Energy is one of the most important issues in the world owing to the steady growth of energy demands, serious concern about fossil fuel exhaustion, and environmental problems. Reverse electrodialysis (RED), which is a technique to capture the useful energy from mixing the solutions of different concentration, is recently recognized as an efficient method to alleviate the current issue associated with the energy. This chapter reviews the RED studies using nanoporous membranes (named as nanofluidic reverse electordialysis, NRED) in place of conventional polymer membranes having no physical pore. This chapter is made up six parts: the mechanism, the fabrication of nanostructures, the influence of operating parameters (concentration gradient and pore size), the materials, the efforts on enhancing performance (surface modification technique and large area platform), the numerical modeling, and, lastly, the feasible applications.
TopIntroduction
Global energy demand has been consistently increasing. According to the report prepared by the U.S. Energy Information Administration (EIA), world energy consumption will grow by 56 percent from 2010 (524 quadrillion British thermal units (Btu)) to 2040 (820 quadrillion Btu). Most of the enlargement in energy consumption occurs in non-organization for economic co-operation and development (non-OECD) countries due to their expanding economy, as shown in Figure 1 (U. S. Energy Information Administration (EIA), 2013).
Figure 1.
Global total energy consumption. The blue bar represents the OECD countries and the red bar is for the non-OECD countries (U.S. EIA, 2013).
Fossil fuels like oil, natural gas, and coal are currently the main energy resources. It is expected that these will be still responsible for more than 80% of the world energy consumption in the future. The fossil fuels, however, are limited resources albeit with the increase in discoverable reserves due to technological advances. Concerns about their depletion (peak oil theory) have been continually raised by many experts during several decades (Mohr & Evans, 2008; Bardi, 2009). Environmental impacts such as climate change issues, resulting from fossil fuel-produced emissions, is also considered as a serious problem. Many countries implement emission regulations to protect the environment (Christoph et al., 2005).
The best solution for such a problems would be to use an energy resource which is sustainable and nonpolluting. Hence, various renewable energy resources including solar, wind, geothermal, hydraulic and ocean energy, have been actively studied (Lund, 2010; Panwar et al., 2011; Shi & Chew, 2012; Esteban & Leary, 2012; Kim et al., 2012; Alkan et al., 2013; Islam et al. 2013; Mahian et al., 2013; Redweik et al, 2013; Nolan et al. 2014). All the techniques are based on energy conversion to obtain a useful energy from intrinsic energy forms like potential, kinetic, and heat having renewable energy resources.
Renewable energy can also be captured with mixing two solutions having different concentrations (Vermaas, 2014). This is relatively an unconventional energy source, which many people have ignored. There however is a huge amount of potential energy. The most promising place for this concentration gradient energy is estuaries where river meets sea. The global river runoff is presently about 37300 km3/year and therefore the theoretical potential energy is estimated to be 1.4~2.6 TW, which is comparable to the current generation of electricity (Vermaas, 2014). Figure 2 shows the potential energy at the major rivers of the world.
Figure 2. Potential energy at the globally major rivers