Environmental Engineering Reference
In-Depth Information
5
Introduction to Solar Energy Conversion
After learning quite a bit about how the suns energy is created, and how that process
might be reproduced on earth, we turn now tomethods of harvesting the energy from
the sun. Heat energy is the easiest form to store and distribute and is useful for
heating water and home. (The advantage of solar water heating is that the vexing
storage and distribution issues are absent in this use of the sun. The saving in
electricity from the grid is equally as valid as adding power from solar farms.) The
more technical challenge is to turn the suns energy into electricity, for consumption
or perhaps conversion into hydrogen as a portable fuel. Two main tracks of electricity
generation are through a heat engine running a generator and direct photovoltaic
conversion. An additional track is to use the suns energy to directly create a fuel,
hydrogen, through a photocatalytic water-splitting process that bears some similarity
to photosynthesis.
5.1
Sun as an Energy Source, Spectrum on Earth
The main properties of the sun as an energy source are its energy density,
1366Wm
2
at the top of the earths atmosphere; the highly directional nature of
its radiation, and its spectrum. For simple estimates, we will take the sun as a black
body at 6000 K.
Starting from the energy density of the radiation given in Chapter 1,
8
phn
3
c
3
1
d
n
u
ð
n
Þ
d
n
¼½
=
½
exp
ð
hn
=
k
B
T
Þ
1
;
ð
1
:
1
Þ
we multiply by
c
/4 to get a power density, Wm
2
, and then convert to wavelength
using
c
¼ l
, to get the power density per unit wavelength (as plotted in Figure 1.3):
½
exp
ðhc=lk
B
TÞ
1
1
d
l:
PðlÞ
d
l ¼½
2
phc
2
=l
5
ð
5
:
1
Þ
As mentioned earlier, the peak in this function is at
l
m
, such that
l
m
T
¼
constant
¼
2.9mmK. The value of
l
m
¼
486 nm and photon energy is 2.55 eV for the solar
spectrum in the visible corresponding to
T
5973 K.
