Materials Today
Volume 42, Januaryā€“February 2021, Pages 178-191
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Hybrid solar-driven interfacial evaporation systems: Beyond water production towards high solar energy utilization

https://doi.org/10.1016/j.mattod.2020.10.022Get rights and content

Abstract

Owing to its promising approach to tackling freshwater scarcity, solar-driven interfacial evaporation (SDIE) which confines the photothermal heat at evaporating surface has attracted tremendous research attention. Optimizing efforts on photothermal conversion and thermal management have greatly improved the SDIE performance. By taking advantage of the heat localization strategy, hybrid SDIE systems have been designed to enhance the solar energy utilization beyond water production. In this review, the development of SDIE and energy flow in hybrid system are discussed. The advanced conceptual designs of different hybrid applications such as electricity generation, fuel production, salt collection, photodegradation and sterilization are comprehensively summarized. Moreover, the current challenges and future perspectives of the hybrid systems are emphasized. This article aims to provide a systematic review on the recent progresses in hybrid SDIE systems to inspire both fundamental and applied research in capitalizing the undervalued auxiliary energy sources for future integrated water, energy and environmental systems.

Introduction

Water shortage poses a growing threat to sustainable economic development and social progresses. The freshwater scarcity situation is exacerbated by the on-going population growth, climate changes and environmental pollution [1]. Although there is an abundant water on Earth, 97% is seawater, which is not suitable for direct drinking, domestic or industrial usage. Therefore, it is of great significance to develop efficient and large-scale desalination technologies to produce freshwater from seawater or even waste water. However, most of the existing desalination technologies such as reverse osmosis (RO) [2], [3], [4], [5] and low-temperature multi-effect distillation (MED) [6], [7] are implemented at great capital expense and high energy consumption, which are unfeasible for remote and off-grid regions. Consequently, addressing the contradiction between water purification and energy consumption becomes a critical issue for sustainable freshwater production.

Solar energy has been widely used since ancient times, and being an inexhaustible and environmentally friendly energy source, it continues to attract immense attention to provide a highly promising way to address numerous challenges [8], [9], [10], [11], [12], [13], [14], [15]. Photothermal effect is a motivating force of the natural hydrologic cycle and atmospheric circulation, which also brings inspirations for tackling water scarcity issues [16], [17], [18]. Solar-driven water evaporation, which uses sunlight as the sole energy source to vaporize water through photothermal effect, circumvents the huge obstacles between freshwater supply and energy consumption. However, the traditional solar-driven evaporation systems usually have low yield due to poor light absorption efficiency and massive heat dissipation [19], [20]. In this context, researchers have spent considerable efforts to develop advanced and efficient solar vapor systems to take full advantage of the sunlight.

In recent years, a highly efficient solar-driven interfacial evaporation (SDIE) system [21], [22], [23] radically different from the common bulk [24] or volumetric [25], [26], [27] evaporation strategies was developed. The solar absorber is located at the waterā€“air interface with confined water path to thermally insulate the absorber from the bulk water. With this, the photothermal heat is confined to the evaporating interface, where only a small volume of water is heated, while the underlying bulk water temperature remains largely unchanged, close to the ambient. Consequently, detrimental heat dissipation is largely suppressed and the temperature of light absorber is greatly increased, boosting evaporation efficiency to over 90% [28], [29], [30], [31], [32], [33]. Besides affording minimal use of photothermal materials, such evaporating strategies also offer the flexibility to tune the vapor flux and vapor temperature, setting the stage for other applications beyond evaporation such as electricity generation [34], [35], [36], [37], [38], fuel production [39], [40], [41], salt collection [42], [43], [44], and so on. So far, there are many excellent and comprehensive reviews that systematically summarized the development of the SDIE systems, meanwhile, in-depth analyses of the materials and system designs that can improve the performance of water evaporation are made available [20], [29], [45], [46], [47], [48], [49], [50], [51], [52], [53]. These meaningful discussions will undoubtedly enlighten and advance the research in SDIE system. However, though many researchers have integrated various applications with the evaporating systems, existing review articles mainly focused on the photothermal materials and system designs that can improve evaporation efficiency, while these hybrid applications merely serve as complements with insufficient or incomprehensive discussions. To date, there are few, if any, articles focusing on important and mutually compatible applications beyond evaporation that are promising in managing the energy-water-environment nexus [8], [20], [54], [55], [56]. In view of this upcoming research area as mankind seek for better performing and multi-function integrated systems, it is necessary and timely to review the recent progresses and gain insights into this interdisciplinary research field to accelerate its development.

The goal of this review is to provide a systematic overview of the recent progresses in hybrid SDIE systems and the relevant advanced concepts that enrich the solar energy utilization. We first provide a comprehensive summary of the state-of-art development of the SDIE with advanced photothermal conversion and thermal management strategies. Next, the energy flow and the types of potential energy sources present in the SDIE system that can be capitalized for other applications will be discussed. We will then present an overview of the recent efforts on utilizing the undervalued energy sources in the SDIE. In terms of application, we classify them into two main types: (1) energy harvesting including electricity generation and fuel production; and (2) other applications including salt collection, photodegradation, sterilization, etc. Finally, we point out the opportunities and challenges of the hybrid applications in the SDIE systems. This review is aimed at providing an in-depth understanding of the various complementary approaches that can fully utilize the energy in a SDIE system, towards high-efficiency solar utilization, beyond freshwater production.

Section snippets

Development of the SDIE

As an emerging solar-driven freshwater collection technology, the unique local heating feature endows SDIE with more flexibility in tuning and optimizing the system. The water evaporation efficiency is mainly determined by two critical factors: (1) photothermal conversion capability which determines the total amount of energy the evaporation system can utilize, and (2) thermal management which shapes the energy flow in system to distribute the photothermal heat for water vaporization. Desired

Solar-driven water-electricity production

Recently, solar-driven water evaporation integrated with electricity generation has attracted tremendous attention, since it may provide renewable and decentralized clean water-electricity solutions especially useful for rural areas and developing countries [15], [16], [45], [56]. In view of the energy flow for typical exclusive water evaporation schemes (Fig. 1), the photothermal heat is localized at the absorber and utilized for solar vaporization, while the internal enthalpy of water body

Salt collection

In a SDIE system, salt concentration at the evaporating interface will increase and eventually precipitate on the solar absorber due to the suppression of ion diffusion. This will severely compromise the light absorption efficiency, leading to a gradual decrease in the water evaporation rate. Diverse salt fouling inhibition strategies such as backwash or chemical cleaning have been introduced [96], [128], [129], [130], however, the concentrated solution after treatment, inevitably constitute to

Conclusion and perspective

With the rapid development of SDIE, various applications have been devised to couple with the freshwater collection systems to expand their functions and improve the overall solar utilization efficiency. In this review, key factors of the SDIE system namely photothermal conversion and thermal management are discussed. By analyzing the energy flow from sunlight absorption to vapor condensation during freshwater production, potential energy sources that come along with the system are discussed.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research is supported by the Ministry of Education, Singapore, under its Academic Research Fund Tier 2 (Award MOE2017-T2-1-140 and MOE2017-T2-2-102).

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