WIND ENERGY SYSTEMS - Cd3wd

Chapter 7—Asynchronous Loads 7–43
efficiency range of 71 to 78 percent. The energy loss appears as low grade heat which must be
removed from the system. The rated current on these large units may be as high as 15,000 A
or even more. There will be hundreds of cells in series to yield a reasonable plant operating
voltage.
These large electrolysis plants represent proven technology with readily available materials.
The cost of the hydrogen produced is rather high, however, because of the inefficient, low
pressure electrolysis process which is used. A substantial increase in the use of electrolytic
hydrogen depends on an improvement of electrolysis efficiency and a decrease in capital costs.
The bubbles being evolved from the electrodes increase the resistance of the electrolyte.
Therefore, one obvious way of improving the efficiency is to increase the operating pressure,
since this compresses the bubbles. This has another advantage for some applications in that
the gases can be produced at pipeline pressures and do not require compression after generation.
The feedwater must be pumped at that same pressure but it requires less energy to
pump the liquid than it does to pump the resultant gas.
The effect of pressure on cell voltage is shown in Fig. 16. The cell voltages of conventional
alkaline electrolyzers (electrolysis cells) are shown as a band in the upper part of the figure.
The horizontal axis is the current density of the cell, in A/m 2 or A/ft 2 . The current density is
the total cell current divided by the electrode area. As the current density increases, the cell
voltage has to increase because of the resistances of the electrolyte and conductors. As the
pressure increases, the electrolyte resistance drops, which lowers the cell voltage and thereby
improves the efficiency. At 400 A/ft 2 , the best conventional electrolyzer has an efficiency of
about 69 percent, while at 1000 psi (6.895 MPa) the efficiency of a pressurized electrolyzer is
87 percent, and at 3000 psi (20.68 MPa) the efficiency is 91 percent. The high pressure cell is
also capable of operating to at least twice the current density of the conventional electrolysis
cells. This reduces the cross sectional area of the cell by a factor of two for a given input
power, which helps to reduce capital costs. Of course, the high pressure container for the cell
will be stronger and more expensive than the container for the low pressure cell. The much
smaller volume of the evolved gases makes it possible for the overall capital cost per unit of
hydrogen to be lower for the high pressure system.
Efficiency also increases with increasing temperature, as shown in Fig. 17. The cell voltage
at 400 A/ft 2 is about 2.07 V at 80 o F and 1.63 V at 400 o F. The efficiency increases from 72
percent to 91 percent with this increase in temperature. The reason for this is that water
becomes more chemically active at higher temperatures, so that it is easier to split into its
constituent elements.
The information on Figs. 7.16 and 7.17 was taken from a high pressure KOH cell developed
at Oklahoma State University[9]. The electrodes were made of solid nickel and were able to
withstand the corrosive action of hot KOH without damage. These researchers discovered
that many materials which would withstand KOH at high pressures or at high temperatures
would not withstand the combination of high pressure and high temperature KOH. A great
deal of research is being performed on various materials to help develop these high efficiency
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001