| 摘要: | 為因應2050淨零排放政策，我國正積極尋找可替代化石燃料的能源。鹽差能(Salinity gradient energy, SGE)是目前我國可投入研發的一種能源，該能源產生主要來自兩相異鹽度水體在進行混合過程中，自由能發生改變，導致一些自由能以滲透壓的形式被放出，而這些能量再轉變為電能。反向電透析(Reverse electrodialysis, RED)即是SGE的其中一種應用，透過交互擺放陰陽離子交換膜後通入濃水和淡水藉此產生膜電位，後以鐵氰化鉀/亞鐵氰化鉀(K3[Fe(CN)6]/ K4[Fe(CN)6])進行氧化還原反應產生電流，但K3[Fe(CN)6]本身屬於易光解性物質，受光照會產生危害環境的氰化物。故考慮到環境安全層面，RED的發展被受限於此。
電容反向電透析(Capacitive reverse electrodialysis, CRED)是衍生自RED的發電技術，將RED的電極板替換成高比表面積之活性碳電極，僅靠活性碳電極吸附之電荷進行電荷之傳輸以達到產生電流之目的，該裝置消除RED系統需要使用K3[Fe(CN)6]的缺點，使RED的發展可能性更邁進一步。
海淡廠和石化製程工廠每天都能產出大量的高鹽度鹵水和製程冷卻水，通常這些廢水都經由處理後排放，並無任何實質用途。若一間石化工廠的CRED處理冷卻水之負荷為0.208 m3/(m2-h)，今將此兩種廢水用於CRED發電，理論每年可產出約737萬度的電力，此度數的產電量以2024年之電力排放係數來看，每年可減少約3,490噸的二氧化碳排放。本研究將模擬海淡廠和石化製程工廠產出之廢水鹽度，固定濃淡水的鹽度為58 g NaCl/L及1 g NaCl/L進行CRED系統之參數研究，包括流速、膜對數、電阻和濃淡水切換間隔，同時對本研究之活性碳電極進行循環伏安法(Cyclic voltammetry, CV)、交流阻抗(Electrochemical impedance spectroscopy, EIS)和恆電位充放電法(Galvanostatic Charge-Discharge, GCD)分析活性碳電極特性，最後以傅立葉轉換紅外光譜法(Fourier transform infrared spectroscopy, FTIR)分析活性碳內含之官能基。CRED參數研究實驗得到於10對膜連接2歐姆電阻，流速為4 cm/s (進流流量1,152 mL/min)，切換間隔定為7分鐘的情況下得到最高功率密度為0.697 W/m2。最終使用海淡鹵水作為濃水、自來水作為淡水並用最佳參數進行CRED實驗，得到功率密度2.26 W/m2，相比使用DI水調配的濃淡水提高200%。;In response to the 2050 net-zero emissions policy, Taiwan is actively exploring alternative energy sources to replace fossil fuels. Salinity gradient energy (SGE) is one such renewable energy source currently under investigation. SGE is generated from the change in Gibbs free energy that occurs when two aqueous solutions of differing salinity are mixed. This energy is released in the form of osmotic pressure and can subsequently be converted into electrical energy.
Reverse electrodialysis (RED) is one of the most prominent technologies for harnessing SGE. It operates by alternately stacking cation and anion exchange membranes between two solutions of seawater (high salinity) and freshwater (low salinity), thereby generating a membrane potential. This potential is typically converted into electrical current via redox reactions using K₃[Fe(CN)₆]/K₄[Fe(CN)₆] as the redox couple. However, K₃[Fe(CN)₆] is photosensitive and can degrade into environmentally hazardous cyanide under light exposure, posing significant safety and environmental concerns. Consequently, the widespread development and deployment of RED systems are limited by the use of this redox electrolyte.
Capacitive reverse electrodialysis (CRED), an emerging technology derived from RED, addresses this limitation by replacing the redox electrodes with high specific surface area activated carbon electrodes. The CRED system generates electricity through the physical adsorption and transport of ions on the carbon electrodes, thereby eliminating the need for K₃[Fe(CN)₆] and enhancing the safety and feasibility of RED-based energy generation.
Desalination plants and petrochemical processing facilities discharge large volumes of high-salinity brine and process cooling water daily. These waste streams are typically treated and discharged without further utilization. If these saline effluents were instead employed in a CRED system, it is theoretically possible to generate approximately 737 MWh of electricity per year. Based on Taiwan’s 2024 electricity emission factor, this corresponds to an annual reduction of approximately 3,490 tons of CO₂ emissions for one factory. In this study, we simulate the waste stream concentrations of desalination and petrochemical plants by fixing the feedwater salinity at 58 g NaCl/L for the concentrated stream and 1 g NaCl/L for the dilute stream. Various CRED system parameters were investigated, including flow rate, membrane pair number, external resistance, and salinity switching interval. The electrochemical properties of the activated carbon electrodes were characterized using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge (GCD) analysis. Additionally, Fourier-transform infrared spectroscopy (FTIR) was employed to identify the surface functional groups of the activated carbon. The highest power density of 0.697 W/m² was achieved under the conditions of 10 membrane pairs, 2 Ω external resistor, 4 cm/s flow velocity, and a switching interval of 7 minutes. In the final experiment, brine from seawater desalination was used as the high-concentration solution and tap water as the low-concentration solution. Under optimized operating parameters, the RED system achieved a power density of 2.26 W/m², representing a 200% increase compared to using DI water. |
