Environmental Engineering Reference
In-Depth Information
hydrogen results in energy plus pure water with no harmful emissions or by-products.
This may not be attractive if electricity is the fi nal product since the round trip effi ciency
is very low (generally below 50%). For this reason there is much interest in using hydro-
gen for transport, but this depends on much improved on board storage systems.
5 .
Flow cells
operate in a mode similar to that of a car battery but without involving the
electrodes. Instead, when the fl ow cell is used as a 'sink', electrical energy is converted
into chemical energy by 'charging' two liquid electrolyte solutions. The stored energy
can be released on discharge. In common with all DC systems connected to the AC
network, a bidirectional power electronic converter is required.
6 .
Batteries.
The lead -acid battery is one of the most developed battery technologies. It is
a low cost and popular choice of electrical energy storage but has disadvantages in terms
of energy density per unit weight, a short cycle life and the dependence of the delivered
energy on the rate of discharge. As a consequence, a large variety of other batteries have
been under intensive development to provide high round trip effi ciency, low life-cycle
cost, high reliability, capability of deep discharge and a large number of charge-discharge
cycles, low maintenance, high power energy density per unit weight and low capital cost.
Although they are generally perceived as too expensive for general inclusion in large
power systems, high temperature batteries such as the Zebra cell [23], are of increasing
commercial interest at the substation level.
7 .
Flywheels.
There has been a number of projects carried out to store energy in very fast
rotating fl ywheels. The major problems that had to be overcome were the maintenance
issues and losses relating to bearings, the low specifi c strength of standard materials such
as steel and the associated severe failure management problems at high speeds. Contem-
porary fl ywheels are made of fi bre-reinforced composites which have powdered magnetic
material introduced into the composite which when magnetized form either the rotor of
a high speed motor/generator or the rotating element of passive magnetic bearings. The
fl ywheel motor/generator is interfaced to the mains through a power electronic converter.
Currently the technology is expensive and only used for niche applications.
8 .
Super capacitors
, alternatively known as ultracapacitors, consist of a pair of metal foil
electrodes, each of which has an activated carbon material deposited on one side. The
activated carbon sides are separated by a paper membrane and then rolled into a package.
Ultracapacitor operation relies on an electrostatic effect whereby charging and discharg-
ing takes place with the purely physical (not chemical) and reversible movement of ions.
As a result there are some fundamental property differences between ultracapacitors and
battery technologies including long shelf and operating life as well as large charge-
discharge cycles of up to 500 000.
9 .
Superconducting magnetic energy storage
(SMES) stores energy within a magnetic fi eld
created by the fl ow of direct current in a coil of superconducting material. Typically, the
coil is maintained in its superconducting state through immersion in liquid helium at
4.2K within a vacuum-insulated cryostat. A power electronic converter interfaces the
SMES to the grid and controls the energy fl ow bidirectionally. With the recent develop-
ment of materials that exhibit superconductivity closer to room temperatures this technol-
ogy may become economically viable.
10.
Heat or cold store.
There is a long tradition of using thermal storage to assist in power
system operation. The UK's old white metre off-peak tariff and the more recent Economy
7 tariff were both primarily for charging storage heaters. More recently, sophisticated
