Environmental Engineering Reference
In-Depth Information
Before Load-Leveling by VRB
After Load-Leveling by VRB
(kW)
(kW)
2500
2500
2000
2000
1500
1500
1000
1000
500
500
0
0
2 4 6 8 10 12
Time
14 16 18 20 22 24
2 4 6 8 10 12
Time
14 16 18 20 22 24
Charging
Discharging
FIGURE 6.4
Load leveling applications of a vanadium redox flow battery (VRB) for Tomari Wind Hills
power station of Hokkaido Electric Power Co., Inc. (Graph courtesy of Sumitomo Electric
Industries Ltd., 2001.)
peak load hours. Figure 6.4 provides a graph of the load leveling capabilities
of a vanadium redox flow battery for a wind turbine application.
8
Properties of Other Electrochemical Storage Devices
Before we discuss vanadium redox flow technology, it is important to first
explain and define fuel cells and flow batteries. Fuel cells convert the chemical
energy available from the oxidation of a fuel (usually hydrogen) by an oxidant
(oxygen from air, for example) directly into electricity. The operating behavior
is similar to that of a primary battery with infinite capacity. A flow battery is a
type of rechargeable secondary battery in which energy is stored chemically
in liquid electrolytes. The electrolytes contain dissolved electro-active species
that flow through a power cell that converts chemical energy to electricity.
Simply stated, flow batteries are fuel cells that can be recharged. From a prac-
tical view, storage of the reactants is very important. The storage of gaseous
fuels in fuel cells requires large, high pressure tanks or cryogenic storage that is
prone to thermal self-discharge. Vanadium redox batteries have the advantage
of being able to store electrolyte solutions in non-pressurized vessels at room
temperature. Vanadium based redox flow batteries hold great promise for stor-
ing electric energy on a large scale (theoretically, they have infinite capacity).
They have many attractive features including independent sizing of power and
energy capacity, long lifetimes, high efficiency, and fast response, and relatively
low cost (small initial investment and reduced operational expenditures).
Vanadium Redox Flow Batteries
A redox flow cell is an electrochemical system that allows energy to be stored
in two solutions containing different redox couples with electrochemical
