Blue Energy and Its Potential: The Membrane Based Energy Harvesting

Abstract

The present energy generation is largely dependent on fossil fuels which results in the emission of greenhouse gases and is also characterized by vulnerability and eminent scarcity. In order to meet the respective concerns, the energy supply should be based on (i) an environmental-friendly non-combustion energy conversion, (ii) a freely available alternative energy source, and (iii) a renewable energy source. In this chapter, the authors want to explore an alternative and the hardly known renewable energy source, i.e. salinity gradient energy. It is the most promising renewable energy source and also termed as 'blue energy'. Estimates from literature predicted coverage of over 80% of the current global electricity demand when applied in all river mouths. From thermodynamic calculations, it can be derived that each m 3 of river water can yield 1.4 MJ when mixed with the same amount of seawater. Two membrane-based processes are available to convert blue energy into electricity: Pressure retarded osmosis (PRO) and Reverse electrodialysis (RED). Blue energy along with its technical and economic potential would be the major focus of this chapter.

Preferential Ion Adsorption in Blue Energy Applications

“…Comparing the optimal energy and the operation conditions for f < 0 and f > 0, we find that the optimal energy for f < 0 (30.7 mJ/m 2 ) is larger than that for f > 0 (28.0 mJ/m 2 ). For f < 0, the globally optimal value for W M occurs with βϵ ≈ 2.8 and H/σ ≈ 3; however, the peak is broad, so the values of βϵ ∈ [2,3] result in similar values for W M . The potential difference ΔV M decreases as βϵ increases for all pore sizes for f < 0 as expected, while the behavior of ΔV M as βϵ increases for f > 0 is a function of the pore size.…”

“…Renewable alternatives to fossil fuels are urgently needed to address energy shortages and avoid further adverse environmental consequences . The ocean provides several potential renewable energy sources, including wave power, ocean thermal energy, tidal energy, marine current energy, and salinity gradient energy. , Salinity gradient energy (or the so-called “blue energy”) is the energy extracted from mixing freshwater and saltwater. Such mixing occurs in large quantities when rivers flow into the ocean, whereby part of the released free energy of the mixing process can be converted to useful work.…”

Harvesting Blue Energy Based on Salinity and Temperature Gradient: Challenges, Solutions, and Opportunities

“…Lower concentration feedwater (i.e., river water) as a flowing aqueous stream meets highly saline draw solution (i.e., sea water) and consequently produces brackish water with moderate salinity. It is possible to quantify the Gibbs free energy of such mixing as Δ G mix = G b − false( G normalf + G normald false) where G b is the Gibbs free energy of the resulting brackish water, and G f and G d are the Gibbs free energies of feedwater and draw solutions (J/mol), respectively. The Gibbs energy of mixing for ideal solutions of different salinities can be determined by the change in standard molar entropy (assuming Δ H mix = 0) by the following equation: Δ G mix = − false( n normalf + n normald false) T Δ S mix , b − false( prefix− n normalf T normalΔ S mix , normalf − n normald T normalΔ S mix , normald false) where n is the number of mixed moles and Δ S mix is the molar entropy of mixing (J/mol K) and is calculated as Δ S mix = − R ∑ i x i ln ⁡ x …”

“…As discussed above, concentration polarization significantly decreases the flow of water across the membranes, which diminishes power generation. The theoretical water flux ( J w , L/m 2 h, LMH) through the membrane is evaluated as a function of the osmotic pressure difference between two solutions (Δπ, bar), the hydrostatic pressure difference (Δ P , bar), and the membrane water permeability coefficient ( A , LMH/bar): J w = A false( normalΔ π eff − normalΔ P false) …”

“…The high membrane price is a major downside of most membrane-based conversion techniques. In addition to the technical advancement of membrane properties, the economic viability must be carefully assessed to make the RED technology feasible . In this regard, the cost of ion-selective membranes is a major obstacle to the commercial viability of RED technology.…”