Elsevier

Renewable Energy

Volume 86, February 2016, Pages 785-795
Renewable Energy

Steady-state investigation of water vapor adsorption for thermally driven adsorption based greenhouse air-conditioning system

https://doi.org/10.1016/j.renene.2015.09.015Get rights and content

Highlights

  • Adsorption uptakes for three adsorbent/water pairs are measured at 20–50 °C.

  • Experimental data are fitted with GAB and D–A adsorption model equations.

  • Steady-state desiccant air-conditioning cycle is evaluated for greenhouses.

  • Effect of regeneration temperature on steady-state moisture cycled is determined.

Abstract

In the present study, water vapor adsorption onto 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 apparatus for greenhouse air-conditioning (AC). The Guggenheim–Anderson–De Boer and Dubinin–Astakhov adsorption models are used to fit the adsorption data of silica-gel and ACP/ACF, respectively. The isosteric heat of adsorption is determined by Clausius–Clapeyron relationship. The adsorbents are evaluated for low-temperature regeneration with aim to develop solar operated AC system for greenhouses. Ideal growth zone for agricultural products is determined by which the steady-state desiccant AC cycle is evaluated on the psychometric chart and adsorption isobars.

Steady-state moisture cycled (MCSS) by each adsorbent is determined for demand category-I, II and III which are based on 60%, 40% and 20% relative humidity of dehumidified air, respectively. In case of demand category-I, the ACP enables maximum MCSS at all regeneration temperatures (Treg), ideally sitting at 47 °C. The ACF enables double MCSS as compared to silica-gel during demand category-II at Treg ≥59 °C. However, the silica-gel is found the only applicable adsorbent for the demand category-III.

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.2nCO2+4n(H2O)fromrootsPhotons 2(CH2O)n+2nO2+2n(H2O)intoair

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

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