Comprehensive Review on Evaporative Cooling and Desiccant Dehumidification Technologies for Agricultural Greenhouses

Abstract

Greenhouses are crucial for maintaining an ideal temperature and humidity level for plant growth; however, attaining ideal levels remains a challenge. Energy-efficient and sustainable alternatives are needed because traditional temperature/humidity control practices and vapor compression air conditioning systems depend on climate conditions and harmful refrigerants. Advanced alternative technologies like evaporative cooling and desiccant dehumidification have emerged that maintain the ideal greenhouse temperature and humidity while using the least amount of energy. This study reviews direct evaporative cooling, indirect evaporative cooling, and Maisotsenko-cycle evaporative cooling (MEC) systems and solid and liquid desiccant dehumidification systems. In addition, integrated desiccant and evaporative cooling systems and hybrid systems are reviewed in this study. The results show that the MEC system effectively reduces the ambient temperature up to the ideal range while maintaining the humidity ratio, and both dehumidification systems effectively reduce the humidity level and improve evaporative cooling efficiency. The integrated systems and hybrid systems have the ability to increase energy efficiency and controlled climatic stability in greenhouses. Regular maintenance, initial system cost, economic feasibility, and system scalability are significant challenges to implement these advanced temperature and humidity control systems for greenhouses. These findings will assist agricultural practitioners, engineers, and researchers in seeking alternate efficient cooling methods for greenhouse applications. Future research directions are suggested to manufacture high-efficiency, low-energy consumption, and efficient greenhouse temperature control systems while considering the present challenges.

1. Introduction

A greenhouse is a modified, environmentally managed structure that creates controlled climates to facilitate plant growth in regions where natural conditions are unsuitable for plant development and production. Growing plants in a greenhouse requires maintaining four essential microclimate parameters: temperature, relative humidity, light intensity, and carbon dioxide concentration [1]. Two parameters, i.e., temperature and humidity control, are deeply studied in this research. Traditional temperature/humidity control practices, like shading practices, ventilation systems, like natural and artificial ventilation, and fogging systems are used, but these systems depend on outside climate conditions and can only partially control the temperature and humidity [2]. Temperature and humidity control are difficult in traditional greenhouse farming due to the constant interaction between external and internal environmental factors. Seasonal variations, regional location, and weather patterns are examples of external factors that might make precise control inside greenhouses challenging. Internally, soil moisture dynamics and plant transpiration make humidity control even more difficult [3]. Due to inadequate temperature and humidity control inside the greenhouse, plants face issues like heat stress, reduced growth rates of leaves and flowers, and overall significant yield losses [4]. Heat stress can negatively impact plant growth and productivity due to inadequate temperature control [5]. A summary of different types of greenhouses based on cost investment, shape, utility and functions, and covering material [6], including semi-buried greenhouses like Walipini or pit greenhouses, passive solar greenhouses [7], and deep winter greenhouses [8], is shown in Figure 1. The main points of each type are described in the figure.

Traditional greenhouse systems depend on the physical involvement of labor, which frequently leads to inadequate climate control and the inefficient use of resources. Desiccant-assisted evaporative cooling systems represent a significant improvement in greenhouse temperature and humidity control. These systems efficiently control temperature and humidity by combining evaporative cooling and desiccant dehumidification [9]. By removing excess humidity from the air, with the help of desiccant materials like silica gel or lithium chloride, desiccants improve the effectiveness of the evaporative cooling system. These systems are suitable in hot and humid regions, where high ambient humidity may make traditional evaporative cooling less effective [10]. Artificial Intelligence (AI) has transformed greenhouse climate control systems through the emergence of smart greenhouses that can make decisions on their own. AI algorithms enhance climate control, irrigation, and fertilizer application by analyzing real-time data from multiple sensors that measure soil moisture, humidity, temperature, and light intensity. This integration increases plant yields, lowers resource usage, and improves energy efficiency [11]. An AI-powered system can protect plant growth trends and proactively modify the environment to satisfy certain plant needs. Modern greenhouses achieve more precise environmental control, which promotes sustainable agriculture practices and increases profitability [12].

The present study gives a state-of-the-art review of greenhouse temperature and humidity control technologies. The study focuses on the significance of the greenhouse VPD and temperature/humidity control. The study explored traditional methods of controlling temperature and humidity, including shading practices, ventilation systems, fogging systems, and vapor compression air conditioning systems. Evaporative cooling systems, like direct, indirect, and Maisotsenko-cycle evaporative cooling systems, and desiccant dehumidification systems, like solid desiccant and liquid desiccant dehumidification systems, have been studied. In addition, integrated desiccant and evaporative cooling systems and hybrid systems have been reviewed. The results demonstrate that the MEC system effectively cools air up to the ambient air’s dew point and wet-bulb temperatures while maintaining humidity, and both desiccant systems efficiently control humidity inside the greenhouse. The integrated system and hybrid systems improve energy efficiency, water consumption, and the stability of the controlled climate in greenhouses. Challenges and future perspectives of temperature and humidity control technologies for greenhouses have been explored.

2. Significance of Greenhouse VPD and Temperature/Humidity Control

The vapor pressure deficit (VPD) is a critical parameter in the greenhouse environment that directly affects plant transpiration and growth, nutrient adsorption, stomatal conductance, and overall physiological performance. An optimal VPD range is necessary to maximize plant productivity because it affects photosynthesis, biomass production, and water-use efficiency. Plants grow well when the VPD falls between 0.4 and 1.25 kPa and grow optimally when the VPD is between 0.8 and 0.9 kPa [13]. A normal and ideal VPD growth zone for plants growing in greenhouses is shown in Figure 2 [14]. The VPD fluctuations from the ideal range affect the plant’s health, growth, and overall productivity. The effect of high and low VPDs on plants growing in greenhouses is shown in Figure 3. So, it is crucial to maintain the ideal VPD range inside the greenhouse for healthy plant growth, development, and high-quality yields.

Temperature is a crucial parameter of greenhouse management that directly affects transpiration, photosynthesis, plant growth, and productivity [15]. It also affects metabolic rate, enzymatic activity, biochemical pathways, plant flowering, and fruiting. Maintaining an ideal temperature range under controlled conditions is required to maximize plant physiological activity and produce reliable yields. Maintaining the temperature within the optimal range, normally 15–30 °C, as given in the literature [14], ensures good growth and production. A psychrometric representation of the ideal and normal temperature zones of plants growing inside the greenhouse is shown in Figure 2 [14]. The optimum temperature control ranges for various plants are different during the day and night. The optimum temperature range varies with the stages of plant growth for different plants [2]. Another study [1] found that the optimal temperature ranges for greenhouses are 21–29 °C during the day and 18.5–21 °C at night. The optimal temperature and relative humidity during the day and night required for the growth of different plants inside the greenhouse are described in

Table 1. Optimum temperature and relative humidity required at day and night times for selected plants to grow inside the greenhouse [2].

Plant Type	Tday (°C)	Tnight (°C)	RH (%)
Tomato	23–27	13–16	50–80
Cucumber	25–30	16–18	70–90
Cabbage	15–16	2	70–80
Peas	25–30	16–18	70–80
Pepper	22–30	14–16	50–70
Beans	22–26	16–18	70–80
Aubergine	25–28	14–16	50–60
Lettuce	24–28	13–16	60–80
Courgettes	20–22	17–18	65–80
Strawberry	20–26	13–16	50–65

, as shown in the literature [2]. High- and low-temperature effects on plants growing inside the greenhouse are shown in Figure 3. Proper temperature control is necessary for plant health and growth to avoid high and low temperatures. Traditional temperature/humidity practices are employed to control the temperature, but advanced systems like evaporative cooling systems and desiccant air conditioning systems provide more effective cooling.

Humidity control is critical for plant growth in greenhouses. It directly affects nutrient uptake, transpiration, and overall plant quality. Although well-developed root systems may withstand high humidity levels of 40–100%, plants can typically grow at a relative humidity level of 20–80% [16]. However, sustaining the ideal humidity range of 50–80% is crucial for healthy plant growth, as it reduces the risk of fungal diseases and balances transpiration and pest swarms [13]. Excessive or insufficient humidity can lead to several issues, including a rise in fungal infections, decreased plant nutrient uptake, and smaller leaves [17]. High- and low-humidity effects on plant growth inside the greenhouses are shown in Figure 3. To avoid these problems, proper humidity control is critical for plant health and growth. Traditional and advanced systems can be employed to control the humidity level, but advanced systems like desiccant-based dehumidifiers offer efficient air dehumidification and can achieve the ideal range within the greenhouse.

3. Traditional Temperature/Humidity Control Practices in Greenhouses

3.1. Shading Practices

Shading practices help partially control temperature and humidity using plastic sheets or green cloths [18]. These practices can control the greenhouse’s temperature and humidity without the need for expensive machinery or skilled staff. These are especially useful for partially regulating the humidity and temperature of greenhouses in hotter areas [19]. The purpose of shading is to lower the greenhouse’s temperature without lowering the amount of sunlight needed for healthy plant growth. Two basic methods to provide shade are (i) shade fabric and (ii) a greenhouse shading compound known as whitewash [19]. It is necessary to consider the benefits and limitations of each method for healthy plant growth.

Shading practices are crucial to control the entrance of sunlight radiation in hot and sunny areas. These practices promote the healthy growth and development of the plant and increase the overall yield [19]. These practices can decrease evapotranspiration, sunlight penetration, greenhouse temperature, and plant stress [20]. The quality and homogeneity of greenhouse plants can also improve by shading. The average length and height of the plants that grow in the greenhouse using these practices are higher compared to those grown without these practices. However, the quality of the solar radiation that is allowed to enter the greenhouse by shading materials affects the growth and development of plants. Proper greenhouse shade practices help reduce evapotranspiration and plant temperature [21]. These practices are considered efficient for partially controlling temperature and humidity. Maintaining temperature and humidity control uniformly throughout a greenhouse is challenging [22].

3.2. Ventilation Systems

Ventilation is a traditional practice of partial temperature/humidity control. Natural and forced ventilation are two main types of ventilation systems. Natural ventilation requires no external power and depends on roof and side-wall openings to control temperature/humidity. It is effortless, environmentally friendly, and cost-effective for controlling greenhouse conditions. The opening area required for adequate ventilation must be 15–25% [23]. Additionally, air fans are used in forced ventilation systems to minimize excessive humidity in the greenhouse environment [2].

3.2.1. Natural Ventilation

A pictorial representation of three types of natural ventilation is shown in Figure 4 [2]. Natural ventilation is a natural process that does not require extra energy for operation. Proper ventilation is crucial for controlling an ideal greenhouse environment. Controlling humidity requires more ventilation than controlling temperature [24]. Natural ventilation controls the greenhouse’s temperature and humidity by replacing the warm and humid inside air with cold and dry outside air [18]. Sensible and latent heat loss during ventilation can be measured using Equations (1) and (2), as reported in the literature [25]:

$$Q_{sensible}=c_p{\rho }_{air}{V}_{vent}\left(T_i-T_o\right)$$

(1)

$$Q_{latent}=L_v{\rho }_{air}{V}_{vent}\left({\omega }_i-{\omega }_o\right)$$

(2)

where Q_sensible is sensible heat loss, c_p is air-specific heat, ρ_air is air density, V_vent is the air exchange rate through the window, Q_latent is latent heat loss, and L_v is the latent heat of vaporization.

Ventilation performance is sensitive to greenhouse design, wind speed and direction, and the location and size of the opening area. The overall vent area should be between 15 and 25% of the greenhouse’s ground area to maintain optimum airflow [23]. This is crucial because the air exchange rate through natural ventilation affects the internal climate. Various studies conducted in the literature on the natural ventilation system used for greenhouse cooling are summarized in

Table 2. Summary of studies conducted in the literature on the natural ventilation system in greenhouses across different countries, highlighting the greenhouse area, plant type, wind speed, wind direction, temperature differences, and ratio of the ventilator opening area.

Ventilation Driving Force	Study Type	Location	AGh (m2)	Greenhouse Type	Plant Type	Wind Speed (m/s)	Wind Direction	∆T (K)	Ratio of the Ventilator Opening Area (%)	Ref.
Wind-driven	Experimental and theoretical	UK	204.8	Four span types	Tomato	<1	Northeast Southeast	281.15–287.15	10 20	[26]
Wind-driven	Numerical modeling through CFD software	Italy	307	Italian type	Ornamental plants	0.67 0.89 0.11	Southeast	275.55–275.95	47	[27]
Wind-driven	Experiments and modeling	France	416	Polyethylene two-span	Tomato	0–8.2	Parallel to the vents	271.15–285.15	7.69	[28]
Wind-driven	Experiments and dynamic tracer gas method analysis	Greece	384	Plastic type	-	0.1–7.6	North South	273.95–287.85	1.2–23.8	[29]
Buoyancy-driven	Simulation by using CFD	France	225	Single-span type	-	0.3	-	275.15	65	[30]
Wind-driven	Numerical investigation using the CFD code	Greece	160	Tunnel type	Tomato	0.2–0.7	Perpendicular to the opening of the tunnel	279.15	22.5	[31]
Buoyancy-driven	CFD simulation/analysis	Israel	960	Multi-span type	Pepper	0.5–3	North to south	279.15–280.15	-	[32]
Wind and stack driven	Ventilation performance analysis by using a neural network	France	30–416 230 210	Tunnel type Richel tunnel type Roof type	Tomato	1.3 1.2 10.6	-	8.1 7.4 9.4	0.8 3 7.5	[33]
Wind-driven	Experiments	France	368	Classically ventilated tunnel Largely open tunnel	Tomato	2.7 in the daytime 2.1 at nighttime	West–northwest East–southeast	273.65 in the daytime 271.35 at nighttime	7 in the daytime, 1 and 3 at nighttime 18 for daytime and nighttime	[34]
Wind-driven	Simulation using CFD software	France	2600	Four-span	Ornamental kalanchoe plants	1.26 for configuration-I 1.40 for configuration-II	West to east West to east East to west	9.1 0.03 2.2	-	[35]
Wind-driven	Experiments and analysis with a three-dimensional CFD model	Spain	882	Parral	-	6	East side West side	-	3.45 10.46	[36]
Wind-driven	Experimental	UK	422.4	Multi-span	Tomato	1–8	-	7	2.704	[37]
Buoyancy driven	Analysis with CFD Model	-	3.3	Mono-span	-	0.3	Perpendicular to the opening area	275.15	25.61	[38]
Wind-driven	Parametric analysis	India	90	Single-span ridge type	Flowers	1–3	-	273.85	15	[39]
Wind-driven	Simulation using GX software	Canada	10,000	Venlo type	Tomato	2.69–4.42	-	-	-	[40]
Wind-driven	Experiment and analysis using ANOVA general linear model	Thailand	200	Net type	Tomato	2.1	-	276.85	1.05	[41]
Wind-driven	Experiments and CFD Analysis	Shanghai	1980	Multi-span plastic	Lettuce	1.3 during summer 2 during winter	Perpendicular to the greenhouse orientation	274.15 during summer 282.05 during winter	-	[42]

. Study type, location, greenhouse area, plant type, wind speed, wind direction, temperature difference, and ventilator opening area ratio are described in the table.

Natural ventilation, however, is partially energy-efficient and inexpensive, but it does have some qualitative limitations. It can partially control the greenhouse temperature/humidity and the minimum temperature/humidity difference achieved by this system. Its performance depends on external weather conditions [2]. It may not be enough to provide cooling during extremely hot weather. The geographic location and orientation of the greenhouse affect the effectiveness of natural ventilation as well. It does not always provide the precise environmental control required for optimal growth

3.2.2. Forced Ventilation

A pictorial representation of a forced ventilation system used for a greenhouse is shown in Figure 5 as given in the literature [2]. The forced ventilation system, based on fans and ventilators, adjusts the inside greenhouse temperature and humidity in such a way as to ensure a stable and controlled environment for plant growth. These systems can provide uniform air distribution within the greenhouse compared to natural ventilation [43]. These are frequently applied to control the greenhouse temperature and humidity during hot summer days and in tropical areas [24]. These systems are less effective in cold climates due to heat loss from the greenhouse; as a result, heating requires extreme energy, making it uneconomical [44]. To protect the inside greenhouse environment from overheating, these systems ensure the proper regulation of the inside temperature. Combining the roof opening and ventilator creates a more comfortable environment for plants within the greenhouse than forced ventilation.

The performance of the forced ventilation systems depends on forced air ventilation fans to move air inside and outside the greenhouse [24]. These systems are often used in commercial greenhouses, using exhaust fans on one side and intake vents on the other to ensure a smooth and effective air exchange. These consistently remove heat by bringing in fresh, cool air, creating an environment suitable for growing plants in ideal climatic conditions.

The cost of forced ventilation systems compared to natural ventilation systems is one of their drawbacks. The loudness of the fan can be an issue, particularly in residential areas [1]. Since these systems rely on a consistent supply of electricity to operate fans and control mechanisms, their higher energy costs can pose a significant disadvantage for large greenhouses or those areas with high electricity prices. Regular maintenance of components such as fans and filters, which require routine cleaning and inspection, is necessary to operate forced ventilation systems at optimal efficiency, thus increasing labor and operating expenses.

3.3. Fogging System

Fogging systems are commonly employed in commercial greenhouses to manage the temperature and humidity inside greenhouses due to their simplicity and ease of operation [45]. These systems humidify the inlet air to provide cooling in the greenhouse. These systems work well in hot, dry areas and are used in conjunction with the cooling process, especially during the summer [18]. The fogging system can employ either high pressure (40 bars) or low pressure (5 bars) to push droplets with a minimum diameter of 200 µm [2]. The fogging systems offer effective cooling that permits suitable temperature control and protects the plant from dryness and heat pressure produced by high temperatures. The key benefit of these systems is the consistency in the environmental conditions generated within the greenhouse without requiring forced ventilation. A pictorial representation of a fogging system used for a greenhouse is shown in Figure 6, as given in the literature [18].

Fogging systems are partially sufficient to maintain greenhouse temperature and humidity, but these have some limitations. These systems need consistent maintenance to ensure nozzles and pumps are working accurately. These systems are not feasible in water shortage areas because they need huge amounts of water to perform effectively [18]. These systems do not perform effectively in humid regions because the air is fully saturated, and extra humidity hinders the system’s capacity to reduce the temperature efficiently. Inadequate management of these systems can increase the humidity of the greenhouse, which is harmful to plant development and production.

The different studies conducted in the literature on fogging systems used for greenhouses are summarized in

Table 3. Summary of studies conducted in the literature on the fogging systems in greenhouses across various countries, highlighting the study type, greenhouse area, operating conditions (temperature and relative humidity), plant type, and greenhouse type.

System Type	Study Type	AGh (m2)	Tamb (°C)	RHamb (%)	TGh (°C)	RHGh (%)	Plant Type	Greenhouse Type	Location	Ref.
Fogging system	Design and experiments	200	32	86	27.6–35.1 29.8–36.9	65.57–91.29 65–87	Mustard green Watercress	Piolet type	Indonesia	[46]
Fogging system	Experiments	-	25–25.9	47–52	20.6–21.9	79–84	Tomato	Single span	Japan	[47]
Fogging system	Experiments	21,730	35.4	30–58	30.5	60–80	Rose	Multi-span plastic	Turkey	[48]
Fogging system	Experiments	26	36–42	-	33–38	-	Tomato	Single span	Japan	[45]
Fogging and circulation system	Experiments	84	28–45	30–80	25–40	30–80	Cucumber	-	Japan	[49]
Combined force ventilation and fogging system	Experiments	868	28–40	-	28	80	Pepper	Even Span-	Israel	[50]
Fogging system	Experiments and mathematical modeling	32	35–38.5	22–30	24.5–28.5 27	60–90 70	-	Two-span	Israel	[51]
Fogging system	Experiments	108	-	-	20–35	80	-	Single-span	Israel	[52]
Naturally vented greenhouse with a pressure fogging system	Experiment simulation	270	35	35	28.1 28.2	73 82	Tomato	Single-span	Arizona	[53]
Fogging system	Experiments	21,648	35.4	25–50	31.6	60–80	-	Multi-span plastic	Turkey	[54]
Fogging system	Experiments	504	-	32	-	40–80	Cucumber	Multi-span	-	[55]
The fogging system with natural ventilation	Experiments	417.6	30.9	60	28	88.2	Tomato	Twin-span	Japan	[56]
Fogging system	Experiments	600	34	>80	<26	<80	-	Aluminized thermal screen type	Arta Western Greece	[57]

. The system type, study type, greenhouse area, ambient temperature and relative humidity, greenhouse temperature and relative humidity, plant type, greenhouse type, and location are described in the table.

4. Vapor Compression Air Conditioning System

A schematic and psychometric representation of a vapor compression air conditioning system is shown in Figure 7. Vapor compression air conditioning (VAC) systems control the temperature and humidity inside greenhouses, particularly in arid and hot regions. Based on a very complex vapor compression cycle, these systems compress, condense, expand, and evaporate a refrigerant that provides consistent cooling with a controlled and stable environment for plant growth [58].

The compressor, condenser, expansion valve, and evaporator are the four primary components of the VAC system. The compressor transfers low-pressure, low-temperature refrigerant vapor into the condenser after sucking it from the evaporator to raise its temperature and pressure [59]. In the condenser, the refrigerant releases heat into the surrounding air, lowering the temperature and transforming it into a liquid. After that, the expansion valve, also known as the refrigerant control valve, decreases the high-energy liquid’s temperature and pressure, enabling it to enter the evaporator gradually. As it enters the evaporator, it undergoes a phase change, absorbs heat from the greenhouse air, and then vaporizes again, reducing the inside temperature [60].

The psychrometric chart shows that these systems use an evaporator to lower the air temperature below the ambient air dew point temperature. These systems demand more energy to deliver sensible heating through condensers, and they also use refrigerants, which are extremely unfavorable to the environment and contribute to global warming and ozone layer depletion. Ultimately, it can be concluded that these systems are inefficient in directly regulating the relative humidity and temperature [13].

The electricity-intensive VAC system contributes to 3.9% of greenhouse gas emissions, of which 27% and 31% are attributable to latent and sensible load drops, respectively, and 37% is attributable to direct and indirect refrigerant releases during and post use, and also account for 10% of global electricity end use while [61]. The VAC systems’ overall effectiveness in providing optimal growing conditions for plants may be affected by certain limitations. One of the main limitations of these systems is the demand for significant energy [62]. These systems use environmentally harmful refrigerants and consume extreme energy [63]. These systems mainly depend on electric power produced from fossil fuels, which are becoming increasingly depleted and unsustainable. These systems are not adapted to regulate the temperature/humidity inside greenhouses in fluctuating climatic conditions. These systems do not perform well in extreme humidity and temperature conditions, which affect the growing conditions required for plants. These systems struggle to maintain the ideal humidity level, which is necessary to protect the plants from diseases and stress in high-vapor-pressure-deficit conditions. These systems are inefficient for dehumidification within the greenhouse. The ineffective control of humidity in these systems enhances the humidity level in the greenhouse. For greenhouse workers, the complexity and operational expenses of these systems might also be problematic. These systems require financial resources and technical knowledge to maintain and services which may not be easily accessible, particularly for smaller operations.

5. Evaporative Cooling Systems

Evaporative cooling systems are energy-efficient, environment-friendly, and sustainable options for air conditioning that require minimum power consumption and low installation cost but provide high thermal comfort [64,65]. These old technologies utilize the latent heat of vaporization to generate a cooling effect. The outside air affects the performance of evaporative cooling systems, which can be determined by measuring the dry-bulb and wet-bulb air temperatures. The high-temperature difference between dry- and wet-bulb air produces a better cooling effect. Direct evaporative cooling, indirect evaporative cooling, and Maisotsenko-cycle evaporative cooling are the three main categories of evaporative cooling systems [66]. These systems do not use energy-intensive mechanical cooling and do not use harmful refrigerants that contribute to global warming and ozone depletion [10]. These systems can be applied to air conditioning, air cooling, and ventilation systems. The economic analysis showed that the payback period of the evaporative cooling system is 2.5 years, as given in the literature [67].

5.1. Direct Evaporative Cooling System

A direct evaporative cooling system is a common type of cooling system in which air is directly in contact with water. This system consists of a fan, a cooling pad, and a water distribution system, making it simple. The fan and pump primarily consume power, which is significantly less than the compressor. The DEC system uses water as the working fluid, making it eco-friendly compared to harmful refrigerants. The DEC system consists of a single wet channel, where the ambient air flows over a moist surface, and water evaporates to absorb heat from the air [68]. As a result, the air becomes cool and more humid due to direct contact between water and air. The fundamental air enthalpy Equation (3), as given in the literature [69], has been used to describe the thermodynamic behavior of this system:

$$h_a=1.006T_a+\omega (2501+1.86T_a)$$

(3)

where the term “1.006 T_a” is the specific enthalpy of dry air, and the term ω (2501 + 1.86 T_a) is the specific enthalpy of saturated water vapors.

A schematic and psychrometric representation of the direct evaporative cooling system is shown in Figure 8 [70]. The DEC system can lower the air temperature to the ambient wet-bulb temperature and raise the relative humidity to approximately 80% [13,71]. The dry-bulb temperature, relative humidity, and humidity ratio of the air at the inlet and outlet fluctuate from one another, but the enthalpy remains constant throughout the process [13]. The wet-bulb effectiveness, expressed in Equation (8), can be used to determine system efficiency [13]. This system achieved a range of wet-bulb effectiveness of 0.75–0.95 [13]. The payback period of the evaporative cooling system is almost 1.21–2.99 years [72].

$${\left({\epsilon }_{wb}\right)}_{DEC}={\displaystyle \frac{T_{in}-T_{out}}{T_{in}-T_{wb}}}$$

(4)

where (ε_wb)_DEC shows the wet-bulb effectiveness of the DEC system, T_in shows the inside temperature, T_out shows the outside temperature, and T_wb shows the wet-bulb temperature of the inlet air.

The DEC system has some drawbacks that limit its application. These systems are only suitable for hot, arid areas where plants require high moisture. Moreover, these systems require more water, which makes them impractical in areas with water shortages. The wet-bulb temperature of the inlet air must be lower than the supply air temperature, which may not suffice to meet cooling demands in humid climates. Moreover, the increase in air humidity in these systems, due to the direct contact between water and supply air, can create uncomfortable conditions for plant growth [10].

5.2. Indirect Evaporative Cooling System

Indirect evaporative cooling is an advanced and efficient cooling system that can lower ambient temperature without increasing humidity. Therefore, it is an excellent choice for air conditioning in scenarios where maintaining a constant humidity ratio is critical [73]. This system consists of two channels: one dry channel and one wet channel. Its working principle is that when ambient air passes through the dry channel, the air becomes slightly cool due to sensible cooling, and when the air passes through the wet channel, the air becomes cool due to isenthalpic cooling [13].

A schematic and psychrometric representation of an IEC system is illustrated in Figure 8, as given in the literature [70]. The IEC system can cool the air below the ambient wet-bulb temperature. However, its overall efficiency is affected by factors like heat transfer rates and airflow balance between the dry and wet channels. Inlet and outlet air show a distinct pattern: the outside dry-bulb temperature and relative humidity are higher than the interior wet-bulb temperature and relative humidity. Further, at the exit of air, enthalpy is less than that of air at the entrance, and the humidity ratio remains the same [13]. The wet-bulb effectiveness, expressed in Equation (5), as shown in the literature [13], has been used to measure the effectiveness of the system, with a typical range of 0.50–0.65 [66]:

$${\left({\epsilon }_{wb}\right)}_{IEC}={\displaystyle \frac{T_{in}-T_{out}}{T_{in}-T_{wb}}}$$

(5)

where (ε_wb)_IEC shows the wet-bulb effectiveness of the IEC system, T_in shows the inside temperature, T_out shows the outside temperature, and T_wb shows the wet-bulb temperature of the inlet air.

The limitations of an indirect evaporative cooling system are characterized as an IEC system that requires a maximum amount of water to operate, which might be difficult in locations with limited water resources. Daily maintenance is essential for keeping these systems operating efficiently, which includes cleaning and replacing filters to prevent blockages. In humid climates, however, indirect evaporative cooling systems are more effective than DEC systems; yet, these do not provide adequate cooling [74].

5.3. Maisotsenko-Cycle Evaporative Cooling System

The Maisotsenko-cycle evaporative cooling (MEC) system is an advanced indirect evaporative cooling system that can cool air down to the ambient wet-bulb and dew point temperatures [66]. The MEC system is a thermodynamic process that uses renewable energy derived from the latent heat of vaporization and redistributes the energy efficiently [75]. This system integrates heat transfer with evaporative cooling to push the temperature lower than DEC and IEC systems. This system also produces saturated hot air as a heat recovery process, improving system efficiency for various applications [76].

The working mechanism of the MEC system, which has three channels, one wet channel and two dry channels, is illustrated in Figure 8 [70]. The ambient air entering the dry channel is cooled without changing humidity, while heat is transported into a wet channel by sensible cooling. The air then cools further when it enters a wet channel, reaching temperatures close to the ambient dew point. The basic idea behind this operation is to redirect cold air into the wet channel as working air, resulting in lower temperatures for both the dry and wet bulbs [66]. The studies conducted in the literature on evaporative cooling systems used for greenhouse air conditioning are summarized in Table 4. The system type, study type, ambient temperature and relative humidity, greenhouse temperature and relative humidity, greenhouse shape, area of the greenhouse, and location of the study are described in the table.

The air conditions in the system’s inlet and outlet follow the pattern shown below: the ambient dew point temperature of the air is lower than the outlet dry-bulb temperature, while the inlet wet-bulb temperature is higher. Also, the outlet air has a higher relative humidity than the inlet air, so its enthalpy is lower. However, the humidity ratio does not change. The dew point effectiveness, expressed in Equation (6), as reported in the literature [13], has been used to measure the performance of the MEC system. The effectiveness of these systems is then quantified in terms of dew point efficiency in the order of 0.50 to 0.65 [13], and wet-bulb efficiency is up to 1.8 [77]. The financial analysis showed that the payback periods of these systems are 2.5 years [78].

$${\left({\epsilon }_{dp}\right)}_{MEC}={\displaystyle \frac{T_{in}-T_{out}}{T_{in}-T_{dp}}}$$

(6)

where (ε_dp)_MEC shows the dew point effectiveness of the MEC system, T_in shows the inside temperature, T_out shows the outside temperature, and T_dp shows the dew point temperature of the inlet air.

Table 4. Summary of performance comparison of evaporative cooling systems used in greenhouses across different countries, highlighting the study types, operating conditions (temperature and relative humidity), greenhouse shape, and area.

System Type	Study Type	Tamb (°C)	RHamb (%)	TGh (°C)	RHGh (%)	Greenhouse Shape	AGh (m2)	Location	Ref.
Direct evaporative cooling	Dynamic simulation using TRNSYS 18 software	33 37 39 40 37 34	37 40 45 42 40 33	26 27 27 27 25 25	78 80 89 90 90 70	Asymmetric	-	Multan Pakistan	[79]
Indirect evaporative cooling	Dynamic simulation using TRNSYS 18 software	33 37 39 40 37 34	37 40 45 42 40 33	28 30 31 31 28 29	62 70 79 79 75 60	Asymmetric	-	Multan Pakistan	[79]
Direct evaporative cooling	CFD modeling	40	23	21	67	Canarian	165	Agadir	[80]
Ventilation with evaporative cooling	CFD modeling	40	34.42	27	54		36	Tunisia	[81]
Maisotsenko-cycle evaporative cooling	Experiments and mathematical modeling	40	32	29.8 32	-	-	-	China	[82]
Direct evaporative cooling	Experiments	33	50–60	<30	60–80	Even span	8.64	Egypt	[83]
Indirect evaporative cooling	Experiments	33	40–60	-	55–75	Even span	8.64	Egypt	[83]
Indirect–direct evaporative cooling (IDEC) system with groundwater	Experiments	50	8	21	62	Even span	5	Baghdad	[84]
Fan and pad EC system Staw pad Celdek pad Sliced wood pad	Experiments and statistical analysis	37.18	20.91	27.74 28.88 30.08	44.97 39.42 36.18	Tunnel type	380	Sudan	[85]
Fan and pad EC system	Experiments and quantified analysis	32.7	65	28	80	Multi-span	2304	Shanghai	[86]
Evaporative cooling system Fogging system Shading screen Whitening treatment	Experiments and performance analysis	20.9	59.3	22.4 23.2 22.6	74 56.3 61.9	Parral	882	Almería	[87]
Fan-ventilated EC system	Experiments	20.9	59.3	28	70	Multi-span	3000	Karditsa	[88]
Fan-pad system supplied by photovoltaic panels	Mathematical modeling	14–30	35–90	<25	-	Span type	300	Mexico	[89]
Fan and pad system	Experiments and performance analysis	32	25	20–27	50–68	Venlo	64	Turkey	[90]
Fan and pad system	Experiments	35	30	25.7	95	Triangular roof block	-	Turkey	[91]
Fan and pad system	Experiments	<35	>10	27–30	60	A gable roof type	240	Iran	[92]
Fan and pad system	Modeling and analysis	36	50	30	-	Single-span ridge	90	India	[93]
Fan and pad system	Experiments	15–35	-	10–30	40–90	Gable	240	Isparta	[94]

Table 4. Summary of performance comparison of evaporative cooling systems used in greenhouses across different countries, highlighting the study types, operating conditions (temperature and relative humidity), greenhouse shape, and area.

System Type	Study Type	Tamb (°C)	RHamb (%)	TGh (°C)	RHGh (%)	Greenhouse Shape	AGh (m2)	Location	Ref.
Direct evaporative cooling	Dynamic simulation using TRNSYS 18 software	33 37 39 40 37 34	37 40 45 42 40 33	26 27 27 27 25 25	78 80 89 90 90 70	Asymmetric	-	Multan Pakistan	[79]
Indirect evaporative cooling	Dynamic simulation using TRNSYS 18 software	33 37 39 40 37 34	37 40 45 42 40 33	28 30 31 31 28 29	62 70 79 79 75 60	Asymmetric	-	Multan Pakistan	[79]
Direct evaporative cooling	CFD modeling	40	23	21	67	Canarian	165	Agadir	[80]
Ventilation with evaporative cooling	CFD modeling	40	34.42	27	54		36	Tunisia	[81]
Maisotsenko-cycle evaporative cooling	Experiments and mathematical modeling	40	32	29.8 32	-	-	-	China	[82]
Direct evaporative cooling	Experiments	33	50–60	<30	60–80	Even span	8.64	Egypt	[83]
Indirect evaporative cooling	Experiments	33	40–60	-	55–75	Even span	8.64	Egypt	[83]
Indirect–direct evaporative cooling (IDEC) system with groundwater	Experiments	50	8	21	62	Even span	5	Baghdad	[84]
Fan and pad EC system Staw pad Celdek pad Sliced wood pad	Experiments and statistical analysis	37.18	20.91	27.74 28.88 30.08	44.97 39.42 36.18	Tunnel type	380	Sudan	[85]
Fan and pad EC system	Experiments and quantified analysis	32.7	65	28	80	Multi-span	2304	Shanghai	[86]
Evaporative cooling system Fogging system Shading screen Whitening treatment	Experiments and performance analysis	20.9	59.3	22.4 23.2 22.6	74 56.3 61.9	Parral	882	Almería	[87]
Fan-ventilated EC system	Experiments	20.9	59.3	28	70	Multi-span	3000	Karditsa	[88]
Fan-pad system supplied by photovoltaic panels	Mathematical modeling	14–30	35–90	<25	-	Span type	300	Mexico	[89]
Fan and pad system	Experiments and performance analysis	32	25	20–27	50–68	Venlo	64	Turkey	[90]
Fan and pad system	Experiments	35	30	25.7	95	Triangular roof block	-	Turkey	[91]
Fan and pad system	Experiments	<35	>10	27–30	60	A gable roof type	240	Iran	[92]
Fan and pad system	Modeling and analysis	36	50	30	-	Single-span ridge	90	India	[93]
Fan and pad system	Experiments	15–35	-	10–30	40–90	Gable	240	Isparta	[94]

6. Desiccant Dehumidification System

Desiccant dehumidification systems absorb moisture from ambient air. Their performance relies on desiccant material. The property of these materials is their ability to expel moisture from water vapors during the dehumidification process. The primary driving force is the difference in vapor pressure between the plant air and the desiccant material [9]. The adsorption capacity of the desiccant material decreases and becomes fully saturated due to the continuous dehumidifying process [95]. Thermal energy sources such as electro-osmotic, waste heat, solar energy, and electric heaters can regenerate the desiccant material [10]. Desiccant dehumidification systems are utilized instead of VAC systems in hot and muggy areas. Solid desiccant dryers and liquid desiccant dryers are two types of desiccant dryers. Each dryer has unique qualities [96].

6.1. Solid Desiccant Dehumidification System

Solid desiccant dehumidification systems are advanced systems that utilize hygroscopic materials, like zeolite, silica gel, and metal–organic frameworks (MOFs), including CU-BTC, MIL-100, MOF-74, UiO-66 [97], CPO27(NI), MIL100(Fe), MIL-101(Cr), and Aluminum Fumarate [98], to absorb moisture from the air and control the humidity level before entering the greenhouse. The solid desiccant wheel consists of a frame lined with a thin layer of desiccant material. The cross-sectional area of the desiccant wheel is divided into two sections: one for dehumidifying the ambient air and another for regenerating the desiccant wheel. The working principle of the solid desiccant dehumidifier is illustrated in Figure 9. The ambient air is dehumidified due to adsorption through the desiccant materials as it passes through the solid desiccant. Due to the continuous dehumidification of the process air, the desiccant is fully saturated with moisture, and then it requires heat for regeneration [99]. Waste heat, solar energy, and electric heaters can be used as thermal energy sources to regenerate the desiccant system [10]. Fuel/electric energy consumption can be reduced using the solar system to regenerate the desiccant. Fuel consumption can be reduced by up to 20% using solar energy for desiccant regeneration [100]. The average solar energy utilization rate can supply 25.14 kW of energy for desiccant regeneration each day [101].

Desiccant dehumidification systems are especially useful in humid conditions that are harmful to plant growth, development, and yield. The efficiency of these systems depends on their potential to use naturally available energy sources and energy efficiency. The thermal effectiveness, dehumidification effectiveness, and regeneration effectiveness of these systems can be measured using Equations (7)–(9), respectively, as given in the literature [102]:

$${\mathsf{\epsilon }}_{{\left(\mathrm{D}\mathrm{W}\right)}_{\mathrm{t}}}={\displaystyle \frac{{\mathrm{T}}_{\mathrm{p}\mathrm{o}}-{\mathrm{T}}_{\mathrm{p}\mathrm{i}}}{{\mathrm{T}}_{\mathrm{r}\mathrm{i}}-{\mathrm{T}}_{\mathrm{p}\mathrm{i}}}}$$

(7)

$${\epsilon }_{{\left(DW\right)}_d}={\displaystyle \frac{{\omega }_{pi}-{\omega }_{po}}{{\omega }_{pi}-{{\omega }_{p0,}}_{ideal}}}$$

(8)

$${\epsilon }_{{\left(DW\right)}_r}={\displaystyle \frac{m_{pro}({\omega }_{po}-{\omega }_{pi})h_{fg}}{{m_{reg}(h}_4-h_3)}}$$

(9)

where ε_(DW)t shows the thermal effectiveness of the desiccant wheel (-), T_pr shows the temperature of the processed outlet air, T_pi shows the temperature of the processed inlet air (°C), T_ri shows the temperature of the regenerated inlet air (°C), ε_(DW)d shows the dehumidification effectiveness of the desiccant wheel (-), ω_pi is the humidity ration of the processed inlet air (kg/kg DA), ω_po is the humidity ration of processed outlet air (kg/kg DA), ε_(DW)r shows the regeneration effectiveness of the desiccant wheel (-), m_pro shows the mass flow rate of the processed air, m_reg shows the mass flow rate of the regenerated air, and h_fg shows the latent heat of vaporization.

Solid desiccant dehumidification systems can be classified into three types: (i) rotary wheel, (ii) fixed bed, and (iii) desiccant-coated heat exchangers [103]. A comparison of the properties, including adsorption capacity, pressure drops, desiccant material utilization, cooling load, heat transfer efficiency, and continuous dehumidification, of these types is shown in

Table 5. Comparison of operational properties of fixed-bed desiccant systems, rotary wheel desiccant systems, and desiccant-coated heat exchanger systems, as reported in the literature [103].

Properties	Fixed Bed Desiccant Systems	Rotary Wheel Desiccant System	Desiccant-Coated Heat Exchanger System
Adsorption capacity	Minimum	Minimum	Maximum
Pressure drops	Maximum	Minimum	Minimum
Desiccant material utilization	Minimum	Maximum	Maximum
Cooling load	Remain constant	Remain constant	Reduce cooling load
Heat transfer efficiency	Minimum	Minimum	Maximum
Continuous dehumidification	More than one bed is required	One single rotary wheel is required	Two desiccant-coated heat exchangers are required

[103].

Solid desiccant dehumidification systems have many benefits. The main advantage of these systems is their energy efficiency, especially in humid climate regions. Using solar energy as a renewable source helps lower operating costs. These systems increase energy recovery by reducing the energy demand required to maintain an ideal temperature and humidity inside the greenhouse. These systems contribute to healthier plant development and higher yield outputs by efficiently decreasing the humidity of ambient air. The consistent dehumidification performance of these systems ensures that plants can grow in ideal conditions in areas where outside temperature and humidity fluctuate. These systems can provide sustainable alternatives that effectively reduce the necessity of mechanical refrigeration.

6.2. Liquid Desiccant Dehumidification System

Liquid desiccant dehumidification systems control the humidity of ambient air. To decrease the humidity level within the greenhouse, these systems absorb moisture from the air by using liquid desiccant solutions like LiCl₂, CaCl₂, MgCl₂, Al₂O₃, Fe₃O₄, and ZnO. The relative humidity and temperature of the air, airflow ratios, and integrated solution are the main environmental parameters that affect the efficiency of these systems. These systems consist of three main steps: (i) dehumidification, (ii) regeneration, and (iii) vapor compression [104]. The dehumidification system consists of a dehumidifier that draws ambient air over a packed bed that holds the liquid desiccant. The humidity of the ambient air is reduced due to adsorption through the desiccant solution as the air passes through the desiccant bed [105]. Due to the continuous dehumidification of the ambient air, the desiccant solution becomes fully saturated with moisture. Moisture is extracted from the desiccant material using a regenerator. Waste heat from the exhaust air is used to heat the solution, which releases the absorbed moisture into the atmosphere. For efficient moisture control and to preserve the desiccant’s efficacy, a regenerated solution is required [104]. These systems can decrease the heating load during the intermediate humidity months, offering an appropriate method to control the temperature and humidity within the greenhouse [106]. A schematic diagram and performance results of the liquid desiccant dehumidification system are shown in Figure 10, as reported in the literature [100].

Liquid desiccant dehumidification systems offer various benefits, particularly in greenhouse applications, such as controlling the temperature and humidity necessary for optimal plant growth. These systems require minimal energy to operate and can be performed efficiently under different climate conditions. Both dehumidification and cooling can be achieved by using these systems within the greenhouse, reducing the overall energy required for greenhouse operations [106]. These systems help maintain the ideal climate conditions for plants, decrease the hazards of heat stress on plants, and protect the plant’s health and yield. These systems can reduce greenhouse gas emissions by dehumidifying the desiccant using renewable energy sources such as solar thermal collectors. These systems can be employed for small and large commercial greenhouses [2]. Economic analysis showed that the payback period of these systems can be 5 years, as given in the literature [107].

Various studies conducted in the literature on liquid desiccant dehumidification systems used for greenhouses are summarized in

Table 6. Summary of studies conducted in the literature on liquid desiccant systems used for greenhouse dehumidification across different countries, highlighting the type of desiccant material, operating conditions (regenerate temperature for desiccant, ambient and greenhouse temperature, and relative humidity), COP of the systems, greenhouse area, and plant types.

System Type	Desiccant Material	Treg (°C)	Tamb (°C)	RHamb (%)	TGh (°C)	RHGh (%)	COP	AGh (m2)	Plant Type	Location	Ref.
Solar-regenerated desiccant evaporative cooling system	CaCl2 LiCl	60	37.5	35	30	55	0.5	250	Lettuce Tomato Cucumber	Gulf	[108]
Solar-regenerated desiccant evaporative cooling system	LiCl MgCl2	35–50	34.2	-	24–28	50–80	0.64–0.74	224	Lettuce	Kolkata Bangladesh Italy India Oman	[109]
Solar-assisted desiccant and nanofluid evaporative cooling system	Al2O3 Fe3O4 ZnO	85	38.6	-	6 °C > Conventional system	-	-	300	Cucumber	Saudi Arabia	[110]
Solar-powered desiccant cooling system	MgCl2	-	34–36	65–71	22.5–24.5	-	0.41	1000	Lettuce Soya Bean Tomato Cucumber	Mumbai Chittagong Messina Muscat Havana	[111]
Desiccant dehumidification system	CaCl2	80	20	60	25	65	0.75	40	Cucumber	Netherland	[112]
Nanofiltration-regenerated assisted liquid desiccant air conditioning.	MgCl2	60–80	-	-	25–33	70–90	5.3 7.6	512	Tomato	Mecca Colombo Bangkok Lahore Cairo	[113]
State point liquid desiccant dehumidifier	Br	50	10–20	40–90	18–26	85	-	5900 4200	Tomato Roses	Ontario Kingsville	[114]
Performance comparison of the mass transfer of two desiccants	LiBr LiCl	55	25.4–35.4 26.9–35.1	46.97–50.60 44.32–56.86	19.7–27.2 21.8–29	-	0.45 0.47	-	-	China	[115]
Counter-flow adiabatic dehumidifier	LiCl CaCl2 LiBr	55	30–42	28.88–48.48	14–30	-	0.13–0.20 0.10–0.15 0.12–0.18	-	-	Greece	[116]
Integrated desiccant air conditioning system	CHKO2	60	30.1–34.7	51.4–70.6	25.1–25.8	-	0.72	-	-	UK	[117]
Desiccant dehumidification system	Silica gel LiCl	80 100 120	26–34	50–80	23–27	65	1–2.6 0.4–2.2	-	-	Singapore	[118]

. The system type, desiccant material, regenerator temperature, ambient temperature and relative humidity, greenhouse temperature and relative humidity, coefficient of performance, greenhouse area, plant type, and location are described in the table.

7. Integration of Technologies for Energy Efficiency Enhancement

7.1. Integrated Desiccant and Evaporative Cooling System

A schematic diagram of an integrated desiccant and evaporative cooling system is shown in Figure 11. Integrated desiccant and evaporative cooling systems efficiently control humidity and temperature and consume low electricity. These systems have no hazardous refrigerant and maintain a high air quality standard. These systems have garnered much attention as a way to extend the use of evaporative cooling. Dehumidification and evaporative cooling are the two primary processes in these systems. Dehumidification removes moisture from the air, while evaporative cooling controls the temperature of the air. Zeolite, alumina, silica gel, hydratable salts, and their combinations are some adsorbent materials used in desiccants [10].

The working principle of desiccant-based evaporative cooling systems is that ambient air flows through a desiccant wheel, where latent heat causes the temperature to rise and adsorption reduces the air’s moisture content. After that, the dehumidified air moves through a heat exchanger, where sensible cooling causes the temperature to drop while maintaining a consistent humidity level. After passing through evaporative cooling systems such as DEC, IEC, and MEC, the pre-cooled air temperature is further dropped. After that, cooled air enters the greenhouses and provides the ideal temperature and humidity for plant growth. The existing air from the greenhouse flows through the heat exchanger, where its temperature rises due to sensible heating. The heated air then flows through the heat source, raising its temperature. After passing through the desiccant, the hot air’s temperature drops, and its moisture content rise due to desorption. The exhaust air is released into the atmosphere [10].

Desiccant-based evaporative cooling systems offer numerous benefits, particularly in suitable climatic regions. These systems reduce reliance on electricity by utilizing low-grade thermal energy sources such as solar heat and waste heat, resulting in lower greenhouse gas emissions and improved air quality. These systems reduce energy costs and have a positive environmental impact due to less energy consumption and fewer carbon emissions [62]. These energy-efficient technologies can provide the greenhouse’s optimal thermal comfort zone. The economic analysis showed that the payback periods of these systems are 3 years [119].

A system diagram and the performance results of a solar-assisted regenerative desiccant air conditioning system with indirect evaporative cooling can be seen in [100], and a system diagram and the performance results of a solar-assisted solid desiccant-integrated Maisotsenko-cycle evaporative cooling system are shown in Figure 12, as reported in the literature [120].

Various studies have been conducted in the literature on integrated desiccant and evaporative cooling systems for various applications, with their result showing that these systems can be applied for greenhouse air conditioning, as summarized in Table 7. The system type, study type, regeneration temperature, inlet temperature, inlet relative humidity, outlet temperature, outlet relative humidity, coefficient of performance, and cooling capacity are described in the table. Some data (outside temperature vs inside temperature, and outside relative humidity vs inside relative humidity) of the table are presented in Figure 13.

Table 7. Summary of experimental and analytical studies conducted in the literature on solid desiccant-based evaporative cooling systems, highlighting the system type, operating conditions (regenerate temperature for desiccant, inside and outside temperature, and relative humidity), coefficient of performance, and cooling capacity of the systems.

System Type	Study Type	Treg (°C)	Tout (°C)	RHout (%)	Tin (°C)	RHin (%)	COP	CC (kW)	Ref.
Desiccant-assisted indirect evaporative system	Analysis through heat and mass transfer equations	70	35	40.46	21	64.45	COPe = >22 COPt = 0.4	2	[121]
Integrated liquid desiccant-assisted indirect evaporative cooling system	Thermodynamic modeling in MATLAB 2021b	-	35	80	26	50	0.93	1.14	[122]
Integrated air conditioning system combining desiccant dehumidification, indirect evaporative cooling	Parametric analysis	70	35	80	16.5	50	0.93	-	[100]
Solar-assisted regenerative desiccant air conditioning with an IEC system	Theoretical and experimental analysis	80	26–34.7	77.8–92.5	23.2–26.1	51.4–59.7	1.81–2.97	1.78–2.90	[123]
Desiccant with a two-stage evaporative cooling system	Simulation-based numerical analysis	73.12 for two-stage 74.81 for single-stage	19.60 for two-stage 17.71 for single-stage	-	26.05 for two-stage 24.79 for single-stage	55.48 for two-stage 59.69 for single-stage	-	-	[124]
Desiccant air conditioning system with IEC	Numerical analysis	45–60	25–40	28.09–50.59	17–21	59.37–68.85	0.7–1.9	-	[125]
Solid desiccant evaporative cooling system configurations Ventilation cycle Recirculation cycle Dunkle cycle Ventilated-recirculated cycle	Performance analysis	70	15–37	26–85	24	60	1.418 0.846 1.144 0.786–0.842	49.48 59.32 56.92 53.4–58.5	[126]
A desiccant evaporative cooling system using a DIEC Configuration-I Configuration-II Configuration-III	Energetic and exergetic analysis	78.8	30–40	37.76–42.75	26	47.67	0.34–0.32 0.56–0.5 0.59–0.58	19.2–19 15.1–15 17.9–17.8	[127]
Solar desiccant-integrated M-cycle evaporative system	Experimental analysis	70	30–40	40.68–45.17	18–22	54.63–78.43	0.91	3.78	[120]
Solar-assisted desiccant-integrated M-cycle evaporative system	Transient seasonal analysis/seasonal basis analysis	58–78	28	58.97	16–18	85.29–88.17	0.78–1.13	8–24	[128]
Standalone M-cycle evaporative system	Experimental and computational analysis	50–60	33.1–36.5	30.29–33.51	21.1–22.3	77.87–80.89	-	1.23	[129]
Desiccant with a dew-point evaporative cooling system	Experimental assessments	50–90	27–35	58.80–71.23	22	-	15.9	-	[130]
Two-stage desiccant air conditioning incorporating M-cycle cooling system	Full transient analysis	41–60 41–80	41	32.24	24	50	1.77	46.2	[131]
Cross-flow dew point evaporative cooler with and without dehumidification	Experimental and mathematical modeling analysis	60–90	30	67.12	25.5	86.34	4.6	2.2	[132]
Desiccant-assisted Maisotsenko-cycle evaporative cooling system	Modeling		39.2	43.75	21.9	100			[133]
MOF-based desiccant with IEC system	Experiments	45–75	27	62	23	90	2.7–6 times > than silica gel		[134]

Table 7. Summary of experimental and analytical studies conducted in the literature on solid desiccant-based evaporative cooling systems, highlighting the system type, operating conditions (regenerate temperature for desiccant, inside and outside temperature, and relative humidity), coefficient of performance, and cooling capacity of the systems.

System Type	Study Type	Treg (°C)	Tout (°C)	RHout (%)	Tin (°C)	RHin (%)	COP	CC (kW)	Ref.
Desiccant-assisted indirect evaporative system	Analysis through heat and mass transfer equations	70	35	40.46	21	64.45	COPe = >22 COPt = 0.4	2	[121]
Integrated liquid desiccant-assisted indirect evaporative cooling system	Thermodynamic modeling in MATLAB 2021b	-	35	80	26	50	0.93	1.14	[122]
Integrated air conditioning system combining desiccant dehumidification, indirect evaporative cooling	Parametric analysis	70	35	80	16.5	50	0.93	-	[100]
Solar-assisted regenerative desiccant air conditioning with an IEC system	Theoretical and experimental analysis	80	26–34.7	77.8–92.5	23.2–26.1	51.4–59.7	1.81–2.97	1.78–2.90	[123]
Desiccant with a two-stage evaporative cooling system	Simulation-based numerical analysis	73.12 for two-stage 74.81 for single-stage	19.60 for two-stage 17.71 for single-stage	-	26.05 for two-stage 24.79 for single-stage	55.48 for two-stage 59.69 for single-stage	-	-	[124]
Desiccant air conditioning system with IEC	Numerical analysis	45–60	25–40	28.09–50.59	17–21	59.37–68.85	0.7–1.9	-	[125]
Solid desiccant evaporative cooling system configurations Ventilation cycle Recirculation cycle Dunkle cycle Ventilated-recirculated cycle	Performance analysis	70	15–37	26–85	24	60	1.418 0.846 1.144 0.786–0.842	49.48 59.32 56.92 53.4–58.5	[126]
A desiccant evaporative cooling system using a DIEC Configuration-I Configuration-II Configuration-III	Energetic and exergetic analysis	78.8	30–40	37.76–42.75	26	47.67	0.34–0.32 0.56–0.5 0.59–0.58	19.2–19 15.1–15 17.9–17.8	[127]
Solar desiccant-integrated M-cycle evaporative system	Experimental analysis	70	30–40	40.68–45.17	18–22	54.63–78.43	0.91	3.78	[120]
Solar-assisted desiccant-integrated M-cycle evaporative system	Transient seasonal analysis/seasonal basis analysis	58–78	28	58.97	16–18	85.29–88.17	0.78–1.13	8–24	[128]
Standalone M-cycle evaporative system	Experimental and computational analysis	50–60	33.1–36.5	30.29–33.51	21.1–22.3	77.87–80.89	-	1.23	[129]
Desiccant with a dew-point evaporative cooling system	Experimental assessments	50–90	27–35	58.80–71.23	22	-	15.9	-	[130]
Two-stage desiccant air conditioning incorporating M-cycle cooling system	Full transient analysis	41–60 41–80	41	32.24	24	50	1.77	46.2	[131]
Cross-flow dew point evaporative cooler with and without dehumidification	Experimental and mathematical modeling analysis	60–90	30	67.12	25.5	86.34	4.6	2.2	[132]
Desiccant-assisted Maisotsenko-cycle evaporative cooling system	Modeling		39.2	43.75	21.9	100			[133]
MOF-based desiccant with IEC system	Experiments	45–75	27	62	23	90	2.7–6 times > than silica gel		[134]

Figure 13. Comparative analysis of different integrated desiccant-assisted evaporative cooling systems. (a) The variation between outside and inside temperatures of air. The green-colored line shows the inside air temperature, and the blue-colored line shows the outside air temperature. (b) The variation between the outside and inside relative humidity of air. The green colored line shows the inside air relative humidity, and the orange-colored line shows the outside air relative humidity. The data taken from the literature [100,120,121,123,125,126,127,128,129,131,132,133].

Figure 13. Comparative analysis of different integrated desiccant-assisted evaporative cooling systems. (a) The variation between outside and inside temperatures of air. The green-colored line shows the inside air temperature, and the blue-colored line shows the outside air temperature. (b) The variation between the outside and inside relative humidity of air. The green colored line shows the inside air relative humidity, and the orange-colored line shows the outside air relative humidity. The data taken from the literature [100,120,121,123,125,126,127,128,129,131,132,133].

7.2. Hybrid Systems

There are three types of hybrid systems: an integrated vapor compression air conditioning system with a desiccant dehumidifier, an integrated vapor compression air conditioning system with an evaporative cooling system, and an integrated vapor compression air conditioning system with a combined desiccant evaporative cooling system.

A hybrid solid desiccant with a vapor compression air conditioning system is shown in Figure 14 [135]. This hybrid system combines a VAC system that delivers efficient cooling with a desiccant dehumidification system, which effectively controls air humidity. The VAC system consists of four processes: compression, condensation, expansion, and evaporation, as described in Section 4. The desiccant dehumidification system utilizes adsorption materials, such as silica gel and zeolite, to absorb moisture from the air and maintain ideal conditions within the greenhouse, as described in Section 6. This hybrid system can separate the cooling and dehumidification processes, allowing them to run more efficiently and improve overall system performance. These technologies are becoming more commonly recognized for controlling greenhouse temperature and humidity. These systems use less electricity than conventional units while effectively controlling humidity levels. The initial installation cost of these systems is high, but they can save 20.2% of energy costs due to less energy consumption, as reported in the literature [136]. These systems create a more sustainable and energy-efficient method of climate control by using the moisture-absorbing properties of solid desiccants in conjunction with the principles of vapor compression refrigeration.

A schematic diagram of the hybrid evaporative–vapor compression system (HEVC), annual energy consumption, and coefficient of performance comparison of the vapor compression cycle (VCC) and HEVC is shown in Figure 15 [137]. Hybrid systems, like the combination of vapor compression air conditioning with evaporative cooling systems, are critical in controlling the temperature and humidity inside greenhouses. VAC systems offer consistent cooling essential for plant health by absorbing heat from the greenhouse and releasing it outside [13]. Evaporative cooling systems are highly energy-efficient and environmentally friendly and employ the natural process of water evaporation to decrease air temperature. This hybrid system allows for temperature and humidity adjustments, which enhance plant output while decreasing resource efficiency [2]. These systems increase energy efficiency while taking advantage of both cooling techniques to produce ideal growing conditions for plants. These systems are becoming increasingly popular in modern farming methods due to offering a more environmentally friendly method of controlling greenhouse temperatures.

A schematic diagram of the single-stage hybrid desiccant air conditioning system, comparison of the coefficient of performance (COP) and energy-saving potential of actual results and simulated results are shown in Figure 16, as reported in the literature [138]. Hybrid systems, i.e., vapor compression air conditioning combined with a desiccant-based evaporative cooling system, show an innovative approach to controlling the temperature and humidity inside the greenhouse, increasing both energy efficiency and productivity. The effectiveness with which these systems control temperature and humidity is an important factor in the successful cultivation of plants. As the demand for sustainable farming practices rises due to resource constraints and climate change, hybrid systems have emerged as an attractive option for modern greenhouse operations. Hybrid systems’ dual purpose enables them to reduce energy consumption and present ideal growing conditions [10]. Even in difficult weather conditions, vapor compression air conditioning effectively cools and dehumidifies the air, while desiccant-based systems absorb moisture to create an ideal environment for plant growth. By removing pollutants from the air, this integration offers a controlled environment and increases air quality and competitiveness in the market [139].

Various studies have been conducted in the literature on hybrid systems for different applications, with the results indicating that these systems can also effectively control greenhouse temperature and humidity, as summarized in

Table 8. Summary of experimental, simulation, and analysis studies on hybrid systems, including system type, operating conditions (regeneration temperature for desiccant, ambient and greenhouse temperature, and relative humidity), coefficient of performance, and cooling capacity of the systems.

System Type	Study Type	Treg (°C)	Tout (°C)	RHout (%)	Tin (°C)	RHin (%)	COP	CC (kW)	Ref.
Hybrid solid desiccant with VAC system	Simulation analysis by using TRNSYS	94	26	50.02	16	50.43	4.5	1.8	[140]
Hybrid solid desiccant with VAC system	Experiments and modeling by using an artificial neural network	98.6–141	26.1–33.2	59.1–86.3	7.5–11.2	76.2–94.6	1.27 for experiments 1.265 for ANN	3.567 for experiments 3.617 for ANN	[135]
Hybrid evaporative VAC system	Feasibility analysis		30–55	40–80	26.7	-	-	3.5	[137]
Hybrid solid desiccant cooling system with passive radiative cooling panels	Numerical analysis	110	31	50	26	50	0.99	3.714	[141]
Hybrid solid desiccant with VAC system	Experimental and simulation	120	33	67	27	65	-	22	[142]
Desiccant evaporative cooling system with VAC system	Experimental analysis	60	32	46.87	22	54.64	4.06	-	[95]
Liquid desiccant with VAC system	Modeling and dynamic simulation	70	33.9	62.4	25	60	0.55	-	[143]
Liquid desiccant with VAC system	Experimental analysis	51	14.6	58	24.5	50.17	0.638	1.758	[144]
Liquid desiccant with vapor compression air conditioning system	Numerical analysis	33–60	33	66.94	22	55	3.32	-	[145]
Liquid desiccant-assisted dehumidification and vapor compression refrigeration air conditioning system	Experimental analysis	-	36.63	77.81	26.07	84.42	2.23	-	[146]
Solid desiccant-assisted VAC	Experimental Simulation	100	30.05 30	54.1 55	19.8 18.9	59.6 64	4.82 5.05	-	[147]
Solid desiccant-assisted VAC system	Mathematical modeling	80 60	43.2 38.6	13.38 37.41	25	50	-	-	[148]
Hybrid indirect evaporative cooling system with mechanical VAC system	Experimentally	-	30–42	38.46–48.85	24–27	62.52–63.35	4.96–6.05	-	[149]

. The system type, study type, regeneration temperature, inlet temperature and relative humidity, outlet temperature and relative humidity, coefficient of performance, and cooling capacity are described in the table.

7.3. EU Subsidies and Life Cycle Assessment of Advanced Climate Control Technologies

Financial incentives, supportive policy frameworks, and technological innovation are necessary for the successful arrangement of energy-efficient temperature control systems in agricultural greenhouses. To encourage the use of solar-assisted evaporative and desiccant cooling systems, the European Union (EU) has launched several national and regional programs. These incentives support the shift to environmentally friendly practices in controlled agriculture and lower the financial obstacles related to initial capital investment [150,151]. Italy’s Conto Termico initiative serves as a prime example of policy-driven support for renewable cooling systems. Installing renewable heating and cooling systems, such as solar-assisted desiccant systems, is financially supported by this incentive program. Payments for solar thermal collector systems are determined by the energy output and collector area under this plan. For example, bigger systems may be eligible for five-year programs, whereas installations utilizing up to 50 m² of collector area are eligible for incentives paid out over two years. This method greatly reduces the payback period of these technologies, making them more economically appealing in both commercial and agricultural contexts [152,153]. The EU has made significant investments through its research and innovation programs, especially Horizon 2020 and LIFE, to assist the development and implementation of sophisticated cooling technologies in addition to national initiatives. To meet 100% of the heating and cooling energy demand in new DHC and up to 60–100% in retrofitted DHC, the WEDISTRICT project intends to demonstrate DHC as an integrated solution that takes advantage of the combination of RES, thermal storage, and waste heat recycling technologies. WEDISTRICT aims to replicate best practices on a wide scale by increasing the value of local resources, such as waste heat and renewable energy, and increasing the efficiency of District Heating and Cooling networks by utilizing new resources [154].

A common technique in design stages to balance embedded and operational emissions is life cycle assessment (LCA). Operational energy refers to the use of heat, water, and electricity, while embedded energy includes the energy used for raw materials, manufacture, transportation, production, maintenance, and disposal [155]. LCA has been used in recent research to support the sustainability credentials of desiccant-assisted evaporative cooling systems, especially in agricultural contexts. LCA on a desiccant–dew point evaporative cooling system combined with a water reclamation unit was carried out in hot and humid conditions [156]. Results show that the hybrid system improved its environmental sustainability by drastically lowering greenhouse gas emissions and water consumption when compared to traditional cooling techniques. By the international standards of series ISO 14040 [157,158], another LCA of an air handling unit desiccant cooling (AHU-DEC) system fitted with hybrid photovoltaic/thermal (PV/T) collectors was completed. The analysis’s goal was to evaluate the solar-assisted AHU DEC system’s energy and environmental performance. Moreover, another case study on the LCA of evaporative heating ventilation air conditioning (HVAC) systems, also called an instant direct cooling system, was performed in Denmark. The LCA results show that the evaporative system has a 24–40% lower total climate impact score than the vapor compression system. This is explained by a 5–12% reduction in the operational climate effect and a 60–71% reduction in the embedded climate impact [155].

8. Challenges and Future Perspectives

Evaporative cooling and desiccant dehumidification systems for agricultural greenhouses are potentially promising for sustainable climate control. Despite this, many challenges must be overcome to improve their effectiveness, economic viability, and widespread implementation. The main challenge is that evaporative cooling depends on ambient climate conditions [159]. Desiccant dehumidification systems demand uniform regeneration of desiccants, which can be energy-intensive, especially in large-scale greenhouse operations [2]. Another crucial issue is the high water consumption of evaporative cooling systems and the energy consumption of the desiccant system for regeneration. The utilization of external energy inputs for the regeneration of desiccant dehumidification units generally increases the operational costs of that process [160]. Furthermore, evaporative cooling systems require regular maintenance to protect against microbial growth and retain the desiccant materials’ efficiency for a long time. Maintaining a consistent temperature, humidity, and CO₂ level across large areas become quite difficult. Poor crop growth and higher losses from insect and disease attacks are the results of these issues. In addition, challenges to the large-scale adoption of advanced systems exist because of their initial system cost and regular maintenance requirements. In reality, these larger areas need more energy for lighting, heating, and cooling, which raises energy costs and greatly affects the environment. As the greenhouse grows in size, the distribution and supply of water and nutrients become increasingly difficult, potentially leading to waste of these resources. Large-scale, conventional greenhouses are prone to several structural issues, such as weight bearing and a propensity to be quite challenging to extend or modify.

Further research should consider improvements in the design of advanced materials and systems to increase consistency and effectiveness. Dehumidification performance can be improved by adopting innovative desiccant materials with higher moisture absorption capacity and lower regeneration energy requirements. Similarly, improved heat exchanger design, the application of coatings, water recycling features of evaporative cooling systems, efficient evaporative media (low pressure drop and high saturation efficiency), optimized watering/spraying, shading, leakproof water tanks, and adjusting fan speed based on real-time conditions can minimize the loss of water associated with these systems and increase operational life. Another promising method to make these cooling and dehumidification technologies more sustainable is to combine them with renewable energy sources, such as solar thermal and photovoltaic systems. The capability of solar energy-assisted cooling systems to decrease the reliance on conventional energy sources makes them more attractive and eco-friendlier for greenhouse temperature and humidity control. The following characteristics of large greenhouse designs also successfully handle scalability issues: Unit homogeneity facilitates expansion and maintenance. Growers can begin with a simple design and add components as needed to meet demand and budgetary constraints. Energy-efficient technology, such as automated climate control systems and insulated panels, can be used to construct scalable systems. Commercial practicability and policy support will contribute significantly to the extensive acceptance of these technologies. More farmers and commercial growers could be encouraged to adopt energy-efficient greenhouse systems with the help of government incentives, subsidies, and financial support. Moreover, techno-economic evaluations and cost–benefit analyses are needed to assess the viability of these systems in different climatic regions and agricultural scales.

9. Conclusions

This study provides a review of greenhouse temperature and humidity control systems, focusing on the importance of the greenhouse VPD and temperature/humidity control. The study explored traditional temperature/humidity control practices, including greenhouse shading practices, ventilation systems, fogging systems, and vapor compression air conditioning systems. Direct, indirect, and Maisotsenko-cycle evaporative cooling systems and solid and liquid desiccant dehumidification systems have been reviewed. In addition, integrated desiccant-assisted evaporative cooling systems and hybrid systems have been reviewed. Results show that in hot and sunny or mild-to-hot climates, shading practices, natural ventilation, and fogging systems are inexpensive and water-efficient solutions that work well for partial climate management. Although VAC systems provide accurate temperature and humidity control, these have significant energy consumption and environmental impact. MEC systems effectively reduce ambient temperature up to the ideal range while maintaining the humidity ratio, and both dehumidification systems effectively regulate humidity and are suitable for greenhouse farming in high-humidity regions. The integrated and hybrid systems improve energy efficiency, water consumption, and the stability of the controlled climate in greenhouses. Regular maintenance, initial system cost, high complexity, and energy consumption for desiccant regeneration are significant challenges to adopting these modern temperature and humidity control systems for greenhouses. Future research should concentrate on developing innovative materials, formulating creative system designs, and integrating renewable energy to maximize these systems for large-scale applications. Policies, financial incentives, and field-scale validation are also essential to promoting the broad adoption of these technologies in commercial greenhouse farming.