Elements of Renewable Energy (Wind energy)
The origins of wind and atmospheric pressure
One square metre (1 m2) of the Earth’s surface on or near the equator receives more solar radiation per year than 1 m2 at higher latitudes.
The curvature of the Earth means that its surface becomes more oblique to the Sun’s rays with increasing latitude, and the Sun’s rays have further to travel, so more of the Sun’s energy is absorbed en route before it reaches the surface. This explains why the tropics are warmer than higher latitude regions. This causes variations in atmospheric pressure, which give rise to the movements of atmospheric air masses that are the principal cause of the Earth’s wind systems.
What is atmospheric pressure?
It’s the pressure resulting from the weight of the column of air above a specified surface area. The unit of atmospheric pressure is the bar, measured using a barometer – usually calibrated in millibars (mbar), i.e. thousandths of a bar.
If we look at a weather map like the one below, similar to those featured in television weather forecasts or in newspapers, we see that there are regions marked ‘high’ and ‘low’, surrounded by contours.
The regions marked ‘high’ and ‘low’ relate to the atmospheric pressure and the contours represent lines of equal pressure called isobars. The high-pressure regions tend to indicate fine weather with little wind, whereas the low-pressure regions indicate changeable windy weather and precipitation (rain or snow).
Energy and power in the wind
The energy contained in the wind is its kinetic energy, and in accordance with basic physical principles the kinetic energy of a moving mass (moving air in this case) is equal to half the mass, m, (of the air) times the square of its velocity, V:
It can also be shown that the power in the wind passing through a wind turbine rotor is proportional to the density of the air, the area of the rotor and the cube of the wind velocity
The density of air is lower at higher elevations such as mountains, and average densities in cold climates may be significantly higher than in hot regions. Wind velocity has a very strong influence on power output because of the ‘cube law’. For example, a wind velocity increase from 6 metres per second (6 ms-1) to 8 ms-1 will more than double the power in the wind. But the power in the wind is not in practice the power that can be extracted by a wind turbine, because of losses in the energy extraction/conversion process.
Wind turbines – types and aerodynamics
Most modern wind turbines come in one of two basic configurations:
1. Horizontal axis wind turbines (HAWTs), examples of which are shown in the figure above. They can be multi-bladed like those which have been used since the nineteenth century for water pumping on farms. Modern HAWTs usually have two or three blades and work at much higher rotational speeds, making them attractive for electricity generation. They range in size from very small machines producing a few tens of watts to very large turbines producing 7.5 MW or more.
2. Vertical axis wind turbines (VAWTs) can harness winds from any direction without having to reposition the rotor. But VAWTs have found little commercial success to date, in part due to issues with power quality, cyclic loads on the tower systems and the lower efficiency of some VAWT designs.
Drag force and Lift force.
The shape of the blades on wind turbines is extremely important, as an object in an air stream experiences a force from the air stream that is equivalent to two component forces at right angles. These are known as the drag force and the lift force.
. At small angles relative to the direction of the air stream – that is, when the “angle of attack” is small – a low pressure region is created on the ‘downstream’ side of the aerofoil section as a result of an increase in the air velocity on that side. This ‘lift’ force acts as a ‘suction’ or ‘pulling’ force on the object, in a direction at right angles to the airflow.
Lift and drag forces are both proportional to the energy in the wind, and modern horizontal and vertical axis wind turbines harness aerodynamic forces in a different way.
In HAWTs the rotation axis is maintained in line with wind direction by a ‘yawing’ mechanism, which constantly realigns the turbine. In a VAWT, the wind direction constantly varies throughout its rotation cycle. This means that the ‘suction’ side reverses during each cycle, so a symmetrical aerofoil has to be used to enable power to be produced irrespective of whether the angle of attack is positive or negative.
Calculating power and energy from wind turbines
The power output of a wind turbine varies with wind speed. Every turbine has a characteristic wind speed–power curve, often simply called the power curve
The energy a wind turbine will produce depends on both its wind speed–power curve, as shown above, and the wind speed frequency distribution.
Factors affecting total generated energy
Current commercial wind turbines typically have annual availabilities in excess of 90%, many have operated at over 95% and some are achieving 98%.
But bearing in mind that other issues can affect the total generated energy produced, some things to consider in discussion with your fellow learners are:
- the reliability of the turbine itself to generate power
- the amount/fluctuation of the wind
- whether any generated power could be lost.
Wind turbines are often described as noisy by opponents of wind energy, but they are not especially noisy compared with other machines of similar power rating (see the figure below).
Noise can be reduced significantly by using acoustic enclosures for the machinery and also using slower rotational speeds to reduce aerodynamic noise.
Electromagnetic interference can sometimes occur if turbines are positioned between some types of radio transmitter, due to reflection of some of the waves. See image below. The extent of this depends mainly on the turbine blade construction material and surface shape.
Future prospects for wind energy
Wind energy looks set to become a major generator of electricity throughout the world. By 2013, total installed wind capacity world-wide had risen to some 318,000 MW. In Europe, the offshore exploitation of wind energy is likely to become one of the most important means of reducing carbon dioxide emissions from the electricity sector.
A European Commission report states:
With additional research efforts, and crucially, significant progress in building the necessary grid structure over the next ten years, wind energy could meet one fifth of the EU’s electricity demand in 2020, one third in 2030 and half by 2050 (Zervos and Kjaer, 2009).
This would require achieving 400 GW wind energy capacity in 2030 and 600 GW in 2050, with the majority (350 GW) of the 2050 capacity coming from offshore turbines.