2.1. Renewable-Energy-Powered Cooling Systems
- (i)Solar Thermal systems
Solar thermal applications in agriculture have the advantage of the heat being generated by solar radiation. They include desalination processes, crop drying, greenhouse heating, as well as solar cooling, which is the most promising technology, given that the peaks of the cooling requirements in greenhouses match the solar radiation peaks. Solar cooling thermal systems use the thermal energy of the sun as an energy source to generate coolness. They have been widespread in protected cropping agriculture for years since they offer a zero-impact technology. Their performance has been consistently investigated and improved by coupling solar collectors to different cooling processes 
. As was detailed in the previous sections, several experimental and numerical studies were performed on solar systems that were based on the evaporative or desiccation cooling processes 
. The results show that solar cooling systems that are adapted for greenhouse units are showing satisfactory results in terms of the efficiency and the economic income. Furthermore, the reviews of the research that focus on the solar cooling processes in greenhouses 
point out that these systems enhance the greenhouse energy efficiency and reduce their dependence on the grid electricity supply. High-performance solar thermal plants, which reach up to 40% 
, consist of integrating concentrating solar collectors (CSCs) into greenhouse cooling systems. The main CSCs that are integrated into protected faming fields are linear Fresnel collectors and parabolic trough solar collectors, which are usually coupled to an absorption chiller. Sonneveld et al. 
performed an experiment on linear Fresnel collectors combined with PV cells to provide the greenhouse with hot water and electric power to be used for cooling and lighting purposes. The linear Fresnel lenses were integrated between the double glass of the southerly oriented roof cover. The Fresnel system splits direct radiation from diffuse radiation, and it concentrates it on the PV modules that are mounted within the focal line of the Fresnel lenses. As a result, the amount of solar energy that is blocked reaches 77%, which leads to a reduction in the greenhouse cooling requirements by about a factor of 4. The Fresnel system also generated 143.89 kWh m−2
of thermal energy, and 29 kWh m−2
of electrical energy, which can be exploited for further cooling by means of an evaporative system.
CSCs are usually mounted with solar tracking systems to collect the maximum radiation, and they are coupled to solar cooling processes, particularly in hot desert locations or rural areas, where electrification is difficult and expensive, and where solar resources are abundant 
. CSCs remain particularly expensive, compared to conventional thermal power generation, and further research and development is needed on this emerging technology for power cooling systems. Alternative ways of improving the operations could be considered and applied, either for the component materials, or for the whole design, in order to enhance the system effectiveness.
- (ii)PV Solar systems
Contrary to solar thermal energy, photovoltaics enable sunlight to be directly converted into electrical power for use in cooling systems, or any other electric equipment, such as pumps, heat pumps, dryers, and artificial lighting. Ghoulem et al. 
demonstrated that a solar cooling system, which was based on a heat pump coupled with PV panels, covered 33.2 to 67.2% of the greenhouse demand in the summer periods. Carlini et al. 
affirm that the efficiency of PV cooling systems ranges between 30% in the summer and 11% in the winter. Actually, the capacity generated by PV cooling systems is dependent on different factors, namely, the location of the panels and their areas, as well as the greenhouse requirements.
The selection of the appropriate panel area and characteristics should accord with the energy demand and the load profile 
. Moreover, the structure and the covering of the greenhouse with large PV panels causes extensive shading, which may contribute to a reduction in the greenhouse temperature and to plant stress in hot climates. This affects the plant growth and productivity 
, as light is considered to be one of the most important sources for photosynthesis.
PV panels, when installed properly and when coupled with cooling systems, show satisfying results, as was demonstrated by Al-Ibrahim et al. 
, who experimented with the use of PV panels of 14.72 kW to cover the electrical needs of a 9 × 39 m greenhouse, which was, namely, an evaporative cooling system. The PV cooling system performance was satisfactorily established since it met the required load of the greenhouse under the hot and arid conditions of Saudi Arabia. As for Ganguly et al. 
, they proved that the cooling solar plant that was tested in India, which combined a fan and pad evaporative system and PV panels, provided the coolness required for a 90 m2
greenhouse. According to them, the PV cooling system constitutes a viable option for powering stand-alone greenhouses in a self-sustained manner. The use of PV systems has expanded in recent years thanks to the decrease in the photovoltaic equipment costs 
. The satisfactory performances of the PV cooling systems will allow this sustainable technology to be promptly implemented worldwide, and specifically in hot and rural locations.
- (iii)Geothermal cooling systems
Geothermal cooling systems, which are often referred to as “shallow geothermal systems”, consist of a ground pipe that is implanted at a depth that is inferior to 100 m, and that exploits the relatively stable low-temperature earth surface to exchange heat and deliver cooling in warm and hot climates 
. Ground heat exchangers are mainly classified into three types: vertical, horizontal, and basket.
The earth–air heat exchanger systems have been studied and tested in several countries, and usually with satisfactory results, such as in Thailand 
, where the cooling performance and condensation impact of a horizontal earth tube system, at a depth of 1 m, was investigated. During the summer season, the generated cooling capacity of the system was about 74.84%, and the COP, which is defined as the ratio of the cooling power to the electrical input power, reached 3.56 
. In Kuwait 
, the greenhouse temperature reduction was about 2.8 °C for a 1.7 m ground-buried heat exchanger. Several studies also focus on geothermal heat pumps, which are vapor compression systems that use the relatively stable earth surface temperature as the heat exchange medium, instead of the outside air temperature, in order to produce either cooling or heating power. Rabbi et al. 
show that geothermal heat pumps outperformed all the other heating methods, except for natural gas. Sanaye and Niroomand 
performed an optimization study of a ground heat pump in Iran, which reached a capacity of 8 to 32 kW, and a coefficient of performance (COP) that varied from 3.9 to 5.4. Boughanmi et al. 
studied the thermal performance of a conic basket heat exchanger, which was implanted at a 3 m depth and coupled to a geothermal heat pump for greenhouse cooling in the Tunisian climate.
The heat pump COP is defined as the ratio of the amount of heat extracted from the greenhouse by the compressor input power. The overall process COP is defined as the ratio between the amount of heat absorbed from the greenhouse and the total electric input power (to the compressor and the pumps). An evaluation of the thermal performance of the system shows that the heat pump COP varied from 3.9 to 4.7, and that the overall process COP ranged between 2.82 and 3.25. The maximum average temperature difference between the inlet and outlet of the geothermal process system was approximately 30 °C. Hence, the greenhouse temperature was decreased by about 12 °C.
2.2. Future Trends in Cooling Systems
- (i)Day-to-night thermal storage
During the last few years, several types of thermal storage have been exploited in greenhouses, which aim to take advantage of the available heat sources (solar gain, ground heat, exhaust heat, etc.). The thermal storage achieved by means of storage mediums, namely, water, rock bed, soil, and phase change materials (PCMs), enhanced the overall thermal performance of the greenhouse 
- (ii)Closed Desiccant Greenhouses
In a closed desiccant greenhouse (Figure 1), the humidity is consistently withdrawn from the hot air by a fluid desiccant that allows for the regulation of the humidity and the temperature, and for the recovery of the heat. A particular surface covering material and design can also be applied in closed desiccant greenhouses in order to guarantee condensation and the recovery of the water vapor that is evaporated by the plants.
Figure 1. Closed desiccant greenhouse (daytime operation).
During the day, the hot humid air in the greenhouse drops in the counterflow with the cold dry fluid desiccant. Then, it reaches the crop zone as cold dry air, while the hot diluted desiccant solution is buffered in the thermal storage. Air cooling is also achieved by means of the evapotranspiration of plants. During the night, the amount of heat stored is used for the desiccant regeneration, as well as for greenhouse heating, and, hence, the evaporation process drags the humid air up into the covering surface, where it condensates and can be recovered. The main advantage of this technology is that it is independent of the air humidity 
. Hence, its implementation is suitable either for hot arid or humid climates. In fact, the first prototype of a desiccant greenhouse, which is mounted in Cairo, Egypt, is under experimentation 
. This emerging technology offers the substitution of energy-intensive mechanical cooling units by a low-cost and economical heat-driven solution.
The thermal energy required for a closed desiccant greenhouse is provided either by the solar thermal energy or the residual heat. The source of the residual heat can be either the return air of the heating loop, or the unexploited heat of an industrial process, or an air-to-air heat exchanger or ground heat exchanger (Figure 2
). A desiccant greenhouse that is coupled to an air-to-air heat exchanger requires less mechanical ventilation, and important heat transfer occurs without resorting to water use. In addition, this system guarantees lower temperature and humidity levels than desiccant greenhouses 
. Actually, this system is being installed at the National Research Institute for Rural Engineering, Water, and Forestry (INRGREF) in Tunisia in order to test its performance and operational capabilities.
Figure 2. Desiccant greenhouse with use of additional residual heat (daytime operation).
Closed desiccant greenhouses can also be coupled to a solar thermal source, such as solar ponds or solar thermal collectors, namely, concentrating solar collector systems that generate high temperatures, such as parabolic trough collectors (PTC) or Fresnel collectors 
. Closed desiccant greenhouses also offer the opportunity of being mounted onto several existing and emerging systems, for instance, CSP plants. The waste heat of the CSP plant is harnessed into the desiccant regeneration, and the cooling energy is transferred to the CSP plant; hence, the coupling of these two technologies could be very effective if the ventilation electricity consumption is lowered. Nonetheless, it is worth developing closed desiccant greenhouse technology in a cost-effective way. For instance, coupling desiccant greenhouses to simple plastic solar absorbers, which are two magnitudes lower in cost than concentrating solar collectors, makes this technology more profitable and convenient for growers. Consequently, this technology is considered to be among the most promising future trends in greenhouses, particularly in hot and arid climates. To this end, the INRGREF has planned to continue future work on testing desiccant systems in a prototype closed greenhouse in Tunisia, using calcium chloride and magnesium chloride desiccants, with relatively low solution concentrations that do not require high temperatures for regeneration.