ABSTRACT
Several renewable energy resources derive from the natural movement of air and water. Therefore the transfer of energy to and from a moving fluid is the basis of meteorology and of hydro, wind, wave and some solar power systems. Examples of such applications include hydropower turbines (Figures 8.3, 8.5 and 8.6), wind turbines (picture on front cover and Figure 9.4), solar air heaters (Figure 6.1) and wave energy systems (Figure 12.14). To understand such systems, we must start with the basic laws of mechan-
ics as they apply to fluids, notably the laws of conservation of mass, energy and momentum. The term fluid includes both liquids and gases, which, unlike solids, do not remain in equilibrium when subjected to shearing forces. The hydrodynamic distinction between liquids and gases is that gases are easily compressed, whereas liquids have volumes varying only slightly with temperature and pressure. Gaseous volumes vary directly with temperature and inversely with pressure, approximately as the perfect-gas law pV = nRT. Nevertheless, for air, flowing at speeds <100ms−1 and not subject to large imposed variations in pressure or temperature, density change is negligible; this is the situation for the renewable energy systems analysed quantitatively in this book. It does not apply to the analysis of gas turbines, for which specialist texts should be consulted. Therefore, throughout this text, moving air is considered to have the fluid dynamics of an incompressible fluid. This considerably simplifies the analysis of most renewable energy systems. Many important fluid flows are also steady, i.e. the particular type of flow
pattern at a location does not vary with time. So it is useful to picture a set of lines, called streamlines, parallel with the velocity vectors at each point. A further distinction is between laminar and turbulent flow (Section 2.5). For example, watch the smoke rising from a smouldering taper in still air. Near the taper, the smoke rises in an orderly, laminar, stream, with the paths of neighbouring smoke particles parallel. Further from the taper, the
flow becomes chaotic, turbulent, with individual smoke particles intermingling in three dimensions. Turbulent flow approximates to a steady mean flow, subject to internal friction caused by the velocity fluctuations. However, even in turbulence, the airflow remains within well-defined (though imaginary) streamtubes, as bounded by streamlines.
