Sustainable Energy from Salinity Gradients

Andrea Cipollina, Giorgio Micale, Andrea Cipollina, Giorgio Micale

• Publisher: Woodhead Publishing
• Year: 2016
• ISBN:
  9780081003237
• Topic: Technology & Engineering
• Subtopic: Renewable Power Resources
• Index: Technology & Engineering

Salinity gradient energy, also known as blue energy and osmotic energy, is the energy obtainable from the difference in salt concentration between two feed solutions, typically sea water and river water. It is a large-scale renewable resource that can be harvested and converted to electricity. Efficient extraction of this energy is not straightforward, however. Sustainable Energy from Salinity Gradients provides a comprehensive review of resources, technologies and applications in this area of fast-growing interest.

Key technologies covered include pressure retarded osmosis, reverse electrodialysis and accumulator mixing. Environmental and economic aspects are also considered, together with the possible synergies between desalination and salinity gradient energy technologies.

Sustainable Energy from Salinity Gradients is an essential text for R&D professionals in the energy & water industry interested in salinity gradient power and researchers in academia from post-graduate level upwards.

For more than ten years the Editors have been sharing substantial research activities in the fields of renewable energy and desalination, successfully participating to a number of European Union research projects and contributing to the relevant scientific literature with more than 100 papers and 2 books on Desalination technologies and their coupling with Renewable Energy. They are intensely working in the field of Salinity Gradient Power, carrying out research with specific focus o.n open-loop and closed-loop reverse electrodialysis and pressure retarded osmosis.

• Covers applications of pressure retarded osmosis, reverse electrodialysis, and capacitive mixing for salinity gradient power in one convenient volume
• Presents the environmental aspects and economics of salinity gradient energy
• Explores possible synergies between desalination and salinity gradient energy

1

Salinity gradient energy

G. Micale; A. Cipollina; A. Tamburini Università degli Studi di Palermo, Palermo, Italy

Abstract

Beyond the most common renewable energy sources today exploited for the production of clean energy, salinity gradients power (SGP) has been attracting the increasing interest of scientists and companies involved in the field. This chapter provides an introduction to SGP, reporting a brief history of the technological developments throughout the years, from the beginning to present. A number of different SGP technologies have been developed in the last decades, all based on the concept of harvesting the energy from the controlled mixing of two solutions at different salinities. A theoretical analysis of the energy potential for SGP places this renewable source of energy among those capable of playing a significant role in the energy sector in the next decades. A classification of all SGP processes is also reported on the basis of: mixing process, energy conversion, and transported species. Finally, an outline of all book chapters is presented.

Keywords

SGP; Gibbs free energy of mixing; Potential; Water-energy nexus; Processes classification

Chapter Outline

1.1 Some history on salinity gradient energy technologies 1

1.2 Theoretical analysis of world potentials for SGE technologies 6

1.3 Classification of SGP technologies 8

1.4 Outline of chapters 12

References 15

Nomenclature

c molar concentration (mol/m³)

G Gibbs free energy of the solution (J)

n number of moles (mol)

P pressure (Pa)

R ideal gas constant (J/(mol · K))

s total number of species

T temperature (K)

V volume of the solution (m³)

x molar fraction

Greek symbols

γ activity coefficient

ΔG variation of the Gibbs free energy (J)

μ chemical potential (J/mol)

Superscript

* of the pure solvent

ω of the solute at infinite dilution

Subscripts

c of the concentrate, or in the concentrate

d of the dilute, or in the dilute

i of the i^th species

m of the mixture, or in the mixture

mix related to the mixing process

tot total (i.e., relevant to all species)

Abbreviations

AccMix accumulator mediated mixing process

AEM anionic exchange membrane

CAPMIX capacitive mixing

CDP capacitive Donnan potential

CEM cationic exchange membrane

DES desalination

ED electrodialysis

ERS electrode rinse solution

GMVP global membrane distillation, valuable resource recovery, and pressure retarded osmosis

HG hydrocratic generator

IEM ion exchange membrane

MEB mixing entropy battery

MFC microbial fuel cell

PRO pressure retarded osmosis

RED reverse electrodialysis

R&D research and development

RO reverse osmosis

RVC reverse vapour compression

SGE salinity gradient energy

SGP salinity gradient power

SSH swelling and shrinking of hydrogels

1.1 Some history on salinity gradient energy technologies

In the 21^st century mankind has to face very fundamental challenges: energy, water, and food must be made available in increasing amounts for the world's growing population. Accomplishing such formidable tasks poses the main issue of sustainability for present and future generations. The quest is open to the broadest exploitation of novel solutions that could turn in the not too distant future into new, sustainable ways to guarantee the availability of such life-essential items.

It should be further observed that the water–energy–food nexus clearly indicates a loop chain where water and energy are both fundamental for food production, water is needed for energy production, and energy is needed for water supply and production (e.g. desalination). As a consequence of this nexus, projections indicate that energy demand is very swiftly growing, with a projected increase of up to 50% by 2035 (Fig. 1.1) (IEA, 2010; FAO, 2011a, b). Such a considerable increase would not be sustainable by the use of fossil fuels (e.g. oil, coal, natural gas), thus posing the priority to develop alternative routes to sustainable production of energy.

Fig. 1.1 Water–energy–food nexus with projections for 2035.

Renewable energy can be harnessed from different sources: solar, wind, geothermal, biomass, hydro, tidal, wave, and marine currents energy. Beyond these, a lesser-known form of renewable energy is the so-called Salinity Gradient Energy (SGE) or Salinity Gradient Power (SGP) (Logan and Elimelech, 2012; Jones and Finley, 2003). This form of energy is available whenever two solutions with different salinity levels are mixed together, as occurs in nature when a river discharges into the sea. Of course the spontaneous mixing of river water into the sea results in the complete dissipation of the energy associated with the mixing process. Conversely, the harnessing of this energy would require a suitable device able to perform a ‘controlled mixing’ of the two streams at different salinity (e.g. river water and seawater). Such operation would thus result in the recovery of the energy available rather than its complete dissipation. Sustainability of SGE is ensured by the hydrological cycle, which guarantees the reestablishment of the original streams and salinity levels. Thus, SGE is a very clean form of renewable energy that does not produce any emission of CO₂ and does not consume the salts contained in the streams. Furthermore, it is suitable for continuous power production as i...