Energy Recovery from Solutions with Different Salinities Based on Swelling and Shrinking of Hydrogels

Abstract

Several technologies, including pressure-retarded osmosis (PRO), reverse electrodialysis (RED), and capacitive mixing (CapMix), are being developed to recover energy from salinity gradients. Here, we present a new approach to capture salinity gradient energy based on the expansion and contraction properties of poly(acrylic acid) hydrogels. These materials swell in fresh water and shrink in salt water, and thus the expansion can be used to capture energy through mechanical processes. In tests with 0.36 g of hydrogel particles 300 to 600 μm in diameter, 124 mJ of energy was recovered in 1 h (salinity ratio of 100, external load of 210 g, water flow rate of 1 mL/min). Although these energy recovery rates were relatively lower than those typically obtained using PRO, RED, or CapMix, the costs of hydrogels are much lower than those of membranes used in PRO and RED. In addition, fouling might be more easily controlled as the particles can be easily removed from the reactor for cleaning. Further development of the technology and testing of a wider range of conditions should lead to improved energy recoveries and performance. ■ INTRODUCTIONSalinity gradients that naturally exist between seawater and river water could provide a large and renewable resource for clean energy production. The theoretical energy of mixing 1 m 3 of river water with a much larger volume of seawater is about 2.5 MJ, which is equivalent to the energy that could be captured from water flowing over a dam more than 250 m in height. 1,2 Worldwide, the potential power production from salinity gradients is estimated to be 1.4−2.6 TW, which is comparable to the current global demand for electrical power (∼2 TW). 3−5Several technologies have been developed to capture salinitygradient energy, including pressure-retarded osmosis (PRO), 6−8 reverse electrodialysis (RED), 9−11 and capacitive mixing (CapMix).12−14 In PRO, water from a low salinity solution (river water) permeates into the highly saline solution (seawater) across a semipermeable membrane, driven by the osmotic pressure difference. This water flow pressurizes the seawater, which can then be used to generate electricity using a hydroturbine.6,15 A RED process is based on using a stack of alternating cation (CEM) and anion exchange membranes (AEM). When waters with different salinities flow through channels separated by CEMs and AEMs, a voltage of ∼0.1 to 0.2 V is generated across each membrane pair due to the ion flux driven by the differences in salt concentrations. This ionic flux is then converted into electrical current through oxidation− reduction reactions at the electrodes. 9,16 The main disadvantage of PRO and RED is that they require use of large surface areas of expensive membranes that foul over time and that can be difficult to effectively clean. CapMix is a relatively new approach to capture energy from solutions with different salinities that does not necessarily require membranes. In this process, seawater and river water alternately are exposed to either plain capacitiv...

Influence of solution concentration and salt types on the performance of reverse electrodialysis cells

“…Energy input to the system (X in , in W) changes with the HC and LC solutions concentration, which was determined from the change in the free energy due to complete mixing of the HC and LC solutions as [12,33]: Energy recovery (E r ) was calculated as the ratio of electric power output of the stack relative to the energy input to the system [9,34]. The electric power output was obtained based on the measured maximum power density of stack and the total membrane area.…”

“…Several technologies have been proposed to capture salinity gradient energy, including pressure-retarded osmosis (PRO) [8], reverse electrodialysis (RED) [9], capacitive mixing (CapMix) [10,11], and hydrogel expansion (HEx) [12]. Each technology uses a uniquely different approach for energy conversion, but one main advantage of RED is that it can be used for continuous and direct electrical current generation from a single reactor.…”

Reducing pumping energy by using different flow rates of high and low concentration solutions in reverse electrodialysis cells

“…Energy recovery (E r ) is a suitable parameter to account for it, which was calculated as the ratio of electrical power output of the stack (P ele , in W) relative to the total energy provided to the system per second (X in , in J/s). The energy input per second X in is estimated from the change in the free energy due to complete mixing of the HC and LC solutions as [26,27] …”

Integrating Reverse‐Electrodialysis Stacks with Flow Batteries for Improved Energy Recovery from Salinity Gradients and Energy Storage

“…The global extractable energy from suitable river mouths is estimated to be 625 TWh per year, which is equivalent to 3 % of the global electricity consumption . Several salinity gradient energy (SGE) technologies have been proposed to capture this energy, including pressure‐retarded osmosis (PRO), reverse electrodialysis (RED), capacitive mixing (CapMix), and hydrogel expansion (HEx) . Among these SGE technologies, RED has the advantage of continuously converting energy into electricity.…”