Page 155 - Applied Photovoltaics

Present CP systems
Photovoltaic powered systems are now considered to be a reliable and economic
solution for CP. This was initially the case for small structures such as small pipelines
or storage tanks, etc. but as photovoltaic prices have fallen, they have been used
increasingly for larger pipelines, well head protection etc.
Low availability system designs are adequate for most structures. A 90% availability
would be expected to extend the life expectancy of the structure being protected by a
factor of 10.
Batteries are used in most systems, although the importance of continuous protection
has not been clearly demonstrated (Ball & Risser, 1988). Float types, such as lead
calcium, are not suitable, and deep cycle types, such as nickel cadmium or deep cycle
lead-acid batteries are therefore used.
The first large photovoltaic-powered CP installation was in Libya in 1979. It was
used for protecting the equipment and structures on oil drilling rigs and pipe lines
(Tanasescqu et al., 1988). This use of photovoltaics for cathodic protection was soon
followed by major oil companies throughout Africa, the Middle East and Asia. In the
USA, Federal Department of Energy regulations require that all underground storage
tanks and pipelines carrying combustible liquids or gases have cathodic protection
(Ball & Risser, 1988), a requirement that provides a large potential market for
photovoltaics.

Unfortunately, poor understanding of photovoltaics on the part of many corrosion
engineers, has and still is preventing its use in many installations. As the advantages
of photovoltaics become better understood, its use will increase. Vandalism and theft
are also problems. For these reasons, pole mounting is preferable, but can lead to
increased wiring losses and higher installation costs, unless specially-designed
integrated systems are used. The German Energy Pillar System, for instance,
combines an aluminium pillar with photovoltaic panels, a step up charge converter
and batteries in a ready-to-install system that, once installed, transfers site data to a
central processing point for monitoring purposes (Korupp & Marthen, 1992).
An example of the design of a CP system follows (based on Tanasescqu et al., 1988).

Fig. 8.4 illustrates the use of cathodic protection along a pipeline. The protection
current must be adjusted so as to keep ǻV min high enough to counteract the
electrochemical potential, which is given by

V v exp rx (8.3)
x
where r is the decay constant, which is a function of the resistance per unit length of
–1
–4
the pipe, and is typically 5 × 10 m .
If L is the length of pipe protected, then
2 § Vǻ max ·
K ln ¨ ¨ ¸ ¸ (8.4)
r © ǻ V min ¹

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