Seite - 113 - in Clean Water Using Solar and Wind - Outside the Power Grid

Wind 113 by both the wind speed and the area S according to the so-called Betz formula: P S v= ⋅ ⋅ ⋅ 1 2 3ρ where ρ is the density of the air. The available energy in the wind is obtained by integrating the power during a time interval Tp, typically one year: E S v dt Tp annual = ⋅ ⋅ ∫12 30ρ The Betz formula explains clearly that the wind turbine power increases significantly not only with the circular area S but also with the tower height. At higher levels the wind is less disturbed by surrounding objects like hills and buildings. Also, wind speeds in the lower atmosphere increase with elevation and this is important given the v3 dependence of extracted energy. A wind generator requires a minimum wind velocity of 3–5 m/s and will deliver the nominal capacity at a wind velocity of 12–14 m/s. Wind turbines have been developed to have a favourable operating condition at 10–12 m/s. These wind towers have a larger rotor diameter per kW of power. At high speeds small-scale generators, for safety reasons, are blocked by a braking system. High-power wind generators turn the rotor blades to stop the rotation and the mechanical brakes are used only for gross failures. There are generators that can adjust to the wind direction so that the power output is constant. The wind generator speed is variable since the wind speed is not constant. The AC power frequency delivered, however, should be constant. Therefore, the rotors of the generator are connected to inverters to control the voltage and frequency of the AC power delivered. Globally wind power has been developing towards larger wind turbines and higher power outputs. The largest commercial turbines today (Vestas) have a power rating exceeding 9 MW. For our purposes of decentralised power sources outside the grid we will consider much smaller systems. Not surprisingly, small wind power units have a higher capital cost per kW compared to large systems. For the considered off-grid applications we will focus on small wind turbines (SWT). There is a Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user