Solar energy
Glass
cover
Collector plate
Tubes
232
Analytical Heat Transfer
Monochromatic solar energy flux (W/m
2
μm)
0.3 0.5
1.5
2.5
2.0
1.0
0
400
2000
1600
1200
800
Solar spectrum outside
the Earth’s atmosphere
Solar spectrum on ground
under a clear atmosphere
O 3
O 3
O 2
H 2 O
H 2 O
H 2 O, CO 2
H 2 O, CO 2
Wavelength (μm)
FIGURE 11.10
Solar spectra outside the Earth’s atmosphere and on the ground.
It is known that H 2 O vapor and CO 2 gas in the atmosphere not only absorb
solar radiation but also absorb radiation from the Earth’s surface at around
300 K that give radiation of wavelengths from 10 to 20 μm with an emissivity
about 1.0. In addition, H 2 O vapor and CO 2 gas in the atmosphere (sky) can
also emit energy at wavelengths of 5–10 μm at the effective sky temperature
around 250–270 K (assume an emissivity of about 0.8–1.0).
Figure 11.11 shows a typical setup for a solar collector. A special glass is
used as a cover for the collector and a specialized coating is used on the
collector plate and tubes where the solar energy is collected to maximize the
performance of the collector.
FIGURE 11.11
A typical setup for a solar collector.