IAN LANG ELECTRONICS
The United Kingdom has the dubious honour of being the windiest country in Europe, and the Government therefore has latched on to this fact and is keen to develop wind farms on a large scale. The topic is highly controversial, and so let's have a balanced look at what is involved in generating electricity from the wind.
First of all, it might be beneficial to look at what wind actually is. It's when two different masses of air, each at different pressures, meet and try to equalise. High pressure is caused by air masses being cooled. As air is a mixture of gases, when cooled it contracts. Low pressure is caused by air being warmed and subsequently expanding. The warming and cooling can have many causes, solar activity, urban centres, and the transition of a mass of air from land to water (over which it cools more rapidly) are a few. Currents from the tropics into the temperate zones are another.
Air will not flow from high to low pressure areas directly, since as the Earth rotates it is deflected in the Northern Hemisphere to the right and in the Southern Hemisphere to the left , so wind flows normally around the high and low pressure areas. This creates a current which will last until the pressures are equalised.
The closer the high and low pressures are, the stronger the winds but the more localised. Nearer to the ground, the frictive effect of the surface causes wind to slow dramatically from that at higher altitudes, and this is more prevalent at night than day because little or no convection current exists at night-time and the surface wind can stop altogether.
Newton says that every action has an equal and opposite reaction and true to this, a strong wind can slow the rotation of the Earth very slightly. This happens in the northern hemisphere's winter, with build-ups of strong westerly winds.
So, how can we extract wind energy and turn it into useful electricity? Put simply, the wind turns an aerofoil, the aerofoil turns a generator and the generator produces electricity.
The above statement, whilst being entirely accurate, is very misleading in its simplicity as there are many factors involved in succesfully building a wind turbine. Not the least of which is which type we are going to use. There are two broad classifications :
Vertical Axis Wind Turbine. (VAWT)
In a VAWT the axis around which the turbine rotates is perpendicular to the ground. There are two main types of VAWT, the first, which you will rarely see, is the Savonius type:
This is a very simple system which consists of two cups. Initially one will face wind direction and the other away. The wind collects in the concave and deflects from the convex causing a pressure imbalance from side to side. The cups are mounted on a rotating shaft, which moves and takes the cups with it. Eventually the shaft rotates 180 degrees, and the second cup is now facing the wind, and the second half of the cycle begins.
The shaft of course is attached to a generator at the bottom of the rig.
You could in theory have more than two cups but eficiency is impaired if you do. The advantages are that they are easy to make and repair, and stand up to very strong winds. The disadvantage is that the force on that bearing is very strong and it has to be a sturdy beast and replaced regularly and that they cannot rotate faster than the windspeed. Thererefore they turn slowly but generate a great deal of torque. They are best suited to very small installations for domestic or light industrial generation as the mechanism does not scale well to large kW schemes due to the fact that the cups and the bearing would become large and unwieldy.
The second type of VAWT is much more common. It is the Darrieus type:
The Savonius above used drag to rotate itself, the Darrieus uses lift. The blades are shaped across the width rather like an aircraft wing, with the leading edge (facing the wind) thicker than the trailing. They are also curved on one side more than the other. An imbalance of pressure is felt on one side of the blade as the wind passes over, causing a lift rather like that on the wing of an aircraft that keeps it up in the air. Consequently the blade moves away, bringing the other one in line and the process repeats. This means they can in fact spin at a speed faster than the windspeed and so are more suited than the Savonius for turning a generator. The centrifugal forces acting on the blades as they turn are enormous and the blades have to be very strong. Bending them in the manner shown from top to bottom helps to spread the load and gives them their distinctive egg-whisk shape.
Since the blades turn fast, they produce little torque and thus the inertia of the turning assembly cannot be overcome by itself alone- in other words they cannot self-start. One solution is to have an electric motor initialise the turning, another is to put a small Savonius array on to start the turn.
Incidentally this is not, as you would think, a recent invention. The patent was issued in 1931. You can see the disadvantage- they are big, they are ugly and need lots of space to be guyed in to stop from blowing over. (You can see the cables emanating from the top of the tower).
You might stumble on a Darrieus somewhere but something you won't be able to miss is the second type of turbine. I see at least one of them every day. It is:
Horizontal Axis Wind Turbine (HAWT)
Yup. It's the big propellors that are fast becoming ubiquitous. These particular ones are Royd Moor Wind Farm and they produce enough energy to power 3300 houses in theory.
In fact they aren't really propellors. They are tri-bladed aerofoil turbines, if you please, and what this means is that they are in fact wings, and there's three of them, and they go round. Here's a drawing of what the profiles of each blade look like:
And as you can see, the principle is the same: you get less pressure on one side than you do on the other, this causes the aerofoil to move, and because it's tethered to the hub it moves the hub and the rotating shaft with it. The blade itself is pretty high tech:
Inside it gets even more complicated:
What do all these bits do? Starting rom the nose, pitch control adjusts the angle of the blades to get the best angle of attack on the blade for the wind. The better the angle, the more energy you can extract. The low speed shaft drives the gear box. Underneath that is a brake. It stops the turbine operating by means of a disc on the high speed shaft. The high speed shaft is driven by the output of the gear box. It turns magnets through a coil, producing electricity (in fact it's a dynamo, just like you used to have on bicycles to work the lights and similar in principle to the alternator on a car). The controller takes readings and does all the adjustments automatically, including the yaw which moves it to face the wind. It can be overriden and the turbine can then be adjusted manually. The anemometer measures the wind speed. You'll see why that's important later.The wind vane keeps it stable and the nacelle covers the working parts and saves them from the elements.
If you're thinking that this is high precision mechanical engineering, you are correct. If you're thinking it won't be cheap, you're right again. A very small one for mounting on your house will not see much change out of £600. Installations like Royd Moor, which need specialist construction and transportation, cost millions.