Elsevier

Desalination

Volume 433, 1 May 2018, Pages 172-185
Desalination

Numerican analysis of an integrated desalination unit using humidification-dehumidification and subsurface condensation processes

https://doi.org/10.1016/j.desal.2017.12.029Get rights and content

Highlights

  • Produce pure water from brackish, saline, brine, waste and seawater is proposed.

  • The mechanism utilizes the benefits of solar still and subsurface condensation method.

  • The system applies renewable energy including shallow geothermal and solar energy.

  • Water production is deeply affected by the addition of solar collector and external condenser.

  • Increasing in the most of inlet parameters has most effect on water production.

Abstract

In the present study a solar water desalination system with humidification-dehumidification (HD) cycle is designed, integrating a solar still (as a solar humidifier) and a new subsurface condensation mechanism. Mass and energy balances are written for both solar humidifier and subsurface condenser in thermodynamic analysis of the system. Obtained nonlinear equations are analyzed numerically to explore the impact of the cycle's parameters on fresh water production. The results of the analysis indicate that the rate of water production can reach above 264.86 (kg/day) and the produced water, passing through the pores on the tubes, can be used to irrigate plant roots or collected as drinking water. According to this design and numerical simulation, it is demonstrated that the new subsurface condensation can be a promising strategy for subsurface irrigation method; and the new mechanism allows farmers to use saline water for agricultural purposes by utilizing solar energy, particularly in arid areas.

Introduction

The demand of fresh water supply is increasing due to the rapid growth of global population and economic development. With limited fresh water resources, desalination of brackish water and seawater offers the potential to meet the increasing water demands. Generally most current water desalination technologies are energy intensive and have high carbon emission. There is a pressing need for industrial scale development of cheaper and more environmentally friendly desalination plants. Therefore, employing methods such as solar humidification-dehumidification (SHD) processes that use renewable energies is deemed to be necessary. In such systems the air is heated by solar energy, then the heated air flows over and is humidified when contacting brackish water. After the air flow is loaded by a large amount of water vapor, the water content can be easily recovered by cooling the humid air in the dehumidification section. In the present plant using a solar still as humidifier helps load the air with high vapor content and volume flow rate of circulating air. Thus, the proposed unit consists of solar still as a solar humidifier, and subsurface condenser as a dehumidifier.

So far, much research has been done to increase the efficiency of solar desalination processes. Chandrashekara and Yadav [1] have discussed various solar thermal desalination methods. They claim that the cost of water production in these plants can be reduced by using solar collectors in the heating processes. They also mentioned that performance of the SHD processes can be increased with high relative humidity available near the sea and high temperature of hot air achieved with solar collectors. Sathyamurthy et al. [2] have reviewed using different solar stills in combinations with solar collectors and discussed how it contributes to improving the efficiency of these plants. They discuss the economic aspects and payback period of different solar stills and concluded that the integration of low-cost solar water heaters and HD cycles is a suitable method for increasing efficiency and reducing production costs. El-Agouz [3] investigate a modification of stepped solar still with continuous water circulation in an experimental research. His results indicate that the productivity of the modified stepped still is higher than conventional stills thanks to the solar radiation absorbers integrated in the system and its daily efficiency is approximately 20% higher. Sharshir et al. [4] offer a hybrid solar desalination system comprising of a humidification-dehumidification unit and four solar stills. They reuse the drain warm water from humidification-dehumidification unit to feed solar stills to prevent massive water loss during the desalination process. Their results show a 50% GOR increase in the system and an approximately 90% improvement in the efficiency of single solar still. In the present study, considering the various advantages of solar stills, application of a single basin solar still as a humidifier is introduced as an innovative addition to the desalination unit. In the conventional solar stills, fresh water is produced through distillation of water vapor on the surface of the glass cover of the basin. Several systems have been introduced in which condensation of vapor is carried out outside the basin by external or internal condensers, which can increase fresh water production. The performance of a modified solar still combined with an external condenser is investigated experimentally by Kabeel et al. [5]. Their results show that integrating the solar still with external condenser increases the distillate water yield by about 53.2%. Recently Rabhi [6] analyzed the efficiency of a solar still with pin fins absorber and external condenser. Daily water production of their systems with condenser (using air flow and external condenser) was 32.18% more than that the conventional still. Moreover, ground heat exchangers which take shallow geothermal energy are widely used and are highly efficient [7], [8], and can be used in the HD desalination processes. Kumar Soni et al. [9] have reviewed the experimental and modeling studies carried out on ground coupled heat exchanger systems. Their study focuses on performance of these systems and conclude that such systems have great potential to conserve significant amount of primary energy and thus mitigate adverse environmental effects through emission reduction. Therefore, in order to increase the efficiency and simplicity of the system, a subsurface condensation mechanism is employed in the present study.

Recently, due to application of desalination technologies in other engineering processes, various combinational methods have been considered [10], [11]. For agricultural purposes, a comprehensive analysis of incorporating a solar desalination unit into greenhouse is provided by Trombe and Foex [12]. An improved version of the same study was later developed by Boutiere and Bettaque [13], [14]. Their system consisted of a two-walled glass roof. Tran and Selsuk [15] found out that the performance of the combined system of SDH process and a greenhouse with a glass roof is not stable. Therefore, they separated the SDH unit entirely from the greenhouse by placing insulating panels below the desalination unit. This provided increased control over the desalination system's surroundings leading to increased water production by the solar desalination system. Chaibi and Jilar [16] conducted an experimental investigation on greenhouse solar desalination system integration in the Tunisian National Research Institute from 2000 to 2003. Their results show that such a system occupies an overall 50% of a wide-span greenhouse's roof surface and is capable of collecting the annual water required for a low canopy crop. Al-Ismaili and Jayasuriya [17] provided a comprehensive and up-to-date review on seawater greenhouse (SWGH) technique. The SWGH recreates the “hydrologic cycle” by evaporating water from saline water source and regains it as freshwater by condensation. Their research also involves investigating the implementation of SWGH unit in Oman. Their results show that SWGH function as a water conservation unit as it reduces the crop water requirement by almost 67% compared to open-field cultivation. Freshwater production from the Oman SWGH ranged between 300 and 600 L/day. However, the SWGH technology has proven to be a promising method of fresh water procurement for irrigation in places where only saline groundwater or seawater is accessible [18], [19].

The present work intends to investigate the numerical and thermodynamic analysis of a new hybrid solar desalination system. Okati et al. [20] investigated the optimum condition of different parameters of a desalination plant on the fresh water production by applying DoE (Design of Experiments). their results which indicates that water production increases with increasing in inlet water temperature of the solar humidifier, the effect of using a flat plate solar collector on increasing the inlet water temperature of the solar humidifier has been evaluated. In addition, as mentioned above, since external condensers help increase fresh water production [1], an external subsurface condensation mechanism is used as a dehumidifier mechanism. There for, the present system operates in a HD cycle. Firstly, energy and mass balance equations are analyzed for different parts of the system in the form of thermodynamic simulation in order to study the impact of various parameters on fresh water production through numerical analysis. The results show that inlet air cross-section, water temperature of solar humidifier, air velocity of inlet air and temperature have significant effects on cycle optimization. Finally, the optimum conditions and effect of different parameters of the cycle on fresh water production are examined.

Section snippets

System overview

There has been a momentum in the past few years in research on producing drinking water from the sea or other sources of wastewater (saline, brackish, flowing and hard underground waters) using solar desalination systems which utilize solar humidification and dehumidification of air. This system integrates a solar flat plate collector and a solar humidifier along with a mechanism of tubes buried under ground that function as condensers (Fig. 1). As Fig. 1 shows, the inlet water enters the solar

Equations and computational model

The equations which are based on thermodynamic principles and the law of conservation of mass and energy are separately written for each part of the process according to the overall details of the mathematical model. The equations are solved by MATLAB software and eventually the results are analyzed and discussed. The purpose of writing the equations and mathematical code is to determine the amount of desalinated water produced according to specific input parameters including: water mass in the

Results and discussion

In the following part, the effect of variations in different quantities on the solar desalinator's functional parameters including airflow velocity, number of pipes buried under ground, inlet water temperature, mass of water existing inside the solar humidifier and cross-section of the inlet airflow are studied and the following results are presented.

Efficiency & cost evaluation

The hourly efficiency of solar stills is calculated by Eq. (39) [6]. Hourly efficiency is defined as the ratio of average latent heat produced by the amount of desalinated water produced per hour to the total solar energy absorbed by the system. Changes in solar radiation levels throughout the day cause changes in system efficiency.ηh=Md×Lw/3600/I

ηh is the hourly efficiency, Md is the hourly productivity (kg/h), Lw is the latent heat of vaporization of water (J/kg), Ab is the basin area, I is

Conclusion

In the present study, in addition to designing a new compound mechanism consisting of a solar humidifier and a subsurface condenser in a humidification-dehumidification cycle, a new subsurface condensation technique is introduced that may significantly change the way agricultural fresh water or drinking water are produced. In this mechanism, a solar still is innovatively used as the humidifier in order to increase the temperature and humidity of the inlet airflow. In addition, in this

Acknowledgment

The authors would like to express their gratitude to the editor and referees of the journal for their valuable comments, criticism, and recommendations on the manuscript and also to Mr. Mohsen Qassemi.

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