tailieunhanh - Ebook Renewable energy resources (2nd edition): Part 2

(BQ) Part 2 book "Renewable energy resources" has contents: Power from the wind, the photosynthetic process, biomass and biofuels, wave power, tidal power, ocean thermal energy conversion, geothermal energy, energy systems, storage and transmission, institutional and economic factors. | Chapter 9 Power from the wind Introduction The extraction of power from the wind with modern turbines and energy conversion systems is an established industry. Machines are manufactured with a capacity from tens of watts to several megawatts, and diameters of about 1 m to more than 100 m. Traditional mechanical-only machines have been further developed for water pumping, but the overriding commerce today is for electricity generation. Such ‘wind turbine generators’ have become accepted as ‘mainstream generation’ for utility grid networks in many countries with wind power potential, . in Europe, the USA and parts of India and China; other countries are steadily increasing their wind power capacity. Smaller wind turbine generators are common for isolated and autonomous power production. The rapid growth of world wind turbine electricity generation capacity is shown in Figure . Since about 2002, much additional generation capacity is being installed at sea in offshore wind farms where the depth is moderate. Later sections will show that in a wind of speed u0 and density , a turbine intercepting a cross-section A of wind front will produce power to its rated maximum according to PT = 1 C A u0 3 2 P () Here Cp is an efficiency factor called ‘the power coefficient’. Note that the power PT is proportional to A and to the cube of wind speed u0 . Thus whereas doubling A may produce twice the power, a doubling of wind speed produces eight times the power potential. The power coefficient Cp also varies with wind speed for individual machines. Since wind speed distribution is skewed, at any one time speeds less than average are more likely than speeds greater than average. Therefore the optimum design size of rotor and generator at a particular site depends on the power requirement, either to maximise generated energy per year or to provide frequent power. Often the average annual power from a wind turbine approximates to the 264 Power from the wind World