From the earliest times wind had played a role in transportation via shipping vessels. But during the medieval period in Europe, wind began to replace water as an alternative way to power mills. A windmill operates via the action of wind blowing upon the vanes or sails attached to a long vertical structure; the spinning motion of the vanes or sails then turns a shaft that can grind grain, pump water, or, as in the wind turbines that are in use today, generate electricity. The first recorded use of windmills for energy dates back to 1180 in northwestern Europe. Around this time, in the Netherlands, pinwheel-shaped windmills peppered Holland's landscape. Their primary role was to pump water from wells. Windmills' efficiency grew tremendously between this period and the late 1800s, when metal was added to their vanes or sails.
The use of windmills involves a basic process in energy production known as conversion. Conversion is the changing of the potential energy of a given energy source, such as wind, into actual work, such as driving a shaft to grind grain or pump water. Conversion generally depends on the interaction between a source of energy and a specific process or device, a converter. For example, the windmill structure itself is a converter: The potential energy of the wind interacts with the mechanical workings of the windmill (the converter), and the wind is converted into the actual work of grinding grain or pumping water. Energy conversion, an important principle at work in the wind energy of earlier ages, is still a concern in the world today. Today, for example, the ease with which the potential energy of oil can be converted into the actual work of, say, powering a vehicle helps oil to maintain its position as a major source of energy, whereas alternative types of fuels, which might involve a more complex conversion process, are slow to enter widespread use, even though they may present fewer safety and environmental hazards. Another principle related to windmill usage that is still important today is efficiency. Efficiency is the degree to which the energy produced by a particular energy source, such as a windmill, surpasses the energy and cost that have been invested to use it for energy in the first place. These principles—conversion and efficiency—were both important aspects of the medieval reliance on windmills. Windmills were not always efficient because winds were not always regular or even available. And the very operation of windmills depended on the ease with which the windmill could convert the potential energy of wind into usable and reliable energy. Energy conversion and energy efficiency, then, are two more examples of modern energy-related issues that were also concerns in the past.
Today these two principles have the most relevance in relation to alternative and renewable energy technologies. For example, the renewable resource of wind power, while an inexhaustible resource, is irregular. Wind is itself directly derived from another source of energy, the Sun, whose unequal warming of the Earth's surface and atmosphere (the thick layer of colorless, odorless gases, or air, that surrounds the Earth) causes shifts in air pressure that result in the formation of wind. Thus, wind's intensity varies by region, time, season, the thickness of the layer of clouds that covers the Sun, and other environmental factors. Wind is not always efficient and not always easily convertible to actual work. A lot depends on the force, and the availability, of wind. In many ways, the inability of the medieval generation and later generations to make the most of renewable resources such as wind led to a buildup of unmet energy needs. This, in turn, opened the way to a time of major change, a significant period in energy history—the Industrial Revolution.
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