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Produced waters (PWs) are oilfield waste streams rich in minerals and hydrocarbons whose production rate is largely increased in last decades following the corresponding increase of energy demand. The high salinity level of PWs inhibits the adoption of cheap biological treatments. Also, desalination techniques based on osmotic membranes would require severe pre-treatments. As an alternative, Reverse ElectroDialysis (RED) and Assisted Reverse ElectroDialysis (ARED) are here proposed for the first time to reduce the salinity level of PWs. RED may also guarantee an operation cost reduction thanks to its energy generation. An ad-hoc model for RED and ARED is here developed in order to deal suitably with PWs. This is done by a calibration and validation with experimental data purposely collected via RED and ARED units fed by real PWs. The model is integrated with economical equations and a techno-economic analysis is carried out in order to identify the best configuration for the desalination purposes. Results suggest that ARED operation mode is the best option guaranteeing a minimum in the controlled dilution cost corresponding to a 1.32 € per m3 of PWs treated, thus leaving room for an affordable future implementation of more sophisticated treatment chains based on bioremediation.

This Chapter treats the application of CFD technique in ED/RED processes. CFD simulations are able to predict flow and mass transfer in channels filled with spacers or membrane profiles, by providing dimensionless correlations useful for process models. At a larger scale, manifolds of the stack and inlet/ outlet zones of the channel can be simulated. Numerical models can include the computation of the electrical potential, thus characterizing the process performance. Direct numerical simulations can elucidate the complex phenomenology of overlimiting transport and of chaotic electrokinetic flows.

The increasing share of renewables in electric grids nowadays causes a growing daily and seasonal mismatch between electricity generation and demand. In this regard, novel energy storage systems need to be developed, to allow large-scale storage of the excess electricity during low-demand time, and its distribution during peak demand time. Acid–base flow battery (ABFB) is a novel and environmentally friendly technology based on the reversible water dissociation by bipolar membranes, and it stores electricity in the form of chemical energy in acid and base solutions. The technology has already been demonstrated at the laboratory scale, and the experimental testing of the first 1 kW pilot plant is currently ongoing. This work aims to describe the current development and the perspectives of the ABFB technology. In particular, we discuss the main technical challenges related to the development of battery components (membranes, electrolyte solutions, and stack design), as well as simulated scenarios, to demonstrate the technology at the kW–MW scale. Finally, we present an economic analysis for a first 100 kW commercial unit and suggest future directions for further technology scale-up and commercial deployment.

Salinity gradients are a non-conventional source of renewable energy based on the recovery of the Gibbs free energy related to the mixing of solutions at different concentrations. Reverse Electrodialysis is a promising and innovative technology able to convert this energy directly into electric current. The worldwide availability of salinity gradients is limited to those locations where water bodies at different salinity levels are present. The present work analyses a number of different scenarios worldwide, in locations where salinity gradients are naturally available or generated by anthropogenic activities. A techno-economic model of the Reverse Electrodialysis process is presented. The model is used to evaluate the energy that can be harvested in each real scenario using a reverse electrodialysis plant and relevant results are reported in terms of power densities and energy yields. Finally, an economic analysis based on the estimation of the Levelized Cost Of Electricity (LCOE) for each scenario is presented, and perspective considerations are reported. Results suggest that competitive values of LCOE may be achieved in some scenarios.

Reverse Electrodialysis (RED) extracts electrical energy from the salinity difference between two solutions using selective ion exchange membranes. In RED, conditions yielding a large net power density (NPD) are generally desired, due to the still large cost of the membranes. NPD depends on a large number of physical and geometric parameters. Some of these, for example the inlet concentrations of concentrate and diluate, can be regarded as “scenario” variables, imposed by external constraints (e.g., availability) or chosen by different criteria than NPD maximization. Others, namely the thicknesses HCONC, HDIL and the velocities UCONC, UDIL in the concentrate and diluate channels, can be regarded as free design parameters and can be chosen so as to maximize NPD. In the present study, a simplified model of a RED stack was coupled with an optimization algorithm in order to determine the conditions of maximum NPD in the space of the variables HCONC, HDIL,UCONC, UDIL for different sets of “scenario” variables. The study shows that an optimal choice of the free design parameters for any given scenario, as opposed to the adoption of standard fixed values for the same parameters, may provide significant improvements in NPD.

Electrochemical processes are considered very effective methods for the treatment of wastewater contaminated by organics resistant to conventional biological processes and various inorganic pollutants. Large sites that treat wastewaters usually deal with a large number of waters often characterized by different salinity contents, that could be potentially used to provide the energy necessary for the electrochemical remediation. Hence, in this work a reverse electrodialysis (RED) process for the treatment of synthetic wastewaters contaminated by organics, without energy inputs, using the salinity gradient of different wastewaters, was studied, for the first time. It was found that two synthetic wastewaters with different NaCl content can be effectively used in a RED system to drive anodic and cathodic processes for the removal of their organic contents without external energy supplies. The effects of salinity gradient, external resistance and set-up of the process was evaluated. Under optimized operating conditions, a fast and high removal of TOC (about 70% every hour) in the anodic compartment and a good stability of operating conditions for all the monitored time (10 h) were achieved. In addition, about 67% of the solution with high salinity used in the stack to provide the salinity gradient was effectively treated in the anodic compartment of the stack.

The coupling of Reverse Electrodialysis with Membrane Distillation is a promising option for the conversion of waste heat into electricity. This study evaluates the performances of the integrated system under different operating conditions, employing validated model and correlations. This work provides a detailed description of the behaviour of a real RED-MD heat engine and indicates the set of inlet concentrations, velocities and equipment size which returns the highest cycle exergy efficiency. These operating conditions were selected for the pilot plant developed within the EU-funded project RED Heat to Power. For the first time, a perspective analysis was also included, considering highly performing RED membranes and future MD module. Relevant results indicate that technological improvements may lead to interesting system performance enhancement, up to an exergy efficiency of 16.5%, which is considerably higher than the values reported in literature so far.

Aim of this chapter is that of describing in detail the REAPower project from the very beginning up to the testing of the pilot plant built at the end of the project.

In the membrane processes, a trans-membrane pressure (TMP) may arise due to design features or operating conditions. In most applications, stacks for electrodialysis (ED) or reverse electrodialysis (RED) operate at low TMP (&lt, 0.1 bar), however, large stacks with non-parallel flow patterns and/or asymmetric configurations can exhibit higher TMP values, causing membrane deformations and changes in fluid dynamics and transport phenomena. In this work, integrated mechanical and fluid dynamics simulations were performed to investigate the TMP effects on deformation, flow and mass transfer for a profiled membrane-fluid channel system with geometrical and mechanical features and fluid velocities representative of ED/RED conditions. First, a conservatively high value of TMP was assumed, and mechanical simulations were conducted to identify the geometry with the largest pitch to height ratio still able to bear this load without exhibiting a contact between opposite membranes. The selected geometry was then investigated under expansion and compression conditions in a TMP range encompassing most practical applications. Finally, friction and mass transfer coefficients in the deformed channel were predicted by computational fluid dynamics. Significant effects of membrane deformation were observed: friction and mass transfer coefficients increased in the compressed channel, while they decreased (though to a lesser extent) in the expanded channel.

In this work, we developed an advanced model useful for the design of profiled IEMs, based on the coupled simulation of local mechanical deformations and of fluid dynamics and associated mass transport phenomena within deformed channels

Reverse electrodialysis in closed loop configurations is a promising membrane technology in the energy conversion and storage fields. One of the main advantages of closed-loop reverse electrodialysis is the possibility of using a wide range of operating concentrations, flow rates and different salts for generating the salinity gradient. In this work, an original exergy analysis of the reverse electrodialysis process was carried out in order to investigate reverse electrodialysis performance in terms of energetic and exergetic efficiency parameters in a wide range of operating conditions. A mono-dimensional model of the reverse electrodialysis process was developed, in which all sources of irreversibility are considered, such as non-ideal membranes permselectivity, ohmic losses and uncontrolled mixing phenomena (salt and water diffusive flux across membranes). For each of them, the influence on the exergy efficiency is quantified and compared. Results also indicate how exergetic and energetic performance are largely dependent on solutions concentration: when high salinity gradient differences are used within the unit, membrane water permeability heavily affects process performance, thus reducing exergy efficiency, though a larger power output can be normally achieved. The more performing flow arrangement for the stack has been found to be the counter-current, though significant differences are observed only for long channels. Finally, performance is improved when short residence time within the stack is attained for the low-concentration solution.

In recent years, water and energy supply issues have boosted a noticeable interest in the scientific community on electromembrane processes such as electrodialysis and reverse electrodialysis. In order to gain an important place in the industrial market, technological challenges on various aspects are involved for the optimization of these processes. In this context, profiled membranes exhibit interesting performances and offer countless geometric alternatives. However, the mechanical behavior of the membranes and its interaction with fluid dynamics has been poorly investigated so far. In membrane-based processes, a trans-membrane pressure (Ptm) between the different solutions flowing through a module may be a design feature or may arise for various reasons, including flow arrangement and differences in physical properties, flow rate or friction coefficient. This leads to local deformations of membranes and channels, affecting flow and mass transfer characteristics, thus causing uneven distributions of flow and mass fluxes, which worsen the process performance. In this work, we developed an integrated model for the numerical simulation of local mechanical deformations and of fluid dynamics and associated mass transport phenomena inside deformed channels. Two diverse profiled membrane types (“overlapped cross filaments”, OCF, and “round pillars”, RP) were simulated under conditions representative of (reverse) electrodialysis and under the assumption of perfectly elastic behaviour. 3-D simulations of a couple of membranes and of the interposed fluid were conducted by the unit cell approach (periodic domain). The Ansys Mechanical 18 (Workbench) and the Ansys CFX 18 software was used. The selected geometries were simulated under Ptm ranging from -0.4 to +0.4 bar, computing expanded and compressed configurations. Then, CFD simulations of the deformed channels were performed, showing significant effects of the deformation on fluid flow and mass transfer. The influence of Ptm was to increase friction under compression conditions (up to ∼2.2-2.5 times) and to reduce it under expansion conditions (but to a lesser extent, i.e. up to ∼50-60%). Overall, compression enhanced mass transfer and expansion reduced it, but with smaller and more complex effects than on friction. The influence of the flow attack angle was negligible for friction, but more significant for mass transfer. In future works the same simulation approach will be adopted in order to compute also the Ohmic resistance in deformed configurations. The simulation results will be implemented in the form of correlations into higher-scale models, in order to study distributions of flow, mass transfer and Ohmic resistance in whole channels. The method proposed can be extended to other membrane applications with minor modifications.

The future energetic supply based on the massive use of renewable sources poses issues linked to fluctuations of power produced and consumed, thus requiring the use of energy storage systems.

Reverse electrodialysis (SGP-RE or RED) represents a viable technology for the conversion of the salinity gradient power into electric power. A comprehensive model is proposed for the RED process using sea or brackish water and concentrated brine as feed solutions. The goals were (i) reliably describing the physical phenomena involved in the process and (ii) providing information for optimal equipment design. For such purposes, the model has been developed at two different scales of description: a lower scale for the repeating unit of the system (cell pair), and a higher scale for the entire equipment (stack). The model was implemented in a process simulator, validated against original experimental information and then used to investigate the influence of the main operating factors and on power output. Feed solutions of different salinities were also tested. A good matching was found between predictions and experiments for a wide range of inlet concentrations, flow rates and feed temperatures. Optimal feed conditions, for the adopted system geometry and membranes, have been found employing brackish water (0.08–0.1 M NaCl) as dilute and brine (4.5–5 M NaCl) as concentrate to generate the highest power density at 40 °C temperature. The model can be used to explore the full potential of the RED technology, especially for any investigation regarding the future scale-up of the process.

The performance of a Reverse ElectroDialysis (RED) system fed by unconventional wastewater solutions for long operational periods is analysed for the first time. The experimental campaign was divided in a series of five independent long-runs which combined real wastewater solutions with artificial solutions for at least 10 days. The time evolution of electrical variables, gross power output and net power output, considering also pumping losses, was monitored: power density values obtained during the long-runs are comparable to those found in literature with artificial feed solutions of similar salinity. The increase in pressure drops and the development of membrane fouling were the main detrimental factors of system performance. Pressure drops increase was related to the physical obstruction of the feed channels defined by the spacers, while membrane fouling was related to the adsorption of foulants over the membrane surfaces. In order to manage channels partial clogging and fouling, different kinds of easily implemented in situ backwashings (i.e. neutral, acid, alkaline) were adopted, without the need for an abrupt interruption of the RED unit operation. The application of periodic ElectroDialysis (ED) pulses is also tested as fouling prevention strategy. The results collected suggest that RED can be used to produce electric power by unworthy wastewaters, but additional studies are still needed to characterize better membrane fouling and further improve system performance with these solutions.

Reverse electrodialysis (RE) is a promising technology for electric power generation from controlled mixing of two differently concentrated salt solutions, where ion-exchange membranes are adopted for the generation of ionic currents within the system. Channel geometry strongly influences fluid flow and thus crucial phenomena such as pressure drop and concentration polarization. Profiled membranes are an alternative to the more commonly adopted net spacers and offer a number of advantages: avoiding the use of non-conductive and relatively expensive materials, reducing hydraulic losses and increasing the active membrane area. In this work, Computational Fluid Dynamic simulations were performed to predict the fluid flow and mass transfer behaviour in channels with profiled membranes for RE applications. In particular, channels equipped with pillars were simulated. The influence of channel geometry on fluid flow and concentration polarization was assessed by means of a parametric analysis for different profile geometries. The unit cell approach along with periodic boundary conditions was adopted to simulate fully developed boundary conditions. Transport equations, valid also for concentrated solutions, were obtained from the rigorous Stefan–Maxwell equation along with the assumptions of binary electrolyte and local electroneutrality. Simulation results show that, in the geometries investigated here, the pumping power consumption is much lower than in a conventional net spacer and very close to that of the empty channel, while calm zones are generated by the profiles, which may accentuate polarization phenomena.

In the examined heat engine, reverse electrodialysis (RED) is used to generate electricity from the salinity difference between two artificial solutions. The salinity gradient is restored through a multi-effect distillation system (MED) powered by low-temperature waste heat at 100 °C. The current work presents the first comprehensive economic and environmental analysis of this advanced concept, when varying the number of MED effects, the system sizing, the salt of the solutions, and other key parameters. The levelized cost of electricity (LCOE) has been calculated, showing that competitive solutions can be reached only when the system is at least medium to large scale. The lowest LCOE, at about 0.03 €/kWh, is achieved using potassium acetate salt and six MED effects while reheating the solutions. A similar analysis has been conducted when using the system in energy storage mode, where the two regenerated solutions are stored in reservoir tanks and the RED is operating for a few hours per day, supplying valuable peak power, resulting in a LCOE just below 0.10 €/kWh. A life-cycle assessment has been also carried out, showing that the case with the lowest environmental impact is the same as the one with the most attractive economic performance. Results indicate that the material manufacturing has the main impact; primarily the metallic parts of the MED. Overall, this study highlights the development efforts required in terms of both membrane performance and cost reduction, in order to make this technology cost effective in the future.

In electro-membrane processes, a pressure difference may arise between solutions flowing in alternate channels. This transmembrane pressure (TMP) causes a deformation of the membranes and of the fluid compartments. This, in turn, affects pressure losses and mass transfer rates with respect to undeformed conditions and may result in uneven flow rate and mass flux distributions. These phenomena were analyzed here for round pillar-type profiled membranes by integrated mechanical and fluid dynamics simulations. The analysis involved three steps: (1) A conservatively large value of TMP was imposed, and mechanical simulations were performed to identify the geometry with the minimum pillar density still able to withstand this TMP without collapsing (i.e., without exhibiting contacts between opposite membranes), (2) the geometry thus identified was subject to expansion and compression conditions in a TMP interval including the values expected in practical applications, and for each TMP, the corresponding deformed configuration was predicted, and (3) for each computed deformed configuration, flow and mass transfer were predicted by computational fluid dynamics. Membrane deformation was found to have important effects, friction and mass transfer coefficients generally increased in compressed channels and decreased in expanded channels, while a more complex behavior was obtained for mass transfer coefficients.

The performances of reverse electrodialysis depend on several factors, including the nature of the electrode material and of the redox processes adopted to make possible the conversion between chemical potential and electric power. In this paper the possible utilization of various redox processes (reduction/oxidation of iron species, oxidation and reduction of water, oxidation of chlorine and reduction of water) was studied in a stack equipped with 10–50 cell pairs and by focused electrolyses in a three compartment cell. The effect of selected redox processes on power density output and eventual contamination of saline solutions flowing in the stack was evaluated in detail. The effect of the number of cell pairs and of the concentration of saline solutions was also investigated.

Reverse Electrodialysis Heat Engine (REDHE) is a promising technology to convert waste heat at temperatures lower than 100 °C into electric power. In the present work an overview of the possible regeneration methods is presented and the technological challenges for the development of the RED Heat Engine (REDHE) are identified. The potential of this power production cycle was investigated through a simplified mathematical model. In the first part of the work, several salts were singularly modelled as possible solutes in aqueous solutions feeding the RED unit and the corresponding optimal conditions were recognized via an optimization study. In the second part, three different RED Heat Engine scenarios were studied. Results show that power densities much higher than those relevant to NaCl-water solutions can be obtained by using different salts, especially those based on lithium ion (i.e. LiBr and LiCl). Results on the closed loop show efficiencies up to about 15% corresponding to an exergetic efficiency of about 85%, thus suggesting that the RED Heat Engine could potentially be a promising technology, with applications mainly in the industry where low-grade heat that has no alternative use can be converted into electricity.

Different computer-based simulation models, able to predict the performance of Reverse ElectroDialysis (RED) systems, are currently used to investigate the potentials of alternative designs, to orient experimental activities and to design/optimize prototypes. The simulation approach described here combines a one-dimensional modelling of a RED stack with a fully three-dimensional finite volume modelling of the electrolyte channels, either planar or equipped with different spacers or profiled membranes. An advanced three-dimensional code was used to provide correlations for the friction coefficient (based on 3-D solutions of the continuity and Navier-Stokes equations) and the Sherwood numbers (based on 3-D solutions of a scalar transport equation), as well as to test simple models for the Ohmic resistances (based on 3-D solutions of a Laplace equation for the electrical potential). These results were integrated with empirical correlations for the transport properties of electrolytes and membranes, and were used as the input for the higher scale model. The overall model was validated by comparison with experimental data obtained in laboratory-scale RED stacks under controlled conditions. This combined approach constitutes a fully predictive, potentially very accurate, and still extremely fast-running, tool for the approximate simulation of all the main variables, suitable for performance prediction and optimization studies.

In this paper an exergy analysis and thermoeconomic cost accounting of a Combined Heat and Power steam cycle integrated with Multi Effect Distillation-Thermal Vapour Compression plant is performed; the goal of the study is to show how these methodologies provide a rational criterion to allocate production costs on electricity and freshwater in such a dual purpose system. After a brief overview on the methodology and a description of reference plant, exergy analysis is carried out to calculate exergy flows and exergy efficiencies at component level. A detailed description of the adopted thermoeconomic model is given. In a first scenario, cost accounting is performed assuming that the concentrated brine is disposed back to sea, thus being its exergy content definitively wasted; furthermore, a sensitivity analysis is carried out in order to assess the changes in the unit cost of electricity and freshwater with several design and operation parameters. In a second scenario, conversely, part of brine exergy is used in a Reverse Electrodialysis unit to produce further electricity. In both cases results show that high unit costs are obtained for the material streams or energy flows which involve major exergy destruction along their production process, particularly freshwater in the former configuration and Reverse Electrodialysis electric output in the latter one.

Technologies for the exploitation of renewable energies have been dramatically increasing in number, complexity and type of source adopted. Among the others, the use of saline gradient power is one of the latest emerging possibilities, related to the use of the osmotic/chemical potential energy of concentrated saline solutions. Nowadays, the fate of this renewable energy source is intrinsically linked to the development of the pressure retarded osmosis and reverse electrodialysis technologies. In the latter, the different concentrations of two saline solutions is used as a driving force for the direct production of electricity within a stack very similar to the conventional electrodialysis ones. In the present work, carried out in the EU-FP7 funded REAPower project, a multi-scale mathematical model for the Salinity Gradient Power Reverse Electrodialysis (SGP-RE) process with seawater and concentrated brines has been developed. The model is based on mass balance and constitutive equations collected from relevant scientific literature for the simulation of the process under extreme conditions of solutions concentration. A multi-scale structure allows the simulation of the single cell pair and the entire SGP-RE stack. The first can be seen as the elementary repeating unit constituted by cationic and anionic membrane and the relevant two channels where dilute and concentrate streams flow. The reverse electro-dialysis stack is constituted by a number of cell pairs, the electrode compartments and the feed streams distribution system. The model has been implemented using gPROMS , a powerful dynamic modelling process simulator. Experimental information, collected from the FUJIFILM laboratories in Tilburg (the Netherlands), has been used to perform the tuning of model formulation and eventually to validate model predictions under different operating conditions. Finally, the model has been used to simulate different possible scenarios and perform a preliminary analysis of the influence of some process operating conditions on the final stack performance.

In the framework of Salinity Gradient Power technologies, Reverse ElectroDialysis (RED) is one of the most promising. In RED, two solutions of different salt concentration are fed into a series of alternate compartments, the one fed by the low salinity solution, the other by the high salinity solution. Compartments are separated by anion and cation exchange membranes alternatively piled to form a stack. Selective ion transport from the concentrate compartment to the dilute one across the membranes allows to generate an ionic current, which is eventually converted into electric current by means of suitable electrode compartments closing the stack. Several options for the feed solutions are possible. Natural waters were first considered as a potential source of salinity gradient, such as: river water and seawater or concentrated brines and low-salinity waters. Another alternative is the use of artificial solutions as feed solutions within closed-loop systems in which deployed salinity gradient of solutions exiting the RED unit is then regenerated by means of a regeneration step (e.g.: using low-grade heat, in RED heat engines; or , using electricity, in RED batteries). A nascent, still less explored alternative is the use of different industrial wastewaters as feed solutions. In fact, in several different scenarios, waste streams to be disposed can actually represent a very effective source of salinity gradient to be exploited before the natural mixing with the receiving water body occurs. In the present work a high salinity waste brine from a fish processing factory and a low-salinity water from a civil wastewater Membrane Bio Reactor (MBR) treatment plant were selected as feed solutions for an extensive investigation on a laboratory-scale RED unit (10 cell-pairs with a 10cmx10cm active membrane area). Firstly, the RED unit performance was characterized with artificial NaCl solutions prepared with the same NaCl concentration as the real streams, in order to have a comparison-reference for tests with real solutions. Subsequently, long-run tests with real feed solutions were performed (firstly feeding real brine and artificial low-salinity feed, then artificial brine and real low-salinity feed and, finally, feeding the stack with both real feed), operating the RED unit in an uninterrupted way for more than 2 weeks for each case. The time-evolution of system performance parameters (pressure drops, stack resistance, power output, etc.) was registered and analyzed in order to assess the relevant effect of the real feed solutions, likely related to fouling/scaling/plugging phenomena in the lab-stack. Also counter washing and chemical washing procedures were periodically (typically, once per week) implemented in order to highlight their effectiveness in restoring process performance. On overall, results collected over a period of more than 2 months, represent an original and unique source of information on the long-time performance of RED systems with such unconventional real feed solutions.

The use of closed-loop salinity gradient power (SGP) technologies has been recently presented as a viable option to generate power using low-grade heat, by coupling a SGP unit with a thermally-driven regeneration process in a closed loop where artificial solutions can be adopted for the conversion of heat into power. Among these, the closed-loop reverse electrodialysis (RED) process presents a number of advantages such as the direct production of electricity, the extreme flexibility in operating conditions and the recently demonstrated large potentials for industrial scale-up. Ammonium hydrogen carbonate (NH4HCO3) is a salt suitable for such closed-loop RED process thanks to its particular properties. At temperatures above 40–45°C, it decomposes into a gaseous phase containing NH3, CO2 and water. Thus, the use of NH4HCO3 solutions for feeding a RED unit would allow their easy regeneration (after the power generation step) just using low-temperature waste heat in a purposely designed regeneration unit. This work aims at presenting an experimental investigation performed on a RED system fed with NH4HCO3 solutions. Laboratory tests were carried out to find the best conditions for maximizing the power density and process performances of a RED unit by investigating a number of operating parameters such as fluid velocity and feed solutions concentration.

The simultaneous generation of electric energy and the treatment of wastewaters contaminated by an organic pollutant resistant to conventional biological processes, Acid Orange 7 (AO7), was achieved for the first time using proper redox processes by reverse electrodialysis using salinity gradients. The stack was fed with two aqueous solutions with different concentrations of NaCl and a synthetic wastewater contaminated by AO7. Various electrochemical approaches including electro-Fenton, electrogeneration of active chlorine (IOAC) and coupled process were performed in a stack equipped with 40–60 cell pairs and studied by focused electrolyses. The effect of the number of cell pairs and of the NaCl concentration of saline solutions was also investigated.

Reverse electrodialysis (RE) is a promising technology for electric power generation from controlled mixing of two differently concentrated salt solutions, where ion-exchange membranes are adopted for the generation of ionic currents within the system. Channel geometry strongly influences fluid flow and thus crucial phenomena such as pressure drop and concentration polarization. Profiled membranes are an alternative to the more commonly adopted net spacers and offer a number of advantages: avoiding the use of non-conductive and relatively expensive materials, reducing hydraulic losses and increasing the active membrane area. In this work, Computational Fluid Dynamic simulations were performed to predict the fluid flow and mass transfer behaviour in channels with profiled membranes for RE applications. In particular, channels equipped with pillars were simulated. The influence of channel geometry on fluid flow and concentration polarization was assessed by means of a parametric analysis for different profile geometries. The unit cell approach along with periodic boundary conditions was adopted to simulate fully developed boundary conditions. Transport equations, valid also for concentrated solutions, were obtained from the rigorous Stefan–Maxwell equation along with the assumptions of binary electrolyte and local electroneutrality. Simulation results show that, in the geometries investigated here, the pumping power consumption is much lower than in a conventional net spacer and very close to that of the empty channel, while calm zones are generated by the profiles, which may accentuate polarization phenomena.

The performances of electrodialysis (ED) and reverse electrodialysis (RED) processes depend on several factors, including the nature of the electrode material and of the redox couple adopted to make possible the conversion between electric power and chemical potential. In this paper, the possible utilization of iron-based redox couples (FeCl3/FeCl2, hexacyanoferrate(III)/hexacyanoferrate(II) and Fe(III)-EDTA/Fe(II)-EDTA) on graphite and DSA electrodes for RED processes was studied by a detailed experimental investigation. The hexacyanoferrate(III)/hexacyanoferrate(II) system was stable for lonf time (more than 12 days) in the absence of light and oxygen at high redox couple concentrations and low current densities both at compact graphite and DSA electrodes. Perfluorinated Nafion cationic membranes were found to be impermeable to the components of the redox couple. Fe(II)-EDTA exhibited a limited electrochemical stability in long term electrolyses at all adopted operative conditions, which discourages the use of the Fe(III)-EDTA/Fe(II)-EDTA for RED applications. The FeCl3/FeCl2 system was, on the other hand, stable for long times (more than 12 days) at acidic pH at compact graphite electrodes. Selemion anionic membranes allowed to confine the redox couple in the electrode compartments with very slow passage of protons to the side compartment (“dilute or concentrated compartment”).

Flows within very thin channels, typically filled with spacers, can be often encountered in many processes such as electrodialysis (ED) and reverse electrodialysis (RED). Although the ED and the RED processes have been studied for a long time, the optimization of the fluid dynamics within the channels is still an open problem. In the present work, realized within the EU-FP7 funded REAPower project, computational fluid dynamics simulations were carried out in order to predict the fluid flow field inside a single ED/RED channel. Some different configurations were tested which includes: an empty channel, a channel provided with a spacer, and a channel filled with a purposely manufactured fiber porous medium. Two types of spacers were investigated: (1) a commercial type made of woven perpendicular filaments and (2) an overlapped perpendicular filament spacer. A sensitivity analysis concerning computational grid size and topology was performed. For the cases investigated, adopting the hybrid grids mainly composed of hexahedral volumes was found to be more reliable and less computational demanding than tetrahedral grids. As concerns the dependence of the pressure drops on the flow rate, the empty channel was found to guarantee the lowest pressure drops at a given fluid flow rate, as expected. Conversely, the woven spacer filled channel was found to provide larger pumping costs. The pressure drops along the channel filled with a porous medium even at low flow rate were very high thus suggesting that this specific configuration may be unsuitable and that further investigations should be devoted to this topic.

We report experiments on a concentration cell with zinc electrodes and ZnCl2 solutions at different concentrations, separated by a porous diaphragm. The cell is aimed at the conversion of the free energy associated to the concentration difference into electrical energy, for renewable and clean energy applications. Usually, the diffusion of the solute across the diaphragm constitutes a waste of free energy, which impairs the voltage generation of the concentration cell with respect to other well-known techniques that work quasi-reversibly, such as reverse electrodialysis or the “mixing entropy battery.” Quite surprisingly, we find that the voltage produced by our concentration cell is significantly higher than the voltage obtained with the other quasi-reversible techniques. We show that the surplus voltage comes from the active transformation of the mixing free energy into electrical energy performed by the liquid junction, and we show the connection with the negative apparent transference number of the zinc ion. This fortunate consequence of using ZnCl2 solution is ultimately related to the formation of complexes. We present the results of a cell for power production, which has excellent performances with respect to known salinity-difference-power methods.

The RED Heat Engine operates as a closed-loop RED system, where limited amounts of artificial saline solutions are used as working fluids. The solutions exiting from the RED unit are regenerated by means of a separation step, which uses low-temperature heat as its energy source. Essentially the system converts low-grade heat to electricity. There are two main alternative regeneration strategies: (i) extract solvent from the high salinity solution and move it to the low salinity solution or (ii) extract salt from the low salinity solution and move it to the high salinity solution. In this thesis are considered the Multi-effect distillation (MED) and the Membrane Distillation (MD) under the solvent extraction strategy, while the use of thermolytic salts with distillation and absorption columns are considered for the solute extraction option. This thesis studied the RED Heat Engine perspectives in terms of: • the sources that can provide the heat necessary for running the system, indicating the characteristics that the system should have to match the market requirements • the life-cycle impacts, providing evidence for the environmental credentials of the technology, compared to alternative electricity supply options, and providing direction to system designs with the minimum environmental impacts. • economic performance, improving the understanding of the factors that affect it, including the design options, operational choices as well as the development over time of the key components in terms of performance and cost

Our society is undergoing a progressive change about the life style and habits. The world population is continuously increasing with 7.6 billion of human beings in 2018, resulting in an increasingly demand of resources in terms of food, water and energy. The exploitation of the planet resources since the first Industrial Revolution, results today in an unsustainable condition, which requires fundamental changes. In particular, in the energy sector the adoption of fossil fuels as the main energy source for human beings’ activities resulted in a strong impact on our planet, leading to climate changes and environmental pollution. Nowadays these aspects have induced society to a substantial challenge to find new sustainable energy sources for the future of human civilization. Low-grade thermal energy, derived from industrial or geothermal sources, represents an interesting resource for energy production. Indeed, huge amounts of low-grade thermal energy are available. Considering the industrial sector an amount ranging between 20-50% of the energetic input of industrial plants, are lost every day, in the form of hot gasses and liquid streams. However, the recovery and re-use of low-grade thermal energy or waste heat is limited due to the lack of efficient technologies for converting low-temperature heat sources into electrical power. Recently, Salinity Gradient Power Heat Engines (SGP-HEs) have been proposed as a viable process for the recovery of low-grade heat. In particular, this PhD thesis focuses on the analysis of Reverse Electrodialysis Heat Engines (RED-HEs), contributing to the activities of the European project “RED-Heat-to-Power” funded by the European Union’s Horizon 2020 Research and Innovation Programme (www.red-heat-to-power.eu). The aim of the project is to study and develop the first prototypes for the conversion of low-grade heat into electricity through a reverse electrodialysis (RED) unit. A reverse electrodialysis heat engine consists of two main units: (i) a power generation unit based on the reverse electrodialysis process, where the salinity gradient between two salt solutions is exploited to produce electricity, and (ii) a regeneration unit where low-grade heat is used to restore the salinity gradient of the reverse electrodialysis solutions exiting from the power generation unit. The restoring of the two solutions can be achieved by means of different strategies, e.g. solvent extraction and salt extraction, as summarized in the following. (a) In solvent extraction, the salt exchanged within the reverse electrodialysis unit is integrated by adding a part of the exhausted dilute stream to the exhausted concentrate stream, then, the resulting solution is fed to the regenerative unit where solvent is recovered by a thermal separation process (e.g. multi-effect distillation, membrane distillation) and transferred to the dilute solution. (b) In salt extraction, the exhausted dilute solution is fed to the regenerative unit where the salt is recovered by using low-grade heat as an energy source and transferred again to the concentrate solution. Rebalancing of the solvent is eventually carried-out to restore the solvent amount in the two streams. The main objective of this thesis is to provide a proof of the ground-breaking concept of a reverse electrodialysis heat engine, demonstrating the technological readiness level of such technology at a lab-scale prototype level. To this aim, both configuration, solvent and salt extraction reverse electrodialysis heat engines were investigated. The first part of this PhD thesis was dedicated to the development of a mathematical model for the RED process, based on a multi-scale modelling approach, considering two different scales of description: (i) a lower-scale model, describing the main phenomena involved in a single repeating unit (cell pair) and (ii) a higher-scale model related to the whole system, including all cell pairs and the relevant interconnections. The mathematical model was validated against experimental results and integrated with exergy analysis tools useful to evaluate the main causes and location of irreversibility sources. Ad hoc experimental campaigns were carried out in order to characterize the behaviour of different salt-water solutions, membrane properties and the operability of lab-scale units. In particular, a purposely-developed test rig was built in order to evaluate the osmotic and activity coefficients of salt-solutions from vapour pressure measurements, then used to determine novel data for caesium and potassium acetate salt solutions. This activity was performed at the University of Edinburgh, during a period abroad of six months. Furthermore, several experimental results on lab-scale reverse electrodialysis unit were carried out in order to provide membrane properties and experimental results for the mathematical model validation. In the case of solvent extraction strategies, validated mathematical models were developed for a reverse electrodialysis heat engine implementing either membrane distillation or multi-effect distillation as regeneration unit. These activities were performed in collaboration with the “Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT)” of Almeria. Finally, an exergy analysis at component level was performed for the integrated RED-MED HE, analysing the impact of the main operating conditions and membranes properties on the exergy efficiency of the system and identifing the main irreversibility sources. In the case of salt extraction strategies, thermolytic salts were selected and studied as working solutions of the RED-HE. Solutions of thermolytic salts have the peculiarity that their ions dissolved in water undergo a degradation process into gaseous species when the temperature is increased over a certain limit. Then, the dissolved thermolytic salt can be removed nearly completely from the solution by means of a thermal desorption process. The stripped gaseous species can be dissolved again in the aqueous solutions through an absorption step, resulting in an aqueous solution consisting of the very same ions of the thermolytic salt. Experimental test-rigs were designed, built and tested to investigate the stripping process of thermolytic salts. Mathematical models were developed in ASPEN Plus® and validated by comparison with experimental results. The model was also used to perform a sensitivity analysis and investigate the performance of the whole regeneration unit, including also the absorption process. Then, a mathematical model of a RED-HE fed by thermolytic salts was developed by coupling the ASPEN Plus® process model for the regeneration unit to the RED process model and used to perform sensitivity analyses. Finally, all the knowledge gained by theoretical and experimental works have made possible the design, construction and operation of the first world prototype of the thermolytic RED HE, demonstrating for the first time the feasibility of the process. The present thesis has been organised in order to cover the main aspects of the RED process and describe the aforementioned objectives. Chapter 1 introduces the concept of salinity gradient power (SGP) and related SGP-HEs, describing the SGP-HE concept and analysing the state of the art of the technology. Section I - Reverse Electrodialysis process Chapter 2 presents an overview of the reverse electrodialysis process, describing the technological fundamentals, experimental tests and modelling. Chapter 3 reports an original exergy analysis of the RED unit and the potential applications of the process in real environments. Chapter 4 presents the analysis on the influence of salt-solution properties on the RED-HE performance, reporting novel osmotic and activity coefficients for potassium acetate and caesium acetate water solutions. Section II - Solvent Extraction: RED with evaporative regeneration unit Chapter 5 and Chapter 6 are dedicated to the solvent extraction reverse electrodialysis heat engine. In particular, Chapter 5 reports the simplified mathematical models used to evaluate preliminary performances of (i) theoretical SGP unit fed by different salt solutions, (ii) RED-MED HE and (iii) RED-MD HE fed by NaCl solutions. Chapter 6 describes the advanced RED-MED-HE model developed to perform exergy analysis on the integrated system. Section III - Salt Extraction: Thermolytic salts Chapter 7 and Chapter 8 reports the analysis on thermolytic salts. In particular, Chapter 7 is focused on the regeneration unit of thermolytic salts presenting mathematical models and experimental assessments. Chapter 8 presents the first operating thermolytic RED heat engine (t-RED HE) and a perspective analysis based a validated process model for the t-RED HE.

In recent years, water and energy supply issues have drawn the attention of the scientific community to electromembrane processes. Electrodialysis (ED) and Reverse Electrodialysis (RED) are two of the most attractive electromembrane technologies for water desalination and electric energy production from salinity gradients, respectively. In order to gain an important place in the industrial market, technological challenges on various aspects are involved in the optimization of these processes. In this context, profiled membranes exhibit interesting performance. However, the mechanical behavior of the membranes and its interaction with fluid dynamics has been poorly investigated so far. In membrane-based processes, a trans-membrane pressure (TMP) between the different solutions flowing through a module may be a design feature or may arise for various reasons (e.g. flow arrangement, differences in physical properties). This may lead to a local deflection of membranes due to their low mechanical stiffness. As a result, the channel geometry may be modified affecting flow and mass transfer characteristics. In this work, we developed an integrated model for the numerical simulation of local mechanical deformation and of fluid dynamics and associated mass transport phenomena inside deformed channels. Profiled membranes with Round Pillars were simulated under typical REDED working conditions. Membranes were assumed to be with perfectly linear elastic behaviour. 3-D simulations of a couple of membranes and of the interposed fluid were conducted by the unit cell approach (periodic domain). The Ansys Mechanical 18 (Workbench) and the Ansys CFX 18 software was used. The profiled membranes were investigated in both expanded and compressed configurations by varying the TMP of 0.1 bar steps from -0.4 to +0.4 bar. Then, CFD simulations of the deformed channels were performed. The influence of TMP was to increase friction under compression conditions (up to 2.2 times) and to reduce it under expansion conditions (i.e. up to 60%). Overall, compression enhanced mass transfer and expansion reduced it, but with smaller and more complex effects than on friction. Channel deformations affect largely fluid dynamics and mass transfer characteristics. Even mild trans-membrane pressures may produce significant variations in the overall process performance. Results will be implemented in a novel higher-scale simulation tool for studying the distribution of flow in whole channels.

Since 70s, the world has been becoming more and more concerned about energy and environmental issues. The need of meeting the current energy demand by guaranteeing the ability of future generation to satisfy theirs have been pushing research not only to exploit alternative energy sources such as renewable energies, but also to improve the efficiency of energy conversion systems. To this regard, developing systems that use efficiently energy is of paramount importance especially in those countries which rely on non-renewable energy sources such as oil, natural gas, and coal. In fact, the reduction of energy consumption will lead not only to the saving of unrestorable and limited energy resources but also to some environmental benefits, such as the reduction of carbon dioxide emissions. To this am, a lot of methods have been developed across scientific research. Among them, Exergy Analysis and Thermoeconomics are well-recognized for supporting the analysis and design of any energy conversion system. This thesis deals with innovative applications of Exergy Analysis and Thermoeconomics in chemical, thermal and cooling energy conversion systems. It aimed at demonstrating the capabilities of both methods to provide insights into design alternatives and operation strategies of energy conversion systems. Exergy Analysis of Reverse ElectroDialsysis process is carried out. This innovative energy system converts salinity gradient into electricity, by interposing selective membranes between two solutions at different concentrations. The analysis aims at providing insights into the effects on the exergy performance of the current membrane properties and of some design and operating parameters. Results will be useful for the future development of this system. A further exergy analysis is focused on the integration of Reverse Electrodialysis process with Multi-effect Distillation Unit. This innovative system allows for the conversion of low-grade heat into electricity. Efforts are still needed to improve the energy performance of this system and to this aim, exergy analysis allows for evaluating the effect of some design and operating parameters. Thermoeconomic Cost Accounting is applied to Multi-Effect Desalination (MED) process, of which energy consumption highly affect the operating costs. In order to gain insights into the cost formation process of the produced freshwater, a thermoeconomic analysis of this system is carried out by adopting a high disaggregation level. The analysis allows for understanding at which extent the irreversibility occurring at each subprocess affects the cost of freshwater. Then, the same system is supposed to be operated in a cogenerative asset realized by coupling it with a steam power plant. In this case, Thermoeconomics provides a method for apportioning cost on electricity and freshwater produced. Another innovative method investigated in this thesis is the Thermoeconomic Diagnosis of energy systems. This Fault Detection and Diagnosis Technique has been extensively applied to thermal power plants, but only recently it has been extended to Heating, Ventilation and Air Conditioning Systems (HVAC). With this respect, most of results achieved so far have been based on a set of virtual-experiments thus not allowing to account for real operation of these system. For the first time, the method is tested using experimental data obtained from a packaged rooftop air conditioning unit. Last but not least, the applications of exergy-based methods to environmental analyses is considered. To this regard, an innovative approach based on integration of Thermoeconomics and Life Cycle Assessment (LCA) is here proposed. The procedure combines the capabilities of these two techniques to account simultaneously for aspects related to thermodynamics of energy conversion processes and to the overall impacts along the plant life cycle, i.e. from raw material extraction to the disposal of facilities. The capabilities of this approach are illustrated by applying it to a water-cooled vapor compression chiller.

Reverse Electrodialysis (RED) is a technology for generating electricity from the difference in salinity between two solutions. RED is usually applied to natural water streams with different salinities, like seawater vs. freshwater. In the RED Heat-to-Power project we explore the option of using artificial water solutions operating in a closed loop where the difference in salinity is regenerated in a separation step powered by heat at temperature ranges between 60 and 100°C. We call this system Reverse Electrodialysis Heat Engine (RED HE). In this paper, first we summarise the possible system configurations and the overall amount of excess heat available in Europe for powering the RED HE process, as described in our previous publications. Then we take a closer look at specific sites, where a RED HE engine could be potentially applied, assessing the amount of waste heat that can be technically and realistically recovered and sizing the RED HE system for those applications. The case studies include the excess heat recovery from: 1. a typical large-sized pulp and paper industry based in Sweden 2. a typical medium-sized food industry as described within the IEE project GREENFOODS 3. a relatively large biogas plant in Germany 4. the on-board auxiliary engine of a medium-sized bulk carrier 5. a gas compressor station in Poland. The results show that the RED HE is suitable to be used by all those sectors. For most cases, the typical heat engines would be of relatively small size, because of the Carnot limits for temperatures at 100°C or lower: Industry 150-600 kW, Biogas plants 5-20 kW, Marine 4-17 kW. On the other hand, the gas compressor stations are suitable for larger applications, in the range of 2-8 MW.