Environmental Engineering Reference
In-Depth Information
three separate laterally displaced devices, to which light is addressed by spectral
splitting optics.
7.4
Summary and Comments on Efficiency
In brief summary, the ideal single-junction cell at best captures only about 30%of the
light energy. Going beyond the single-junction cell, a well-developed technology is to
stack two or three junctions in series (tandem), engineered so that light not converted
in the
first junction may be converted in the second or third junction. This approach
brings the measured efficiency for a (concentrated) tandem cell up to the vicinity of
41.8%. Tandem solar cells, typically based on GaAs, are commercially available but
are expensive. On the other hand, in conjunction with mirrors or lenses to
concentrate light, as mentioned in connection with Figure 7.7, this approach may
possibly be competitive, even with electricity from coal-powered plants. Light
dispersing optics has also been demonstrated to separate the spectrum and to steer
different spectral regions to locations of appropriate solar cells.
The second, less developed, idea, is to use dye molecules in wide-area receiving
plates, to funnel reemitted light to a dedicated array of solar cells. A large inventory of
dye molecules has long been available, and this particular use of dyes is based on the
most fundamental role of a dye, to convert blue or green photons to red
photons. This proposal could turn out to be important.
The third approach, in principle, to improve ef
ciency is tomultiply the number of
charges produced per photon. This could make use of the blue portion of the solar
spectrum, not fully utilized in the single-junction cell, on the conventional assump-
tion of one charge per photon, with excitons. This effect is well known in the physics
of the Li-drifted germaniumdetector, a device formeasuring the energy of a cosmic
ray or other energetic particle by counting how many charges it generates by the
excitonmultiplication process in a pure sample of germanium. We will return to this
topic in Chapter 8, and to the further possibility of an intermediate band solar cell
(see the second curve in Figure 7.1).
7.5
A Niche Application of Concentrating Cells on Pontoons
A niche application of concentrating solar cells has been described [98] that exploits
strengths and overcomes weaknesses of these devices. Solar cell ef
ciency decreases
with temperature, which occurs in locations where the solar energy is strongest.
Multijunction tandemdevices de
nitely need cooling because of the concentration of
energy, a factor of several hundred.
Concentrating devices need tracking that can be expensive. The pontoon-mounted
concentrating solar cell arrangement shown in Figure 7.17 addresses and exploits
these aspects. There are many locations where standing water is left open to the sun,
