Vegetable Oil: From Kitchens to Power Lines
Jim_Kling
PhotoDisc
Petroleum-based (mineral) oils are unglamorous, yet vital mainstays of
industry. They are used as lubricants and coolants, among other applications.
But
they are nonrenewable, hazardous, and expensive to clean up spilled. Those
drawbacks have industry considering alternatives, including vegetable oils from
crops such as rapeseed and soybeans.
An article in the March issue of Tribology and Lubrication Technology (1)
describes the use of vegetable oils in one important application: electrical
transformers, which transform voltage from the high levels, used to transport
power over long distances with minimal loss of power, to the lower levels
required
for local use. Waverly (Iowa) Light and Power and California’s Sacramento
Municipal Utility District (SMUD) have committed to using only transformers
that
use vegetable oils.
The decision stems from several incentives. About 40% of existing
transformers are contaminated with PCBs, mandating expensive environmental
cleanups when
spills occur. The U.S. Environmental Protection Agency requires that even
PCB-free mineral oil spills be cleaned up. Vegetable oils can be used to
retrofit
existing transformers because they are fully miscible with mineral oils and
perform the same cooling and insulating functions.
Vegetable oils are gentler on the cellulose-based paper that insulates the
transformer coils. Inevitably, atmospheric water finds its way into transformer
oil, and residual acid in the paper catalyzes hydrolysis and degradation of
the cellulose. Vegetable oils dampen that process because they have a greater
capacity for carrying water than conventional oils. As a result, water tends to
be drawn out of the insulating paper and into the surrounding oil, protecting
the paper from hydrolysis. Tests performed by Cooper Power Systems, a
manufacturer of medium- and high-voltage electrical equipment, showed that
vegetable
oils increased the lifetime of a similar paper five- to eight-fold.
A SMUD-sponsored study suggested that in spite of their initial high cost,
transformers using bio-based oils are 10–20% cheaper in the long run than
conventional transformers because a typical transformer using the bio-based oil
would last 40 years instead of 30.
Vegetable oils could also make it possible to use smaller transformers and
still handle sufficient current during peak demand times. Acid-catalyzed
cellulose hydrolysis is accelerated by the heat produced by the flow of current
through the transformer. Using an oil that limits water exposure could allow
utilities to ramp up the current, transforming more power without reducing the
lifetime of the transformer.
Bio-oils are less of a fire hazard than petroleum-based oils. Although
individual transformers rarely catch fire, they are so ubiquitous that a fire
occurs
somewhere just about every day. Mineral oil burns easily and can stoke such
fires, but vegetable oil is far less flammable.
These latest developments represent something of a revival for vegetable
oils. Late 19th-century developers of oil-based transformers considered using
vegetable oils, but they oxidized too quickly and had no price advantage over
mineral oils. In the 1990s, new processing methods made vegetable oil more
resistant to oxidation; and genetic engineering techniques promise to further
reduce
oxidation by increasing the content of oleic acid, an antioxidant. One soybean
variety has an oleic acid content of 80%, as compared with the average of
18%. In an oxidation chamber, the oil lasted 192 hours. Conventional oil lasted
just 7.
Plant oils are well positioned for a takeover. In the United States, the glut
of soybean oil has depressed crop prices. Companies like Cooper Power Systems
and the agricultural commodity supplier Cargill hope that transformer life,
fire safety, and environmental concerns could extend the demand into larger
transformers and transmission lines. As production of vegetable oil increases,
it should also gain a price advantage over mineral oil.
(1) Fields, S. Powering up with bio-based oils. Tribology & Lubrication
Technology 2004, 60(3), 30–35 (not available online).
This article first appeared on May 17, 2004.
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