Earth solar radiation striking

Table 14.1 Characteristics of solar radiation striking the earth s surface...

The processes affecting solar radiation or radiation emitted by the Earth s surface are still quite poorly understood. Let us take radiation emitted by the Sun. When solar radiation strikes the atmosphere, part of it is reflected out into space, while the rest penetrates into the atmosphere and is refracted the refracted portion is, in turn, either sent out into space or in towards Earth. Moreover, multiple gases absorb, in a highly irregular manner, radiations of certain wavelengths. Of the radiation that finally reaches Earth, some types are refracted once again, others are absorbed, while yet a second source of radiation is supplied by the clouds and sky. From this (grossly oversimplified) summary we can see the complexity of the situation. We may know some of the processes, but we must admit that most of them escape us. 

The average daily incident solar radiation, or insolation, that strikes the earth s surface worldwide is about 220 W/m (1675 Btu/ft ). The annual insolation on 0.01% of the earth s surface is approximately equal to all energy consumed (ca 1992) by humans in one year, ie, 321 x 10 J (305 X 10 Btu). In the United States, the world s largest energy consumer, annual energy consumption is equivalent (1992) to the insolation on about 0.1 to 0.2% of U.S. total surface. 

The rate of heat conduction is further complicated by the effect of sunshine onto the outside. Solar radiation reaches the earth s surface at a maximum intensity of about 0.9 kW/ m. The amount of this absorbed by a plane surface will depend on the absorption coefficient and the angle at which the radiation strikes. The angle of the sun s rays to a surface (see Figure 26.1) is always changing, so this must be estimated on an hour-to-hour basis. Various methods of reaching an estimate of heat flow are used, and the sol-air temperature (see CIBSE Guide, A5) provides a simplification of the factors involved. This, also, is subject to time lag as the heat passes through the surface. 

As discussed in Chapter 3, solar radiation passing through the atmosphere to the earth s surface is both scattered and absorbed by gases and particles. The intensity of radiation striking the surface can be expressed in the form of a Beer-Lambert law ... 

As seen in Eq. 1, the water-splitting reaction has an overall energy requirement of 4.92 eV per O2 molecule formed (or +474.7 kj/mol O2 formed). The most abundant solar radiation to strike the earth s surface falls in the visible range (750-400 nm) and fortunately, these photons are energetic enough (1.65-3.1 eV)27 so that as little as two photons are required to drive this process thermodynamically. When broken down into redox half-reactions (5 and 6), the multi-electron nature of reaction 1 is readily apparent. 

Figure 30.4 also shows the radiation level in Tulsa for an object positioned horizontal to the earth s surface. For this case, the radiation varies much more dramatically from about 150 to 290 Btu/hr-fh (473 to 915 W/m ) throughout the year. Peak radiation levels occur during the summer months mainly because the view allows more solar radiation to strike an area positioned horizontal to the earth. The amount of radiation that strikes an area positioned horizontal to the earth can be described by the Lambert cosine law ... 

Ozone absorbs uv radiation from 200 to 360 nm. This leads partly to a reversal of reaction 11-2 and thus a steady state concentration is established. The net result of all these processes is absorption and conversion to heat of considerable solar uv radiation that would otherwise strike the earth s surface. Destruction of any signifi-... 

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