The wind is a proven energy source it is not resource-limited in the United States, and there are no insolvable technical constraints. However, even at that production cost, wind-generated electricity is not yet fully cost-competitive with coal- or natural-gas-produced electricity for the bulk the market. Wind energy is a commercially available renewable energy source, with state-of- the-art wind plants producing electricity at about $0.05 per kWh. This is reversed during the night, when the cool air over the quickly cooled land flows toward the water, where the air over the land is less dense because water retains the Sun’s heat longer. Near the world’s bodies of water, the cool air over the water flows to the land. The satellite picture taken by NOAA shows different temperatures in different spots of the Earth-blue indicates the coolest and red indicates the warmest (Figure 3.4). 1173.1 Introduction higher pressure from one area to flow to another. 116 CHAPTER 3 Wind Energy Conversion Systems Warm air Air above land rises Warm air above the sea rises Cool land breeze Seabreeze Cool air FIGURE 3.3 The coastal wind flow. 115 FIGURE 3.2 A lift-type American windmill. ![]() This causes the Renewable Energy System Design Copyright © 2014 Elsevier Inc. The wind blows because the heating rates of the Earth differ, therefore, as the rate of evaporation of air over one area is dif- ferent than another, there is a pressure differential (Figure 3.3). As long as the Sun is heating the Earth, there will always be winds because tem- perature differences drive air circulation. That is about 50 to 100 times more than the energy converted into biomass by all plants on Earth. About 1 to 2% of the energy coming from the Sun is converted into wind energy. In other words, the Earth receives 1018 watts (W) of power. All renewable energy (except tidal and geothermal power), and even the energy in fossil fuels, ultimately comes from the Sun, which radiates 100,000,000,000,000 kilowatt hours (kWh) of energy to the Earth per hour. By 1910, wind turbine generators were producing electricity in many European countries. By 1900, small electric wind systems were developed to generate direct current, but most of these units fell into disuse as inexpensive grid power was extended to rural areas during the 1930s. Most of them were used to pump water for farms and ranches. In the United States, millions of windmills were erected as the American West was developed during the late nineteenth century (Figure 3.2). Within several centuries before Christ, simple windmills were used in China to pump water. There is evidence that wind energy was propelled boats along the Nile River as early as 5000 BC. ![]() Since earliest recorded history, wind power has been used to move ships, grind grain, and pump water. These were used for grinding grains and pumping water. Several hundred years later in Europe, horizontal windmills were operated with a manual mechanism that rotated the whole windmill to face the wind. These windmills were fixed permanently to face the prevail- ing coastal winds. During the tenth century, horizontal axis-mounted windmills first appeared in the Mediterranean region. They had a number of arms on which sails were mounted, with the sails initially made from bundles of reeds. The first wind machines were probably vertical axis windmills used for grinding grain in Persia dating back to 200 BC (Figure 3.1). ![]() With wind turbine generators, this mechanical energy is converted into electricity and in windmills this energy is used to do work such as pumping water, mill grains, or drive machinery. CHAPTER Wind Energy Conversion Systems 3 3.1 INTRODUCTION Wind energy conversion systems (WECS) are designed to convert the energy of wind movement into mechanical power.
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