As the world’s energy demands
increase, Yale University researchers are examining alternative and sustainable
power generation techniques. Menachem Elimelech, the Roberto C. Goizueta
Professor of Chemical and Environmental Engineering, has published extensively
on using engineered osmosis to address the growing demand for energy. Now, in a
co-authored piece with Professor Bruce Logan of the Pennsylvania State University
in Nature, Elimelech examines three water-based methods for electricity
generation and the challenges that must be met before they can be used for
widespread application.
The researchers highlight
pressure-retarded osmosis (PRO), which uses the flow of water through membranes
to produce pressurized water. The water than generates electricity using
mechanical turbines. Past challenges to PRO have included inadequate flow from
existing membranes; cellulose-acetate membranes designed specifically for use
with PRO systems have significantly increased the achieved power density, while
the researchers suggest that thin-film composite polyamide membranes will allow
additional power density breakthroughs.
Membranes continue to be the main
challenge to widespread use of PRO, however, with a need for both low-cost
materials and those that offer minimal internal concentration polarization,
which decreases power density. In addition, the researchers note that PRO
requires extensive pretreatment of water to avoid fouling the membranes.
In examining reverse electrodialysis
(RED), where electricity is generated directly from the difference in salinity
between seawater and fresh water, the researchers note that water splitting, an
element of the RED process, releases toxic chlorine gas from seawater, as well
as potentially explosive hydrogen gas. The engineers suggest, however, that
iron-based oxidation-reduction, or redox, could remove the need to split water
in RED processes. They also highlight advances in increasing the approach’s
power density as well as energy efficiency based on improvements in both
membrane materials and their spacing and architecture. They note that the cost
of ion-exchange membranes remains the primary challenge for commercial use of
RED, but cite the development of new materials and increased production of
membranes over the last decade as a driving factor in cost decreases, and
speculate that a global increase in demand could lower costs.
Finally, the engineers examine
microbial fuel-cell (MFC) technologies that generate energy from organic
materials in wastewater, which can effectively make a wastewater treatment
plant a power plant as well. While low power densities have limited efforts for
the use of MFC systems in competition with other wastewater treatment systems
in the context of power generation, the researchers note that modified MFCs can
also be used for chemical remediation and produce hydrogen, methane, and
acetate. Still, widespread use of such systems is challenged by electrode cost
and longevity as well as instability and cost of cathodes.
The researchers conclude that all
three of the examined processes require inexpensive, effective, and uniquely tailored
membranes to achieve widespread use. With such advances, they suggest that
salinity gradients, wastewater and biomass provide available sources of energy
for renewable and sustainable power generation.
Source: Yale University