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Flexible Thin-Film PV for Metal Roofs

Ma  Special  Feature  Jun17 2 Lowrez

Designers are finding firm success with flexible PV

Today, solar energy is becoming as visible as the sun. Flexible, thin-film photovoltaic (PV) products are a vital component of this movement. They incorporate very thin layers of photovoltaic material placed on a glass superstrate or a metal substrate. Thin-film solar cells can consist of several technologies, including cadmium telluride, copper indium gallium selenide and amorphous thin-film silicon.

Cadmium telluride is the predominant thin-film technology. With about 5 percent of worldwide PV production, it accounts for more than half of the thin-film market. Thin-film PV thickness varies from a few nanometers to tens of micrometers. This is much thinner than flexible films' rival technology, the conventional, first-generation crystalline silicone solar cell, which uses wafers of to 200 micrometers. One of the main advantages to flexible thin-film

PV products is that they can manufactured in long, continuous rolls, or incorporated onto a flexible substrate, such as laminates, shingles and roofing tile.

 

FLEXIBLE DESIGN AESTHETICS

Metal roofing is a very good platform for thin-film PV technology. When installed on a metal roof, thin-film PV can adhere directly to the roof surface between the raised seams. Its connectors are easily hidden underneath the roof's ridge cap, where they are protected from snow and ice. The result is an aesthetically pleasing solar roof and a clean, sleek look without obtrusive racks.

Compared to traditional crystalline module, flexible thin-film PV products' appearance is more uniform. When applied directly to a dark-colored standing seam metal roof, flexible, thin-film PV is hardly discernible. Because of its low profile and uniform color, thin-film PV appeals to those with a concern for aesthetics-architects, designers, and end users-for streamlined building-integrated applications and maximum freedom of design.

"Aesthetically, I prefer thin-film PV with metal roofs because of the minimal profile that thin film provides," says Arielle Schechter, architect, PLLC, AIA, Chapel Hill, N.C. "One of the aesthetic advantages of metal roofing is the seam-and-joint profile, which forms shadow lines and an interesting rhythm. Traditional crystalline panels are usually top mounted and hide the roof profile. Unlike the more traditional crystalline panels, the thin film sits right on the roof versus above the ribs, allowing the metal roofing profile to still be visible. Thin film allows one to keep the metal roof profile prominent as a design element."

Certain low-load capacity roofs can't support conventional crystalline panels and rack systems because of the PV's added weight. However, thin-film PV can weigh as little as 7 ounces per square foot. In addition to roofs, thin-film PV can integrate better into architectural features like building façades and windows.

Solar installers benefit from flexible PV because its high-performance, peel-and-stick adhesive application does not require racking assembly, ballast or roof penetrations. This means an easier installation with a faster learning curve. The factory-applied, butyl-based self-adhesive has a tested and proven track record. With it, the flexible modules become an integrated part of the roof system producing the same wind uplift and seismic performance characteristics of the roof system itself. Labor costs are also lower because project staging, loading and installation can be completed faster with fewer workers than with conventional rack-mounted glass modules.

In spite of their thin design, flexible PV modules are exceptionally durable. Most can be walked on and many have a warranty of 20 years. Asphalt shingle roofs don't have as long of a life span and will fail much sooner than metal panel roofs. When asphalt shingle roofs fail, building owners will be faced not only with the re-roofing expense, but also with complete removal and re-installation of the solar array. Metal is one of the most durable roofing materials available. Metal roofs will likely last longer than the life span of solar panels, which reduces expense and hassle in the long term.

 

ADVANTAGES AND EFFICIENCY

Thin-film PV roof laminates are ideal for facilities with challenging metal roof issues and have the following benefits:

  • Produce energy even in low-light or when partially shaded
  • Seamless integration onto buildings
  • Lower installation costs
  • Can fit around curved structures
  • No roof penetrations
  • Best resistance to wind uplift
  • Durable, nonbreakable
  • Lightweight, with no need for structural reinforcements
  • Low maintenance
  • Lays flat, no tilt required
  • Vandal- and theft-resistance
  • No wind load
  • No mounting hardware

 

In spite of its many advantages, thin-film PV did have one initial setback. The first-generation of commercial thin-film PV materials had a sunlight-toelectricity conversion less than crystalline module PV. First-generation, single- and multicrystalline modules had typical conversion efficiencies between 12 percent and 17 percent. But, early thinfilm technologies only had half that, approximately 6 percent to 8 percent. This resulted in a larger area required to produce a power output equal to crystalline modules. However, second-generation thin-film modules made of copper indium gallium selenide have shown higher conversion lab efficiency above 20 percent. This thin-film module has a 2 percent share of the overall PV market, but is growing.

"Thin-film PV efficiency has increased," says Mark Hagerty, president of Michigan Solar & Wind Power Solutions, Commerce Charter Township, Mich. "The appearance seems to be the same; less panels are needed. Thin film will win out in the long run since it can be tailored to the building for building integrated solar-the shingles and siding can be thin film-they cannot be crystalline silicone."

Some thin-film technologies provide better shade tolerance and low-light performance than crystalline modules. They contain triple-junction cells to implement bypass diodes across each cell within the module (not just within the module junction box), allowing current to flow around any shaded cell. And each sub-cell of the multi-junction cell can capture different light wavelengths, resulting in higher-power production in cloudy weather and diffused light. Other thin-film modules (without bypass diodes on every cell) have better shade tolerance than crystalline modules due to cell shape.

Flexible, thin-film PV mounted on a metal roof aids in building energy savings. Thin-film PV can absorb some sunlight and reflect the remainder, which contributes to cooling. To help achieve LEED certification, thin films gain points in renewable energy production, green power and carbon offsets, heat island reduction, and building life cycle impact reduction. Building owners who have thin-film PV mounted on their metal roofs can often receive tax credits and incentives.

 

EMERGING THIN-FILMS

There are emerging thin-film PV technologies classified by the National Renewable Energy Laboratory. Many use organic materials, often organometallic compounds, as well as inorganic substances. While they have not yet been commercially applied, and are still in the research and development phase, many think they could achieve low-cost, high-efficiency solar cells.

These emerging photovoltaics, often called thirdgeneration photovoltaic cells, include:

  • Copper zinc tin sulfide solar cell
  • Dye-sensitized solar cell, also known as Grätzel cell
  • Organic solar cell
  • Perovskite solar cell
  • Polymer solar cell
  • Quantum dot solar cell

 

FLEX-FILM PV MARKET GROWTH

According to a recent report from Market Research Future titled, "Thin Film Photovoltaic Market Overview, Manufacturing Cost Structure Analysis, Growth Opportunities & Restraints 2016-2027" increasing demand for electricity globally and technological advancement are primary driving factors in the growth of the thin-film photovoltaic cells market. Thin-film photovoltaic cells' cost effectiveness and a significantly renewable source of energy has gained much popularity in recent times with many new market players entering the market.

The report gives a clear picture of current market scenarios, which includes historical and projected market size in terms of value and volume, technological advancement, macro economical and governing factors in the market. It claims that Asia- Pacific is the leading region for thin-film photovoltaic cells market mainly due to several thin-film PV cell projects expected underway in this region. Increasing demand for power and awareness regarding renewable energy sources is also expected to drive the market in this region. It also reports that Europe and North America are other dominant regions due to technological advancements.

This market overview only goes to further emphasize that when used on metal roofs, flexible thinfilm PV produces an operational and environmentally friendly roof system that generates renewable, clean, green energy, one that will continue to do so for many years to come.

 

Sidebar: Ask an Architect

What I find incredible about thin-film photovoltaics (PV) is how manufacturers and developers of these products truly listen to the industry (especially designers, since we have our eye on aesthetics) and have created thinner, higher-performing output products that have become visually nearly gone, out of sight. [PV films] have integrated itself seamlessly into the building design and many of us would not even notice what the product is or does!

I feel PV films are moving to a more seamless appearance, higher output of energy and some types of integrated product are within the metal panel systems themselves such that I foresee being nearly impossible to differentiate what the eye may be seeing … times are exciting in this field!

Something I learned by taking an online class about thin-film PV products is the amazing history of development of products used, i.e., from silicon base materials to cell structure development to Gallium base materials. The research and development side is not failing us in trying to develop a higher-output product for less expense. Other thin-film technologies I learned about that are very exciting in development are the quantum dots, nanocrystal technology and organics. The reach of thinking is incredible.
W. Scott Schellhase, AIA, NCARB, APA, LEED AP BD+C, vice president, SLS Partnership Inc., Lubbock, Texas