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Understanding wind energy collection

Dec. 26, 2024
How to better control turnbines to unleash their full potential

Wind energy is one of the least expensive and cleanest method of electricity generation. Automating  wind turbines operations is an interesting challenge for our profession. In my "Ask the Experts" column, coming in January 2025, I’ll continue  discussing wind turbines, but will focus on the sensors and control loops used to guarantee their safe and optimized operation. Here, I focus on the process of wind energy collection because it must be fully understood before it can be properly controlled.

Wind is fueled by solar energy because it’s generated by temperature differences in the atmosphere, which causes pressure differences that move air towards the poles. The driving force is combined with the effects of Earth's rotation, the Coriolis effect, and the consequences of humidity and land-surface variations, which result in the complex wind patterns we experience. The total kinetic energy content of these air flows is immense compared to mankind's total energy needs, which it exceeds by about a million times.

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The main advantages of using wind energy is its fuel is free and there are no carbon emissions. Disdvantages include aesthetics, noise (older units), low energy density (area requirement = 10 times that of solar parks), and relatively short life spans (about 20 years). 

Areas with high wind energy potential include a 50-mile wide strip on seashores. Because the wind is often blowing at night or when solar energy isn’t available, total energy production becomes more continuous if the solar and wind farms are combined. It not only reduces the size of the needed energy storage, but also reduces the land requirement for combined hybrid plants (Figure 1), an important financial consideration in locations where land is expensive.

The Adani Green Energy Limited (AGEL) power plant in India combines wind and solar power to provide clean electricity to more than a million homes in the region. AGEL’s energy plant at Khavda, Gujarat, begins with wind-energy generation of an initial 250 megawatts (MW), enhancing Khavda’s combined operational capacity to 2,250 MW, which is similar to a nuclear power plant.

Compared to the size of other energy producers, the share of wind-based electricity generation capacity is still small. In the U.S., it’s about 4% of the total. Until the most recent U.S. election, it was planned to reach 30 gigawatts (GW) by 2030, but that is now unlikely. The total global solar electricity generation capacity is about 600 GW.

Globally, the use of the free, clean and inexhaustible wind and solar generation capacities were quickly increasing because the cost of both wind and solar electricity costs were dropping (Figure 2). This was the case until recently. Today, the costs of installed wind sytems are increasing, and only solar system costs are decreasing. Fossil fuels and nuclear electricity costs are nearly twice as high, and they too will probably continue to rise.

Wind turbine components and operation

As shown in Figure 3a, turbine blades are curved, so wind blowing over the top must travel a longer distance, and therefore the pressure drop over the curved surface is higher than on the flat side below. This causes a pressure difference that generates the lifting force that spins the blades. The energy generated increases with the area swept by the blades (longer blades equal more area). Because wind speed is slower near the ground, it generates more power as hub height increases. As a result, if wind speed doubles, generated power increases eight-fold (cubic relationship). The lifting force also increases if the angle of the "chord line" of the blades is increased. This is the case up to an angle of around 10° to 15°. Any greater angle, and "drag" forces start to exceed lift and the turbine stalls.

Electric power generation begins when wind speed exceeds a minimum "cut-in" value (Region I in Figure 4a). As wind speed increases, electricity production also increases (Region II), until the turbine blades reach the maximum ("nominal" or "rated") speed, which corresponds to the maximum power generation of the turbine. When the wind speed increases further (Region III), rotation is slowed by increasing the chord line angle (α the blade pitch) to prevent the speed of the blade tips from exceeding safety limits. The longer the blade, the more difficult it is to maintain the integrity of its tips. Such a breaking accident occurred recently when a blade of a GE Vernova turbine broke at the Vineyard Wind farm off the coast of Massachusetts. As shown in Figure 4a, rated power production continues throughout Region III, until the wind speed reaches the maximum allowed (cut-out) velocity and the turbine's is stopped.

Figure 4b shows the actual yearly power production of one of today's largest offshore wind turbines. Its production naturally varies with the yearly average wind speed at the location.

The faster the blade rotation and the larger the hub height and the rotor diameter, the higher the wind turbine's nameplate capacity, which today can be up to about 3.5 MW or more (Figure 5). The speed of the blade tips must not exceed about 80 m/s (180 mph) because, as the Vineyard Wind accident shows, higher speeds can damage blade tips. Operating noise level is also a consideration.In most locations, it must be kept under 40-60 dB. Sonic booms that occur at speeds of 761 mph must always be prevented.

Today, the hub heights on utility-scale, land-based units is around 100 m and their heights are also increasing. Turbine efficiency increases with hub height because the blades reach higher, where wind speeds are faster than near the ground (where vegetation, etc, slows it). On the other hand, increasing hub heights requires stronger structures to support the weight of the nacelle.

Offshore wind farms

The design and control requirements of wind turbine installations become more challenging when a wind farm is moved from solid ground onto the ocean because land-based units only need to respond to changes in the wind’s aerodynamic thrust forces, while offshore units must also respond to the changing hydrodynamic thrust forces of  waves and account for  interactions with mooring lines.

As shown in Figure 6, types of offshore installations vary with ocean depths. Besides keeping the hub stable, another control challenge is protecting the cable that transports generated electricity to the grid. One condition to protect against is keeping the cable from twisting due to turbine rotation. Eventually, a total control package will have to be developed, serving the optimization and safety of wind turbines. The overall goal of this optimization envelope, which is to maximize electricity production up to but not exceeding safety limits, is like those  in other industries. However, because aerodynamic and hydrodynamic forces are large and unpredictable, development of the required algorithms will represent a major challenge to our automation profession.Otherwise, accidents like the one at Vineyard Wind will keep happening.

About the Author

Béla Lipták | Columnist and Control Consultant

Béla Lipták is an automation and safety consultant and editor of the Instrument and Automation Engineers’ Handbook (IAEH).

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