Voltage reversal

Figure 3-17. Rectifier-DC motor driver (a) unidirectional or field-reversing (b) bidirectional operation with armature voltage reversing [10].

S. Ye, Fuel cell anode structure for voltage reversal tolerance, US Patent 7608358, assigned to BDF IP Holdings Ltd. (Vancouber, BC, CA), October 27, 2009. 

The practical application of ultrasonics requires effective transducers to change electrical energy into mechanical vibrations and vice versa. Transducers are usually piezoelectric, ferroelectric, or magnetostrictive. The application of a voltage across a piezoelectric crystal causes it to deform with an amplitude of deformation proportional to the voltage. Reversal of the voltage causes reversal of the mechanical strain. Quartz and synthetic ceramic materials are used. 

A current operational problem is damaging anode voltage reversal following fuel starvation. A flow of patents covers the topic (Knights etal., 2000 2001a Colbow etal., 2001 Taylor etal., 2001). The problem, relevant to both the PEFC and the DMFC, and to all manufacturers, is discussed by Johnson Matthey in Ralph etal. (2003). 

No Ballard competitor mentions voltage reversal, presumably due to inadequate test experience. 

United Technologies Fuel Cells is engaged in DMFC development, in competition with Ballard/Johnson Matthey. It is a part in the project by Renault to develop the Scenic vehicle fuel cell. Neither for its PEFC, nor for its DMFC (and MCFC), does UTC Fuel Cells offer product-coloured illustrations. Moreover, its literature or listed web site does not deal with the cell voltage reversal problem, mentioned in Ballard patents above in connection with fuel cell bus operation. Accordingly it is not possible for the author to portray the UTC Fuel Cells scheme of things. 

Knights S etal, 2001a, Fuel Cell Anode Structure for Voltage Reversal Tolerance. WO 01/15247. 

We mentioned there is a voltage reversal across the switch, when we try to interrupt its current. Let us try to understand this better now. 

Figure 1-8 How the Voltage Reverses on Attempted Interruption of Inductor Current...

So hypothetically, if a circuit is wired in a certain way, and the conditions are right, it is certainly possible that voltage reversal won t occur, so long as current continuity can still be maintained. 

However, we must be clear that if and when a converter reaches a steady state, voltage reversal will necessarily occur at every switch transition. 

From V = LAI/At, it is clear that if the current is ramping up for a positive (i.e. applied) voltage, the current must ramp down if the voltage reverses. So the following equations must apply (magnitudes only)... 

One mitigatimi is to provide sufficient reactant to prevent the fuel cell from starvation, just as the conventimial approach used in chemical fuel cells. Making MFCs operate in continuous mode may keep the MFCs from voltage reversal. For DC-DC converters, because of the low output power of microscale MFC, the efficiency of DC-DC converter may be low thus improving the efficiency of DC-DC converter is preferred. 

Reversible anode voltage Reversible cathode voltage Total... 

Figure 13.7 I An electric potential has a fixed polarity and voltage. Reversing the proles of a battery with respect to a voltmeter changes the sign on the measured voltage but does not acm-ally influence the electrochemical reaction in the battery.

Logan, B.E. and Oh, S.E. (2007) Voltage reversal during microbial fuel cell stack operation. J. Power Sources, 167 (1), 11-17. 

When the power switch Q1 opens, which is referred to as the off time (denoted by tofr)> the inductor voltage reverses polarity, and the output side (the right-hand end) flies back above the input voltage and is clamped by diode D1 at the output voltage. 

We hardly ever see some of the metals like hthirrm, sodium and potassium in metallic form because they react rapidly and catch fire in the presence of air and moisture. A table of oxidation potentials shows which metals are most reactive and which least so. The table also allows a user to work out the voltage to be obtained from various chemical cells. Sometimes you will firrd the table printed as a table of reduction potentials with the positive and negative signs of the voltages reversed. However, in that form, it serves the same pitrpose. 

The cells in a stack are connected in parallel with respect to the gas flow but in series with respect to the current. The implication is that in complete absence of gas flow in a cell, cell voltage reversal will occur which will induce a rapid damage (e.g., due to carbon corrosion). As a consequence, individual cell voltage... 

Capturing power at higher voltages from arrays of microbial fuel cells without voltage reversal. Energ Environ Sci 4(11) 4662 667... 

Mitigation of possible cell-voltage reversal caused by temporary hydrogen undersupply during fast transients, which can be achieved by incorporation of efficient... 

Diode Cathode i Positive anode to cathode voltage Reverse anode current Recovery time before turning off Turn off and on dqiend on circuit conditions High power Ci abilities... 

---