Technology
Spotlight on Systems Research
Four universities strive to improve the way lighting systems, solar-harvesting technologies, and HVAC work
(archrecord.construction.com - June 2005)
By Ted
Smalley Bowen
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| This
diagram shows the design of a flexible solar cell containing
a layer of “quantum dots” that harvest sunlight,
pioneered by the University of Toronto. |
Behind every technological breakthrough
that grabs headlines are scores of smaller-scale studies aimed
at improving the way existing products and systems work. Often,
the innovations in product or system design that result from
such studies are difficult to envision: Who could have guessed
that the chunky blocks of plastic that passed for mobile phones
15 years ago would evolve into the multifunctional, slim-as-a-credit-card
fashion accessories they are today? In this feature—really
a series of four featurettes—we highlight research projects
in energy efficiency that point the way toward substantial
improvements in the way buildings use (or harvest) power.
How about thin, flexible solar cells that can be ordered by
the roll, like paper? Or using your laptop to dim the lights
and turn off the air-conditioning in your office when you
step out at lunchtime? The science behind these scenarios
is there, even if all the technological details and cost issues
haven’t been resolved yet. As energy prices remain uncertain,
it’s likely that owners will have more incentives in
the future to employ strategies that curtail energy usage,
whether for retrofits or new construction. Imagining what
form those solutions might take—as these researchers
are doing—is half the fun. - Deborah Snoonian, P.E.
Tapping solar radiation’s unseen
benefits
Most designers think of sunlight as
a destructive force when it comes to surface treatments. They
look for UV-stable paints and coatings, and calculate life-cycle
costs with the expectation of regularly replacing exposed
surfaces. But recent advances in materials science point to
coverings—even paints and fabrics—that double as
solar cells. Instead of worrying about the deleterious effects
of the sun, designers could look forward to using a variety
of building materials that have embedded energy-producing
capacity.
Researchers at the University of Toronto,
in Canada, have expanded the range of solar radiation that
such materials can harvest, tapping infrared rays as well
as the visible spectrum of light (current solar technology
works in the visible spectrum only). This could boost the
efficiency of new photovoltaic materials and make them more
affordable; it also opens the way for cheap infrared cameras,
which could figure in building-security systems.
The researchers’ infrared-active
colloidal “quantum dots” are made up of lead sulfur
nanocrystals and semiconducting plastic. By changing the size
of the nanocrystals, the researchers can “tune”
the quantum dots to absorb wavelengths from 800 to 2,000 nanometers.
Within five years or so, architects
and builders might be able to specify rolls of thin, lightweight,
and flexible plastic solar sheeting, made by spraying a solvent
containing the nanocrystals onto a thin, flexible substrate
within a controlled manufacturing environment. Manufacturers
might also choose to coat glass or metal surfaces with the
solvent, according to lead researcher Ted Sargent, a University
of Toronto professor of electrical and computer engineering.
Applying the solvent like paint to such materials in the field
won’t work, says Sargent, because the process needs to
be carried out in a controlled, clean environment to be successful.
Lightweight and flexible solar cells
would do away with many of the limitations of current silicon-based
PV cells, which are heavy, breakable, bulky, and relatively
expensive to install. The quantum dots could also be used
to make thermal photovoltaic cells, tapping infrared radiation
from fuel-fired sources, and for medical diagnostics, using
infrared light to screen for cancer, according to the researchers.
The ability to harness infrared radiation
could make solar energy more practical in more geographic
areas, “assuming there’s some total power-production
rate threshold that has to be met before the approach becomes
economical in a given area,” Sargent said. “There
is a mild advantage in that some infrared light makes it better
through clouds, but the main point is that harvesting infrared
as well as the visible wavelengths results in more power harvested.”
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| Traditional
silicon-based solar panels (above) are often derided for
being clunky and expensive. Researchers at Georgia Tech
are making organic solar cells that are thin and flexible
(below). |
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|
Photography:
© Royalty-Free/CORBIS (top);
Courtesy Georgia Institute of Technology (bottom) |
The quantum dots represent an early
stage in the evolution toward commercially available solar cells.
But their internal quantum efficiency—the amount of photons
absorbed that actually reach the electrical circuit and are
turned into usable energy—is just 3 percent, compared to
90 percent for most PV cells now on the market. The researchers
are working on increasing this number, along with the quantum
dots’ absorption of external light and their external power
efficiency, or the ability to harvest the sun’s power efficiently
over the entire spectrum, absorbing more light at multiple wavelengths
and ensuring that the efficiencies are additive, Sargent said.
Researchers are also addressing the
environmental trade-offs in making solar cells, a process
that’s energy-intensive and involves hazardous chemicals.
The lead sulfide nanoparticles in the Toronto study “need
to be encapsulated, and an end-of-life strategy is needed,
such as recycling of the materials,” Sargent said. He
noted that the lead sulfide is “a showcase for the technology.
The approach illustrates the value of infrared harvesting
cheaply and flexibly. Once we or others develop even more
innocuous materials that do the same thing, they will be adopted.”
Creating a process for making any material
a solar collector by applying quantum dots is a step in the
right direction, said Alexis Karolides, an architect and green-building
consultant with the Rocky Mountain Institute. “Instead
of asking how much can we increase the efficiency of current
photovoltaic technology, we need to ask what’s possible,”
she said.
Down the road, embedded solar cells
and solar sheeting will need to be integrated with building
control systems and power storage technologies like hydrogen
fuel cells, according to Sargent. “Presumably, the days
when the sun is shining don’t correspond identically
with your power needs—so you might think of looking at
power harvesting and storage problems together, in an integrated
fashion.”
Going solar could mean going organic
Despite their advantages, there are
many reasons to look for alternatives to existing silicon-based
photovoltaic (PV) cells. Heavy, bulky, brittle, and aesthetically
compromised, the older designs require clean-room manufacturing
facilities, and are made using some less-than-clean materials
and processes. Transportation and installation can also be
expensive, resulting in higher capital costs.
Thin-film solar cells made of amorphous
silicon and other materials address some of the drawbacks
of current PV cells, including weight and flexibility—but
some of these newer technologies raise environmental and safety
questions of their own. More promising, but earlier in development,
are organic solar cells, which have the potential to be relatively
cheap, easier and cleaner to produce, and more versatile than
existing solar technology.
Researchers at the Georgia Institute
of Technology have developed a lightweight, flexible, organic
photovoltaic cell using pentacene, a polycrystalline organic
semiconductor, and the carbon molecule C60. Pentacene is often
used in research on transistors, and C60 is in the family
of carbon molecules commonly referred to as “buckyballs,”
named for their resemblance to Buckminster Fuller’s designs.
The Georgia Tech organic solar cell
consists of a glass plate, layers of indium oxide, pentacene,
C60, and bathocuproine, and an aluminum electrode. According
to lead researcher Bernard Kippelen, a professor of electrical
and computer engineering at the university, it can be produced
inexpensively...
(continued....)
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