# CHAPTER

Energy2green Wind And Solar Power System

Get Instant Access Latitude (deg)

Figure 2.107. Height and latitude distribution of heat added to the atmosphere as a result of condensation of water vapour (C: top) and radiation (R: bottom). Both are in units of (cP deg day-1) and averaged over longitude and the period December-February (based on Newell et al., 1969).

Latitude (deg)

Figure 2.107. Height and latitude distribution of heat added to the atmosphere as a result of condensation of water vapour (C: top) and radiation (R: bottom). Both are in units of (cP deg day-1) and averaged over longitude and the period December-February (based on Newell et al., 1969). Figure 2.108. Height and latitude distribution of heat added to the atmosphere by turbulent transport of sensible heat from the Earth's surface, in units of (cP deg day-1) and averaged over longitude and the period December-February (based on Newell et al., 1969).

Separation of scales of motion

The averaging procedure discussed in conjunction with (2.15) presumes that a "large" and a "small" scale of motion are defined. This may be properly done by performing a Fourier analysis of the components of the wind veloc ity vector, or of the wind speed \v\. It is convenient to consider the variance of the wind speed (because it is proportional to a kinetic energy), defined by < ~ • ~ >. The brackets <> denote average over a statistical ensemble, corresponding to a series of actual measurements. Writing the spectral decomposition in the form Figure 2.109. Components of the net energy flux passing the atmosphere, averaged over longitude and the year. E ™" is the net radiation flux, Lvra the heat gained by condensation of water vapour and - ESens the turbulent flux of sensible heat from the Earth's surface to the atmosphere (i.e. the same quantities appearing in Figs. 2.107 and 2.108, but integrated over height). The deviation of the sum of the three quantities from zero should equal the heat gain from horizontal transport of sensible heat (Fsens) (based on Sellers, 1965).

ft ^ i» ^ 1 (v • v)=J S(o)do=J o S(o)d(log o), one obtains a form-invariant spectral function S(o), which is shown in Fig. 2.110, based on a modern version of a pioneering measurement effort made by van der Hoven (1957). The peaks exhibited by the spectrum vary in magnitude with the height of measurement (cf. Fig. 3.37).

A striking feature of the spectrum in Fig. 2.110 (and the analogues for other heights) is the broad gap between o ~ 0.5 h-1 and o ~ 20 h-1. A large volume of measurements have confirmed that the existence of such a gap is an almost universal feature of the wind speed spectrum. Its significance is to provide a clear distinction between the region of large-scale motion (o • *0.5 h-1) and the region of small-scale (eddy) motion (o • *5 h-1). The existence of the gap makes the time averaging procedure in (2.13) easy and makes the exact choice of At insignificant over a reasonably large interval, so that the resulting large-scale motion is not critically dependent on the prescription 