Mavi Enerji

Energy has been the cornerstone of human civilization's development and progress over the centuries. Energy is the key to power, one of the most important issues of the future, and has profound implications for humanity's socio-economic-political sphere, becoming a determining factor in international affairs. The rising global population and resulting industrialization have significantly increased energy demand. While global primary energy consumption in 1800 was 5,652.78 terawatt-hours (TWh), by 2018 it had reached 157,063.77 TWh. The majority of primary energy consumed by the planet in 2018 was obtained by burning non-renewable and rapidly depleting fossil fuels [1]. Given the depletion of fossil fuel reserves, the increasing difficulty of accessing and processing them, and their negative environmental impacts, renewable energy sources have become increasingly important. As technology advances, research and development into methods for utilizing renewable energy sources—which rely on energy circulation in existing processes—is growing alongside traditional energy production methods that depend on dwindling raw materials and cause problems such as global climate change and environmental pollution. One of these methods, Reverse Electrodialysis (RED), traces its history to the membrane accumulator system implemented by Manecke [2] in 1952. This work is thought to have inspired the first theoretical approaches that contributed to the idea of power generation using salinity gradient differences in subsequent periods. Salinity gradient energy is the energy obtained from the difference in salt concentration between freshwater and saltwater. The first work recognized as the initial strategy proposed and tested for RED was conducted by Pattle [3] two years after Manecke's work. As freshwater and saltwater pass through a membrane module called a "hydroelectric battery," an electromotive force is generated as a result of selective ion separation by anion exchange membranes (AEM) and cation exchange membranes (CEM) connected in series [3]. Figure 1 presents a flow diagram of a typical RED system [4]. A typical RED system contains pumps, electrode solution, dilute and concentrated solutions, electrodes, spacers, seals, and ion exchange membranes. The pumps continuously recirculate the electrode solution along with the concentrated and dilute solutions. Seals are used to prevent liquid solutions from leaking, while spacers allow ions to move freely. RED is a system in which energy generation occurs through an ion transport mechanism via ion exchange membranes [5], [6]. Ion transport takes place through sequentially arranged cation and anion exchange membranes. Positively charged ions pass through the CEM while negatively charged ions pass through the AEM in the reverse direction, creating positive and negative potential differences at the two ends respectively [7]. The ionic current generated in the RED system is converted to electric current through redox reactions occurring at the electrodes located at both ends (Figure 1) [8]. [caption id="attachment_143106" align="aligncenter"] Figure 1. Schematic representation of a typical RED system [4][/caption]The fundamental logic of energy generation and potential in the RED system actually rests on the first law of thermodynamics. This law is also known as the "law of energy conservation." Energy cannot be created from nothing; existing energy cannot be destroyed; it only transforms from one form to another. In other words, the energy we theoretically expend to obtain freshwater from saltwater (e.g., seawater) should be recoverable when freshwater (e.g., river water) mixes with saltwater (e.g., seawater) [9]. Although energy generation in the RED system depends on the salt concentration of these waters, theoretically 2.5 MJ of energy is expected from mixing 1 m3 of seawater with 1 m3 of freshwater. This is equivalent to the energy produced by a dam with a water height of approximately 250 meters [10], [11]. Salinity gradient energy, also known as blue energy, has enormous potential of approximately 2.8 terawatts (TW) globally [12]. The worldwide RED potential currently has production capacity nearly equivalent to the electrical energy consumed globally today. RED is a vast renewable energy source with no negative environmental impact, non-polluting and carbon-free. It directly relates to the planet's dynamic water cycle and depends on the energy generated when solutions with two different salt concentrations interact [12]–[14]. There are currently several reasons why this potential is not being utilized. The primary reason is that RED systems typically use membranes prepared for electrodialysis devices, and these membranes are thought to have negative effects on salinity gradient and energy production [15]. Membrane thickness, RED system design, the distance created between membranes, and power density are also important factors affecting system performance [16]–[18]. One of the findings from Pattle's work in the 19th century was that at lower temperatures, membrane internal resistance is higher and power output is lower. In other words, RED systems are expected to be more suitable in climates where seasonal temperature differences are not significant and water temperature is relatively higher [3]. When evaluating the potential of the RED system, one should not limit freshwater sources to rivers and saltwater sources to seas and oceans alone. Although the flow of rivers into seas, oceans, or similar water sources represents a significant potential, treated wastewater can be used as the freshwater source in this system, while concentrated solutions from desalination facilities can be used as the saltwater source. Recently, interest in these alternative feed water trials has also increased. This is also quite important in terms of the reusability of treated wastewater. Today, treatment plants in many coastal cities discharge treated wastewater into the sea. Istanbul is one of the best examples of this situation. With a population exceeding 15 million, Istanbul is one of the world's most densely populated metropolises. According to the activity report published by İSKİ (Istanbul Water and Sewerage Administration) in 2021, the annual amount of wastewater treated at wastewater treatment plants in Istanbul is 1,498,276,404 m3, with 608,660,129 m3 of this wastewater being treated biologically and advanced biologically. The same report indicates that the annual electricity consumption at wastewater treatment plants is 380,068,959 kWh, while the annual electricity generation at these plants is 146,769,893 kWh (URL-1). Even the data from Istanbul alone demonstrates that a significant and clean energy source is being wasted [19], and when considered globally, we realize we are not utilizing an enormous energy source. An important factor here is the proper treatment of wastewater to high quality. For this reason, biological and advanced biological treatments increase the effectiveness of these processes while minimizing environmental harm. The number of laboratory and pilot-scale studies on RED systems in the literature is increasing daily. These studies generally focus on new module and membrane designs, process optimization, and minimizing the challenges the process will face. The first pilot-scale study was conducted in Italy in 2014 with a system containing 125 membrane pairs. Using seawater and saturated brine samples, a maximum power density of 1.3 W/m2 was achieved [20], [21]. In 2014, the world's first RED power plant was established in the Netherlands through a partnership between Wetsus, Fujifilm, and RedStack. At this facility, which uses custom-made membranes, 50 kW of electricity was generated using IJsselmeer water (freshwater) near Afsluitdijk, the Netherlands' largest dam, and Wadden Sea water (saltwater). Researchers noted that this power plant has the potential to generate energy capable of meeting the energy needs of 500,000 households (URL-2). Studies are being conducted not only for electricity generation at RED facilities but also on their direct application as fuel cell-based power sources. The most significant of these is a pilot-scale facility in Fukuoka, Japan, where hydrogen production using RED was achieved, with high-efficiency hydrogen generation realized by utilizing the salinity gradient difference between seawater and treated wastewater. Thus, it was demonstrated that the salinity gradient difference can be directly used for efficient gas fuel generation for fuel cell-based power sources. The system combining electricity generation and hydrogen generation with RED is called RED-H2. One of the promising solutions proposed for constraints such as the irregular nature of energy produced from renewable sources and the inability to store excess electricity production capacity is "Power to Gas (P2G)" technology. With P2G technology, electrical energy is converted into gases such as hydrogen or methane, stored, and used when needed [22]–[24]. Demonstrating the applicability of reverse electrodialysis technology for hydrogen production through the RED-H2 system is quite exciting in this regard. Turkey's geographical location makes it possible to conduct full-scale studies. The Black Sea is located to Turkey's north, the Sea of Marmara to its northwest, the Aegean Sea to its west, and the Mediterranean to its south. Additionally, some lakes in Turkey on land, such as Lake Tuz and Lake Burdur, have water as saline as seawater. Among the world's first 20 river mouths with the highest energy density (energy potential per cubic meter of freshwater), the Great Menderes ranks first and the Ceyhan River ranks third. However, in addition to high energy density, there must be a stable/balanced salinity gradient difference and high recoverable energy value [25]. It should not be forgotten that obtaining the desired amount of energy from the system depends on multiple factors, such as the provision of appropriate operating conditions and system efficiency. The Mediterranean stands out because of the high salinity of seawater and high average seawater temperatures that would positively affect RED system performance. Turkey is considered to have an important and advantageous geographical location for the application of RED technology. RED technology can be applied at industrial scale at locations where river basins in Turkey discharge into the sea using concentrated salt solutions, or at wastewater treatment plant discharge points to the sea, and has promising potential in terms of increasing the use of local resources in energy generation. However, regional-scale studies reflecting different environmental conditions are important for determining energy generation potential. In conclusion, salinity gradient energy is clean energy produced through chemical energy resulting from the mixing of two solutions with different salinity levels. Utilization of this renewable energy potential can also contribute to bridging the country's energy deficit. RED has significant potential for achieving Turkey's goal of meeting its energy needs through socially, economically, and environmentally appropriate methods.       References [1] A. Zoungrana and M. Çakmakci, "From non-renewable energy to renewable by harvesting salinity gradient power by reverse electrodialysis: A review", International Journal of Energy Research, v. 45, no. 3. 2021. [2] G. Manecke, "Membranakkumulator", Zeitschrift für Phys. Chemie, v. 201, no. 1, pp. 1–15, 1952. [3] R. E. Pattle, "Production of Electric Power by mixing Fresh and Salt Water in the Hydro-electric", Nature, v. 174, p. 660, 1954. [4] R. Ortiz-Imedio, L. Gomez-Coma, M. Fallanza, A. Ortiz, R. Ibañez, and I. Ortiz, "Comparative performance of Salinity Gradient Power-Reverse Electrodialysis under different operating conditions", Desalination, v. 457, pp. 8–21, 2019. [5] D. Kim, K. Kwon, D. H. Kim, and L. Li, Energy Generation Using Reverse Electrodialysis: Principles, Implementation, and Applications. 2019. [6] D. A. Vermaas, Energy generation from mixing salt water and fresh water. 2014. [7] D. A. Vermaas, E. Guler, M. Saakes, and K. Nijmeijer, "Theoretical power density from salinity gradients using reverse electrodialysis", Energy Procedia, v. 20, pp. 170–184, 2012. [8] J. Veerman, M. Saakes, S. J. Metz, and G. J. Harmsen, "Reverse electrodialysis: A validated process model for design and optimization", Chem. Eng. J., v. 166, no. 1, pp. 256–268, 2011. [9] J. N. Weinstein and F. B. Leitz, "Electric power from differences in salinity: The dialytic battery", Science (80-. )., v. 191, no. 4227, pp.557–559, 1976. [10] J. W. Post et al., "Salinity-gradient power: Evaluation of pressure-retarded osmosis and reverse electrodialysis", J. Memb. Sci., v. 288, no. 1–2, pp. 218–230, 2007. [11] K. Nijmeijer and S. Metz, "Chapter 5 Salinity Gradient Energy", Sustainability Science and Engineering, v. 2, no. C. 2010. [12] J. Veerman, "Reverse Electrodialysis: design and optimization by modeling and experimentation", 2010. [13] S. Pawlowski, J. Crespo, and S. Velizarov, "Sustainable power generation from salinity gradient energy by reverse electrodialysis", in Electrokinetics Across Disciplines and Continents: New Strategies for Sustainable Development, 2015. [14] J. G. Hong, W. Zhang, J. Luo, and Y. Chen, "Modeling of power generation from the mixing of simulated saline and freshwater with a reverse electrodialysis system: The effect of monovalent and multivalent ions", Appl. Energy, v. 110, pp. 244–251, 2013. [15] A. H. Avci et al., "Sulfonated polyethersulfone based cation exchange membranes for reverse electrodialysis under high salinity gradients", J. Memb. Sci., v. 595, p. 117585, 2020. [16] D. A. Vermaas, M. Saakes, and K. Nijmeijer, "Doubled power density from salinity gradients at reduced intermembrane distance", Environ. Sci. Technol., v. 45, no. 16, pp. 7089–7095, 2011. [17] N. Y. Yip, D. A. Vermaas, K. Nijmeijer, and M. Elimelech, "Thermodynamic, energy efficiency, and power density analysis of reverse electrodialysis power generation with natural salinity gradients", Environ. Sci. Technol., v. 48, no. 9, pp. 4925–4936, 2014. [18] H. K. Kim, M. S. Lee, S. Y. Lee, Y. W. Choi, N. J. Jeong, and C. S. Kim, "High power density of reverse electrodialysis with pore-filling ion exchange membranes and a high-open-area spacer", J. Mater. Chem. A, v. 3, no. 31, pp. 16302–16306, 2015. [19] A. Zoungrana, O. K. Türk, and M. Çakmakci, "Energy coverage of Ataköy-Ambarlı municipal wastewater treatment plants by salinity gradient power", J. Water Process Eng., v. 38, no. July, p. 101552, 2020. [20] H. Tian, Y. Wang, Y. Pei, and J. C. Crittenden, "Unique applications and improvements of reverse electrodialysis: A review and outlook", Applied Energy, v. 262. 2020. [21] M. Tedesco, C. Scalici, D. Vaccari, A. Cipollina, A. Tamburini, and G. Micale, "Performance of the first reverse electrodialysis pilot plant for power production from saline waters and concentrated brines", J. Memb. Sci., v. 500, pp. 33–45, 2016. [22] M. Higa et al., "Sustainable hydrogen production from seawater and sewage treated water using reverse electrodialysis technology", Water Pract. Technol., v. 14, no. 3, pp. 645–651, 2019. [23] M. Götz et al., "Renewable Power-to-Gas: A technological and economic review", Renew. Energy, v. 85, pp. 1371–1390, 2016. [24] G. Gahleitner, "Hydrogen from renewable electricity: An international review of power-to-gas pilot plants for stationary applications", Int. J. Hydrogen Energy, v. 38, no. 5, pp. 2039–2061, 2013. [25] O. A. Alvarez-Silva, A. F. Osorio, and C. Winter, "Practical global salinity gradient energy potential", Renew. Sustain. Energy Rev., v. 60, pp. 1387–1395, 2016. URL-1 İSKİ (Istanbul Water and Sewerage Administration), "2021 Activity Report", https://www.iski.gov.tr/web/assets/video/Genel%20Kurul%20Konuşması/2021%20FAALİYET%20RAPORU.pdf URL-2 Dutch Water Sector, "Dutch King Opens World's First RED Power Plant Driven on Fresh-Salt Water Mixing", https://www.dutchwatersector.com/news/dutch-king-opens-worlds-first-red-powerplant- driven-on-fresh-salt-water-mixing (2014)     Prof. Dr. Mehmet Çakmakci Yıldız Teknik Üniversitesi Faculty of Civil Engineering Department of Environmental Engineering   Dr. Ali Zoungrana University of Manitoba Faculty of Civil Engineering Department of Environmental Engineering   Research Assistant Oruç Kaan Türk Yıldız Teknik Üniversitesi Faculty of Civil Engineering Department of Environmental Engineering