Page 56 - Design of Solar Thermal Power Plants

2.2 THE SOLAR CONSTANT AND RADIATION SPECTRUM 49

2. Radiant intensity (W/sr, where sr refers to steradian) is the quotient
of the radiation power leaving the point radiation source (or
radiation source panel) and the respective solid angle element
within the solid angle element in a given direction.
2
3. Radiance (W/[sr$m ]) is the quotient of the panel’s radiant
intensity on a specific point on the surface in a given direction
and the orthographic projection area of the respective panel on the
surface perpendicular to the given direction.
3
4. Spectral irradiance (W/m ) is the quotient of irradiance within the
range of infinitesimal wavelength and the respective wavelength
range.
2
3
5. Radiant exposure (MJ/m ;1 MJ ¼ 10 kW s ¼ 0.28 kWh) is the total
irradiation or accumulated irradiation value within a certain
period (such as a day or month).
6. Global radiation is the sum of downward direct solar radiation and
scattered solar radiation received on a horizontal surface within a
2p solid angle.
7. Solar altitude refers to the altitude angle at the solar disk center,
namely the angular distance from the horizon at the observation
point along the azimuth circle where the sun is located to the solar
disk center.
8. True solar time is time calculated based on the sun’s actual position
in the sky and is also known as “apparent time.” The time when the
sun passes through the local meridian line is called the midday of
the local true solar time (with an hour angle of zero). The interval
between two passes of the sun through the local meridian line is
deemed a true solar day. A true solar day is not always the same
length.
9. Because true solar time varies by specific day, for practicability one
must use the average day of solar days for an entire year, also
known as the “mean solar day.”
10. The mean solar day is 24 h on average and is referred to as the
“mean solar time.”

2.2.2 Solar Radiation Spectrum

The sun is the central body of the solar system and can be deemed a full
radiator with a surface temperature of 5777K. It is the source of light and
thermal energy for Earth; it continuously transfers an enormous amount
of thermal energy to Earth in the form of radiation. Solar radiation is
resolved into monochromatic elements that are distributed based on
wavelengths or frequencies in sequence from short to long wavelengths,
including cosmic lights, g- and X-rays, and ultraviolet, visible, infrared,
and radio radiation. As in energy science, the wavelengths of commonly
used solar radiation fall into a range of 0.15e4 mm, which can be divided
into three major regions: the ultraviolet region with a shorter wavelength,
the infrared region with a longer wavelength, and the visible light region