Shunt current

Neben-schliessung, /., -schluss, m. Elec.) shunt, shunting, also shunt circuit, -schluss-schaltung, /. shunt connection, -schlu s-strom, m. Elec.) shunt current, -serie, /. subsidiary series, secondary aeries, subordi-... 

Fig. 23.7 Shunt currents, local anodes and cathodes due to electrolysis and cell design.

Cell construction, current feeders and power supply are less expensive for the hipolar connection (see also Fig. 12), but the different potentials of all electrodes can be problematical (e.g. shunt currents, additional by-products, and corrosion in the channels). 

Fig. 11.1. Two basic types of current ampliflers. (a) Feedback picoammeter. It consists of two components, an operational amplifier (op-amp) A, and a feedback resistor 1 fb- a typical value of the feedback resistor used in STM is 10 fl. The stray capacitance Cfb is an inevitable parasitic element in the circuit. In a careful design, Cfb 0.5 pF. The input capacitance Cm is also an inevitable parasitic element in the circuit. Those parasitic capacitors, the thermal noise of the feedback resistor, and the characteristics of the op-amp are the limiting factors to the performance of the picoammeter. (b) An electrometer used as a current amplifier (the shunt current amplifier). The voltage at the input resistance is amplified by the circuit, which consists of an op-amp and a pair of resistors R, and R2. The parasitic input capacitance Cm limits the frequency response, and the Johnson noise on Rm is the major source of noise. Also, the input resistance for this arrangement is large.

The simplest extension to real device operation in a stationary state consists in introducing losses via a series resistance Rs, which represents contact resistances, Ohmic losses in the front contact grid and in the rear contact, and a parallel resistance Rp, which includes any current bypassing the membranes (junction), and even shunt currents through short-cuts. 

The total current density for the real diode is now composed of three contributions, the photoinduced short-circuit current, the diode current and the shunt current through Rp ... 

Katz, M., Analysis of electrolyte shunt currents in fuel cell power plants, J. Electrochem. Soc., 125, 515, 1978. 

Roche, R.P. and Nowak, M.P., Integrated Fuel Cell Stack Shunt Current Prevention Arrangement, U.S. Patent 5,079,104, January 7, 1992. 

Boyer, T.D. Transjugular intrahepatic portosystemic shunt Current status. Gastroenterology 2003 124 1700-1710... 

Kerlan, R.K., LaBerge, JJM., Gordon, R.L., Ring, E.J. Transjugular intrahepatic portosystemic shunts current status. Amer. J. Radiol. 1995 164 1059-1066... 

A bipolar electrode consists of an anode in electrical contact with a cathode—it is polarized in the electric field of the cell. Bipolar cells have two feeder electrodes and a number of bipolar electrodes between them. Ideally, the current supplied to the system can be multiplied by the number of anodes or cathodes to calculate the charge that is passed to the electrolyte. In reality, some current losses occur due to bypass or shunt currents, so-... 

The main component of voltaic inefficiency is usually the ohmic loss, compounded by mass-transport-related overvoltage. The extent of coulombic losses depends on the system and may be due to parasitic electrode and/or chemical reactions, self-discharge, and/or shunt currents (in flowing systems wth a common electrolyte). The efficiency, as measured at battery terminals differs from the effective value if a part of the battery s energy is used to operate auxiliaries (e.g., pumps) or if thermal losses are involved (high-temperature batteries). 

Another source of inefficiency are the shunt currents arising in all series-connected systems with a common electrolyte. In the EDA system, these parasitic currents are minimized to some (model computed) 5% power loss by maximizing the hydraulic resistance. 

Shunt currents are minimized in an innovative way by use of a protective current applied from the battery to the common electrolyte flowing through the manifold channels. The energy loss connected with this type of protection is reported as <5%. 

The current utilization refers to the fraction of the total current that passes through an electrodialysis membrane stack that actually is used to transfer anions or cations from a feed solution. The current utilization is always less than 100% due to (1) co-ion intrusion into the ion-exchange membrane (i.e., no ion-exchange membrane will completely exclude co-ions), (2) osmotic and ion-bound water transport (water will flow into the concentrate compartment due to osmosis and the electro-osmotic drag of water molecules with the transporting ions), and (3) shunt currents that skirt around the membranes and pass through the stack manifold. 

Concentration Dependence of Freezing Potential and Shunt Current. As has been pointed out, in Group I solutes the freezing potential curve in any experimental mn builds up to a maximum, after which it declines as the concentration on the liquid side of the phase boundary increases. The height of this maximum is itself a function of the initial solution concentration and shows a maximum value at an optimum concentration that depends on the particular solute. 

The simplest explanation for shunt currents is that the electrolytes in the manifolds, or the manifolds themselves if they are metallic, provide parallel paths to the current flowing in the cells. Thus, with few exceptions, some shunt currents will flow in the connecting piping of bipolar cell stacks. As shown in Fig. 5.5 A, the distribution of shunt... 

The typical problems associated with shunt currents include ... 

Not all the shunt current returns to the electrolyzer. Some leaks from the system and does not take part in electrolysis. Since the production rate depends on the current supplied, there is a production loss caused by shunt currents. In order to accurately determine current efficiency, the actual current received by each cell in the circuit needs to be known. One of the uses of shunt current models, discussed in the next section, is the estimation of the shunt currents as well as the current in each cell. For well-designed chlor-alkali plants, the shunt current loss will usually be less than 2% and frequently less than 1%. The shunt currents that bypass the center cells do no useful electrolysis, but wiU cause IR heating of the electrolytes. For production of molten metals, where shunt cinrent loss could be much higher than 2% because of the high conductivity, such IR heating could be of some benefit, but the economic trade-off between the choice of bipolar and monopolar cells for such an application needs to be carefully considered [8]. 

Failure to understand the nature of shunt currents in bipolar cell stacks frequently leads to corrosion problems. Corrosion occurs most frequently where shunt currents leave a metal component of the cells or piping system, but corrosion can also occur where the shunt currents re-enter a metal component. Such corrosion can occur at nearly any metal component of an electrolyzer, but the following are the most common cell nozzles, manifold ports, edges of the main cathode in a unit cell adjacent to an inlet or outlet, metal cell components electronically in contact with the cathode and adjacent to an inlet or outlet of the catholyte compartment, and pipe walls adjacent to the flanges of a manifold when these manifold flanges are located near the center of a DC circuit. 

P.G. Grimes, R. J. Bellows, and M. Zahn, Shunt Current Control in Electrochemical Systems—Theoretical Analysis. In R.E. White (ed.). Electrochemical Cell Design, Plenum Press, New York (1984), p. 259. 

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