Environmental Engineering Reference
In-Depth Information
1 Introduction
Energy is not only the material foundation that sustains the advance of human
civilization, but also the indispensable requirement for the development of modern
society. With the rapid growth of the global economy, our society has become
increasingly interdependent on energy. As data show 70 % of the world's annual
energy consumption comes from fossil fuels, there can be no doubt that our
reserves of fossil fuels are finite, and the resulting environmental pollution is
worsening. Thus, we need to promote the harnessing of clean, renewable energy
sources such as solar, wind, geothermal, and tidal energy. However, these
renewable resources are commonly intermittent in time and diffusive in space; the
electricity produced must be stored and made available on demand. So large-scale
electric energy storage (EES) systems will be an essential part of the future
renewable energy-based grid. Meanwhile, another pressing demand is to find
appropriate
EES
for
the
electrification
of
transportation
that
does
not
emit
greenhouse gases.
Electrochemistry can contribute to EES in several diverse ways. Electro-
chemical energy storage technologies include flow redox batteries, super capaci-
tors, and rechargeable batteries (Pb-acid, Ni-Cd, Na-S, and Li-ion batteries, etc.),
showing great advantages of high efficiency, low cost, and flexibility. Hereinto, Li-
ion batteries have currently been considered as one of the most promising tech-
nologies due to their long lifetime and high energy density. However, for wide-
spread EES applications, there is increasing concern about the cost and limitation
of lithium terrestrial reserves. As a result, great efforts have been made to explore
new low-cost and reliable electrochemical energy storage technologies.
Due to the global lithium resource constraint, scientists have turned their
attention to rechargeable Na-based batteries. As Na is positioned in the same
period with Li, its electrochemical properties are similar to Li. Na has very neg-
ative redox potential (-2.71 V, vs. SHE) and a small electrochemical equivalent
(0.86 gAh
-1
), which makes it the most advantageous element for battery appli-
cations after lithium. Na element instead has very abundant and widespread supply
as the crustal abundance of Na is 2.64 %, compared to 0.006 % of Li. Moreover,
there is 3.5 % of NaCl in the ocean, showing inexhaustible Na resource, thus Na is
an ideal energy storage material.
In the past decades, high-temperature Na battery system such as Na/S and Na/
NiCl
2
(ZEBRA battery) systems have been commercially developed for electric
vehicles and MWh scale electric storage due to their high energy density and low
cost. Na/S batteries produced by NGK Ltd. in Japan have entered in the market
since 2000. The biggest units with a power output of 8 MW have been built and
more than a hundred Na/S energy storage stations are operating over the globe.
The biggest market of ZEBRA batteries is thought to be transportation. Benz
Corp in German has carried out longtime tests since the 1990s, where the index
of the ZEBRA batteries can fulfill the medium-term target of USABC. The
ZEBRA battery powered vehicles have passed over 3,200 thousand miles' test.
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