A review of upgradation of energy-efficient sustainable commercial greenhouses in Middle East climatic conditions

1.1 Conventional greenhouse HVAC systems

Existing greenhouse systems, i.e., conventional greenhouse systems, are considered as experiencing high heat loss rates due to the high energy demands of the materials used to construct the greenhouse envelopes. As Ganguly and Ghosh (2011) explained, greenhouses that have cooling and ventilation systems need constant supply of electricity. The use of fan-driven cooling pad system is basically made to take away the heat generated by the supplying components as the pad and fan systems which consist of basic exhaustive fans at one end of the greenhouse and a circulating water pump with cooling pad at the other end. In order to maintain the constant temperature level in the greenhouses all the vents' opening and closing are being interfaced with automatic climate control module to trigger the same as per the requirement. Thus, this driven cooling pad system is helpful in removing energy from the air, which lowers the temperature of the air introduced into the controlled farming indoor space (Baptista et al. 2010). The cooling process was highly inefficient as it seemed to be not achieving its maximum productivity and also leading to a high amount of wastage of resources and failing to make the best use of limited time and resources. This high level of inefficiency is seen in the process of recirculation as it has been observed that the conditioned closed space in the conventional greenhouse is done without the use of air grills on the top or sides, which reduces its efficiency and leads to a poor and inefficient process that require much more time and resources. In many mechanical systems, the use of axial fans with metal blades reduces their efficiency of operation. Deployment of a large workforce to maintain and regulate the indoor variables like temperature, relative humidity etc., is also forms a vital factor gar is giving significant affect in the value of net realization which is mainly caused by range recurring of a cooling pad along with the recirculation system maintenance. Figures 2 and 3 represent the 3D and sectional views of a conventional greenhouse.

Figure 2

3D view of the exterior of the conventional greenhouse ventilation.

Figure 3

Sectional view of the conventional greenhouse ventilation.

In the recent years, a large number of researchers have dwelt on the control of factors affecting the indoor conditions of the greenhouses. Researchers investigated the efficiency of using specifically designed HVAC systems (Bot 1983) as well as the use of wet pads (Coelho et al. 2005) and spray cooling (Bot 1989a). Natural ventilation is taken as the most fundamental and vital technique that facilities education in the usage of energy while making buildings. This phenomenon is based on the fact that the rage of cooling power and capacity of an ambient air could be easily harnessed to increase theoverall level of indoor thermal comforts which is a vital factor to be analyzed while designing the air conditioning systems. In order to take advantage of either deriving force or combining both, the building need to be designed in a proper way (Beven and Binley 1992). Further, being an old technology, the natural ventilation still serves and leads to enhanced experience of recent resurgence of better meeting the interest mainly in Europe where a large number of major research initiatives are conducted to evaluate this vital concept of natural ventilation.

1.2 Sustainable greenhouse

Greenhouse structures enhance developing situations of vegetable, fruits, and decorative vegetation. Commercial greenhouses protect flora from unfavourable atmospheric dealers and, collectively with appropriate gadget, impact, and in the end modify the crop micro-climate, consequently increasing the marketplace accessibility of products, enhancing their high satisfactory, and permitting high yield. Thus, the consistent global growth within the region protected by means of greenhouses has led to enhanced need for growing sustainable blanketed horticulture. Sustainable greenhouse horticulture may be achieved via means of various cultivation techniques good enough to control gadgets and also by progressive substances aimed at lessening the agro-chemical substances, electricity use, and water intake.

2.1 Greenhouse ventilation

For the successful growth of crops in agricultural greenhouses, there is a need to control and maintain the temperature and humidity at optimum levels. These optimum conditions are required for the successful survival and yield of the crops. Settles (2000) analysed a basic greenhouse model to identify the controlled parameters such as temperature, relative humidity, and air velocity for plant growth in early days. The control parameters mainly consist of the factors that can be controlled or managed as per the need. With respect to the current greenhouse ventilation, the various controllable factors comprise temperature, relative humidity, and air velocity for plant growth, which are taken into full control and can be managed as per the need and requirement.

In a common way, the structure used as a ventilation vent that forms a part of the greenhouse would have a much significant impact on the level of the ventilation of the greenhouse. Greenhouse structure, design, and geometry are modelled to withstand the adversity of external factors such as the wind, rain, snow, etc. Along with the protection from environmental factors, internal factors which includes the dead and live loads for the entire greenhouse also shall be designed in an efficient way to maximize the optimal solar radiation for the crop growth. The structural components of the greenhouse and their geometry seem to directly affect or associate with the solar radiation transmission (Bot 1989b). The geometry of the greenhouse designs is basically reflected in terms of the ratio of the expected length and width, which is assumed to be 30 m and were around 10, 10, and 8, respectively, to maintain the proper geometry and design of greenhouse. Various studies have been carried out to understand the effect of the vent structure and orientation in commercial greenhouses (Albright et al. 2001). Generally, on the basis of greenhouse design and construction, the ventilation system comprises side, roof, and a combination of two in greenhouses.

Commercial greenhouses that utilize conventional air-conditioning systems are not as effective as in terms of cooling; and with humidity controls, they ought to be operating at half of their microclimate control potential. The concept of the microclimate is mainly defined as a fundamental factor which is based on the temperature, humidity, and speed of moving air. Thus, the microclimate control potential system is mainly designed for having a climate control of a sustainable greenhouse for increasing or lowering the temperature and relative humidity. The use of natural ventilation is made but the forced ventilation is not taken into account as the number of additional equipment and tools required for forced ventilation results in increased cost of greenhouse. Thus, to maintain the cost efficiency and to ensure a greenhouse with a normal use of technique, only natural ventilation is implicated and forced ventilation is not used (Coelho et al. 2005).

2.2 Greenhouse cooling using evaporative systems

Various researchers have explored the utilization of evaporative systems in temperature management of greenhouses. An evaporative system mainly comprises an intake chamber along with various filter(s) with supply fan which acts as a heat exchanger in order to maintain the proper cooling and temperature inside a greenhouse. The use of exhaust fan, water sprays, recirculating water pump, and water sump are made to have a proper and efficient evaporative cooling systems which are mainly characterized by low energy use compared with a refrigeration cooling system. A typical evaporative cooling system components and the flow of operations have been shown in Figures 4 and 5. Joudi and Hasan (2013) studied the effectiveness of using absorption-type evaporative coolers in Baghdad. Besides this, it has also been observed that the use of evaporative cooler is also vital to ensure effective cooling along with air-condition as it has been understood that evaporative coolers are utilized to perform both functions. Thus, making efficient utilization of evaporative coolers is better and suitable while covering HVAC as it performs both the important and crucial functions of cooling and conditioning the air (Linker et al. 2011).

Figure 4

Box-type evaporative cooling system.

Figure 5

Working principle of an evaporative cooler.

The annual total amount of coal required for maintaining and running an HVAC system basically ranged from 160.34 to 466.78 t/ha, while the total cost of the system is between 37,412 and 108,916 $/ha. Apart from this, the total annual fixed cost on the basis of the cost per hectare was taken to be between 10,325 and 14,328 $/year whereas the total variable cost fluctuates and varies from 20.1 to 30.9 $/ha. Further, the total cost which was calculated as the total annual and hourly costs per hectare ranged from 65,891.5 to 151,220.6 $/year and from 23.8 to 34.2 $/ha, respectively (López-Cruz et al. 2012a). The upgradation, therefore, aims at minimizing this cost by using efficient evaporative coolers, where the cooling load itself is reduced by using a covering material appropriate for Middle East’s climatic condition. Figure 6 shows the structure of such an upgraded greenhouse.

Figure 6

Single span greenhouse with individual box-type evaporative coolers and ventilation fans (V-flow fans) – sectional view and 3D view.

An in-depth analysis of these factors are discussed as a part of qualitative analysis while concentrating and focusing on further evaluation of these factors to be used in the commercial greenhouse (López-Cruz et al. 2012b). Existing technologies implement the evaporative cooling technique to manage temperatures. One such technology system that reduces air temperature without altering the air moisture content is the indirect evaporative cooling system. In such systems, the process air stream does not make a direct connect with the cooling fluid stream although there is sensible heat transfer between them leading to the cooling of one stream. Thus, an analysis can be made that the indirect evaporative cooling system has an efficiency of 60–70% as compared with the direct evaporative coolers, as it costs nearly about one-half and also makes use of only about one-quarter of energy compared other cooling systems; thus, it is more efficient than other systems (López-Cruz et al. 2014). Thus, here the discussion about the thickness and width of cooling pad structure is made, based on that a requirement to select the proper packing material has been identified in order to meet the desired water flow (Luo et al. 2005). The improved technology using Maisotsenko cycle (M-cycle) can provide a better result in energy-friendly evaporative cooling operation. This cycle is mainly taken as an indirect evaporative cooling–based cycle that makes utilization of a much smarter geometrical configuration for leading and ensuring better air distribution. The energy source of this type of cooler is water in place of electricity; thus, the usage of M-cycle-based coolers ensures a more significant way for energy saving as it leads to more than 80% saving in terms of average consumption of electricity (Mashonjowa et al. 2013a).

The M-cycle in greenhouse is used as an air-conditioning technology that offers better opportunities for energy conservation and reduction of greenhouse gas emissions, thus leading to better growth of crop. Kreid et al. (1978) had investigated the finned evaporative coolers along with the range of other condensers (Mashonjowa et al. 2013b). The above discussions have been summarized in Table 2.

To sum up, based on the research available on upgradation of commercial greenhouses, evaporative cooling systems are suitable for high-temperature and low relative humidity ambient conditions. Because of such climatic conditions in the Middle East, we have used custom-designed box-type evaporative coolers for better performance in terms of yield management in commercial greenhouses. Effective results from the evaporative coolers require the cooling pads to be saturated at all times. The discharge between the fans and cooling pads also has to be negotiated effectively employing proper motor controls. This can be achieved with box-type individual evaporative cooling configurations, making this more energy saving, simple, environment friendly, and cost-effective air-conditioning strategy.

2.3 Greenhouse heating

Different greenhouse heating systems have been adopted in the past, such as direct air heaters, central pipe surface heating systems, geothermal heating, etc. Ghosal and Towar developed a greenhouse model with geothermal energy suitable for freezing temperature conditions. In this study, the authors realize a remarkable temperature rise of 14–23°C while it was freezing outside the greenhouse. Carlini et al. (2010), through their experimental study conducted in Viterbo, Italy, proved the feasibility of geothermal plant through a TRNSYS simulation and verified the results, thereby confirming the efficacy of using geothermal solutions. Table 3 summarizes the list of reviewed papers, with their major findings and conclusions.

Various options for greenhouse heating have been mentioned in the literature. One of the major drawbacks in many of the upgradation practices were the level of CO₂ emission. Qerimi et al. (2020) carried out a solar thermal energy model for the upgradation of urban areas to avoid CO₂ emission mainly from buildings and other sources. Upgradation using solar thermal energy system similar to other buildings in urban areas to reduce the CO₂ emission is an innovative option where the controlled farming can convert into more sustainable way. In our upgraded greenhouse, HVAC control systems are capable of controlling the CO₂ level without any additional CO₂ generators which is a significant contribution towards sustainability.

3.1 Greenhouse covering materials

The covering material for the greenhouse, which is integral to its design structure, is chosen based on the effective control of yield management. This is similar to the shading system associated with the structure. A study was conducted in United Arab Emirates to interpret the results from different combinations of covering materials which include Solexx twin-wall covering, poly film plastic greenhouse covering, single-walled polycarbonate, and twin-walled polycarbonate. Structures have been tested using two different combinations of polycarbonate covering materials with the conventional covering material using polyethylene (Udink ten Cate 1984). The experiments concluded that multilayer polycarbonate combinations of covering materials are providing a higher yield of up to 20% compared to the conventional polyethylene sheet. A vital parameter of greenhouse cultivation is light. Nageib et al. (2012) experimented with various shading options in cucumber farming, which is taken as the most high-yield vegetable during the course of greenhouse methodology. In the commercial greenhouses, cucumbers are grown, which tend to have lower percentages of defective growth and higher commercial yield than the ones which are grown in the open field. Abu-Zahra and Ateyyat (2016) chose shading materials with proper optical properties based on the photosynthetically active radiation of the crop.

3.1.3 Covering material upgradation

Behera et al. (2016), in their study, reviewed the properties of polyethylene sheets used for greenhouses and covering material upgradations. From this study, the major inference is that a polycarbonate sheet with two layers of 2 mm each in operation provides better result than a single layer of same thickness. Real-time cultivations were carried out for one complete cycle of cucumber growth in those three different greenhouses. Real-time cultivation results emphasized that the crops harvested from the polycarbonate sheet-mounted greenhouse yielded more with good quality, and this has also been verified by spectrometric analysis by making use of filed devises like the temperature sensors and PAR meters (Tap 2000).

Test setup was in three different greenhouses with polyethylene as material 01 while the other two comprise polycarbonate of different thicknesses 2 mm and 4 mm as materials 02 and 03, respectively (Stanghellini and de Jong 1995). A general methodology of covering material selection has been shown in Figure 7. Hochmuth (1993), in his study, outlined the various properties of polyethylene sheets and their correlation to the agronomical gradients which are defined as different parameters for the plant growth. Teitel (2007) observed the effect of thickness of the covering materials on plant growth and concluded that polycarbonate sheets produce the best results in terms of temperature control and crop yield.

Figure 7

Flow chart of the covering material selection methodology.

Figures 8 and 9 show the experimental setup comprising greenhouses with polyethylene and polycarbonate as the covering materials (Wang and Liang 2006). The first greenhouse has a polyethylene covering while the second one has a polycarbonate covering of 4 mm thickness. The double-film-covered setup works well with heat storage–release systems; the covering materials coated both sides with thermal absorbent coatings that can manage the heat generated from the underground thermal systems and this has been verified in the study conducted by Wang and Liang (2006).

Figure 8

Polyethylene sheet covering material.

Figure 9

Polycarbonate covering material.

Hochmuth (1993) successfully carried out the spectrometric analysis of various combinations of the available shading options and recorded the optimal combinations. Results of a spectrometric analysis using the licor-LI190 R sensors are summarized in Table 4.

Table 4

Spectral analysis of the covering materials as reported by Hochmuth (1993) and Subin et al. (2018)

Material	Range	Wavelength (nm)	Transmittance (%)		Reflectance (%)	Absorptance (%)
			Total	Direct
2 mm double-wall polyethylene sheet	PAR	400–700	90	66	13.4	7.2
2 mm polycarbonate sheet	PAR	400–700	63	41	23.4	14.2
4 mm double-wall polycarbonate sheet	PAR	400–700	27.2	11.8	56.1	17.2

Abdel-Ghany et al. (2015) studied the effect of the overall shading on indoor plant growth and analysed the U-value characteristics (the thermal transmittance of a material or an assembly is expressed as a U value). Nelson (1998) analysed the shade curtain components with detailed upgradation plans suitable for plant growth while Radojević et al. (2014) emphasized the effect of energy balance in HVAC systems upon shading upgrades. Rumsey recorded a detailed study about the high-density polycarbonate shade screens and the advantages of the same in terms of energy efficiency as well as the crop protection from the sunlight (Speetjens 2008). Sanford (2011) indicated how shade curtains are useful in reducing heating and cooling costs. Since the curtain thicknesses range from 0.5 mm to 3 mm, which can be controlled using a manual arrangement of roller or sensor for automated climate control. Curtain influences the incoming transmission of light and heat retention within greenhouse, and both these factors highly influence the total energy usage of greenhouse (Figures 10 and 11).

Figure 10

Micrograph of the upgraded covering material with HDPE-reinforced energy curtains.

Figure 11

Greenhouse using HDPE fibre-reinforced shade curtains.

With the use of curtains, the total energy costs related to commercial greenhouse might decline by 25–40%. Apart from this, proper use of climate screens assists in optimizing the overall climate in greenhouse that will lessen the cost as well as the intensity of activities associated with crop management including spraying, watering, and pruning (Sethi and Sharma 2007a). A summary list of the reviewed articles along with their major findings and innovations is detailed in Table 5.

Table 5

Reviewed articles, approaches, and parameters studied

Author	Approach	Aspects of study
Nageib et al. (2012)	Shading	Declared the importance of crop shading in the growth of apricots
Abu-Zahra and Ateyyat (2016)	Shading methods	Analysed how shading techniques affect the survival of cucumbers
Abdel-Ghany et al. (2015)	Shading properties and plant characteristics	Researched about the correlation with the photosynthetic active region and the shade properties
Taleb (2014)	Passive cooling	Recommended the use of passive cooling to reduce energy consumption
Nageib et al. (2012)	Shading	Declared the importance of crop shading in the growth of apricots
Sanford (2011)	Shade curtains	Studied greenhouse structure and materials and identified the benefits that curtains present to heating and cooling cost reductions
Teitel (2007)	Covering materials	Analysed suitable materials for greenhouse covering and explored the best combination for the polycarbonate sheet

Based on the available literature on upgradation of covering materials, the major challenge was the selection of covering materials and its thickness. However, based on the methodology shown in Figure 7, a polycarbonate sheet of 2 mm thickness provides good yield in the Middle East climatic conditions with a reasonable capital investment. Moreover, a specially designed shade curtain supplements the covering materials efficiently. The transparent energy screen and traditional shade curtains are meant for the retention of energy and enable increased level of light transmission. Proper installation of curtains and screens are important for maximizing cost saving. This also improves the yield by up to 20% and maintains the temperature, ensuring proper growth of crops.

3.2 Greenhouse direction or orientation

An all-year greenhouse model has been developed; and in 24° E–W direction, the results were successful. Thermal modelling of the successful model has been carried out in the previous studies by researchers and proved the abovementioned recommendations (Panwar et al. 2012). Selection of optimal shape of the greenhouse in conjunction with the orientation may provide an improved result in the arid climatic conditions especially in Middle East regions (El-Maghlany 2015). E–W direction with arch-shaped combination can provide a good crop yield (Sengar et al. 2013). Greenhouse location selection is crop-specific and based on the required temperature requirement of the crops.

3.3 Greenhouse irrigation systems

Irrigation is defined as the artificial application of water to soil by different systems of pumps, tubes, and sprays. Different types of irrigation systems uniformly supply water to the whole field. The water for irrigation comes from the ground through surface water, wells or springs, reservoirs, lakes, rivers, and other sources. Some common irrigation system types include surface irrigation in which the water is distributed across and over the land through gravity and no mechanical pump is included (Sethi and Sharma 2007b). Localized irrigation is another type of irrigation in which the water is distributed through piped network under low pressure and applied on each plant. Apart from this, sprinkler irrigation is also one of the types in which water is distributed through overhead high-pressure guns or sprinkler from a central location in the field. In addition to these types of irrigation, other types include drip irrigation, centre pivot irrigation, lateral move irrigation, manual irrigation, and sub-irrigation.

4.1 Climate control module

Climate control module with a microcontroller in the sensing unit was tested in the field to evaluate their data transmission with PLCs in the beginning. A scientifically designed control system in a commercial greenhouse is becoming a necessity for high crop yield by controlling the field requirements of the crops with minimal human interference (Udink ten Cate 1985). Irrigation and the crop transpiration rates are controlled using the respective sensors mounted at various locations of the greenhouses and aided in accurate measurement monitoring necessary to trigger corrective measures in greenhouses (van Ooteghem 2005, 2007, 2010). Temperature and relative humidity sensors can log the variance between the set value for the crop and this can improve the crop quality as well as avoid typical phenomena like tip burn for the crops due to the sudden fluctuations in temperatures. Figure 12 illustrates a typical climate control module schematic in a commercial greenhouse.

Figure 12

Climate control setup (Subin et al. 2020).

Implementation of a nonlinear multivariable transfer function control system model of a greenhouse using thermodynamic laws in conjunction with the variables directly affecting temperature and relative humidity can maintain the greenhouse indoor conditions in an optimal manner. Mamdani model-based fuzzy proportional integral derivative (PID) has been developed (Subin et al. 2020) to compare the performance and productivity level of a PID and proportional integral (PI) in order to accomplish a smooth and better controllable action. A typical control system schematic using fuzzy logic controllers for commercial greenhouses is shown in Figure 13. A concise list of the reviewed articles and their major findings and conclusions are shown in Table 6.

Figure 13

Schematic for an upgraded greenhouse using fuzzy logic controllers (Subin et al. 2020).

Bluetooth technology has been used to control the indoor agriculture ambient conditions in our upgraded greenhouse. Based on the previous studies conducted, upgradation in control system using PID and PLCs maintained the required indoor ambient conditions. By implementing a Mamdani model-based fuzzy PID controller, the yield improved by up to 20% with properly selected HVAC systems. Further enhancement is possible using artificial intelligence and neural network systems which is beyond the scope of this study.