Out of all the possible renewable power opportunities there is little doubt that Wind Power is the most exploitable. There are many conventional types of energy production, such as coal, gas, nuclear, which are being supplanted by wind energy in 21st century power production.
Of the two commercially viable renewable energy sources of Wind and Solar it is Wind energy that represents the most cost – effective solution, especially in temperate climates.
Although when we think of wind energy we think of giant (possibly offshore) turbines creating many Megawatts of electricity it is important to remember that Wind Energy has been exploited for centuries. From the renaissance onwards wind energy used to be employed in Dutch windmills, these used to be used for water pumping and primarily grain grinding. They provided energy to communities for centuries before being made obsolete with the advent of the steam engine.
Modern turbines are obviously infinitely more efficient than 16th century windmills but the conversion of wind to rotational energy is much the same. Whilst these historic windmills worked on the principal of thrust to ‘push’ the sails modern turbines operate based on the aerodynamic principles of a turbine to create a pressure difference of an aero foil to create a vastly more efficient turbine. In addition, modern turbines can not only turn into the wind but control the pitch of their blades to maximise the efficiency of the aero foil in relation to the wind speed.
The issue with Wind energy however is the same as it was in the 16th century, if there is no wind there is no power, whether that be to produce electricity or for the grinding of flour. As Wind Energy is inherently by nature unreliable it is classed as a low-quality form of energy, the disorderly motion of air molecules and the inconsistently in wind rates means the consistency of the power given by wind remains unreliable and inconsistent.
The aim of modern turbines is to convert the low-quality energy of Wind Power to an efficient high-quality output of electrical energy. Much like the conversion of heat energy into mechanical energy within an internal combustion engine the conversion of wind power into electro mechanical power has an efficiency limit. A slight change in wind speed will make a large difference to the power available from it and therefore Wind Power’s viability is limited to parts of the world with consistently high winds. As wind speed is critical to the amount of power available from the wind, irrespective of the efficiency of any turbine the amount of power that can be extracted is always limited by that factor.
Each turbine will require a certain amount of wind to turn. The point at which the turbine will start to make rotational energy is referred to as the Cut-in wind speed. At this point it will start producing power. The thing to remember with Wind Energy however is that the power output will always be proportional to the power contained in the wind and that the electrical output is exponentially affected by wind speed.
As the wind speed increases the output of the turbine obviously increases, in low to moderate winds the power output from a turbine can be highly efficient. However, in high wind speed conditions another issue can become of concern, namely that you reach the power generation capacity, the rated power of the generator attached to the turbine. It is essential that as the wind speed increases that turbines do not allow the rated output of the generator to be exceeded. If this were to happen then the generator could quite feasibly overheat, under overload conditions the probability of the machine being permanently damaged or even catching fire becomes very real. To a large extent a modern wind turbine can reduce or increase the speed of the input shaft driven by the wind turbine by passing the rotational force through a gearbox. This effectively gives the turbine a broader window of operation for power generation, In low wind speed conditions the speed of the input shaft is increased at the cost of torque but this is usually still sufficient to drive the generator.
To ensure the power output remains equal or less than the rated power irrespective of wind speed modern turbines close at their full capacity. This process is called cut-out or furling wind speed and is essential to protect the generator. This is usually achieved through a brake system situated between the gearbox and the generator. It not only serves to protect the generator but also the rotors that could overcome their rotational capacity in very high winds and simply break apart.
For small wind turbines of less than 1 kw output rotor blades can be constructed from wood, for large wind turbines the use of composite materials and fibre glass which gives the structural strength and lightness that allows the rotors to be transported to the site. The size of a blade for a 1 MW turbine can be the size of a football field and as such this creates some serious issues that require overcoming to produce the turbines. The size of a facility required to construct one single piece of glass fibre that size as well as keeping a consistent taper and strength throughout the structure make the construction of the blades a highly specialist job.
In addition to the construction of the blades the tower to which the blades are mounted must be able to not only support the not insignificant weight of the turbines, gearbox and generator but also resist the forces of thrust from the wind that will act upon it for many years, essentially trying to topple it over. As such it is imperative that the towers are very strong and capable of resisting the forces. Normally the height of a tower is twice the diameter of the blades so for large capacity turbines the towers are truly massive, this is done to raise the turbines into clean air and to higher windspeeds further from the ground.