Will heat be the winner in de-icing turbine blades?
When it comes to keeping wind turbines operating smoothly in icy conditions, there are several approaches being considered and tried, from the highly technical to the old school ‘shiver’ technique.
By Susan Kraemer
Some of the best wind resources in the world are in some of the coldest regions. These regions, with the best potential for wind energy because of their sparse population and favourable wind conditions, are expected to host 72 per cent of new wind farms through 2017, according to Finland’s State Technical Research Center Valtion Teknillinen Tutkimuskeskus (VTT).
But howling winds and temperatures as cold as -35 C can place enormous demands on wind turbine systems - and on the incomes of wind farm operators, because ice build-up on blades can reduce electricity generation by as much as 20 per cent.
There are a variety of methods that have been considered or are in use for reducing ice build-up on wind turbine blades. The passive solutions include hydrophobic coatings, or blades that have been sprayed with a chemical or painted black to maximise heat absorption.
The active solutions break down to either mechanical means, such as controlled blade acceleration and deceleration to shake off ice, and several options for thermal solutions, including microwaves, heated air inside the blades and built-in electric heating foils.
Several factors make de-icing technology essential in cold climates. Not only can operations themselves be affected, impacting generation income, but when clumps of ice fall off turbine blades they can also represent a real danger to workers or to farm animals far below. In addition, the weight of ice on blades can also cause unbalanced rotor spinning. This can lead to mechanical failures down the line.
Both Vestas and Siemens are currently backing thermal solutions, trying two thermal approaches; heated air inside the blade, and electric heating elements embedded in the blade itself.
Siemens took a look at the options years ago and has collected data about the operation of wind turbines in extreme climate zones since 1994. In 2012, it offered a system for de-icing blades using a heating mat integrated into the blade. This heating mat has no wiring within the blade, but is electrically conductive and is installed close to the surface of the blade, to be switched on when necessary to melt the ice and let the machine continue operation.
Siemens patented its heating mat in 2012.
Vestas combines sensors, databases, and communications to operate a hot air flow unit that runs inside the blades.
Rather than directly detecting actual ice build-up, the VDS algorithm deduces it from a measurement of several variables, including temperature, humidity, wind speed, and turbine output. When the output is below par for the wind conditions and temperatures are low enough for ice to be the cause, de-icing is initiated automatically by the Vestas De-icing System (VDS).
“VDS will provide significant value to those who want to harness the potential of wind power in colder climates with icing risk, locations such as North America as well as the northern and central regions in Europe, areas previously not economically feasible due to the risk of ice affecting power production,” says Vestas Chief Technology Officer Anders Vedel.
VDS is available for several models of Vestas 3.3MW turbines. The system is fully integrated with its control systems, and serviceable from within the hub and inside the blade. It does require installing electrical components in the blade’s leading edge. And it is a flexible system. Clients can tailor it to their preferred de-icing strategy, either allowing it to trigger automatically or triggering it manually - ensuring full monitoring control of the system.
Vestas first order is for 4 VDS units in Austria this year, after testing a prototype in Canada in 2013. It has a relatively low 150kW efficiency cost in a 3.3MW turbine.
But is the market ready?
But not everyone feels the need for thermal solutions, even in the very cold climates that should be the ideal market.
Todd Simmons, General Manager of Wind Operations at Minnesota Power says the conditions they see in Minnesota and North Dakota - where winter temperatures can get down to -34C - rarely impact operations. “We probably only have icing conditions that impact our operations between five and eight per cent of the time on an annual basis,” he explains.
Their blades have no special coating to either protect or radiate additional heat, but they do have a rotor torque sensor that is designed to detect imbalance caused by icing that will shut the unit down for a short time, at which point the unit will wait for the prescribed duration before restarting.
“We typically wait until temperatures change enough that the units can shed the ice or in some instances we ‘shiver’ the blades. Just start and stop them quickly in order to flex the blades enough to break the ice free,” he says.
It remains to be seen what will turn out to be the ultimate solution to getting ice off turbine blades. It all comes down to personalised choice and a utility or farm operator’s appetite for operational risk and down time.