Elsevier

Desalination

Volume 491, 1 October 2020, 114580
Desalination

Enhanced performance of a direct contact membrane distillation (DCMD) system with a Ti/MgF2 solar absorber under actual weather environments

https://doi.org/10.1016/j.desal.2020.114580Get rights and content

Highlights

  • A solar DCMD system with a 5-stack Ti/MgF2 solar absorber was performed.

  • A Ti/MgF2 solar absorber exhibited enhanced solar-thermal performance.

  • Distillate water successfully produced under actual weather conditions.

  • Compatibility to commercial solar collector was evaluated.

Abstract

In this study, we report on a solar membrane distillation (MD) system with a 5-stack Ti/MgF2 solar absorber adapted as a heat source under actual weather conditions. The 5-stack Ti/MgF2 solar absorber showed light absorption of 85% over the wavelength range from 0.3 to 2.5 μm. This consequently induced greater heat and a two-fold greater water heating capability up to 80 °C for water in a closed water tank without insulation under a 1-sun illumination. The enhanced solar absorption of the Ti/MgF2 solar absorber showed a 12% improvement in permeate flux of the integrated MD system compared with a system without a solar absorber. Under actual weather conditions, the solar DCMD system with the Ti/MgF2 solar absorber successfully produced distillate water in the range of 0.51–4.78 L/m2·day depending on weather conditions. Despite unfavorable solar irradiance conditions in autumn, the integrated DCMD system with the Ti/MgF2 solar absorber proved superior to a conventional commercial evacuated-tube solar collector in terms of daily production of distillate water and solar energy requirements for certain amounts of distillate.

Introduction

Demand for alternative sources of fresh water is globally increasing due to population growth, industrialization, water contamination, and climate change [[1], [2], [3], [4]]. Therefore, many countries have explored seawater and water reuse as alternative water resources [[3], [4], [5], [6], [7]]. To produce fresh water from seawater and reclaimed wastewater, advanced treatment through desalination processes such as reverse osmosis or thermal desalination is essential [[1], [2], [3], [4], [5], [6], [7]]. While these technologies successfully remove salts and other dissolved contaminants from seawater and reclaimed wastewater, they require a large amount of energy [[1], [2], [3]]. Therefore, the ability to utilize renewable energy is essential for desalination [2], especially in less-developed areas with inferior water and energy infrastructure.

Recently, membrane distillation (MD) has attracted a great deal of attention for desalination, because of its low fouling potential and potential ability to use low-grade heat sources such as solar or geothermal energy [[8], [9], [10], [11], [12]]. This hybrid membrane process utilizes thermal energy: the difference in temperature on each side of the membrane drives vapor flux across the hydrophobic and porous membrane, generating pure water upon condensation [9].

Solar-driven MD can be explained structurally as integration of an MD system and solar technology [2]. Use of renewable energy sources in the desalination process will mitigate environmental issues and reserve conventional energy for other applications [13,14]. Solar absorbers for thermal systems integrated with MD systems are worth studying because MD is a thermal process driven by heat sources.

Solar absorbers have been widely studied for photothermal applications such as water vaporization and solar desalination [[15], [16], [17]]. Since the solar absorber is a critical component of solar MD, a high-performance solar MD cannot be realized without effective light absorption and photothermal conversion of solar radiation in solar absorbers. An ideal solar absorber absorbs all incident light of the solar spectrum like a black body, without reflection or transmission.

Many approaches have been studied for solar absorbers with plasmonic nanostructures, semiconductors, and carbon-based materials to enhance solar absorption and corresponding photothermal conversion efficiency [[16], [17], [18], [19], [20], [21], [22], [23]]. The solar absorbers based on SiO2 cermets, such as TiN-SiO2 and Mo-SiO2 cermets, have shown high light absorbance (up to 94%) with excellent spectral [20,21]. Recently, metals and dielectric (SiO2 and Al2O3) composites have been developed for enhanced photothermal conversion efficiency based on plasmonic absorption [22,23]. Zhou et al. [22] achieved light absorbance of 99% with an outstanding broad range (400 nm to 10 mm) by combining an aluminum oxide nanoporous template. The light absorption properties of a solar absorber consist of a metal/dielectric multilayer structure that can be easily tailored by choosing proper metal/dielectric combinations and their thicknesses [24]. This kind of approach based on the plasmonic effect provides relatively simple design methods for designing broadband solar absorbers as well as narrowband sensors.

In previous efforts by our research team, a facile, scalable method of fabricating a mechanically robust and stretchable solar absorber with broadband absorption was proposed and evaluated with optical simulation [24]. In the present study, we applied the new type of 5-stack Ti/MgF2 solar absorber as a heat source to improve the performance of a direct contact membrane distillation (DCMD) system for decentralized water supply. Performance of the solar absorber was evaluated in terms of solar absorption and heat ability for water. We also investigated the effectiveness of the integrated MD system combined with the solar absorber under actual weather conditions compared with an MD system using a commercial solar collector. The results confirmed enhancement of fresh water production with the Ti/MgF2 solar absorber-integrated MD system for decentralized water supply to households or small-scale communities.

Section snippets

Solar absorber fabrication and solar absorber modules

It has been reported that Ti/MgF2 multi-layered structures show enhanced solar absorption depending on structure, i.e., thicknesses of Ti and MgF2, and number of alternating stacks [24]. The 5-stack Ti/MgF2 film structure has been adapted for solar absorbers to increase solar absorption and consequently increase the temperature of water underneath it. In this study, 5-stack Ti/MgF2 multi-layer films were deposited on a substrate using an electron beam evaporator (KVE-T8897, Korea Vacuum Tech,

Efficient heat transfer of the solar absorber

A vital component of the solar DCMD system is the solar absorber, which absorbs solar radiation and converts it into thermal energy for MD. Previous studies on solar absorber design have been conducted independently without integration with an MD system [24]. The mechanically robust solar absorber design proposed here is believed to provide sufficient thermal energy through the MD system. The optical absorption spectra of solar absorbers with different thicknesses (Type-A: 7.3 nm/96.5 nm and

Conclusions

To enhance the performance of a solar DCMD system, we proposed a solar absorber functionalized with a Ti/MgF2 very thin film structure as a heat source for the system and evaluated the effectiveness of the integrated DCMD system with this solar absorber. The Ti/MgF2 solar absorber effectively absorbed most of the desired wavelengths of solar irradiance and raised the temperature of the water to a sufficient level required to drive the MD system. Under outdoor conditions in autumn, the

CRediT authorship contribution statement

Jaewon Shina:Investigation, Visualization, Writing - original draft.Hye Jin Lee:Resources, Investigation, Visualization, Writing - original draft.Byung Min An:Data curation, Visualization, Writing - review & editing.Junki Kim:Investigation.Jinsoo Cho:Investigation.Dasom Wang:Resources, Validation.Kyung Guen Song:Conceptualization, Supervision, Writing - review & editing, Funding acquisition.Won Jun Choi:Conceptualization, Supervision, Writing - review & editing.Jeong Min Baik:Supervision.

Acknowledgments

This research was supported by a grant (code 18CTAP-C116746-02) from Korea Agency for Infrastructure Technology Advancement funded by Ministry of Land, Infrastructure and Transport of Korean Government and partially by the Technology development Program of MSS (S2717300).

References (30)

Cited by (12)

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    A series of desalination research based on SMD systems have been reported. For example, Shin and co-workers [14] designed an SMD system with a 5-stack Ti/MgF2 solar absorber adopted as a heat source to generate distillate water in the range of 0.51–4.78 L·m−2·day−1. Han et al. [15] designed the bio-derived ultrathin hierarchical membranes (carbon nanotubes functionalized carbonized eggshell membrane) and extended them to MD technology to obtain a high flux of 1.15 kg·m−2·h−1.

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Co-first authors: Jaewon Shin and Hye Jin Lee (#, authorship equally shared).

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