Progress and prospects in reverse electrodialysis for salinity gradient energy conversion and storage
Graphical abstract
Introduction
Energy is one of the key resources determining the overall socio-economic development: a robust energy supply must be secured for a sustainable growth and an improved living standard. Global energy consumption is increasing at an exponential rate: it is estimated that global energy consumption will rise by 48% between 2012 and 2040 [1]. Moreover, the anticipated population increase in the coming decades will generate two billion new energy consumers in emerging economies by 2050 [2], [3]. During the same time interval, global energy-related carbon dioxide emissions are expected to increase by 46%, corresponding to 45 billion metric tonnes [1], [3], [4]. In this context, the development of alternative energy resources able to alleviate the skyrocketing demand for clean energy and related environmental issues are urgently required [5].
Salinity gradient energy or Salinity Gradient Power (SGP), also referred to as ‘Blue Energy’, is generated by converting the chemical potential difference between two salt solutions with different concentrations into electrical or mechanical energy [6], [7], [8], [9], [10], [11], [12]. It is a completely clean and sustainable energy source with no toxic gas emissions. Commonly (but not exclusively) river and seawater are used as salt solutions. Other potential resources of SGP involve brine solutions from anthropogenic sources (industrial streams, solar ponds [13], [14], [15]) or natural sources (e.g. the Dead Sea), saline wastewater from industrial processes or domestic source, thermolytic solutions (e.g. ammonium bicarbonate), etc. The SGP concept was first proposed by Pattle in 1954 [16]. A number of patents were filled and pioneering research was performed at that time [9], [16], [17], [18], [19]. However, technological progress was hampered mainly by the unavailability of suitable membranes. The fast-growing rate of today’s membrane market, in conjunction with the increasing demand for renewable energy, currently represents the main driving force of interest in SGP.
The unexploited estuarial SGP released by mixing seawater and river water, which has an estimated maximum global potential of 2.6 TW, is the second largest marine-based energy source next to ocean waves (see Table 1) [7], [8], [20]. With reference to the most relevant water bodies, the gross global potential of salinity gradient energy is estimated to be higher than 27,000 TWh/year, the extractable part of which being around 2000 TWh/year which is more than 10% of the total global potential of renewable energy resources [21]. This estimate may vary, depending on technical and operational factors, such as flow rate, temperature, recovery rate, efficiency, salinity levels, fouling behavior and other aspects, including ecological and legal constraints [22]. Of the total global potential of SGP, about 0.98 TW is estimated to be available for extraction. In addition, efficient utilization of the global SGP could potentially yield 38 Mt/yr of hydrogen, a clean and versatile energy carrier [23]. The SGP potential from wastewater discharged into an ocean is estimated to be 18 GW [7]. Novel SGP applications based on closed-loop systems using excess waste heat could also potentially enable the production of more than 120,000 GWh/year [24].
Unlike intermittent wind and solar energy sources, SGP can be exploited continuously 24 h per day and 365 days a year. The natural water cycle illustrated in Fig. 1 exemplifies the concept of SGP as a renewable energy resource that potentially originates when river water, brackish water, seawater or brine are mixed with each other.
This review focuses on RED technology which is currently witnessing a significant development and, so far, the only pilot plant producing power from SGP using RED technology [28], [29], [30]. Furthermore, positive net power density values have been already achieved in RED stacks operated over extended periods of time with either natural river water and seawater [31], [32], [33] or brackish water/seawater and brine [29], [30], [34], [35]. The present work presents a detailed critical assessment of the potential of RED to harvest salinity gradient energy. Major research advances from the past decades up to now are presented and discussed, with a special focus on material development, system design, membrane stability against fouling, process optimization, and advanced applications in the logic of process intensification. The most significant research outcomes in terms of ion exchange membrane preparation, characterization and testing in RED are reported. Suitable membrane materials still remain the key components determining the overall performance and economics of RED for commercial success. Applications based on hypersaline salt solutions are evaluated and compared with respect to conventional RED operations using river water and seawater. The applicability of RED for sustainable production of water and hydrogen in integrated process schemes with RO, MD, bio-electrochemical systems and water electrolysis technologies is presented, including potential use as an energy storage device (concentration gradient flow battery). Technological challenges and economic aspects, outlining the future research perspectives required for large-scale implementation are also discussed.
Section snippets
Thermodynamic potential of salinity gradients
The Gibbs energy of mixing (ΔGmix) is released when two solutions of different salinity are mixed. For the i-th component in a solution, the chemical potential μi (J/mol), i.e. its partial molar Gibbs energy, is defined as [36]:where v is the partial molar volume (m3/mol), Δp the pressure difference (Pa), z the valence (equiv./mol), F the Faraday constant (96,485 C/equiv.), ΔΨ the electrical potential difference (V), R the gas constant (8.314 J/(mol K), T the
Technologies for salinity gradient power
SGP can be generated by different technologies: Reverse Electrodialysis (RED) [21], [34], [35], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], microbial RED [50], [51], [52], [53], capacitive mixing (CAPMIX) [54], [55], mixing entropy batteries (MEB) [56], pressure retarded osmosis (PRO) [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68] and vapour pressure difference utilization (VPD) [69]. RED and PRO are the two promising membrane-based
Reverse electrodialysis
Despite the fact that the concept of RED technology was reported long ago in 1954 [16], the most remarkable trend of research advances has been recorded from 2007 onwards [42], [49]. Fig. 3 shows the number of papers on RED published yearly over the past decade, and the key research topics so far have covered process analysis, testing and optimization [39], [41], [70], [71], [72], [73], [74], [75], [76], stack design [77], [78], [79], [80], [81], membrane design and development [82], [83], [84]
Advances in power density
The maximum Pd is a crucial parameter determining the commercial feasibility of RED. Progressive improvements in power density have been reported from the time of early investigations till now (Fig. 29). Very low power density (up to 0.17 W/m2) was reported at the early stage of RED research activity by Weinstein and Leitz [9], who carried out experiments with seawater and river water. Driven by improvement in membrane materials and manufacturing, significant enhancement has emerged since 2009.
Advanced applications of RED
RED systems integrated with other technologies are emerging as an attractive option in the production of key resources like water and hydrogen, in addition to electrical energy generation. RED technology can be integrated with different technologies in desalination [35], [38], [47], [99], [100], [106], [316], [317], [318], [319], bio-electrochemical systems [51], [53], [106], [277], [320], [101] and water electrolysis [23], [105], [277] (Table 7).
Closed-loop RED system
Operation of RED in open-loop with natural water or wastewater streams has some drawbacks, such as the requirement of extensive pretreatment and fouling-control strategies. Additionally, there could be an environmental impact due to the withdrawal of natural water from ecosystems and to some extent the discharge of outlet effluents into natural waters [7], [8], [376].
During the RED process, the mixing of the HCC and LCC occurs in the stack as a result of ion transport, as well as water
Pilot-scale reverse electrodialysis developments
Significant contributions and improvement in performance of RED have been recorded at lab scale in the past. Conversely, large-scale RED advances and scale-up is limited. In fact, some research on industrial-scale development of RED were recently foreseen in R&D projects [28], [29], [160]. REDstack BV (The Netherlands) is the company which operated the first 5 kW RED pilot plant together with Frisia Zout BV (a European salt company) [381]. On October 2013, REDstack BV, Fujifilm Manufacturing BV
Economic aspects
The economic and financial feasibility of RED has been preliminarily investigated for different scenarios in comparison with other renewable energy resources [81], [272]. It was observed that the current membrane price of 50 €/m2 makes RED energy more expensive than other energy sources, such as solar and wind power [81]. However, if the membrane price could be reduced to 4.3 €/m2 by using cheap raw materials and manufacturing procedures in the near future, the cost of produced electricity
Concluding remarks and future prospects
The major limitation of RED at the current state-of-the-art is the availability of low resistance ion-conductive membrane materials at a low cost (<4 €/m2) and with high permselectivity (>95%) for operation under real conditions. Addressing these issues by the design of novel, high performance IEM materials at an affordable cost highly impacts the possible commercial implementation of RED technology. Prospective, low cost hydrocarbons to be further investigated could be polyolefin,
Acknowledgements
The financial support of The Education, Audiovisual and Culture Executive Agency (EACEA) under the Program “Erasmus Mundus Doctorate in Membrane Engineering”-EUDIME (FPA 2011-0014) is kindly acknowledged. Ramato A. Tufa acknowledges the financial support of the European Union’s Horizon 2020 Research and innovation pro-gramme under the Marie Skłodowska-Curie Actions IF Grant agreement No. 748683. This work was also supported by the Associated Laboratory for Sustainable Chemistry- Clean Processes
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