Wind energy is the energy present in the flow of air around the earth. The wind is driven by the temperature and pressure differences in the atmosphere arising from heating of the earth by the sun and it is further guided by topography. The energy stored in the atmosphere is in three forms, kinetic, potential, and thermal. They all play a part in the motion of wind, but the main interest is in the kinetic energy from the wind's motion.
The term wind energy is most often used to refer to the generation of useful energy from wind. This can be electrical, as in wind turbines, or mechanical, as used in wind pumps, used often in simple agriculture. The kinetic energy of the wind is harnessed, initially by conversion to a rotation; this rotation can be used in a generator, or directly for a mechanical task, such as pumping, or milling.
The wind has been used for thousands of years as a source of energy. Sailors capture it in the sails of their ships, and weather has therefore had important historical consequences, for example, the Spanish Armada was defeated with the help of stormy weather around the British Isles. The Netherlands are famous for the use of windmills with four cloth sails. These were used for pumping water to drain polders forming agricultural land. In the UK, Norfolk borrowed this Dutch expertise to do the same, leaving distinctive features on the flat lowland landscape.
In poor agricultural communities, and in remote areas all over the world, small turbines with many blades harness wind energy for pumping water. For example, in Egypt, there is a long history of irrigation of the Nile valley with machinery. Archimedes's Screw can be turned by mule, human, or wind energy.
Windmills have long been used for milling flour (the origin of the word?). Wooden shafts and cogs would take the rotation from the sails, and turn the millstones. Once captured, the power could also be used for lifting loads. There used to be hundreds of thousands of these distinctive buildings all over Europe. The number now is far less, although there is a growth in the number of wind turbines which may number a few tens of thousands across Europe and out in its shallow seas.
The use of wind energy is the most economic method of electrical power production widely considered to be environmentally sound. There is a small carbon cost in building the turbines, but this is paid back in terms of 'saved carbon' within a few weeks or months of operation over a thirty-year life span. The machines can be disassembled leaving little impact on the original landscape.
Denmark, Germany, Spain, India, and the USA have large installations of wind power, although it is principally Europe that is leading the way with quiet, efficient and modern machinery. Opposition to the use of wind power is growing slowly, and results in research into possible negative impacts of its use. For example, one study by an anti-wind group in the (pro-industry!) USA foresaw a significant impact on climate because of the reduced wind speed due to wind turbines. This has some basis in that wind turbines do slow the wind down in order to extract kinetic energy; however, making global climate predictions based on this effect is extremely challenging.
Peaks and troughs
Wind generators suffer a disadvantage that wind strength varies by time and by season, and may not match the load. Wind generators may therefore need to be matched with suitable enegery storage devises that can release the energy at the times when it is most wanted, including:
