A small California
company called BioSolar is
introducing plastic components for photovoltaic (PV) cells based on cotton and
castor oil that are described as less costly than plastics based on petroleum.
"Designed specifically for cost-sensitive, economical PV
solar cell modules, we expect the BioBacksheet-C to be instrumental in driving
down the cost per watt of solar power," says David Lee, chief executive officer
of BioSolar. "An initial full production run was recently completed at one of
our manufacturing partners and samples will be available for all interested PV
module manufacturers shortly."
The substrate of BioBackSheet-C is a cellulosic plastic
coated with a proprietary material to reduce the water vapor transmission rate
and to increase abrasion resistance. The two-component system aimed at
crystalline silicon photovoltaic applications is said to possess high
dielectric strength.
BioSolar's products attack one of the conundrums of the
emerging solar cell industry - they use millions of square feet of materials
that are based on petroleum or require use of toxic chemicals to produce.
BioSolar's approach is also unusual in the green world because it saves money -
at least according to the company's business plan.
"Our goal is to sell any given configuration at a savings of
20-25 percent versus the prevailing material in the market," says Lee. "With our first product, we are achieving
that goal."
Most biobased plastics target packaging markets and cost a
premium of 30 to 500 percent above the materials they replace. In the solar
cell market, materials are generally much more expensive, and in some cases,
have been in short supply.
One material in BioSolar's gunsite is DuPont's Tedlar
polyvinyl fluoride (PVF), a thermoplastic fluoropolymer that is structurally
very similar to polyvinyl chloride. It has excellent vapor permeability and is
very resistant to weathering, key requirements for the backsheet of a
photovoltaic cell. Tedlar constructions often replace glass.
Costly Structures
Tedlar is typically used as the outside layer of a sheet that
includes polyester to reduce costs and improve mechanical strength and
dielectric properties. Price of the sheet is around $1.60 a square foot.
Tedlar is solvent-cast using an industrial solvent called dimethyl
acetamide (DMAC), which can produce systemic injury when inhaled or absorbed
through the skin in sufficient quantities over a prolonged period of time. If
Tedlar is burned, corrosive hydrogen fluoride fumes can be released. Another
important issue: Tedlar has been in very
short supply for the past two years because of its growing use in photovoltaic
cells and aircraft interiors.
In August, DuPont announced plans
to boost Tedlar production by 50 percent.
"Our capacity expansions are critical steps in growing the
Tedlar business and maintaining our market leadership in backsheets for solar
panels that deliver the long-term, reliable power supply that our customers
have come to expect from their investment in renewable power generation," says David
B. Miller, group vice president - DuPont Electronic & Communication
Technologies.
There are at least 10 companies that make backsheet
materials for solar cells. Some of them are testing other biobased plastics such
as polylactic acid. Stan Levy, a former DuPont official who is now the chief
technical officer at BiSolar, says PLA falls short as a solar cell material.
"It is designed as a biodegradable product, and is almost destroyed during the
damp heat test (used to test materials)." In addition, extruded PLA film can be
brittle. Levy spent 27 years at DuPont working on many premiere films,
including Teflon, Mylar and Kapton.
In other markets, such as automotive, PLA is alloyed
with a petroleum-based engineering plastic to boost durability. The sole
advantage is reduction of the carbon footprint. Such a solution could also be
introduced for the PV market.
Two Employees
A fascinating aspect of BioSolar is that the company has
only two employees, yet is developing and introducing a broad slate of
bio-based back sheets that offer different properties and economics.
The two bioplastics deployed by BioSolar are available from
well-known suppliers. The castor oil-derived nylon 11 was developed in Europe in the 1940s and has been used for industrial
applications for more than 50 years. BioSolar will initially buy nylon 11 from
the primary supplier, Arkema, but other major companies are developing capacity
as demand grows for the unique bioplastic. DuPont
and BASF,
for example, are now offering the material.
Significant new technology may be required to process the
biomaterials for photovoltaic applications. Rowland
Technologies, a leading extruder of high performance films, is working on
thin nylon 11 films. Lee would not disclose the name of BioSolar's supplier of
cellulosic materials.
The films using cellulose are manufactured much the same way
that electrical insulation papers are made, using Fourdrinier machines.
Cellulosic polymers go back to the 19th Century when they used to
replace ivory in billiard balls. In fact, they were the first successful
thermoplastic polymer, predating plastics such as polyethylene by more than 80
years.
"Special types of cellulosic films have been used in
dielectric applications for more than 80 years and meet or exceed the UL
specifications for use in PV modules," says Levy. They are inexpensive, but
have a high water vapor transmission rate (WVTR). BioSolar plans to laminate
nylon 11 to the cellulosic film to reduce its WVTR and to improve wear
properties.
BioSolar isn't the only game in town in the growing
photovoltaic market. Several manufacturers are developing new materials'
configurations to improve economics and performance. And DuPont obviously
doesn't agree with BioSolar's assessment of Tedlar. DuPont is investing more
than $120 million in the Tedlar expansion.
In addition, some manufacturers may require several years of
field testing to determine the worthiness of the new biomaterials for
applications expected to last 15 years or longer.
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