Page 226 - Uninterruptible Power Supplies

Kinetic Energy as an Alternative Power Source
224 Chapter Eight

~
= = ~

Figure 8.1

Over time developments have enabled us to reconsider the use of
kinetic energy. Recent developments in circuitry and components have
now made it feasible to consider kinetic energy as an alternative to
chemical battery systems.
Clearly, any such alternative system must show advantages and also
ensure the utmost reliability. Manufacturers must illustrate advan-
tages in space, which is nearly always at a premium, and in its ability
to form a reliable energy source for the stated autonomy time required
by the circuit—usually the time between the loss of mains and the
availability of full power from the standby diesel generator set. In addi-
tion, for the system to be economically viable, it must be efficient; there
are losses both in the bearings and windage losses which normally are
not inconsiderable.
Since the stored energy in a flywheel is proportional to the square of
its rotational speed, manufacturers must require the flywheel to operate
at much higher speeds than previous designs, and in this area advances
made for other applications may be used for this field.
Quite a few new materials have been developed which will allow
higher speeds with reliability and a list is shown below.

3
Material Specific strength (in )
Graphite/epoxy 3,509,000
Boron/epoxy 2,740,000
Titanium and its alloys 1,043,000
Wrought stainless steel 982,000
Wrought stainless steels 931,000
7000 series aluminum alloys 892,000

Specific strength is a measure of material strength to weight density.

However, the costs involved in using many of these materials are pro-
hibitive and in designs currently available steel is used, although this
limitation may be solved in the future.
Variable-speed drives are now available which enable us to drive
machines economically at much higher speed. So, circuits are available
which can harness the kinetic energy inherent in high-speed flywheels

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.