WIND ENERGY SYSTEMS - Cd3wd

Chapter 7—Asynchronous Loads 7–31
to be built in large quantities at acceptably low costs.
Major difficulties seem to be in developing the metal to ceramic seals and in developing
the β-alumina electrolyte. Corrosion is a major problem. One problem with the electrolyte
is the tendency to accumulate metallic sodium along its grain boundaries, which shorts out
the cell. These problems seem to have been largely overcome, so the beta battery may be a
major contributor to utility load leveling and to wind energy systems in coming years.
The number of possibilities for battery materials seems almost limitless[4]. The batteries
discussed in this section seem to have the highest probability of wide use, but a technological
breakthrough could easily move another battery type into the forefront. Whatever the
ultimate winner is, these batteries will be used as system components, like transformers, by
the utilities. If the utility has enough batteries on its system, it may not be necessary to
physically place batteries at wind turbines for storage purposes. Of course, if it is desired to
install large wind turbines on relatively low capacity distribution lines, batteries may be very
helpful in matching the wind turbine to such a line.
The engineer designing the battery installation will be concerned with a number of parameters,
including the voltage, current, storage capacity in kWh or MWh, the energy density
per unit area of base (footprint), weight, height, reliability, control, heating and cooling requirements,
safety, and maintenance. If the batteries are to be installed at a typical utility
substation, the desired total capacity will probably be 100 or 200 MWh. The total battery
voltage will probably be in the range of 2000-3000 V dc. These voltages would make maximum
use of modern power semiconductors and would reduce current requirements as compared with
a lower voltage installation.
A reasonable footprint is about 300 kWh/m 2 and a height of 6 m would probably be
imposed by mechanical constraints. A height of 2 m may be better in terms of maintenance if
the greater land area is available. The batteries should be capable of accepting a full charge in
4 to 7 hours and should be able to deliver all their stored energy in as little as 3 hours. They
should be capable of more than 2000 charge-discharge cycles and should have an useful life
of more than ten years. The energy efficiency, defined as the ratio of the ac energy delivered
during discharge to the ac energy supplied during charge, should be at least 70 percent. This
definition of efficiency includes both the efficiency of the individual cells and the efficiency of
the power conditioning equipment.
Example
Your company is considering installing a 100 MWh beta battery installation at a substation.
Individual cells are 0.8 m tall and occupy a rectangular space that is 5 cm on a side. Each cell can
store 200 Wh of energy. The energy efficiency is 70 percent, and you assume all the losses occur during
the charge cycle. That is, if you put in 200/0.7 Wh during charge, you get back 200 Wh per cell on
discharge. The battery installation is to be charged during a five hour period and discharged during a
three hour period. Half the losses are used to maintain battery temperature and the other half can be
used to provide thermal input to a 25 percent efficient turbine generator. The cells are to be mounted
two high, so the total height requirement is less than 2 m. The battery voltage is 2500 V dc.
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001