C haptEr 9 design Environments and systems
380
done via thermoelectric and photovoltaic materials. Thermoelectric
materials convert the heat collected from the sun into electricity.
Photovoltaics, on the other hand, convert solar radiation into electric energy. In the case of thermoelectrics, the overall conversion
process would operate as follows: First the infrared (wavelength =
800–3000 nm) spectrum of sunlight is collected and turned into
heat or stored in a thermal bath. Subsequently, thermoelectric materials convert the heat into electricity. A very important parameter
associated with these materials is the conversion efficiency, which
is related to a quantity called the Figure of Merit, ZT, (see equation
9.1 in Section 9.3)
Most current thermoelectric materials exhibit ZT values of 1. These
materials generate conversion efficiencies of around 7% or 8%. To
improve the efficiency to around 15%, novel materials with ZT =
2–3 are required. In this regard, nanostructured materials seem
to be good candidates. One-dimensional nanomaterials, such as
carbon nanotubes, molybdenum sulfide (MoS 2 ), tungsten sulfide
(WS 2 ), titanium sulfide (TiS 2 ) nanotubes, as well as bismuth telluride (Bi 2 Te 3 ) nanowires (see Figure 9.51), have seen rapid advances.
The reason behind this behavior is related to quantum-size effects
induced by nanoscale dimensions. In other words, dimensional
confinement of electrons may lead to an increase in the electron
density of states at the Fermi level, thereby increasing the Seebeck
coefficient and consequently the parameter ZT.
In addition to 1-D nanomaterials, nanocomposites filled with
Bi 2 Te 3 nanotubes are also likely to improve the current ZT values
due to enhanced phonon boundary scattering. With regard to solar
energy conversion, all thermoelectric nanomaterials would need
to work at temperatures close to 700°C. Thermoelectric materials
of this type are also discussed in Section 9.3. Besides thermoelectric materials, photovoltaic materials are also an essential technology for solar energy conversion (see Figure 9.52). Currently, the
production of photovoltaic materials is primarily associated with
crystalline silicon (94% of the market). Devices made of these firstgeneration materials are basically diodes limited to a conversion
efficiency of around 30% and a current cost of $1–2/Watt. Secondgeneration photovoltaic technologies are mainly those capable of
achieving good efficiencies combined with low costs, but these are
still in the early phases of commercialization. They include thin
films, solar concentrators, and organic photovoltaics. Thin films
were initially made of amorphous silicon, representing 5–6% of
the total photovoltaic market. These devices are made of multiple
layers, some at the nanoscale (see Figure 9.53). Because the techFigure 9.51
Scanning electron microscopy (SEM) planar and
cross-section view of Bi 2 Te 3 nanowires. (Courtesy
of Kalapi G. Biswas and Timothy D. Sands, Purdue
University.)
Figure 9.52
Photovoltaic solar installation. (Courtesy of
Argonne National Laboratory.)
380
done via thermoelectric and photovoltaic materials. Thermoelectric
materials convert the heat collected from the sun into electricity.
Photovoltaics, on the other hand, convert solar radiation into electric energy. In the case of thermoelectrics, the overall conversion
process would operate as follows: First the infrared (wavelength =
800–3000 nm) spectrum of sunlight is collected and turned into
heat or stored in a thermal bath. Subsequently, thermoelectric materials convert the heat into electricity. A very important parameter
associated with these materials is the conversion efficiency, which
is related to a quantity called the Figure of Merit, ZT, (see equation
9.1 in Section 9.3)
Most current thermoelectric materials exhibit ZT values of 1. These
materials generate conversion efficiencies of around 7% or 8%. To
improve the efficiency to around 15%, novel materials with ZT =
2–3 are required. In this regard, nanostructured materials seem
to be good candidates. One-dimensional nanomaterials, such as
carbon nanotubes, molybdenum sulfide (MoS 2 ), tungsten sulfide
(WS 2 ), titanium sulfide (TiS 2 ) nanotubes, as well as bismuth telluride (Bi 2 Te 3 ) nanowires (see Figure 9.51), have seen rapid advances.
The reason behind this behavior is related to quantum-size effects
induced by nanoscale dimensions. In other words, dimensional
confinement of electrons may lead to an increase in the electron
density of states at the Fermi level, thereby increasing the Seebeck
coefficient and consequently the parameter ZT.
In addition to 1-D nanomaterials, nanocomposites filled with
Bi 2 Te 3 nanotubes are also likely to improve the current ZT values
due to enhanced phonon boundary scattering. With regard to solar
energy conversion, all thermoelectric nanomaterials would need
to work at temperatures close to 700°C. Thermoelectric materials
of this type are also discussed in Section 9.3. Besides thermoelectric materials, photovoltaic materials are also an essential technology for solar energy conversion (see Figure 9.52). Currently, the
production of photovoltaic materials is primarily associated with
crystalline silicon (94% of the market). Devices made of these firstgeneration materials are basically diodes limited to a conversion
efficiency of around 30% and a current cost of $1–2/Watt. Secondgeneration photovoltaic technologies are mainly those capable of
achieving good efficiencies combined with low costs, but these are
still in the early phases of commercialization. They include thin
films, solar concentrators, and organic photovoltaics. Thin films
were initially made of amorphous silicon, representing 5–6% of
the total photovoltaic market. These devices are made of multiple
layers, some at the nanoscale (see Figure 9.53). Because the techFigure 9.51
Scanning electron microscopy (SEM) planar and
cross-section view of Bi 2 Te 3 nanowires. (Courtesy
of Kalapi G. Biswas and Timothy D. Sands, Purdue
University.)
Figure 9.52
Photovoltaic solar installation. (Courtesy of
Argonne National Laboratory.)