Figure 1
Article by Jeremy Taylor, product system engineer, SunModo
It’s easy to think about solar systems getting hot—their potential is realized when the sun beats down on them. Temperatures on roofs can reach beyond 200°F. But in most climates, systems get cold, too. Even in Hawaii, panels and racking can experience temperature swings totaling more than 100°F. Concern about temperature becomes relevant when discussing mechanical connections for weatherization and human safety.
Typically, solar panels have accounted for temperature swing, and the mechanical expansion and contraction associated with it, through flexibility in construction materials and, on a relatively small scale, in each module. To appropriately account for temperature fluctuations on a system level, however, the racking must also be considered.
The issue becomes most critical when mechanically attached materials, such as aluminum rails and a wood deck, expand at different rates. A steel roof with metal purlins whose structure remains in the shade is also worth considering. How elevated the array is above the roof will also influence how much ventilation it gets, contributing to temperature. This is important because attachments can loosen their water seals or shear standoffs and hardware over time. For a full set of calculations, please visit SunModo’s website for an application note. In general, the rules of thumb mentioned below are useful.
If the installation location is determined to be a mild climate with ambient temperature change of less than 60°F across the year, then creating a thermal measure every 80 in. is suggested. In extreme climates (101 to 130°F ambient change) and moderate climates (60 to 100°F ambient change) this should be accounted for at every 40 in. Systems with shorter rails experience less accumulated change than engineered deflection values can handle.
Figure 2
Installers have two options. An installation can be designed as separate arrays with stand offs and end clamps stopping and starting at the thermal break. Or installers can use a thermal splice, sliding clip and bonding strap, which makes the array appear continual.
If the latter option is chosen, it is imperative that the gap be set correctly. Over-setting the gap would create a problem almost as bad as no thermal break at all: the rails could become disconnected. So it is important to note the gap chart (see figure 1) and use an IR temp gun to account for the actual installation temperature of which the expanding and contracting rails will begin their movement.
A helpful tool from SunModo can help measure a system with thermal gaps (see figure 2). When sunlight-elevated rooftop temperatures are thoroughly considered, many factors are influenced, including system performance, conductor sizing and, most importantly, worker health. In the same light, planning for thermal expansion in arrays will keep attachments weatherized and PV racking lasting a lifetime.