Page 153 - Applied Photovoltaics

8.5 CATHODIC PROTECTION
The corrosion of oil and gas pipes, wells, chemical storage tanks, bridges etc. is a
major problem, particularly in remote areas away from grid-connected power.
Cathodic protection (CP) involves the use of an electrical current to counteract
natural, corrosive electrochemical currents. Water and acids in soil act as electrolytes
to allow the transfer of electrons to the metal structures, which act as anodes and are
consequently oxidised, or corroded. An externally-applied counteracting current
allows the metal structure to become cathodic, thereby eliminating corrosion.
A similar effect can be achieved using a sacrificial anode near the metal structure,
provided the sacrificial anode is more anodic; that is, it establishes a greater
electrochemical potential than the structure to be protected.

8.5.1 System sizing
In designing a photovoltaic-powered CP system, the following aspects must be
considered (Tanasescqu et al, 1988):
1. The load is the amount of current required to overcome the open circuit
potential between the metal (anode) and the surrounding electrolyte.
2. The current requirement is determined by the area of bare metal. In general,
metal structure coatings will have a specified integrity factor. Typical of a
good plastic coating is an integrity factor of 99.999%, which means a
5
2
2
structure with a surface area of 10 m has 1 m exposed.
3. The resistance of the CP circuit determines the voltage necessary to provide
the current from (2). This can be measured, but will vary with moisture
content, temperature, compactness and even salt content of the soil.
Resistance measurements are complicated by such factors as:
x pipe capacitance
x electrochemical polarisation, owing to ionic conductivity of the soil
(depolarisation time can be 16–18 hours for a well insulated pipe)
x ground bed resistance (R) where
> 2 L ln r L S 2 @ ȡ ln
R (8.1)
2 ʌ L
for a horizontal bed, such as with a high water table, or
> 4 r 1 @ ȡ ln
L
R (8.2)
2 ʌ L
for vertical beds, such as with a deep well or bore hole, where L is the anode
column length, ȡ is the soil resistivity, S is the depth and r is the radius of the
anode.
Owing to such uncertainties and variability over time (see Fig. 8.3) and distance, a
safety factor for both the voltage and current needs to be included in the load
determination.

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