POWER October 2014 - 48

OPERATIONS & MAINTENANCE
Offshore Operation Adds Challenges
Though no commercial wind farms have
yet been installed in U.S. waters, 2,277
offshore turbines worth a combined nameplate
capacity of 7.03 GW had been installed
worldwide at the end of 2013. All
were in the Northern Hemisphere-most
in northwestern Europe, and the rest in
China, Japan, and South Korea-where
they are prone to a number of cold climate
issues.
Like onshore turbines, offshore turbines
can be debilitated by ice formation on rotors
and other components, but that risk
is increased by added water in the air,
rainfall, and sea sprays. This " atmospheric "
icing is dependent on the elevation of
the turbine rotor over sea level and on the
size and type of wind turbine.
Another distinct challenge for offshore
wind turbine structures in cold climates
is caused by ice-covered waters. Ice pack
on the sea surface can cause mechanical
shocks and increased vibrations to turbine
structures that may result in additional operational
loads. Threats include ice-induced
vibration, ice thickness, and ice drift.
" The magnitude of the ice load and
dynamic excitation due to ice failure are
usually dominating the structural design
in ice-covered sea areas, " note Maria Titemperatures
safely above freezing.
The same is not true for smaller projects,
especially for run-of-river generation, in part
because reduced flows during winter, when
much of a plant's watershed may freeze over,
mean that intake water may be drawn at or
very near the surface.
One of the unique wintertime risks is a phenomenon
known as frazil ice. Frazil ice forms
when supercooled water (water that is below
freezing but has not frozen) mixes through a
body of water in turbulent conditions. Small
ice crystals form throughout the flow and can
rapidly turn it slushy. If frazil ice is drawn into
a turbine, damage can result.
According to studies by the U.S. Bureau
of Reclamation, which operates many of the
federally owned dams in the U.S., prevention
of ice intake problems is mostly a matter of
good design, but seasonal preparation is also
important. If possible, an ice cover should be
induced upstream of the intakes. This will
keep intake water above freezing and prevent
the formation of frazil ice. Ice booms
and floating screens, which can form floating
" ice blockades " can be useful tools.
48
kanmäki, Jaakko Heinonen, Vilho Jussila,
and Juha Kurkela of the VTT Technical Research
Center of Finland. A site-specific
determination of the ice conditions and
an overall dynamic simulation of the wind
turbine with the ice interaction can enable
a cost-effective structural design
(Figure 1), they suggest. A common way
of decreasing ice loads is to include in the
substructure a cone that switches the ice
failure mode from crushing to bending.
Though energy intensive, trash rack
heating can prevent ice formation and is a
common method at plants in cold regions.
However, it is only effective when begun
before frazil ice begins to form. Upstream
temperatures should monitored, and these
temperature sensors should be checked regularly
to ensure proper operation.
Certain modern trash rack materials have
been shown to greatly reduce frazil ice accumulation.
Where it has been a problem in the
past, retrofitting nonmetallic trash racks may
be warranted.
Where frazil ice has already formed in and
around intakes, mechanical removal may be
necessary. The trash rack cleaning system
should be checked and tested well before
cold weather begins.
Solar Systems
With most solar systems (especially solar thermal)
being located in areas with mild climates,
severe winter weather is not usually a concern.
However, cold weather can affect solar photovoltaic
(PV) systems in other areas.
Most generators are used to lower PV
www.powermag.com
1. In cold waters. To make offshore
wind turbine foundations hardier in icy waters,
Finnish company Suomen Hyötytuuli
Oy began testing a new gravity-type iceresistant
foundation in the Pori sea area of
the Northern Baltic Sea in 2010. (This image
shows the turbine in heavy pack ice at the
end of February 2011.) The foundation consists
of a light steel shell with a ring footing,
ice-breaking cone, and crushed rock fill.
Courtesy: Eranti Engineering Oy
outputs in winter, but a snowstorm can cut
output to nothing as long as the panels are
covered in snow. Steeply inclined panels,
as with most utility-scale installations, usually
clear themselves quickly when the snow
stops, but rooftop panels may need to be
manually cleared, unless the sun returns after
the storm. Preparations need to be in place
for snow removal if a quick return to production
is important.
Because PV panels are of solid-state
construction with no moving parts or fluids
inside, they are much less vulnerable to extreme
winter weather than other generation
sources. That does not mean they are weatherproof,
however.
PV panels that undergo repeated freezethaw
cycles, especially with snowfall, are
vulnerable to water intrusion as freezing melt
water can force open small gaps in panel covers,
mounting brackets, and electrical connections
over time. Regular inspections of such
systems for weather damage are mandatory,
given that water intrusion into solar electrical
systems can cause serious problems.
PV panel mountings also need to be regularly
inspected for integrity, especially if the
system may be exposed to high winter winds.
This is a particular concern for freestanding
pole-mounted panels.
One positive side effect of cold weather
is that solar PV panels actually perform
more efficiently in colder temperatures,
though overall output is usually lower due
to reduced sunlight.
Storage Batteries
The lithium-ion (Li-ion) batteries that make
up most renewable on-site energy storage are
relatively weather-hardy, but they still need
attention during severe weather. Most such
utility-scale batteries currently operating are
deployed in containerized units that employ
HVAC systems to maintain optimal internal
temperatures, since Li-ion batteries in general
operate less efficiently in cold temperatures.
Proper maintenance and inspections of
the HVAC system, as well as prompt snow
removal from HVAC intakes, is important.
Such systems that are deployed in areas seeing
severe winter weather need to be hardened
so that they can continue operating in
sub-zero conditions. A properly designed and
maintained battery storage system should not
be overly challenged by winter storms. For
example, AES reported to POWER that its
Laurel Mountain facility in West Virginia,
currently the largest Li-ion storage system
in operation, weathered the 2014 polar vortex
storms with availability well above other
generation assets in PJM. ■
-Sonal Patel and Thomas W. Overton,
JD are POWER associate editors.
POWER | October 2014
http://www.powermag.com
POWER October 2014

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