by Rick Duncan, PhD, PE
Eliminating fossil fuel consumption in buildings is best done by achieving both energy conservation and energy generation. Sprayed polyurethane foam (SPF) roofing provides a method for accomplishing the first goal, and photovoltaic (PV) systems offer high performance for the second. Given the core energy solutions SPF roofing and PV systems each offer, roofing assemblies that incorporate both are increasingly being specified in the built environment.
Combining SPF and PV systems can be quite effective in roofing projects for both commercial and residential structures. There are, however, considerations that must be addressed to ensure optimal performance when looking to join these two powerful systems on the roof of a building.
Rooftop PV installation types for use with SPF
Rooftop PV systems may either be installed on racks or adhered directly to the roof surface. When combining photovoltaics with SPF, it is generally not advisable to employ the latter method. Solar heat and water can accumulate between the PV and roof coating and negatively impacting coating performance. Moreover, panels applied directly to a low-slope roof will not properly align with the sun—therefore, performance is compromised.
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 of rack offers 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.
The type of PV panel predominantly used with SPF roofing is crystalline silicon (cSi). A typically rigid panel with glass and metal frames, cSi may be applied via rack installation methods—unlike other dominant PV panel types.
Electrical safety and heat buildup
PV 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 two wires from the back 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. To do this, the whole system must be shut down properly, as a precautionary measure to prevent shocks from harming workers or arcing between electrical connections. This ‘shutdown’ procedure—provided by the inverter manufacturer—must be followed with precision as part of a lock-out/tag-out program. Under no circumstances should SPF contractors ever disconnect or decommission a PV panel or system unless trained and qualified to do so.
PV panels convert approximately 15 to 20 percent of light to electricity, leaving any remaining unconverted energy to be released as heat. Additionally, these panels become more effective when their temperature drops. Due to these factors, the majority of rooftop PV installations are done with the goal of encouraging airflow under panels, which in turn reduces the temperature of the panels, improves conversion efficiency, and allows them to release heat effectively.
PV panels installed 100 to 127 mm (4 to 5 in.) above the roof will leave the temperature of the roof unchanged. Instead, they provide shade to the surface, potentially extending the life of SPF roof coatings.