Environmental Engineering Reference
In-Depth Information
Fig. 4.1
Secondary battery
vs. fuel cell in the hydrogen
energy system
energy which, in theory, may be converted to work with an efficiency of 100 %.
Also, unlike the direct use of solar energy itself, electricity can be transported by
transmission lines. However, electricity is not suitable for demands varying over
time. When supply and demand are different, some means of storing excess supply
or providing at times of shortage is also required.
As one means for compensating for the fluctuation of electrical energy obtained
from sources such as wind power and solar cells, development of secondary (or
rechargeable) batteries is thriving. In addition to the classical lead-sulfur batteries,
sodium-sulfur batteries have entered practical use for the purpose of leveling large
power fluctuations. In recent years, lithium-ion batteries have started to be used
as a power source for electric vehicles or hybrid cars, which can also contribute to
averaging energy demand. Several new types of batteries have been proposed to
achieve larger capacity per volume or per weight and shorter charge/discharge time.
In the large scale wind and solar power renewable energy future, it will be neces-
sary to store a greater proportion of the power. However, battery function is pro-
vided by the electrode active material in the battery cell, so to accumulate a larger
amount of electricity over longer periods, it is necessary to use a larger battery
cell body. From the point of initial investment and resource consumption footprint,
to increase capacity and size endlessly is impossible, which creates the demand
for more effective means of storing renewable energies on a large scale. The con-
cept of a hydrogen energy system or 'hydrogen society' may provide a solution to
these problems (Fig.
4.1
). Hydrogen can be produced from water via electrolysis
or other methods, and directly or indirectly from renewable energy resources. It
can be stored in a tank as a pressurized gas or as a liquid, or in a form of hydride
compound. When used, it emits only water. Hydrogen thus can be regarded as a
potentially ideal energy carrier and storage medium.
The essential device to complete the hydrogen energy system, by converting the
chemical energy of hydrogen to electricity, is the fuel cell. A fuel cell consists of
electrochemical cells as the secondary battery, but without having active materi-
als inside. Since the fuel and oxidant (air) can be continuously supplied from the
outside, the size of the fuel cell is not limited by the stored energy but only by the
