Renewable energy generation and hydrogen production from concentrated brine by reverse eectrodialysis

Abstract

Salinity Gradient Power-Reverse Electrodialysis (SGP-RE) is among the emerging membrane-based technologies for renewable energy generation. In RE, cation exchange membranes (CEM) and anion exchange membranes (AEMs) are alternatively aligned to create a high concentration compartment (HCC) and low concentration compartment (LCC). When the compartments are feed by a low concentration and high concentration solution, salinity gradient is created which initiates the diffusive flux of ions towards electrodes. Electricity is generated by the redox process occurring at the electrodes. The total voltage generated (open circuit voltage, OCV) is proportional to the number of membrane pairs (cells). One of the challenges pertaining to the Ohmic losses when using very low concentration salt solutions like river water can be reduced by working with highly concentrated brines (Chapter 1). Investigation of the performance of RE under realistic high-salinity conditions is crucial for implementation of RE under natural condition. The most abundant ions in natural waters involve sodium, magnesium, calcium, chloride, sulfate, and bicarbonate. Under this condition, the presence of multivalent ions, in particular Mg2+, have a lowering effect on OCV and hence a reduction of power density. This could be attributed to the enhancement of cell resistance in the presence Mg2+ ion resulting in an increase of membrane resistance. The SGP potential and comparable decrease in power density of RE operated with solutions mimicking real brackish water and exhaust brine from a solar pond depicts the pretreatment requirement in RE for better performance (Chapter 2). Seawater reverse osmosis (SWRO) is the most widespread technology for fresh water production in many parts of the world. Extensive research have been carried out to tackle the technological challenges coming along with the expansion of SWRO practice with time, specifically the reduction of energy consumption. The integrated application RE in desalination technologies in the logic of process intensification is an interesting approach towards low energy desalination. Simultaneous production of energy and desalted water is possible by hybrid application of Direct Contact Membrane Distillation (DCMD) and RE units operated on the retentate stream from a SWRO desalination plant. The use of concentrated brine for energy recovery also leads to Near-Zero Liquid Discharge from desalination systems. This avoids the adverse ecological effect of discharging hypersaline solution into natural water bodies. Thus, integrated application of RE with RO and DCMD for simultaneous water and energy production represent an innovative approach towards low energy desalination and Near-Zero Liquid Discharge paradigm (Chapter 3). The possibilitity to exploit the chemical potential of sulfate wastes by SGP-RE can be a promising alternative renewable energy source. The key challenge remains the property of membrane in sulphate solution. Although the trends in the variation of desirable membrane properties (high permselectivity and low resistance) in Na2SO4 test solutions with varying operating conditions remain similar with that of NaCl test solution, their performance is comparatively low. This has a negative impact on the performance of the RE mainly on the obtained OCV and power density. Hence, design of well optimized and high performance membranes is required for practical applicability of SGP-RE for renewable energy generation from sulfate bearing waste resources (Chapter 4). Ion exchanging membranes (IEMs) are key components in RE. Low resistance and highly permeable ion exchange membranes are required for optimal performance of RE system. For practical applications of RE under real condition, IEMs which are less susceptible to fouling are required. There is a potential risk of fouling (for example, scaling of sparingly soluble salts) of IEM operated in concentrated brine. Operations under real conditions also require feed quality control, as the presence of multivalent ions negatively impact RE performance. The variation in Total Organic Carbon (TOC) and Total Hardness (TH) of feed samples may alter the membranes physico-chemical and electrochemical properties. In addition, long term stability of IEMs in concentrated brine govern their life time. Investigation on fouling and stability of IEMS, specifically in concentrated brines, would be essential to set a clear pretreatment requirement for the performance of RE under natural conditions (Chapter 5). For techno-economic optimization and feasibility study of RE, performance of large scale (industrial scale) systems need to be investigated under varying experimental conditions. Comparative assessment of operating conditions like feed concentration, flow velocity and temperature in a small scale RE and large scale RE systems is essential. In general, the trends in OCV and power density for industrial scale operations remain more or less similar to that of membrane based water and energy technologies (based on the difficulties to meet sustainability criteria) helps in identification of technological gaps and strategic solution (Chapter 9). Future research on RE will be focusing on optimal design and development of high performance membrane in hyper-saline solution. This will extend from design of highly permeable and low resistance ion exchange membranes to the development of fouling resistant and stable membrane, particularly in concentrated brine. The relationship between physicochemical membrane properties and fouling tendency under hyper-saline environment need to be assessed. The effect of other multivalent ions in seawater like SO4 2- and Ca2+ on the performance of RE under extreme operating conditions should be clearly outlined. For integrated applications in desalination technologies, for example with DCMD, the risk of scaling and fouling for practical applications should be investigated deeply. Better membranes and module designs are required for membrane desalination systems in general. For efficient application of RE in hydrogen technologies, specifically with APE water electrolysis, development of highly conductive and durable anion selective membranes as well as highly active and stable catalysts in corrosive alkaline environment is of future research interest. Above all, well established technoeconomic evaluations of a standalone and integrated applications of RE is essential in order to evaluate the feasibility of scale-up and commercialization of the technology as a renewable energy source (Chapter 10).