Environmental Engineering Reference
In-Depth Information
between the two electrolyte tanks is permeable to hydrogen ions, enabling ion
exchange during the charge/discharge cycles. Cycle efficiencies over 80 per cent
have been achieved, and long cycle lifes are expected, although the energy density
is comparatively low. As yet mostly demonstration systems have been tested - a
1 MW vanadium bromide system has been investigated in Castle Valley, Ohio for
voltage/reactive power support (van der Linden, 2004).
Alternatively, the polysulphide bromide battery utilises sodium bromide and
sodium polysulphide electrolytes separated by a polymer membrane that only
permits sodium ions to penetrate. A storage time over 12 hours has been achieved,
with a cycle efficiency of 75 per cent, enabling daily charge and discharge cycles.
A demonstration project at Little Barford, England was proposed to provide
120 MWh of energy at 10 MW rating to Southern Electric's 33 kV distribution
network. Finally, the zinc bromide configuration has a cycle efficiency of about
75 per cent, and many small-scale units have been built and tested. Here, the two
electrolyte storage reservoirs of a zinc solution and a bromine compound are
separated by a microporous polyolefin membrane. Multi-kWh designs are available
for assembly complete with necessary plumbing and power electronics, while lar-
ger installations (2 MWh) have been used for substation peak management.
5.5.1.4 Compressed air storage
In an open-cycle or combined-cycle gas turbine plant, incoming air is compressed
by the gas turbine compressor before being ignited with the incoming fuel supply.
The exhaust gases are then expanded within the turbine, driving both an electrical
generator and the compressor. If suitable large-scale storage is available, such as an
underground mine, salt cavern, aquifer, etc., the compressor alone can be operated
at off-peak times to create a ready supply of compressed air. Alternatively, an
underground storage complex can be created using a network of large diameter
pipes. Later, the compressed air can be released as part of the generation cycle,
providing a cycle efficiency of approximately 75 per cent (Kondoh et al. , 2000).
Commercial installations are still few, but include a 290 MW unit built in Hundorf,
Germany in 1978 and a 110 MW unit in McIntosh, Alabama, built in 1991. The
Hundorf installation was originally intended as fast-acting reserve for a nearby
nuclear power station, but has since been modified to provide a grid-support role
(van der Linden, 2004). In contrast, the much greater cavern capacity of the
McIntosh installation enables it to generate continuously for 24 รพ hours.
In 2004, a 2,700 MW plant was proposed for Norton, Ohio, consisting of
9 300 MW generating units and an existing limestone mine 700 m beneath the
surface with a cavern volume of 120 10 6 m 3 , providing a storage capacity of
43 GWh (van der Linden, 2004). Independent of this, an underground aquifer has
been selected near Fort Dodge, Ohio, to store compressed air at 36 bar driven by a
100 MW wind farm. A separate section of the aquifer provides storage of natural gas,
enabling purchase when prices are low, for supply to 200 MW of generating capacity.
All of these plant operate on natural gas, but conceptually, at least, more
environmentally friendly schemes can be envisaged using gasified biofuels, for-
estry residue, etc. as the fuel supply. With the future growth of clean coal
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