Environmental Engineering Reference
In-Depth Information
V
oc
Maximum
power
V
oc
V
j
ss
j
ss
Solar irradiance (
I
)
Current density (
j
)
(a)
(b)
Figure 7.14
(a) The open-circuit voltage
V
oc
and short-circuit current density
j
ss
of a photovoltaic cell as
functions of solar irradiance. (b) The voltage versus current density characteristics of a photovoltaic cell for
a fixed value of solar irradiance.
1 kW/m
2
under the conditions for maximum power. Typical efficiencies are in the range of 15-25
%, with
V
oc
∼
300-400 A/m
2
.
The photovoltaic cell is but one component of a practical system for converting sunlight directly
to useful electric power. Many individual cells must be wired together to produce output power
at desirable voltage and current levels, and the array of cells must be mounted in a manner that
optimizes the solar irradiance input. Photovoltaic power is direct-current (DC) power and must be
converted to alternating-current (AC) form for most uses. These other constituents of a practical
photovoltaic power supply add to its capital cost, although at the present time the basic cell cost is
so high that it constitutes by far the major component of system cost.
0
.
5 V and
j
ss
∼
7.6
WIND POWER
The use of wind to provide mechanical power for grinding grain and propelling ships and boats
dates from ancient times, three or four millennia ago. Substantial improvement in wind technology
within the past thousand years, especially for transoceanic vessels, made possible the migration
of populations to western and southern continents and the initiation of intercontinental trade. The
technology of sailing vessels had reached a high level in the mid- to late nineteenth century when
it was suddenly displaced by steel-hulled, fossil-fuel-powered steamships that greatly transformed
intercontinental travel and trade.
At one time, wind-powered mills for grinding grain and pumping water were common in
western Europe, numbering some 10,000 by the twelfth century. By present standards their design
was rudimentary and their use, like that of sailing vessels, was displaced by the advent of industrial
power in the nineteenth century.
