Elsevier

Desalination

Volume 453, 1 March 2019, Pages 77-88
Desalination

Towards the first proof of the concept of a Reverse ElectroDialysis - Membrane Distillation Heat Engine

https://doi.org/10.1016/j.desal.2018.11.022Get rights and content

Highlights

  • A comprehensive performance analysis of a RED-MD heat engine was carried out.

  • RED and MD units were modelled via validated theoretical model and correlations.

  • The RED-MD system efficiency shows a maximum at Chigh,in = 2 M and Clow,in = 0.01 M.

  • A high-performance system was simulated improving RED membranes and MD heat recovery.

  • A maximum exergy efficiency of 16.5% was achieved with highly-performing equipment.

Abstract

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.

Section snippets

Introduction and literature review

Nowadays, an increasing research interest is devoted to the exploitation of salinity gradient energy, i.e. the chemical energy associated to the mixing of two solutions at different salt concentration. Among the SGP technologies, two have reached a significantly high technology readiness level, namely Pressure Retarded Osmosis (PRO) and Reverse Electrodialysis (RED) [[1], [2], [3], [4], [5]]. The former employs membranes, which are selective toward the passage of the solvent, while the latter

Mathematical modelling

The layout of the coupled RED-MD system is sketched in Fig. 2. The integrated system was designed considering the two processes as connected in a discontinuous way, due to the different scale (or working times) of the two units employed, a commercial RED stack and commercial MD modules.

More precisely, the whole system operates cyclically. In each working cycle, the RED unit is supposed to work in continuous for 5 h, during that a dilute and concentrate solutions are fed in the unit with fixed

Results and discussion

The results section consists of two main parts: one is devoted to describing the performances of an integrated system composed of a real RED and a real MD unit. Conversely, the second part deals with the combination of highly performing equipment. All the simulations were performed fixing some parameters, e.g. RED stack number of cell pairs (equal to 50) and RED membrane width and length (i.e. 0.1 and 0.88 m, respectively), which correspond to the size of a real RED equipment. In all cases, the

Conclusions and future perspectives

In this work, an integrated RED-MD system is investigated by carrying out modelling activities. For the first time, a multi-scale model for Reverse Electrodialysis units has been coupled with correlations derived from a model describing a commercial MD unit along with suitable mass and energy balances. The aim of the work was that of identifying the operating conditions and the process configurations maximizing the cycle efficiency in order to drive the design of the first RED-MD heat engine

Nomenclature

    A

    membrane area [m2]

    aw

    water activity [−]

    b

    membrane width [m]

    C

    solution molarity [mol/m3]

    AΦ, α, b

    Pitzer's model constants [(kg/mol)1/2]

    fγ, Bγ, Cγ, BΦ

    Pitzer's model coefficients [−]

    AΛ, ΒΛ, ΧΛ

    coefficients of Jones and Dole's equation

    D

    salt diffusivity in solution [m2/s]

    Dmembr

    salt diffusivity through the membrane [m2/s]

    ΔP

    distributed pressure drops [Pa]

    E

    voltage [V]

    f

    friction coefficient [−]

    i

    current [A]

    J

    molar flux [mol/(m2 s)]

    J'

    volumetric flux [m/s]

    m

    solution molality [mol/kg]

    Mw

    molecular weight [g/mol]

    Ncell

Acknowledgements

This work was performed within the RED-Heat-to-Power project (Conversion of Low Grade Heat to Power through closed loop Reverse Electro-Dialysis), funded by EU within the Horizon 2020 research & innovation programme, grant agreement No: 640667. www.red-heat-to-power.eu.

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