Spray Polyurethane Foam and Photovoltaic Roofing Systems

Spray polyurethane foam and photovoltaic systems are increasingly utilized together as
a joint solution for energy savings. With the continued push toward sustainability and growing
movements, like net-zero-energy construction, SPF and PV systems are a logical combined solution for the generation of renewable energy, the conservation of heating and cooling energy, and the elimination of the structure’s dependence on fossil-fuel-consuming electricity sources. Regardless of whether net-zero energy is the end goal, SPF and PV combined in roofing can be quite effective for many structures. Here are some considerations when looking to join these two powerful systems on the roof of a building.

ROOFTOP PV INSTALLATION TYPES FOR USE WITH SPF

Installation of PV systems on SPF roofing will inevitably create additional foot traffic. It is important to protect heavily trafficked areas with additional coating and granules or walk pads.

Installation of PV systems on SPF roofing will inevitably create additional foot traffic. It is important to protect heavily trafficked areas with additional coating and granules or walk pads.


Rooftop PV systems can vary significantly in size. Large-footprint buildings can employ PV systems rated from 50 kilowatts to 1,000 kW or larger while residential rooftop PV systems are commonly 3 kW to 5 kW.

Rooftop PV systems may be installed on racks or adhered directly to the roof surface. When looking to combine PV with SPF, it is generally not advised to adhere or place the PV panels directly onto the roof surface. Solar heat and water can accumulate between the PV and roof coating which could negatively impact coating performance. Moreover, panels applied directly to a low-slope roof will not be properly aligned with the sun to achieve optimal performance.

Non-penetrating rack systems may be placed directly on a rooftop and held in place with ballast. Racks may also be installed with penetrating supports that require flashings. Each type provides advantages and disadvantages. For example, ballasted racks may block water flow and affect drainage while penetrations require leak- and maintenance-prone flashings. SPF is unique in that it easily self-flashes around penetrating supports.

PV EXPLANATION

PV cells are the basic unit used to convert light to electricity. Many PV cells are bundled together to make a PV panel, or module. PV panels are grouped electrically to create a PV string. Depending on the system size, two or more strings are combined to create a PV array.

The dominant type of PV panel used with SPF roofing is cSi, or crystalline silicon. cSi is a typically rigid panel with a glass and metal frame and may be applied, unlike other dominant PV panel types, via rack installation methods.

A PV system includes many components in addition to the panels. Components include racks, rails, rooftop attachment devices, grounding systems, wiring and wiring harnesses, combiner boxes, inverter(s) and connection to the main electrical panel. Components may also include control modules and storage batteries for off-grid PV system installations.

ELECTRICAL SAFETY

Photovoltaic panels must be handled and maintained with caution. Electricity is produced when a single panel is exposed to light; however, because a panel is not part of a circuit, that electricity will not flow until the circuit is complete. A worker may complete the circuit by connecting the two wires from the backside of a PV panel.

When maintaining a PV system, it may become necessary at some point to disconnect or remove an individual panel from a string or an array. The whole system must be shutdown properly as a precautionary measure to prevent shocks from occurring to workers and arcing between electrical connections. This “shutdown” procedure must be followed with precision as part of a lock-out/tag-out program. This procedure is provided by the inverter manufacturer. Under no circumstances should SPF contractors ever disconnect or decommission a PV panel or system unless they are trained and qualified to do so.

HEAT BUILDUP

Photovoltaic panels convert approximately 15 to 20 percent of light to electricity, leaving the remaining unconverted energy to be released as heat. Additionally, PV panels are more effective when their temperature drops. It is for these reasons that the majority of rooftop PV systems are installed to encourage airflow under panels, which reduces the temperature of the panels, improves conversion efficiency and releases heat effectively. Photovoltaic panels installed 4 to 5 inches above the roof will not change the temperature of the roof and, instead, provide shade to the surface of that roof. This additional shade may extend the life of SPF roof coatings.

LOAD

PV panels add weight to a rooftop and this must be factored into the design and installation. Existing structures should be analyzed by a structural engineer to determine if the additional weight of the PV system is acceptable.

Rack-mounted arrays with penetrating attachments are fairly lightweight at 2 to 3 pounds per square foot, and ballasted arrays add 4 to 6 pounds per square foot. However, with the latter, more ballast is utilized at the perimeters and corners of a PV array. Thus, localized loading from ballast may reach as high as 12 to 17 pounds per square foot, which must be considered. Most SPF roofing systems have a compressive strength of 40 to 60 psi.

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Solar Market to Grow 75 Percent by 2019

Led by China, the solar industry will grow at a CAGR of 8.3 percent from 37.5 GWp in 2013 to 65.6 GWp in 2019, but emerging trade disputes involving the Asian giant, as much as global policies, cast a shadow over short-term prospects, according to Lux Research.

China became the biggest solar market in the world with 11.8 GWp installations in 2013, and has been key to faster-than-expected global recovery. Since the competitive bankruptcy-ridden cost environment of 2012, module supplier margins have increased, with most Tier-1 suppliers topping 10 percent toward the end of 2013 and 15% in the first quarter of 2014.

With solar now fairly common in most parts of the world, it reaps the rewards of direct incentives but also faces uncertainty due to pressure on trade activity with China,” said Matthew Feinstein, Lux Research Senior Analyst and the lead author of the report titled, “Solar Market Size Update 2014: Reform for the Long Haul.”

“Furthermore, as an increasingly commonplace electricity source, most major markets are dealing with some combination of these dynamics, complicating the status of policy globally,” he added.

Lux Research analysts evaluated the growth trajectory of the solar industry, besides weighing policy and other challenges. Among their findings:

Growth is fastest in the Americas. At a CAGR of 16.3 percent, the Americas will be the fastest-growing region in the world as its new installations market nearly triples from 5.3 GWp in 2013 to 15.4 GWp in 2019. The U.S. will pace the rest of the Americas, growing from 4.7 GWp to 11.7 GWp but South America will grow 10-fold to 2.5 GWp in 2019. The Asia-Pacific region will grow at a lower 8.2 percent CAGR but will account for over 50 percent of global demand, led by China, Japan and other emerging markets.

Cost cuts will be sustained. With cost cuts critical to the sustained growth of the industry, incremental increases in efficiency are on course from technologies such as passivated emitter rear contact (PERC), heterojunction with intrinsic layer (HIT) and selective emitter (SE). System costs will drop by between $0.36/Wp for utility-scale and $0.60/Wp for residential by 2019. This will translate to a 20 percent cut in total system costs.

X-Si remains technology of choice. Crystalline silicon (x-Si) will dominate the solar market through 2019 even though other module technologies such as copper iridium gallium diselenide (CIGS), copper zinc tinc sulfide (CZTS), cadmium telluride (CdTe) and thin, flexible, epitaxial silicon (epi-Si) have the potential to become major threats in the future. X-Si, with an 84.6% market share, will grow from 31.6 GWp in 2013 to 55.7 GWp in 2019, growing at a CAGR of 8.45%. CdTe and CIGS will be a distant second – growing to 4.8 GWp and 4.2 GWp, respectively, in 2019.

The report, titled “Solar Market Size Update 2014: Reform for the Long Haul,” is part of the Lux Research Solar Intelligence service.