Energy Recovery from Solutions with Different Salinities Based on

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Energy Recovery from Solutions with Different Salinities Based on Swelling and Shrinking of Hydrogels Xiuping Zhu, Wulin Yang, Marta C. Hatzell, and Bruce E. Logan* Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

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.

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INTRODUCTION Salinity 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 m3 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).3−5 Several 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 © 2014 American Chemical Society

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 capacitive electrodes12,14 or porous electrodes coated with ion exchange polymers.13,17 Energy is extracted from cycles of electrical current flow that occur along with changes in cell voltages produced using solutions with different salt concentrations. To date, the energy recovered by several different types of CapMix approaches is quite low,12,13,18 with the best results obtained in systems that use either ion exchange membranes or polymer coatings on the electrodes, which are also subject to fouling. Higher power has recently been obtained with CapMix, but only when it was used in conjunction with a bioelectrochemical system (BES), such as a microbial fuel cell.19 Here we describe a new approach to capture energy from two solutions with two different salinities based on extracting work done during the swelling and shrinking of poly(acrylic acid) hydrogel particles through alternating exposure to solutions with high and low salt concentrations. A related idea of energy capture through contraction of a material was proposed some time ago by Sussman and Katchalsky using Received: Revised: Accepted: Published: 7157

February 21, 2014 May 23, 2014 May 27, 2014 May 27, 2014 dx.doi.org/10.1021/es500909q | Environ. Sci. Technol. 2014, 48, 7157−7163

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Figure 1. (A) Scheme of recovering energy from salinity gradient using poly(acrylic acid) hydrogels and (B) photo of used reactor.

collagen fibers to turn a shaft in a concentrated LiBr solution.20 The process examined here is somewhat different, as it is based on the work done by expansion (swelling) rather than contraction, in a piston-type process (lifting weights), with very different materials (hydrogels in low concentration NaCl solutions). Hydrogels are three-dimensional networks of polymer chains that can entrap large volumes of water (over 99.9%) due to the high concentration of hydrophilic groups.21−23 Hydrogels can easily be synthesized by inverse suspension polymerization or free-radical polymerization,21,22 and their cost is relatively low as they are extensively used as water-retaining agents for plant growth, disposable diapers, tampons, and sanitary napkins.23−25 In the presence of fresh water, hydrogels will take up water due to the osmotic pressure and hydrogen bonding of the carboxylic groups of the polymer with water molecules.24,25 Poly(acrylic acid) hydrogel is a special kind of superabsorbent polymer that can absorb water to several hundred times its own mass, creating a swelling pressure ranging from 0.20−4.23 MPa.23,26 When an external load less than swelling pressure is applied, hydrogels can do work against a load due to its expanding volume. The other attribute of these polymers that make them useful here is that swollen hydrogels can be dehydrated by immersing them in salt water, due to both the change in osmotic pressure and charge neutralization of the polymer chains.25,27 These properties of the poly(acrylic acid) hydrogels therefore make it possible to create a cyclical process whereby energy can be extracted through swelling and shrinking of hydrogels when seawater and river water alternatively flow through the hydrogels (Figure 1). The feasibility of this new approach was examined by varying hydrogel particle diameters, the mass of the gel used, solution salinity ratios, solution flow rates, and the weights of the external load.

a glass bottle immersed in ice water. Then, 2% cross-linker and 0.1% initiator were added based on the moles of acrylic acid, and water was added to keep the cross-linker concentration at 0.02 M. The mixed solution was sparged with N2 gas and sealed using a stopper. Then, the sealed glass bottle was put into a 70 °C oven for reaction. After 4 h, the formed hydrogel was washed using DI water several times, cut into small pieces, and dried at 70 °C in open air until a constant weight was reached. Finally, the dried hydrogel was crushed into small particles and sieved to recover size fractions of 300−600, 600−850, and 850−1180 μm. Reactor Configuration and Operation. A modified glass syringe filled with hydrogels was used to examine energy recovery from solutions with high and low salt concentrations (Figure 1). The glass tube had a diameter of 3 cm and a volume of 70 mL. Dried hydrogels with different weights of 0.12, 0.24, or 0.36 g, were added into the tube. A plastic plunger was inserted into the tube with holes drilled through the bottom to let solution flow out. The high concentration (HC, 35 g/L NaCl, 54 mS/cm) or low concentration (LC, varied to produce salinity ratios of 50, 100, 200, or infinite with distilled water) solutions were pumped into the bottom of the reactor and pumped out from the top through the holes at the bottom of the plunger at flow rates of 1, 2, or 5 mL/min. In order to recover energy, different weights were added to the plunger (total external loads of 50, 110, 210, or 310 g). Calculations. The recovered energy, E (J), was calculated based on the work done against the external load, W1 (J), and the work done for upward movement of the gravity center of the hydrogels, W2 (J),28 as follows:

MATERIALS AND METHODS Synthesis of Poly(acrylic acid) Hydrogel. The polymer hydrogels were synthesized by free-radical polymerization of monomer acrylic acid (Sigma-Aldrich, 99%) and cross-linker N,N′-(1,2-dihydroxyethylene)bis(acrylamide) (Sigma-Aldrich, 97%), using the initiator potassium persulfate (Sigma-Aldrich, ≥99%), in an aqueous solution at 70 °C for 4 h.21,22 Acrylic acid was first neutralized by 60% with 11 M NaOH solution in

where m1 (kg) is the external load, g0 (9.81 m/s2) the gravitational constant, h1 (m) the linear movement of the plunger calculated based on the expanding volume and the diameter of the tube, m2 (kg) the average weight of the swollen and deswollen hydrogels calculated according to their average volumes and an assumption of hydrogel density in water of 1000 kg/m3, and h2 = h1/2 (m) the displacement of the center of gravity of the hydrogels.

E = W1 + W2 = m1g0h1 + m2g0h2

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7158

(1)

dx.doi.org/10.1021/es500909q | Environ. Sci. Technol. 2014, 48, 7157−7163

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Figure 2. Volume change of hydrogels over time in one cycle with different: (A) particle sizes (hydrogel mass 0.12 g, infinite salinity ratio, external load 50 g, flow rate 5 mL/min); (B) salinity ratios (particle size 300−600 μm, hydrogel mass 0.12 g, external load 50 g, flow rate 5 mL/min); (C) hydrogel masses (particle size 300−600 μm, salinity ratio 100, external load 50 g, flow rate 5 mL/min); (D) external loads (particle size 300−600 μm, hydrogel mass 0.36 g, salinity ratio 100, flow rate 5 mL/min); and (E) solution flow rates (particle size 300−600 μm, hydrogel mass 0.36 g, salinity ratio 100, external load 210 g).

The energy efficiency, ηE, was calculated as the ratio of the recovered energy relative to the energy consumed by the system, as follows: ηE =

E × 100 X − X out in

examined for energy recovery, with the smallest particle sizes demonstrating the most effective energy recovery. The hydrogels were initially fully expanded due to storage in DI water. When the HC solution (35 g/L NaCl) was pumped through the hydrogels (5 mL/min), the volumes quickly decreased to 10−20 mL in 5 min (Figure 2A). The hydrogels with the smallest particle sizes shrank the fastest, likely due to their larger total surface area. In contrast, when the HC was switched to the LC solution (DI water), the volumes of all the hydrogels substantially increased, especially for the hydrogels with the smallest particle sizes. After 60 min, hydrogels expanded to 44 mL (300−600 μm), 38 mL (600−850 μm), or 33 mL (850−1180 μm) (Figure 2A). This shrinking-swelling cycle with these two HC and LC solutions was repeated three more times, with good reproducibility in the last three cycles (see Supporting Information (SI) Figure S1). Only a single representative cycle (the second cycle) is shown in Figure 2A in order to clearly illustrate the duration of the shrinking and swelling processes. The different performance in the first cycle was likely due to the slightly different initial condition of the hydrogels (fresh hydrogels in DI water for Cycle 1 versus used hydrogels in NaCl for cycles 2 to 4). The expanded volumes of the last three cycles with an external load of 50 g were used to calculate the energy

(2)

where Xin (J) is the total energy provided to the system. This energy was estimated from the change in the free energy due to complete mixing of the HC and LC solutions,29 as follows: ⎛ aiin,HC aiin,LC ⎞ ⎟⎟ X in = RT ∑ ⎜⎜VHCciin,HC ln + VLCciin,LC ln ai ,M ai ,M ⎠ i ⎝

(3)

where R (8.314 J mol−1 K−1) is the gas constant, T (298 K) the absolute temperature, V (L) the volume of solution, c (M) the molar concentration of ionic species i in the solution, a the activity of ionic species i in the solution, and the subscript M indicates the mixed solution. Xout (J), the energy leaving the system, was calculated using eq 3 based on effluent conditions.

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RESULTS Hydrogel Particle Size. Hydrogels with three different particle sizes (300−600, 600−850, and 850−1180 μm) were 7159

dx.doi.org/10.1021/es500909q | Environ. Sci. Technol. 2014, 48, 7157−7163

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Figure 3. Average recovered energy for the last three cycles over swelling time for hydrogels with different: (A) particle sizes (hydrogel mass 0.12 g, infinite salinity ratio, external load 50 g, flow rate 5 mL/min); (B) salinity ratios (particle size 300−600 μm, hydrogel mass 0.12 g, external load 50 g, flow rate 5 mL/min); (C) hydrogel masses (particle size 300−600 μm, salinity ratio 100, external load 50 g, flow rate 5 mL/min); (D) external loads (particle size 300−600 μm, hydrogel mass 0.36 g, salinity ratio 100, flow rate 5 mL/min); and (E) solution flow rates (particle size 300−600 μm, hydrogel mass 0.36 g, salinity ratio 100, external load 210 g).

= 200), 12 mJ (SR = 100), and 7 mJ (SR = 50) (Figure 3B). The energy efficiencies decreased with salinity ratios, from 0.04% (DI water) to 0.01% (SR = 50) (Figure 4B). Based on these results, an SR of 100 was used in further tests. Hydrogel Mass. In order to initially examine the scalability of this process, hydrogel masses were increased from 0.12 to 0.24 or 0.36 g. Larger swelling volumes were obtained with an increase in hydrogel mass, but more time was needed to shrink or swell the bed of hydrogel particles to a steady state volume (Figure 2C). When the HC solution flowed through the hydrogels, it took about 10 min for 0.36 g of the hydrogel to shrink from 62 mL to a nearly steady state condition of 18 mL, compared to 5 min for the other two conditions (from 36 to 13 mL for 0.24 g, and 17 to 8 mL for 0.12 g). When the LC solution (0.35 g/L NaCl) was pumped into the reactor, the time required to swell the bed was longer, with 60 min needed to reach 62 mL using 0.36 g, 30 min to reach 36 mL with 0.24 g, and 20 min to reach 17 mL with 0.12 g. The average recovered energy (last three cycles) increased with hydrogel mass, with 68 mJ using 0.36 g, compared to 30 mJ (0.24 g) and 12 mJ (0.12 g), all after 60 min (Figure 3C). On the basis of the nearly linear increase in energy recovery with hydrogel mass, it was estimated that 1 g of hydrogel could be used to recover 214 mJ of energy per cycle, although the reactor here was limited to 0.36 g of hydrogel based on the expansion volumes. The

recoveries (Figure 3A). The energy recovered over time increased in accordance with the increase in the volumes of the hydrogels. After 60 min, the recovered energies were 44 mJ (300−600 μm), 35 mJ (600−850 μm), and 27 mJ (850−1180 μm) for hydrogels with different particle sizes (Figure 3A). The energy recovered in 1 h increased inversely with particle size, with a range of 0.02% to 0.04% (Figure 4A). The hydrogels with the smallest particle sizes (300−600 μm) had the highest energy recovery since they had a larger total surface area, and less time was needed for water to swell these smaller particles. Particles smaller than 300−600 μm were not examined as they could have flowed out through the holes on the bottom of the plunger used to release water. Therefore, hydrogels with a particle size of 300−600 μm were used in subsequent tests. Solution Salinity Ratio. LC solutions containing some salt, rather than using DI water, were examined at salinity ratios of 200 (0.18 g/L NaCl), 100 (0.35 g/L NaCl), and 50 (0.70 g/L NaCl) using a constant HC solution (35 g/L NaCl). The swelling volumes of hydrogels with these different salinity ratios were much smaller than those obtained using DI water (Figure 2B). After 30 min of water flow using these LC solutions, the volumes of hydrogels stabilized at 20 mL (SR = 200), 16 mL (SR = 100), and 11 mL (SR = 50), compared to 38 mL (DI water). As a result, the average energy recoveries based on the last three cycles for the different salinity ratios were 16 mJ (SR 7160

dx.doi.org/10.1021/es500909q | Environ. Sci. Technol. 2014, 48, 7157−7163

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Figure 4. Recovered energy in 1 h (red diamond) and energy efficiency (bar) for hydrogels with different: (A) particle sizes (hydrogel mass 0.12 g, infinite salinity ratio, external load 50 g, flow rate 5 mL/min); (B) salinity ratios (particle size 300−600 μm, hydrogel mass 0.12 g, external load 50 g, flow rate 5 mL/min); (C) hydrogel masses (particle size 300−600 μm, salinity ratio 100, external load 50 g, flow rate 5 mL/min); (D) external loads (particle size 300−600 μm, hydrogel mass 0.36 g, salinity ratio 100, flow rate 5 mL/min); and (E) solution flow rates (particle size 300−600 μm, hydrogel mass 0.36 g, salinity ratio 100, external load 210 g).

Solution Flow Rate. Higher flow rates can result in faster energy recovery rates (more power), but this reduces energy efficiency due to a larger rate of solution consumption and higher hydrodynamic energy losses. Therefore, solution flow rates were reduced from 5 mL/min to 2 mL/min or 1 mL/min. As expected, the shrinking and swelling rates of the hydrogels decreased due to the reduction in flow rates (Figure 2E). The recovered energy over 60 min declined with flow rate, from 151 mJ at 5 mL/min, to 143 mJ at 2 mL/min, and 124 mJ at 1 mL/ min (Figure 3E). However, the energy efficiency substantially increased from 0.07% (5 mL/min) to 0.34% (1 mL/min) due to the reduced volume of solution used (Figure 4E). Additionally, the energy needed for pumping solutions through hydrogels in one cycle decreased from 148 mJ (5 mL/min) to 28 mJ (1 mL/min), based on estimated pressure drops for the given heights and solution volumes used. A maximum net energy of 96 mJ was obtained with the lowest flow rate of 1 mL/min. Considering both of energy recovery rates and efficiency, 1 mL/min was the most optimal rate here, although improvements in energy efficiencies might be obtained using lower flow rates than those examined here.

increase in energy efficiency with hydrogel mass was approximately linear, from 0.02% (0.12 g) to 0.04% (0.36 g) (Figure 4C). It is therefore expected that much higher energy efficiency could be obtained with a larger scale system. External Load. The load used on the plunger can impact energy recovery, with an optimum ranging from too light a load (little work done) to too much resistance (less expansion). To examine conditions for optimizing energy recovery, the weights added to the plunger were varied between 50 and 310 g. As expected, an increase in the external load reduced the swelling volume of the hydrogels (Figure 2D), from 62 mL (50 g) to 52 mL (110 g), 46 mL (210 g), and 28−37 mL (310 g) (60 min swelling in 0.35 g/L NaCl). The recovered energy increased from 68 mJ (50 g) to a maximum of 151 mJ (210 g), but it decreased to 140 mJ at the highest load (310 g) (Figure 3D). The energy efficiency was also highest (0.08%) with an external load of 210 g (Figure 4D). Thus, the optimum external load with this system, under the tested conditions, was 210 g. The external load would likely need to be changed to optimize energy recovery for larger-scale systems operated under different conditions. For example, a larger external load could be used for a larger system, if there was sufficient mechanical strength of the hydrogels, depending on the salinity ratios used. 7161

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DISCUSSION The energy recovered using hydrogels varied due to a number of factors. The most optimal conditions were based on using the smallest particle sizes (300−600 μm) with the highest mass load of particles possible in our system (0.36 g), a median but higher external load (210 g), and a solution flow rate of 1 mL/ min. Although deionized water provided the most optimal energy recovery, under more realistic conditions where the low conductivity solution had a salt concentration similar to that of fresh water, 124 mJ was obtained in 60 min with a salinity ratio of 100. It is estimated that 344 mJ of energy could be recovered with 1 g of hydrogels in 1 h (0.1 mW/g-hydrogel) using solutions with salinities similar to those of seawater and river water. This energy recovery, normalized by the mass of hydrogels, is lower than other more developed salinity gradient energy technologies, such as PRO and RED on the basis of the mass of membranes needed for these processes. The power density produced using PRO has been increasing from