Tidal Current Energy

266 J. Salminen et al.
Cathode
Figure 15.3 . A battery discharging under load.
Table 15.3 .
Some primary battery types .
Electrolyte
or ionic
conductor
LOAD
Electrically powered
system or device
Anode
� �
Cell type General applications Operating potential/V Energy density/(W�h�kg � 1 )
Zinc–MnO2 [15]
Zinc–air [15]
Lithium coin cell
(Li–MnO 2 ) [16]
Portable electronics 1.5 130–160
Hearing aids, lightweight
applications
1.3–1.4 300
Small, low-power
electronics (watches,
time keeping)
3.0 280
secondary cells. Since these are plated electrodes, they are extremely susceptible
to dendritic growth upon recharge and are therefore cannot be recharged in a safe
or effective manner. Secondary batteries typically undergo chemical conversions
that do not radically alter the physical nature of the electrodes.
Overall, batteries have lower energy densities than liquid fuels and their
power output cannot compete with combustion engines or gas turbines for
large-scale production. However, for very small devices (10 cm 3 ) the benefits of a
closed system and a system where the reactor and the storage tank are coupled
are great. Batteries require no plumbing as it is not necessary to shuttle reactants
to and from the cell. Batteries are also not prone to sudden mechanical failures.
These attributes make batteries particularly well suited to military, medical and
biological applications, where size and reliability are of paramount concern.
Primary batteries are intended for short to moderate lifetime applications
(a few days to a couple of years) and are used in applications where frequent
charging is either impossible or less favorable than replacing the cell outright.
Table 15.3 lists a few types of primary cells, applications, operating potentials
and energy densities.
Common rechargeable batteries include lead–acid, nickel–metal hydride and
lithium-ion batteries. Lead–acid battery technology is well proven and is more
than a century old. These batteries are capable of being cycled thousands of times