When considering energy storage, chances are you aren’t thinking about the pitch-control system in a wind turbine. But these systems, standard in most utility-scale turbines, include an important power-storage component.
Because of the large rotors on these wind turbines, blades are exposed to wind speeds that vary from the highest point to the lowest. This results in uneven forces on each blade, and additional mechanical stress on the turbine and its components. A good pitch-control system helps manage these fluctuating forces and extends the life of a turbine.
Power storage for a pitch system needs sufficient capacity and a reliable cycle life. It has one advantage: a smaller system need not rely on batteries. About 65% of electric-pitch systems in turbines now use ultracapacitors instead of batteries for storing electric energy and that percentage is rising with time.
The pitch for ultracapacitors
Pitch controls in a turbine work by orienting the rotor blades at an angle that captures the maximum amount of wind power while acting as a safety mechanism that protects the rotor from spinning out of control.
Traditionally, turbine manufactures relied on hydraulics for these systems. More recently, many have opted for electric-pitch systems with battery backup. Both options require costly maintenance checks that limit their demand in the industry. The former because of the use of hydraulic fluid and the need to monitor a system for potential hairline cracks (from the high-pressure pump), and the latter because of a battery’s high-power density and relatively short lifespan. Batteries are also heavy, which make lugging them up an 80 or 90-m tower an unenviable job.
“Most turbine operators have to replace batteries in an electric pitch-control system every 18 to 36 months,” said Chad Hall, Co-founder and VP at Ioxus, a power technology company. “Throughout its lifetime, a battery’s efficiency drops to 70% and can go as low as 50% from the charge-discharge cycles. Fortunately, there is another option for power storage is efficient and trims maintenance costs.”
Enter ultracapacitors, which aren’t a new option in the wind industry. In fact, turbine manufacturers were some of the early adopters of these systems and for good reason. Ultracapacitors provide a lighter weight option than traditional batteries and offer a faster power charge with an efficiency that hovers around 98%. But it’s taken almost a decade for ultracapacitors to prove their value and gain market acceptance.
“That length of time is understandable,” he said. “People felt safe with batteries. They know how they will react and their life expectancy. Ultracapacitors required a longer learning curve, but they’re designed to last a minimum of 10 years without maintenance in wind turbines. Of the millions of cells in use in turbines today, I don’t actually know of a single failure in the last 10 years.” Hall expects some of the top-name turbine manufacturers to fully adopt ultracapacitors in the next couple of years.
The main reason ultracapacitors outperform batteries in efficiency and reliability is because they store energy as an electric charge rather than chemically, and so they can survive hundreds of thousands of more charge-and-discharge cycles. In a turbine, ultracapacitors also eliminate the need for slip rings when batteries are not mounted within the hub. The power-storage system is installed right in the hub, helping to reduce the weight and structural requirements usually required to fit a battery.
“When we first started in the wind industry, pitch-control systems used what they called 16V/58F modules that consisted of six 350-Farad cells in series,” Hall explained. The Farad unit measures how much electric charge is accumulated on a capacitor. He said wind turbines could include up to 90 per blade and that each module had six bolts and two wires, which translated into countless installations hours. “This made no sense. So we looked at a lot of the pitch-control systems, which were all divisible by 80V, and we created an ultracapacitors module that can stack up…two for 160, three for 240, four for 320, and so on.”
Hall and his team were able to reduce the bolts down to a dozen or less per module stack and the wires down to two per stack, substantially cutting installation costs. This also allowed for the removal of the battery heater because ultracapacitors work at low temperatures and don’t need one.
While this was an expensive venture in the early days of ultracapacitor-powered pitch systems, costs have dropped 99% in the last 10 years because of better materials, manufacturing, and processing. “Historically, we had to rely on return on investment just like turbine owners. But today ultracapacitors are at cost parity with a lead-acid battery at an original installation,” Hall said. He added that battery prices have also come down, but only by 30% and without a significant change in performance.
Nevertheless, Hall doesn’t deny there is a place for batteries. “They are workhorses and will always be around. The choice isn’t necessarily one or the other, ultracapacitors or batteries. There’s a place for both.”
Hall gives the example of a wind farm that would integrate storage using an ultracapacitor to handle short, under 30-second charges and a battery that could handle the long-term energy storage needs. “Ideally, both devices would work in combination,” he said. “Battery life extends two to eight times when an ultracapacitor works alongside it, taking care of all of the high discharge and recharge currents. Then the battery can do what it does best: maintain long-term storage.”
Combining storage systems
In Ireland, a country that has a target goal of 40% renewables within the next five years, ultracapacitors are already working alongside batteries to support the grid in an energy storage system.
The Irish distribution network is essentially an island grid that relies heavily on wind power, except for two smaller power lines that connect it to Great Britain. More than half, and on some days up to 75%, of its electricity is generated from renewable sources. Impressive but with variable wind speeds, the grid needed stability measures.
With the help of German renewables’ developer Freqcon, Ireland deployed its first combined ultracapacitor and energy storage facility last year for the Tallaght Smart Grid Testbed in South Dublin County. The Testbed uses a microgrid stabilizer for voltage and frequency stability, in combination with lithium-ion batteries and ultracapacitors for active power support in the grid’s distributed network.
“The Tallaght Smart Grid Testbed will show that energy storage is the key to minimizing grid instability issues as more renewable energy sources come online,” said Wolfgang Beez, Senior Product Marketing Manager of Maxwell Technologies, a U.S.-based producer of power delivery and energy storage solutions. Maxwell provided the ultracapacitors for the project. “With an increasing number of new-generation capacity stemming from wind and solar farms, advanced energy storage systems that use technology, such as ultracapacitors, are critical for the success of reliable distributed energy generation.”
Norbert Hennchen, CEO of Freqcon agreed. In a press statement about the project, he said: “The market for grid-tied energy storage systems is growing, and fast frequency response is a valuable system service to the grid. Ultracapacitors are the ideal technology to do this.”
The electrostatic energy storage mechanism of ultracapacitors lets them charge and discharge in fractions of a second and perform over a broad temperature range (-40 to +65°C). But an ultracapacitor can store only about five percent as much energy as a lithium-ion battery of similar size. Working together, the two can optimize power use, the ultracapacitor taking care of the rapid high-discharge and recharge current, and the battery standing up to the long-term storage needs.
“By combining two complementary technologies to provide a service that neither would be able to do as efficiently alone, you come up with a cost-efficient, fast-frequency response system that also provides backup power with the batteries,” said Beez. “We are looking forward to seeing more of these systems deployed in the future.”
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Filed Under: Energy storage, Projects