WIND ENERGY SYSTEMS - Cd3wd

Chapter 7—Asynchronous Loads 7–29
through the battery. One possible configuration for the battery is shown in Fig. 12. This
sketch shows a battery with three cells, so with an open circuit voltage of 1.8 V/cell, the
total voltage would be 5.4 V. The two interior plates are called bipolar electrodes. They are
made of thin sheets of nonporous carbon. The same sheet acts as the positive electrode for
one cell and the negative electrode for the adjacent cell. During charge, the surfaces marked
with a + will oxidize bromide ions to bromine gas, which is dissolved in the electrolyte. At
the same time zinc ions will be deposited as metallic zinc on the surfaces marked with a −.
During discharge, the dissolved bromine gas and the metallic zinc go back into solution as
zinc bromide.
Figure 12: Diagram of a zinc bromide battery.
The bipolar electrodes have no need to be electrically connected to anything, so current
flow can be completely uniform over the cross section of the battery. This simplifies the
electrical connections of the battery and makes assembly very easy. It also makes the battery
more compact for a given stored energy or a given power density.
There has to be a microporous separator in the middle of each cell to reduce the transport
rate of dissolved bromine gas across the cell to the zinc on the negative electrode. Any gas
that reacts with the zinc directly represents an efficiency loss to the system since the electron
transfer necessary to produce the zinc and bromide ions does not produce current in the
external circuit. For the same reason, the electrolytes for the two halves of each cell are kept
in separate reservoirs. One reservoir will contain electrolyte with dissolved bromine while the
other will not. The discharge cycle will continue until all the dissolved bromine is converted
to bromide ions, so total discharge is possible without damage to the battery.
Most secondary batteries with zinc anodes have life problems due to the formation of zinc
dendrites. This does not occur with this battery because the bromine will react with any
dendrites as they form in the separator. Therefore, long cycle life should be possible.
Bromine gas is toxic, but the strong odor gives ample warning of a leak before the injury
level is reached. The development difficulties include deterioration of the positive electrode,
which limits cycle life. Another difficulty is the high self-discharge rate, where early versions
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001