Classes of Flow Batteries
Various classes of flow batteries exist including the redox (reduction-oxidation) flow battery, a reversible fuel cell in which all electroactive components are dissolved in the electrolyte. If one or more electroactive components are deposited as a solid layer the system is known as a hybrid flow battery; that is, the electrochemical cell contains one battery electrode and one fuel cell electrode. The main difference between these two types of flow batteries is that the energy of the redox flow battery, as with other fuel cells, is fully decoupled from the power, because the energy is related to the electrolyte volume (tank size) and the power to the electrode area (reactor size). The hybrid flow battery, similar to typical batteries, is limited in energy by the size of the battery electrode (reactor size).
Energy producing electrochemical cells are generally divided into two categories. Cells that can be discharged only, with irreversible electrochemical reactions, are termed primary cells, while rechargeable cells with reversible reactions are termed secondary cells (see also primary and secondary batteries). Using this historical convention, a redox flow battery is better described as a secondary fuel cell or regenerative fuel cell, with the fundamental difference between batteries and fuel cells being whether energy is stored in a solid state electrode material (batteries) or in the electrolyte (fuel cells). This difference leads to the decoupling of energy and power in a fuel cell described above.
The misnomer of "redox flow battery" rather than "reversible fuel cell" has led to a great deal of confusion in understanding and terminology. For example, the processes of solid-state diffusion and intercalation in a Lithium Ion Battery do not apply to redox flow batteries, but the heterogeneous electron transfer in a fuel cell does. In industrial practice, fuel cells are usually, and unnecessarily, considered to be primary cells, such as the H2 / O2 system, with limited examples of reversible systems (i.e., the unitized regenerative fuel cell on NASA's Helios Prototype). The European Patent Organisation classifies redox flow cells (H01M8/18C4) as a sub-class of regenerative fuel cells (H01M8/18).
Examples of redox flow batteries are the vanadium redox flow battery, polysulfide bromide battery (Regenesys), and uranium redox flow battery. Hybrid flow batteries include the zinc-bromine, zinc-cerium and lead-acid flow batteries. Redox fuel cells are less common commercially although many systems have been proposed.
Read more about this topic: Flow Battery
Famous quotes containing the words classes of, classes, flow and/or batteries:
“There are four classes of idols which beset men’s minds. To these for distinction’s sake I have assigned names—calling the first class Idols of the Tribe; the second, Idols of the Cave; the third, Idols of the Market-Place; the fourth, Idols of the Theatre.”
—Francis Bacon (1561–1626)
“When we of the so-called better classes are scared as men were never scared in history at material ugliness and hardship; when we put off marriage until our house can be artistic, and quake at the thought of having a child without a bank-account and doomed to manual labor, it is time for thinking men to protest against so unmanly and irreligious a state of opinion.”
—William James (1842–1910)
“Opinionated writing is always the most difficult ... simply because it involves retaining in the cold morning-after crystal of the printed word the burning flow of molten feeling.”
—Gavin Lyall (b. 1932)
“Postmodernism is, almost by definition, a transitional cusp of social, cultural, economic and ideological history when modernism’s high-minded principles and preoccupations have ceased to function, but before they have been replaced with a totally new system of values. It represents a moment of suspension before the batteries are recharged for the new millennium, an acknowledgment that preceding the future is a strange and hybrid interregnum that might be called the last gasp of the past.”
—Gilbert Adair, British author, critic. Sunday Times: Books (London, April 21, 1991)