black shade cloth significantly improved
the cooling performance, compared to an unshaded greenhouse. Their results reveal that
the energy gain reached 15.9 kW, that the greenhouse temperatures were lowered by 2.6
°C, and that the electricity consumption was reduced by 157 Wh. However, an important
constraint of this system is its relatively high water consumption, which could reach about
67% of the greenhouse water needs, which is beyond the irrigation requirements [103].
Therefore, using nonconventional water, such as pretreated wastewater or sea water,
could be considered to compensate for the water consumption excess.
3.5.3. Fogging Systems
Fogging is a simple and common cooling method that is used mainly in commercial
greenhouses. This system provides cooling by humidifying the ambient air inside the
greenhouse that is to be conditioned by pressurizing and spraying water through small
nozzles in the fogging pipe that is mounted at a high level in the greenhouse (Figure 7).
The fogging systems are usually applied as a complementary system to the principal cool-
ing process, especially during the summer season, and they show better results in hot and
dry climate conditions [104]. The fogging systems could operate at a high pressure (40
bars), propelling droplets of 10–30 µm, or at a low pressure (5 bars), propelling droplets
with minimum diameters of 200 µm [34].
Figure 6.
Schematic of cooling by roof evaporative system. Reprinted with permission from Ref. [
97
].
Copyright 2019 Elsevier.
All of the numerical and experimental studies that have been performed on roof
evaporative systems assert that using this system efficiently could be a cost-effective
solution to improving climate conditions for greenhouse cooling under hot and arid climate
conditions [
98
–
100
]. Limited studies have been conducted to investigate the performance
of roof evaporative cooling for greenhouses in hot and dry regions. The reported results
show that the addition of the roof water flow over the greenhouse roof can lower the
greenhouse inner temperature by up to 6
◦
C [
98
,
101
]. Willits and Peet [
102
] found that the
application of a film of water under an external black shade cloth significantly improved
the cooling performance, compared to an unshaded greenhouse. Their results reveal that
Agronomy 2022,12, 626 14 of 31
the energy gain reached 15.9 kW, that the greenhouse temperatures were lowered by
2.6 ◦C
,
and that the electricity consumption was reduced by 157 Wh. However, an important
constraint of this system is its relatively high water consumption, which could reach about
67% of the greenhouse water needs, which is beyond the irrigation requirements [
103
].
Therefore, using nonconventional water, such as pretreated wastewater or sea water, could
be considered to compensate for the water consumption excess.
3.5.3. Fogging Systems
Fogging is a simple and common cooling method that is used mainly in commercial
greenhouses. This system provides cooling by humidifying the ambient air inside the
greenhouse that is to be conditioned by pressurizing and spraying water through small
nozzles in the fogging pipe that is mounted at a high level in the greenhouse (Figure 7). The
fogging systems are usually applied as a complementary system to the principal cooling
process, especially during the summer season, and they show better results in hot and dry
climate conditions [
104
]. The fogging systems could operate at a high pressure (40 bars),
propelling droplets of 10–30
µ
m, or at a low pressure (5 bars), propelling droplets with
minimum diameters of 200 µm [34].
Agronomy 2022, 12, x FOR PEER REVIEW 15 of 32
Figure 7. Schematic of cooling by a fogging system [97].
Several studies were performed on the distribution of the spray nozzles, their optimal
diameter, and the length of the spray cycles and flowrates, as well as on the feasibility of
applying a water recovery process. Perdigones et al. [105] proved that at equal spray cycle
duration, and mounting the fogging pipe below the shading screens, allowed for higher
values of relative humidity (71.8%) than mounting it above the shading device (58.59%).
Tiny water droplets of 2–60 µm were recommended by Ohyama et al. [106] to avoid the
water droplets dripping off of the plant leaves. Many researchers [33,104–107] agree that
fogging systems provide an efficient cooling process that allows for adequate climate con-
trol and that prevents the plant dehydration and heat stress caused by high temperatures.
Moreover, compared to fan and pad systems, the main advantage of the fogging systems
is the uniformity of the climate conditions that they generate inside the greenhouse, with-
out requiring forced ventilation [34]. However, as it is a fresh-water-consuming technol-
ogy, it is not highly recommended in countries that suffer from water scarcity. Perdigones
et al. [105] propose a pulse-width modulation (PWM) method to lower the water con-
sumption. PWM allows for fogging control by varying the cycle duration as a function of
the inner temperature. This method can reduce the water consumption by 8–15%, com-
pared to fogging systems with fixed cycle durations. A.M., Abdel-Ghany, and Kozai
[33,107] compared three fogging cycles, with fixed fogging rates of 10 g/s, and fixed fog-
ging spans of 30, 60, and 90 s, which were separated with time intervals of 90, 180, and
270 s, respectively. The experiments show that using a fogging cycle of 60 s on/180 s off
provided a higher cooling efficiency. Nevertheless, the main disadvantages of fogging
systems that should be considered are, on the one hand, the high-quality water they re-
quire in order to prevent the clogging of the spray nozzles, which leads to higher costs,
especially in water-scarce countries. On the other hand, the pressure that is required for
the fog system requires a higher rate of pumping energy compared to droplet evaporators.
The time that it is in contact with the air is also shorter, compared to good evaporators,
which also results in higher pumping costs.
3.6. Desiccant Systems
Desiccant cooling systems are effective for applications in multiple climates, includ-
ing in hot and arid regions. The main principle of these systems is based on the combina-
tion of the cooling and the dehumidification of the ambient air by using desiccant materi-
als that could be solid or liquid. The commonly used cooling-based dehumidification sys-
tems have very high energy consumptions, depending on the design of the structure and
the systems, the local climate, the temperature settings, the controls, and the growing
strategies [5,8,51]. Therefore, using desiccant systems coupled to solar devices is an effec-
tive and energy-saving process. In fact, solar thermal collectors coupled with desiccant
systems are largely used in building and agriculture sectors as an alternative to conven-
tional air dehumidification and cooling systems [108–110].
Figure 7.
Schematic of cooling by a fogging system. Reprinted with permission from Ref. [
97
].
Copyright 2019 Elsevier.
Several studies were performed on the distribution of the spray nozzles, their optimal
diameter, and the length of the spray cycles and flowrates, as well as on the feasibility
of applying a water recovery process. Perdigones et al. [
105
] proved that at equal spray
cycle duration, and mounting the fogging pipe below the shading screens, allowed for
higher values of relative humidity (71.8%) than mounting it above the shading device
(58.59%). Tiny water droplets of 2–60
µ
m were recommended by Ohyama et al. [
106
] to
avoid the water droplets dripping off of the plant leaves. Many researchers [
33
,
104
–
107
]
agree that fogging systems provide an efficient cooling process that allows for adequate
climate control and that prevents the plant dehydration and heat stress caused by high
temperatures. Moreover, compared to fan and pad systems, the main advantage of the
fogging systems is the uniformity of the climate conditions that they generate inside the
greenhouse, without requiring forced ventilation [
34
]. However, as it is a fresh-water-
consuming technology, it is not highly recommended in countries that suffer from water
scarcity. Perdigones et al. [
105
] propose a pulse-width modulation (PWM) method to lower
the water consumption. PWM allows for fogging control by varying the cycle duration
as a function of the inner temperature. This method can reduce the water consumption
by 8–15%, compared to fogging systems with fixed cycle durations. A.M., Abdel-Ghany,
and Kozai [
33
,
107
] compared three fogging cycles, with fixed fogging rates of 10 g/s, and
fixed fogging spans of 30, 60, and 90 s, which were separated with time intervals of 90, 180,
and 270 s, respectively. The experiments show that using a fogging cycle of 60 s on/180 s
off provided a higher cooling efficiency. Nevertheless, the main disadvantages of fogging
Agronomy 2022,12, 626 15 of 31
systems that should be considered are, on the one hand, the high-quality water they require
in order to prevent the clogging of the spray nozzles, which leads to higher costs, especially
in water-scarce countries. On the other hand, the pressure that is required for the fog
system requires a higher rate of pumping energy compared to droplet evaporators. The
time that it is in contact with the air is also shorter, compared to good evaporators, which
also results in higher pumping costs.
3.6. Desiccant Systems
Desiccant cooling systems are effective for applications in multiple climates, including in
hot and arid regions. The main principle of these systems is based on the combination of the
cooling and the dehumidification of the ambient air by using desiccant materials that could be
solid or liquid. The commonly used cooling-based dehumidification systems have very high
energy consumptions, depending on the design of the structure and the systems, the local
climate, the temperature settings, the controls, and the growing strategies [
5
,
8
,
51
]. Therefore,
using desiccant systems coupled to solar devices is an effective and energy-saving process.
In fact, solar thermal collectors coupled with desiccant systems are largely used in building
and agriculture sectors as an alternative to conventional air dehumidification and cooling
systems [108–110].
The solar desiccant concept was first proposed by Lof [
111
]. The two system phenom-
ena, which characterize the reaction between the vapor reaction and the desiccant solution,
are based on either the absorption or adsorption processes. The absorption is a chemical
reaction between the humidity and the liquid solution, while the adsorption phenomena,
which uses a solid desiccant, occurs on the desiccant surface, without any chemical reaction.
For the regeneration, a low-energy level is required to regenerate the absorbed humidity.
Liquid desiccant is widely used for dehumidification and cooling purposes as an
alternative to conventional vapor compression systems. Moreover, most of the research
deals with the performance of liquid rather than solid desiccants, given the ability of
liquid-desiccant systems to be more easily integrated into the greenhouse roof.
A comparison between desiccant and conventional vapor compression systems is
presented in Table 3. Conventional vapor compression systems cool the air below its dew
point in order to condensate the water vapor and remove the moisture. The dehumidified
greenhouse air is then heated to the setpoint temperature. These deep cooling and reheating
electrically driven processes are energy consuming compared to desiccant processes, which
can be driven by low-grade thermal energy. Desiccant systems are considered to be an
environmentally friendly technology, as they can be operated by solar energy or waste
heat, and they restrict the use of hazardous refrigerants, which are widely used in vapor
compression systems [
108
,
112
–
115
]. Moreover, these systems are able to remove all of
the airborne contaminants (dust, spores, bacteria, viruses) because of the direct contact
between the air and the highly concentrated desiccant solution. Therefore, high indoor air
quality is reached, compared to the vapor compression systems that generate large humid
surfaces that breed bacteria [108].
Liquid desiccant dehumidification technology has been shown to be very effective in
controlling the humidity level, but it is often more complex to operate and control compared
to conventional systems [
108
,
113
,
115
,
116
]. The evaluation of the installation cost is also still
under debate. Some researchers have concluded that the vapor compression installation
cost is higher than that of desiccant systems [
117
,
118
], and others have found that vapor
compression systems have the lowest installation costs [
112
,
116
]. Thus, reducing the initial
costs should be considered in order to achieve the economic benefits of desiccant cooling
applications in hot climate regions.
Agronomy 2022,12, 626 16 of 31
Table 3. Comparison between desiccant and conventional systems [113,116,119].
Parameter Conventional Vapor
Compression System Desiccant System
Performance High Low
Operation cost High Low
Energy source Mainly electricity Low-grade thermal energy
Environmental safety Low High
Control over humidity Average Accurate
Indoor air quality Average Good
System control Average Complicated
Ghoulem et al. [
97
] were motivated to use a solar regenerator combined with a desic-
cant to dehumidify and cool the climate inside the greenhouse to ensure crop growth for the
humid and hot regions by dehumidifying and cooling (Figure 8). Ghosh and Ganguly [
120
]
numerically investigated desiccant evaporative cooling for agricultural greenhouse appli-
cations to be used in humid and hot regions. They modeled a partially closed loop with
a recirculation of a partial quantity of the return air, and the desiccant regeneration was
realized by a solar roof. The results show that the coefficient of performance (COP) of the
system, which is defined as the ratio between the refrigerating power delivered by the
system and the heating power required [
120
], ranges between 0.64 and 0.74 during the most
humid periods while maintaining a required temperature for the growth of lettuce crops.
Agronomy 2022, 12, x FOR PEER REVIEW 17 of 32
during the most humid periods while maintaining a required temperature for the growth
of lettuce crops.
Figure 8. Schematic of solar-assisted desiccant cooling system [97].
Davies [2] studied the coupling of the incoming air desiccation to the evaporative
cooling in a greenhouse under hot climate conditions in the Gulf state of Abu Dhabi. The
proposed arrangement consists of adding a desiccant pad that is placed immediately up-
stream of the first evaporator pad of an evaporative cooling system. The system is also
powered by solar energy, which ensures the regeneration of the desiccant solution. Davies
[2] claims to have reduced the greenhouse summer temperature by 5 °C. Thus, the gener-
ated climate was suitable for the cultivation of lettuce between 3 and 6 months, and for
the cultivation of tomatoes and cucumbers for about 7 months during the whole year.
In Saudi Arabia, Abu-Hamdeh and Almitani [121] studied the performance of cou-
pling desiccation with evaporative cooling, and of integrating different nanofluids with
various concentrations in the cooling system. The nanofluid pipes were embedded in the
desiccant pad to improve the heat exchange process of the cooling system. They found
that the energy effectiveness of the enhanced desiccant evaporative system, which was
expressed through the NTU method, reached 50.10%. The daily maximum temperature
inside the greenhouse was reduced by about 6 °C, compared to a conventional evapora-
tive cooling system.
Table 4 summarizes the main research carried out on desiccant systems for green-
house cooling [108–110,122].
Figure 8.
Schematic of solar-assisted desiccant cooling system. Reprinted with permission from
Ref. [97]. Copyright 2019 Elsevier.
Davies [
2
] studied the coupling of the incoming air desiccation to the evaporative
cooling in a greenhouse under hot climate conditions in the Gulf state of Abu Dhabi.
The proposed arrangement consists of adding a desiccant pad that is placed immediately
upstream of the first evaporator pad of an evaporative cooling system. The system is
also powered by solar energy, which ensures the regeneration of the desiccant solution.
Davies [
2
] claims to have reduced the greenhouse summer temperature by 5
◦
C. Thus, the
generated climate was suitable for the cultivation of lettuce between 3 and 6 months, and
for the cultivation of tomatoes and cucumbers for about 7 months during the whole year.
In Saudi Arabia, Abu-Hamdeh and Almitani [
121
] studied the performance of coupling
desiccation with evaporative cooling, and of integrating different nanofluids with various
concentrations in the cooling system. The nanofluid pipes were embedded in the desiccant
pad to improve the heat exchange process of the cooling system. They found that the energy
effectiveness of the enhanced desiccant evaporative system, which was expressed through
Agronomy 2022,12, 626 17 of 31
the NTU method, reached 50.10%. The daily maximum temperature inside the greenhouse
was reduced by about 6 ◦C, compared to a conventional evaporative cooling system.
Table 4summarizes the main research carried out on desiccant systems for greenhouse
cooling [108–110,122].
Table 4. Summary of studies of desiccant systems in greenhouses.
Locations System Characteristics Desiccant Greenhouse
Area (m2)Performance
Saudi Arabia [121]
Solar-assisted
desiccant-evaporative
system; liquid
desiccant.
Liquid desiccant
nanofluids 300
A decrease of 6 ◦C of the
maximum indoor temperature
compared to a conventional
evaporative cooler.
The energy effectiveness
reaches 50.10%.
The Gulf,
Abu Dhabi [3]
Solar regeneration
desiccant-evaporative
system.
Liquid desiccant,
CaCl2LiCl 250
A decrease of 5 ◦C
of the maximum indoor
temperature; increase
in crop yield; extension
of the growing season.
Australia [123]
Solar-assisted desiccant
system. Liquid desiccant -
Good agreement between
experimental and numerical
results; satisfactory
performance.
India, Bangladesh,
Italy Cuba [124]
Solar-powered
desiccant system.
Liquid desiccant,
MgCl21000
A reduction of 5.5–7.5 ◦C
of the indoor temperature;
extension of the
growing season.
Netherlands [125] Desiccant system. Liquid desiccant,
CaCl2LiCl 40 RH is maintained between
75% and 85%.
India [120]
Solar-powered
desiccant evaporative
system.
Liquid desiccant,
LiCl 224
Indoor temperature is
maintained below 27 ◦C;
optimum growing conditions
for crops.
Greece [126]
Hybrid system: air–air
heat pump and
desiccant
Liquid desiccant,
CaCl263
RH is maintained below 80%;
increase in total water losses.
4. Innovations and Emerging Technologies for Greenhouses
The emerging technologies consist of innovative systems, renewable energy processes,
and coupled systems that aim to allow for and control suitable indoor conditions for crop
growth in greenhouses, the production of fresh water, and, mainly, a reduction in the water
and energy consumption.
4.1. Water Recovery in Greenhouses
Agriculture is the largest water-consuming sector, with a rate of 70% of the global
fresh water [
127
]. Furthermore, given the relevant decreases in the groundwater reserves
because of high consumption rates and the warranty of water availability in the face
of climate change, the development and improvement of irrigation water management
techniques has become a major priority in agriculture development, particularly in arid
areas. Protected cultivation has contributed significantly to the enhancement of water
efficiency, as it produces higher yields with reduced water consumption, as well as the
efficient use of other resources, such as fertilizers, pesticides, and labor, compared to open-
field cultivation [
4
,
128
,
129
]. Water management, which is correlated with issues such as
the limitation of freshwater resources, efficient water use, and water desalination, consists
Agronomy 2022,12, 626 18 of 31
of water recovery monitoring techniques that apply closed, or semiclosed, air cycles in
the greenhouse design. Another source of the water supply consists of recovering the
wastewater that is treated and recycled. Wastewater can be collected from any water-
consuming process, such as industrial processes or those of households. Hence, converting
wastewater or the loss of water by evapotranspiration into irrigation water could be an ideal
strategy for transforming crop agriculture from its status as the greatest water consumer, to
the status of water producer.
4.1.1. Condensation on a Cold Surface
This technology is based mainly on collecting the condensate of humid air. The existing
systems of water recovery are based on the following two conditions: taking advantage
of the high values of the air humidity that are achieved in greenhouses, especially in
closed ones; and decreasing the greenhouse temperature by cooling processes. Actually,
condensation occurs when the humid air of the greenhouse comes into contact with a
surface that is at a temperature that is lower than its dew point. The water content of the
air is thus removed by means of condensation. The relative humidity of the air in a closed
greenhouse is mainly affected by the ventilation and the evapotranspiration of the plants.
Hence, it remains a great challenge to apply humidification with the aim of collecting more
droplets, without causing crop damage. Indeed, water recovery could be achieved by
coupling a condensation process to the natural convection or mechanical cooling systems,
or even to air-to-air heat exchangers. On the basis of the developed dynamic model for
predicting the energy and mass exchanges in a greenhouse as a function of the dynamic
environmental factor, Yildiz and Stombaugh [83] prove that condensation, in the case of a
closed heat pump system that was used for greenhouse cooling in the United States, could
ensure a water recovery reaching 1.17 kg of water day
−1
m
−2
during the summer season,
which is the same amount as the daily water transpiration. They found that the closed
loop system was the most water-conserving system, since all the transpired water could be
recovered on the coils, making the overall water consumption in this system null. Moreover,
the cooling process maintained the greenhouse temperature at 20
◦
C during the day, and at
18
◦
C during the night, with an energy consumption of 0.69 kWh day
−1
m
−2
. In the arid
region of Oman [
94
], the performance of an evaporative cooling system for the recovery
of freshwater was investigated by utilizing two condensers placed after the second pad
of the system (Figure 9). Perret et al. [
94
] found that the relative humidity often reached
100%, and that the water temperature was reduced by 3
◦
C through the two cooling pads.
The dew point temperature was higher than the temperature of the condensers, by about
4
◦
C, and condensation was therefore observed on the dehumidifier, but the amount of
condensate was negligible, and was not measurable. This was justified by the high air
flowrate through the condensers, and improvements in the condenser design were, thus,
highly recommended.
Agronomy 2022, 12, x FOR PEER REVIEW 19 of 32
This technology is based mainly on collecting the condensate of humid air. The ex-
isting systems of water recovery are based on the following two conditions: taking ad-
vantage of the high values of the air humidity that are achieved in greenhouses, especially
in closed ones; and decreasing the greenhouse temperature by cooling processes. Actu-
ally, condensation occurs when the humid air of the greenhouse comes into contact with
a surface that is at a temperature that is lower than its dew point. The water content of the
air is thus removed by means of condensation. The relative humidity of the air in a closed
greenhouse is mainly affected by the ventilation and the evapotranspiration of the plants.
Hence, it remains a great challenge to apply humidification with the aim of collecting
more droplets, without causing crop damage. Indeed, water recovery could be achieved
by coupling a condensation process to the natural convection or mechanical cooling sys-
tems, or even to air-to-air heat exchangers. On the basis of the developed dynamic model
for predicting the energy and mass exchanges in a greenhouse as a function of the dy-
namic environmental factor, Yildiz and Stombaugh [83] prove that condensation, in the
case of a closed heat pump system that was used for greenhouse cooling in the United
States, could ensure a water recovery reaching 1.17 kg of water day−1 m−2 during the sum-
mer season, which is the same amount as the daily water transpiration. They found that
the closed loop system was the most water-conserving system, since all the transpired
water could be recovered on the coils, making the overall water consumption in this sys-
tem null. Moreover, the cooling process maintained the greenhouse temperature at 20 °C
during the day, and at 18 °C during the night, with an energy consumption of 0.69 kWh
day−1 m−2. In the arid region of Oman [94], the performance of an evaporative cooling sys-
tem for the recovery of freshwater was investigated by utilizing two condensers placed
after the second pad of the system (Figure 9). Perret et al. [94] found that the relative hu-
midity often reached 100%, and that the water temperature was reduced by 3 °C through
the two cooling pads. The dew point temperature was higher than the temperature of the
condensers, by about 4 °C, and condensation was therefore observed on the dehumidifier,
but the amount of condensate was negligible, and was not measurable. This was justified
by the high air flowrate through the condensers, and improvements in the condenser de-
sign were, thus, highly recommended.
Figure 9. Water recovery in the evaporative cooling system of a seawater greenhouse in Oman [94].
4.1.2. Advanced Desalination Processes
Several studies have focused on desalination in greenhouses as a key process of water
management, and they have investigated several techniques to improve its efficiency and
sustainability, particularly in the water-scarce countries [130]. Desalination processes are
generally coupled to solar processes in order to guarantee low costs and zero-energy in-
tegrated systems. Buchholz et al. [131] and Zaragoza et al. [132] performed an experi-
mental study in Spain under the Watergy project, which achieved a controlled indoor cli-
mate and water recovery. The solar-assisted system is based on water and air cycles, with
Figure 9.
Water recovery in the evaporative cooling system of a seawater greenhouse in Oman.
Reprinted with permission from Ref. [94]. Copyright 2005 Elsevier.
Agronomy 2022,12, 626 19 of 31
4.1.2. Advanced Desalination Processes
Several studies have focused on desalination in greenhouses as a key process of water
management, and they have investigated several techniques to improve its efficiency and
sustainability, particularly in the water-scarce countries [
130
]. Desalination processes are
generally coupled to solar processes in order to guarantee low costs and zero-energy inte-
grated systems. Buchholz et al. [
131
] and Zaragoza et al. [
132
] performed an experimental
study in Spain under the Watergy project, which achieved a controlled indoor climate
and water recovery. The solar-assisted system is based on water and air cycles, with the
additional usage of a heat exchanger that harnesses the temperature difference between the
night cooling and the indoor air to condensate the water vapor on the envelope surface.
The 200 m
2
closed greenhouse ensured thermal energy capture, water recycling, water
desalination, and advanced horticultural use. The greenhouse control climate allowed for a
75% savings in the water consumption. El-Awady et al. [
133
] performed an experimental
investigation of an integrated solar greenhouse that combined the methods of desalina-
tion and wastewater treatment, along with water condensation harvesting, in the arid
area of Giza in Egypt. The proposed prototype showed satisfying results for the zones
suffering from water shortages, given that the greenhouse that was studied provided a
low-cost solution, on the one hand, for indoor climate control, and, on the other hand, for
the production of freshwater that was suitable either for drinking or irrigation purposes.
In Muscat, Oman, the seawater greenhouse studied by Al-Ismaili and Jayasuriya [
134
]
combined humidification–dehumidification and solar desalination processes. The fresh-
water production is about half of the irrigation demands:
300–600 L day−1
of a salinity
lower than 0.020 dS
·
m
−1
. Furthermore, multiple studies have aimed to integrate the de-
salination system into the roof of the greenhouse. Chaibi and Jilar [
135
] demonstrated
that this technique aims to maximize the grower economic return, and they proved that a
roof-integrated desalination system in Tunisia guaranteed the plant water supply, with a
rate that ranged from 1 to 1.6 kg day
−1
m
−2
, and a system efficiency of about 40% [
136
].
Davies and Paton [
2
] studied the temperature variation trends in a seawater greenhouse in
Dubai, where the cooling was achieved by an evaporative system that was equipped with
fans and two cooling pads. An array of plastic pipes, which were used for the greenhouse
shading, provided the back evaporative pad with hot seawater to boost the freshwater
production, and the condenser was fed with cooled seawater from the front pad. The
greenhouse mean temperature and the radiant temperature were decreased, respectively,
by 1
◦
C and
7◦C
. Moreover, the freshwater production was enhanced by 63% [
2
]. In
conclusion, water production is a necessary design objective, especially in hot and arid
countries, where water scarcity is increasing.
4.2. Advanced Cooling Systems
4.2.1. Renewable-Energy-Powered Cooling Systems
(i)
Solar Thermal systems
Solar thermal applications in agriculture have the advantage of the heat being gen-
erated by solar radiation. They include desalination processes, crop drying, greenhouse
heating, as well as solar cooling, which is the most promising technology, given that the
peaks of the cooling requirements in greenhouses match the solar radiation peaks. Solar
cooling thermal systems use the thermal energy of the sun as an energy source to generate
coolness. They have been widespread in protected cropping agriculture for years since
they offer a zero-impact technology. Their performance has been consistently investigated
and improved by coupling solar collectors to different cooling
processes [4,108,109]
. As
was detailed in the previous sections, several experimental and numerical studies were
performed on solar systems that were based on the evaporative or desiccation cooling
processes [
3
,
121
,
123
,
124
,
133
,
137
]. The results show that solar cooling systems that are
adapted for greenhouse units are showing satisfactory results in terms of the efficiency
and the economic income. Furthermore, the reviews of the research that focus on the solar
cooling processes in greenhouses [
5
,
64
,
138
–
141
] point out that these systems enhance the
Agronomy 2022,12, 626 20 of 31
greenhouse energy efficiency and reduce their dependence on the grid electricity supply.
High-performance solar thermal plants, which reach up to 40% [
8
], consist of integrating
concentrating solar collectors (CSCs) into greenhouse cooling systems. The main CSCs that
are integrated into protected faming fields are linear Fresnel collectors and parabolic trough
solar collectors, which are usually coupled to an absorption chiller.
Sonneveld et al. [103]
performed an experiment on linear Fresnel collectors combined with PV cells to provide the
greenhouse with hot water and electric power to be used for cooling and lighting purposes.
The linear Fresnel lenses were integrated between the double glass of the southerly oriented
roof cover (Figure 10). The Fresnel system splits direct radiation from diffuse radiation, and
it concentrates it on the PV modules that are mounted within the focal line of the Fresnel
lenses. As a result, the amount of solar energy that is blocked reaches 77%, which leads
to a reduction in the greenhouse cooling requirements by about a factor of 4. The Fresnel
system also generated 143.89 kWh m
−2
of thermal energy, and 29 kWh m
−2
of electrical
energy, which can be exploited for further cooling by means of an evaporative system.
Agronomy 2022, 12, x FOR PEER REVIEW 21 of 32
lighting purposes. The linear Fresnel lenses were integrated between the double glass of
the southerly oriented roof cover (Figure 10). The Fresnel system splits direct radiation
from diffuse radiation, and it concentrates it on the PV modules that are mounted within
the focal line of the Fresnel lenses. As a result, the amount of solar energy that is blocked
reaches 77%, which leads to a reduction in the greenhouse cooling requirements by about
a factor of 4. The Fresnel system also generated 143.89 kWh m−2 of thermal energy, and 29
kWh m−2 of electrical energy, which can be exploited for further cooling by means of an
evaporative system.
CSCs are usually mounted with solar tracking systems to collect the maximum radi-
ation, and they are coupled to solar cooling processes, particularly in hot desert locations
or rural areas, where electrification is difficult and expensive, and where solar resources
are abundant [8]. CSCs remain particularly expensive, compared to conventional thermal
power generation, and further research and development is needed on this emerging tech-
nology for power cooling systems. Alternative ways of improving the operations could
be considered and applied, either for the component materials, or for the whole design, in
order to enhance the system effectiveness.
Figure 10. (a) The PV modules of the system; (b) the linear Fresnel collectors of the system; (c) the
Fresnel greenhouse (The Netherlands) [103].
(ii) PV Solar systems
Contrary to solar thermal energy, photovoltaics enable sunlight to be directly con-
verted into electrical power for use in cooling systems, or any other electric equipment,
such as pumps, heat pumps, dryers, and artificial lighting. Ghoulem et al. [97] demon-
strated that a solar cooling system, which was based on a heat pump coupled with PV
panels, covered 33.2 to 67.2% of the greenhouse demand in the summer periods. Carlini
et al. [142] affirm that the efficiency of PV cooling systems ranges between 30% in the
summer and 11% in the winter. Actually, the capacity generated by PV cooling systems is
dependent on different factors, namely, the location of the panels and their areas, as well
as the greenhouse requirements.
The selection of the appropriate panel area and characteristics should accord with the
energy demand and the load profile [143]. Moreover, the structure and the covering of the
greenhouse with large PV panels causes extensive shading, which may contribute to a
reduction in the greenhouse temperature and to plant stress in hot climates. This affects
the plant growth and productivity [144,145], as light is considered to be one of the most
important sources for photosynthesis.
PV panels, when installed properly and when coupled with cooling systems, show
satisfying results, as was demonstrated by Al-Ibrahim et al. [146], who experimented with
the use of PV panels of 14.72 kW to cover the electrical needs of a 9 × 39 m greenhouse,
which was, namely, an evaporative cooling system. The PV cooling system performance
was satisfactorily established since it met the required load of the greenhouse under the
hot and arid conditions of Saudi Arabia. As for Ganguly et al. [90], they proved that the
cooling solar plant that was tested in India, which combined a fan and pad evaporative
system and PV panels, provided the coolness required for a 90 m2 greenhouse. According
to them, the PV cooling system constitutes a viable option for powering stand-alone green-
houses in a self-sustained manner. The use of PV systems has expanded in recent years
Figure 10.
(
a
) The PV modules of the system; (
b
) the linear Fresnel collectors of the system; (
c
) the Fresnel
greenhouse (The Netherlands). Reprinted with permission from Ref. [
103
]. Copyright 2011 Elsevier.
CSCs are usually mounted with solar tracking systems to collect the maximum radia-
tion, and they are coupled to solar cooling processes, particularly in hot desert locations
or rural areas, where electrification is difficult and expensive, and where solar resources
are abundant [8]. CSCs remain particularly expensive, compared to conventional thermal
power generation, and further research and development is needed on this emerging tech-
nology for power cooling systems. Alternative ways of improving the operations could
be considered and applied, either for the component materials, or for the whole design, in
order to enhance the system effectiveness.
(ii)
PV Solar systems
Contrary to solar thermal energy, photovoltaics enable sunlight to be directly converted
into electrical power for use in cooling systems, or any other electric equipment, such as
pumps, heat pumps, dryers, and artificial lighting. Ghoulem et al. [
97
] demonstrated that a
solar cooling system, which was based on a heat pump coupled with PV panels, covered
33.2 to 67.2% of the greenhouse demand in the summer periods. Carlini et al. [
142
] affirm
that the efficiency of PV cooling systems ranges between 30% in the summer and 11%
in the winter. Actually, the capacity generated by PV cooling systems is dependent on
different factors, namely, the location of the panels and their areas, as well as the greenhouse
requirements.
The selection of the appropriate panel area and characteristics should accord with the
energy demand and the load profile [
143
]. Moreover, the structure and the covering of
the greenhouse with large PV panels causes extensive shading, which may contribute to a
reduction in the greenhouse temperature and to plant stress in hot climates. This affects
the plant growth and productivity [
144
,
145
], as light is considered to be one of the most
important sources for photosynthesis.
PV panels, when installed properly and when coupled with cooling systems, show
satisfying results, as was demonstrated by Al-Ibrahim et al. [
146
], who experimented with
the use of PV panels of 14.72 kW to cover the electrical needs of a 9
×
39 m greenhouse,
which was, namely, an evaporative cooling system. The PV cooling system performance
Agronomy 2022,12, 626 21 of 31
was satisfactorily established since it met the required load of the greenhouse under the hot
and arid conditions of Saudi Arabia. As for Ganguly et al. [
90
], they proved that the cooling
solar plant that was tested in India, which combined a fan and pad evaporative system and
PV panels, provided the coolness required for a 90 m
2
greenhouse. According to them, the
PV cooling system constitutes a viable option for powering stand-alone greenhouses in a
self-sustained manner. The use of PV systems has expanded in recent years thanks to the
decrease in the photovoltaic equipment costs [
90
]. The satisfactory performances of the
PV cooling systems will allow this sustainable technology to be promptly implemented
worldwide, and specifically in hot and rural locations.
(iii)
Geothermal cooling systems
Geothermal cooling systems, which are often referred to as “shallow geothermal
systems”, consist of a ground pipe that is implanted at a depth that is inferior to 100 m,
and that exploits the relatively stable low-temperature earth surface to exchange heat
and deliver cooling in warm and hot climates [
147
]. Ground heat exchangers are mainly
classified into three types: vertical, horizontal, and basket.
The earth–air heat exchanger systems have been studied and tested in several coun-
tries, and usually with satisfactory results, such as in Thailand [
148
], where the cooling
performance and condensation impact of a horizontal earth tube system, at a depth of 1 m,
was investigated. During the summer season, the generated cooling capacity of the system
was about 74.84%, and the COP, which is defined as the ratio of the cooling power to the
electrical input power, reached 3.56 [
148
]. In Kuwait [
149
], the greenhouse temperature
reduction was about 2.8
◦
C for a 1.7 m ground-buried heat exchanger. Several studies
also focus on geothermal heat pumps, which are vapor compression systems that use the
relatively stable earth surface temperature as the heat exchange medium, instead of the out-
side air temperature, in order to produce either cooling or heating power.
Rabbi et al. [18]
show that geothermal heat pumps outperformed all the other heating methods, except for
natural gas. Sanaye and Niroomand [
150
] performed an optimization study of a ground
heat pump in Iran, which reached a capacity of 8 to 32 kW, and a coefficient of performance
(COP) that varied from 3.9 to 5.4. Boughanmi et al. [
151
] studied the thermal performance
of a conic basket heat exchanger, which was implanted at a 3 m depth and coupled to a
geothermal heat pump for greenhouse cooling in the Tunisian climate.
The heat pump COP is defined as the ratio of the amount of heat extracted from the
greenhouse by the compressor input power. The overall process COP is defined as the
ratio between the amount of heat absorbed from the greenhouse and the total electric input
power (to the compressor and the pumps). An evaluation of the thermal performance
of the system shows that the heat pump COP varied from 3.9 to 4.7, and that the overall
process COP ranged between 2.82 and 3.25. The maximum average temperature difference
between the inlet and outlet of the geothermal process system was approximately 30
◦
C.
Hence, the greenhouse temperature was decreased by about 12 ◦C.
4.2.2. Future Trends in Cooling Systems
(i)
Day-to-night thermal storage
During the last few years, several types of thermal storage have been exploited in
greenhouses, which aim to take advantage of the available heat sources (solar gain, ground
heat, exhaust heat, etc.). The thermal storage achieved by means of storage mediums,
namely, water, rock bed, soil, and phase change materials (PCMs) (Figure 11), enhanced the
overall thermal performance of the greenhouse [8].
Day-to-night thermal storage is an innovative recovery process that extracts heat
during the day for the purpose of using it at night. It consists of a passive system that
stores the rising water that is to be served for cooling the air flow that falls through a
heat exchanger during the day. The stored heat is released back inside the greenhouse
during the night by the buffered water that flows in the opposite direction of the water
loop (back to cold storage). As a result, the thermal storage is cooled, and the cooling
Agronomy 2022,12, 626 22 of 31
capacity for the next day is recharged. As for diurnal cooling, this is achieved by naturally
low air temperatures inside the closed greenhouse, as well as by plant evaporation. A
first prototype of the day-to-night storage concept was reported by Buchholz et al. [
131
]
and Zaragoza et al. [
132
] in Almeria, Spain (Figure 12), and it showed satisfactory results.
Under the hot climate conditions of the region, the day-to-night storage system succeeded,
without any extra energy to maintain the temperature during the day between 20 and
35 ◦C
, which is a suitable range for crop growth. The water consumption was also reduced
by 75%. Accordingly, day-to-night thermal storage constitutes a basic innovative device
that can be mounted in any cooling system and that can generate important energy savings.
Agronomy 2022, 12, x FOR PEER REVIEW 23 of 32
Figure 11. Thermal-storage-based systems used in greenhouses: (a) rock bed storage [152]; (b) PCM
storage.
Day-to-night thermal storage is an innovative recovery process that extracts heat dur-
ing the day for the purpose of using it at night. It consists of a passive system that stores
the rising water that is to be served for cooling the air flow that falls through a heat ex-
changer during the day. The stored heat is released back inside the greenhouse during the
night by the buffered water that flows in the opposite direction of the water loop (back to
cold storage). As a result, the thermal storage is cooled, and the cooling capacity for the
next day is recharged. As for diurnal cooling, this is achieved by naturally low air temper-
atures inside the closed greenhouse, as well as by plant evaporation. A first prototype of
the day-to-night storage concept was reported by Buchholz et al. [131] and Zaragoza et al.
[132] in Almeria, Spain (Figure 12), and it showed satisfactory results. Under the hot cli-
mate conditions of the region, the day-to-night storage system succeeded, without any
extra energy to maintain the temperature during the day between 20 and 35 °C, which is
a suitable range for crop growth. The water consumption was also reduced by 75%. Ac-
cordingly, day-to-night thermal storage constitutes a basic innovative device that can be
mounted in any cooling system and that can generate important energy savings.
Figure 12. Air circulation in the greenhouse cooled by the day-to-night thermal storage system
Adapted from [131].
(ii) Closed Desiccant Greenhouses
In a closed desiccant greenhouse (Figure 13), the humidity is consistently withdrawn
from the hot air by a fluid desiccant that allows for the regulation of the humidity and the
temperature, and for the recovery of the heat. A particular surface covering material and
Figure 11.
Thermal-storage-based systems used in greenhouses: (
a
) rock bed storage; (
b
) PCM
storage. Reprinted with permission from Ref. [152]. Copyright 2018 Elsevier.
Agronomy 2022, 12, x FOR PEER REVIEW 23 of 32
Figure 11. Thermal-storage-based systems used in greenhouses: (a) rock bed storage [152]; (b) PCM
storage.
Day-to-night thermal storage is an innovative recovery process that extracts heat dur-
ing the day for the purpose of using it at night. It consists of a passive system that stores
the rising water that is to be served for cooling the air flow that falls through a heat ex-
changer during the day. The stored heat is released back inside the greenhouse during the
night by the buffered water that flows in the opposite direction of the water loop (back to
cold storage). As a result, the thermal storage is cooled, and the cooling capacity for the
next day is recharged. As for diurnal cooling, this is achieved by naturally low air temper-
atures inside the closed greenhouse, as well as by plant evaporation. A first prototype of
the day-to-night storage concept was reported by Buchholz et al. [131] and Zaragoza et al.
[132] in Almeria, Spain (Figure 12), and it showed satisfactory results. Under the hot cli-
mate conditions of the region, the day-to-night storage system succeeded, without any
extra energy to maintain the temperature during the day between 20 and 35 °C, which is
a suitable range for crop growth. The water consumption was also reduced by 75%. Ac-
cordingly, day-to-night thermal storage constitutes a basic innovative device that can be
mounted in any cooling system and that can generate important energy savings.
Figure 12. Air circulation in the greenhouse cooled by the day-to-night thermal storage system
Adapted from [131].
(ii) Closed Desiccant Greenhouses
In a closed desiccant greenhouse (Figure 13), the humidity is consistently withdrawn
from the hot air by a fluid desiccant that allows for the regulation of the humidity and the
temperature, and for the recovery of the heat. A particular surface covering material and
Figure 12.
Air circulation in the greenhouse cooled by the day-to-night thermal storage system.
Adapted from [131].
(ii)
Closed Desiccant Greenhouses
In a closed desiccant greenhouse (Figure 13), the humidity is consistently withdrawn
from the hot air by a fluid desiccant that allows for the regulation of the humidity and
the temperature, and for the recovery of the heat. A particular surface covering material
and design can also be applied in closed desiccant greenhouses in order to guarantee
condensation and the recovery of the water vapor that is evaporated by the plants.
Agronomy 2022,12, 626 23 of 31
Agronomy 2022, 12, x FOR PEER REVIEW 24 of 32
design can also be applied in closed desiccant greenhouses in order to guarantee conden-
sation and the recovery of the water vapor that is evaporated by the plants.
Figure 13. Closed desiccant greenhouse (daytime operation).
During the day, the hot humid air in the greenhouse drops in the counterflow with
the cold dry fluid desiccant. Then, it reaches the crop zone as cold dry air, while the hot
diluted desiccant solution is buffered in the thermal storage. Air cooling is also achieved
by means of the evapotranspiration of plants. During the night, the amount of heat stored
is used for the desiccant regeneration, as well as for greenhouse heating, and, hence, the
evaporation process drags the humid air up into the covering surface, where it conden-
sates and can be recovered. The main advantage of this technology is that it is independent
of the air humidity [9,153,154]. Hence, its implementation is suitable either for hot arid or
humid climates. In fact, the first prototype of a desiccant greenhouse, which is mounted
in Cairo, Egypt, is under experimentation [137]. This emerging technology offers the sub-
stitution of energy-intensive mechanical cooling units by a low-cost and economical heat-
driven solution.
The thermal energy required for a closed desiccant greenhouse is provided either by
the solar thermal energy or the residual heat. The source of the residual heat can be either
the return air of the heating loop, or the unexploited heat of an industrial process, or an
air-to-air heat exchanger or ground heat exchanger (Figure 14). A desiccant greenhouse
that is coupled to an air-to-air heat exchanger requires less mechanical ventilation, and
important heat transfer occurs without resorting to water use. In addition, this system
guarantees lower temperature and humidity levels than desiccant greenhouses [84]. Ac-
tually, this system is being installed at the National Research Institute for Rural Engineer-
ing, Water, and Forestry (INRGREF) in Tunisia in order to test its performance and oper-
ational capabilities.
Figure 13. Closed desiccant greenhouse (daytime operation).
During the day, the hot humid air in the greenhouse drops in the counterflow with
the cold dry fluid desiccant. Then, it reaches the crop zone as cold dry air, while the hot
diluted desiccant solution is buffered in the thermal storage. Air cooling is also achieved
by means of the evapotranspiration of plants. During the night, the amount of heat stored
is used for the desiccant regeneration, as well as for greenhouse heating, and, hence, the
evaporation process drags the humid air up into the covering surface, where it condensates
and can be recovered. The main advantage of this technology is that it is independent of the
air humidity [
9
,
153
,
154
]. Hence, its implementation is suitable either for hot arid or humid
climates. In fact, the first prototype of a desiccant greenhouse, which is mounted in Cairo,
Egypt, is under experimentation [
137
]. This emerging technology offers the substitution of
energy-intensive mechanical cooling units by a low-cost and economical heat-driven solution.
The thermal energy required for a closed desiccant greenhouse is provided either by
the solar thermal energy or the residual heat. The source of the residual heat can be either
the return air of the heating loop, or the unexploited heat of an industrial process, or an air-
to-air heat exchanger or ground heat exchanger (Figure 14). A desiccant greenhouse that is
coupled to an air-to-air heat exchanger requires less mechanical ventilation, and important
heat transfer occurs without resorting to water use. In addition, this system guarantees
lower temperature and humidity levels than desiccant greenhouses [
84
]. Actually, this
system is being installed at the National Research Institute for Rural Engineering, Water, and
Forestry (INRGREF) in Tunisia in order to test its performance and operational capabilities.
Agronomy 2022, 12, x FOR PEER REVIEW 24 of 32
design can also be applied in closed desiccant greenhouses in order to guarantee conden-
sation and the recovery of the water vapor that is evaporated by the plants.
Figure 13. Closed desiccant greenhouse (daytime operation).
During the day, the hot humid air in the greenhouse drops in the counterflow with
the cold dry fluid desiccant. Then, it reaches the crop zone as cold dry air, while the hot
diluted desiccant solution is buffered in the thermal storage. Air cooling is also achieved
by means of the evapotranspiration of plants. During the night, the amount of heat stored
is used for the desiccant regeneration, as well as for greenhouse heating, and, hence, the
evaporation process drags the humid air up into the covering surface, where it conden-
sates and can be recovered. The main advantage of this technology is that it is independent
of the air humidity [9,153,154]. Hence, its implementation is suitable either for hot arid or
humid climates. In fact, the first prototype of a desiccant greenhouse, which is mounted
in Cairo, Egypt, is under experimentation [137]. This emerging technology offers the sub-
stitution of energy-intensive mechanical cooling units by a low-cost and economical heat-
driven solution.
The thermal energy required for a closed desiccant greenhouse is provided either by
the solar thermal energy or the residual heat. The source of the residual heat can be either
the return air of the heating loop, or the unexploited heat of an industrial process, or an
air-to-air heat exchanger or ground heat exchanger (Figure 14). A desiccant greenhouse
that is coupled to an air-to-air heat exchanger requires less mechanical ventilation, and
important heat transfer occurs without resorting to water use. In addition, this system
guarantees lower temperature and humidity levels than desiccant greenhouses [84]. Ac-
tually, this system is being installed at the National Research Institute for Rural Engineer-
ing, Water, and Forestry (INRGREF) in Tunisia in order to test its performance and oper-
ational capabilities.
Figure 14. Desiccant greenhouse with use of additional residual heat (daytime operation).
Closed desiccant greenhouses can also be coupled to a solar thermal source, such
as solar ponds or solar thermal collectors, namely, concentrating solar collector systems
that generate high temperatures, such as parabolic trough collectors (PTC) or Fresnel
collectors [
8
]. Closed desiccant greenhouses also offer the opportunity of being mounted
onto several existing and emerging systems, for instance, CSP plants. The waste heat of the
Agronomy 2022,12, 626 24 of 31
CSP plant is harnessed into the desiccant regeneration, and the cooling energy is transferred
to the CSP plant; hence, the coupling of these two technologies could be very effective if
the ventilation electricity consumption is lowered. Nonetheless, it is worth developing
closed desiccant greenhouse technology in a cost-effective way. For instance, coupling
desiccant greenhouses to simple plastic solar absorbers, which are two magnitudes lower
in cost than concentrating solar collectors, makes this technology more profitable and
convenient for growers. Consequently, this technology is considered to be among the
most promising future trends in greenhouses, particularly in hot and arid climates. To
this end, the INRGREF has planned to continue future work on testing desiccant systems
in a prototype closed greenhouse in Tunisia, using calcium chloride and magnesium
chloride desiccants, with relatively low solution concentrations that do not require high
temperatures for regeneration.
5. Conclusions and Recommendations
The present paper provides an updated literature review of the climate control meth-
ods and cooling systems, with a particular focus on their reliability under hot and arid
climate conditions. The main criteria of the performance evaluation are the effectiveness of
these systems in generating suitable climate conditions for crops, and the decline in the
energy and water consumption.
In the greenhouse conception, particular attention must be paid to the shape, orienta-
tion, covering materials, and the opening patterns, in accordance with the geographical loca-
tion. For hot and arid climates, the E-W orientation is the most recommended at all latitudes,
as the greenhouse receives higher radiation in winter than in summer. However, other
directions can be proven more appropriate when the stormy wind direction
is considered.
The climate management of the greenhouse environment is crucial to guaranteeing
suitable growing conditions for the crop. The climate control passes through the monitoring
of the solar radiation, air temperature, relative humidity, and carbon dioxide concentration
inside the greenhouse, either manually or automatically, or via smart methods that guaran-
tee advanced remote control. The use of a basic or a smart greenhouse monitoring system
depends on the aimed accuracy and effectiveness. Advanced control methods that are
integrated into the greenhouse with effective cooling systems guarantee valuable energy
savings and water use regulation.
The outcomes of our review on the cooling systems that are used in greenhouses under
hot and arid climates show that the cooling effect of ventilation systems is more effective
when mechanical ventilation is used with the appropriate monitoring of the greenhouse
openings in terms of choosing the adequate areas and positions of the openings, and also
in controlling the time and duration of the opening, depending on the outside ambient
temperature conditions.
The performance of cool roof and fan and pad systems, which are widely used tech-
niques in hot and arid regions, can be enhanced by the use of saline water under the use
of techniques that prevent clogging. The use of a second pad, or a condenser, in the fan
and pad system, as well as coupling the evaporative system to a desiccant system, offers
the opportunity to achieve a higher cooling efficiency and lower energy consumption than
when using only a desiccant or an evaporative system, particularly if a solar loop is applied
for the desiccant regeneration.
The progress in achieving the high energy and cost efficiency of cooling could be made
by improving the heat transfer processes, the heat storage methods, and the water recovery
techniques. Multiple advanced cooling systems are emerging as viable solutions for hot
and arid regions, particularly when they are powered by renewable resources, such as solar
collectors, PV modules, concentrating collectors, or geothermal systems.
The adoption of the day-to-night thermal storage concept in greenhouses by means
of water, rock bed, soil, or PCMs reduces the climate control energy demands and, conse-
quently, enhances the overall thermal performance of the greenhouse.
Agronomy 2022,12, 626 25 of 31
The implementation of closed desiccant greenhouses in hot regions is proposed as a
low-cost and economical heat-driven solution that can replace energy-intensive mechan-
ical cooling units. This emerging technology can be coupled to a solar thermal source,
an air-to-air heat exchanger, or a ground heat exchanger, in order to achieve maximum
energy efficiency.
With consideration to the improved water efficiency in protected cultivation, further
research efforts continue to be performed, with a specific focus on the advanced processes
of combined evaporation–condensation, which is aimed at the recovery of the irrigation
water in greenhouses. The development of these technologies, which are closely linked
to modified or advanced cooling systems, will lead to energy-efficient and cost-effective
protected agriculture systems.
Author Contributions:
Conceptualization, M.S., M.T.C. and M.B.; investigation, M.S. and Z.S.;
methodology, M.T.C. and M.S.; writing—original draft preparation, M.S. and Z.S.; writing—review
and editing, M.T.C. and M.B.; supervision, M.T.C. and M.B.; project administration, M.T.C. and
M.S.; funding acquisition, M.T.C. All authors have read and agreed to the published version of
the manuscript.
Funding:
The project leading to this work received funding from the European Union’s Horizon
2020 research and innovation program, under grant agreement No. 101000801 (Project LC-FNR-
06-2020 TheGreeFa, G.A. 101000801). The authors acknowledge the funding from the Tunisian
Ministry of High Education and Scientific Research, under the support of the PRIMA program
(CONSIRS project).
Informed Consent Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
COP Coefficient of performance
DLI Daily light integral (mol m−2d−1)
NTU Number of transfer units
Psat Vapor pressure at saturation (Pa)
PAR Photosynthetically active radiation (W/m2)
PPFD Photosynthetic photon flux density (µmol m−2s−1)
RH Relative humidity (%)
T Temperature (◦C)
VPD Vapor pressure deficit (Pa)
CSC Concentrating solar collectors
CSP Concentrating solar power
E-W East-West
GCC Gulf Cooperation Countries
HRV Heat recovery ventilator
INRGREF National Research Institute for Rural Engineering: Water and Forestry
MENA Middle East and North African area
NIR Near infrared radiation
N-S North-South
PTC Parabolic trough collectors
PV Photovoltaic
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