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Performance analysis of a partially closed solar regenerated desiccant assisted cooling system for greenhouse lettuce cultivation
Abstract and Figures
In this paper, a novel scheme of a partially closed solar regenerated liquid desiccant assisted evaporative cooling system has been proposed. The objective of the system is to provide suitable conditions inside greenhouses for cultivation of high value temperate crops like lettuce throughout the year in hot and humid climates of tropics and subtropics. The partially closed system re-circulated a fraction of return air from the greenhouse and ventilated a certain amount of ambient air. A forced parallel flow solar regenerator has been used to re-concentrate the weak desiccant solution. A thermal model has been developed to predict the greenhouse air temperature, vapour pressure deficit and the coefficient of performance of the system. The predicted greenhouse temperature has been compared with that of a reference model study available in literature. A very good agreement is established on comparing the temperatures predicted by the two models with a root mean square error of 0.82 °C and average percentage difference being 2.4%. The COP of the cooling system varied between 0.64 and 0.74 for daylong operation in the most humid month of the year (July). The maximum greenhouse temperature predicted using our proposed model was about 27 °C for year round system operation for the place under consideration (Kolkata). The predicted vapour pressure deficits were also in the optimum range. Thus, optimum or viable lettuce growing conditions were predicted by our model all year around. The study consequently reinforces the viability of cultivation of target plantation (lettuce) in hot and humid climates prevailing in the tropical or subtropical countries like India with our model.
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Supplementary resource (1)
... Different cooling methods are used, like direct or indirect evaporative cooling, chilled water circulation, etc. (Buker and Riffat 2015). In some desiccant cooling systems, a specific portion of processed air supplied returns to the space as cold air mixed with fresh atmospheric air (Ghosh and Ganguly 2017;Song and Sobhani 2020;Mandal and Ganguly 2021). This mixed air is cooled in a direct evaporative cooler and supplied to the sensible heat exchanger. ...
... The system restricted the maximum greenhouse temperature to 21.9 0 C when the outside air temperature was about 39.2 0 C. (Ghosh and Ganguly 2017) proposed and analyzed the performance of a partial closed solar liquid desiccant system coupled with an indirect evaporative cooler for Lettuce cultivation inside the greenhouse in Kolkata, India. The given system could maintain the maximum greenhouse temperature around 27 0 C, even on July's hot and humid days. ...
... The following assumptions are made for the thermal model development: The amount of solar flux immersed in the greenhouse soil is assumed to be negligible (Ghosh and Ganguly 2017). ...
This paper proposes a novel scheme of biomass regenerated two-stage desiccant-supported greenhouse cooling in the tropical and sub-tropical countries. The system's goal is to provide the ideal thermal environment inside a greenhouse for the growth of different kinds of Orchid. The two-stage desiccant-based cooling is used in the proposed system to obtain a low humidity ratio inside the greenhouse. The desiccant is regenerated using a biomass-based heating system. The first law analysis and the exergy analysis of the system's individual components have been included in the study. The results of the thermal model (greenhouse temperature) have been compared with that of a reference model study available in the literature. With a mean absolute error for the humidity ratio and greenhouse air temperature of 4.3 percent and 4.5 percent, respectively. The model's predictions are in good agreement with the results of the reference model. The performance study of the proposed system reveals that the maximum temperature of the greenhouse air can be limited within the defined values even during July and September, when both the ambient humidity and the temperature are high. The proposed system COP th varies from a minimum of 0.54 to a maximum of 1.02 for a typical hot and humid day of July. The system can exhibit maximum exergy efficiency of 33% in the morning, while the ultimate exergy destruction occurs at the DW 1 (39.25%) and regeneration heater (21.69%). The payback period of the proposed system is 8.2 years considering 15 years’ life span.
... The study revealed that LiCl could perform better than CaCl 2 for high humidity inlet conditions. Ghosh and Ganguly (2017) proposed a scheme of a partially closed solar regenerated liquid desiccant assisted evaporative cooling system for the cultivation of Lettuce. The performance analysis was done for the climatic condition of Kolkata, India, using a thermal model. ...
... In Equation (28), the term K l represents latent heat transfer coefficient (W/m 2 K), while e gh and e 6 represent the water vapor pressure inside the greenhouse air and the process air respectively at the inlet of the greenhouse. The latent heat transfer coefficient inside the greenhouse air can be estimated as (Ghosh and Ganguly 2017): ...
... In Equation (32), L c and u i represent the characteristic length (m) of the target plantation and average velocity (m/ s) of air inside the greenhouse. The average air velocity can be estimated as (Ghosh and Ganguly 2017): ...
This article presents a thermo-economic analysis of a biomass heating-based two-stage desiccant dehumidifying system for distributed fan-pad evaporative cooled floriculture greenhouse. The proposed system aims to maintain a suitable environmental condition inside the greenhouse for Orchid cultivation in the sub-tropic and tropical regions. The thermal model of the present system has been developed based on the first law and second law of thermodynamics to predict the air temperature inside the greenhouse. The results of the desiccation system have been validated with the results of the reference model with a mean absolute error of 0.14 g/kg and 0.42 °C for humidity ratio and temperature of the process air, respectively. A comparative study has been made with the considered reference models for both days long and round the year operations. From the performance analysis of the present system, it is observed that the maximum air temperature inside the greenhouse can be restricted within the specified limits. The COPth of the proposed system is found to vary from a minimum value of 0.54 to a maximum value of 1.02. The payback period of the proposed system is estimated to be 12 years.
... C p.da and C p.wv are specific heat of dry air and water vapour. (η φ ) is obtained from equation (6) with specific correlation coefficients In Eq.s (7) through (11). Similarly, (η h ) can be calculated using equation (12) with respective correlation coefficients In Eq.s (13) through (17). ...
... Thus, the weather data for Kolkata as available from the Indian Meteorological Department, New Delhi [9] have been used for the simulation. Roof area (A gf ) 240 m 2 [11] Effectiveness of the heat exchange (ε h ) 0.8 [5] Greenhouse volume (V g ) 728 m 3 [11] Diameter of the desiccant wheel (D) ...
... Thus, the weather data for Kolkata as available from the Indian Meteorological Department, New Delhi [9] have been used for the simulation. Roof area (A gf ) 240 m 2 [11] Effectiveness of the heat exchange (ε h ) 0.8 [5] Greenhouse volume (V g ) 728 m 3 [11] Diameter of the desiccant wheel (D) ...
In this paper, a novel scheme of biomass regenerated desiccant supported greenhouse cooling with distributed fan-pad evaporative system has been proposed. The system aims to provide suitable thermal condition inside the greenhouse for cultivation of high value flowers like varieties of Orchid which require a temperature between 21 °C to 26 °C and humidity from 50 to 70% for the sub-tropical climate prvailing in the plains of India. In the proposed system two stage desiccant based cooling is used to obtain a low humidity ratio. A biomass based heating system is coupled for regeneration of the desiccant. Thermal modeling of desiccant wheel and greenhouse are done to estimate air temperature inside the greenhouse. A comparative study has been made based on estimated greenhouse air temperature with the results of the reference model available in the literature. From the performance analysis of the proposed system, it is revealed that the maximum temperature of the greenhouse air can be restricted within 25 °C and 25.8 °C for a hot and humid day prevailing in the month of July and September respectively. However the system can maintain a lower temperature of 19 °C during the morning time which is more conducive for target plantation (Orchid) in the given region.
... Although regeneration can be carried out in packed beds [14,[19][20][21][22], direct contact solar regenerators, where the solution itself is the fluid collecting the heat from a surface exposed to solar radiation, are preferred due to its higher effectiveness and lower overhead of system components [18,23]. Recent studies on the use of direct solar regenerators in LDCS have been reported by several authors in the literature [23][24][25][26][27][28]. Alizadeh and Saman [28] presented a theoretical model of a forced parallel flow solar regenerator using an aqueous solution of CaCl 2 as the desiccant and an experimental study on the same reported by Alizadeh and Saman [23] confirmed the accuracy of the theoretical model developed previously and the results were also consistent with other findings reported in the literature. ...
... The present work includes features from that model and couples a DPIEC additionally, to enhance the performance of the system with respect to handling sensible cooling load. Ghosh and Ganguly [27] proposed a LDCS for greenhouse application and showed through simulations that optimum/viable greenhouse microclimatic conditions could be maintained for growing temperate crops like lettuce year-round in hot and humid climates. The present study develops on this previous work and extends the application to provide human thermal comfort inside office buildings with high internal load, where the design criteria and dehumidifier configuration are wet bulb 0 at 0 • C completely different. ...
... The present study develops on this previous work and extends the application to provide human thermal comfort inside office buildings with high internal load, where the design criteria and dehumidifier configuration are wet bulb 0 at 0 • C completely different. Secondly, this study focuses on a more detailed thermal analysis of each and every component of the system and relaxes many of the restricting assumptions applicable to the previous model study as in reference [27]. The current thermal model is also more dynamic and flexible which makes it suitable for simulating the system operation under varied ambient and design conditions. ...
In this paper, we provide thermal analysis and design methodology of a liquid desiccant assisted dew point indirect evaporative cooling system. The purpose of the system is to serve as an alternative for conventional vapour-compression based building air conditioning systems in providing satisfactory human thermal comfort conditions in the hot and humid climatic regions. The main features of the study are the following: i) novel incorporation of a forced parallel flow direct solar regenerator and a dew point indirect evaporative cooler within the same air conditioning unit; ii) detailed thermal modelling of each of the system components with lesser simplifying assumptions with respect to earlier works; iii) large cooling capacity (∼18.8 TR) under harsh climate; and, iv) a comprehensive year-around case study for system operation in a hot and humid location (Kolkata, India). Our thermal model is validated with a reference model study. The maximum room air temperature predicted by the current system for yearlong analysis is 26.7 °C. The thermal COP of the system for diurnal operation in the most humid month of a calendar year (July) varied between 0.40-0.96. The cooling system can prevent overheating of the conditioned space, as specified by ASHRAE Standard 55-2017, throughout the year.
... The evapotranspiration rate depends on the internal greenhouse conditions and the growth stage of the crop. Previous studies showed that the FAO Penman-Monteith equation, which was initially developed for open-field agriculture, can also be used to calculate the reference evapotranspiration rate, ET 0 , for indoor crop production [4,41,42]. In accordance with the study by [41], adaptations for applica- Fig. 1. ...
... In combination with literature research into greenhouse cooling methods, companies and experts in this field were consulted in order to give insight into current operational cooling systems and the required greenhouse conditions (Table B1, Appendix B). A standard method for cooling greenhouses in regions with high temperatures is evaporative cooling [42,43]. It should be mentioned that the cooling pad can be operated with salt water. ...
Land desertification, water scarcity, and food security challenges in arid zones are intensifying, driving the need for sustainable agricultural solutions like aquaponics. This study investigated innovative water and energy-saving strategies using an integrated dynamic model for an on-demand industrial aquaponics system in Israel. The model evaluated the performance of a recirculating aquaculture system (RAS), hydroponics system (HPS), and desalination unit (DU) by adjusting physical and operational parameters to optimize water and nutrient use efficiency, energy consumption, and yield. Optimizing the system design resulted in an aquaponics system with approximately 420 m 3 RAS, 6.85 ha HPS and 40 m 3 /d DU, achieving phosphorus use efficiency of 96 %, a water use efficiency of 97 %, freshwater input of 1.5 L/day/m 2 , and energy consumption of 0.56 kWh/day/m 2. To mitigate the challenges of extreme arid climates, evaporative cooling combined with outdoor shading and mechanical cooling was found to be a feasible option to control temperature and humidity in the greenhouse. Dehumidifi-cation technologies further improved system performance by recovering 22 % freshwater from seawater and increasing nitrogen use efficiency by 18 %. Achieving daily energy self-sufficiency required 4500 m 2 photovoltaic panels and 5000 m 2 solar heating system. While the system model was initially devised with a specific focus on conditions in Israel, it has been designed with scalability, allowing it to be adapted and applied extensively across diverse peri-urban regions and arid zones globally.
... It is necessary to control the temperature, humidity and many parameters in the greenhouse. To this end, a number of researchers have proposed additional systems for solar greenhouses [1][2][3][4][5][6][7][8] ...
... here, п -is the number of animals; 1 W -the amount of water separated from one animal; tW k -correction factor taking into account the release of moisture by animals depending on the air temperature in the room (table 1). ...
... Ghoulem et al. [97] were motivated to use a solar regenerator combined with a desiccant to dehumidify and cool the climate inside the greenhouse to ensure crop growth for the humid and hot regions by dehumidifying and cooling (Figure 8). Ghosh and Ganguly [120] numerically investigated desiccant evaporative cooling for agricultural greenhouse applications to be used in humid and hot regions. They modeled a partially closed loop with a recirculation of a partial quantity of the return air, and the desiccant regeneration was realized by a solar roof. ...
... They modeled a partially closed loop with a recirculation of a partial quantity of the return air, and the desiccant regeneration was realized by a solar roof. The results show that the coefficient of performance (COP) of the system, which is defined as the ratio between the refrigerating power delivered by the system and the heating power required [120], ranges between 0.64 and 0.74 during the most humid periods while maintaining a required temperature for the growth of lettuce crops. Davies [2] studied the coupling of the incoming air desiccation to the evaporative cooling in a greenhouse under hot climate conditions in the Gulf state of Abu Dhabi. ...
This work is motivated by the difficulty of cultivating crops in horticulture greenhouses under hot and arid climate conditions. The main challenge is to provide a suitable greenhouse indoor environment, with sufficiently low costs and low environmental impacts. The climate control inside the greenhouse constitutes an efficient methodology for maintaining a satisfactory environment that fulfills the requirements of high-yield crops and reduced energy and water resource consumption. In hot climates, the cooling systems, which are assisted by an effective control technique, constitute a suitable path for maintaining an appropriate climate inside the greenhouse, where the required temperature and humidity distribution is maintained. Nevertheless, most of the commonly used systems are either highly energy or water consuming. Hence, the main objective of this work is to provide a detailed review of the research studies that have been carried out during the last few years, with a specific focus on the technologies that allow for the enhancement of the system effectiveness under hot and arid conditions, and that decrease the energy and water consumption. Climate control processes in the greenhouse by means of manual and smart control systems are investigated first. Subsequently, the different cooling technologies that provide the required ranges of temperature and humidity inside the greenhouse are detailed, namely, the systems using heat exchangers, ventilation, evaporation, and desiccants. Finally, the recommended energy-efficient approaches of the desiccant dehumidification systems for greenhouse farming are pointed out, and the future trends in cooling systems, which include water recovery using the method of combined evaporation–condensation, as well as the opportunities for further research and development, are identified as a contribution to future research work.
... Fig. 6. Schematic diagram of the closed solar system proposed by Ghosh and Ganguly (2017). Right reserved by Elsevier Rightslink . ...
... They found that, the average inside air velocity was 0.44 m/s, and ranged from 0.3 to 1.8 m/s at 1 m crop height, which is the desired air velocities range. Ghosh and Ganguly (2017) proposed a partially closed solar regenerated liquid desiccant assisted evaporative cooling system (Fig. 6). This system was made to provide a suitable condition for cultivation under greenhouse of high value temperate plants like lettuce all over the year under harsh climatic conditions. ...
... Ghosh and Ganguly [65] evaluated the benefits of a solar-assisted, solid-desiccant system for a 571 greenhouse in a subtropical climate using a silica gel wheel as the active desiccant material with an 572 Figure 5. Schematic representation of a solar assisted liquid desiccant system with evaporative pad cooling for greenhouse crops (Modified from Davies [176]). ...
... Ghosh and Ganguly [65] evaluated the benefits of a solar-assisted, solid-desiccant system for a greenhouse in a subtropical climate using a silica gel wheel as the active desiccant material with an auxiliary fan-pad. This combination restricted the maximum temperature inside the greenhouse to 26 • C during the monsoon season. ...
The projected increase of the world’s population, coupled with the shrinking area of arable land required to meet future food demands, is building pressure on Earth’s finite agricultural resources. As an alternative to conventional farming methods, crops can be grown in protected environments, such as traditional greenhouses or the more modern plant factories. These are usually more productive and use resources more efficiently than conventional farming and are now receiving much attention—especially in urban and peri-urban areas. Traditionally, protected cropping has been predominantly practised in temperate climates, but interest is rapidly rising in hot, arid areas and humid, tropical regions. However, maintaining suitable climatic conditions inside protected cropping structures in warm climates—where warm is defined as equivalent to climatic conditions that require cooling—is challenging and requires different approaches from those used in temperate conditions. In this paper, we review the benefits of protected cropping in warm climates, as well as the technologies available for maintaining a controlled growing environment in these regions. In addition to providing a summary of active cooling methods, this study summarises photovoltaic (PV)-based shading methods used for passive cooling of greenhouses. Additionally, we also summarise the current humidity-control techniques used in the protected cropping industry and identify future research opportunities in this area. The review includes a list of optimum growing conditions for a range of crop species suited to protected cropping in warm climates.
... Ali et al. [14] optimised a liquid desiccant system for closed greenhouses, emphasising continuous dehumidification and regeneration. Ghosh and Ganguly [15] reported a coefficient of performance (COP) of 0.64 to 0.74 in a liquid desiccant system for lettuce cultivation, highlighting energy efficiency improvements. Hassanien et al. [16] reviewed solar energy technologies for greenhouse cooling, noting benefits in desert regions, while Masoud [17] highlighted energy consumption reductions of up to 50 % with liquid desiccant systems compared to traditional cooling. ...
Effective thermo-hygrometric control is essential for buildings and greenhouses, particularly in green and low-carbon production. There is growing interest in integrating heat pumps with liquid desiccant systems to enhance energy efficiency and reduce system size in air moisture control. The current study investigates a novel liquid desiccant system combined with a heat pump for continuous, energy-efficient dehumidification and regeneration in horticultural crop cultivation in greenhouses. The uniqueness of the developed system lies in its integration, contrasting with previous liquid desiccant systems designed with separate dehumidifier and regenerator. A semi-theoretical model is developed and validated using in-house experimental datasets to simulate novel liquid desiccant system performance. Results demonstrate that the system effectively maintains air humidity levels, with a maximum enthalpy demand for dehumidification of 99.1 kJ/kg at a solution temperature difference of 40 • C during summer. The minimum achieved moisture content with lithium chloride, calcium chloride, and potassium formate is 7.64 g/kg da , 8.1 g/kg da , and 7.78 g/kg da , respectively, while regeneration produces maximum moisture contents of 23.5 g/kg da , 23.9 g/kg da , and 23.7 g/kg da. The system's maximum effectiveness reaches 76 %, 75 %, and 74 % for lithium chloride, calcium chloride, and potassium formate, respectively. When dehumidification demand exceeds 1,500 h annually, the payback period is five years or less, making the system suitable for new installations and retrofits. A case study considering outdoor conditions in the northeastern United Kingdom presents simulation results for two distinct scenarios, demonstrating the system's potential in real-world applications.
... Mat. SM.1 and the initial parameters are shown in Table 2. (Franco et al., 2014) Transmittance greenhouse cover 0.8 - (Franco et al., 2014) Absorption rate at which radiation entering the greenhouse 0.7 -( Ghosh and Ganguly, 2017) Latent heat of vaporization 131 Evaporative pad cooling is a proven method for regulating greenhouse temperatures in 132 high-temperature regions, effectively using salt water as the operating medium (Ghosh 133 and Ganguly, 2017;Van Beveren et al., 2015). As the airflow leaves APS after cooling, 134 the air undergoes dehumidification, allowing fresh water to be recovered and collected 135 (Zhu, 2023). ...
... We run the simulations for energy performance and we get the results that our energy needs can be fulfilled with PV energy, and we will have some extra energy left throughout the whole year. But in case of any absurd situation, we will have a grid connection as a backup [10]. ...
In this study, we used the Internet of Things (IoT) and artificial intelligence to develop a smart greenhouse environment monitoring and disease detection system. We have developed an automated conveyor belt-type hardware system that works in conjunction with an Arduino Uno base central processing unit and a hybrid (solar and grid-based) power management system. The central processing unit is composed of temperature, humidity, light intensity, darkness, and soil moisture sensors. A high-quality camera was also mounted to the conveyor belt hardware to capture the images of plants and a display unit was also installed with a central processing unit to visualize the data in real-time. The YOLOV8 image processing was performed on a remote computer and the expected results were shown on an external monitor. The design of the proposed system helps us automatically monitor the greenhouse environment and detect disease in the observed plants. The solar-powered automated hybrid system not only saves money on electricity but also on labor-intensive maintenance. The current cutting-edge smart farm monitoring system will not only stimulate the next generation of farmers but will also aid the economic growth of agricultural-based countries such as Bangladesh.
... C, and its maximum reaches 34.9 C at 12:40p.m. In other studies, desiccant material and evaporative cooling systems have been investigated as suitable methods for reducing the indoor greenhouse temperature in tropical and subtropical climates and reported similar results [11]. ...
... 40% water saving was achieved through intelligent greenhouse design using evaporative cooling compared to conventional design for arid regions [17]. Numerical investigation of a partially closed desiccant evaporative cooling loop powered by solar roof revealed that during most humid periods the coefficient of performance of the cooling system ranges between 0.64 and 0.74 [18]. The average temperature of greenhouse was lowered by 6 • C and energy consumption was reduced by 40% compared to conventional cooling system using evaporative cooling through liquid desiccant in KSA [19]. ...
the study presents the design and thermal performance evaluation of a novel solar greenhouse with
humidification-dehumidification unit, water-cooled heat exchanger and variable mixing ratio in the Mediter-
ranean climate. The greenhouse is designed to provide proper microclimatic conditions for crops, produce fresh
water through condensing water vapor released from the plants, and save energy using the semitransparent PV
panels. Results showed that on a typical summer day, the solar greenhouse ensures proper microclimatic con-
ditions all day long by reducing the temperature by 11.14 ◦C compared to conventional greenhouses, while
maintaining acceptable values of relative humidity and producing 70 L of fresh water per day. An operation
schedule was established based on varying return-to-fresh air ratio to ensure optimal system performance. The
operation schedule showed that proper microclimatic conditions can be reached through ventilation for 13 h per
day. To enhance the net-energy performance, the produced water can be used for irrigation and the return air can
be used for cooling the PV panel
... This, in return, results in a reduction in the humidity and temperature levels. Ghosh and Ganguly [173] proposed a partially closed solar regenerated liquid desiccant coupled with an evaporative cooling system. The cooling system was proposed to enable an optimal environment for cultivating lettuce in hot and humid regions. ...
The demand for food items is increasing to match population growth rates. Meanwhile, many countries struggle to guarantee a full year-round production of crops because of the harsh weather conditions in cold and hot seasons. Greenhouses are used to mitigate the harsh weather conditions and allow for a full year-round production of crops by controlling the microclimate and ensuring that the crops acquire the required level of temperature, humidity, water, and light. Several design factors play a crucial role in the successful deployment of the greenhouse. These parameters include shape, orientation, structure, cover material, and climate control technologies. Climate control strategies include cooling and heating technologies. This paper presents a comprehensive review of cooling technologies for greenhouses, especially ones deployed in hot climates. Technologies such as natural ventilation, shading and reflection, evaporative cooling, desiccant cooling, and combined cooling technologies are presented and discussed. Moreover, this paper explores recent advancements in the field of cooling technologies and systems. The paper also explains existing 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 suitable cooling technology for their specific regions.
... Therefore, solely using the EH for desiccant regeneration is not recommended for DAC systems. Ghosh and Ganguly [45] developed a solar LDAC system using the solar collector/regenerator which was an FPSC integrated with a liquid desiccant regenerator. Solar collectors which directly heated the liquid desiccant were used by Wang et al. [46] and Zhou et al. [47]. ...
With independent temperature and humidity control, desiccant air-conditioning (DAC) systems have multi-aspect advantages (e.g., high-efficiency moisture control, no ozone-depleting coolants use and easy renewable integration etc.) compared with conventional vapor-compression cooling systems, thereby attracting increasingly more research attention. Recently, many studies have been conducted to improve DAC system overall performance by integrating various energy sub-systems/technologies, improving system configurations and optimizing system designs and controls. This study, therefore, provides a timely and systematic review on the abovementioned studies. The study first overviews three typical DAC system configurations, their operating processes, and the commonly used performance indicators. Second, the study summarizes the main integration approaches of DAC systems. Detailed comparative analysis has been provided regarding these integration approaches. Third, the study presents the DAC system configuration improvements in four main categories, and comparatively analyzes their performance enhancements. Last, the study briefly reviews the DAC system design and control optimizations. Meanwhile, based on the DAC system latest developments, the study provides discussions on pressing issues and recommendations for future works. The study can help improve the understanding of the latest developments on DAC system integration and design for overall performance improvements. Associated discussions and recommendations may help focus the needed efforts on solving the pressing issues and outstanding problems, thereby leading to DAC system further performance improvements.
... The solar cooling system could supply significant energy-saving opportunities for greenhouses' cooling demands especially in hot climates as it reduces the electricity and water consumption by exploiting the contemporaneity between the cooling requirements and the solar energy availability. Numerous researchers have proposed solar systems for cooling greenhouses, especially in hot and desert areas, to reduce energy costs [31][32][33]. A solar-powered absorption refrigeration was developed by Puglisi et al. [34] to cool a greenhouse in Italy. ...
The most important goal of a greenhouse is high yield production outside the cultivation season which is possible by maintaining optimum conditions specially temperature at each stage of a crop's growth. In the warm seasons, temperature in the greenhouse may exceed the optimal temperature for cultivated plants. In order to have the desired temperature for a greenhouse in summer, it must be cooled as quickly as it gains heat from the ambient. For reducing the internal heat of the greenhouse and heat discharge, the greenhouse needs to be coupled with a proper cooling system based on the climatic conditions of the region. This article focuses on the current greenhouse cooling methods and compares them with modern, renewable energy-based methods with the aim of using clean energy, reducing the possible potential of environmental damage, as well as reducing energy consumption and also using winter energy storage like solar energy and applying for the greenhouse cooling demands in warm seasons.
... Other researchers proposed to use rock-beds to store excess heat during daytime, and found the system was able to improve tomato yields by 22% compared to a conventional greenhouse [15]. Solar-desiccant assisted evaporative cooling systems were also studied for year-round greenhouse lettuce cultivation in hot and humid climates prevailing in tropics and subtropics [16]. ...
This paper presents a technoeconomic analysis of a solar combined heat and power (S-CHP) system based on hybrid photovoltaic-thermal (PVT) collectors for distributed cogeneration in a greenhouse tomato-farm in Bari, Italy. The thermal and electrical demands of the greenhouse of interest are currently fulfilled by a gas-fired CHP system that features an internal combustion engine (ICE) prime mover, and partially by an auxiliary gas boiler and electricity from the grid. A PVT-water S-CHP system is designed and sized based on a transient model, with hourly weather data and measured demand data given as inputs. Annual simulations are performed to predict the transient behaviour of the S-CHP system and to assess the system’s energy outputs. The economic profitability of such solution is also evaluated by considering the investment costs and cost savings due to the reduced on-site energy consumption. The results show that, with an installation area of 30,000 m ² , the PVT S-CHP system is able to cover up to 73% of the annual thermal demand of the greenhouse, while delivering a net electrical output 2.6 times that of the annual electrical demand. This performance is similar to that achieved by the equivalent ICE-CHP system (92% and 2 times, respectively). Furthermore, the total annual cost saving of the PVT S-CHP system is more than 6 times higher than that of the ICE system, due to the much lower fuel cost of the PVT system. Similarly, the potential CO 2 emission reduction associated with the PVT system is considerably higher, at 3010 tCO 2 /year saved (vs. 86 tCO 2 /year). The payback time of the PVT system is not significantly longer than that of the ICE system (10.4 years vs. 8.4 years), but its levelized cost of energy is much lower (0.076 €/kWh vs. 0.132 €/kWh) due to the higher annual cost savings. These results indicate that such PVT S-CHP systems have an excellent technoeconomic potential in the proposed greenhouse applications and could be competitive over conventional fossil-fuel-based ICE-CHP systems in terms of energetic, economic and also environmental metrics.
... The validation of the proposed thermal model is carried out by comparing the results predicted by our model with a similar reference model study [14]. Figure 2 shows the comparison between the present and reference model for a typical day in August (16 th of August). ...
In this paper, exergy analysis of a novel solar powered liquid desiccant assisted air
conditioning system is presented and simulated. The system aims to provide suitable thermal
comfort conditions inside large office buildings with high internal loads situated in the hot and
humid tropical/subtropical countries of the world. The system consists of process and
regenerating air streams, a liquid desiccant solution loop and a cooling water loop. The primary
objective of this study is to present the exergy of cooling capacity along with the overall
exergy efficiency of the proposed system. The study helps to quantify the optimum operating
and design parameters for system operation based on the second law of thermodynamics. For
the base case, which is representative of a hot and humid climatic condition, the proposed
system is able to maintain the room air conditions within the moderate thermal acceptability
criterion. The exergy of cooling capacity and exergy efficiency for the base case is about 2900
W and 2 % respectively. Parametric analyses show that the system performs the best under
conditions of high ambient insolation and temperature, low ambient humidity and a process air
to desiccant solution mass flow rate of about 3 in the dehumidifier.
... Other researchers proposed to use rock-beds to store excess heat during daytime, and found the system was able to improve tomato yields by 22% compared to a conventional greenhouse [15]. Solar-desiccant assisted evaporative cooling systems were also studied for year-round greenhouse lettuce cultivation in hot and humid climates prevailing in tropics and subtropics [16]. ...
This paper proposes a novel scheme of biomass-regenerated two-stage desiccant-supported greenhouse cooling in tropical and subtropical regions. The system’s goal is to provide the ideal thermal environment inside a greenhouse for the growth of different kinds of Orchids. Two-stage desiccant-based cooling is used in the proposed system to obtain a low humidity ratio inside the greenhouse. The desiccant is regenerated using a biomass-based heating system. The first law analysis and the exergy analysis of the system’s individual components have been included in the study. The results of the thermal model (greenhouse temperature) have been compared with those of a reference model study available in the literature. The mean absolute error for the humidity ratio and greenhouse air temperature is 4.3% and 4.5%, respectively. The model’s predictions are in good agreement with the results of the reference model. The performance study reveals that the maximum greenhouse air temperature can be restricted within 26 °C even during the peak sunshine hours for a typical day in May which represents the peak summer season in India. The proposed system COPth varies from a minimum of 0.54 to a maximum of 1.02 for a typical hot and humid day of July. The proposed system can exhibit a maximum exergy efficiency of 33% at 6 AM for a representative day in July. The maximum exergy destruction occurs at the DW1 (39.25%) and regeneration heater (21.69%). The payback period of the proposed system is 8.2 years, considering a 15-year life span.
This paper presents multi-objective optimization of a biomass heating-based two-stage desiccant-supported greenhouse cooling system used for Orchids cultivation in hot and humid weather conditions. The simulation model has been developed considering thermodynamics, economic, and environmental aspects. The thermal coefficient of performance (COPth) of the system and greenhouse temperature have been predicted for the five most impactful months (March, May, August, September, and December) corresponding to the respective seasons of Spring, Summer, Monsoon, Autumn, and Winter of a calendar year. The system maintains the peak average greenhouse temperature at a maximum of 26°C during the prominent sunshine period (12 hours) in May while ensuring a minimum of 18°C during nighttime. In terms of system components, the residue boiler stands out as the significant contributor to exergy destruction (45%), followed by regeneration heater 1 (22%), DW1 (7%), and the heat recovery water heater (6%) during the critical operational month of August. Multi-objective optimization has also been conducted using the optimization toolbox provided in MATLAB-R2017a to determine the optimal performance and operating conditions of the Two-stage Desiccant Cooling System (TSDC) system. The optimal conditions display the corresponding total cost rate, considering capital and maintenance costs, operating cost, CO2 penalty costs, and exergetic efficiency.
By 2050, the human population will have risen massively, so the demand for food and agricultural field will be the greatest global challenge in the future. However, biodiversity is declining as it experiences the impacts of deforestation, river pollution, the release of greenhouse gases, and many more detrimental effects that include global warming - the long-term increase in the planet’s temperature. The agricultural field is primarily dependent on climate and thus climate change could affect agriculture in various ways. The situation demands alternative approaches to overcome and concurrently maintain food security. Hence, this paper presents a review of the adaptation measures employed by the agricultural sector to address climate change. The focus is Southeast Asia as climate change has been progressively more threatening in this region. Several adaptive measures and gaps were discovered, as presented in this paper. The present review will be a viable resource for communities, especially local farming communities in Southeast Asia, which are facing a future of climate change.
Humidity is one of the climate parameters in greenhouse food production. Maintaining the humidity levels within the optimal growth range enhances yield. Moreover, excessively high relative humidity levels lead to diseases and deterioration of the crops. This paper presents a state-of-the-art review of the various dehumidification technologies available in the agricultural industry. Several novel conceptual designs from the literature are also discussed. The principal humidity control approaches utilized in greenhouses are ventilation (natural and forced), maintaining a high temperature, condensation on a cold surface, and adsorption by hygroscopic materials. The most common method for dehumidification is ventilation due to its minimal infrastructure. Although this method is considered the simplest, it causes additional sensible heating loads, particularly in colder climates. The added heating load can be reduced, ideally eliminated, by employing heat recovery systems. Furthermore, dehumidification by controlled condensation on a cold surface enables the capture and re-use of the latent energy released in condensation. By adsorption of water vapor using hygroscopic material, the latent heat of condensation is converted to sensible heat, which can be used for space heating in the greenhouse. Such a system can reduce the greenhouse humidity while maintaining a more uniform temperature profile over the crop canopy and reducing energy consumption. Finally, it is essential to emphasize that an appropriate dehumidification method should be able to prevent water vapor condensation on plant surfaces, and also its operational cost should be as low as reasonably achievable to remain beneficial for growers.
Greenhouse technology is becoming an increasingly indispensable and a viable solution for modern methods of crop production. Technological advances have lessened the effect of severe weather conditions on the yield of greenhouse crops in hot climates. Cooling is crucial to guarantee the required range of temperatures and humidity inside the greenhouse. The purpose of this work is to review the design and systems used for greenhouse cooling applications in hot climates. Theoretical and practical aspects related to greenhouse cooling techniques are presented: working principles, working conditions and performance parameters. The review revealed that the combination and simultaneous usage of natural ventilation, evaporative cooling and shading has the potential to reduce greenhouse energy requirement and provide optimum indoor conditions required to maximise crop yields in greenhouses in hot climates. Hybrid cooling systems must be assisted by an effective control strategy to ensure that the required temperature and humidity levels and distributions are maintained in the greenhouse. The status of the research on the use of numerical modelling for the design of greenhouses in hot climates or conditions was investigated and the future challenges facing the development of greenhouses in hot climates identified. Recommendations for future research and development are proposed.
Improvement of greenhouse insulation, for example through using a double inflated wall or thermal screens, leads to changes in the microclimate of the protected cultivation. The low air exchange rate induces an increase in the inside air humidity and the development of condensation on the walls and roof which causes water to drip on to the crops. Consequently, the occurrence of fungal diseases and physiological disorders increases and greenhouse air dehumidification is necessary to combat these.In this paper, the greenhouse air dehumidification possibilities are analysed with both experimental and theoretical evaluations of the water vapour transfer between the different sub-systems (soil, crop, inside air, roof, outside air, heat pump), considered as sources or sinks of water vapour. The water vapour fluxes were estimated from measurements of temperature, heat flow, humidity and radiation.A dynamic model of water vapour exchange was developed and compared with the experimental results. It showed that the heat pump did not reduce significantly the inside air humidity, but eliminated almost completely the water condensation on the roof (and dripping of water on to the crops). This model allows the determination of the size of the dehumidification system required, according to the characteristic parameters of the greenhouse and of the crop, namely, air temperature and humidity set points, air changes, crop leaf area index, stomatal and aerodynamic resistance.
A simple linear model has been developed, based on the greenhouse crop heat and mass balances allowing for the calculation of inside air temperature and humidity together with crop temperature. This model is valid for a mature non-stressed crop with low-temperature difference between inside and outside. It can numerically be inverted in order to identify the greenhouse ventilation function together with the soil heat storage parameter, by fitting measured and calculated data of crop temperature and inside air temperature and humidity, and by deducing crop transpiration rate.This model was tested in summer in a ‘classically ventilated tunnel’ with a small opening surface (6%), and in a ‘largely opened tunnel’ with a large opening surface (18%). Measurements were carried out when the tomato crop was mature. On the basis of the experimental measurements, the coefficients of efficiency for ventilation were determined and used to validate the model with respect to inside air measurements. The identified values for the ventilation coefficients are in agreement with the values reported in the literature. Likewise, calculated inside air speed deduced from ventilation in both tunnels was also in good agreement with the measured values. It is shown that this approach allows for a precise estimation of ventilation and transpiration rates using only simple measurement devices such as temperature and air humidity sensors.
A greenhouse climate model, incorporating the effects of natural ventilation and evaporative cooling (fog-system), is proposed and discussed. Linearization of the greenhouse heat and water balance equations leads to a simple system of two equations with two unknowns (the temperature and humidity differences between inside and outside air) which represent quite well the complex coupling mechanisms between ventilation and fog observed in situ. The model predicts that a minimum inside temperature can be reached for a certain combination of these cooling processes. Crop temperature and transpiration were also estimated using the Penman-Monteith approach and the energy balance of the crop. Good agreement between measured and computed values of air temperature, air humidity, crop temperature and transpiration was observed. From a practical point of view this approach may permit improvement in on-line climate control.
A new and simple greenhouse crop transpiration model enabling predictions from outdoor conditions is presented and the parameters involved are discussed with respect to different types of greenhouse-crop systems. This transpiration model was validated against experimental data measured in a soil-less tomato crop cultivation in Avignon in summer conditions, when the greenhouse is open, and early spring climatic conditions, when the greenhouse is kept closed and the inside air strongly confined. The evapo-condensation phenomena on the greenhouse cover, particularly important when the greenhouse is closed, are not considered in the water vapour balance. Model estimation improved from spring to summer and comparisons with previous transpiration models have shown that considering outside climate instead of inside climate as a boundary condition implied a deterioration of transpiration model performances especially when greenhouse air is confined. This deterioration is primarily due to simplifications introduced during the model derivation and model performances were satisfactory when the greenhouse air was closely coupled to outdoor conditions.
Greenhouse technology is a viable option for sustainable crop production in the regions of adverse climatic conditions. High summer temperature is a major setback for successful greenhouse crop production throughout year. The main intent of the paper is to present a comprehensive review on the design and technology for cooling of greenhouse during summer months. Effect of characteristic design parameters on greenhouse microclimate and the applicable cooling technologies have been discussed. A detailed survey of literature revealed that, apart from cooling, studies on greenhouse design, evaluation of new cladding materials for greenhouse covering and natural ventilation with respect to local climate and agronomic condition is necessary to achieve desirable benefits. Analysis of the earlier studies revealed that a naturally ventilated greenhouse with larger ventilation areas (15–30%), provided at the ridge and sides covered with insect-proof nets of 20–40 mesh size with covering material properties of NIR (near infrared radiation) reflection during the day and FIR (far infrared radiation) reflection during night is suitable for greenhouse production throughout year in tropical and subtropical regions. Based on the review, salient areas in need of further research are focused.
Growth chamber studies were undertaken with a tipburn-sensitive cultivar of romaine lettuce (Lactuca sativa L. cv. Lobjoits Green Cos) grown under a photosynthetic photon flux density of 320 micromoles s-1 m-2 for 16 hours; light and dark temperatures were 26.0 degrees and 12.5 degrees C, respectively. As the relative humidity (RH) during the light period was decreased from 74% to 51%, growth was retarded, Ca concentration increased, and the onset of tipburn delayed. Decreasing RH during the dark period from 95% to 90% reduced growth and resulted in lower Ca concentrations and earlier tipburn development. Further decreases from 90% to 65% caused no additional change in growth or tipburn response. Root temperatures of 23.5 degrees, compared with 15.0 degrees, slightly increased Ca concentration but induced earlier tipburn development. Ca concentrations were increased and tipburn delayed by humidity conditions which provided large diurnal fluctuations in water potential in the plant and which encouraged root pressure flow during the dark period. Elevated root temperatures did not provide expected increases in Ca accumulation in young leaves.
Humidity imparts an adverse effect upon a conventional fan-pad ventilated greenhouse in the tropical and subtropical countries. In the present work, a novel scheme of a desiccant assisted distributed fan-pad ventilated greenhouse system has been proposed for the cultivation of varieties of Gerbera. A thermal model of the proposed system has been developed to predict the greenhouse temperature and compare the same with a reference model study available in literature. To regenerate the desiccant materials, solar thermal energy is used which is harnessed using a number of flat plate collectors. Study reveals that, while the maximum temperature inside the conventional greenhouse without desiccation is about 28.8 °C, the same can be maintained below 27 °C even during the peak sunshine hours of the summer season with the proposed system for the place under consideration (plains of Indian sub-continent). During the monsoon season (June), the maximum greenhouse temperature can be restricted within 26.6 °C with the present system, while during the same period the temperature of the conventional fan-pad ventilated greenhouse reaches about 28.8 °C. A cumulative cash flow model has also been included in the study to examine the payback period and the Net Present value (NPV) of the proposed system. From the economic analysis, it is observed that the payback period of the system is about 6 years, while the NPV is about 0.15. The system thus reinforces the viability of the proposed system both from technical and economic point of view.
This article is about using solar energy and liquid desiccant to provide evaporative cooling systems of spaces occupied by plants in high ambient humidity climate. The system took full benefit of the regeneration of liquid desiccant by pure solar energy. The effect of airflow on the predicted and measured average daily maximums of greenhouse temperatures obtained using desiccant system and those obtained with conventional evaporative cooling for the month of June was investigated and reported. The desiccant evaporative cooling system lowered the average daily maximum temperatures in the greenhouse by about 6 °C relative to conventional evaporative cooling system. Furthermore, shell and tube heat exchanger configuration was adopted to simulate pipes implanted in the desiccant pad and fed with nanofluids supplied from a cooling tower. The effects of changing volume fractions of nanoparticles on energy effectiveness and heat transfer coefficient of an assumed shell and tube heat exchanger have been analyzed and discussed. Improvements on convective heat transfer coefficient of 7.20–14.40%, 6.20–12.30%, and 5.50–9.01% were obtained for 0.01–0.04 volume concentrations of Al2O3-W, Fe3O4-W, and ZnO-W nanofluids, respectively. In addition, energy effectiveness has been analyzed and enhancements of 27.50–50.10%, 25.01–40.10%, and 24.00–32.02% were obtained for volume fractions from 0.01 to 0.04 of ZnO, Fe3O4, and Al2O3 nanoparticles, respectively, suspended in water with constant mass flow rates of fluids. Dynamic indicators (life cycle and annualized life cycle costs) were applied to evaluate the economic-effectiveness of this energy supply system. The total life cycle cost was found to be /year.
Humidities between 1.0 kPa and 0.2 kPa vapour pressure deficit (vpd; 55 to 90% r.h. at 20°C) have little effect on the physiology and development of horticultural crops. Humidities lower than these lead to plant water stress and thus reduce growth, whereas higher levels encourage disease and may cause disorders of growth and development. In some conditions, e.g. in polluted atmospheres, high humidity may contribute to further damage to the crop. There may, however, be occasions when a high humidity is desirable, e.g. in generating root pressure to avoid calcium deficiency in fruit or young leaves; when using pathogenic fungi to control insect pests; and in the propagation of plants from leafy cuttings or in tissue culture.
Experiments and theoretical modelling have been carried out to predict the performance of a solar-powered liquid desiccant cooling system for greenhouses. We have tested two components of the system in the laboratory using MgCl2 desiccant: (i) a regenerator which was tested under a solar simulator and (ii) a desiccator which was installed in a test duct. Theoretical models have been developed for both regenerator and desiccator and gave good agreement with the experiments. The verified computer model is used to predict the performance of the whole system during the hot summer months in Mumbai, Chittagong, Muscat, Messina and Havana. Taking examples of temperate, sub-tropical, tropical and heat-tolerant tropical crops (lettuce, soya bean, tomato and cucumber respectively) we estimate the extensions in growing seasons enabled by the system. Compared to conventional evaporative cooling, the desiccant system lowers average daily maximum temperatures in the hot season by 5.5–7.5 °C, sufficient to maintain viable growing conditions for lettuce throughout the year. In the case of tomato, cucumber and soya bean the system enables optimal cultivation through most summer months. It is concluded that the concept is technically viable and deserves testing by means of a pilot installation at an appropriate location.
The dimensionless leaf area index (LAI) is a fundamental crop characteristic. Since the direct measurement of LAI or leaf area is labour intensive and destructive, fast and reliable indirect methods have been devised to estimate LAI of different crops. The objective of this work was to test indirect methods for the non-destructive estimation of LAI in kohlrabi (Brassica oleracea var. gongylodes L.) and lettuce (Lactuca sativa var. capitata L.). Focusing on the gap fraction methodology, digital photographs and simultaneous radiation interception measurements were taken using a Li-Cor plant canopy analyser (LAI-2200) on 12 sampling dates from planting to harvest, with concurrent destructive estimations of the leaf area. Several geometric protocols of the LAI-2200 and inversion algorithms of the accompanying software were evaluated. Very good indirect-direct LAI relationships were obtained for kohlrabi (R2 > 0.97, n = 12) and lettuce (R2 > 0.99, n = 9) for the most suitable protocols and algorithms.
This article presents modeling and experimental results on a recently proposed liquid desiccant air conditioner, which consists of two stages: a liquid desiccant dehumidifier and an indirect evaporative cooler. Each stage is a stack of channel pairs, where a channel pair is a process air channel separated from an exhaust air channel with a thin plastic plate. In the first stage, a liquid desiccant film, which lines the process air channels, removes moisture from the air through a porous hydrophobic membrane. An evaporating water film wets the surface of the exhaust channels and transfers the enthalpy of vaporization from the liquid desiccant into an exhaust airstream, cooling the desiccant and enabling lower outlet humidity. The second stage is a counterflow indirect evaporative cooler that siphons off and uses a portion of the cool-dry air exiting the second stage as the evaporative sink. The objectives of this article are to (1) present fluid-thermal numerical models for each stage, (2) present experimental results of prototypes for each stage, and (3) compare the modeled and experimental results. Several experiments were performed on the prototypes over a range of inlet temperatures and humidities, process and exhaust air flow rates, and desiccant concentrations and flow rates. The model predicts the experiments within ±10%.
NREL has developed the novel concept of a desiccant enhanced evaporative air conditioner (DEVap) with the objective of combining the benefits of liquid desiccant and evaporative cooling technologies into an innovative 'cooling core.' Liquid desiccant technologies have extraordinary dehumidification potential, but require an efficient cooling sink. DEVap's thermodynamic potential overcomes many shortcomings of standard refrigeration-based direct expansion cooling. DEVap decouples cooling and dehumidification performance, which results in independent temperature and humidity control. The energy input is largely switched away from electricity to low-grade thermal energy that can be sourced from fuels such as natural gas, waste heat, solar, or biofuels.
A solar energy refrigeration and air conditioning system for cooling an enclosed area comprising a low temperature vapor generator, condenser and evaporator specifically configured for use with a low boiling point refrigerant to use solar energy as a means of vapor generation is described. The low temperature vapor generator is coupled to an externally heated water source through a first fluid drive means. The low temperature vapor generator and evaporator feed refrigerant vapor to the condenser through means of an ejector while the liquid refrigerant is recirculated from the condenser to the low temperature vapor generator and evaporator by a second fluid drive means. Chilled fluid from the evaporator is circulated through a coil contained in an air handling system to cool the enclosed area. Of course, refrigerant could be evaporated in the evaporator by exposing evaporator heat exchange surfaces directly to the air in the duct thus cooling air directly.
An open process of summer air conditioning by dehumidification-humidification is presented and simulated. Air dehumidification is achieved in an absorption column through direct contact with a hygroscopic solution over a cooling tube bank inside which water flows, coming from a cooling tower. After heat exchange with the cooler outlet air, the processed air is adiabatically humidified. The diluted solution is reconcentrated in a desorber by direct contact with external air over a heating tube bank, inside which water flows, coming from an external heating or heat recovery system. A heat exchanger recovers sensible heat between inlet and outlet air. The independent variables controlling the process and their variation effects on its performance are defined through the study of three different modes of operation. The system performance for a certain number of cases are presented as well.
The regeneration system represents a vital part of any desiccant air conditioning system. The need of a solar assisted desiccant regeneration system is more important today. In this paper, an experimental study of a novel regeneration system modified from solar tilted still is carried out. A corrugated blackened surface is used to heat the desiccant and an air flow is used to regenerate calcium chloride solution. The effect of the liquid to air flow rate ratio; the desiccant temperature; the desiccant concentration and the inlet air humidity ratio on the evaporation rate has been studied experimentally. A wide range of liquid to air flow rate ratios are employed. The optimum value of the liquid to air flow rate ratio for higher evaporation rate is reported.
This study is motivated by the difficulty of cultivating crops in very hot countries and by the tendency for some such countries to become dependent on imported food.Liquid desiccation with solar regeneration is considered as a means of lowering the temperature in evaporatively-cooled greenhouses. Previous studies demonstrated the technical feasibility of the desiccation–evaporation process, but mainly in the context of human dwellings.In the proposed cycle, the air is dried prior to entering the evaporative cooler. This lowers the wet-bulb temperature of the air. The cooling is assisted by using the regenerator to partially shade the greenhouse. The heat of desiccation is transferred and rejected at the outlet of the greenhouse.The cycle is analysed and results given for the climate of the The Gulf, based on weather data from Abu Dhabi. Taking examples of a temperate crop (lettuce), a tropical crop (tomato) and a tropical crop resistant to high temperatures (cucumber) we estimate the extension in growing seasons relative to (i) a greenhouse with simple fan ventilation (ii) a greenhouse with conventional evaporative cooling.Compared to option (ii), the proposed system lowers summers maximum temperatures by 5°C. This will extend the optimum season for lettuce cultivation from 3 to 6 months of the year and, for tomato and cucumber, from 7 months to the whole year.
A forced flow solar collector/regenerator is one of the effective ways of regenerating the weak liquid solution in an open cycle liquid desiccant air conditioner using solar energy. In this system, the weak solution flows over the absorber plate of a tilted collector/regenerator as a thin liquid film. The forced air stream, which flows parallel or counter to the solution film, removes the moisture which is evaporated from the liquid solution due to absorption of solar energy. The absorber plate of the collector/regenerator is blackened and glazed to enhance the solar energy absorption and protect it from the environment. To evaluate the thermal performance of the solar collector/regenerator, a computer model has been developed using calcium chloride as the desiccant. A parametric analysis of the system has been performed to calculate the rate of evaporation of water from the solution as a function of the system variables and the climatic conditions.
The major energy requirement associated with any liquid desiccant-based systems is the low-grade energy for desiccant regeneration. This paper presents the results from a simplified model of a packed bed regeneration process in which the desiccant solution is heated in any of the two ways. With method A, the desiccant solution is heated in a heat exchanger with a fluid (water) heated by any low-grade thermal energy such as solar energy or waste heat sources. While in method B, the desiccant solution is heated by a conventional energy source such as a line heater. A closed form solution is obtained for both methods of heating through two dimensionless performance parameters to estimate the water evaporation rate from the weak desiccant solution to the scavenging air stream in terms of known operating parameters. Good agreement is shown to exist between the predictions from the simplified model and the experimental findings available in the literature. The influences of the heating fluid (water) inlet temperature and the effectiveness of the heating fluid-to-desiccant heat exchanger on the performance of the regenerator are studied for method A whereas the effects of energy input on the evaporation rate of water with the scavenging air flow rate are investigated for method B and the results are reported in this paper.
This paper describes current trends in solar-powered air conditioning, which has seen renewed interest in recent years due to the growing awareness of global warming and other environmental problems. Closed-cycle heat-powered cooling devices are based mainly on absorption chillers, a proven technology employing LiBr–water as the working fluid pair. Recent developments in gas-fired systems of this type make available double- and triple-effect chillers with considerably higher COP than their single-effect counterparts, which makes it possible to reduce the amount of solar heat required per kW of cooling. These systems require, however, high-temperature solar collectors. The principles of multi-staging absorption systems are described. An economic comparison is provided which shows the total system cost to be dominated by the solar part of the system. At current prices, the high COP, high temperature alternative is still more costly than the low temperature one. Open-cycle desiccant systems employing either solid or liquid sorbents are described. While the main thrust in research on novel closed-cycle absorption systems has been toward increasing the operating temperature in order to improve efficiency through multi-staging, open-cycle absorption and desiccant systems have been developed for use with low temperature heat sources such as flat plate solar collectors. A novel open-cycle (DER) system is described, which makes it possible to use the solar heat at relatively low temperatures, for producing both chilled water and cold, dehumidified air in variable quantities, as required by the load.
One of the main components of a liquid desiccant cooling system is the regenerator. In a liquid desiccant air conditioner, outside air is dehumidified by liquid desiccant and cooled within the absorber. The diluted desiccant solution thus obtained has to be concentrated for reuse, by passing through the regenerator and the cycle is, consequently, repeated. The regenerator used in this application is a forced parallel flow type solar collector/regenerator. The regenerator has been designed and optimized and the prototype of the solar collector/regenerator has been built and tested. Calcium chloride has been used as the absorbent solution. The results of the tests conducted as a parametric analysis indicate that the air and solution mass flow-rates and the climatic conditions affect the regenerator performance. Furthermore, a comparison between the experimental data obtained and a previously developed model for a forced parallel flow solar collector/regenerator reveals that the experiments are in good agreement with the model predictions. Finally, it was concluded that the proposed solar collector/regenerator performs satisfactorily under the summer conditions of Adelaide, Australia.
A nominal 10.5-kW (3-ton) open-cycle liquid desiccant dehumidification system has been designed, installed, and successfully operated at the Solar Energy Applications Laboratory, Colorado State University. Packed bed units were used to dry the air in the dehumidifier and to concentrate the desiccant in the regenerator. Liquid distribution in the regenerator was studied for two systems: a gravity tray distributor, and a spray nozzle system. Higher capacities (40–50% increase) and lower pressure drop (30–40% reduction) for the air flow were observed with the spray system. Cooling capacities of 3.5–14.0 kW (1.0–4.0 refrigeration tons) were achieved for both the regenerator and dehumidifier. Functional relationships correlating the independent variables to the rate of vaporization in the regenerator and rate of condensation in the dehumidifier were obtained by statistical analysis of the experimental data. These studies thus provide data and correlations useful for design guidance and performance analysis of similar open-cycle liquid desiccant cooling systems, particularly for the liquid/vapor contact units.
This paper reviews the available worldwide cooling technologies for agricultural greenhouses and discusses the representative applications of each technology. Relevant information about the system characteristics, application and performance of the existing greenhouse cooling technologies such as ventilation (natural and forced), shading/reflection, evaporative cooling (fan-pad, mist/fog and roof cooling) and composite systems (earth-to-air heat exchanger system and aquifer coupled cavity flow heat exchanger system) is collected and presented in detail. As per the collected information, the pros and cons of each technology are also discussed. Finally, some important conclusions are drawn (based on the collected information) regarding the performance of each discussed system.
This paper describes a relatively simple model for the preliminary design of an air dehumidification process occurring in a packed bed using liquid desiccant through dimensionless vapor pressure and temperature difference ratios. An expression is derived using the aforementioned ratios to predict the water condensation rate from the air to the desiccant solution in terms of known operating parameters. The model predictions were compared against a reliable set of experimental data available in the literature, with very good agreement. The effects of the cooling water inlet temperature and the desiccant-to-water heat exchanger effectiveness on the performance of the dehumidifier are also studied and the results are presented in this paper.
India is the first major tropical country which has set out to widen her industrial dimensions. The prevailing wet bulb temperatures in many parts of the country vary between 80–85°F. In this process the scientists and engineers have to face the problem of the heat of the process added to the heat of the atmosphere. This often results in working conditions beyond the limits of human endurance and a considerable drop in productivity of operators. The high wet bulb temperatures rule out the possibility of using evaporative cooling and the high cost of maintenance and operation eliminate the use of airconditioning with refrigeration. The object of this study is to explore the possibilities of the utilization of solar energy in conjunction with different systems for the dehydration and cooling of air, and to endeavour to establish one which will be most suited under the Indian conditions. An adsorption or absorption system of dehumidi-fication with sensible and evaporative cooling of air appears to show promise.
Desiccant systems have been proposed as energy saving alternatives to vapor compression air conditioning for handling the latent load. Use of liquid desiccants offers several design and performance advantages over solid desiccants, especially when solar energy is used for regeneration. For liquid–gas contact, packed towers with low pressure drop provide good heat and mass transfer characteristics for compact designs. This paper presents the results from a study of the performance of a packed tower absorber and regenerator for an aqueous lithium chloride desiccant dehumidification system. The rates of dehumidification and regeneration, as well as the effectiveness of the dehumidification and regeneration processes were assessed under the effects of variables such as air and desiccant flow rates, air temperature and humidity, and desiccant temperature and concentration. A variation of the Öberg and Goswami mathematical model was used to predict the experimental findings giving satisfactory results.
A packed column air-liquid contactor has been studied in application to air dehumidification and regeneration in solar air conditioning with liquid desiccants. A theoretical model has been developed to predict the performance of the device under various operating conditions. Computer simulations based on the model are presented which indicate the practical range of air to liquid flux ratios and associated changes in air humidity and desiccant concentration. An experimental apparatus has been constructed and experiments performed with Monoethylene Glycol (MEG) and Lithium Bromide as desiccants. MEG experiments have yielded inaccurate results and have pointed out some practical problems associated with the use of Glycols. LiBr experiments show very good agreement with the theoretical model. Preheating of the air is shown to greatly enhance desiccant regeneration. The packed column yields good results as a dehumidifier/regenerator, provided pressure drop can be reduced with the use of suitable packing.
Growing demand for air conditioning in recent years has caused a significant increase in demand for primary energy resources. Solar-powered cooling is one of the environmentally-friendly techniques which may help alleviate the problem. A promising solar cooling method is through the use of a liquid desiccant system, where humidity is absorbed directly from the process air by direct contact with the desiccant. The desiccant is then regenerated, again in direct contact with an external air stream, by solar heat at relatively low temperatures. The liquid desiccant system has many potential advantages over other solar air conditioning systems and can provide a promising alternative to absorption or to solid desiccant systems.Earlier work by the authors included theoretical simulations and preliminary experiments on the key components of the liquid desiccant system. The objective of the present study has been to construct a prototype system based on the knowledge gained, to monitor its performance, identify problems and carry out preliminary design optimization. A 16 kWt system was installed at the Energy Engineering Center at the Technion, in the Mediterranean city of Haifa. The system comprises a dehumidifier and a regenerator with their associated components operating together to dehumidify the fresh (ambient) air supply to a group of offices on the top floor of the building. LiCl-water is employed as the working fluid. The system is coupled to a solar collector field and employs two methods of storage – hot water and desiccant solution in the regenerated state. The performance of the system was monitored for five summer months under varying operating conditions. The paper describes the operation of the experimental system and presents the measured data and the calculated performance parameters.
A novel dew point evaporative cooling system for sensible cooling of the ventilation air for air conditioning application was constructed and experiments were carried out to investigate the outlet air conditions and the system effectiveness at different inlet air conditions (temperature, humidity and velocity) covering dry, temperate and humid climates. The results showed that wet bulb effectiveness ranged between 92 and 114% and the dew point effectiveness between 58 and 84%. A continuous operation of the system during a typical day of summer season in a hot and humid climate showed that wet bulb and dew point effectiveness were almost constant at about 102 and 76%, respectively. The experiment results were compared with some recent studies in literature.
An open cycle absorption refrigeration system is simulated and analyzed. The open cycle differs from the closed cycle in that the open cycle regenerates the weak absorbent solution by evaporating refrigerant to the earth's atmosphere rather than to a condenser. The solar collector used for the open cycle is one in which the weak absorbent solution flows as a fluid film over a flat, open, black surface. The absorbent solution is heated by the black surface and is regenerated by water evaporating to the atmosphere. It was found that the relationship between the collector length and the solution mass flow rate was tied to environmental factors such as wind and humidity when optimizing system performance. The system performance was simulated for five cities using actual weather data. The overall daily cooling COP's (cooling/incident solar) ranged from 0.09 to 0.45 for various conditions.
The main drawback of greenhouse evaporative cooling systems based on cooling pads and extracting fans is the thermal gradient developed along the direction of the airflow. High-temperature gradients of this type can markedly affect plant growth, and growers often combine cooling pads with shading. To predict the temperature gradients along a greenhouse, a simple climate model is proposed which incorporates the effect of ventilation rate, roof shading and crop transpiration. In order to calibrate the proposed model, measurements were performed in a commercial greenhouse equipped with fans and pads and shaded in the second half. Experimental data show that the cooling system was able to keep the greenhouse air temperature at rather low levels. However, due to the significant length of the greenhouse (60 m), large temperature gradients, (up to 8°C) were observed from pads to fans. The model was calibrated by fitting temperatures in the middle and at the end of the greenhouse. The model was validated on experimental data different from those used for the calibration and then it was used to study: (i) the influence of different ventilation rates combined with shading on air temperature profiles along the greenhouse length; and (ii) the influence of the outside air temperature and humidity on the performance of the cooling system. High ventilation rates and shading contribute to reduce thermal gradients. Despite its simplicity, the model is sufficiently accurate to improve the design and the management of the cooling pad systems.
Carbon Dioxide in Greenhouses
- T J Blom
- W A Straver
- F J Ingratta
- S Khosla
- W Brown
Blom, T.J., Straver, W.A., Ingratta, F.J., Khosla, S., Brown, W., 2002. Carbon Dioxide in
Greenhouses [WWW Document].
- A Ghosh
A. Ghosh, A. Ganguly
Solar Energy 158 (2017) 644-653