Solar energy in progress and future research trends

Radiation

A hot body may lose heat by radiation through emission of electromagnetic waves The maximum power which can be radiated from a body at a given temperature is called the black body radiation corresponding to that temperature. The radiation power, P, from a black body increases as the fourth power of the absolute temperature, T, of the same body and it is given by Stefan’s law as

P = sT (70)

where s = 5.67 X 10—8 W/m2 K4 is the Stefan’s constant. Heat flux in the case of radiation from a black body is presented in Table 9 for different absolute temperatures.

Similar to sun’s radiation heat can be radiated from the surfaces of heated materials. The amount of radiation is first dependent on the temperature of the radiating body and then

Table 9

Black body radiation

Surface temperature (K)

Heat flux (W/m2)

6000

73.5 X 106

3000

4.6 X 106

2000

9.1 X 105

1073

75.0 X 103

873

32.9 X 103

673

11.6 X 103

473

2.84 X 103

353

1.10 X 103

333

880

300

459

Radiation amount is also dependent on the surface material emission. Most materials used in the building construction have high emissions of approximately 0.9, which means that they radiate 90% of the theoretical maximum for a given temperature. Usually, the total heat loss from combined effects of conduction, convection and radiation is referred to as the U-value. Its unit is the amount of loss per area per centigrade degree. Typical U-values are provided in Table 10.

Solar energy in progress and future research trends

Spatial solar radiation estimation

In the previous sections, the modeling of solar radiation is discussed on a given site. However, in practical solar energy assessment studies, it is also necessary to have spatial (multiple …

Solar thermal collectors and applications

Extensive fossil fuel consumption in almost all human activities led to some undesirable phenomena such as atmospheric and environmental pollutions, which have not been experienced before in known human history. …

Heat transfer and losses

As explained earlier the easiest way of solar radiation collection is for low temperature heating purposes. It is well known that black surfaces absorb solar radiation more than any other …

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