Electrolyzer voltage

Solid Oxide Electrolysers (SOE) are in development for steam electrolysis. As electrolysis is an endothermic process, a supply of waste heat can be used beneficially to reduce the electrolyzer voltage, and thus increase its electrical efficiency. Combination with nuclear power generation and geothermal heat sources is often encountered in development programs for SOE. 

It has been already said that the back electromotive force makes up for the electrolyzing voltage, as long as E < Ev. This is, however, not exactly expressed as actually some current flows through the system even before the decomposition voltage Ep is reached (see Fig. 16, line OB). This small current, called residual current, is caused by the fact that the products of electrolysis (chlorine and hydrogen) do not permanently remain in the electrodes, as theoretically assumed, but a small quantity is lost by dissolution in the surrounding electrolyte and by diffusion into the bulk of electrolyte. Those losses are compensated by the residual current. 

Electrolyzer exit temperatures rise as operating current is increased. Operators should adjust feed brine and caustic temperamies as the load approaches 2kAm in order to bring the exit temperatures under control. Design exit temperatures typically are 87°C, with an acceptable range of 80-90°C. The required feed temperatures depend on electrolyzer voltage and other technical requirements. It may be found convenient to maintain feed brine and caustic temperatures equal. 

Electrolyzer voltages, for all practical purposes, are linear with current density over the normal operating range. The following equation, which is similar to Eq. (8.5), applies ... 

Most of the voltage savings in the air cathode electrolyzer results from the change in the cathode reaction and a reduction in the solution ohmic drop as a result of the absence of the hydrogen bubble gas void fraction in the catholyte. The air cathode electrolyzer operates at 2.1 V at 3 kA/m or approximately 1450 d-c kW-h per ton of NaOH. The air cathode technology has been demonstrated in commercial sized equipment at Occidental Chemical s Muscle Shoals, Alabama plant. However, it is not presentiy being practiced because the technology is too expensive to commercialize at power costs of 20 to 30 mils (1 mil = 0.1 /kW). 

Tetraphenylethane (2). A solution of diphenylacetic acid 1 (2f fS g, O.f mol) In OMF was electrolyzed for 17 h and the voltage was increased from f 20 to 220 V dunng electrotysts. The reaction mixture was poured into water and extracted with EtjO. The extract was filtered, washed and the solvent evaporated. Recrystallization from CHCI3 gave 4 g of 2 (24%), mp 207-208 C. 

Two directions of current flow in galvanic cells are possible a spontaneous direction and an imposed direction. When the cell circuit is closed with the aid of electronic conductors, current will flow from the cell s positive electrode to its negative electrode in the external part of the circuit, and from the negative to the positive electrode within the cell (Fig. 2.2a). In this case the current arises from the cell s own voltage, and the cell acts as a chemical source of electric current or battery. But when a power source of higher voltage, connected so as to oppose the cell, is present in the external circuit, it will cause current to flow in the opposite direction (Fig. 2.2b), and the cell works as an electrolyzer. 

In cells working as electrolyzers, the positive electrode is the anode and the negative electrode is the cathode. All the factors listed above (for the case of a battery) wiU now produce an increase in voltage relative to the OCV (Fig. 2.3fc) ... 

The power consumption of the electrolyzer is generally calculated by multiplying the applied current by the stack voltage that are both continuously measured. However, this power consumption refers to DC current and therefore to the electrolysis stack. The AC power consumption is not usually measured, and most manufacturers assume a 5-10% energy loss for the AC/DC converter. 

Electrolyzers are generally current-controlled, which means that a certain DC is imposed according to the desired hydrogen production. In a wind-hydrogen system, the wind turbine power available for the operation of the electrolyzer is generally known therefore, the power input should be transformed to a current input. The voltage-current relation of an electrolyzer is not very simple because it depends on the temperature, pressure, and other construction characteristics. For a given electrolyzer, it is possible to experimentally establish the I-V curve at different temperatures and pressures, and deduce a temperature-dependent current-power curve. 

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