Blue Energy: Current Technologies for Sustainable Power Generation from Water Salinity Gradient

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Blue energy: Current technologies for sustainable power generation from water salinity gradient

ARTICLE in RENEWABLE AND SUSTAINABLE ENERGY REVIEWS · MARCH 2014 Impact Factor: 5.51 · DOI: 10.1016/j.rser.2013.11.049

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Zhijun Jia Baoguo Wang Chinese Academy of Sciences Tsinghua University

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Available from: Zhijun Jia Retrieved on: 09 September 2015 Renewable and Sustainable Energy Reviews 31 (2014) 91–100

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Renewable and Sustainable Energy Reviews

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Blue energy: Current technologies for sustainable power generation from water salinity gradient

Zhijun Jia a,b,c, Baoguo Wang a,b,n, Shiqiang Song a,b, Yongsheng Fan a,b a The State Key Laboratory of Chemical Engineering, Tsinghua University, Beijing 100084, China b Department of Chemical Engineering, Tsinghua University, Beijing 100084, China c National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100084, China article info abstract

Article history: “Salinity energy” stored as the salinity difference between seawater and freshwater is a large-scale Received 7 June 2013 renewable resource that can be harvested and converted to electricity, but extracting it efﬁciently as a Received in revised form form of useful energy remains a challenge. With the development of membrane science and technology, 2 September 2013 membrane-based techniques for energy extraction from water salinity, such as pressure-retarded Accepted 18 November 2013 osmosis and reverse electro-dialysis, have seen tremendous development in recent years. Meanwhile, many other novel methods for harvesting exergy from water mixing processes, such as electrochemical Keywords: capacitor and nano-ﬂuidic energy harvesting systems, have been proposed. In this work, an overview and Power extraction state-of-the-art of the current technologies for sustainable power generation from the water salinity Water salinity gradient are presented. Characteristics of these technologies are analyzed and compared for this Pressure-retarded osmosis particular application. Based on these entropic energy extracting methods, the water salinity, as the Reverse electro-dialysis “ ” Electric double-layer capacitor blue energy , will be another source of renewable energy to satisfy the ever-growing energy demand of Faradic pseudo-capacitor human society. & 2013 Elsevier Ltd. All rights reserved.

Contents

1. Introduction...... 91 2. Theoretical analysis of available work from salinity mixing process ...... 92 3. Methods for harvesting energy from salinity difference ...... 92 3.1. The pressure-retarded osmosis ...... 92 3.2. Reverse electro-dialysis (RED)...... 94 3.3. Electric double-layer capacitor ...... 96 3.4. Faradaic pseudo-capacitor ...... 97 3.5. Other technologies...... 99 4. Conclusion and outlooks ...... 99 Acknowledgements ...... 99 References...... 99

1. Introduction global energy consumption [1]. Harvesting clean energy from the environment to satisfy the ever-growing energy demand of the More and more renewable energies are required for reducing human society is of great importance for the survival and sustain- pollution, carbon dioxides emission, and the fossil energy part in able development of the human civilization [2,3]. Technologies for harvesting renewable energy such as solar, wind and geothermal sources have attracted great attentions and have developed n Corresponding author at: Tsinghua University, The State Key Laboratory of extensively recently. The “salinity energy” stored as the salinity Chemical Engineering, Haidian District, Beijing 100084, China. Tel.: þ86 10 62788777; fax: þ86 10 62770304. difference between seawater and freshwater is another large-scale E-mail address: [email protected] (B. Wang). renewable energy source that can be exploited [4,5]. When a river

1364-0321/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.rser.2013.11.049 92 Z. Jia et al. / Renewable and Sustainable Energy Reviews 31 (2014) 91–100 runs into a sea, spontaneous and irreversible mixing of freshwater energy change of the two solutions, is obtained at xE0.4 and is and seawater occurs, thereby increasing the entropy of the system. equal to ΔGE 851.9 J L1. This entropy change can be utilized to convert part of the thermal To calculate the maximum extractable energy per unit volume energy of the fluids into electrical energy [6]. It is calculated that of freshwater, which is considered as the limiting resource in this from each cubic meter of river water that flows into the sea, 2.3 MJ process, the Gibbs free energy of mixing is plotted with respect of of free energy is dissipated in the process, some portion of which the volume of river water consumed in Fig. 1(B). For the excess of can be harvested [6,7]. Worldwide, the potential for energy seawater, the maximum energy extractable is ΔGE 2500 J Lof extraction from this “salinity potential” resource (for all river freshwater (xE1). This means that a power plant processing effluents combined) amounts to around 2.42.6 TW, close to 400 m3 s1 of freshwater could produce power up to 1 GW. present-day global electricity consumptions [7,8]. Since the Fig. 1(C) shows the theoretical power output of the main rivers 1950s, it has been recognized that it is possible to interpose a over the world by the mixing process. It is apparent that it flows suitable device between the flow of freshwater and saltwater, in with tremendous energy in the mixing process when these rivers order to exploit the free energy stored in the salinity difference as a run into the sea. Amazon is the largest river in the world and its completely renewable energy source, so-called “blue energy”. annual average flow rate reaches 179,000 m3/s. When it runs into Already described techniques are pressure-retarded osmosis (PRO), the sea, the power produced in the mixing process will be up to reverse electro-dialysis (RED), concentration electrochemical cells 437.5 GW, which is 31 times of the installed capacity of Itaipu and devices exploiting difference in vapor pressure [4,5,9–12]. Hydroelectric Power Station, the largest hydroelectric power Recently this “blue energy” has received renewed interest; station in South America. At the estuary of Yangzi River in China, many new techniques were employed for the energy extraction a power output of 85 GW, about 4.7 times that of Sanxia Hydro- from the salinity difference, such as electrochemical capacitor and electric Power Station, runs away with the freshwater. If the nano-fluidic diffusion techniques [6,13,14]. Hence, this review exergy can be harvested effectively, it will provide sufficient provides an overview of the current status, and gives some energy for the development of human society. opinions about the main challenges and future trends of these technologies for harvesting energy from the entropy change. It is anticipated that this review will attract more attentions for this 3. Methods for harvesting energy from salinity difference renewable “blue energy”. 3.1. The pressure-retarded osmosis

2. Theoretical analysis of available work from salinity The osmosis pressure difference between river water and mixing process seawater is about 23 atm under ordinary conditions, equivalent to the hydrostatic head of 231 m dam [1,18–21]. Utilizing specific The “blue energy” that can be extracted from the mingling of devices, the large-scale salinity energy can be converted to freshwater and seawater is best illustrated by its reverse process mechanical energy or electricity directly. “desalination”–energy is required to extract freshwater from Pressure-retarded osmosis (PRO) is a novel opinion with a long seawater [15,16]. Therefore, in theory the reverse of any desalina- history, which can extract salinity-gradient energy by using semi- tion process should release energy. The theoretical non-expansion permeable membranes to allow the transport of water from a low work that can be produced from mixing a relatively concentrated concentration solution (such as river, brackish or wastewater) into salt solution s (seawater) and a dilute salt solution r (river water), a high concentration draw solution (seawater, brine water). Sea- at constant pressure p and absolute temperature T, to give a water can also be used as the low-concentration solution with brackish solution m,isdefined by the Gibbs energy of mixing brines produced from seawater desalination as the draw solution. ΔmixG [7,17]: The migration of water from the low-concentration solution to the high-concentration solution could increase the static energy ΔmixG ¼ Gm ðGs þGrÞð1Þ of the high-concentration side which can be utilized to promote Assuming that solutions are ideal dilute, the Gibbs energy of the the turbine. Theoretically the maximum extractable energy during mixing process can be calculated just from the change in molar the irreversible mixing of a dilute stream with saline draw Δ ¼ entropy (i.e., mixH 0): solutions is substantial, ranging from 0.75 kWh to 14.1 kWh per – ΔmixG ¼ðns þnrÞTΔmixSm ðnsTΔmixSs nrTΔmixSrÞð2Þ cubic meter depending on the low-concentration stream [18 25]. Fig. 2 shows a typical simplified PRO process [1]. Filtered where n is the amount (moles) and Δ S represents the contribu- mix freshwater and seawater are pumped into modules containing tion of the molar entropy of mixing to the total molar entropy of semi-permeable membranes. In the modules freshwater migrates the mixing electrolyte solution, according to through the semi-permeable membrane into the pressurized sea- fl ΔmixS ¼R∑ ci ln ci ð3Þ water. Then the ow of the mixing solution is split into two i streams, where one is depressurized by a hydropower turbine to where R is the universal gas constant, which is 8.314 J/mol K, and generate power and the other passes through a pressure exchan- c indicates the mole fraction of component i (i¼Naþ ,Cl). For a ger in order to pressurize the incoming seawater. As can be seen in unit of volume, the difference in the Gibbs free energy between Fig. 1, the plant has a high-pressure loop for the salt and brackish the solution after mixing and the separate ones before mixing can water and a low-pressure side for the freshwater. The two key be calculated as a function of the volumetric fraction of seawater, components in a traditional PRO plant are the pressure exchanger x, which is shown as follows: and the membrane. The energy efficiency of both these components is very important for the energy cost of this “blue energy”. Δ G ¼ 2RT½c ln ðc Þxc ln ðc Þð1xÞc ln ðc Þ ð4Þ mix m m s s r r With employment of high–energy-efficient membranes and pres- By considering the activity coefficient over the range of salt sure exchangers, a PRO power plant generates about 1 MW from concentrations examined here (0.024 M for river water and 0.6 M each cubic meter per second of freshwater that passes through the for seawater) to be unity, the Gibbs free energy change is shown in membranes [23–26]. Fig. 1 as a function of the volumetric fraction of seawater, x. The Actually the energy extracted by PRO is always limited by the minimum in Fig. 1(A), which is the maxima of mixing entropic property of the membrane. On the one hand, efficiency of the PRO Z. Jia et al. / Renewable and Sustainable Energy Reviews 31 (2014) 91–100 93

Fig. 1. Gibbs free energy of mixing as a function of the volumetric fraction of seawater for per unit volume of (A) ﬁnal solution, (B) low concentration solution and (C) the theoretical power output of the main rivers over the world.

Fig. 2. Simplified process layout for a typical osmotic power plant. is affected by the permeable property of the membrane. The (ICP), which significantly reduces permeate-water flux and therefore membrane is not completely impermeable to the solutes. Energy power density [5,27,30–33]. Cellulose–acetate membranes specifi- will be lost by the irreversible migration of salts to the freshwater, a cally designed for forward osmosis have substantially reduced ICP loss which can be expressed as a reduction in the effective osmotic effects, stimulating resurgence in PRO research and development pressure. On the other hand, the development of PRO has been also [27–30]. The power density has increased from less than hindered for many years by the lack of a membrane capable of 12.7 W m2 for river water and seawater; using seawater allowing an adequate flow. The bulky support layers of the reverse- reverse-osmosis brine, the power density reached 45.1 W m2 osmosis membranes cause severe internal concentration polarization [27,29–31].Thin-film composite polyamide membranes may allow 94 Z. Jia et al. / Renewable and Sustainable Energy Reviews 31 (2014) 91–100

Fig. 3. Schematic diagram of the pilot PRO plant, constructed by Statkraft [36]. the power density to reach up to 5.710 W m 2, because of their results by Tanugi et al. [39], it indicates that the single-layer graphene higher intrinsic water permeability and lower ICP than cellulose– can effectively separate salt from water and water can flow across a acetate membranes [32–35]. graphene membrane at rates in the range of 10–100 L/cm2/day/MPa The Norwegian state power company Statkraft SF, a company while still rejecting salt ions, which is 2–3 orders of magnitude with a strong tradition in hydropower, believes that osmotic higher than diffusive RO membranes. It could satisfy the property power can become a significant renewable energy source and requirement of the semi-permeable membranes used in PRO. If this has engaged in the PRO technology development aiming at cost- grapheme membrane can be used in the osmotic power plant, it may effective power productions [1,36–38]. The pilot PRO plant, con- be a great development in the energy output and energy efficiency of structed by Statkraft Company as shown in Fig. 3, generated the osmotic power technique. roughly 1 W m2 using the commercial, asymmetrical cellulose– acetate membranes [1,37,38]. This is much lower than the target power density of 5 W m2 needed to make PRO economically 3.2. Reverse electro-dialysis (RED) viable. Nonetheless, the biggest challenge the technology faces is, and still remain, the performance of the fragile membranes. The RED is a non-polluting, sustainable technology, which can permeation of the salt across the membrane will decrease the convert the free energy generated by mingling the two aqueous energy efficiency of the system, and contamination of the mem- solutions into electrical power directly with no other auxiliary brane also will deteriorate the performance of the osmotic power equipments [40–42]. The idea of RED was first proposed by Pattle plant. Therefore, these challenges indicate the need to develop in the 1950s as the reverse process of electro-dialysis and its fouling-resistant and solutes-impermeable membranes with tai- working principle is shown in Fig. 4 [5,40]. In the working process, lored surface properties and membrane modules with improved seawater and freshwater are pumped into arrays or stacks of hydrodynamic mixing to mitigate fouling. membranes with alternating anion-exchange and cation-exchange With the development of core material, properties of mem- membranes. Owing to the concentration difference of solutions brane will be improved greatly. Graphene, which consists of a 2D and the ion-selectivity of these membranes, an electrochemical sheet of sp2-bonded carbon atoms in a hexagonal honeycomb potential is generated directly. The difference in electrochemical lattice, is the ultimate thin membrane. Potential advantages of potential as a result of the positive ions moving one way and the graphene over existing RO membranes include negligible thick- negative ions moving the other is turned into an electrical current ness and high mechanical strength, which may enable faster water in the stack. An electrochemical redox couple, such as Fe2 þ /Fe3 þ , transport and a wider range of operating conditions than these which circulates in the electrode compartments, is employed to membranes used previously in PRO. According to the computational carry out the current to the external circuit [43]. Z. Jia et al. / Renewable and Sustainable Energy Reviews 31 (2014) 91–100 95

Fig. 4. The working principle of the RED [5].

Table 1 The summary and performance of some RED systems in recent years.

Membrane Thickness Spacer thickness Brine water Dilute water Power density Reference (mm) (μm) (W/m2) Anion Cation Company Country

1 FAD FKD Fumatech Germany 0.082 200 30 g/L 1 g/L 0.93 [50–53] 2 FAD FKD Fumatech Germany 0.080 200 30 g/L 1 g/L 1.17 [50–53] 3 Heterogen. Heterogen. Qianqiu China 0.580 200 30 g/L 1 g/L 0.49 [50–53] 4 Homogen. Homorgen. Qianqiu China 0.250 200 30 g/L 1 g/L 1.05 [50–53] 5 AMV CMV Asahi Japan 0.130 200 30 g/L 1 g/L 1.18 [50–53] 6 ACS CMS Tokuyama Japan 0.130 200 30 g/L 1 g/L 0.60 [50–53] 7 AMX CMX Tokuyama Japan 0.150 200 30 g/L 1 g/L 0.65 [50–53] 8 AMX CMX Tokuyama Japan 0.150 190 35.4 g/L 0.56 g/L 0.46 [50–53] 9 AMH CMH Mega Czech 0.230–0.250 240 30 g/L 1 g/L 0.80 [50–53]

For reverse electro-dialysis, the theoretical value of the poten- conductivity (S/m), κd is the diluted compartment conductivity (S/m) tial over the membrane for an aqueous monovalent electrolyte and Rel is the electrode resistance (Ω). (e.g. NaCl) can be calculated using the Nernst equation [7]: The final stack power output of the RED stack can be found from Kirchhoff's law and is defined as follows [7,44–46]: 2α RT a ΔV ¼ av ln c ð5Þ theo 0 zF ad ðV Þ2R W ¼ I2R ¼ load ð8Þ load ð þ Þ2 ΔVtheo indicates the theoretical membrane potential, αav, is the Rstack Rload average membrane perm-selectivity of an anion and a cation Here I is the current and R is the load resistance (Ω). exchange membrane pair (V), R is the gas constant (8.314 J/ load To generate a maximum power output (W ), R needs to be (mol K)), T is the absolute temperature (K), z is the electrochemical max load equal to R . In that case, Eq. (8) changes into Eq. (9), which valence, F is the Faraday constant (96485 C/mol), a is the activity stack c shows the relationship between the open circuit potential, the of the concentrated solution (mol/L) and a is the activity of the d maximum power output and the stack resistance: diluted solution (mol/L). The overall potential of the system with N pairs of membranes is the sum of the potential differences over ð 0Þ2 ¼ V ð Þ each pair of membranes [7]: Wmax 9 4Rstack 0 ¼ Δ ð Þ V N V theo 6 Combination of Eq. (9) with Eqs. (6) and (7) finally yields Eq. (10) which relates the maximum power output of the RED stack to the Stack resistance can be defined as the sum of the resistances of individual contributions of each component: the individual stack components as shown in the following ððα = Þ ð = Þ2Þ ¼ avRT zF ln ac ad ð Þ equation (9) [7,17]: Wmax NAð þ þð =κ Þþð =κ ÞÞþ 10 Raem Rcem dc c dd d Rel N dc dd Rstack ¼ Raem þRcem þ þ þRel ð7Þ More specifically, it relates the maximum power output of the A κc κd system (Wmax) to the average membrane selectivity (αav) and the where N is the number of membrane pairs, A is the effective membrane resistance (Raem and Rcem). Therefore, the performance 2 membrane area (m ), Raem is the anion exchange membrane resistance of RED is affected by the properties of the membrane and the 2 2 (Ω m ), Rcem is the cation exchange membrane resistance (Ω m ), dc is structure of the stack [47–49]. the thickness of the concentrated compartment (m), dd is the thickness In recent years, substantial advances have been made to of the diluted compartment (m), κc is the concentrated compartment increase RED power density and energy efficiency by improving 96 Z. Jia et al. / Renewable and Sustainable Energy Reviews 31 (2014) 91–100 membrane materials, spacing and architecture. Table 1 shows the capacitor is brought into contact with the freshwater. The charged summary and performances of some RED systems in recent years. ions diffuse away from the electrodes, against the electrostatic As shown in Table 1, the highest power density for an RED force. Therefore, the electrostatic energy of the whole system will stack is 1.18 W/m2, neglecting the electrode over-potentials and increase. The surplus energy can be extracted from the capacitor, pumping losses. When these energy losses are taken into account, thus converting the salinity difference into usable energy. there is a lower net power density of about 0.16–0.26 W/m2, which Fig. 6 shows the schematic view of the EDL capacitor designed is much lower than the target power density for RED economical by Brogioli for energy extraction from the water salinity [6]. application [50–53]. The other main challenge for the commercia- As shown in Fig. 6, the cell contains two electrodes, and is filled lization of RED is the cost of ion-exchange membranes. Therefore, with the changeable solution. Two water reservoirs, with two for the practical application target of RED, much more attention is solutions at different NaCl concentration representing seawater needed to pay on the research about the low-cost membrane with and freshwater, are connected to the cell by pumps, which are excellent performance and the optimal system architecture. activated in sequence, so that the concentration of the solution in

3.3. Electric double-layer capacitor

“Capacitive energy extraction” is a new and interesting method to convert the free energy released from the mixing of freshwater with seawater into electricity. The technique is based on the electrochemical double layer (EDL) capacitor technology. EDL is described by the Gouy–Chapman–Stern theory currently, which models the ion distribution close to planner electrodes as the sum of an adsorbed EDL (Stern EDL) and a diffuse EDL [6,54–57]. In the diffuse EDL, charged ions reach the equilibrium between diffusion, which tends to equalize the ion concentration, and electro- staticforcethattendstoincreasethechargeimbalanceclosetothe surface on the contrary. At long distance from the electrode the charge is completely screened, so that the electric field is present only inside the diffuse EDL [6,54,55]. The relation between the surface charge density s and the potential difference φ between Fig. 6. Scheme of the EDL capacitor [6]. the electrode and the bulk solution is as follows: ! 2k T s φ ¼ B sinh 1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð11Þ e 8CNAε0εrkBT where KB is the Boltzmann constant, T is the temperature, e is the electron charge, NA is the Avogadro constant, ε0 is the electric constant, εr is the relative dielectric constant and C is the electrolyte concentration; the equation is valid for a symmetric, monovalent electrolyte such as NaCl. According to Eq. (11), the relationship between charge and voltage is shown in Fig. 5. The decrease of concentration in C leads to an increase of potential. Therefore, when the EDL capacitor, which is constituted by active carbon electrodes, is immersed in seawater, it stores the charges in the EDLs constituted by counter ion distributions close to the electrode surfaces. Then the EDL

Fig. 7. Cycle for extracting energy from salinity difference [6].

Fig. 5. Charge-voltage cycle at different EC-voltage V0, represented by dotted lines. The area enclosed by each cycle represents the extracted electrical energy from switching between 1 mM and 500 mM salt solutions. The lines labeled with “sea water” and “fresh water” represent the charge-voltage relations obtained from the Gouy–Chapman–Stern theory, respectively, for 500 mM and 1 mM with η¼0.99 [6]. Fig. 8. Representation of the cycle on the potential versus charge graph [6]. Z. Jia et al. / Renewable and Sustainable Energy Reviews 31 (2014) 91–100 97

Fig. 9. Schematic view of the flow cell for capacitive energy extraction from the sequential flow of saline and freshwater, and of the required electrical circuit [54]. the cell can be changed from a low concentration to a high 3.4. Faradaic pseudo-capacitor concentration and vice versa. In the work of Brogioli, the activated carbon electrodes, an A pseudo-capacitor is based on charge storage brought by extremely porous conductor, are shaped in 2 mm-diameter disks, Faradaic charge transfer process of the capacitor electrodes 0.1 mm thick. A switch can temporarily connect the electrodes to a [58–63]. The pseudo-capacitor consists of a couple of reversible charge or a discharge circuit. The load is represented by the resistor electrochemical systems where the salts in the electrolyte and the R¼1kΩ, through which charge and discharge current flow. electrodes are the reactants [17,60–64]. Therefore, the ions can In order to extract energy from the capacitor, a cycle with four be stored on the electrodes. Compared with the EDL capacitors, phases is performed, as shown in Fig. 7. the pseudo-capacitor has a higher energy density and a simpler structure. Phase A: The cell is filled with seawater. The capacitor is In order to extract the free energy in the water salinity, a novel

charged, towards voltage φcharge¼300 mV. pseudo-capacitor was constructed according to Eq. (12) by Cui Phase B: The circuit is open. The cell is ﬂushed with freshwater; [17,65]:

the voltage increases up to φB¼333 mV. 5MnO2 þ2Agþ2NaCl2Na2Mn5O10 þ2AgCl ð12Þ Phase C: The capacitor is discharged, towards voltage φ ¼300 mV. It is apparent that a sudden voltage drop In this device, two different electrodes were employed: an anionic discharge appears, due to internal resistance in the capacitor, followed by electrode, silver electrode, which interacts with Cl selectively; and a þ a nearly exponential decay. cationic electrode, MnO2 electrode, which interacts with Na selec- Phase D: The circuit is open. The porous electrodes are brought tively. In this electrochemical system, the two separate electroche- into contact with saltwater. The voltage decreases down to mical reactions can be written as follows: φ ¼274 mV. þ D 5MnO2ðαÞ þ2NaðεÞ þ2e 2Na2Mn5O10ðαÞ ð13Þ

The relationship between voltage across the electrodes of the 2AgClðβ′Þ þ2e 22Agðα′Þ þ2Clε ð14Þ capacitor and electric quantity in the energy extracting cycle is α ε β′ shown in Fig. 8. By a calculation based on the graph shown in where is the Na2xMn5O10 phase, is the electrolyte phase, is the α′ Fig. 8, it can be noticed that the net energy generated by the AgCl phase, and is the silver phase. The potential of the two salinity difference is about 5 μJ, equivalent to 2.2% of the energy it reactions with respect to the normal hydrogen electrode (NHE) is stores [6]. given by The second generation of the laboratory-scale EDL capacitor RT aNa;ε E þ ¼ E þ ;0 þ ln ð15Þ designed by Brogioli is shown in Fig. 9, which consists of dense F aNa;a graphite current collectors (250 μm), porous carbon electrodes (270 μm, porosity 65%), and an open-meshed polymer spacer RT ÂÃ E ¼ E ; ln a ;ε ð16Þ (250 μm). All materials are cut in pieces of dimension 6 6cm2 0 F Cl and assembled, after which the entire stack of all layers is firmly where Eþ and E are the potentials of the electrodes, Eþ ,0 and E,0 compressed. An aqueous NaCl solution is pumped into a small hole arethestandardpotentialsoftheelectrodes,αNa,α is the activity of (1.5 1.5 cm2) located in the exact middle of the stack, and flows the sodium in the solid phase α, αNa,ε is the activity of the sodium outward through the spacer channel, leaving the cell on all four ions in the electrolyte, and αCl,ε is the activity of the chloride ions in fl 1 sides. The total ow rate, which is 1 ml s , is constant during all the electrolyte. The difference between the two potentials is ΔE. experiments. The electrical circuit includes a potentiostat, that If the activity of sodium in the solid phase is fixed (no current), one simulated the EC, operating at a voltage V0 and a resistance obtains R¼11 Ω, constituting the load, in series with the flow cell [54]. RT RT In the experiment with the second generation setup, the ΔE ¼ ΔE ða ;αÞþ2 ln ½C þ2 ln ½γ ð17Þ 0 Na F NaCl F NaCl extracted energy is of the order of about 2 J for EC-voltages of 6 Δ V0 40.5 V, which is of the order of 10 times the amount extracted where E0 is the standard cell voltage, CNaCl is the concentration of γ fi in the micro-scale prototype as previously reported [54–57]. NaCl, and NaCl is the mean activity coef cient of NaCl. The depen- γ – Although the performance of the energy extracting capacitor is dence of NaCl on CNaCl can be described by the Debye Hückel law, far from the practical application, it provides a strong platform for whichisasfollows: pffiffiffiffiffiffiffiffiffiffiffi further development. Moreover, if the solution cycling rate of ½γ ¼ A pCNaClffiffiffiffiffiffiffiffiffiffiffi ð Þ different frequencies can be reached, the power production will ln NaCl 18 1þB C get different orders to satisfy different applications. And the NaCl energy and power performances of EDL capacitors can be further where A and B are the constants. improved by suitably engineering the structure of the porous As shown in Eq. (17), ΔE will be increased by the increase of the electrode material. Therefore, EDL capacitor will be a competitive NaCl concentration. Therefore, the electrodes described in Cui's technique with membrane-based methods. device are charged in a low ionic strength solution (river water) by 98 Z. Jia et al. / Renewable and Sustainable Energy Reviews 31 (2014) 91–100

Fig. 10. Schematic representation of the working principle behind a complete cycle of the mixing entropy pseudo-capacitor.

Fig. 11. Typical form of a cycle of the pseudo-capacitor voltage (E) versus charge (q) during one cycle, demonstrating the extractable energy. removing the Na þ and Cl ionsfromtherespectiveelectrodes initially, as shown step 1 in Fig. 10.Successively,thelowionic strength electrolyte is exchanged for a concentrated electrolyte

(seawater), which is accompanied by an increase in the open Fig. 12. Schematic illustration of the net diffusion current generation. potential of the cell (Fig. 10 step 2). With the higher open potential, the “salinity pseudo-capacitor” is discharged, as the anions and cations are reincorporated into their respective electrodes (Fig. 10 consumed in step 1. The energy gain is given by the integral along step 3). Then the high ionic strength solution is removed and the cycle of the voltage with respect to the charge: substituted by the dilute electrolyte (river water), which results in W ¼∮ ΔEdq ð19Þ a decrease in potential difference between the electrodes (Fig. 10 C step 4). The exchange of solutions could be carried out by a flow In the laboratory-scale salinity pseudo-capacitor, the geometrical process, which could be attractive for large-scale energy extraction. electrode surface area in contact with the solutions was 1 cm2. The The exergy in water salinity can be extracted in the cycle as distance between positive and negative electrodes was 1 cm. The shown in Fig. 10, as shown by a schematic of the expected shape of pseudo-capacitor was charged and discharged at 250 μAcm2 the pseudo-capacitor voltage (E) versus charge (q) during one with a time limit of 20 min. In the case of seawater (0.6 M NaCl cycle in Fig. 11. In the close cycle, no energy is produced or solution) and river water (0.024 M NaCl solution), the energy consumed during step 2 and step 4. During step 1, the capacitor density produced by the prototype device is about 29 mJ cm-2 requires energy to drive the ions out of the crystal structure, while (power density 105 mW m2) [17]. during step 3 the capacitor produces energy by incorporating the Although the performance of the pseudo-capacitor for energy ions. Therefore, the energy gain is due to the fact that the same harvesting from water salinity is far from the practical application, amount of charge is released in step 3 at a higher voltage than that with the capacitor structure optimization and the development of Z. Jia et al. / Renewable and Sustainable Energy Reviews 31 (2014) 91–100 99 materials, the pseudo-capacitor is also a promising technology 4. Conclusion and outlooks whose practical application can make a significant contribution to the field of renewable energy production. This process of generat- A brief overview of current entropic energy extraction techni- ing electrical energy could also be reversed, and exploited as a ques from water salinity is discussed in this paper, including basic method for water desalination. theories, structures, and applications. It is proved that the potential energy, stored as the salinity difference between seawater and river water, can be extracted as a renewable energy resource to 3.5. Other technologies satisfy the ever-growing energy demand of human society. The membrane-based systems are the most suitable technolo- In nature, some organisms have the inherent ability to convert the gies for large-scale applications. In order to improve the efficiency salinity energy into bio-electricity [66]. One remarkable example is of these energy harvesting systems, the performance of mem- the electric eel, which is capable of generating considerable electric branes should be optimized. shocks with highly selective ion channels and pumps on its cell fl membrane [67]. In recent studies, a fully abiotic nano- uidic energy (1) The semi-permeable membrane should have a good water fi harvesting system, which ef ciently converts Gibbs free energy in transport flux. the form of a salinity gradient into electricity, has been constructed to (2) Ion selectivity is another important property of these membranes. – mimic the gating functions of the biological counterparts [14,68 76]. (3) The membrane should be resistant to fouling when using The power generation mechanism can be understood as shown in natural seawater and river water. Fig. 12 [14]. In a single synthetic nano-pore, the counter-ions are (4) The membrane should also have a good mechanical property. preferentially transported over the co-ions, known as ion-selectivity. Under the concentration gradient, the ions diffuse spontaneously For commercial energy production, the break-even point for across the channel; in the spontaneous process, part of the Gibbs free these membrane's performances is about 5 W/m2. energy can be extracted continuously from the nano-fluidic system The capacitor and nano-fluidic diffusion techniques could by means of a net diffusion current. extract the electrical work from water salinity differences directly, In the prototype of the energy harvesting device, the nano- without the need for secondary energy converters or batteries. In a fluidic diffusion process was conducted on ion-track-etched single capacitor, an electrode with a large specific surface and a high nano-pore with a large opening (base) of 1.2 μm in diameter and charge capacity is very important to system’s performance. For the a small opening (tip) of 41 nm, which is embedded in a nano-fluidic diffusion technique performance, the great ion selec- polyimide membrane. 1 mM and 10 mM KCl solutions were placed tivity and water transport flux of the membrane are the restrain- at the tip and the base side of the conical nano-pore. With a series ing factors. Although these techniques have the potential to bring change of the condition, the short circuit current (I ) and the open 0 cost-efficient harvesting of this renewable source of energy closer circuit potential (V ) were measured in the nano-fluidic diffusion oc to realization, the performances still have a long way to go before process. The maximum energy production capability of the nano- being used in real applications. With the improvement of the heart pore fluidic systems is then calculated by P ¼I V . All the max 0 oc materials, such as electrode materials and membranes, these results are shown in Table 2 [14]. techniques will extract the entropic energy much more efficiently As shown in Table 2, the maximum resulting power with and realize the real application. individual nano-pore approaches about 10 pW. As a theoretical The water salinity power is almost completely benign to the analysis, the power generation can be further improved by increasing physical environment. The energy extracted does not seem to play the pore number, engineering the pore geometry, and tailoring the any significant role in any natural process. Furthermore, no deleter- surface chemistry of the nano-pore. Therefore, materials with uni- ious substance, including heat, is added to the output stream. It is a form nano structure are promising for constructing the energy completely environment-friendly power source. With increasing extraction device. From a perspective of potential application, the efficiency,wecanforeseethatthewatersalinitypowermaybea nano-fluidic power source can be used to supply other micro-/nano- kind of ideal energy source for human society. scale-sized devices, which will greatly reduce the size and weight of theentirenano-system[77]. By connecting such individual nano- fluidic structures in series (to build up voltage) and in parallel (to build up current), the hierarchical integrated system may serve as a Acknowledgements building block for a practical clean-energy-recovery plant [78,79].For improving the performance of the nano-fluidic system, further The authors gratefully acknowledge financial support from chemical modification could be applied to construct low adhesive the National Natural Science Foundation of China (21076112, water/solid interfaces on the nano-pore wall to eliminate bio-fouling. 21276134) and the National 863 Project (2012AA051203)

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