Electricity: Part 5 - Wind Turbines

Wind turbines are increasing in number because they produce electricity with reasonable environmental impact. But how green are they really?

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In the good old days, the traditional windmill was built to grind corn, to pump water or to drive a sawmill. The wind turbine is basically a good old-fashioned windmill that has been optimized to drive an electrical generator.

The performance of wind turbines is entirely bounded by the laws of aerodynamics and good designs reach towards those bounds. The most fundamental of these bounds is that the available power is proportional to the area of the blade disk. The other is that the available power goes as the cube of the wind speed.

As the disk area goes as the square of the diameter, it follows that large size is advantageous, provided that the larger stresses involved can be resisted. It also follows from the cube law that a wind turbine will have to generate in two modes with respect to its rated power. In one mode the wind speed is less than is needed to deliver full power and the turbine should operate at maximum efficiency to extract as much power as possible. In the other mode for its own protection the turbine must deliver no more than its rated power and the efficiency is no longer relevant.

There are many different types of wind turbine but they can be distinguished by the orientation of the blade axis. The horizontal axis wind turbine (HAWT) is more efficient, but needs to be steered into the wind. The vertical axis wind turbine (VAWT) is less efficient but does not need to be oriented.

As wind power is free, it changes the meaning of efficiency. Perhaps a better metric is the energy available over the proposed lifetime of the turbine divided by the unit cost, both financial and environmental.

Matching something as capricious as the wind to the precision of a synchronous ac power grid is a non-trivial problem and various control solutions have evolved. Early wind turbines used alternators that were driven by step-up gearing from the rotor shaft. In order to be synchronous with the 50- or 60Hz grid, the rotor could only turn at one speed.

The speed of the rotor was controlled by adjustment of the collective pitch of the blades. The output of the alternator was controlled by adjustment of the field current. Once the alternator was in-phase with the grid it could be connected and would then run at constant speed, using the blade pitch to adapt to changes in wind speed.

It was always possible to tell if a wind farm used synchronous machines, because all the rotors would turn at exactly the same speed.

The turbine rotor is an aerodynamic device like an airscrew or a helicopter rotor. It is basically a rotating wing having an airfoil section. Some energy is wasted driving the airfoil through the air. This is known as profile drag and is unavoidable. The goal is to extract the most useable power with respect to the profile drag. This is governed by the relationship between the wind speed along the axis of rotation and the tip speed of the blade at right angles to that.

If one visualizes the rotor screwing its way through the air, the tip speed ratio effectively relates to the fineness of the thread. There is an optimum tip speed ratio at which the rotor is at its most efficient. The amount of twist in the blades will be determined for that condition. Above and below that ratio efficiency falls. With a synchronous generator, the rotor speed is fixed, so only one wind speed delivers maximum efficiency.

The solution to that problem is to use power electronics between the alternator and the grid so they can operate at different frequencies. This allows the rotor to run at whatever speed allows the most power to be generated.

As the wind speed increases, the maximum power level will be reached and from then on the control system operates to prevent damage. It will do this by limiting the alternator output and by using the collective pitch to feather the blades and prevent over speed.

Fig.1 - As hydropower can be drawn at anytime, provided there is water in the lake, it can be used to maintain supply of electricity when wind or solar power is insufficient.

Fig.1 - As hydropower can be drawn at anytime, provided there is water in the lake, it can be used to maintain supply of electricity when wind or solar power is insufficient.

With twisted blades, at very coarse pitch settings, one part of the blade can have a sensible angle of attack and continue delivering power, whereas another part of the blade will have a huge angle of attack and will be stalled. In extreme winds it is possible to feather the blades so that the average angle of attack is zero and they can be brought to a halt by a combination of dynamic braking and mechanical brakes.

It is not much fun being a wind turbine blade, as the smooth airflow needed to provide constant forces hardly ever happens. Turbulence exposes the blades to fluctuating forces that shortens their lives. Disposing of time-expired wind turbine blades is a significant problem that has not fully been solved, resulting in large numbers of old blades simply being buried.

Early wind turbines required gearboxes to raise the rotor speed up to what the alternator needed. The gearbox was heavy, expensive and subject to wear. The development of extremely powerful rare earth magnets meant that the direct drive wind turbine became a possibility. In the direct drive machine the rotor and the rotating section of the alternator rotate together on the same bearing and the gearbox is eliminated.

Direct drive wind turbines with permanent magnet alternators and power electronics can operate over a range of rotor speeds, so that for a given wind speed they can extract more power by running at an advantageous tip speed ratio.

Wind turbines are an indirect form of solar power in that it is heat from the sun that makes the wind blow. Wind turbines and photo-voltaics have in common that they are both low-density power sources. What this means is that not only do they require significant area but the output is also statistical rather than constant.

In other words, solar and wind turbine installations produce power when they can and not necessarily when it is needed. In the real world that means they cannot alone provide viable energy solutions. Either they need to work alongside a different type of electricity source, or they need to be equipped with some sort of energy storage system.

Fig.2 - In a pumped storage system, surplus solar and wind power pumps water uphill so it can be released later.

Fig.2 - In a pumped storage system, surplus solar and wind power pumps water uphill so it can be released later.

There is no doubt that photo-voltaic panels and wind turbines themselves have a low carbon footprint. The carbon debt of manufacture is soon offset by the zero carbon operation. However, when it is realized that solar and wind power cannot work alone, the carbon footprint of what they must work with has to be considered.

Fig.1 shows a system in which photovoltaics, wind turbines and hydro-electric power are combined. Unlike PV and wind turbines, the potential energy of water can be released at any time, provided there is some water up there. All of the power available from PV and wind is used, and any shortfall is made up using hydropower. Such an approach is used in, for example, Sweden, which has considerable hydropower.

Fig.2 shows an extension of this idea, which requires the hydro-electric system to be reversible. When there is a power surplus from PV and wind, water is pumped uphill. When there is a power shortfall, the water is allowed to run down again to generate power.

Whilst it is an ideal, putting Figs. 1 and 2 into practice is not so easy, especially as places that have plenty of sunshine tend not to have much water. Fill a dam with water in such places and someone will want to drink it or irrigate fields.

That's a pity, because the environmental damage due to building a dam is spread over a very long operational lifetime. Often PV and wind power are levelled using thermal electricity stations.

The alternative form of energy storage is a battery. Electricity companies are faced with uncertainty in demand for electricity, which has peaks and troughs and short spikes and the further uncertainty from solar and wind power doesn't help. Batteries are seen as one solution to leveling the difference between supply and demand. While there is no doubt that the idea works, the problem is that the batteries needed are huge.

For example, a 3 megawatt-hour lithium battery used in power levelling holds about 10 gigajoules of energy when charged. That's the equivalent of two tons of TNT.

The real problem, however, is the embodied carbon, also known as the carbon debt, of the batteries, along with other environmental damage due to mining of the materials used in battery manufacture. Embodied carbon applies to all manufactured products and represents the carbon dioxide emitted during the entire process leading up to delivery. The embodied carbon content of batteries is one of the skeletons in the cupboard of clean electricity.

Once the respectably green photo-voltaic panel or wind turbine is combined with battery storage, which must often be the case for practical reasons, the respectability and greenness takes a huge hit. More will be said about that when the topic of batteries is considered.

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