Greenhouse cooling technologies were introduced.
The latest developments and technical specifications in greenhouse design considerations and technologies were reviewed.
Mapping sustainable development goals to greenhouses was discussed.
Future directions for the development of greenhouses were identified.
Abstract
Demand for food is rising to match population growth rates. Meanwhile, many countries are struggling to guarantee crop production all year round due to harsh weather conditions in the cold and hot seasons. Greenhouses are used to mitigate harsh weather conditions and control the microclimate, ensuring that crops achieve the required level of temperature, humidity, water and light, enabling year-round crop production. Various design factors play a crucial role in the successful placement of the greenhouse. These parameters include shape, orientation, structure, cladding material and climate control technologies. Climate control strategies include cooling and heating technologies. This article provides a comprehensive review of cooling technologies for greenhouses, especially those used in hot climates. Technologies such as natural ventilation, shading and reflection, evaporative cooling, desiccant cooling and combined cooling technologies are presented and discussed. Furthermore, this article reviews recent developments in the field of refrigeration technologies and systems. The paper also describes current strategies to save energy and improve the efficiency of greenhouses. The results of this review are expected to guide researchers in the process of selecting the most appropriate cooling technology for their region.
Introduction
Air, water and food are basic needs for the survival of all living things [1]. With the unprecedented increase in the global population, the demand for food has increased several-fold, highlighting the importance of modern agricultural practices that are as independent as possible from the external environment [2], [3]. Food supply shortages left 811 million people without food in 2017 and around 820 million people without food in 2018 [4]. Moreover, the situation is expected to worsen, especially with armed conflicts around the world. The production and preservation of food is crucial for ensuring the food supply to society [5]. However, many countries struggle to guarantee crop production all year round due to harsh weather conditions during the cold and hot seasons. In countries with scorching weather conditions, the intensity of sunlight radiation causes high temperatures, which limits food production [6]. These countries focus on food imports as it is difficult to produce crops regionally. However, due to the COVID-19 pandemic, many border restrictions and lockdowns have been imposed to limit its spread, food transportation between countries has been restricted and harvests have slowed in many regions. Moreover, the COVID-19 pandemic threatens the world’s food security, leaving 265 million people without food by the end of 2020 [7].
Greenhouses are used to alleviate harsh weather conditions and allow year-round crop production by controlling the microclimate and ensuring that crops obtain the required level of temperature, humidity, water and light [5], [8]. 9]. It also provides protection against dust, dirt, animals and rain, as well as ensuring a high quality animal supply [5]. As a result, greenhouses will ensure the availability of regional food supplies. An important tool used to influence plant production in greenhouses is climate control [10]. Climate control maintains an optimal environment for plants to survive and grow by keeping the temperature, amount of light, humidity and CO2 absorption as needed. Indoor climatic conditions depend not only on outdoor climatic conditions but also on the types of crops grown in the greenhouse. Most species grown in greenhouses require temperatures ranging from 17 to 27 °C [11]. This is a problem for countries with hot, humid and arid climates, living in hot and arid regions [12], [13]. In the summer months, the ambient temperature in such countries can reach 50 °C and the humidity can exceed 90%, making it necessary to use cooling methods to control the temperature and humidity levels in greenhouses. In addition, it is very important to regulate and maintain humidity levels within the desired range because low humidity levels will inhibit plant growth. In contrast, high humidity levels will increase the occurrence of plant fungal diseases that limit plant growth and development [14]. Therefore, the desired range of relative humidity in hot and arid regions is between 50% and 85% [15].
The aim of this review is to conduct a comprehensive survey of the existing literature focusing on cooling technologies for greenhouses. The innovation here stems from a change in the common perception of the function of the greenhouse when used in warmer climates, where cooling is necessary. The concept of a greenhouse usually refers to an enclosure that traps thermal energy through the modulation of incoming solar energy wavelengths. This happens by taking advantage of the available sun in areas with temperatures too low to allow crops to grow during the cold seasons, or in areas where they are not native due to low temperatures. Cooling such greenhouses for the above-mentioned applications is counterintuitive, especially given that global warming caused by the accumulation of carbon dioxide in the lower layers of the atmosphere is caused by the “greenhouse gases” produced and trapped. In warmer climates, the definition of greenhouse is currently used to refer to a protected environment for growing crops, and hence different ways are designed to control the temperature inside this enclosed space. Some mechanisms are passive and rely on reducing the solar radiation reaching inside the enclosure, while others require expending energy to eliminate the cooling load. This study includes comparisons and discussions on a wide range of greenhouse cooling technologies. These technologies are categorized according to their energy requirements, passive design and optical properties of the coating materials. This relates to discussions on natural ventilation technologies, shading and reflection techniques, evaporative cooling technologies and desiccant cooling systems. Other review papers have previously been conducted to analyze and discuss different greenhouse cooling technologies, including Ghani et al. [9], Ghoulem et al. [16] and Kumar et al. [17]. However, this study focuses on recent advances in technologies used to cool greenhouses. The results of this analytical review will help researchers to select the most appropriate greenhouse cooling technology suitable for specific geographical regions. There are many components to optimize in a greenhouse and these are listed in Figure 1 as the main design parameters to consider in the review of the literature. These parameters include cover materials, enclosure design, orientation and structure, climate control and irrigation/nutrient distribution.
Chapter snippets
Considerations in greenhouse design
Controlling all microclimatic conditions of a greenhouse will lead to optimum crop growth rate, which in turn will lead to various economic, social and environmental advantages and disadvantages [18]. In hot/arid climates, greenhouses consume more electricity as cooling systems become a necessity to achieve the required temperature inside the greenhouse [19]. Electricity consumption in greenhouses can cause some environmental negative impacts such as ozone depletion, pollution and climate change.
Mapping sustainable development goals to greenhouses
The United Nations (UN) organized a conference to present the UN’s policy framework for Sustainable Development in 2015. The Framework states that achieving the 17 Sustainable Development Goals (SDGs) and 169 related targets will ensure sustainable world development. The UN’s policy framework has been streamlined to ensure that the needs of future generations do not compromise the needs of current generations [42]. In order to map these 17 sustainable targets to greenhouses,
Greenhouse cooling technologies
During the summer season, many countries face many challenges in growing crops, even using protected growing facilities such as greenhouses. Greenhouses therefore benefit from cooling technologies that play an important role in controlling the microclimate of the greenhouse. But choosing the most appropriate cooling technology for the greenhouse environment is a challenge many growers face. Selecting the optimum cooling technology requires consideration of the crop
Fan and pad evaporative cooling systems
Direct evaporative cooling systems are classified as fan and pad systems and misting/fogging systems. These evaporative cooling systems are used to improve the environmental conditions in a greenhouse [38]. Fan and pad evaporative cooling systems are well-established cooling technologies introduced in 1954 to equip greenhouses in hot regions with the necessary cooling requirements. These systems use exhaust fans, evaporator pads and circulation pumps to achieve cooling.
Misting evaporative cooling systems
The misting/fogging system is the second type of direct evaporative cooling system that maintains a favorable environment for optimum plant growth. Misting systems use small nozzles to spray fine water droplets into the greenhouse; the water droplets range in size from 2 to 60 µm. Keeping the size of the droplets within this range ensures that the droplets evaporate quickly before they reach the plant leaves. The high evaporation rate of water droplets leads to significant reductions in air pollution.
Combined cooling technologies
Often the use of a single cooling technology is insufficient to control the greenhouse microclimate in regions with extreme weather conditions. This is why many researchers resort to combining multiple compatible cooling technologies to achieve the desired results. Combined cooling technologies provide the optimum amount of cooling required by the greenhouse. Furthermore, the combined technologies reduce energy use and greenhouse temperature [16]. As a result, a suitable
Future directions
This review of greenhouse cooling technologies provides a summary of the process of selecting the best cooling technology for specific geographical regions. To ensure minimum energy input, natural ventilation is sought, combined with the appropriate choice of optical properties. This includes selecting the cover material, choosing the orientation of the greenhouse for proper use of natural shading and reflection, using evaporative, desiccant and combined cooling technologies to reach the lowest level.
Results
This paper presents a comprehensive survey focusing on current cooling technologies used in greenhouses. These cooling technologies are responsible for providing a suitable greenhouse environment for crop production in hot and dry weather conditions. After a thorough review of these technologies, it became clear that the key criterion for evaluating the performance of these technologies is the ability to develop a suitable microclimate to maximize crop yields. Obviously,
Declaration of Competitive Interest
The authors declare that there are no competing financial interests or personal relationships that would appear to influence the work reported in this article.
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