Prepare Your Renewable Plant for Cold Weather Operations

Although renewable generation is typically less affected by winter weather than gas- and coal-fired plants, it is far from weatherproof. Understanding the risks and preparing properly for them can avoid most problems.   

Last winter’s polar vortex was a reminder that, despite several years of mild winters, colder months can still pack a wallop. Decades of coal, gas, and nuclear plant operations have taught plant operators the dangers of bad weather, but the relative novelty of utility-scale wind and solar generation means many newer owners are unaware of what can happen to performance when temperatures plunge and the snow begins to fall. While there may be no pipes to freeze in a solar plant or wind turbine, for example, there are still risks from severe winter weather.

Wind Farms

Though many wind turbines are installed in regions of the world that see bitterly cold winters, wind power isn’t immune to extreme cold events.

During the February 2011 Southwest cold snap, an unprecedented 16% of wind units within Texas grid operator Electric Reliability Council of Texas’ operation region reportedly failed—709 MW due to blade icing and another 1,237 MW because frigid temperatures exceeded turbine limits.

And although PJM Interconnection lauded wind power’s “positive impact on supply” during the January 2014 polar vortex, saying it contributed to PJM’s ability to maintain reliability, data shows that wind energy generation dropped precariously during demand peaks during the Jan. 6–9 extreme weather event. The Midcontinent Independent System Operator’s system also braced for the plunge in wind output following Jan. 6, crediting accurate wind forecasts. However, it records that it saw 1,000 MW of wind power “cutouts” as a result of the extreme cold on Jan. 6. The Federal Energy Regulatory Commission later speculated that the bulk of those wind turbine failures were due to “models reaching their minimum operating temperatures.”

Bracing for Low Temperatures. Wind turbines are typically designed, per international standards for turbine design, to operate within ambient air temperatures of –10C while operational and –20C at standstill. (For offshore considerations, see the sidebar.) As the North American Electric Reliability Corp. (NERC) notes, turbines have an automatic shutdown feature to protect components if that range is exceeded.

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 Tikanmäki, 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.

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 ice-resistant 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

Wind generators in places like Canada typically install “cold weather packages” to extend temperature ranges, using up to 200 kW to 300 kW of parasitic power per turbine at conditions below –20C for heating components such as the nacelle space, yaw drive and pitch motors, and the gearbox, slip ring, controller and control cabinet, and battery. GE’s 2010-introduced Cold Weather Extreme package for its 2.5 x l turbine, for example, ensures operations in temperatures to –30C and a “survival mode” to –40C.

According to NERC, “it does not appear that [cold weather packages] were used in the Southwest” during the 2011 event. It recommends that all entities investigate the purchase of these packages in preparation for extreme cold events.

Heating could also prevent another insidious cause of turbine failure: When a turbine is not running, oil that is stationary in radiator passages can quickly cool, and its viscosity can increase. Even if wind turbines are not being used, an important lesson worth learning is that the turbines should be cycled online to provide flow of cooling oil, NERC says. All cooling equipment for radiators on wind turbines should also be disabled for cold weather events.

Preventing Icing. Icing on wind turbines affects three different aspects simultaneously: the design (aerodynamics, load, control system, and material), the safety (ice throw, unbalanced rotor spinning, over-power, and fatigue), and performance (annual energy output, wind measurements, and design life duration). But it can also affect wind sensors—rendering ineffective wind-measuring equipment—as well as increase noise levels and generally decrease a turbine’s cost-effectiveness.

A deep freeze could have long-term implications for a wind turbine, warns Jeff Nelson, a TransAlta wind engineer. Turbines can eventually fault and shut down; in some cases they have had up to 250 millimeters of ice. That could mean an extended period of downtime (if ambient temperatures drop significantly and ice cannot be shed) of up to three weeks.

For TransAlta, key takeaways from previous icing debacles at its 96-MW Kent Hills wind farm in Canada’s northeastern province of New Brunswick include the imperative to catch icing events early, using instrumentation, ice detectors, anemometer cameras, or/and blade accelerometers. Twenty-four-hour monitoring of icing conditions is also essential.

And here too cold weather packages can prove valuable. Some offer, in addition to technical solutions that widen operating temperature ranges, active or passive de-icing or anti-icing systems for rotor blades. Systems use a variety of methods, including heat, water-resistant coatings, and controlled blade acceleration/deceleration to shake the ice off.

Last year, for example, Vestas introduced a de-icing system that can detect an icing condition (using algorithms measuring several variables, including temperature, humidity, wind speed, and turbine output), determine whether it’s worth shutting down the turbine to perform de-icing, and initiate a de-icing procedure if it is worthwhile. The outer third of each turbine blade essentially includes a small heating element and fan that uses up to 150 kW to run.

Hydroelectric Plants

Of all generation resources, hydroelectric plants may be the least affected by nasty winter weather. Indeed, many countries around the world that rely heavily on hydropower, such as Norway, Iceland, Canada, and Russia, must also deal with severe winters. Though reservoirs may freeze over, penstock intakes are usually deep enough to withdraw water at temperatures 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.

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 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 freeze-thaw 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.

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