Bài giảng Green energy course syllabus - Chapter 5: Green electric energy - Nguyễn Hữu Phúc
A material or device that is capable of converting the
energy contained in photons of light into an electrical
voltage and current is said to be photovoltaic.
A photon with short enough wavelength and high enough
energy can cause an electron in a photovoltaic material to
break free of the atom that holds it.
If a nearby electric field is provided, those electrons can be
swept toward a metallic contact where they can emerge as
an electric current.
The driving force to power photovoltaics comes from the
sun, and it is interesting to note that the surface of the
earth receives something like 6000 times as much solar
energy as our total energy demand.
Spurred on by the emerging energy crises of the 1970s, the
development work supported by the space program began to
pay off back on the ground.
By the late 1980s, higher efficiencies (Fig. 8.1) and lower
costs (Fig. 8.2) brought PVs closer to reality, and they began
to find application in many offgrid terrestrial applications
such as pocket calculators, off-shore buoys, highway lights,
signs and emergency call boxes, rural water pumping, and
small home systems.
• While the amortized cost of
photovoltaic power did drop
dramatically in the 1990s, a
decade later it is still about
double what it needs to be to
compete without subsidies in
more general situations.
Figure 8.1 Best laboratory PV cell efficiencies for various technologies.
Figure 8.2 Possible evolution of turn-key PV system
prices
By 2002, worldwide
production of
photovoltaics had
approached 600 MW
per year and was
increasing by over 40%
per year (by
comparison, global
wind power sales were
10 times greater).
Figure 8.3 World production of photovoltaics is
growing rapidly, but the U.S. share of the market is
decreasing. Based on data from Maycock (2004).
Critics of this decline point to the
government’s lack of enthusiasm to fund PV
R&D. By comparison, Japan’s R&D budget is
almost an order of magnitude greater.
Before we can talk about solar power, we need
to talk about the sun
Need to know how much sunlight is available
Can predict where the sun is at any time
Insolation : incident solar radiation
Want to determine the average daily insolation
at a site
Want to be able to chose effective locations and
panel tilts of solar panels
The sun
◦ 1.4 million km in diameter
◦ 3.8 x 1020 MW of radiated electromagnetic energy
Blackbodies
◦ Both a perfect emitter and a perfect absorber
◦ Perfect emitter – radiates more energy per unit of
surface area than a real object of the same
temperature
◦ Perfect absorber – absorbs all radiation, none is
reflected
Plank’s law – wavelengths emitted by a
blackbody depend on temperature
3.74108
E
(7.1)
14400
5 exp
1
T
•λ = wavelength (μm)
• Eλ = emissive power per unit area of
2
•bTla=ckabbosodlyut(We /temm-pμemra)ture (K)
Visible light has a wavelength of between 0.4 and 0.7 μm, with
ultraviolet values immediately shorter, and infrared immediately longer
The earth as a blackbody
Area under curve is the total radiant pFoiwgureer7.e1mitted
Total radiant power emitted is given by the
Stefan –Boltzman law of radiation
E AsT4
(7.2)
•E = total blackbody emission rate (W)
• σ = Stefan-Boltzmann constant = 5.67x10-8
2
4
•WT /=mab-sKolute temperature (K)
•A = surface area of blackbody (m2)
The wavelength at which the emissive power
per unit area reaches its maximum point
2898
max
(7.3)
T
•T = absolute temperature (K)
• λ = wavelength (μm)
• λmax =0.5 μm for the sun , T = 5800 K
• λmax = 10.1 μm for the earth (as a blackbody), T = 288 K
Figure 7.2
As sunlight passes through the
atmosphere, less energy arrives at the
earth’s surface
• h1 = path length through atmosphereFwigiutreh7.s3un
directly overhead
• h2 = path length through atmosphere to spot on
• sβu=rfaaclteitude angle of the sun
h2 1
air mass ratio m =
h sin
(7.4)
1
Figure 7.3
Air mass ratio of 1 (―AM1‖) means sun is directly
overhead
AM0 means no atmosphere
AM1.5 is assumed average at the earth’s surface
• m increases as the sun
appearslower in
the sky.
• Notice there is a large
loss towards the blue end
for higher m, which is why
the sun appears reddish at
sun rise and sun set
One revolution every 365.25 days
Distance of the earth from the sun
360(n 93)
d 1.5108 10.017sin
km (7.5)
365
n = day number (Jan. 1 is day 1)
d (km) varies from 147x106 km on Jan. 2 to
152x106 km on July 3 (closer in winter, further
in summer)
Note that the angles in this chapter are in
degrees
In one day, the earth rotates 360.99˚
The earth sweeps out what is called the ecliptic
plane
Earth’s spin axis is currently 23.45˚
Equinox – equal day and night, on March 21
and September 21
Winter solstice – North Pole is tilted furthest
from the sun
Summer solstice – North Pole is tilted closest to
the sun
Tải về để xem bản đầy đủ
586 trang
24200
Free
Bạn đang xem 20 trang mẫu của tài liệu "Bài giảng Green energy course syllabus - Chapter 5: Green electric energy - Nguyễn Hữu Phúc", để tải tài liệu gốc về máy hãy click vào nút Download ở trên
File đính kèm:
- bai_giang_green_energy_course_syllabus_chapter_5_green_elect.pdf
