Salinity Gradient Energy Cheatsheet

Cheatsheet Content

Salinity Gradient Energy (SGE) SGE harnesses the chemical potential energy released when two solutions of different salinities mix, such as freshwater and seawater. This energy can be converted into electricity. 1. Pressure Retarded Osmosis (PRO) Principle: Freshwater flows through a semi-permeable membrane into seawater, increasing pressure on the seawater side. This pressure drives a turbine to generate electricity. Process: Freshwater reservoir ($C_1$) and Seawater reservoir ($C_2$, $C_2 > C_1$). Semi-permeable membrane separates the two. Water flows from freshwater to seawater side due to osmotic pressure ($\Delta\Pi$). Increased volume/pressure on seawater side drives a hydro-turbine. Osmotic Pressure Formula: $\Delta\Pi = i \cdot R \cdot T \cdot \Delta C$, where $i$ is van 't Hoff factor, $R$ is gas constant, $T$ is temperature, $\Delta C$ is concentration difference. Pros: High energy potential; relatively mature technology; can be integrated with desalination brine. Cons: Requires durable membranes; fouling issues; membrane degradation; high capital cost. 2. Reverse Electrodialysis (RED) Principle: Uses alternating cation and anion exchange membranes to create an electric potential from ion movement between saltwater and freshwater. Process: Stacks of alternating Cation Exchange Membranes (CEM) and Anion Exchange Membranes (AEM). Seawater and freshwater flow through alternating compartments. Cations ($Na^+$) pass through CEMs, anions ($Cl^-$) pass through AEMs. Ion movement generates an electric current between electrodes at the ends of the stack. Voltage Generation: Each membrane pair generates a small voltage (e.g., $0.1-0.2V$). Total voltage is sum across all pairs. Pros: Direct electricity generation (DC); scalable; no moving parts; lower operating pressure than PRO. Cons: Membrane cost and efficiency challenges; fouling; pH sensitivity; limited power density. 3. Capacitive Mixing (CapMix) Principle: Exploits changes in electrode capacitance when exposed alternately to freshwater and saltwater, generating energy during charge/discharge cycles. Types: Capacitive Energy Extraction (CNE): Based on double layer capacitance. Capacitive Mixing (CapMix) with Ion-Exchange Membranes (CCM): Uses membranes to enhance ion selectivity. Process (CNE): Electrodes charged in freshwater (low capacitance). Electrodes discharged in saltwater (high capacitance), releasing more energy. Cycle repeats. Energy Output: $E = \frac{1}{2} C V^2$, where $C$ is capacitance, $V$ is voltage. Energy gain comes from $C_{salt} > C_{fresh}$. Pros: No moving parts; potentially low maintenance; environmentally friendly. Cons: Still in research phase; low power density; electrode degradation; complex control. 4. Vapor Pressure Difference Method Principle: Uses osmotic pressure differences to create vapor pressure gradients, which can drive turbines or other energy conversion systems. Mechanism: Salinity difference creates a vapor pressure difference across a membrane or interface. This drives water evaporation from the low-salinity side and condensation on the high-salinity side, or drives a gas flow. Pros: Can integrate with thermal processes (waste heat utilization); potentially less membrane fouling issues compared to PRO/RED. Cons: Complex system design; generally less efficient than PRO or RED for direct electricity generation; requires careful heat management. 5. Membrane Distillation with Osmotic Gradient Principle: Combines thermal and osmotic processes. Water vapor passes through a hydrophobic membrane, driven by a partial pressure difference, which is further enhanced or utilized by an osmotic gradient. Process: A hot saline feed is on one side of a hydrophobic membrane. A cold draw solution (e.g., concentrated brine) or permeate is on the other side. Vapor pressure difference drives water vapor across the membrane. The osmotic gradient can assist in drawing water or creating a pressure for energy conversion. Pros: Hybrid approach for both desalination and energy; useful for treating highly saline solutions; can utilize low-grade waste heat. Cons: High thermal energy requirement (though often from waste heat); membrane wetting issues; complex system integration.