Environmental Engineering Reference
In-Depth Information
flow is also generated at 23
C
¼
59 kW. If water-coolingwas employed, a substantial supply of water at 73
Fon a winter
day might be useful for home heating, but typically such exhaust energy is wasted.
All thermal solar power systems, unlike nonconcentrating photovoltaics, require
cooling. In practice for large plants, thismeans that water is needed. It is noted that in
California the use of drinking water for this purpose is illegal, suggesting that
common practice is to dump the cooling water.
Smaller installations based on Stirling engines heated by plastic Fresnel lenses are
possible, for example, for a remote household power supply, typically to charge
batteries. An acrylic Fresnel lens of 0.5m diameter and 2mm thickness is inexpen-
sive, and it is possible to set up a Stirling engine so that fossil fuel may alternatively
heat the Stirling intake when the sun is not shining. Based on the performance noted
above, peak sun power would, at 30% efficiency, generate at best 60W. While this
might be more expensive than a larger area of photovoltaic cells, the latter would not
allow fossil fuel backup.
¼
73
F, evidently
P
exhaust
¼
0.69
26.5 kW/0.31
5.4
Generations of Photovoltaic Solar Cells
The single junction solar cell was described in Chapter 3, before Figure 3.17. The
limiting ef
ciency of a single junction cell is 31% at one sun illumination, but as high
as 40.8% with concentrated sunlight. The Si single-crystal solar cell shown in
Figure 3.17 is the most common
first-generation solar cell. Elaborations on this cell
reaching 24% ef
ciency will be described in Chapter 6, but the bulk of the Si cells are
characterized, as in Figure 5.5, in a range 15
-
20%. The crystalline Si solar cell is built
on a wafer of Si, which is typically 200
-
300
thick, cut from a large crystal using a
wire saw that wastes also one wire-width of the Si single-crystal boule (http://www.
omron-semi-pv.eu/en/wafer-based-pv/wafer-preparation/slicing-the-ingot.html.).
The cost of chip-based cells is correspondingly high, here estimated in the range of
$3.50/peak watt.
An in
uential summary and prediction of the costs in solar cell types, dating from
2003, is shown in Figure 5.6. This
figure also shows the fundamental limits on the
ef
ciency of cells. It is clear that a thin-
lm solar cell structure noted as type II in the
gure will cost less, because the thickness of the active portion of the cell is reduced to
a few microns, limited by the absorption of light in the material in question. Thin
films of silicon can be deposited by several methods, representing the most common
form of thin-
lm type II cell, shown as type II in Figure 5.6. This class of single
junction cell is here characterized at $1/W with ef
ciency in the vicinity of 10%.
These devices are inherently less expensive than those based on single crystals of Si
because they can be deposited in depths of only a few
m
m on wide area substrates of
glass ormetal foil. There are large continuingmarkets for smaller scale cells or panels
where the ef
m
ng
tiles, power for roadside signs and telephones, see-through amorphous panels for
windows, or auto sun-roofs, which have been served by inexpensive amorphous
ciency is less important than price, such as watches, calculators, roo
