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<br />3. OVERVIEW OF SMALL-SCALE HYDROELECTRIC TECHNOLOGY <br /> <br />In this chapter, the principles underlying the generation of elec- <br />tricity from falling water are outlined and the major general types of SSH <br />plants are described. Regional and environmental considerations are dis- <br />cussed for each of the different types of SSH projects. The discussion <br />below is a general introduction to the more detailed discussion in the <br />following four chapters. <br /> <br />3.1 HYDROPOWER PRINCIPLES <br /> <br />Water acquires potential energy during the hydrologic cycle when it is <br />raised to an elevation above sea level, from which it falls as rain. Run- <br />off, flowing to the sea, expends much of its potential energy as kinetic <br />energy in collisions with stream banks and bottoms, in turbulence, and in <br />carrying a sediment load. Hydroelectric plants capture some of the energy <br />of water as it fall s from a high potent i a 1 energy 1 eve 1 to a lower one. <br />The water falls through a "wheel," or turbine, which converts the potential <br />energy into rot at i ng mechan i ca 1 (k i net i c) energy; the generator is used to <br />change the mechanical energy into electrical energy. <br /> <br />The measure of the total amount of energy available over a given <br />stream reach is the difference in elevation (gross head) times the amount <br />of the discharge. In other words, both the height through which the water <br />falls and the amount of falling water contribute to the energy available at <br />a given site. Neither all the head nor all the flow can be captured effec- <br />tively in a hydroelectric plant. Plants normally are able to capture about <br />80% of the available energy (Creager and Justin 1950). <br /> <br />Comparatively simple formulas have been develop,ed to calculate the <br />power available at a given site. The standard formula1 (Creager and Justin <br />1950) is: <br /> <br />IThe equivalent formula in conventional units is more familiar to American <br />engineers: <br /> <br />PkW = Q(H - hf) e/11.8 <br /> <br />where P = power (kW) <br />Q = discharge (ft3/sec) <br />H = gross head between intake and draft tube (ft) <br />hf = head losses within the system <br />e= a fraction for overall system efficiency, usually about 0.80 <br />11.8 = a constant related to the weight of water. <br /> <br />17 <br />