Steady-state investigation of water vapor adsorption for thermally driven adsorption based greenhouse air-conditioning system
Introduction
The absolute humidity in the agricultural greenhouses used to increase continuously due to the photosynthesis and evapo-transpiration processes by which plants remain in danger of insects/pests/fungus attack and condensation/dripping of water vapors. Generally photosynthesis occurs during daytime whereas evapo-transpiration continues throughout the day and night times. Photosynthesis is the most important process in plants by which the plant makes carbohydrate using the carbon dioxide in the presence of light energy. During this process, the plant releases half of the water vapors into the air which was sucked from the plant roots as expressed by Eq. (1). Furthermore, the greenhouse temperature tends to increases continuously because of the presence of sunlight.
Evapo-transpiration is the summation of evaporation from the soil and transpiration from plant. Hagishima et al. [1] reported the average transpiration rate of 150–456 g/day for three plants having leaf area of 0.99–1.47 m2. Thus it can be concluded that the greenhouse humidity increases day and night times. The plant's growth and/or flowering are highly influenced by the relative humidity (RH) as well as CO2 level in the greenhouse [2], [3]. In greenhouses, sufficient amount of CO2 is always required for effective photosynthesis which also limits the applicability of return air utilization for any kind of greenhouse air-conditioning (AC) system. The required relative humidity for a plant depends on its ideal vapor pressure deficit (VPD) that may vary depending upon the plant growth stage, maturity stage, danger of insect/pest/fungus attack, extreme weather conditions, and water stresses etc. [4]. For example, Short et al. [5] found the different ideal VPDs for five growth stages of greenhouse tomatoes including germination, seedling, vegetative, early and mature fruiting. The optimizations of air humidity ratio, intake solar radiation intensity, CO2 enriched outdoor air, and rated crop water requirement for effective photosynthesis and evapo-transpiration bring the temporal variability in the sensible and latent load of AC.
The greenhouse environment involves in higher relative humidity AC as compared to AC for human's thermal comfort as shown in Fig. 1 [4], [6], [7], [8]. Desiccant AC systems are getting lots of attention in order to control the humidity in various air-conditioning applications e.g. greenhouses [9], [10], [4]; buildings [11], [12]; automobiles [13]; wet markets [14]; marine ships [15], [16]; museums [17], [18]; hospitals, product storage and preservation etc. [19]. Being free from refrigerants, it enables zero ozone depletion and global warming potential. In addition to higher air quality it can be operated on low grade waste heat or renewable thermal energy sources. Fig. 2 shows that the desiccant AC in comparison with the conventional vapor compression AC has the ability to achieve the sensible and latent load of AC distinctly, which gives the opportunity to fulfil above mentioned greenhouses AC demands. Desiccant AC combines the desiccant dehumidification (1→D) and low-cost evaporative cooling (D→2) [19]. On the other hand conventional VAC cools the air below the dew point (1→V1:V2) so the heating is required from (V2→2) in order to obtain the desired conditions of temperature and humidity [14], [20], [21], [22]. It can be noticed from Fig. 2 that it is unnecessary to over cool the air below the dew point in case of desiccant AC which results in energy saving. According to a feasibility study [23], the electricity saving of 24% is obtained by desiccant AC when the system is operated under humid climate of Thailand. In another study, the use of desiccant cooling system for wet markets of hot and humid Hong Kong yields the energy cost savings, and CO2 emission reduction from 1% to 13% [14]. Furthermore, the system payback period of less than 5 years can be obtained by utilizing the system intelligently [19]. Hence, it can be concluded from the referenced studies that the desiccant AC has a huge potential in greenhouse AC application because of the high humidity application.
The carbons are rarely studied for conventional desiccant AC because of the little water vapor adsorption uptake at normal relative humidity range. However, most of the carbons adsorb water vapors at higher relative humidity [24] which could be interesting for greenhouse AC. In this regard, the present study experimentally investigates the water vapor adsorption by two kinds of micro porous carbon based adsorbents (CBAs) which present the distinctive water vapor adsorption trends at higher RH. In addition to high porosity the both CBAs enable high structural stability and experimental repeatability. Moreover a commonly used hydrophilic adsorbent silica-gel is considered for comparison point of view. The trends of water vapor adsorption isotherms by the CBAs are well-known. However from our evidences, the presently studied powder type carbon based adsorbent (ACP) processes very high adsorption uptake as compared to other published carbon/water pairs. As the total adsorption uptake always influences the performance of adsorption heat pump systems, hence the present work will be worthy for the greenhouse AC application. The study evaluates the steady-state desiccant AC cycle on the psychometric chart and adsorption isobar for each adsorbent. The effect of regeneration temperature on steady-state moisture cycled and adsorbent to air mass fraction is determined for three demand categories which are based on RH of dehumidified air.
Section snippets
Materials
Adsorbents used in the present study are: (i) RD type silica-gel [25] which is a famous hydrophilic adsorbent provided by Fuji Silysia Chemical Ltd., Japan (ii) pitch based activated carbon powder (ACP) of type Maxsorb-III [26] which is a highly porous adsorbent provided by Kansai Coke & Chemicals Co. Ltd., Japan, and (iii) pitch based activated carbon fiber (ACF) of type A-20 [27] which is a fibrous adsorbent with high porosity and ease of handling. The surface area, micropore volume and pore
Adsorption equilibrium
In the previous comparative study by the authors [30], six adsorption models are analyzed for the regression analysis of the experimental data at 30 °C. The studied adsorption models are Brunauer–Emmett–Teller (BET); Guggenhein, Anderson, De-Boer (GAB); Oswin; Freundlich; Peleg; and Dubinin–Astakhov. The resulted fitting error by each adsorption model is shown in Fig. 4. It can be seen that the Guggenheim [31], Anderson [32], De-Boer [33] (GAB) and Dubinin–Astakhov (D–A) [34], [35] adsorption
Adsorption isotherms
Adsorption of water vapor onto silica-gel and two kinds of CBAs had been experimentally measured at 20, 30 and 50 °C using a volumetric method based adsorption measurement apparatus. The resulted adsorption isotherms are presented in Fig. 7(a)–(c) together with the system uncertainty in adsorption uptake measurement. The horizontal error bar values were very small which are unable to be shown in Fig. 7. It can be seen that the studied CBAs enable conventional trends of water vapor adsorption by
Conclusions
Water vapor adsorption uptake by silica-gel, activated carbon powder (ACP), and activated carbon fiber (ACF) has been experimentally measured at 20, 30 and 50 °C using a volumetric method based adsorption measurement unit for greenhouse air-conditioning (AC) application. The adsorption data is successfully fitted with Guggenheim–Anderson–De Boer (GAB) and Dubinin–Astakhov (D–A) equations for silica-gel and ACP/ACF, respectively. The isosteric heat of adsorption (Qst) is determined by the
References (55)
Effect of relative humidity on growth and flowering of some greenhouse plants
Sci. Hortic.
(1986)- et al.
JAPIEST: an integral intelligent system for the diagnosis and control of tomatoes diseases and pests in hydroponic greenhouses
Expert Syst. Appl.
(2008) - et al.
Three years experimental comparative analysis of a desiccant based air conditioning system for a flower greenhouse: assessment of different desiccants
Appl. Therm. Eng.
(2015) - et al.
Comparative experimental analysis and modelling of a flower greenhouse equipped with a desiccant system
Appl. Therm. Eng.
(2012) - et al.
Construction and initial operation of the combined solar thermal and electric desiccant cooling system
Sol. Energy
(2009) - et al.
Development and construction of the novel solar thermal desiccant cooling system incorporating hot water production
Appl. Energy
(2010) - et al.
High energy efficiency desiccant assisted automobile air-conditioner and its temperature and humidity control system
Appl. Therm. Eng.
(2006) - et al.
Site verification and modeling of desiccant-based system as an alternative to conventional air-conditioning systems for wet markets
Energy
(2013) - et al.
Development of a new marine rotary desiccant airconditioning system and its energy consumption analysis
Energy Procedia
(2012) - et al.
Energy and exergy performance analysis of a marine rotary desiccant air-conditioning system based on orthogonal experiment
Energy
(2014)
A coupled numerical approach on museum air conditioning: energy and fluid-dynamic analysis
Appl. Energy
Energy saving strategies in air-conditioning for museums
Appl. Therm. Eng.
An overview of solid desiccant dehumidification and air conditioning systems
Renew. Sustain Energy Rev.
Performance analysis of four-partition desiccant wheel and hybrid dehumidification air-conditioning system
Int. J. Refrig
Simulation of a hybrid liquid desiccant based air-conditioning system
Appl. Therm. Eng.
Experimental investigation of a liquid desiccant system for solar cooling and dehumidification
Sol. Energy
Feasibility study of desiccant air-conditioning system in Thailand
Build. Environ.
Water adsorption on carbons — Critical review of the most popular analytical approaches
Adv. Colloid Interface Sci.
Experimental investigation of the silica gel–water adsorption isotherm characteristics
Appl. Therm. Eng.
Adsorption of ethanol onto parent and surface treated activated carbon powders
Int. J. Heat Mass Transf.
A study on the kinetics of ethanol-activated carbon fiber: Theory and experiments
Int. J. Heat Mass Transf.
Experimental investigation of activated carbon fibers/ethanol pairs for adsorption cooling system application
Appl. Therm. Eng.
Adsorption properties of a natural zeolite–water pair for use in adsorption cooling cycles
Appl. Energy
Adsorption in micropores
J. Colloid Interface Sci.
Application of the GAB model to the moisture sorption isotherms for dried fruits
J. Food Eng.
The cooling performance of a building integrated evaporative cooling system driven by solar energy
Energy Build.
Performance Evaluation of the Desiccant Bed Solar Dryer
Energy Procedia
Cited by (65)
Analysis of spatial-temporal trends and causes of vapor pressure deficit in China from 1961 to 2020
2024, Atmospheric ResearchA critical review on efficient thermal environment controls in indoor vertical farming
2023, Journal of Cleaner ProductionEvaluating the emerging adsorbents for water production potential and thermodynamic limits of adsorption-based atmospheric water harvesting systems
2023, International Communications in Heat and Mass Transfer