Blue Energy Extraction Using Salinity Gradients: A Critical Evaluation of Case Studies

By Mihir Kumar Purkait, Mukesh Sharma, Pranjal Pratim Das and Chang-Tang Chang

Blue Energy Extraction Using Salinity Gradients presents a collection of case studies on real-world power plants from around the world that involve sustainable energy extraction via salinity gradients. Using real-world examples, the book explains and demonstrates the fundamentals, technologies, processes, and application of salinity gradient energy extraction methods, and offers practical solutions.

The opening chapter of the book provides an overview of the fundamentals and technologies of salinity gradient energy. Each Subsequent chapter analyses a real-world salinity gradient power plant from a different region of the world and includes examples from developed and developing economies on three continents. For each case study, key aspects of performance are evaluated, and the benefits and operational challenges are discussed. Validated mathematical models are also included to improve readers understanding of how to control operating parameters.

Blue Energy Extraction Using Salinity Gradients provides a unique perspective on the commercialization of salinity gradient energy extraction, and is an invaluable resource for students, researchers, and industry engineers.

• Reviews the latest technologies, progress, and developments in sustainable energy generation using salinity gradients
• Provides real-world case studies from working power stations around the globe, focusing on the practical challenges that are faced by their implementation
• Critically evaluates the potential for energy generation using salinity gradients, which regions yield the greatest potential, and supports this understanding with mathematical models
• includes examples of full-scale osmotic power extraction

• Language: English
• Publisher: Elsevier Science
• Release date: Apr 19, 2024
• ISBN: 9780443216138

Mihir Kumar Purkait

Dr. Mihir Kumar Purkait is a Professor in the Department of Chemical Engineering at the Indian Institute of Technology Guwahati, Assam, India. His current research activities are focused in four distinct areas viz. i) advanced separation technologies, ii) waste to energy, iii) smart materials for various applications, and iv) process intensification. In each of the area, his goal is to synthesis stimuli responsive materials and to develop a more fundamental understanding of the factors governing the performance of the chemical and biochemical processes. He has more than 20 years of experience in academics and research and published more than 300 papers in different reputed journals (Citation: >16,500, h-index = 75, i-10 index = 193). He has 12 patents and completed 43 sponsored and consultancy projects from various funding agencies.

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Chapter 1

Potential for power production using salinity gradients

Abstract

Salinity gradient power (SGP) is a type of renewable energy that harnesses the energy released when freshwater and saltwater mix. This occurs naturally when rivers flow into oceans or where seawater and freshwater meet underground. While the technology to generate power from salinity gradients is still in its early stages, there is significant potential for its global use. According to the International Energy Agency, SGP could theoretically generate up to 2000 TWh per year globally, which is equivalent to approximately 10% of the world's electricity demand. Among the several developed technologies, such as the pressure retarded osmosis process (PRO), reverse electrodialysis (RED), capacitive mixing, battery mixing, and other hybrid processes, PRO and RED are preferred for several small-scale and large-scale prototypes and functioning plants. However, it is important to note that the practical potential of SGP will depend on several factors, such as the location, the size of the mixing area, and the efficiency of the technology used. This chapter summarizes the technologies used to generate energy using salinity gradients. It serves as an introduction to the adapted methodologies for several case studies globally, which have been discussed in the later sections. Overall, while SGP is still a developing technology, it has significant potential to contribute to the world’s renewable energy mix and help reduce reliance on fossil fuels.

Keywords

Salinity gradient; renewable energy; pressure retarded osmosis; reverse electrodialysis; water management; energy sustainability

1.1 Fundamentals of the blue energy

1.1.1 Sources of osmotic power

The blue energy is the energy retrieved at the intersection between two different water sources with variable salinity levels and is referred to as the salinity gradient energy. For example, a river that discharges into the sea and comprises freshwater can be a source of energy from the salinity gradient. Two technologies have garnered the most attention and have been recognized to be among the most powerful strategies to use salinity gradients to generate energy. Membranes (Bulasara et al., 2011; Purkait et al., 2005; Singh et al., 2019) are used as a separator in the technologies of reversed electro dialysis (RED) and pressure retarded osmosis (PRO), which allow preferential transport of species. For instance, in PRO, freshwater is converted into extremely concentrated seawater using a semipermeable membrane. Using the pressure surge created at the seawater side to spin the blades of the turbine for generating electricity. With compartments that are alternately filled with freshwater and seawater in the case of the RED, anion and cation exchange membranes (AEMs and CEMs) are used to transport ions. The Gibbs free energy is captured during the mixing of the fresh and the saline water. This form of energy is a sudden and irreversible method that boosts system entropy and has the potential to be a renewable energy source (Sharma, Das, Chakraborty, et al., 2022; Sharma, Das, Sood, et al., 2022). The available Gibbs free energy can be theoretically expressed by the relation (1.1).

Equation (1.1)

where ΔHmix and ΔSmix indicate the energy and entropy change caused by mixing. It is shown how mixing the two separate solutions with various salt concentrations could result in nonexpansion work. Vermaas et al. (2014) examined the energy generated when two solutions of different concentrations are combined and discovered that it is dependent on the difference in concentration between the two systems (Vermaas et al., 2014).

1.1.2 Fundamentals of salinity gradient technology

Saline and fresh water are mixed arbitrarily, and the system quickly finds chemical equilibrium without effectively conserving the released energy. Henceforth, only a suitable technique can be employed to harvest energy from varying salinity. Pattle was the first to delve into great detail about the reverse electrodialysis technique of energy generation utilizing salinity gradients (Pattle, 1954). The feed and draw solutions are separated from each other in PRO by a special thin-film membrane. In this scenario, the shift in salt concentrations initiates the osmotically driven water permeation mechanism from the feed to the draw side of the solution. This is also used to pressurize and drive the hydraulic turbines mechanically to generate power.

1.1.3 Global potential for energy harvesting

The most significant prerequisite for a prosperous future is the utilization of renewable energy sources. There are a range of sources for generating renewable energy, such as wind, geothermal, small-scale hydro, wave, biomass, tidal, solar, and salinity gradient energy. Table 1.1 demonstrates that a significant quantity of osmotic power, such as 20,800 MW² and 2080 MW² from the Amazon and Ganges rivers, may be produced. The Blue Energy or salinity gradient energy is the least used of these renewable energy sources. The driving mechanism behind the salinity gradient energy is the Gibbs free energy gradient (Sharma et al., 2019). It is the power generated when solutions with variable salt concentrations, like seawater and pristine river water, are combined. Of all the marine renewable energy sources, it has the highest energy potential and is entirely renewable and sustainable (Sharma et al., 2018).

Table 1.1

Source: From Sharma, M., Chakraborty, A., Kuttippurath, J., & Yadav, A. K. (2018). Potential power production from salinity gradient at the Hooghly Estuary System. Innovative Energy & Research, 07(03). https://doi.org/10.4172/2576-1463.1000210.

1.2 Principles of blue energy

1.2.1 Pressure retarded osmosis

PRO can generate electricity and desalinate water at the same time. However, extracting energy using different salt-concentrated streams is often favored (Lee et al., 1981). Loeb first suggested the PRO as a promising technique for harnessing energy back in 1973 (Loeb & Mehta, 1978), but it took researchers about 30 years to actually take notice. Since then, multiple studies have been performed to examine the evolution of the process, and numerous initiatives to attain commercialization have been attempted. In 2009 StatKraft, a Norwegian energy business, built a pilot plant to test the salinity gradient energy generation process with freshwater and saltwater. In subsequent developments, the megaton project in Japan (SWRO-PRO) and the global MVP (M for membrane distillation, V for valuable resource recovery, P for PRO) in Korea (MD-PRO) implemented different hybrid techniques along with a variety of desalination methods to decrease the significant energy requirement of processes like membrane distillation (MD) and seawater reverse osmosis (SWRO) (Yip et al., 2016). In fact, Salt Power in Denmark asserts to be the very first osmosis-based commercialized energy generation innovation unit globally (Cobos & Søgaard, 2021).

A semipermeable membrane makes up the PRO system, which enables preferential species transfer. The process’ operation is shown schematically in the illustration of Fig. 1.1. Freshwater can travel through the semipermeable barrier without being trapped. The solute particles are simultaneously rejected in relation to the water permeability when considering the water permeation flow through the membrane.

Figure 1.1 Functioning of pressure retarded osmosis process. From Yip, N. Y., Brogioli, D., Hamelers, H. V. M., & Nijmeijer, K. (2016). Salinity gradients for sustainable energy: Primer, progress, and prospects. Environmental Science and Technology, 50(22), 12072–12094. https://doi.org/10.1021/acs.est.6b03448.

In their investigation, Yip and Elimelech (2014) explained the occurrence of concentration polarization as well as the phenomena of salt permeation in the opposite direction of water flux. Discussions were held regarding the two forms of concentration polarization known as internal concentration polarization (ICP) and external concentration polarization (ECP). ICP is a phenomenon that develops when a concentration lump occurs inside the membrane, while ECP arises on the exterior of the membrane. Osmotic energy generated per unit surface area of the membrane is evaluated as the available power density and is determined as the product of ΔP and Jw, that is, W=Jw ΔP (Achilli et al., 2009). The two main elements that influence the effectiveness of the PRO system are concentration polarization and membrane fouling.

1.2.2 Reverse electrodialysis

A membrane serves as the process’s central component in the energy extraction technique known as RED. Several studies were conducted to enhance the properties of the ion-exchange membranes (Sharma et al., 2021; Sharma et al., 2022) and the other polymeric membranes (Sontakke et al., 2022). In fact, several other techniques such as electrocoagulation (Das, Anweshan, & Mondal, Sinha, et al., 2021; Das, Anweshan, & Purkait, 2021; Das, Mondal, et al., 2021), membrane-based processes (Chakraborty et al., 2023; Das, Mondal, et al., 2023; Dhara, Shekhar Samanta, et al., 2023) and adsorption (Murchana Changmai et al., 2018; Duarah et al., 2022; Taghizadeh et al., 2013) along with bio-based processes (Das, Deepti, et al., 2023; Dhara, Das, Uppaluri, Purkait, Maulin, et al., 2023; Dhara, Das, Uppaluri, Purkait, Sillanpää, et al., 2023; Sharma, Das, Chakraborty, et al., 2023; Shekhar Samanta et al., 2023) were used to treat water/wastewater (Das, Dhara, et al., 2023; Das, Sontakke, et al., 2023; Samanta et al., 2022a; Shekhar Samanta et al., 2023) apart from generating energy using salinity gradients. This also involved the incorporation of a wide range of nanoparticles (Changmai et al., 2022; Samanta et al., 2022b) to enhance the properties of the membrane (Das et al., 2022) and for applications in energy storage (Mukesh Sharma et al., 2023). A case study on extracting salinity gradient energy using the RED process for the Hooghly estuarine region in India was also conducted by Sharma et al. (2018). Anion exchange membrane (AEM) and cation exchange membrane (CEM) are two separate ion-exchange membranes (IEMs) that are alternately positioned between electrodes in this method of extracting renewable energy. AEM and CEM are arranged alternately, and the dilute and concentrate compartments are alternately filled in a basic stack of RED. Spacers are used to keep the membranes’ alternately arranged membranes’ continuous spacing. Because they are so highly selective, the membranes employed in the procedure enable preferential species transfer.

As the CEM and AEM are positioned alternately, the cation and anion migration occur in various directions, resulting in the establishment of an electric potential between both the electrodes and the conversion of salinity gradient energy into electrical energy using an appropriate redox pair. To boost the efficiency of the RED stack, it is necessary to tune the various parameters that determine how the electric potential develops. The recovered total power density in the stack and the power used to pump the solutions are traded off to get the total power density. IEMs are thought of as the system’s beating heart because of their substantial contribution to improving system efficiency. Ion separation and the electrochemical redox reaction are both affected by IEMs and electrodes working together (Nijmeijer & Metz, 2010). The properties of electrodes and IEMs are also influenced by other parameters such as materials, thickness, surface condition, morphology, and structures; as a result, it is essential to use high-quality materials and manufacturing processes. According to Turek et al. (2008) studies, to optimize the process to the commercial scale, effort should be placed on fabricating low-cost, reduced resistance IEMs up to a hundredfold (Turek et al., 2008). To make this type of energy extraction technology commercially viable, problems, including membrane fouling, locational geographic constraints, and electrode solution leakage, must be resolved. Additionally, the elevated cost of the IEMs is one of the main barriers to the power extraction method employing RED; therefore, a significant advance in the development of low-cost, high-performance IEMs is needed to make this source of energy affordable.

1.3 Hybrid processes

1.3.1 Capacitive mixing

CapMix is the newest of the three technologies, having only been first exhibited in 2009 (Brogioli et al., 2011; Brogioli, 2009). As an electrical double layer (EDL) capacitor, CapMix uses a porous electrode pair in the electrolyte in contrast to PRO and RED, which are primarily based on membranes (Brogioli et al., 2011). A simplified illustration of CapMix’s four phases is shown in Fig. 1.2A. In phase I, electrodes that are submerged in hard carbon (HC) solution are charged by an external electric potential (Marino et al., 2015). To maintain local electroneutrality, oppositely charged ions in the electrolyte gather next to the electrode surface (shown by the blue arrows of phase I in Fig. 1.2A). To raise the concentration of ions of the EDL at the electrode–solution interface, energy is used during charging. In phase II, the circuit is opened, and the HC solution—which is partially diluted because of ions trapped in the EDL—is replaced with the lead carbon (LC) solution. Expanding the diffuse layer thickness reduces the EDL capacitance by reducing the ionic strength of the neighboring electrolyte (Brogioli et al., 2011). As a result, even if the capacitive electrode pair’s capacity to hold charge remains unchanged, the cell’s electric potential rises.

Figure 1.2 Capacitive mixing (CapMix) indicating (A) charging, open circuit, and discharging, (B) plot of cell potential against charge. From Yip, N. Y., Brogioli, D., Hamelers, H. V. M., & Nijmeijer, K. (2016). Salinity gradients for sustainable energy: Primer, progress, and prospects. Environmental Science and Technology, 50(22), 12072–12094. https://doi.org/10.1021/acs.est.6b03448.

When the circuit is shut off, there is controlled mixing and ion diffusion from the EDL into the bulk LC solution, which partially raises cyclic lead carbon (CLC). Charges retained at the electrodes are expelled through an external load resistor as a result of the drop in the EDL ion concentration, creating useful work. As the discharge occurs at a bigger potential difference than the charging phase, more energy is produced because phase III’s useful work exceeds phase I’s energy consumption. Phase IV of the cycle involves opening the circuit, draining the LC solution, and replacing it with the HC solution. A representative cell voltage across the electrode pair for each of the four stages is shown in Fig. 1.2B (directional arrows indicate the progression of the controlled mixing). The net energy produced in a CapMix cycle is given by the integral difference between the phase I and phase III cell potentials, spanning the charge transferred, and is indicated by the blue patterned region in Fig. 1.2B. The cyclic process is continued by reusing the HC and LC solutions until concentration equilibrium is achieved, that is, cHC=cLC, to access the whole salinity gradient for additional productive work generation. Thus ion adsorption on porous electrodes in a high-salinity solution and consequent desorption in a low-salinity environment allows for controlled mixing in CapMix (Marino et al., 2015).

1.3.2 Battery mixing

Comparable to CapMix, battery mixing (BattMix), also referred to as mixing entropy batteries, uses faradaic electrodes instead of inert porous electrodes to convert the chemical energy in salinity gradients to electricity (Marino et al., 2015). The cathodic and anodic electrodes for the technique’s initial demonstration were MnO2|Na2Mn5O10 and Ag|AgCl, with NaCl serving as the electrolyte (La Mantia et al., 2011). By applying an external voltage to the appropriate electrodes’ respective Na+ and Cl−, the BattMix cell is charged (i.e., dilute aqueous NaCl). The LC solution is switched out for the HC solution to start the discharge process, and the electrochemical potential causes Na+ ions to facilitate the interaction into the MnO2 cathode while Ag(s) is oxidized to Ag+ ions at the anode. As the energy produced when the cell empties into an external load is greater than the energy used in charging, excess power is produced from the controlled blending of the salinity gradient. In BattMix and CapMix, the redox electrodes and capacitive electrodes function as electrical accumulators that store and discharge charges. As a result, the technologies for capturing salinity gradient energy known as accumulator mixing (AccMix) comprise BattMix and CapMix (Marino et al., 2015; Yip et al., 2016). BattMix is a relatively new technology compared to PRO, RED, and CapMix, but it is included in our assessment as it has demonstrated significant promise and is gaining research attention.

1.3.3 Other salinity gradient technologies

Other methods for harnessing the energy of blending from salinity gradients involve mechanochemical contraction turbines using replenished collagen fibers, vapor pressure difference turbines, hydro-voltaic cells, forward osmosis-electrokinetic power generation, osmotic power generation using dialysis cassettes, osmotic power generation using hydrogels, controlled mixing through ion-selective nanopores, nanochannels, (Buonomenna, 2022). However, either there is not enough information on these subjects to make an accurate assessment, or published studies only produced a small amount of power, which made it difficult to support the procedures’ potential feasibility. Therefore this book does not include these other technologies.

1.4 Progress and prospects in theoretical to field scale setup

The world’s first salinity gradient energy pilot plant, which opened in November 2009, is built on PRO (Kumaravel & Abdel-Wahab, 2018), demonstrating the method’s technological and economic development compared to RED and CapMix at the time. The prototype PRO system at Tofte, Norway, originally aimed to produce 10 kW of energy and supply fresh water from a lake and seawater by gravity (Kumaravel & Abdel-Wahab, 2018). For the seawater–river water salinity gradient power to be commercially appealing, Statkraft, the international renewable energy company with its headquarters in Norway and operating the pilot plant, states that the power density of the PRO membranes required to be 4–5 W/m² and estimates the cost of energy generation at 0.05–0.10 €/kWh (Bui et al., 2021). The claimed power densities were 33.7 W/m² (Bui et al., 2021), and the business declared in 2011 that it will collaborate with the producer of membranes Nitto Denko/Hydranautics to create and provide PRO membranes competent of achieving 5 W/m² (Wang et al., 2019). To add the power generated to the national grid, Statkraft and the Israeli water business Israeli desalination Technologies agreed to design and build a larger 2 MW PRO plant in Sunndalsra, Norway, in 2013. This was done in 2013.

In RED, two types of IEMs are used to create a stack of alternating AEMs and CEMs. Saltwater is then flowed across the stack, with fresh water flowing on one side of the stack and concentrated brine on the other. The membranes allow the selective passage of ions, creating a voltage potential that can be harnessed to generate electricity. In PRO, a similar stack of membranes is used, but instead of saltwater, fresh water and saltwater are separated by the membranes. The freshwater side is put under high pressure, which drives it through the membranes and into the saltwater side, where it mixes with the saltwater and reduces the osmotic pressure. The energy released by this mixing can be harnessed to generate electricity.

Both RED and PRO require specialized IEMs that are selective for specific ions, such as sodium or chloride ions. These membranes are typically made from polymers like sulfonated polysulfone or perfluorinated sulfonic acid membranes, which have good ion selectivity and high stability in saltwater environments. As illustrated in Table 1.2 and Table 1.3, several membranes were developed and tested for extracting energy using the RED