1. Introduction

Global warming and climate change are emerging environmental issues [1]. Rapid population growth, industrialization, and economic growth have made it necessary to use a lot of fossil fuels, and fossil fuels have emitted greenhouse gases, polluting the environment [2-3]. Industrialization also caused water pollution and increased the need for clean water [4]. At this time when eco-friendly energy technologies and clean water production are needed to prevent climate change and replace fossil fuels, a desalination is one of the promising technologies [5].

Recently, a technology producing electricity from the osmotic pressure is emerging, which is called pressure retarded osmosis (PRO) [6]. Energy is released from the mixing of two aqueous solutions with different salinities and transported to hydraulic pressure in PRO [7]. Unlike fossil fuels, it is less damaging to the environment because it would not generate dangerous pollutions to the surroundings [8]. Also, it has such a large range to use that various source of water could be utilized [9].

In 1954, it was first described to generate electricity from the osmotic pressure conceptually and the study has been conducted since then [10]. However, generating electricity from low-grade heat energy has some difficulties because of current technology limitations and the high cost of membranes. Also, harnessing energy from sources with variable and low temperature is theoretically difficult regarding the second law of thermodynamics [11]. Membrane, a critical component of PRO, has been used for water desalination and reverse osmosis [12-14].

In this review, we discuss several technologies to increase the energy efficiency of osmotic heat engine to harvest salinity gradient power, especially about desalination membranes and their modeling. Fig. 1 shows the schematic diagram of desalination membrane process using thermal energy. Table 1 represents the summary of desalination membrane used for power generation.

2. Desalination membranes for power generation

To address the problem of the lack of a sufficient semipermeable membrane, Tong et al. synthesized a free-standing membrane consisting of graphene oxide (GO) to generate electricity in an osmotic heat engine [15]. GO dispersions were synthesized through a modified Hummer’s method, while a vacuum filtration process is used to synthesize the free-standing graphene oxide membrane (GOMs). They tried to synthesize thinnest GOM to generate a high water permeability while thickest GOM as to make a sufficient mechanical strength. They determined the membrane thickness using cross-sectional SEM images and analyzed GO chemistry using a K-Alpha X-ray photoelectron spectrometer system. Fig. 2 shows the XPS and SEM images of GO and free-standing GOM.

A modified RO test cell was applied to measure the membrane water and salt permeability, using DI water and salt solutions, respectively. In Tong’s study, to determine suitable coefficients for PRO, the porous frit was substituted by a porous mesh-type SEPA CF spacer in the RO testing cell. Especially, the salt permeability coefficient was computed by B=, by measuring R (the membrane salt rejection). Fig. 3 shows power density and energy efficiency values of GOM. The synthesized GOM with 4.4 L/m² h bar of water permeability coefficient showed high power density (20.0 W/m²) under hydraulic pressure of 6.90 bar and with 2 M draw solution of ammonium bicarbonate solution. Tong et al. concluded that the free-standing GOM is well qualified to be applied in the osmotic heat engine, but they mentioned that further research on stable GOM with higher burst pressure is needed. The way to increase energy efficiency is by eliminating the membrane support layer while the internal concentration polarization is minimized. Ammonium bicarbonate solution used as the working fluid is an efficient way to achieve high power density and the system energy efficiency increases if the applied hydraulic pressure increases.

Using GOMs in PRO can provide a new way to develop salinity gradient power (SGP) technology [16]. Tong et al. synthesized GO sheets and the free-standing GOMs using the same methods mentioned earlier. The free-standing GOMs had a smooth surface, as confirmed from the SEM images and showed outstanding mechanical strength and average water permeability coefficient. The thinnest free-standing GOM has good stretch resistance and rigidity and stretch resistance (GOM-1 tensile strength 174.5MPa). In general, the water permeability coefficient decreases significantly if the GOM thickness increases. The highest water permeability coefficient of GOM-1 was calculated by applying RO setup with modified cross flow, 4.27 L/m² h bar because of the smallest thickness (1.73 mm). To characterize the free-standing GOM, water and salt flux of the GOM-1 was quantified in the FO system. The higher draw solution concentration, the higher water permeability coefficient and the higher reverse salt flux were measured. At the same time, lower membrane structure parameter was measured, which can reduce ICP, a main factor cutting down the membrane performance in PRO.

Also, the free-standing GOMs can minimize ICP and increase water flux with high draw solution concentration. Tong et al. obtained a power density of 24.62 W/m² at a hydraulic pressure of 6.90 bar using 3 M and 0.017 M of NaCl as a draw and a feed solution, respectively. This indicates that applying GOMs with higher water flux, superior mechanical strength, and high draw solution concentration can create higher power density, which will serve as the foundation of the PRO for sustainable development.

For development of osmotic electricity, membranes must have features including high ionic flux and highly efficient ion rectification with long-term stability in seawater [17]. Chen et al. showed that bio-inspired ion-selective membranes can be employed for osmotic energy. They fabricated high strength aramid nanofibers in the presence of boron nitride (BN) nanosheets. BN nanosheets were hydroxylated to have better interaction with Aramid fibers. Different composition of aramid/boron nitride (ABN) nanocomposite membrane are fabricated to check the effect of filler. Assembled ABN30 membranes show a uniform arrangement of BN nanosheets. The presence of hydroxylated group enhances the hydrophilicity of the composite membrane without affecting the flexibility. Hydrophilicity of the membrane was checked by contact angle measurement. X-ray diffraction (XRD) is used to analyze the structures of the ABN membranes. It showed self-assembly pattern of BN nanosheets. The intensity and position of diffraction peak of 2D boron nitride nanosheets in ABS are intact.

The tensile strength of the ABN membrane reached higher, due to the strong interfacial interaction through hydrogen bonds between aramid nanofibers and hydroxylated surface of BN nanosheets. Additionally, thermal stability of ABN is higher than neat aramid membranes, with a higher decomposition temperature. The power density of 0.6 W/m² is obtained from the membranes with wide-ranging temperature (0.0~95°C) and pH (2.8~10.8), which is important for the economic feasibility of osmotic engine.

N. Y. Yip et al. propose the fabrication of thin-film composite membranes to obtain an efficient membrane for PRO [18]. Nonsolvent-induced phase separation of polysulfone (PSf) on poly(ethylene terephthalate) (PET) was utilized to fabricate the thin, porous support layer, and the polyamide active layer was created on top of the PSf support layers through interfacial polymerization. The morphology of the membranes is shown in Fig. 4.

The TFC-PRO membranes performance is related to the interplay between the support layer and the active layer with minimized ICP, low salt permeability and high water permeability. Fig. 5 shows experimentally measured water fluxes for the fabricated membranes, LP#1, MP#1, and HP#1. Yip et al. also suggested the new model to predict the power density by calculating the water flux in PRO, which incorporates external concentration polarization (ECP). They showed that a hand-cast membrane with balanced permeability and selectivity (A = 5.81 L/m² h bar, B = 0.88 L/m² h) achieved the highest potential peak power density (10.0 W/m²) from a river water feed solution and seawater draw solution. It is because of the ability of the support layer to prevent the accumulation of leaked salt, an average salt permeability, and the high water permeability of the active layer. On the other hand, membranes with greater selectivity and lower water permeability (A= 1.74 L/m² h bar B=0.16 L/m² h) showed a lower peak power density (6.1 W/m²), while membranes with a higher permeability and lower selectivity (A= 7.55 L/m² h bar B=5.45 L/m² h) performed poorly (6.1 W/m²), because of severe reverse salt permeation.

3. Novel salt-form PRO systems

The research to be carried out to obtain osmotic heat engine running is about the appropriate application circumstances, system performance and thermodynamic efficiencies [19]. Therefore, Tong et al. conducted a research on the efficiency of osmotic heat engine system (with NH₄HCO₃ solution as the working fluid) and the feasibility to generate electricity from diverse heat sources. It was reported that high energy efficiency (h_th) and exergy efficiency (h_x) were reached 4.61% and 17.90% respectively, at lower operating temperature (323K) and with high draw solution concentration (2 M) and low feed solution concentration (0.1 M). Also, an even greater energy return was calculated from the low-grade industrial waste heat (54.7) than a solar thermal energy (1.3-2.2). Low energy return from the solar thermal energy was obtained because the large area of the flat-plate solar collector is needed. It is appropriate to harvest energy from industrial waste, at lower operating temperature and with high draw solution concentration and low feed solution concentration.

The power density and concentration of the draw solution (DS) are important factors to choose an adequate draw solute [20]. Gong et al. found the data set that evaluated three inorganic draw solutes, 13 W/m² for MgCl₂, 14 W/m² for NaCl, and lower power densities for MgSO₄. It indicates that draw solutes with more diffusive salts achieve higher water flux and induce ICP. However, the outstanding draw solute for osmotic engines is depended not just on the solution properties, but on the complex interaction between the membrane properties and solution properties, which means both the selectivity and diffusivity of the membrane affect transport. In conclusion, various membrane is needed to evaluate which draw solute is best satisfied for osmotic engine and storage application.

McGinnis et al. investigated using concentrated ammonia- carbon dioxide draw solution in PRO process, an osmotic heat engine, to produce high osmotic pressure and make efficient water flux [21]. The internal concentration polarization would be removed, and high membrane water flux and effective mass transport would be obtained by using deionized water working fluid at low temperature, which can let membrane power density be greater than 200 W/m² and maximum thermal efficiency be 16% of Carnot efficiency. Furthermore, more effective power generation is possible with the mixture of a highly concentrated NH₃/CO₂ draw solution and a deionized working fluid. In addition, the employment of an ammonia-carbon dioxide osmotic heat engine might be an economically competitive because the power production is possible from various energy sources, even from the low temperature heat sources which cannot be used by other heat engines.

Hydro-osmotic power (HOP) has a high potential of osmotic energy to generate electricity [22]. Fig 6 shows a change in power density as a function of DS inlet hydraulic pressure. By using an appropriate membrane and setting more than 25 bar of the osmotic potential difference, a cost of £30 M/MWh of clean electricity could be gained, which means that the system permeability and osmotic pressure difference have a critical impact on the productivity of HOP plant. Furthermore, Sharif et al. investigated different operational conditions for pilot plant. They showed that the membrane system constitutes 50~80% of the HOP plant cost and that research on suitable membranes is needed to increase the feasibility of the process. The higher membrane permeability, the lower the capital cost and the higher the power productivity. In addition, the water permeability is a critical factor to evaluate the HOP process feasibility. The interaction between the properties of the fluid and the membrane properties will have a greatest impact on developing HOP plant technology.

4. Energy production from heat source

The heat-based SGP such as thermal-driven electrochemical generator (TDEG) have recently received more attention than conventional PRO but the former system is based on the electrochemical reaction. In recent study by Luo et al., the concept of a novel system of waste heat conversion using TDEG is proposed to utilize waste heat, which consists a reverse electrodialysis (RED) stack and a distillation column [23]. Luo et al. obtained a maximum power density (0.33 W/m², ionic flux efficiency (88%), and energy efficiency (31%) at the optimal condition of LC concentration (0.02 M) and flow rate (800 mL/min), by using NH₄HCO₃ solutions as working fluids. The feasibility of NH₄HCO₃ made it validated to generate electricity for TDEG, a promising way to produce energy from waste heat.

The existing technologies are not suitable for energy production from heat source of lower grade with different heat output and with a small difference of temperature between the source and the environment [24]. Straub et al. suggested using thermo-osmotic vapor transport through hydrophobic, nanoporous membranes to create energy from low-grade heat sources. Power densities of 3.53 0.29 W/m² are obtained in the process pushing the vapour flux to a hot reservoir (60°C) to a pressurized cold reservoir (13 bar, 20°C) through the membrane. The efficiency of a continuous closedloop system would be bigger than 50% of the Carnot efficiency. This process has a great merit over other systems because it is possible to generate energy from low and changing source temperatures and low temperature differences (less than 40°C). There is a specific limit for working fluid, so water can be used to make it environment friendly. It would be achievable that a lot of energy can be producible from low-grade heat sources by further technical developments (improved pressure resistance of small pore size vapor-gap membranes, better heating configurations, new type of working fluids and batch operations, etc.).

5. Conclusions

For generating electricity from the recovery of waste heat from salinity gradient, research on PRO process and membranes has been conducted. In this review, studies on seawater desalination membranes and theoretical modeling of desalination technologies through efficient membranes were discussed. Membranes in PRO are the crucial parts that determine the energy efficiency of PRO engines. A free-standing GOMs can be applied in PRO process, in that they can minimize ICP and achieve high water flux. Further research on stable GO membranes with higher burst pressure is required to increase power density. Bio-inspired nanocomposite membranes and thin-film composite membranes are also applied. A hand-cast membrane with balanced permeability and selectivity showed the highest potential power density when the experiment is conducted with thin-film composite membranes. Modeling of the process of recovery of waste heat from salinity gradient was discussed with PRO heat engine. It is well-qualified to generate energy from industrial waste at lower operating temperatures and high concentration differences. Draw solute and water permeability are also the main factors to affect the power density of osmotic heat engines. Draw solutes with more diffusive salts showed to achieve higher water flux and induce ICP. Above all things, the complicated interplay between the membrane properties and solution properties should be significantly considered to select the finest draw solute for osmotic engines. Clean water and energy can be produced from low-grade heat sources by further technical developments of PRO. Increasing water permeation and reducing reverse salt flux by developing better membranes and draw solutes are required to enhance the PRO performance. Accordingly, further studies focused on developing new membranes should be continued.