Environmental Engineering Reference
In-Depth Information
external circuit attached to the battery. The positive electrodes are often made from
lithium iron phosphate.
Lithium-ion rechargeable batteries became the most important storage techno-
logy in portable applications in recent years. They have a 90-95% efficiency, and
their energy density is superior to all other commercial rechargeable batteries, be-
ing in the capacity range of 250-350 watt-hour/liter (100-200 watt-hour/kilogram).
Lead-acid batteries have a capacity of 50-100 watt-hour/liter (5 to 50 watt-hour/
kilogram).
Lithium-ion batteries come usually in 6.2 kilowatt-hour modules, which can
be fitted together for several megawatt-hours of electricity. Some of the present
electric cases on the market operate at 24 kilowatt-hours. These modules can be
charged in as few as 30 minutes.
The properties of nickel-cadmium and nickel-metal hydride batteries lie
between lead-acid and lithium-ion batteries.
Renewable energies such as wind and solar are only intermittently available, so it
is necessary to have a means to store the electricity that they generate.
Among the many energy-storage technologies available, pumped hydro has the
highest rated power (around 1,000 MW) and discharge time (100 hours). Com-
pressed air energy storage (CAES) comes next with 1-100 MW and a discharge
time of 1-10 hours. Lithium-ion and a number of other batteries have a rated power
between 0.01 and 1 MW and discharge times of 0.1-10 hours.
The most commonly used storage technology is compressed air energy storage
(CAES). Progress has been made recently using adiabatic CAES in which the heat
generated during compression can be stored and used again during decompression,
considerably improving the efficiency of the system. For load leveling, compressed
air stored in large underground salt caverns appears to be an economical and tech-
nically feasible option. Cost for long term storage with hydrogen are low due to its
very high energy density.
Hydro systems are the most economical option, but because of geographic con-
ditions and public acceptance, there is a limited potential for pumped hydro power
plants.
